'Impact Resistance'. In: Encyclopedia of Polymer …nguyen.hong.hai.free.fr/EBOOKS/SCIENCE...

IMPACT RESISTANCE

Introduction

Impact resistance is a measure of the ability of a material, specimen, or structureto withstand a sudden load without failure. The impact resistance of a specimenor structure is therefore a complex function not only of intrinsic factors such as themechanical properties of the material but also of extrinsic factors such as geometry,mode of loading, load application rate, environment, and, quite importantly, thedefinition of failure.

The issue of stress geometry or mode of loading is all-important: a materialhighly resistant to failure in one mode of loading can fail catastrophically in an-other. Therefore, the impact resistance of a structure is relevant only with respectto a particular mode of loading. A soundly designed structure can fail unexpect-edly if the geometry of a structural component is altered by the gradual changeof shape, material wear, or crack growth, or if the mode of loading is changed, aswhen a force is applied in an unforeseen manner. Thus, it is often necessary to testthe material or structure under conditions more severe than those of actual use.It is also prudent to follow certain well-established design guidelines. Last butnot least, the use of damage tolerant materials may contribute to the avoidanceof catastrophic failure.

Impact often implies a high loading rate, but the type of failure usuallyassociated with impact can also occur under apparently innocuous conditions,even at low rates of loading. In addition to high strain rates, low temperaturesand sharp notches tend to reduce fracture resistance. When the rate of strainenergy accumulation is higher than can be contained in or dissipated by thematerial in the vicinity of a crack or a flaw, sudden and unstable fracture canoccur, often entailing crack speeds of hundreds of meters per second. Higher

528Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

Vol. 6 IMPACT RESISTANCE 529

speeds are more likely to cause unstable fracture, other conditions being equal,for then even small flaws, less severe geometries, or milder temperatures canbecome critical. This is partly due to the viscoelastic nature of polymers. Undertruly high speed conditions, ie, when inertial effects are involved, the deformationand fracture behavior are complicated by wave propagation and constraint effects(1–4).

In contrast to impact resistance, the fracture toughness of a solid polymer ismuch better defined for a given set of quasi-static low strain rate testing con-ditions (see also FRACTURE; FATIGUE and ASTM D5045-99). Toughness can of-ten be improved by the incorporation of a soft, elastomeric phase into the rigidpolymer matrix. The art and science of material modification for the purpose oftoughening is extremely complex (5). Enhancement of toughness improves impactresistance.

The performance of a material or a structure also depends on local variationsdue to processing conditions, such as cooling rate, shear stress, and melt-flowpaths, which result in orientation and residual stress. Multiphase materials canexhibit even greater local variations in material properties than those observedin single phase polymers. A structure produced by a given process may possesssignificant morphological differences from test specimens. Ultimately, it may benecessary to test the impact resistance of components.

Since the impact toughness of a material, as it is commonly called, dependson the technique of measurement, the testing instruments and the technique mustbe considered first.

Testing Machines and Techniques

Several types of machines are in use for different types of tests, offering advan-tages and disadvantages as well as different information. Impact strength is mea-sured by many different empirical methods, some of which may not be appropri-ate for the performance evaluation of finished products. More sophisticated andmeaningful testing devices are under development but remain mostly in researchlaboratories. The following is a description of the machines and techniques thathave gained wide currency. However the foregoing should serve to warn the readerto be cautious about the validity of the results for applications. For this and otherreasons a number of standardized test methods are used to approximate actualuse conditions.

Standard Test Methods. Many impact tests are sufficiently standardizedto have ASTM designations (see Table 1).

Pendulum-Type Instruments. Pendulum-type machines are used fornotched or unnotched specimens that may be of different sizes and supportedas a cantilever (Izod) or as a bar supported at its ends (Charpy). These tests arealso referred to as flexed-beam impact tests.

The Izod impact test is based on an old, established test originally designedfor metals in which a notched-bar specimen is tested in cantilever fashion with anexcess energy pendulum machine. Izod (ASTM D256), Charpy (ASTM D6110 andResearch Report D20-1034, to become a new standard), and the tensile-impact(ASTM D1822) tests can be performed with pendulum machines.

530 IMPACT RESISTANCE Vol. 6

Table 1. Standard Tests for Impact Resistancea

Test Designation Description

Brittlenesstemperature

ASTM D746 The temperature is determined at whichplastics and elastomers exhibit brittlefailure under impact.

Falling weight ASTM D3029 Impact resistance indicated by energy tobreak or crack rigid plastics by means ofa falling weight (tup); constant dropheight and variable weight (tup)b arerecommended.

Falling weight ASTM D1709 Similar to D3029 but for measuring impactresistance of polyethylene film byfree-falling round-headed dart.

Falling weight ASTM D2444 For impact resistance of thermoplastic pipeand fittings by falling weight (tup).b

Fracture toughness ASTM D5045 For plane-strain fracture toughness.High rate stress/

strain (tension)ASTM D2289 Area under stress–strain curve measures

impact resistance at testing speeds up to254 m/min.

Izod impact ASTM D256c Energy to break a notched cantilever beamspecimen upon impact by a pendulum.Notch tends to promote brittle failure.Unnotched impact strength is obtainedby reversing the notched specimen in thevise. Notch sensitivity can be determinedby using Method D.

Charpy impact ASTM D6110 (also,Research ReportD20-1034)

Similar to Izod impact test. Notchedspecimen is supported on two ends andstruck by a pendulum in the middle, athree-point-bend setting.

Tensile impact ASTM D1822 Recommended for plastic materials tooflexible, too thin, or too rigid to be testedby ASTM D256. Measures energy tobreak by “shock in tension” imparted bya swinging pendulum.

aRef. 6.bDesignation of the weight.cIncludes Charpy impact testing.

In these machines a mass is attached to the end of an arm that rotates abouta pivot point; a striker is attached to a mass. The arm is raised to a predeterminedposition. When the arm is released, it swings downward and strikes the specimen,which is mounted rigidly in a vise or a fixture. The precise mounting position andthe type of fixture depends on the test. The mass, length of the arm, and angleat the raised position determine the amount of available energy. The total energymust be high enough to give a complete break. Typical pendulum machines of theIzod and Charpy types are depicted in Figures 1 and 2, respectively; specimengeometries are shown in Figure 3. After the striker breaks the specimen at thebottom of its swing, it continues to swing upward until all the kinetic energy is


Point of impact

90o

Fig. 1. Izod-type pendulum impact machine, ASTM D256.

converted to potential energy. The energy absorbed by the specimen, Es, is foundby

ES = EI − ER − EF − Ew

where EI is the initial energy available, ER the energy at the maximum angulartravel, EF the energy dissipated by friction, and Ew the kinetic energy to removethe broken, detached half-specimen as it is thrown forward by the pendulum. Thelast one is usually small; EF can be found by performing the test without a speci-men in place. Machines are usually equipped with a dial with calibration marksmounted in the plane of rotation in such a way that EI and ER can be read di-rectly. An “instrumented” pendulum machine is equipped with a load cell, usuallya piezoelectric quartz device for high frequency response, mounted on the strikeror on the mounting block to which the specimen vise is attached. The electric sig-nal from the load cell is amplified, digitized, and recorded by electronic memory.The energy is calculated from the load–time relationship by a computer connectedto the instrument. The total energy can be displayed by a digital readout, or the


Point of impact

90o

Fig. 2. Charpy-type pendulum impact machine, ASTM D256.

entire load–time trace can be displayed on a cathode-ray tube screen. In the latterinstance, a microcomputer is attached and the information can be processed asdesired.

The Izod test is used mostly in the United States and the United King-dom [British Standard (BS) 2782, Method 306A]. The notched specimen is firmlyclamped in a vertical position in a vise fixed in the base of the apparatus (Fig. 1).A pendulum fitted with a striker head of 3.2-mm radius, falling from a height of0.61 m (ASTM) or 0.31 m (BS), hits the specimen horizontally at a point abovethe notch. At least 10 samples are required to obtain a satisfactory assessment ofimpact strength.

The U.S. test method has now been accepted as an International Standardiza-tion Recommendation ISO/R180, 2000. The material from which the test pieces arecut must be prepared under carefully controlled conditions to ensure developmentof full strength and freedom from strain caused by uneven heating or rapid cooling.The cutting of the notch requires special care, particularly with notch-sensitivematerials. Small variations in the notch can give wide variations in impact values.The notch should be cut with a tool in good condition and shaped smoothly andaccurately to the dimension called for in the test specification. Errors are caused


Fig. 3. Geometry of testing and specimen support arrangement for (a) Charpy and (b)Izod tests specified in ASTM D256 (7). Courtesy of Applied Science Publishers Ltd.

by variations in clamping pressure, failure to strike the specimens squarely, andthe condition of the cutter and the cutting techniques for machining notches (8,9).Impact strength decreases linearly with increasing clamping pressure. A taperof 13 min of an arc on the specimen induces errors up to 25% of the expectedimpact strength of a perfectly parallel specimen. Variations in excess of 10% inexpected impact strength were observed from differences in the notch cutter. Tor-sional stresses caused in the Izod specimen by skewness between the pendulumstriker head and the surface of the specimen increase impact strengths even forsmall angles of skewness (Table 2).

Table 2. Effect of Skew Striking on ImpactStrength of Poly(methyl methacrylate) Sheeta,b

Impact strength, J/mc ofnotchd at clamping pressure

Angle ofskewness 16.5 MPae 49.6 MPae

10′ 26.05 22.4250′ 27.75 25.781◦10′ 28.02 26.954◦50′ 29.78 29.365◦10′ 29.78 29.52aRef. 8.bPerspex, Imperial Chemical Industries.cTo convert J/m to ft·lbf/in., divide by 53.38.dBritish standard.eTo convert MPa to psi, multiply by 145.


Table 3. Energy for Crack Initiation and Propagation for Standard Cast PMMAa,b

Energy, %Notched impactstrength, J/mc Crack propagation Crack initiation Energy ratiod

ASTM 17.08 69 31 2.22BS 24.02 49 51 0.96aRef. 8.bPerspex, Imperial Chemical Industries.cTo convert J/m to ft·lbf/in., divide by 53.38.dCrack propagation to crack initiation.

The total breaking energy as measured by the Izod test comprises the follow-ing: energy to initiate fracture; energy to propagate fracture across the specimen;energy for plastic deformation; energy to throw the broken fragment away fromthe test piece; and energy lost through friction and vibration (9).

The energy to initiate and propagate fracture depends largely on the notchgeometry. The energy for plastic deformation is important in materials that, evenwhen notched, break by ductile failure. The energy to separate the broken frag-ments is important in materials of low impact strength and high density, eg, min-eral filled phenolic resin. The effect is corrected by placing the fragment on thestriker head of the machine, striking it again, and subtracting the energy absorbedfrom the original apparent impact strength. However, in being thrown forward thefragment may adopt different modes of movement, and may rotate only under thefirst strike. For high impact, low density materials, this source of error is lessimportant. Points of increased stress or of stress concentration can be introducedduring machining; their effect is less for the ASTM specimen than for the BSspecimen (8).

As can be seen in Table 3, in the ASTM test the energy required for crackpropagation in PMMA is more than twice of that required for crack initiation;in the BS test they are about equal. Therefore the ASTM test results are muchless likely to be affected by the specimen preparation than are BS test results.The BS specimens are more susceptible to imperfections in the notch surfaceand to the direction of machine marks (ie, stress-concentrating imperfections).Moreover, since a machined surface is never as smooth as a molded surface, molded(notched) specimens are expected to give higher values. This has been confirmedwith specimens prepared from PMMA moldings where molded notch values arehigher than machined notch values by a factor of 1.8. In addition, specimenswith molded notches, especially those from crystalline polymers, exhibit higherimpact strength than those with machined notches because of the difference inmorphology or thermal stresses due to rapid cooling near the surface and slowercooling in the interior.

In the Charpy impact tester the specimen is mounted on a span support andstruck centrally with a swinging pendulum (see Fig. 2). The results are expressedin terms of breaking energy per unit of cross-sectional area.

The Charpy pendulum impact test is most commonly employed in the Eu-ropean continent. Both the standard test bar and the small standard test barare specified in the German Standard DIN 53453. The sample is notched and is


supported horizontally against the stops at either end. The pendulum striker hitsthe sample centrally behind the notch and the excess energy of the pendulum ismeasured by the angle of the subsequent swing. A large number of tests is neces-sary to give an average value. The Charpy-type test used in the United States isASTM D6110 (Research Report D20-1034); in the United Kingdom, it is BS 2782,Methods 306 D and E.

Although the Charpy impact test is accurate and reproducible, significanterrors may occur (10). Testing and machine variables tend to produce abnormallyhigh values, since they retard the pendulum swing or otherwise cause excessenergy losses. The recorded energy may be in error by as much as 100–200% atlow energy and 15–20% at high energy levels. Differences are usually caused by thecondition of the test machines; methods of machining and finishing the specimens;and cooling and testing (5). With strict controls and carefully prepared specimens,an average impact value should be accurate to within 5% for values up to 27.1 J(20 ft·lbf).

Pendulum-type machines are inexpensive and simple to use and maintain.Data can be produced from which fracture toughness GIc (more correctly known asthe critical strain-energy release rate in Mode I) can be calculated (see FRACTURE

and ASTM D5045-99 and later in this article). However, these machines are lim-ited in the range of impact speeds, with a maximum of 3.4 m/s. Furthermore, thecommon practice of reporting only the total energy does not allow easy interpre-tation of the data, which can be overcome by using the instrumentation describedpreviously. The use of instrumentation as described previously is often helpful forthis purpose.

Falling-Weight Impact Instruments. Falling-weight methods are usuallyemployed for sheet specimens. The weight may be gradually increased in mass, ordropped from increasing heights on the same specimen or onto a series of speci-mens. Like the pendulum tester, the machine is driven by gravity. The specimen,ordinarily in the form of sheet, is subjected to a direct blow from a falling weight.The weight is raised to a known height and is allowed to fall almost freely insidea guide tube or along a set of guide rails (see Fig. 4). A striker is mounted beneaththe weight or in a guide resting on top of the specimen; the Gardner impact tester,usually preferred, is of the latter type. In recent years drop towers of considerableheight have become commercially available. However, as the terminal velocity de-pends on the square root of the drop height, the maximum velocity that can beconveniently obtained is limited. In the case of the drop tower an arrangementsimilar to that described for the instrumented pendulum impact tester is neces-sary to determine the load applied to the specimen and the terminal velocity. Sincethe maximum velocity achieved with this type of machine is considerably higherthan that available from the pendulum-type machine, vibration and inertial ef-fects can interfere with the recording of the load signal. Therefore all supportingcomponents must be as rigid as possible, and the piezoelectric load cell assemblymust be of low mass and high rigidity to produce a high natural frequency. In ad-dition to the load, the velocity of the falling weight must be determined. Frictionaleffects can result in significant errors when calculating the velocity by assumingfree fall. The velocity must therefore be measured directly by a linear array of,for example, photoelectric sensors positioned near the end of the fall, just abovewhere the specimen is struck. An accelerometer attached to the falling weight


Controlhousing

Hoist chain

Heightlimitswitch

Hammer

Guide post (2)

Support post (2)

Brake switch

Anvil specimensupport area

Base

Stop block (2)

Heightindicatorscale

Releaselatchhousing

Fig. 4. Typical drop-weight testing machine (11). Courtesy of ASTM.

measures load and velocity separately. The electric signal from the accelerometeris recorded digitally as before. The absorbed energy and the force are calculatedfrom the change in velocity caused by the impact. The results are obtained byprocessing the acceleration signal in the computer. The accelerometer must bemounted near the tip of the striker to minimize interference from reflected shockwaves. An accelerometer is preferred over a load cell at high test velocities.

In the falling-weight test (BS 2782, Method 306B) also known as a dart-drop test or Gardner impact test, a spherical ball striker, 12.7 mm in diameter, isfastened to a load-carrying device to which weights can be attached. The strikerassembly slides freely in vertical guides and is released from a predeterminedheight to strike centrally on a specimen, which is supported on the base of theequipment. In the so-called staircase method the load is increased by fixed in-crements on successive specimens, each struck once, until a specimen fails. Theweight is then reduced by fixed increments until a specimen withstands the im-pact, after which it is increased again by fixed increments until a specimen failsagain. This procedure is repeated on at least 20 samples to determine the energy


FailureDropheight t,m

Impact,J

2.74

2.64

2.54

2.44

2.34

2.24

2.13

2.03

1.93

1.83

1.73

1.63

366

352

339

325

312

298

285

271

258

244

230

217

x

x x

x x

x

x x x

x x

x x x

x x

x x

x

x x x

x x

x

o

o o

o o

o

o

o o

o o

o o o

o o

o o o

o

o o

o o

o

Nofailure

9

914

142

20

0

F50 = 268 J

Fig. 5. Typical results of a dart-drop test using the “staircase” method of analysis (12).To convert J to ft·lbf, divide by 1.35. Courtesy of the Society of Plastics Engineers.

level at which 50% of the specimens break; the result is quoted as the impactstrength for 50% failure (F50) (see Fig. 5). An important source of uncertainty inthis test is the criterion used to determine failure. Sometimes failure is obvious,as when the specimen shatters; other times failure is indicated by a small crack.As this is usually a visual test, whether or not failure has occurred depends onthe judgment of the testing technician.

In the Probit method (13), a set of energy levels for the falling weight is se-lected. A series of specimens is tested at each level, and the failure–impact energyrelation determined. With the help of a detailed failure–impact energy curve asimple control test can be carried out by testing a small number of specimens at asingle fixed energy level. This procedure is far more reliable and economical thanthe F50 values.

In the common drop-weight test, force and displacement are not recordedby electronic instruments. On the other hand, in fully instrumented procedures aweight–height combination at which the specimen is consistently broken is foundby trial and error, or by choosing a height that produces a desired terminal velocityand adjusting the weight to break the specimen consistently. A number of droptests are then performed under identical conditions. Each test is recorded by anelectronic instrument. The exact number of tests required depends on the diver-gence of the data and the confidence limit desired. This method requires fewerspecimens. The velocity can be varied as desired (within the limits of the tester),and an entire set of data can be obtained at the same velocity. The deformationbehavior, including the point of crack initiation, can in many cases be deducedfrom the load–deflection curve, reducing operator errors. Furthermore, the abilityof a material to decelerate a moving mass can be assessed.

Many materials that give low Izod impact values on notched specimens failin a ductile manner when tested in sheet form. Notched impact strength and theimpact properties of sheet are not correlated because the stress states and thematerial responses differ.

The cause of failure of sheet material in practice is more likely to be revealedby a falling-weight impact test than by an Izod test (14). Falling-weight tests thus


give a better guide to practical behavior unless the molded article contains ribs,sharp corners, and similar geometry. Any anisotropy present, such as excessiveorientation leading to weakness in a given direction, is revealed in the test. Forexample, an oriented injection-molded specimen may fail with a single straightcrack at a low energy, whereas a substantially unoriented compression moldedspecimen of the same material would fail with cracks in several directions afterhaving withstood much higher impact energies.

In falling-dart testers impact is produced by means of a pointed weight ordart. This method assesses the toughness of plastic film under certain specifiedconditions, such as sample geometry, dimensions of the dart head, and velocity. Inone such apparatus, a perfectly flat specimen is clamped by a vacuum and heldin uniform tension. The flat test film is placed over the specimen holder and keptwrinkle-free by the vacuum. The dart is automatically released from an electro-magnet as soon as the correct pressure differential is reached. Darts of variousweights are dropped, and the percent failure is plotted against dart weight onprobability graph paper. A straight line is drawn through the plot. The intersec-tion of the line with 50% failure is the weight in grams for F50 impact failure.ASTM D1709 gives a formula for the calculation of impact strength without agraph. In the examination of tubular film the edge fold as well as the face ofthe film may be tested. The U.S. specification for the falling-dart method (ASTMD1709-01) refers specifically to polyethylene film, whereas the U.K. standard (BS2782, Method 306F) covers all flexible plastic film. In an equipment modificationtwo electromagnetic counters are provided: one records the number of specimenstested and the other, which is operated by a push switch, the number of breaks. Fortests of thick film, a clamping ring is provided in addition to the vacuum clamp-ing to prevent film slippage, which is especially important in this case. In bothspecifications the dart, which has a 38-mm-diameter hemispherical head, fallsfreely 0.66 m to strike the specimen. The weight of the dart is adjusted with looseweights to determine the limit required to fracture 50% of the specimens. Othermore recent versions are commercially available, usually equipped with digitalrecording devices and computer analysis of the results.

Tensile-Impact Instruments. A standard Izod impact-testing machine, avariation of the swinging pendulum, is easily adapted at little cost to tensile impacttesting. The conventional Izod vise is replaced by one capable of holding the fixedend of a tensile specimen and a metal crosshead is secured to the other end ofthe specimen (see Fig. 6). The swinging pendulum strikes the crosshead removingboth the crosshead and half of the specimen. Development of this instrument ledto the specification ASTM D1822-99 (tensile impact energy to break plastics andelectrical insulating material). This method can be used to measure the impactresistance of test pieces cut from actual moldings, and help to assess directionaldifferences in stress that arise from flow-induced molecular orientation producedduring mold filling. It is also very useful for the evaluation of very small samples.The energy required to produce failure can be obtained by varying the height offall or the mass of the falling weight or by measuring the excess swing of thependulum. Transducers and electronic instruments can provide load–deflectioncurves. The results show reasonable correlation with the falling-weight test offilm and with the performance of moldings and sheet. Tensile impact instrumentsrequire less material than the usual falling-weight impact test. At relatively low


Pendulum

Pendulum

Tossed crosshead clamp

(a)

Pendulumhead

Crossheadclamp

(b)

Anvil

Anvil

Anvil

Fig. 6. Tensile impact machines. (a) Specimen in base; (b) specimen in head.

speeds they often give good correlation with falling-weight tests. This correlationis due to the fact that in both tests the tensile mode of stress is generated bythe testing geometry. The falling-weight test, however, generates a biaxial tensilestress that can more easily give rise to failure. This is a consequence of the highercomponent of hydrostatic tensile stress in the biaxial tensile test, which favorsthe cohesive mode of failure.

Special Test Machines. Hydraulically and pneumatically driven ma-chines have found increasing use in research and quality control laboratories;ASTM standards have not yet been established. These machines require higheroperator skills and are more expensive, but offer better control and produce moreinformation.

Hydraulic and pneumatic impact testing machines differ in the way in whichthe impact energy is derived, in the amount of energy and maximum velocity thatcan be developed, and in the amount of deceleration suffered by the striker. Ahydraulic fluid or a gas under high pressure is stored in an accumulator. Upon re-ceiving an electrical signal, a valve is opened, and the fluid or gas is transferred toa cylinder, wherein a low friction piston is connected to the load application device.The pressure difference between the two sides of the piston causes acceleration toa high velocity. Different designs in the valve and pressure return arrangement,and, increasingly, high speed digital electronics and signal processors allow thevelocity to be controlled to varying degrees of success. The maximum load capacityof the testing machine depends on the pressure of the fluid, piston area, and pres-sure drop in the valve and upon expansion. The maximum velocity depends on thepressure, the flow rate allowed by the valve, the size of the accumulator, the in-ertia of the moving mass, and the distance allowed for acceleration. Commercialhydraulically driven impact testers are now capable of generating a maximumspeed of 22 m/s at a maximum load of 900 kg (see Fig. 7); different designs areavailable including computers for control and data acquisition and analysis. Possi-ble tests include tensile impact, puncture (flexed-plate) impact, and flexed-beamimpact. A pneumatic machine is shown in Figure 8; anvils for various types ofimpact tests are shown in Figure 9. The high compressibility of gases means thatthe striker could decelerate significantly if the force required to deform the testspecimen is high.


Fig. 7. Hydraulic high speed impact tester. (a) Specimen enclosed in environmental cham-ber; (b) hydraulic drive unit at the end of the rails for component testing. Courtesy of InstronCorp., Canton, Mass.

A principal advantage of hydraulic pneumatic machines employing feedbackcontrol is that a higher fixed velocity can be maintained. Much depends, of course,on the specifics of the design.

In pendulum machines the velocity upon striking the specimen is initiallyconstant at 2.4 m/s in the Izod test, and 3.4 m/s in the Charpy test. After the speci-men is struck the velocity decreases, depending on the amount of energy absorbed.This problem is more serious in tough specimens. In drop-weight tests, the veloc-ity upon impact depends on the drop height, and yet the test results often reportonly the product of weight and drop height, ignoring the impact velocity. Sincepolymeric materials are rate sensitive in their mechanical behavior, tests usinggravity-driven machines could produce misleading results because the terminalvelocity just prior to striker contact with the specimen could vary widely. The use


Stabilizer

Pressure regulatorimpactor velocity control

Adjustable anvil height

Impactor

Adjustable stripper plate

Force transducer

Three-way viewingthrough transparentsafety panels

Pneumatic cylinder

PositionsensorDoorsefetyinterlock

Anvil

Firingbutton

Impactabsorbingfeet

Height210 cm

Fig. 8. Pneumatic impact tester. Various types of impactors, anvils, and force transducersare available for different applications. Courtesy of ICI Australia Operations Pty Ltd.,Dingley, Victoria, Australia.

of the special machines described here avoids such problems. (This test standardhas been discountinued in 1992.)

Product Testing. Standardized specimens are specially prepared forfalling weight tests, swinging-pendulum tests, tensile impact tests, and others.However, frequently results from such tests fail to predict the performance of ac-tual fabricated components in service. For this reason, the conventional formsof impact testing are more useful as control or material comparison tests. Thislimitation has become more serious in recent years with the rapid introduction ofmany new polymers and their blends. One reason for the difference in performanceis the fact that the fabricating process can influence impact strength. A typicalexample is the effect of flow-induced orientation in thin-walled moldings. The flowbehavior of the material during the filling of the mold may be more important thanthe basic impact properties of the unoriented material in governing the impactbehavior of the thin-wall moldings. Specimen geometry is also important becausethe stress state and rate of stress buildup are governed by the size and shapeof the article. Therefore the impact strength of the finished product may need tobe tested under appropriate conditions in addition to the usual control and qualitytests.


Izod Charpy Film test

Flat plate Plaque test V block

Fig. 9. Accessory anvils for gravity, hydraulic, and pneumatic testers. Courtesy of ICIAustralia Operations Pty Ltd., Dingley, Victoria, Australia.

Mechanical Behavior

Tensile Impact. The tensile impact test subjects a specimen to uniaxialtension at a strain rate of approximately 102s− 1. This strain rate is 3–6 ordersof magnitude higher than that encountered in conventional uniaxial tension test-ing (7,15,16). The effect of strain rate on uniaxial tensile behavior depends onthe scale of deformation and the temperature range. At small strains the ten-sile modulus E increases with strain rate, but the magnitude of change dependson the temperature range. If the relaxation process is significant, eg, the glasstransition or a strong secondary relaxation occurs near the test temperature, themodulus would also be rate dependent. The change can be about a factor of 2 for asecondary relaxation, and an order of magnitude or more for a temperature some-what below Tg. If no significant relaxation occurs near the test temperature, Ecan be unaffected by the strain rate. At room termperature, PMMA has a strain-rate senstitive modulus whereas PC has not, although at sufficiently high ratesstrain-rate sensitivity develops. At strains beyond 1–2%, some materials becomebrittle even at low strain rates. An example is commercial PS, which typicallycontains small amounts of foreign material that serves as nucleation centers forcraze initiation. Pure PS without surface flaws may be susceptible to shear yield-ing; increasing strain rate reduces flaw tolerance. If the material is not brittleenough to fracture, it eventually reaches a yield point defined as the maximum inthe load–elongation curve. This yield point is not necessarily due to shear yield-ing. Impact-modified polystyrene, for example, exhibits a yield point independent


DelrinLexan

Nylon-6,6

Poly(vinyl chloride)

Teflon TFE

Teflon FEP

PolypropylenePolyethylene

Polypropylene

TFE

FEP

100

90

80

70

60

50

40

30

20

10

00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

True

str

ess

σ, M

Pa

True strain ε

Fig. 10. True stress–true strain behavior of polymers. Delrin is an acetal resin; Lexan,a polycarbonate; Teflon TFE designates the tetrafluoroethylene homopolymer; Teflon FEPis a copolymer of tetrafluoroethylene and hexafluoropropylene (17). To convert MPa to psi,multiply by 145. Courtesy of Polymer Engineering and Science.

of shear yielding. If the load maximum is due to shear yielding, further strainingresults in a neck because of strain softening (16). Some crystalline polymers donot exhibit a maximum in the load–elongation curve but merely a knee because ofthe strengthening effect of the spherulites. Yield strength usually increases withstrain rate (15,16). The strain-rate sensitivity of the shear yield stress behaves inmuch the same fashion as the modulus; it also depends on a significant relaxationprocess near the test temperature. After neck formation the strain-rate sensitivitystabilizes and neck extension occurs (16). On the other hand, thinning of the neckresults in ductile fracture, which can also be due to high strain rates.

Examples of uniaxial tensile true stress–true strain behavior of polymers de-formed at low strain rates are shown in Figure 10. Common engineering stress–strain curves cannot be used to determine the elongational strain because theshape for a given set of testing conditions depends on the specimen geometry.Some polymers show ductile deformation after yield by a process of necking andcold drawing. The apparent neck extension depends on the length of the specimengauge section if the conventional method of calculating engineering strain ε =�l/l0 is used, where �l is the elongation and l0 the original gauge length. The truevalue of the extensional strain can be ascertained only by direct measurementof the strain in the necked section. If the specimen dimensions are identical and


LoadEnergy

Displacement

0

Fig. 11. Typical load and energy data from tensile impact test on polypropylene usingan instrumented pendulum tester. The dotted line shows the accumulated energy as thedeformation increases. Courtesy CEAST SpA, Torino, Italy.

the extents of necking comparable, the apparent maximum engineering straincan be used as a relative ductility index. Ductility usually decreases with strainrate or decreasing temperature. At a given strain rate and temperature, ductilitydepends on molecular weight and amount of filler or impurity in the material.Stresses applied orthogonal to the principal stress direction can also affect duc-tility. Tensile impact toughness, a measure of the total energy absorbed by a dog-bone-shaped specimen in uniaxial tension, is the integrated value of the typicalforce–elongation curve. It is highly sensitive to the ductility of the material at thetesting rate. The tensile impact behavior of polypropylene is shown in Figure 11.The curve is typical of a pendulum impact tester equipped with transducers anddigital storage instruments. The strain-softening behavior is clearly discernibleeven at such a high strain rate. The advantage of electronic instrumentation canbe seen immediately.

Flexed-Plate Impact. In flexed-plate impact, a ball, dart, or striker isdriven by gravity or pressure into a thin plate supported by or clamped over anannular area. The nose of the dart is usually hemispherical, though sometimessquare. A flex-plate impact test corresponds closely to conditions of actual use.However, geometric factors, such as specimen thickness and dart-to-annular open-ing ratio, cannot be readily standardized, and furthermore, the geometry itselfcan cause a change in the deformation mode (14). The progression of deformationprocesses associated with the load–deflection curve can be seen in Figure 12. InFigure 12A the specimen failed in a ductile manner and the plastic deformed ex-tensively around the dart nose. The remaining unsupported material was colddrawn. Partially deformed specimens and finite-elements analysis reveal yielding


Load

C

B

A

Displacement

Fig. 12. Typical load–deflection data from puncture impact tests. (A) ductile failure; (B)cracking started at about 50% of the maximum load; (C) brittle failure (18).

under the dart early in the deformation process, at about 25% of the maximumload. This initial yielding is marked by a slight decrease in the slope. Subse-quently a yielded zone in the shape of a hemispherical dome develops around thedart nose. As deflection continues, the net area undergoing yielding also increases,which results in geometric hardening, ie, the load increase is due to the change inspecimen geometry as described above. The processes of material softening andgeometric hardening eventually combine to result in a slight increase in slope. Atthis stage of the deformation the unsupported area of the specimen is akin to thatof a membrane rather than a plate. The stress in the membrane is essentiallyuniaxial tension in the radial direction. Finally, excessive thinning takes placesomewhere in the specimen, the exact location being dependent on the geometry,and rupture causes the load to drop precipitously (18,19). Thus, the total energyabsorbed, which is the area under the load–deflection curve, depends in a verycomplex manner on both the testing geometry and the yield and deformation be-havior of the material. The curve of Figure 12B is obtained when the specimendevelops a crack during the membrane-stretching process. The sudden release ofelastic energy upon cracking causes the load-cell assembly to vibrate at its nat-ural frequency and results in the “ringing” signal. The crack is stabilized by the


specimen geometry and does not propagate. Consequently the specimen is ableto carry an increasing load even though, by some definitions, the specimen wouldbe considered to have failed (18). The curve in Figure 12C is obtained when thespecimen fails in a totally brittle manner; no decrease in the slope of the load–deflection curve is discerned. Cracking occurs on the tension side opposite thearea where the dart or striker contacts the plate. Little or no plastic deformationoccurs anywhere on the specimen. Brittle fracture of this type occurs more readilyin thick plates even for materials normally considered to be ductile. This obser-vation serves to warn against treating such test results as material properties.Clearly, the test specimen and specimen holding fixture contribute importantly tothe test results.

In addition to plate thickness, decreasing temperature, increasing deforma-tion rate, or environmental aging can transform ductile-type failure (Fig. 12A) tobrittle-type failure. In the case of a ball or a hemispherical dart impact test, itis commonly assumed that the resultant stress state at the center of the plateis biaxial tension. This assumption is erroneous because the normal force on theplate is significant and could lead to over optimistic values on ductile-to-brittletransition conditions, whether expressed as a critical temperature or aging time.This behavior is caused by normal stress suppressing brittle failure (15,18,20).

Flexed-Beam Impact. Flexed-beam impact tests are sometimes carriedout on unnotched specimens, though notched specimens are preferred. The Izodand Charpy tests are examples of flexed beam tests. These two testing geometriesare different, but both types of notches create a severe stress concentration. Manymaterials that are tough in the tensile and flexed-plate impact tests become brittlein notched flexed-beam tests because the stress state near the base of the notchfavors a brittle response due to the high component of hydrostatic tensile stressmentioned above. For a given specimen thickness, the impact toughness dependson the notch tip radius (see Fig. 13). Subjecting a thin, notched specimen to bendingcreates a tensile stress concentration along two orthogonal directions: the mainone along the specimen length, the other parallel to the notch or crack direction.The magnitude of the former depends on the notch sharpness. The minor stressis due to constraints created by notch-free sections of the specimen, which are ata lower stress. Because the specimen is thin, it contracts freely in the thicknessdirection; a state of plane stress is said to exist in this instance. The biaxial tensionstate is more favorable than the uniaxial tension state for craze or crack formationbecause of the larger hydrostatic or dilatational stress component in the formercase. Many polymers that are ductile in uniaxial tension exhibit brittle behaviorin this stress state; other polymers are able to shear-yield. Shear yielding creates alarge plastic zone at the notch tip, and subsequent failure is due to ductile tearingof the material. This mode of failure usually absorbs a large amount of energy.Some impact-modified materials are able to form a plastic zone ahead of the notchcontaining of a high density of crazes. Considerable energy can be absorbed bythis mechanism. The load–deflection behavior of tough plastics is illustrated inFigure 14.

Subjecting a thick notched, notch-sensitive specimen to bending creates atensile stress component that is perpendicular to the specimen faces. This thirdstress is due to constraint in the thickness direction because the material ahead of


Fig. 13. Impact strength as a function of notch tip radius for different polymers (21).To convert kJ/m2 to ft·lbf/in.2, divide by 2.10. POM = polyoxymethylene. Courtesy of ThePlastics and Rubber Institute.

the notch is unable to contract freely in this direction; the constraint effect is saidto be due to plane strain. The triaxial tension state is even more favorable thanthe biaxial tension state for the formation of crazes, again because of the largerdilatational stress component in the former. With sufficiently large thickness andsharp cracks few unmodified polymers are able to deform in a completely ductilemanner owing to the intrinsically low cohesive strength of polymers relative totheir yield strengths. The fracture is due to craze formation and rapid crack prop-agation. At intermediate thicknesses or notch radii the specimen can deform inmixed mode: plane-strain in the specimen center and plane-stress near the sur-faces. In the plane-stress region, plastic deformation zones are often found. Theirsize determines the amount of energy absorbed by the specimen; the plane-strainzone, by contrast, absorbs little energy.


Supertough nylon

HIPS

Polycarbonate

ABS

0 5 10 15 20

Displacement, mm

0

100

200

300

400

500

For

ce, N

Fig. 14. Load–deflection behavior of a number of tough polymers tested in the notchedCharpy geometry at low rates (22). To convert N to kgf, multiply by 0.102. Courtesy of theSociety of Plastics Engineers.

The impact toughness of a material, as measured by a flexed-beam test on anotched specimen, depends on the thickness of the specimen and the radius of thenotch, and therefore cannot be considered a material property. Furthermore, thetransition from predominantly plane-stress to predominantly plane-strain is un-predictable when complex geometries are encountered in structural components.Because this transition can create unexpected and catastrophic failures, somepolymers normally considered to be very tough and ductile may benefit from im-pact modification. The modified versions are less sensitive to geometric variations.

The transition from plane-stress to plane-strain can also be brought aboutby impact rate (23), temperature (Fig. 15), molecular weight, and thermal history(24). The effects of thickness and rate are shown by an example in Figure 16. Inthis example notched polycarbonate (PC) specimens are tested at various bendingspeeds. The thick (6.4 mm) specimens are uniformly brittle, whereas the thin(3.2 mm) specimens are uniformly ductile. The intermediate thickness (4.4 mm)specimens exhibit a ductile-to-brittle transition at about 0.3 cm/s. In toughenedPC even the 6.4-mm-thick specimens are tough up to about 40 cm/s (see Fig. 17).

Fracture Toughness from Flexed-Beam Impact Tests. As statedabove, the so-called impact toughness values obtained from Izod and Charpy testsare not material properties because they depend on specimen thickness, notchdepth, notch radius, and other factors unrelated to material properties. These


Unnotched

c = 2 mmρ

ρ

ρ

c = 1 mm

c = 0.25 mm

40

30

20

10

0−80 −60 −40 −20 0 20 40 60

Tb

Temperature, °C

Impa

ct s

tren

gth,

kJ/

m2

Fig. 15. Effect of test temperature on the impact strength of PVC specimens containingvarious notch radii (21). Tb = brittle temperature. To convert kJ/m2 to ft·lbf/in.2, divide by2.10. Courtesy of The Plastics and Rubber Institute.

values are therefore useful only for the comparison of different materials, and areuseless for design calculations. By contrast, fracture toughness (ASTM D5045-99),which is a material property, is useful in design calculations. Consequently it is de-sirable to have fracture toughness values that are taken under impact conditions,and especially if they could be obtained from the Izod or Charpy test machines.

The fracture toughness GIc can be determined from the impact energy U if thegeometry of the specimen is known (25) and if the specimen fails in plane-strain.To determine if the latter condition is satisfied one could examine the fracturesurface. If the top edges of the fracture surface remain in the planes of the sidesof the specimen and no sign of large-scale plastic drawing in front of the notch tipis observed then this condition is probably satisfied. Another signature for plane-strain fracture is the load–deflection curve, which would have the shape of a righttriangle. This points out the desirability of using an instrumented tester.

In this analysis it is assumed that linear elastic fracture mechanics (LEFM)applies. When a specimen containing a sharp notch of depth a is subjected to


010−1 100 101 102

cm/s

100

200

300

400

500

600

700

800

Frac

ture

ene

rgy,

J/m

Fig. 16. Effect of test speed on the fracture energy of polycarbonate specimens of variousthicknesses tested in the notched Charpy geometry. ◦, 3.2 mm; �, 4.4 mm; �, energy upto load maxima for 4.3 mm; �, 6.4 mm (22). To convert J/m to ft·lbf/in., divide by 53.38.

bending with a load P, a deflection x results. The energy U stored is given by

U = Px/2 (1)

When the crack grows, compliance C = x/P increases as the stored energy isreleased. Thus, if it is known how C changes with a the strain-energy release rateG can be calculated from equation 1

U = P2C/2 (2)

For a specimen of uniform thickness B,

G = 1B

dUda

= 1B

dUdC

dCda

(3)

When fracture occurs, the critical strain-energy release rate Gc is calculated:

Gc = P2

2BdCda

(4)


0

100

200

300

400

500

600

700

800

10−1 100

cm/s

101 102

Fra

ctur

e en

ergy

, J/m

Fig. 17. Effect of test speed on the fracture energy of an impact-modified PC as inFigure 16. ◦, 3.2 mm; �, 4.4 mm; , energy up to load maxima for 6.4 mm (22). Toconvert J/m to ft·lbf/in., divide by 53.38.

In an impact test, usually only the energy U is determined. Combining equa-tions 2 and 4 gives

U = Gc BC

dC/da(5)

In practice, U is determined for a series of specimens with various notchdepths a. It is therefore more convenient to express equation 5 in terms of

Gc = UBDC

dCd(a/D)

(6)

where D is the specimen width. The dimensionless compliance function

φ = CdC/d(a/D)

can be determined experimentally from quasi-static bending tests, and has beencalculated analytically for this geometry (26). Equation 6 can then be rewrittenas

U = Gc BDφ (7)

predicting that a linear relationship should exist between the impact energy Uand BDφ, the slope of which should be equal to Gc. G can then be obtained by


Table 4. Strain-Energy Release Ratesa

Gc, J/m2b

Material Charpy Izod

PolystyreneGeneral purpose 0.83 × 103 0.83 × 103

HIPS 15.8 × 103c 14.0 × 103c

PVCDarvic 110 1.42 × 103 1.38 × 103

Modified 10.05 × 103 10.00 × 103

PMMA 1.28 × 103 1.38 × 103

Nylon-6,6 5.30 × 103 5.00 × 103

Polycarbonated 4.85 × 103 4.83 × 103

PEMedium densitye 8.10 × 103 8.40 × 103

High density f 3.40 × 103 3.10 × 103

Low density 34.7 × 103 34.4 × 103

ABSLustran 244 49.0 × 103c 47.0 × 103c

aRef. 25. Courtesy of Polymer Science and Engineering.bTo convert J/m2 to ft·lbf/in.2, divide by 2.10 × 10− 3.cJc as defined in Ref. 1.dSpecimens cut in the extrusion direction.eDensity = 0.940; MI = 0.2.f Density = 0.960; MI = 7.5.

determining U for a series of specimens with various notch depths if the relation-ship is indeed linear. If the impact tester is instrumented, a further criterion wouldbe a rapid drop in load P after reaching a peak, ie, the load–deflection curve shouldhave the shape of a right triangle. If the notch is too blunt and a significant amountof plastic flow occurs, which would cause the top of the load–deflection curve toround off, a correction factor may be needed, ie, the crack length a is increased bythe plastic zone size rP (25,27):

af = a+ rP

The plastic zone size can usually be determined from the appearance of thefracture surface and is distinguished by a whitened or very rough area directlyahead of the original notch tip. The Gc, values for some plastics for both the Izodand Charpy geometries (25) are in Table 4. With the same method, the GIc of anumber of polymers was determined (3) at different rates (Fig. 18).

The procedure outlined above can be useful if the specimens are preparedcarefully to ensure that LEFM applies. The specimen should be as thick as ispracticable, and the notches should be very sharp. If a razor blade is used forcutting, a compressive plastic zone must be avoided at the notch tip. This can beaccomplished by slicing the notch tip rather than by pressing the blade into thenotch. If the fracture surface contains plastic flow, the crack size most likely has


60402000

50

100

150Gc = 1.6 kJ/m2

Fra

ctur

e en

ergy

, mJ

8080 60402000

100

200

300

400

500

600

Gc = 5.6 kJ/m2

Fra

ctur

e en

ergy

, mJ

0

1000

2000

150100500

Fra

ctur

e en

ergy

, mJ

RateIncreasing

BD�, 10−6 m2

(c)

BD�, 10−6 m2

(b)

BD�, 10−6 m2

(a)

Fig. 18. Fracture energy as a function of BDφ for (a) PMMA at 20◦C; (b) PTFE at 25◦C(◦, Charpy; •, Izod); (c) HDPE at 20◦C and several rates (�, 26 s− 1; , 122 s− 1; •, 620s− 1) (3). To convert J to ft·blf, divide by 1.355. To convert kJ/m2 to ft·lbf/in.2, divide by 2.10.Courtesy of The Institute of Physics, Great Britain.

to be corrected. The method for determining fracture toughness can be extendedto tensile impact of a notched specimen (28).

Improving the Impact-Resistance of Materials

Impact resistance is not an absolute term. It depends on the design of the struc-ture as much as it does on the intrinsic material toughness. Nevertheless theimpact resistance of a material can be increased by modifying it with tougheners.This increase is sometimes at the cost of reduced modulus and strength, and per-haps fatigue resistance. The impact toughness of commercial plastics is shown inTable 5.

High Impact Polystyrene (HIPS), Acrylonitrile–Butadiene–StyreneCopolymer (ABS), and Modified Poly(xylenol ether) (PXE). Polystyreneis a transparent, rigid, easily processable, inexpensive material. Its chief disad-vantages are extreme brittleness and high susceptibility to crazing by organicliquids. It was also among the first materials to be produced in a toughened form,commonly known as HIPS, in order to allow the material to be used in applicationswith moderate loadings such as electrical applicances.

In the most common production method rubbery polybutadiene (PBD) is pre-cipitated from a styrene–polybutadiene solution in styrene during the polymeriza-tion of styrene. The precipitated phase itself typically contains 2–10-µm particlesconsisting of a rubbery sphere containing spheroidal polystyrene inclusions thatare separated by thin membranes of PBD rubber (see Fig. 19). It is believed thatthis type of composite particle has an advantage over solid rubber spheres be-cause for a given weight fraction, it maximizes the effective volume fraction of therubbery phase, thus enhancing the impact toughness without causing undue re-duction in modulus. The rubbery phase can also be introduced by blending PS with


Table 5. Typical Notched Izod Impact and Drop-Weight Impact Toughnessa

Notched Izod, J/mb Falling dart,c N·md

Thermoplastic 22◦C −40◦Ce 22◦C −40◦Ce

Acetal 53–53Nylon, amorphous 587–1121 213–480 f

Nylon-6g 53–81Impact modified nylong 961 294 f 65 57 f

Nylon-6,6g 27–64Toughened nylon-6,6g 907–117433% glassg 219Nylon/ABS 998 96 f 73 35 f

Nylon/PPO 213 133 f 51 40Polyarylate 294 213Polycarbonate 641–854 133–598Polycarbonate/ABS 454–641 267–614 f 38–61 30–46 f

Polycarbonate/PBTg,h 694–854 160–641 56–57 49 f

Polycarbonate/PET 907 694 f 50 f

Polyester, aromatic (LCP)i 53–534Thermoplastic polyester, PBTh 43–53Impact modified PBTh 801–854 747–801 f 43–54 43 f

Impact modified PBT/PET 801–854 5430% glass-reinforced PET, 101 96

thermoplastic polyesterToughened 30% glass PET 139–235 123–160Polyetheretherketone 59–81Polyetherimide 32–53 34–36Polyethersulfone 53–81Poly(phenylene oxide) modified 267–534 133–187 15–34 3–11Poly(phenylene sulfide) 27–75Poly(phthalate carbonate) 534Polysulfone 69 64Polyurethane, engineering 53–213

thermoplasticaRef. 6.bTo convert J/m to ft·lbf/in., divide by 53.38.cGardner or falling dart.dTo convert N·m (J) to in.·lb, divide by 0.113.eUnless otherwise statedf At −29◦CgDry as molded.hPoly(butylene terephthalate).iLCP indicates liquid crystalline polymer.

a triblock copolymer such as poly(styrene–butadiene–styrene) (SBS). The tough-ening is believed to be due to the nucleation and controlled growth of extensivecrazing at the rubber–PS interface (29). The appearance of stress-whitening is dueto the high density of crazes generated by deformation. The stress–strain behaviorof ABS, PS, and a typical HIPS are shown in Figure 20. It is readily apparent thatHIPS has much lower modulus and tensile strength, but much higher elongation


Fig. 19. Electron micrograph of an osmium tetroxide stained ultrathin section of HIPS.This micrograph shows the typical morphology of the dispersed rubber particles. Thethin lines connecting the particles are crazes formed after the material has undergonedeformation.

0

10

20

30

40

Str

ess,

MP

a

1.00.750.50.25

Elongation, cm

HIPSMIPS

ABS

PS

Fig. 20. Typical uniaxial tensile stress–strain behavior of PS, medium-impact PS (MIPS),high-impact PS (HIPS), and ABS (30). To convert MPa to psi, multiply by 145. Courtesy ofSpringer-Verlag.


than PS. At the cost of a decrease in modulus and strength, ductility and impacttoughness are greatly enhanced because of the higher volume fractions of rubberparticles in the tougher materials.

Acrylonitrile–butadiene–styrene polymers (qv), the impact-modified versionof SAN (styrene–acrylonitrile copolymer), are produced in the same manner asHIPS. Block copolymers such as SBS may also be used as the second phase. Themorphologies of ABS and HIPS are very similar except that for optimal impacttoughness the rubber particles in ABS are smaller than those in HIPS, ca 1–5µm in diameter. The difference between ABS and HIPS, as far as mechanicalbehavior is concerned, is due to the fact that SAN is more ductile than PS. Con-sequently, the impact toughness of ABS is higher than that of HIPS. Because ofcompositional and morphological differences, these materials exhibit a range ofmechanical properties. For a given volume fraction of rubber particles in ABS,higher AN (acrylonitrile) content enhances toughness. The toughening mecha-nism is believed to consist of the formation of numerous shear bands in additionto massive crazing (29); both are due to the presence of the rubber particles. Incertain ABS grades deformation in tension causes the formation of cavities insidethe rubber particles, leading to the formation of shear bands from these cavitatedparticles (31).

The toughest member in this category of plastic materials is a blendof poly(xylenol ether) [PXE, poly(2,6-dimethylphenylene oxide)] and PS and arubbery impact modifier; PXE is also known as PPO [poly(phenylene oxide)](Table 6). However, PPO, a registered trademark, can be confused withpoly(propylene oxide), and therefore PXE is the preferred abbreviation. PXE iscompletely miscible with PS and forms a true solid solution (32); Tg is high and thematerial is ductile. Melt viscosity is also very high. Processability is improved byblending with PS; the viscosity of the mixture is approximately the weighted aver-age of the two components, whereas Tg varies with the composition (32). Comparedwith the weighted average, moduli and yield strengths are higher but the fracturetoughness is lower (32,33). Commercial grades are usually impact-modified by adispersion of rubbery particles, which are sometimes obtained by directly blend-ing PXE with HIPS instead of PS. The stress–strain behavior of HIPS/PS/PXEblends is shown in Figure 21. The PXE/HIPS blends exhibit a good overall com-bination of mechanical properties. The toughening mechanism is believed to bea combination of shear banding and massive crazing (29). The PXE/HIPS blendsare much more expensive than HIPS or ABS. Furthermore, they require precise

Table 6. Composition of HIPS/PS/PXEBlends (wt%) Investigated

Blend HIPS PS PXE

A 50 50 0B 50 37.5 12.5C 50 25 25D 50 12.5 37.5E 50 0 50


D

E

C

B

A

10

0

20

30

40

50

Str

ess,

MN

/m2

0 10 20 30 40 50 60 70 80

Strain, %

Fig. 21. Uniaxial tensile stress–strain behavior of HIPS/PS/PXE blends at 20◦C showingthe effects of matrix composition. Strain rate 4 × 10− 4 s− 1 (29). To convert MN/m2 to psi,multiply by 145. Courtesy of Applied Science Publisher Ltd.

processing control in injection molding because of the relatively high melt viscosityof commercial PXE.

The terms HIPS, ABS, and PXE refer to families of polymers, within whichthere are variations in composition, morphology, and properties. This is especiallytrue of PXE blends, which exhibit behaviors in between those of HIPS and PXE.Processing and applications must be specific to each grade. As in other materials,pigments, flame retardants, and especially reinforcing fillers can have significanteffects on impact toughness. The relevant properties must be determined for eachcomposition.

Since the toughness of these materials depends on the presence of rubberparticles, the destruction or oxidation of these particles reduces impact tough-ness. Excessively high temperatures or shear rate during processing and pro-longed exposure to oxidative environments destroy or cross-link the rubber andmust be avoided. Prolonged exposure to ultraviolet radiation degrades these ma-terials because of the decomposition of the PBD rubber and sometimes PXE. Sat-urated elastomers resist oxidative degradation better than unsaturated PBD. Insuch instances, the low temperature impact toughness of the material might beimpaired because the Tg of the saturated rubber is usually higher than that ofPBD. UV stabilizers may improve the resistance of PXE to sunlight. The rub-ber particles act as effective sites for the nucleation of crazes in cyclic fatigue.


Therefore fatigue initiation occurs more readily in the impact-modified versionsthan in the unmodified versions; crack propagation is slower in the former(30).

Polycarbonates, Other Ductile Glassy Polymers, and Their Impact-Modified Versions. A number of glassy polymers possess some degree of ductil-ity at or near room temperature at moderate rates of strain (<100% s− 1). Ductilityis defined here as the percentage of permanent elongation after yield in a uniax-ial tension test. These polymers include polycarbonate (PC), polysulfone (PSU),polyethersulfone (PES), some polyesters, and others as well as copolymers de-rived therefrom. Some low Tg difunctional epoxies also belong in this category.Poly(vinyl chloride) (PVC), which may possess a low degree of crystallinity, andwhich loses its ductility more readily under adverse external conditions such aslow temperatures and impact rates than other members in this category, may beincluded here. The class of materials discussed here, whether impact-modified ornot, generally exhibits high values of tensile-impact toughness compared with thestyrenic materials discussed previously. Drop-weight impact toughness may alsobe high. However, a flexed-beam impact test (ie, Izod or Charpy) performed onnotched specimens of these materials may produce brittle behavior. Using typicalnotched Izod impact geometries (notch depth of 2.5 mm, notch tip radius of 0.25mm) and a specimen thickness of 3.2 mm, some of these materials fail in a brittlemode and exhibit impact toughness values that are surprisingly low, given thefact that these same materials are ductile in uniaxial tension. Other materials,PC for example, fail in a ductile mode and absorb large amounts of energy whenthe specimen thickness is 3.2 mm or less, but dramatically change to the brittlefracture mode when the specimen thickness is increased or the temperature low-ered. The specimen thickness effect is due to the difference in constraints at thesurfaces and in the interior of the specimen. The temperature effect is not wellunderstood, but is undoubtedly due to the different temperature dependencies ofthe shear yield stress and crazing stress. These dependencies may be correlatableto molecular relaxations.

The need for impact modification of this type of materials is not based onbrittleness but on notch sensitivity. Impact modification does not necessarily en-hance toughness when tensile impact and drop-weight impact tests are the cri-teria. Impact modification is achieved by melt blending a second soft, usuallyelastomeric, phase, which may be in the form of a dispersion of spherical particlesor a network-like structure. To aid the dispersion of the second phase, compati-bilization is usually required; that is, the elastomer is copolymerized or alteredchemically to reduce the interfacial energy between the two polymer phases. Inad-equate compatibilization can lead to unstable morphologies after melt processing,which can in turn produce weak knit lines in injection-molded articles, or skinand core morphologies that interfere with the proper functioning of the processedarticle and reduce the impact resistance of the molded articles.

It appears that the toughening effect near a crack tip occurs as follows: afterinitial deformation, cavitation occurs within or around the soft phase in a zonesurrounding the crack tip. This zone is known as the process zone. The newlycreated surface around these cavities in the process zone allows the latter to growplastically, thus absorbing some energy. Localized shear deformations, often re-ferred to as shear bands, may also grow from these cavities. If the volume fraction


of the soft phase is sufficiently large, these shear bands may merge or coalesce.Since the localized deformation in shear bands cannot be recovered, it is knownas plastic deformation. If the plastically deforming material is adjacent to a sur-face, eg, a crack, then the unloading that occurs there absorbs additional energy(34). The latter may be a significant mechanism of energy absorption in materi-als that exhibit this mechanism. However it may not occur if the crack growth isterminated by unstable fracture, which often occurs, due to the geometry of crackgrowth or to very high rate of deformation.

Cavitation in or around the soft phase causes stress-whitening, which is oftenobserved at and near the fracture surface of broken specimens. Stress-whiteningis sometimes mistakenly attributed to crazing. While profuse crazing could cer-tainly give rise to intense whitening, the latter could also be due to the occurrenceof numerous cavities associated with the rubber phase. Cavitation (including craz-ing) is an essential step in the toughening process because it relieves the triaxialtension around each cavity initially built up at the crack tips. After the triaxialtension is relieved by cavitation, the stress that remains is more favorable forshear yielding. Since these materials are more prone to shear yielding than tocrazing, ductile fracture is restored.

Evidence for the initial deformation process described above can be seen inthe three micrographs of an epoxy toughened with elastomer, shown in Figure 22.In Figure 22a, cavitated rubber particles can be seen in a scanning electron mi-crograph of the fracture surface. In Figure 22b, the cavitated rubber particles canbe clearly discerned beneath the fracture surface. The shear bands between therubber particle cavities are revealed by viewing the specimen between crossed-polarizers in Figure 22c.

Semicrystalline Polymers and Their Toughened Versions. Semicrys-talline polymers are different in mechanical behavior from glassy polymers. Con-ventional melt processing techniques may produce semicrystalline morphologiesin many polymers. The polymers in this category include high density polyethy-lene (HDPE), polypropylene (PP), polyacetals (POM), polyamides (PA, or nylons),poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), and morerecent materials such as poly(phenylene sulfide) (PPS), and polyetheretherketone(PEEK). Low modulus polymers such as low density polyethylene (LDPE) and cer-tain thermoplastic elastomers can be included. Crystallinity is typically between30 and 40%, though it can be as low as a few percent and as high as 80% inHDPE. Mechanical behavior of semicrystalline polymers depends strongly on thedegree and distribution of crystallinity and the differential cooling rates betweenthe surface and the interiors of the specimens. In molded articles, the interior ofa thick section contains many large spherulites. Since, in the presence of a notch,the interior is in plane-strain, resulting in triaxial tension, large spherulites in-variably cause brittleness, apparently due to weak interspherulitic boundaries. Invery thick sections voiding can also occur during the crystallization process. Some-times, in the mistaken belief that larger cross-sections provide greater strengthand a more gentle radius reduces stress concentration, a designer specifies verythick sections at the junction between two surfaces. This causes the growth ofvery large spherulites and voids, resulting in brittleness. Brittleness in this cir-cumstance may be avoided by using thin sections. Stiffness can be overcome bythe proper choice of cross-sectional geometries.


Fig. 22. (a) Scanning electron micrograph of the fracture surface of an elastomer-modifiedepoxy. (b) Optical micrograph of a thin section of the same epoxy. (c) The same opticalmicrograph viewed between crossed-polarizres.

Enhancement of impact toughness in some semicrystalline polymers is sim-ilar to techniques used for some glassy polymers. Usually an elastomeric phase ismelt-blended into the host polymer, resulting in a dispersion of spheroidal elas-tomeric particles or a network. Compatibilization is obtained by copolymerizationor chemical reaction at the interface. The toughening mechanisms may includeboth massive crazing and cavitation-plastic deformation, depending on the poly-mer blend, morphology, and degree of adhesion. The elastomeric phase may alter


the morphology of the semicrystalline phase, for example, by nucleating addi-tional spherulites, perhaps of a different crystalline structure, thus improving theinherent toughness of the crystalline phase by reducing the spherulitic size. Iftoughening is caused by the formation of shear bands, deformation of spherulitescontributes to energy absorption.

Incompatible Blends of Rigid Polymers. Some blends are composed ofrigid polymers, one or both of which may be rubber-modified. Examples of thesematerials include PC/ABS, PVC/ABS, PC/PBT, and nylon/PXE. Other combina-tions are also possible. An important difference between these materials and thePS/PXE blend described above is that the latter is thermodynamically compatible,and behaves as a homogeneous material, whereas the former are thermodynami-cally incompatible. The incompatibility causes the components, though intimatelymixed, to form a continuous-dispersed phase morphology, or a cocontinuous mor-phology; in the latter neither component can be considered a dispersion. The ABSblends are tough, presumably because the ABS component is already toughened.The commercial versions of the other two examples contain an elastomeric impactmodifier. The morphologies and mechanical behaviors of these materials have notyet been studied in detail. However, because of the potential for being able to com-bine the desirable properties of each of the components, this type of material willundoubtedly become increasingly important.

BIBLIOGRAPHY

“Impact Resistance” in EPST 1st ed., Vol. 17, pp. 574–629, by C. A. Brighton and D. A.Lannon, British Geon Ltd; “Impact Resistance” in EPST 2nd ed., Vol. 8, pp. 36–68, byAlbert F. Yee, University of Michigan.

1. J. G. Williams, Fracture Mechanics of Polymers, Halsted Press, New York, 1984, p.237ff.

2. P. E. Read, in E. H. Andrews, ed., Developments in PolymerFracture-1, Applied SciencePublishers, London, 1979, Chapt. “4”.

3. J. M. Hodgkinson and J. G. Williams, Phys. Technol. 13, 152 (1982).4. S. N. Kukureka and I. M. Hutchings, in Proceedings of the Conference on High Energy

Rate Fabrication, Leeds, U.K., 1981, p. 29.5. C. B. Bucknall, Toughened Plastics, Applied Science Publishers Ltd., London,

1977.6. Guide to Engineered Materials, American Society for Metals, Metals Park, Ohio,

1986.7. A. J. Kinloch and R. J. Young, Fracture Behavior of Polymers, Applied Science Publish-

ers, London, 1983, p. 107.8. C. E. Stephenson, Brit. Plastics 30, 99 (1957).9. C. E. Stephenson, Brit. Plastics 34, 543 (1961).

10. N. H. Fahey, Mater. Res. Std. 1, 871 (1961).11. J. K. Rieke, in R. E. Evans, ed., Physical Testing of Plastics, Correlations with End-Use

Performance (ASTM STP 736), American Society For Testing and Materials, Philadel-phia, Pa., 1981, pp. 59–76.

12. W. J. Moritz, in Proceedings of the Society of Plastics Engineers, 33rd ANTEC, 1975,p. 540.


13. S. Turner, Mechanical Testing of Plastics, 2nd ed., The Plastics and Rubber Institute,London, 1983, p. 199ff.

14. S. Turner, Mechanical Testing of Plastics, 2nd ed., The Plastics and Rubber Institute,London, 1983, p. 139ff.

15. I. M. Ward, Mechanical Properties of Solid Polymers, 2nd ed., John Wiley & Sons, Inc.,Chichester, U.K., 1983, p. 329ff.

16. P. B. Bowden, in R. N. Haward, ed., Physics of Glassy Polymers, Halsted Press, NewYork, 1973, Chapt. “5”, p. 287ff.

17. S. Bahadur, Polym. Eng. Sci. 13, 266 (1973).18. L. M. Carapellucci, A. F. Yee, and R. P. Nimmer, in Proceedings of the Society of Plastics

Engineers, 44th ANTEC, 1986, p. 622.19. R. P. Nimmer, Polym. Eng. Sci. 23, 155 (1983).20. P. B. Bowden and J. A. Jukes, J. Mater. Sci. 3, 183 (1968).21. P. I. Vincent, Impact Tests and Service Performance of Thermoplastics, Plastics Insti-

tute, London, 1971.22. A. F. Yee, J. Mater. Sci. 12, 757 (1977).23. G. C. Adams and T. K. Wu, in Proceedings of the Society of Plastics Engineers, 39th

ANTEC, 1981, p. 185.24. J. T. Ryan, Polym. Eng. Sci. 18, 264 (1978).25. E. Plati and J. G. Williams, Polym. Eng. Sci. 15, 470 (1975).26. W. F. Brown Jr. and J. F. Srawley, ASTM Special Technical Publication, ASTM, Philadel-

phia, Pa., 1966, p. 12.27. J. G. Williams and M. W. Birch, in Proceedings of the 4th International Conference on

Fracture (ICF4), Vol. 1, Part IV, 1977, p. 501.28. L. V. Newman and J. G. Williams, Polym. Eng. Sci. 20, 572 (1980).29. Ref. 5, p. 182ff.30. J. A. Saver and C. C. Chen, in H. H. Kausch, ed., Crazing in Polymers (Advances in

Polymer Science, Vol. 52/53), Springer-Verlag, New York, 1983.31. H. Breuer, F. Haaf, and J. Stabenow, J. Macromol. Sci., B: Phys. 14, 387

(1977).32. A. F. Yee, Polym. Eng. Sci. 17, 213 (1977).33. A. F. Yee and M. A. Maxwell, J. Macromol. Sci., B: Phys. 17, 543 (1980).34. A. F. Yee, in Proceedings of the International Conference on Toughening of Plastics II,

London, 1985, p. 19.1.

GENERAL REFERENCES

A. J. Kinloch and R. J. Young, Fracture Behavior of Polymers, Applied Science Publishers,London, 1983. This is a particularly comprehensive treatise and contains much up-to-date(early 1980s) information.I. M. Ward, Mechanical Properties of Solid Polymers, 2nd ed., John Wiley & Sons, Inc.,Chichester, U.K., 1983. Provides many of the background concepts, mostly from a polymerphysics point of view.J. G. Williams, Stress Analysis of Polymers, 2nd ed., Halsted Press, Chichester, 1980.Presents fundamental mechanics using relatively simple mathematics. The last chapteris particularly relevant.C. B. Bucknall, Toughened Plastics, Applied Science Publishers, London, 1977. Containsmid-1970s state-of-the-art information on the science and technology of impact-modifiedplastics.

Vol. 6 INITIATORS, FREE-RADICAL 563

ASTM Standards, Philadelphia, Pa. The current edition of this series contains detailedinformation on testing techniques and data analysis.D. R. Paul and C. B. Bucknall, Polymer Blends, John Wiley & Sons, Inc., New York, 2000.

ALBERT F. YEE

University of MichiganHUNG-JUE SUE

Texas A&M University

INFRARED SPECTROSCOPY. See VIBRATIONAL SPECTROSCOPY.

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