Properties Materials 7

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www.manpower.gov.om Chapter 4 -63-

1-DESTRUCTIVE TESTS

In order to determine the properties of materials many standard tests have been devised. This tutorial covers the main tests for determining mechanical properties.

Testing can be divided up into Destructive and Non Destructive and as the names imply, one destroys the sample and the other does not. Non destructive tests may be carried out on the actual component or structure to see if it has any defects (e.g. checking aircraft skins for cracks). Destructive tests may use a specimen from a batch of new bulk material to check the batch or the specimen may be made from a structure that has been in use (e.g. checking material that failed unexpectedly).

1-1TENSILE TESTING

The tensile test is conducted in order to find the following properties of a material.

The yield or proof stress The ultimate tensile stress The elastic range

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The ductile range The modulus of elasticity

Ductility is indicated by two results from the test as follows. The % elongation. The % area reduction.

There are other properties that can be discovered for materials with more complex behaviour.

1-2 TEST SPECIMENS

Let's now look at Figure . the gauge length (L0) is the length over which the elongation of the specimen is measured. The minimum parallel length (Lc) is the minimum length over which the specimen must maintain a constant cross-sectional area before the test load is applied. The lengths L0. Lc, Li. and the cross-sectional area (A) are all specified in BS 18.

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The elongation obtained for a given force depends upon the length and area of the cross-section of the specimen or component, since.

Elongation=applied force x LE x A

Where L = length

A = cross-sectional area

E = elastic modulus

Therefore if the ratio [ L/A ] is kept constant (as it is in a proportional test piece), and E remains constant for a given material, then comparisons can be made between elongation and applied force for specimens of different sizes.

1-3 Tensile test results

Let's now look at the sort of results we would get from a typical tensile test on a piece of annealed low-carbon steel. The load applied to the specimen and the corresponding extension can be plotted in the form of a graph, as shown in Figure

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From A to B the extension is proportional to the applied load. Also, if the load is removed the specimen returns to its original length. Under these relatively lightly loaded conditions the material is showing elastic properties.

From B to C it can be seen from the graph that the metal suddenly extends with no increase in load. If the load is removed at this point the metal will not spring back to its original length and it is said to have taken a permanent set. Therefore, B is called "limit of proportionality ", and if the force is increased beyond this point a stage is reached where a sudden extension takes place with no increase in force. This is known as the " yield point" C.

The yield stress is the stress at the yield point; that is, the load at B divided by the original cross-section area of the specimen. Usually, a designer works at 50 per cent of this figure to allow for a 'factor of safety'

From C to D extension is no longer proportional to the load, and if the load is removed little or no spring back will occur. Under these relatively greater loads the material is showing plastic properties.

The point D is referred to as the 'ultimate tensile strength' when referred extension graphs or the 'ultimate tensile stress' (UTS) when referred to stress-strain graphs. The ultimate tensile stress is calculated by dividing the load at D by the original cross-sectional area of the specimen. Although a useful figure for comparing the relative strengths of materials, it has little practical value since engineering equipment is not usually operated so near

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to the breaking point.

From D to E the specimen appears to be stretching under reduced load conditions. In fact the specimen is thinning out (necking) so that the 'load per unit area' or stress is actually increasing. The specimen finally work hardens to such an extent that it breaks at E.

In practice, values of load and extension are of limited use since they apply to one particular size of specimen and it is more usual to plot the stress- strain curve.

Stress and strain are calculated as follows.

stress(σ) load

areaofcros section

steain(ε) extension originallength

The physicist Robert Hooke found that within its elastic range the strain produced in a material is proportional to the stress applied. It was left to Thomas Young to quantify this law in terms of a mathematical constant for any given material.

strain α stress

stress

therefore strain = constant(E)

This constant term (E) is variously known as 'Young's modulus', the 'modulus of elasticity' or the 'tensile modulus'. Thus:

Tensile or Compressive Stress

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E=

Strain

(force) (original cross-sectional areal

= (change in length)(original length)

Increase in lengthElongation%= x100

Original length

Increase in length (elongation)= length at failure- original lengthLu - Lo

Elongation%= x100 Lo

Example 1Calculate the modulus of elasticity for a material which produces the following data when undergoing test.

Applied load=35.7 kN, Cross-sectional area = 25 mm2

Gauge length=28 mm Extension= 0.2 mm

stressE=

strainWhere 35.7kNstress (σ)= =1.428 MPa

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25mmAnd 0.2mmstrain (ε) = =0.007

28mmTherefore

35.7 X28E = =199.92 kN / mm2

25 X0.2= 200 GPa (approx.)This would be a typical value for a low-carbon steel.

The diagram shows typical Force – extension graphs for some materials.

Elongation in gauge length of a standard test piece at the point of fracture when subjected to a tensile test to destruction.

increase in length Elongation % = X 100

original length

The increase in length is determined by fitting the pieces of the fractured specimen together carefully and measuring the length at failure.

increase in length (elongation) = length at failure — original length

Figure . shows a specimen for a soft, ductile material before and after testing. It can be seen that the specimen does not reduce in cross-sectional area uniformly, but that server local necking occurs prior to fracture. Since most of the plastic deformation and,

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therefore, most of the elongation occurs in the necked region, doubling the gauge length dose not double the elongation when calculated as a percentage of gauge length. Therefore it is important to use a standard gauge length if comparability between results is to be achieved.

Elongation is calculated as follows.

Elongation.

Lu - LoElongation % = X100

Lo

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2-The compression testBecause of the presence of submicroscopic cracks, brittle materials are often weak in tension, as tensile stress tends to propagate those cracks which are oriented perpendicular to the axis of tension. The tensile strengths they exhibit are low and usually vary from sample to sample. These same materials can nevertheless be quite strong in compression. Brittle materials are chiefly used in compression, where their strengths are much higher. A schematic diagram of a typical compression test is shown in figure

shows a comparison of the compressive and tensile strengths of gray cast iron and concrete, both of which are brittle materials.

Tensile and compressive engineering stress-strain curves for gray cast iron and concrete.

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2-1Ductility testing

The percentage elongation, as determined by the tensile test, has already been discussed as a measure of ductility. Another way of assessing ductility is a simple bend test. There are several ways in which this test can be applied, as shown in figure 13. The test chosen will depend upon the ductility of the material and the severity of the test required.

Close bend test The specimen is bent over on itself and flattened. No allowance is made for spring back, and the material is satisfactory if the test can be completed without the metal tearing or fracturing. This also applies to the following tests.

Angle bend test The material is bent over a former and the nose radius of the former and the angle of bend (θ˚) are fixed by specification. Again no allowance is made for spring back.

180 ˚ bend test This is a development of the angle bend test using a flat former as shown. Only the nose radius of the former is specified.

Bend tests: (a) close bend; (b) angle bend; (c) 180 :C bend

2-2Impact testing (toughness testing)

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Figure (a)Impact tests consist of striking a suitable specimen with a controlled blow and measuring the energy absorbed in bending or breaking the specimen. The energy value indicates the toughness of the material under test.

Figure (b)

Figure (b) shows a typical impact testing machine which has a hammer that is suspended like a pendulum, a vice for holding the specimen in the correct position relative to the hammer and a dial for indicating the energy absorbed in carrying out the test in joules (J).

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When the heavy pendulum, released from a known height, strikes and breaks the sample before it continues its upward swing. From knowledge of the mass of the pendulum and the difference between the initial and final heights, the energy absorbed in fracture can be calculated, as shown in figure (a) the schematic drawing of the impact test machine.

Figure( c) shows how a piece of high carbon steel rod will bend when in the annealed condition, after hardening and lightly tempering, the same piece of steel will fracture when hit with a different hammer.

Figure(c).

Figure(c). Impact loading: (a) a rod of high-carbon (1.0%) steel in the annealed (soft) condition will bend struck with a hammer (UTS 925 MPa); (b) after hardening and lightly tempering, the same piece steel will fracture when hit with a hammer despite its UTS having increased to 1285 MPa.

There are several types of the impact tests and the most famous type is the Izod test. Figure (d)

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In the Izod test, a 10mm square, notched specimen is used, it is preferred to use a specimen that have a more than one or two and even three notched in the same specimen. The striker of the pendulum hits the specimen with a kinetic energy of 162.72 J at a velocity of 3.8m/s. Figure(E) shows details of the specimen and the manner in which it is supported

Figure(E)

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Figure(E) Izod test (a/I dimensions in millimeters); (a) detail of notch; (b) section of test piece (at notch); (c) position of strike Since test use a notched specimen, useful information can be obtained regarding the resistance of the material to the spread of a crack which may originate from a point of stress concentration such as sharp comers, undercuts, sudden changes in section, and machining. Marks in stressed components. Such points of stress concentration should be eliminated during design and manufacture

A second type of impact test is the Charpy test. While in the Izod test the specimen is supported as a cantilever, but in the Charpy test it is supported as a beam. It is struck with a kinetic energy of 298.3 J at a velocity of 5m/s. The Charpy impact test is usually use for testing the toughness of polymers. Figure( F) .shows details of the Charpy tes: manner in which it is supported.

Figure( F)

Charpy test (all dimensions in millimeters)

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3-Hardness testing

Hardness has already been defined as the resistance of a material to indentation or abrasion by another hard body ( good hardness generally means that the material is resistant to scratching and wear) . It is by indentation that most hardness tests are performed. A hard indenter is pressed into the specimen by a standard load, and the magnitude of the indentation (either area or depth) is taken as a measure of hardness.

Hardness tests are commonly used for assessing material properties because they are quick and convenient. However, a variety of testing methods is appropriate due to differences in hardness among different materials. The most well known hardness tests are Brinell and Rockwell

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3-1The Brinell hardness test

In this test, hardness is measured by pressing a hard steel ball into the surface of the test piece, using a known load. It is important to choose the combination of load and ball size carefully so that the indentation is free from distortion and suitable for measurement. The relationship of the Brinell hardness [HB] which is between load P (kg), the diameter D (mm) of the hardened ball indenter and the diameter d (mm) of the indentation on the surface is given by the expression.

HB=Load (Kg )

surface curved of Areaindentation of

HB=P ( Kg)

1/2 π x D (√D2−d2)(mm2 )For different aterimals, the ratio p/D2 has been standardized in order to obtain accurate and comparative results such as:

k= PD2

Where K is. a constant; typical values of K are.Ferrous metals K = 30Copper and copper alloys K = 10

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Aluminum and aluminum alloys

K = 5

Lead, tin and white-bearing metals

K = 1

Figure( j) shows how the Brinell hardness value is determined. The diameter of the indentation is measured in two directions at right angles and the average taken. The diameter is measured either by using a microscope scale, or by a projection screen with micrometer adjustment.

Figure( j)

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To ensure consistent results, the following precautions should be observed.

The thickness of the specimen should be at least seven times the depth of the indentation to allow unrestricted plastic flow below the indenter.

The edge of the indentation should be at least three times the diameter of the indentation from the edge of the test piece.

The test is unsuitable for materials whose hardness exceeds 500 HB, as the ball indenter tends to flatten.

There are a definite relationship between strength and hardness so it is possible to measure the tensile strength from the hardness test.

3-2 The Vickers hardness test

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This test is preferable to the Brinell test where hard materials are concerned, as it uses a diamond indenter. (Diamond is the hardest material known - approximately 6000 HB.) The diamond indenter is in the form of.a square-based pyramid with an angle of 1 36˚ between opposite faces.Since only one type of indenter is used the load has to be varied for different hardness ranges. Standard loads are 5, 10, 20, 30, 50 and100 kg. It is necessary to state the load when specifying a Vickers hardness number. For example, if the hardness number is found to be 200 when using a 50 kg load, then the hardness number is written as HV (50) = 200.

Figure (h) shows the measuring screen for determining the distance across the corners of the indentation. The screen can be rotated so that two readings at right angles can be taken and the average is used to determine the hardness number (HD). This is calculated by dividing the load by the projected area of the indentation

Figure (h)

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H v (mm2 )=Load ( Kg)

indentation of area Surface

H v=P ( Kg )

d2/2sin 12(1360 )(mm2 )

H v= 1854 Pd2

Where P is the load in Kg and d (mm) is the diagonal of the impression made by the indenter made by the diamond.

3-3The Rockwell hardness test

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Although not as reliable as the Brinell and Vickers hardness tests for laboratory purposes, the Rockwell test is widely used in industry as it is quick, simple and direct reading. Universal electronic hardness testing machines are now widely used which, at the turn of a switch, can provide either Brinell, Vickers or Rockwell tests and show the hardness number as a digital readout automatically. They also give a "hard copy' printout of the test result together together with the test conditions and date.

In principle the Rockwell hardness test compares the difference in depth of penetration of the indenter when using forces of two different values. That is, a minor force is first applied (to take up the backlash and pierce the skin of the component) and the scale are set to read zero. Then a major force is applied over and above the minor force and the increased depth of penetration is shown on the scales of the machine as a direct reading of hardness without the need for calculation or conversion tables. Figure( g) shows a typical Rockwell hardness testing machine.The standard Rockwell test can not be used for very thin sheet and foils and for these the Rockwell superficial hardness test is used.

Figure( g)

4- Shoe Scleroscope

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The test piece must be small enough to mount in the testing machine, and hardness is measured as a function of indentation. However, the scleroscope is not like other types of hardness tests based their measure on the ratio of applied load divided by the resulting impression are [ like Brinell and Vicker well] or by the depth of impression [ like Rock well].The scleroscope is an instrument that measures the rebound height of.

A hammer droped from a certain distance above the surface of the material to be tested. The hammer consist of a weight with diamond indenter attached to it. The scleroscope therefore measures the mechanical energy absorbed by the material when the indenters strikes the surface. The energy absorbed gives an indication of resistance to penetration, which matches our definition of hardness. As shown in figure (k) The primary use of the sclerscope seems to be in measuring the hardness of large parts of steel, large rolls, casting and gears. And since the seclroscope can be carried to the work piece, it is useful for testing large surfaces and other components which could not easily be placed on the testing tables of any other testing machines.

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figure (k)

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The table below shows the different in the ways of measuring the hardness numbers for methods we mentioned above

Exercises

1- What types of destructive tests

2- Calculate the modulus of elasticity for a material which produces the following data when undergoing test.

Applied load=25.7 kN, Cross-sectional area = 20 mm2

Gauge length=25 mm Extension= 0.2 mm

3- What types of hardness tests

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