ASU All Material Project Report_VV

8/9/2019 ASU All Material Project Report_VV

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Project Report

Report

Vipin VijayakumaranMechanical and Materials Engineering

Arizona State University, Tempe, AZ

December 2012


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Summary

The mechanical behavior of various polymer matrix composites was evaluated through tensile

tests and 3-point bending tests at different temperatures (77 F !"# F and -"#F$% The failure

mechanisms for the different materials in form of plates and plies are briefly described% Thetesting methodologies and design of specimens have been obtained following &ST' and

'ilitary standards whenever possible% & comparison between the various materials based on

results from engineering stress-strain curves obtained from tensile testing and maximum stress-strain curves from 3-point bending is offered% The 3-point bending tests indicated that the crac

initiation load is within elastic regime and after initiation the crac propagates gradually for

textile based materials and rapidly in case of brittle materials (carbon)epoxy$% Failure under 3-point bending was deduced to be associated to both fracture of fibers within individual plies on

the tension side and bucling of individual plies on the compression side% The latter seems to

dominate the behavior and load drops observed after an initial maximum in the load-deflection

curves are liely associated to this failure mechanism% &n interesting result for 3-point bending is

that the performance of most of the samples regardless of resin improved at lower temperaturesas compared to room temperature which is consistent with behavior dominated by fibers within

the plies% This behavior was different in the case of *yneema +,# where delamination of theindividual fiber bundles occurred%

.xperimental procedures

Tensile tests were performed at room temperature 7!%! / (!"# F$ and -0!%! / (-"# F$

following &ST' *3#31)*3#31' 2 # Standard Test 'ethod for Tensile 4roperties of 4olymer'atrix /omposite 'aterials5 as closely as possible% Typical dimensions for tensile specimens are

shown in Table !

Table !6 Sample dimensions for tensile specimens

auge 8ength (mm$ #%##

auge 9idth (mm$ !:%;#

Thicness (mm$ 7%"#

/ross section area (mm:$ 1;%:;

*yneema and Spectra Shield samples were tested with a reduced thicness of 0 mm

(approximately$% Specially designed fixtures were used to impart gripping pressure on the

specimens% *etailed drawings of the grips are provided later in this report% &n alternative designfor the gripping fixtures to use with these materials is provided in the &ppendix%

For 3 point bending tests were performed at room temperature 7!%! / (!"# F$ and -0!%! / (-"# F$ following &ST' *7:";


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Table :6 Sample dimensions (in mm$ for 3 point bending tests

Total ,eam 8ength :#3%:

Supported ,eam 8ength !0:%;

,eam 9idth !:%3

,eam Thicness !#%07

Tests were performed using computer controlled servo-hydraulic frames under stroe control%

Samples were connected to the frames using wedge grips and tested until failure (3 samples for

each temperature condition$% 8oad-displacement data were captured digitally during the tests and

used to obtain engineering stress-strain curves% & displacement rate of :%0; mm)min (#%! in)min$was used for the crosshead velocity while conducting the tests%

For tensile testing and 3 point bending tests at non-ambient conditions the &ST' standardmentioned above was complemented with the following reference6 'il +andboo - !7 - !F -

=olume !->4olymer 'atrix /omposites uidelines for /haracteri?ation of Structural 'aterials%>

Section "%0%3 of these guidelines contains detailed information regarding testing at non ambienttemperatures and some of the corresponding procedures used are as follows6

& ,.'/@ environmental chamber was used to perform the test at high and low temperatures% &

liAuid nitrogen tan with flow regulator valves was connected to achieve the lower testingtemperatures%

+igh)low temperature conditioning and soaing time6 according to the 'il +andboo the test

chamber needs to be heated at desired temperature and it should be stabili?ed at that particulartemperature using a thermocouple in direct contact with the middle of the samples (a B-type

thermocouple was used in this wor$% The recommended soa time for the temperature ranges

used here is 0-!# minutes for dry conditions and : minutes for wet conditions% &ll the tests wereconducted using these procedures for high and low temperature testing%

Tensile Tests for all plies were conducted following &ST' *3#31)*3#31' 2 # Standard Test

'ethod for Tensile 4roperties of 4olymer 'atrix /omposite 'aterials5% Specially designed

fixtures were used for some of the materials to avoid slipping during testing% The geometry of the

grips used which were manufactured from carbon steel is shown in Fig% !% Two of these gripsforming a mating pair were used to sandwich5 the plies in between the peas and valleys of the

undulations and them the fixtures were held to the servo-hydraulic load frame using hydraulic

grips%


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Figure !6 Cndulating grips used for tensile testing of plies%

4reliminary tensile tests with the full thicness specimens of the *yneema +,# and SpectraShield SD-3!3" caused easy slipping-out of the composite material from the hydraulic grips via

delamination of the outermost plies from the hydraulic grips of the machine when gripping

pressure was applied directly to the outer layers of the gripping section of the samples%&dditional grips as shown in Figure : were designed to avoid slipping of the composite material

from the grips% The geometry of these grips made of is identical to the gripping section of the

*ogbone samples% Two of these grips are part of a set of identical grips which


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Figure : 6 &dditional grips for Tensile testing of *yneema +,# E Spectra Shield SD 3!3"

Desults and *iscussion

eneral results will be briefly described and discussed in this section% ndividual datasheets

listing values of important mechanical properties individual stress-strain curves as well aspictures of failed specimens after testing have been provided separately%

Bevlar G4 +!7#

Bevlar G4 +!7# consisting of high performance Bevlar !:1 fibers combined with a proprietary

thermoplastic resin matrix core exhibited relatively high tensile strength at low temperatures%Figure ; illustrates this behavior with the .ngineering Stress vs .ngineering Strain for the

Tensile test of the Bevlar G4 +!7# over the range of testing temperatures% This behavior is

similar to other Bevlar B': fibers H!I (Fibers n%d%$%These behaviors were similar through all therange of temperatures for the 3 point bending tests as well and can be seen in the data sheetsprepared for this material% The mechanism of failure was very similar over the range of

temperatures tested and consisted of delamination of the outer fiber layers and fracture of the

internal fiber bundles after that% Figure 0 shows the Bevlar G4 +!7# *ogbone specimen aftertensile testing illustrating the mechanism of failure described above%


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0 0.2 0.4 0.6

0

20000

40000

60000

80000

100000

120000

Kevlar XP H170 Tensile Test at Room Temperature(77F) ,High Temperature(160F) an !o" Temperature(#60F)

Specimen 1 _RT

Specimen 2 _RT

Specimen 3 _RT

Kevlar XP H170 _HT_2a

Kevlar XP H170 _HT_2b

Kevlar XP H170 _HT_2c

Kevlar XP H170 _CT_3a

Kevlar XP H170 _CT_3b

Kevlar XP H170 _CT_3c

$ngineering %train(in&'in&)

$ngineering %tress (psi)

Figure ; 6 .ngineering Stress vs .ngineering Strain for Bevlar G4 +!7# Tensile Tests at alltemperatures

Figure 06 *ogbone specimen of Bevlar G4 +!7# after Tensile testing at room temperature


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For the 3 point bending tests similar to the tensile tests Bevlar +!7# exhibited higher flexuralstrength at low temperature% This can again be due to higher strength of the Bevlar fibers at 8ow

Temperature% Figure " illustrates the evidence for this behavior with the .ngineering Stress vs

.ngineering Strain at the temperatures tested% Figure 7 shows the 3 point bending sample aftertests done at room temperature%

0 0.010.020.030.040.050.06

0

500

1000

1500

2000

2500

Kevlar XP H170, point ening tests at various temperatures

Kevlar H170_2a_T

Kevlar H170_2b_T

Kevlar H170_2c_T

Kevlar H170_1a_HT

Kevlar H170_1b_HT

Kevlar H170_1c_HT

Kevlar H170_1a_RT

Kevlar H170_1b_RT

Kevlar H170_1c_RT


$ngineering %tress(psi)

Figure "6 .ngineering Stress vs .ngineering Strain for 3 point bending tests of Bevlar G4 +!7#at different temperatures%


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Figure 76 Bevlar G4 +!7# 3 point bending sample after test done at room temperature

*yneema +,(+ard ,allistic$ #

*yneema +,# is made of ultrahigh-molecular-weight polyethylene fibers(C+'94.$ aligned

in the polymer matrix forming flat sheets% The tensile test samples were tested with the speciallydesigned grips as described in the experimental procedures% Tensile tests were conducted on

specimens with reduced thicness% Specimens with half and Auarter thicness of the original

thicness of the plates provided% The strength of the *yneema plates were found to beindependent of the thicness% The hydraulic grips were used at their maximum pressure to hold

the samples by their gripping section%

The mechanism of failure with regard to the tensile test of the *yneema +,# specimens at

room temperature consisted of delamination of the specimen% This resulted in gradual load drop

in the load displacement curve after maximum load was reached as each ply delaminated with

further extension of the specimen% Figure 7 shows a sample of *ogbone *yneema +,# tested

at Doom Temperature illustrating the mechanism of failure described above%

Figure 6 *ogbone sample of *yneema +,# after test at room temperature%


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Figure 16 'aximum Stress vs 'aximum Strain curves of 3 point bending tests done at (-"#F$

/arbon .poxy

For carbon epoxy composites (,1 and ,!#$ materials due to stress concentration effects failure

occurred near the shoulder which strongly suggests that a lower bound of the tensile strengthwas recorded while testing these materials% & revised geometry for the specimen could be used to

try to overcome this issue% n particular a gauge length with an hourglass shape and a very largeradius would reduce stress concentration effects% The revised geometry could be tested using

&ST' standards to obtained more accurate values of tensile strength for these carbon)epoxy

composites%

The behavior of carbon)epoxy composite under tensile loading was influenced by temperature

which might be due to changes in the matrix and interfacial regions as temperature was changed%

The matrix dominated properties and interfacial properties are mainly affected by temperatureand degradation in these properties might lead to overall change in composite strength and

stiffness%

n fiber-reinforced polymer matrix composites the coefficient of thermal expansion of the matrix

is an order of magnitude greater than that of fibers% /arbon composite has a negative coefficient

of thermal expansionJ as the matrix contracts due to low temperature the fibers tend to elongateand the reverse is true as well with elevated temperature%H:I (/hawla :##1$

From figure 1 it is clear that tensile strength of carbon)epoxy at elevated temperature isapproximately :;K lower as compared to room temperature% t is interesting to now that tensile

strength at low temperature is increased by approximately K with reference to room

temperature as shown in figure !#%

0 0&00&0.0&060&0,

0

0000

.0000

60000

,0000

100000

$10 Carb%n &p%'" 4800 RT_1a $10 Carb%n &p%'" 4800 RT_1b

$10 Carb%n &p%'" 4800 RT_1c $10 Carb%n &p%'" 4800 HT 2a

$10 Carb%n &p%'" 4800 HT 2b $10 Carb%n &p%'" 4800 HT 2c



Figure 1% Stress-Strain curves for /arbon).poxy /omposite (DT vs% !"# F$

n figures 1 E!# the sudden load drop for each specimen after reaching maximum load


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indicates fracture of the specimen%

0 0&0 0&0. 0&06 0&0,0

0000

.0000

60000

,0000

100000

10000

210 3aron $po+ .,00 RT41a 210 3aron $po+ .,00 RT41

210 3aron $po+ .,00 RT415 210 3aron $po+ .,00 !T4a

210 3aron $po+ .,00 !T4 210 3aron $po+ .,00 !T45



Fig

ure !#% Stress-Strain curves for /arbon).poxy /omposite (DT vs% -"# F$

The behavior of ,1 and ,!# materials is similar in all aspects% 'aterial ,!# which was

fabricated with ;## psi rolling pressure is :1K stronger than ,1 (:;## psi$ in terms of tensile

strength at room temperature% The failure mechanism delamination in this case is identical forboth materials% Figure !! shows delamination of a tensile specimen tested at room temperature

(DT$%

Figure !!% Failure of ,!# (similar for ,1$ tensile test sample tested at DT


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,ending stress-strain curves for ,!# material tested at room and elevated temperatures are

illustrated in figure !:% The linear elastic regime holds until the initial pea load and then the

load drops gradually until final fracture% The initial part corresponds to a regime where cracs arenot propagating so the pea curve liely corresponds to an initiation load% &s cracs start

propagating the load drops an indication that crac initiation taes more load than crac

propagation% The flexural strength at elevated temperature is approximately 0#K lower ascompared to room temperature%

0 0&01 0&0 0&00

000

10000

1000

0000

000

0000

210 3aron $po+ .,00 RT41a

210 3aron $po+ .,00 RT41

210 3aron $po+ .,00 RT415

210 3aron $po+ .,00 HT4a

210 3aron $po+ .,00 HT4

210 3aron $po+ .,00 HT45/aimum %train(in&'in&)

/aimum %tress(psi)

Figure !:% ,ending /urves for ,!# tested at DT and +T (DT vs% !"# F$

Figure !3 shows failure of beam under 3 point bending test at low temperature% The failure

mechanism is the same at all environmental conditions% &fter the initiation load is reached crac

propagates rapidly from the tensile side of the sample at an angle liely due to variations in the

stress state as the curvature of the sample increases%


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Figure !3% Failure of beam (,!#$ tested at low temperature

The flexural strength of ,!# at low temperature is comparatively low by :K to room

temperature however it is approximately !%0 times stronger than elevated temperature% Figure !;illustrates this behavior with the 'aximum Stress vs 'aximum Strain curve for the 3 point

bending tests done at Doom Temperature(77F$ vs 8ow Temperature(-"#F$%

0 0&01 0&01 0&0 0&0 0&00

000

10000

1000

0000000

0000

000

.0000

.000

0000

210 3aron $po+ .,00 RT41a 210 3aron $po+ .,00 RT41

210 3aron $po+ .,00 RT415 210 3aron $po+ .,00 !T4a

210 3aron $po+ .,00 !T4 210 3aron $po+ .,00 !T45

/aimum %train(in&'in&)

/aimum %tress(psi)

Figure !;% 'aximum Stress vs 'aximum Strain /urves of ,!# tested at DT and 8T (DT vs% -"#

F$

Tensile Testing of 4lies


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Tensile testing of plies of various materials was carried out in accordance with &ST'

*3#31)*3#31'-#>Standard test 'ethod for Tensile 4roperties of 4olymer 'atrix /omposite

'aterials>%For some of the specimens with a smoother surface finish additional grips were usedto prevent slipping of the specimens from the Laws as described in the experimental procedures

Bevlar !!:;17

The Cltimate Fracture strength of Bevlar !!:;17 plies exhibited some significant variations

from test to test% This variation can be attributed to the number of longitudinal and transversefiber bundles along the gauge length of the specimen and the direction of the bundles with

respect to the tensile force applied% Figure !0 shows the fiber bundles after the tests were tensile

was conducted at room temperature% The strength of the fiber bundles was directly related to thenumber of longitudinal fiber bundles along the gauge length% 9ith reference to Figure !0 there

are 3 longitudinal fiber bundles which showed higher strength than those with fewer bundles

along the gauge length%

Figure !0 6 4ly of Bevlar !!:;17 after test at room temperature

/onclusions and Decommendations for Future Testing

*yneema +,# demonstrated the highest fracture strength (approximately 1#### psi$ of all the

materials tested at room temperature% Spectra Shield SD 3!3" also displayed very high fracturestrength (approx% 7#### psi$% /onsidering all the aramid fibers Bevlar G4 +!7# showed overall

higher fracture strength and higher flexural strength (indicated by the 3 point bending tests$ than

Bevlar B': over the range of temperatures%

Degarding the different processing conditions of Bevlar B': i%e rolling pressure from :;## psi

for ,3 E ,; and ;## psi for ,0 E ," did not show any significant effect on the fracture


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strength% Similarly for the : types of *yneema +,# (,! E ,:$ and of ;## psi(,: E ,$ did

not seem to have a significant effect on the fracture strength of the specimens%

The failure mechanism of the *yneema +,# 3 point bending tests consisting of bucling of the

sample causing deformation and subseAuent delamination reAuires further investigation% To

further document the exact nature of this failure */ must be used to obtain a map of the straindistribution through the thicness of the sample% iven the successful results obtained from the

*/ done on the different Bevlar samples */ for the *yneema +,# would also provide

further indications on formation of deformation bands and delamination of the sample% */ onthese samples would also confirm the results reported for *yneema +,# in this report%

&ppendix

The tensile testing of *yneema +,# and Spectra Shield SD 3!3" proved to be time consuming

and difficult as the materials were strong enough to cause fracture to the additional grips that

were designed with tool steel after testing only a few specimens% 'ultiple sets of grips had to bemanufactured at various stages to test all the specimens% For a more robust and cost effective

testing procedure the thicness of the grips was increased and the specimen thicness was

decreased to half of its original value as shown in Figure%

For future testing of *yneema +,# and similar materials the specimen geometry and grips

reported in H3I (8es?e /?echowsi :#!:$% The specimen geometry and grips are shown in Fig%!" and Fig%!7 respectivey% The rounded shoulders and thinner gauge length would be easier totest%

Figure !"6Specimen eometry


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Figure !7 6 Mew rips

2iliograph+(1)*iber+, !-P%n /#vance#. Kevlar Aramid Fiber Technical Guide.!-P%n.

(2)Cala, S. Caraceriai%n an# m%#elin %5 e eec %5 envir%nmenal#era#ai%n %n 7e'-ral +ren %5 carb%n ep%'" c%mp%+ie+. MS Thesis,2009 10:11.

(3)e+e; Cec%+;i,

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