TENSILE  TESTING  OF  COMPOSITE  MATERIALS  –  SR2  

Submitted by Arjun Radhakrishnan

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SUMMARY The tension test is conducted on three uni-directional carbon fiber composite specimens. The load and its corresponding displacement are noted and the strain gauges are utilized to obtain longitudinal and transverse strain in the specimen. The strain in the specimen is measured using three independent instruments; Strain gauge, extensometer and the testing machine displacement. The stress-strain to failure is plotted. The longitudinal tensile strength, longitudinal tensile modulus and Poisson’s ratio are calculated. The current test is compared with a previous one of similar conditions. The reliability of the test is discussed. INTRODUCTION Testing of a composite material is entirely different when compared to a metal. The main two reasons are1:

1. The anisotropic behavior of the composite material. 2. Viscoelastic effects of the matrix material.

The testing standards currently being used are a derivative of the metal testing methods. Different standardization organizations have proposed their own methods; ASTM D3039, BS2782, CRAG2 (300,301 & 302) and ISO 527. The industries have developed their own set of tests for material characterization and quality assurance. The reliability of the test on composite materials is highly dependent on the manufacturing process and the environment. The volume fraction of the fibre influences the test properties hence porosity can be in the specimen can give a lower value of strength. The gripping at the ends should not damage the fibres hence it is end tabbed. The end tabs are usually GRP but differs with various standards. The end tabs provide a friction grip so as to load the specimen and also prevents the stress concentration from affecting the outer fibres.

Strain gauges work on the principle of Wheatstone bridge. The strain gauges are bonded to the specimen using an adhesive, as shown in fig 2. When the specimen is loaded the strain gauges experiences the elongation too. The strain gauge is calibrated and the change in resistance is converted to the strain on the specimen as shown below3: Length ∝ Resistance of the circuit

Figure  1:  Nomenclature  of  specimen  

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Gauge factor, GF =(ΔR/RG)/ε The GF is known so strain can be calculated. The data acquisition device does this calculation. It should be clearly understood that the strain gauge measures the surface strain only.  

 Figure  2:  strain  gauge

OBJECTIVES Apply tension on the given composite specimens and:

• Plot the stress-strain to failure for the given composite specimens. • Calculate the longitudinal secant modulus, tensile strength and Poisson’s ratio. • State the reliability of the test • Discuss the observations on the response of the specimen under tension.

PROCEDURE Specimen preparation:

• The specimens are uni-directional CFRP (T300/914 from Hexcel). • The CRAG2 300 standards are used for tensile test of the specimens as shown

in fig 3.

 

• GRP  end  tabs  are  bonded  using  adhesives  onto  the  ends  of  the  specimen  to  prevent  failure  due  to  stress  concentration  at  the  grips.

• Specimen  length,  width,  thickness  and  end  tab  lengths  are  measured  and  averaged.

• Strain  gauging: o The   surface   of   the   specimen   is   roughened   so   that   the   adhesive  

bonds  the  strain  gauge  well. o The  surface  of  is  cleaned  using  a  wipe  and  solvent. o A   FCA   3-­‐11(Table   2   lists   the   details)   biaxial   foil   strain   gauge   is  

fixed  on  to  the  center  of  the  specimen  using  adhesives o The  wires  are  soldered  on  an  electrical  pad  and  the  output  is  taken  

from  the  electrical  pad.

Figure  3:  CRAG  300  standard  specimen  

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o When  soldering,  care  must  be  taken  not  to  tighten  the  strain  gauge  output  as  this  might  cause  apparent  stresses.

o The  gauge  resistance  is  verified.

Table  1:  Details  of  FCA  3-­‐11  Strain  gauge4  

Type 90o , 2 element cross stacked Gauge length (mm) 3 Gauge width (mm) 1.7 Gauge resistance (Ω) 120

Testing machine: • The   specimen   is   fixed   into   the   tensile   testing   machine.   The   three  

specimens  are  tested  in  two  different  testing  machines  and  loading  rates  as  shown  in  Table  2.

Table  2:  Loading  rates  

Specimen number Testing machine used Loading rate (mm/min) 1 Instron 1 2 Zwick 1 3 Zwick 5

• The strain gauges are connected to the system to collect the transverse and

longitudinal strains on the specimen. • A contact extensometer of gage length 50 mm is clipped on to the specimen to

measure the displacement. • The extensometer is removed at approximately at 0.5 % strain to avoid

damage to it.

 Figure  4:  Specimen  loaded  on  to  the  testing  machine

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Observation: • The specimen is taken to failure and the visual and audio responses during the

test are studied. • The data is processed and the strain on the specimen is independently

obtained from machine displacement, extensometer and strain gauge. • The stress-strain to failure is to be plotted • The tensile properties are calculated • The results are to be interpreted and its validity is to be discussed. • Comparison with the similar previous test on specimens 1a, 2a and 3a is to be

done. Where specimens 1a, 2a and 3a where tested in the same manner as specimen 1,2 and 3 respectively.

RESULTS Dimensions:

Table  3:  Specimen  dimensions  

Specimen Length (mm)

Width (mm)

Thickness (mm)

End tab length (mm)

1 99.5 15.03 2.13 51 2 99.5 14.97 2.22 51 3 100 15.05 2.22 51

Stress – Strain plots: The stress-strain is plotted to the last valid data available.

 Figure  5:  Specimen  1  -­‐  Stress  Vs.  Strain

 Figure  6:  Specimen  2    -­‐  Stress  Vs.  Strain

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 Figure  7:  Specimen  3  -­‐  Stress  Vs.  Strain  

 Figure  8:  Specimen  1  and  1a  comparison  (Strain  gauge)  

 Figure  9:  Specimen  2  and  2a  comparison  (Strain  gauge)  

 Figure  10:  Specimen  3  and  3a  comparison  (Strain  gauge)  

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 Figure  11:  Extensometer  data  for  specimen  1  and  1a  

 Figure  12:  Extensometer  data  for  specimen  2  and  2a  

 

 Figure  13:  Extensometer  data  for  specimen  3  and  3a  

Tensile properties and observation:

 Figure  14:  Calculation  of  secant  modulus

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 Figure  15:  Calculation  of  Poisson's  ratio

TS –tensile strength E1T – Secant Modulus at 0.25% strain, as shown in fig 14. υ1T – Poisson’s ratio at 0.25% of longitudinal strain, as shown in fig 15.

Table  4:  Properties  and  observation  

Specimen 1 2 3 M* E+ S# M E S M E S

E1T (GPa) 52 120 128 108 120 128 1.76 136 120 υ1T 0.336 0.36 0.293

Longitudinal TS (GPa)

2.21 2.15 2.12

Failure mode Near one of the end tab and explosively causing failure of the material. A crackling noted before failure.

The ply came off one by one with initial ply being close to the strain gauge. A crackling sound characteristic of delamination was noted before the explosion

Delamination of an inner ply. The ply came out and fell over in between the cracks in the material.

*Machine data +Extensometer data #Strain gauge data Failed specimens:

 Figure  16:  Specimen  3  failed  piece  

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 Figure  17:  Anomaly  occurring  for  specimen  3  test  

DISCUSSION The current test and the CRAG 300 are compared in table 5. The three specimens where tested in as per CRAG standards but certain points where not strictly adhered to and they are listed in the table 5. The machine seems to indicate very low stresses and high strain unlike the extensometer and the strain gauge for all the specimens, as shown by fig 5,6,7. The machine is likely to have higher errors due to its poor response. The extensometer and strain gauge reading seems to correlate well to each other, as shown by fig 5,6,7. The extensometer data seems to be incrementing in steps as shown in fig 11,12 and can be attributed to slipping. This effect seems to less at high loading rates as indicated by fig 13. The strain gauges exhibit a linear behavior for all the specimens.

Table  5:  Comparison  of  CRAG  and  current  test  

STANDARD CRAG METHOD 3001

CURRENT TEST COMMENTS

Thickness 1.0 mm with tolerance of 0.04

Mean of the three specimens at 2.19 mm

This accounted by CRAG standards as nearest mouldable thickness. A thicker specimen may cause the adhesion at the end tabs to fail before the material itself.

Testing speed To cause failure in 30-90 s

The rates where 1mm/min and 2 mm/min

The failure has not exactly occurred in the specified time limit. This effect may have resulted in an increased effect of creep on the results.

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Table  6:  Properties  of  specimen  1a,  2a  and  3a  

Specimen 1a 2a 3a M* E+ S# M E S M E S

E1T (GPa) 56 120 124 16 120 128 16 120 116 υ1T 0.36 0.376 0.336

Longitudinal TS (GPa)

2.03 2.15 2.11

The specimen 1 and 1a seems to have had different region of initiation of failure. As shown in fig 8, the strains reduce for specimen 1 unlike 1a, indicating delamination ripping of the strain gauge from the specimen. The visual observation noted that the initiation was possibly near to the end tab and the resulting shock wave caused the specimen to explosively fail5. The specimen 2 and 2a seem to exhibit the exact similar trend and is almost super-imposed, as shown in fig 9. Similar to the previous specimens, strain gauge failed even before the failure load is touched. The strains reduce while indicating an increase in stress before completely failing. The crackling sound before the failure possibly indicates delamination. The stress continued to increase as the remaining fibres where taking the load until complete failure. The specimen 3 and 3a have a different trend, as there was an anomaly in the mode of failure, as shown in fig 10. The delaminated ply in specimen 3 testing came off and fell into the crack causing a kind of reinforcement effect in the transverse direction, as seen in fig 17. This occurred after the failure of strain gauge hence it could be noticed that stress is increasing while the strain remains constant until failure. Specimen 3a exhibits a smooth trend with an increase in stress with strain leading to failure after reaching a maximum.

Table  7:  Standard  deviation  of  data  for  6  specimens  

Instrument Standard deviation of tensile modulus Testing machine 39.045

Extensometer 6.532 Strain gauge 5.059

Table  8:  Comparison  with  literature  

Test Literature5

Method CRAG ASTM D-3039 Tensile strength (GPa) 2.17 * 1.860+ Tensile modulus (GPa) 123.33^ 135+

Tensile strain 1.6 %^ 1.3% *Mean value +Normalized to 60% fiber volume ^mean of readings from strain gauge and extensometer As shown in table 7, standard deviation indicates that the strain gauge data is more consistent. But, the extensometer read consistently 120 GPa for five of the six specimens.

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Table  9:  Short  summary  of  possible  errors  

Source Error Effect Strain gauge Improper soldering Apparent stresses

Improper bonding to specimen

Lower values of strains

Temperature sensitivity Increased strains Trans-sensitivity 0.2% error

Extensometer Slipping Improper clamping but it is inherent to the device

End tabs Improper bonding Shearing at the end tabs rather than tensile failure at gage length

Test machine High clamping force Shearing at the end tabs rather than tensile failure at gage length

Low clamping force The specimen slips resulting in improper data

CONCLUSION The specimens 1,2 and 3 can be said to have had a valid failure through visual inspection. The stress-strain plot of specimen 1a, 2a and 3a indicates a valid failure, although not visually inspected. The mean tensile strength is recorded to be 2.17 GPa and tensile modulus is 123.33 GPa. The mean Poisson’s ratio is 0.357. The results show slight variation from literature indicating that the testing is dependent on the environment. Both the extensometer and strain gauges are reliable measuring devices. The data acquired indicates major failures to be delamination or failure near the end tabs. REFERENCE

1. Hodgkinson, J. M., Mechanical testing of advanced fibre composites, 2000 pg. 43-73, edition 2000

2. P T Curtis (ed), CRAG Test Methods for the Measurement of the Engineering Properties of Fibre Reinforced Plastics, Royal Aircraft Establishment, Farnborough, UK, Technical Report 88012, 1988.

3. Hannah, R.L. and Reed, S.E., Strain gauge user’s handbook, 1992 pg. 35 to 36 4. Technical data sheet No. CFA 001 Torayca T300 data sheet 5. Meggyesi, Joseph P., Tensile testing SR2 laboratory inference