etude - ryan sager - short beam testing of composite laminates with and without swcnts

8/12/2019 Etude - Ryan Sager - Short Beam Testing of Composite Laminates With and Without SWCNTs

1/14

Short Beam Testing of Composite

Laminates with and without SWCNTsRyan Sager

Daniel Ayewah


2/14

Summary

Fibrous composites offer outstanding material properties and adaptability for use in the

aerospace industry. With the discovery of carbon nanotubes, the prospect of enhancing

existing mechanical and thermal properties of composite materials has led to a massiveeffort to understand the effects nanotubes have on existing materials. An important

material property associated with composite laminates is the interlaminar shear strength.

This property relates the amount of shear stress a specific material will handle before

individual plies fail in shear. Because carbon nanotubes have exceptional stiffness andtensile strength, it is proposed that adding them to the fiber-matrix interface of composite

laminates will enhance material shear strength properties.

After receiving two composite plates made of T650/Epon 862/W carbon fiber/epoxy, one

12-ply material with Single-Walled Carbon Nanotubes (SWCNTs) sprayed along the

midplane and the other, 12-ply material without SWCNTs, the Texas A&M University

Nanotechnology Research Group decided to test the effect SWCNTs have on theinterfacial shear strength of composite laminates. The short beam shear test described by

the American Society of Testing and Materials [1] offers an easy and repeatable method

for testing the apparent interlaminar shear strength of composite materials and wasperformed on 5 specimens of each material.

Results for the testing revealed anticipated load-displacement curves resulting ininterlaminar failure of each specimen. The specimens without SWCNTs produced

slightly higher average short beam shear strength of 57.158 MPa with a coefficient of

variation of 1.223% and failed in shear along the specimen midplanes. The specimenswith SWCNTs produced slightly lower average short beam shear strength of 56.680 MPa

with a coefficient of variation of 1.378% and failed in shear along two ply interfacesneighboring the midplane.

Although the specimens without SWCNTs produced slightly higher short beam strength

values, the difference was less than 1% and is insignificant. In previous testing, results

were obtained which indicated that SWCNT-infused specimens failed in shear away fromthe nanotube coated midplane whereas blank specimens failed in shear along the

midplane. This suggested that the nanotubes created a strengthening effect in shear.

However, results obtained from this test reveal no difference in failure between SWCNTspecimens and blank specimens. Both failed in shear slightly above and below the

midplane. It is recommended that additional short beam shear testing be performed on

existing material and that other testing such as the double-cantilever test or additionalshort beam shear tests be performed on future materials.

Introduction

Over the course of two months, short beam shear testing was performed on compositelaminates at Texas A&M University. The composite plates used in the testing were

created by Grace Rojas at the Air Force Research Laboratory in Dayton Ohio during the


3/14


4/14

rotate, allowing free lateral motion of the specimen. Load was applied in the center of

the specimen at the rate described above through the use of a 6.0 mm diameter steeldowel. The beam was loaded until fracture, and the fracture load was taken as a measure

of the apparent shear strength of the material. Displacement was measured from the

relative movement of the loading head through the use of the integrated MTS linear

displacement gauge. The test set-up can be seen in Figure 3 below.

Figure 3 Short beam test set-up

Displacement and load data were automatically logged by computer through the use of

the MTS TestStar software package. A predicted load-displacement curve was observedfor each specimen. As load was applied, a linear deflection response was observed until

a maximum load was achieved. At this point, the applied force drops dramatically

indicating the specimen has failed. This maximum load was taken as a measure of theapparent shear strength of each specimen. Five specimens of each configuration weretested. Short beam shear strength was calculated for each specimen based on the formula

[1]:

Fsbs= 0.75 X Pm/ (b X h)

where:Fsbs = short-beam strength, MPa

Pm = maximum load observed during the test, N

b = measured specimen width, mmh = measured specimen thickness, mm


5/14

Test Results and Discussion

Figure 4 and Figure 5 describe the results obtained during testing. The load-deflection

curves obtained for each of the tested specimens demonstrate good repeatability andcorrespondence between each test. The 12-ply blank specimens resulted in an average

maximum load of 3294 N, with a standard deviation of 71.7 N and a coefficient ofvariation of 2.2%. The average short beam strength was 57.4 MPa with a standarddeviation of 0.8 MPa and a coefficient of variation of 1.4%. This represents acceptable

data correlation within the test.

The 12-ply specimens exhibited ideal brittle failure modes during their tests. A linearslope on the load-displacement curve followed by a sharp decrease in load represents the

interlaminar shear failure desired during these tests. The average maximum load for the

12-ply nanotube-infused specimens was 3257 N with a standard deviation of 35.5 N anda coefficient of variation of 1.1%. The average short beam strength for the 12-ply

specimens was 56.2 MPa, with a standard deviation of 1.2 MPa and a coefficient of

variation of 1.7%, resulting in a difference in strength between the SWCNT and blankspecimens of 0.8%.


6/14

Short Beam Test Results for 12-ply Blank Composite Laminate

-500

0

500

1000

1500

2000

2500

3000

3500

4000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Displacement (mm)

Force(N)

Test 1

Test 2

Test 3

Test 4

Test 5

Specimen # Maximum Load (N)

Short Beam

Strength (Mpa)

1 3185.456 56.8

2 3287.962 56.6

3 3279.831 57.2

4 3348.029 58.0

5 3369.000 58.5

Average 3294.056 57.4

S.D. 71.7 0.8C.V. (%) 2.2 1.4

Figure 4 Results for short beam shear test of blank specimens


7/14

Short Beam Test Results for Composite Laminate w/ SWCNTs

-500

0

500

1000

1500

2000

2500

3000

3500

4000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Displacement (mm)

Force

(N)

Specimen 1

Specimen 2

Specimen 3

Specimen 4

Specimen 5

Specimen # Maximum Load (N)

Short Beam

Strength (Mpa)

1 3061.157 56.2

2 3097.638 56.2

3 3372.381 57.8

4 3298.279 56.4

5 2912.309 56.4

Average 3148.353 56.6

S.D. 186.1 0.7

C.V. (%) 5.9 1.2

Figure 5 Results for short beam shear test of SWCNT-infused specimens

Microscopic analysis of the failed specimens reveals interesting results. In previous

testing performed at Texas A&M by the authors, blank specimens of the same material asthat discussed in this test failed in shear along the midplane. This was the expected

failure as the maximum shear force should be experienced along the midplane. However,

in the current set of tests, evidence of midplane shear failure could not be found in theblank specimens tested. Instead, shear failure occurred directly above (top surface of ply

6) the midplane. Figure 6 illustrates the failure mode of the blank specimens used in the

test. Failure is clearly evident on the top surface of the 6th

ply whereas there is nocracking present along the midplane.


8/14

Figure 6 Failure mode of blank specimen

Forty-five degree cracks are also prevalent throughout the specimen. This can beexplained as tensile failure from the principal stresses associated with near-pure shear.

When a combination of stresses is present, principal stresses result at a given angleaccording to the following formulas:


9/14

Pure shear results in a pof 45 degrees.

Figure 7 illustrates the failure mode for the 12-ply SWCNT-infused specimens.

Interlaminar shear failure is evident throughout the specimen; however failure did not

occur at the midplane. Instead, interfacial shear failure occurred above and below themidplane.

Figure 7 Failure mode of SWCNT specimen


10/14

Conclusions and Recommendations

The preceding tests demonstrated very good repeatability and data correlation betweenindividual specimens. Shear failure modes were also observed as expected although not

along the midplane. These observations lead to the conclusion that the tests were

successful however results are not definitive. Although the short beam strengthassociated with the blank specimens are greater than those associated with the SWCNT-

infused specimens, the difference in strengths of 0.8% fell within the standard deviation

for the tests and is therefore not significant.

The observation that failure did not occur along the midplane of either the blank or

SWCNT specimens, but rather slightly above and below leads to a failure in the ability to

compare strengthening effects of nanotubes in the midplane. It is suggested that otherexperiments, such as the double-notched shear test or short beam shear testing on non-

woven fabric composites should be performed to verify results.


11/14


12/14


13/14

12 Ply T650/Epon862 w/ SWCNT's Short Beam Test

-500

0

500

1000

1500

2000

2500

3000

3500

4000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Displacement (mm)

Force

(N)


14/14

Specimen No. 1

Material Carbon Fiber Metric English

Dimensions

Depth d 4.572 mm 0.1800 in

Length L 27.940 mm 1.1000 in

Width b 8.928 mm 0.3515 inMax Load 3061.157 N 688.1756 lb

Short Beam Strength 56.245 MPa 8157.606 psi

Specimen No. 2


Dimensions

Depth d 4.610 mm 0.1815 in

Length L 28.194 mm 1.1100 in

Width b 8.966 mm 0.3530 in

Max Load 3097.638 N 696.3769 lbShort Beam Strength 56.205 MPa 8151.814 psi

Specimen No. 3


DimensionsDepth d 4.585 mm 0.1805 in

Length L 28.194 mm 1.1100 in

Width b 9.538 mm 0.3755 in

Max Load 3372.381 N 758.1416 lbShort Beam Strength 57.842 MPa 8389.275 psi

Specimen No. 4


Dimensions

Depth d 4.572 mm 0.1800 in

Length L 27.927 mm 1.0995 in

Width b 9.589 mm 0.3775 in

Max Load 3298.279 N 741.4828 lb


Specimen No. 5


Dimensions

Depth d 4.597 mm 0.1810 in

Length L 27.902 mm 1.0985 in

Width b 8.420 mm 0.3315 in

Max Load 2912.309 N 654.7133 lb


etude - ryan sager - short beam testing of composite laminates with and without swcnts

Documents