Measurement Systems for Compression Tests of Composite Materials

Carbon fiber reinforced plastic (CFRP) has an exceptionally high specific strength, even among composite materials, and finds applications in aeroplanes and certain transport aircraft to improve fuel consumption by reducing weight. An essential parameter that is always tested in the design of composite materials is compressive strength. However, since it is difficult to test compressive strength, a wide range of test methods are available.

The combined loading compression (CLC) method found in ASTM D6641 is an important compression test method. The CLC method can be carried out with a simple jig structure, untabbed strip specimens, and can be used to evaluate strength and measure elastic modulus at the same time. The compression testing of CFRP according to ASTM D6641 was then performed.

Measurement System

The CFRP specimen used was T800S/3900. Other information on the specimen is shown in Table 1. Table 2 shows the test equipment used. As shown in Figure 1, the specimen was attached to the jig based on the CLC method in ASTM D6641 and compressed using a compression plate. A photograph of the specimen is shown in Figure 2. A strain gauge was attached on the front and rear in the middle of the specimen, as shown in Figure 2. Outputs from the rear and front strain gauges confirmed that the specimen was aligned straight in the jig during specimen attachment. Using a torque wrench, the specimen was attached and fastened in place uniformly. The test was carried out with the test speed set to 1.3 mm/minute.

Test Fixture

Figure 1. Test Fixture

Specimen

Figure 2. Specimen

Table 1. Specimen Information

. .
Length : 140 mm
Width : 13 mm
Thickness : 3 mm
Lamination Method : [90/0] 4S

Table 2. Experimental Equipment

Testing Machine : AG-Xplus
Load Cell : 50 kN
Test Jig : CLC test fixture

Test Results

Measurements were made twice, and stress-strain curves are shown in Figure 3. The strain used here is the mean strain taken from the rear and front sides of the specimen. Figure 4 shows the relationship between the first strain measurement and time, demonstrating the outputs acquired from the strain gauges. As shown in Figure 4, the outputs from both strain gauges were similar up to around 40 seconds, which proves that the test was indeed successful. After around 0.5% strain, a small amount of deviation between the strain gauges arises which is caused by a slight specimen flexure. The test results are shown in Table 3. 640.7 MPa was the mean compressive strength and 72.9 GPa was the mean elastic modulus. The mean of the strain gauge outputs was used to calculate the elastic modulus.

Stress-Strain curves (n=2)

Figure 3. Stress-Strain curves (n=2)

Displacement-Time curves (1st)

Figure 4. Displacement-Time curves (1st)

Table 3. Test Results

. Compressive Strength [MPa] Elastic Modulus [GPa]
1st 629.9 71.4
2nd 651.4 74.3
Mean 640.7 72.9

Conclusion

The test system was successfully used to perform compression testing of a CFRP according to ASTM D6641. Compressive strength and elastic modulus can be determined relatively easily for CFRPs because this standard test method enables the testing of untabbed strip specimens.

This information has been sourced, reviewed and adapted from materials provided by Shimadzu Scientific Instruments.

For more information on this source, please visit Shimadzu Scientific Instruments.

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