Material Characterization Testing

3.1.2.1 Plain Weave Coupon Tension Tests

A two-ply plain weave CFRP laminate plate was fabricated using Patz Materials & Technologies IM7 GP-6k carbon fiber fabric prepreg with GP2-61-2 resin. A Stahl’s Hotronix heat press was used to cure the plate, shown in Figure 3.4(a), by pressing it for one hour at 107°𝐶 under 0.138 MPa of pressure followed by three hours at 177°𝐶 under 0.552 MPa of pressure. After allowing the plate to return to room temperature, the plate was cut into coupons with dimensions shown in Figure 3.4(b). Coupon tension testing was performed in accordance with ASTM 3518/D3518M-94.

(a) (b)

Figure 3.4: (a) Heat press setup for fabricating flat coupons, and (b) schematics of tensions tests for unidirectional on-axis, plain weave on-axis, and plain weave off-axis

The coupon dimensions were 12.7 mm by 177.8 mm with fibers oriented at either 0° or ±45° to the loading direction. Gripping tabs were glued to the ends of the coupons to prevent slippage, and the coupons were tested in tension using an MTS Bionix servo- hydraulic machine, shown in Figure 3.5(a). Two Digi-Key CEA linear strain gages were fastened to the middle of each specimen to measure the strain in the loading and transverse directions, as shown in Figure 3.5(b). Strain was measured within a range of ±5% with a resistance of 350 Ohms and a resistance tolerance of ±0.3%. Displacement control was applied with a crosshead rate set to 2 mm/minute.

(a) (b)

Figure 3.5: (a) MTS Bionix testing machine, and (b) a close-up of a plain weave coupon tested in tension.

The variables recorded during testing were load 𝑃, axial displacement 𝛿, longitudinal normal strain 𝜀_𝑥, and lateral normal strain 𝜀_𝑦. Shear stress 𝜏₁₂ was calculated as:

𝜏₁₂= 𝑃 2𝐴

(3.1)

where 𝐴 is defined as the cross-sectional area of the coupon. Shear strain 𝛾₁₂ was calculated as:

𝛾₁₂= 𝜀_𝑥− 𝜀_𝑦 (3.2)

The in-plane shear modulus of elasticity 𝐺𝑐ℎ𝑜𝑟𝑑 was calculated using the following equation:

𝐺₁₂𝑐ℎ𝑜𝑟𝑑 =𝛥𝜏12 𝛥𝛾₁₂

(3.3)

where 𝛥𝛾12 is the difference between two shear strain data points in the linear portion of

the shear strain plot, and 𝛥𝜏₁₂ is the difference in applied stress between the two data points in the linear portion of the plot. Ultimate in-plane shear strength 𝑆₁₂ was calculated using:

𝑆12 =

𝑃_𝑚𝑎𝑥 2𝐴

(3.4)

where 𝑃𝑚𝑎𝑥 is the maximum load before specimen failure. Additionally, ultimate tensile

strength +𝑆11 was calculated by dividing the peak load by the cross-sectional area of the

coupon. Young’s modulus in the fill tow direction 𝐸₁₁ and Young’s modulus in the warp tow direction 𝐸₂₂ were taken to be equal and calculated as the initial linear slope of the stress versus axial strain plot. The in-plane Poisson’s ratio 𝜈₁₂ was determined by the difference between two points of the transverse strain data along the initial linear portion of the transverse strain versus axial strain plot divided by the difference between the two data points of axial strain.

3.1.2.2 Unidirectional Coupon Tension Tests

unidirectional material was an IM7 carbon fiber preimpregnated with an 8552 epoxy resin. The coupons were tabbed and cut with the same dimensions as the plain weave coupons with the fibers oriented parallel to the loading direction. Testing was performed in accordance with ASTM D3039/D3039M–14. The ultimate tensile strength +𝑆₁₁, Young’s modulus in the fill tow direction 𝐸₁₁, and the in-plane Poisson’s ratio 𝜈₁₂ were determined following the same procedure as described for the plain weave material testing.

3.1.2.3 Fiber Volume Fraction Tests

Determination of the fiber volume fraction of the three-layer FlexLam CFRP laminate was performed in accordance with ASTM D3171–15 using nitric acid to dissolve the epoxy matrix. Specimens of the tape spring laminate were cut to dimensions of approximately 25 mm by 25 mm, as shown in Figure 3.6(a), which weighed on average 0.4776 grams. Each specimen was placed into a glass beaker with approximately 100 mL of 69.5% nitric acid from KMG Electronic Chemicals, Inc. and heated to 80°𝐶 for 6 hours. The mixture was then filtered and vacuumed, rinsed three times with distilled water, and cleaned with acetone. The remaining fibers were placed on a ceramic plate, covered with perforated aluminum foil, and heated at 100°𝐶 for 1 hour. The cooled fibers from each specimen, shown in Figure 3.6(b), were then weighed.

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Figure 3.6: (a) Tape spring laminate specimen before testing, and (b) carbon fibers of specimen after epoxy disintegration.

The fiber volume fraction was calculated as: 𝑉_𝑟 = 𝑀𝑓 𝑀_𝑖 × 100 × 𝜌𝑐 𝜌_𝑟 (3.5) 𝜏12 = 𝑃 2𝐴 (3.1)

where 𝑉_𝑟 is the fiber volume fraction, 𝑀_𝑓 is the final mass of the fibers, 𝑀_𝑖 is the initial mass of the laminate specimen, 𝜌_𝑐 is the density of the composite, and 𝜌_𝑟 is the density of the IM7 carbon fibers, reported by Hexcel to be 1.78 g/cm3 (Hexcel, 2014).

3.1.2.4 Epoxy Master Curves

To numerically describe the stress relaxation behavior of the CFRP tape spring, a master curve of the epoxy used in the matrix of the plain weave laminae was developed. This was done through a series of stress relaxation tests performed with a dynamic mechanical analysis (DMA) machine, shown in Figure 3.7, in the Polymers and Composites Laboratory at the University of New Mexico (UNM). The PMT-F7 resin specimens were first molded and cured by Patz Materials & Technologies at 177°𝐶 for two hours (Patz, 2014). The cured epoxy was then transported to the UNM Physics Laboratory where it was cut and shaved down to rectangular specimens with dimensions of 20 mm by 5 mm by 1 mm. The epoxy specimens were then tested for stress relaxation in tension using the DMA, TA Instruments Q800 (Garner et al., 2015). Throughout testing the temperature was increased from 30°𝐶 to 240°𝐶 by increments of 5°𝐶. After the machine equilibrated at each temperature for 5 minutes, the specimen was displaced 0.1 µm over 10 minutes before the temperature was increased to the next value and the specimen displacement was reset to zero. The modulus of the epoxy was measured as a function of time for each temperature investigated. Using the time-temperature superposition principle (TTSP) (Findley, 1976), the modulus versus time data for each temperature of testing was shifted to generate the

(a) (b)

Figure 3.7: (a) DMA machine, and (b) an epoxy specimen tested for stress relaxation in tension. Images courtesy of Amy Garner.