Unidirectional composites were tested to failure at various strain rates.
Experimental results reveal that the transverse compressive strength increases with the increment of strain rate for both CFA graphite/epoxy and S2/8552 glass/epoxy composites. Furthermore, the transverse compressive strength could be expressed as a semi-logarithmic function in terms of the normalized strain rate.
The failure was found to initiate at around 25-35∘between the shear failure plane and the loading direction. SEM micrographics on failure surfaces indicate that for CFA graphite/epoxy, the fiber/matrix debonding is the dominant failure mode; however, matrix failure is the main failure mechanism in the S2/8552 glass/epoxy composites.
The fiber/matrix debonding can significantly reduce the transverse compressive of fiber composites. Moreover, for PPG A15037L graphite/epoxy composites, due to the improvement of the interfacial bonding, apparently the corresponding transverse is increased as compared to the CFA graphite/epoxy composites.
According to the Mohr-Coulomb type criterion together with the angle of shear failure plane, the out-of-plane shear strength was calculated. Base on viscoplasticity theory, the shear strain rate was evaluated. It was found that the out-of-plane shear strength was also significantly affected by a shear strain rate. Again, the out-of-plane shear strength was modeled using a semi-logarithmic function in terms of the normalized shear strain rate.
In addition, for E-glass/epoxy composites and nanocomposites, it was found that the corresponding in-plane shear strength and transverse compressive strength was enhanced which could be a result of the modified interfacial bonding improved by the dispersed organoclay.
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Appendix A
Investigating rate dependent behavior of PPG A15037L graphite/epoxy composites.
In this appendix, the rate sensitivity of the failure strength of PPG material subjected to transverse and off-axis loading will be investigated and the results will be in comparison to the CFA graphite/epoxy material presented early in Chapter 2. In addition, the associated failure mechanism for the material system will be determined using SEM microscopy.
A.1 Specimen Preparation of PPG Graphite/Epoxy Composites
Forty five ply prepreg of unidirectional PPG graphite/epoxy composite (from Ad group, Taiwan) were laid up and the final lay-up is 6.5 mm thick. It is noted that in the composite material system, the fiber is HTA-12K (from Toho Tenax, Japan) and the epoxy type is Bisphenol A, Novalak, and rubber modified epoxy. In addition, the fiber volume fraction is 65.23%. With the appropriate curing process, the laminate had been cured in hot press. The curing process includes two steps, in which the first was heating from room temperature to 90℃ in fifteen minutes then continuing thirty minutes and the second was continuing in
constant temperature for sixty minutes after heating up to 145℃. The pressures were 75.85psi and 142.13psi respectively for the duration of curing process, and it resulted in about 6.0 mm laminate.
The edges of the laminates were cut out using a cutting machine. The off-axis block
specimens had the dimensions of 10.0×6.0×6.0 mm and were cut from the laminates using the precision diamond saw (IsometTM 1000 Precision Saw). The fiber orientations considered included 15, 30, 45 60 and 90 degrees with respect to the loading direction. The specimens were lapped on a lapping machine (Secular LM15) with the 14.5 µm abrasive slurry
(EXTEC® aluminum oxide power) to ensure having smooth, flat and parallel loading surfaces.
In order to reduce the friction on loading surfaces of specimens, both contact surfaces were coated with titanium layer by DC sputter machine (ULVAC Co. Japan). First, the specimens were adhered on the wafer in order to be coated in the sputter. Subsequently, Argon gas flow rate, deposition pressure, deposition rate and bipolar dc power were kept constant at 170 sccm (standard cubic centimeter per minute), 2.6×10-3Pa, 8.56nm/min and 300W, respectively.
After coating on the surfaces, the other sides of the surfaces were exchanged to adhere on the wafer. According to the same process, both loading surfaces of all specimens were coated with a 1.2μm titanium film.
A.2 Compressive Failure Test