Moisture Characterization of Thermoplastics and Thermosets for use as Matrices in Fiber Reinforced Composites

Epoxy matrix composites are widely used in aerospace industry for lighter aircraft while thermoplastic-matrix composites have traditionally been underutilized despite their excellent fracture resistance, impact strength, and the ability to be recycled. Toray Advanced Composites (Morgan Hill, CA) wants to investigate the structural integrity of thermoplastic matrices in common aircraft operating environments. An experiment was conducted to observe the effect of moisture absorption on thermoplastic composites’ mechanical strength. PEEK, PPS, and PEI matrices were compared to an epoxy matrix in this experiment. Samples were submerged in 160ºF distilled water to accelerate moisture absorption. For two months, the changes in mass and dimension were recorded at frequent intervals to compare the absorption characteristics of each matrix. Short-beam shear tests were conducted weekly to examine moisture’s effect on interlaminar shear strength. Results indicated that epoxy had significantly higher saturation point and dimensional strain compare to the thermoplastics. At saturation, epoxy also had a greater decrease in interlaminar shear strength (34.0%) than the thermoplastics (PEEK: 9.11%, PPS: 16.24%, PEI: 10.18%). The coefficient of moisture expansion (CME) for each material was the slope of a linear trendline through a percent moisture content versus percent strain plot. Epoxy had the highest CME of 1.075, followed by PPS: 1.012, PEI: 0.532, and PEEK: 0.192. Although not directly proportional, higher CME seemed to correlate with higher strength loss

Recrystallization of epitaxial GaN under indentation

We report recrystallization of epitaxial (epi-) GaN(0001) film under indentation.Hardness value is measured close to 10 GPa, using a Berkovich indenter. Pop-in burst in the loading line indicates nucleation of dislocations setting in plastic motion of lattice atoms under stress field for the recrystallization process. Micro-Raman studies are used to identify the recrystallization process. Raman area mapping indicates the crystallized region. Phonon mode corresponding to E2(high) close to 570 cm-1 in the as-grown epi-GaN is redshifted to stress free value close to 567 cm-1 in the indented region. Evolution of A1(TO) and E1(TO) phonon modes are also reported to signify the recrystallization process.Comment: 10 pages, 3 figures