Specimen size effect in off-axis compression tests of fiber composites

Specimen size effects in off-axis compression tests were studied by conducting experiments with small block off-axis specimens of low modulus S2 glass fiber reinforced composites and high modulus AS4 carbon fiber reinforced composites. It was found that the off-axis compressive strength of the glass/epoxy composite decreased by a small amount (<5%) when increasing either specimen width or thickness. However, an appreciable reduction in off-axis compressive strength of the high modulus carbon/epoxy composite was observed as specimen width or thickness increased with lapped specimens. However, when a thin layer of titanium was applied on both contact ends of the specimen, the contact friction was significantly reduced, leading to much smaller reductions in off-axis compressive strength as the specimen width or thickness increases. It can be concluded that specimen end surface friction can produce specimen size effect in off-axis compression tests, and the specimen size effect is negligible when the friction between the specimen end and loading element is minimized.