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. (C) 2007 Elsevier Ltd. All rights reserved.