Effects of specimen size and sample aspect ratio on the elastic stiffness of graphite/epoxy laminates

Effects of specimen height and sample aspect ratio on the elastic stiffness of uniaxially compressed graphite/epoxy laminates of [0]48, [±10]12s, [±15]12s, [±30]12s and [±45]12s architectures have been studied by using an end-loading fixture. Tall samples allowed all fibers to be unconstrained on at least one side while short samples constrained a portion of the fibers at the specimen ends against the platen interfaces. For all of the fiber orientations tested, a rather counter-intuitive size effect was discovered in that the specimens with smaller aspect ratios and heights displayed a more compliant response than their taller counterparts with identical ply orientations. A systematic investigation found this effect to be genuine, and not attributable to the loading machine, data acquisition system, or the laminate-to-laminate variability stemming from composite manufacturing. An in-depth investigation which included measurement of the surface strain profile along the specimen height attributed the increased compliance of the shorter samples to the frictional sliding of the sampleʹs loading ends in the thickness direction. This conclusion was drawn by noting that the shorter specimens displayed much smaller in-plane Poisson ratios near the specimen ends compared with taller specimens (because of larger loading end-induced in-plane constraints) even though all specimens irrespective of their heights and aspect ratios accommodated the same overall axial strain. While these effects highlight the importance of specimen geometry and fixturing considerations for off-axis compression testing, the vast data set presented here should be of interest to the composite designers.