Scaling Effects in Sublaminate-Level Scaled Composite Laminates

Abstract

A series of tensile tests has been performed to determine the effect of specimen size on the mechanical response of composite laminates that were scaled at the sublaminate level. The two material systems studied were AS4/3502 and APC-2. In the case of AS4/3502, three generic layups were considered: (±30/902L,, (±45/0/90)™, and (90/0/90/0)™ deg, where n = 1, 2, 3, or 4. For the APC-2 system, only the (±45/0/90),,.? deg layup was studied with n = 1 and 4. The effect of specimen size was examined with respect to first-ply failure strain, delamination onset strain, and ultimate stress and strain. In general, the strength of scaled specimens increased with increasing specimen size. Models from the literature were applied to attempt to explain the variations observed. It was found that the effect of ply constraint is not addressed properly in existing theories. A systematic study examining the effect of specimen size on the tensile response of several composite layups, employing ply-level scaling, has been presented by Kellas and Morton.2 In agreement with the behavior of many brittle materials, a strength degradation was observed with increasing specimen size. The strength degra- dation with increasing specimen size was more pronounced in the matrix-dominated layups where off-axis plies carry a significant amount of the load. It is clear that the onset of damage, such as matrix cracking, depends on ply thickness.513 Kellas and Morton2 stated that this dependence on ply thickness was responsible for the observed strength trends. In a sublaminate-level scaling study,3 it was found that the first-ply failure stress and the strength of (±45/ ±45)nj deg laminates increased significantly with increasing speci- men size. The increase in strength was attributed to the diminishing effect of the two weak surface plies (where first-ply failures initi- ated) as the laminate thickness increased. First-ply failure initiated in the surface plies due to the reduced shear constraint that led to in-plane, normal-to-the-fiber stresses at the free edge. As an extension to the earlier research on ply-level scaled laminates,2 the present work was initiated to study three simi- lar generic layups that have been scaled at the sublaminate level. Two carbon-fiber-reinforced composite systems were considered: AS4/3502 and APC-2, which consists of the same fiber (AS4) in a PEEK matrix. Three layup configurations using the graphite/epoxy material system were investigated. Because of limited resources, only one layup was investigated using the APC-2 system. A quasi- isotropic layup was chosen.