ORIENTATION CHANGES IN KEVLAR(R)-49 UNDER AXIAL-COMPRESSION

The orientation of crystals in a single Kevlar(R) 49 fibre has been me asured by X-ray diffraction using a microcompressive device and the Co rnell High Energy Synchrotron Source (CHESS). The data show that the o rientation with respect to the fibre axis decreases with increasing ax ial compressive strain. The birefringence has been measured on the sam e sample with a polarizing optical microscope and a tilting compensato r. The results show that the birefringence decreases with increasing a xial compressive strain. This also is consistent with a decreasing cry stal orientation with increasing strain. Both effects are slightly non linear and a graph of the full width at half maximum of the crystal or ientation versus birefringence is nearly linear. The decreasing crysta l orientation in compression is opposite to the increasing orientation with increasing tensile strain. The tensile effect increases the fibr e modulus. The compressive effect, on the other hand, should decrease the modulus. The former has a limiting effect, but the latter could ha ve the opposite and might contribute to compressive failure. Copyright (C) 1996 Elsevier Science Ltd.