THRESHOLD DAMAGE CRITERIA FOR THIN AND THICK LAMINATES SUBJECTED TO LOW VELOCITY IMPACT LOADS

The effects of low velocity impact on laminated composite panels are presented in this study. In the experimental program, 16 ply, 32 ply and 48 ply quasi-isotropic laminated composite panels were subjected to low velocity impact loads. The laminates were held in a special fixture, which was designed to simulate simply supported boundaries. Impact experiments were conducted using a Dynatup low velocity impact testing machine where a drop-weight system was used to strike each panel at 900 incidence under controlled conditions of impact velocity. The impactor used was of a constant weight and tip diameter. The impact height and hence the velocity and energy were used as variables in the study. Preliminary impact tests were performed to establish the incipient damage (lower bound) and visible back face damage and spalling (upper bound) energy for each of the 16 ply (thin), 32 ply (moderately thick) and 48 ply (thick) quasi-isotropic graphite/epoxy laminates. Seven energy levels were selected to study the progressive deformation and damage mechanics. It is shown that threshold damage criteria can be established by using a simple FFT smoothening of impact load-time history data.