衝撃曲げ試験によるＰＭＭＡの動的破壊じん性値の測定

Abstract

With an increasing use of polymeric materials in structural components, fracture prediction of these materials under various loading conditions is becoming of great importance. The authors reported previously that the crack-tip singular stress field of a viscoelastic body was characterized by three parameters: the stress intensity factor KI(t), the strain intensity factor TI(t), and the energy release rate J. In this paper, the results of impact fracture tests on PMMA specimens are presented to examine the applicability of these parameters.The three-point bend specimens were fractured at 20°C by a falling steel sphere or cylinder over the range of loading rates 103MN/m3/2s≤KI(t)≤104MN/m3/2s. The fracture initiation time was measured by a strain gauge mounted near the crack-tip or a painted electrical resistance circuit in the path of the crack. The contact force between the specimen and impactor was analyzed by applying Hertz's theory to the local deformation near the contact point, and the fracture mechanics parameters were determined by using the simple formulas which had been derived by the authors using Timoshenko's beam theory.The critical values of the fracture mechanics parameters at crack initiation were compared with the quasi-static results reported in the previous paper. It was found that the critical values of KI(t) and J increased with loading rate, while the critical value of TI(t) was constant over the range of loading rates 10-3MN/m3/2s≤KI(t)≤104MN/m3/2s. An examination was also made on the fractography of the specimens, and the critical value of the crack opening displacement was found to be constant. This fact is consistent with the above experimental results, since the strain intensity factor criterion is equivalent to the crack-tip opening displacement criterion.