7075-Al合金の不連続変形に及ぼす試験機剛性低減効果

Abstract

It has been clarified in a previous report that in the case of high machine-stiffness, the stress drop and time interval increase with increasing spring constant ratio α. However, it is unknown whether such a dependence may apply to low-stiffness machine systems. The purpose of this report is to clarify the influence of the stiffness on the plastic elongation and stress drop in the discontinuous deformation using 7075-Al alloy and to discuss it together with the previous results obtained using a high-stiffness machine.The method to lower the stiffness of the testing machine is to insert a spring between the load cell and the specimen grip. The cross head speed was selected so that the plastic strain rate approximately may equal 3×10−5 s−1. Tensile tests were performed at 303 K.It was found that the form of the stress-strain curve changed from a periodical to a stepped one as the stiffness is decreased. The stress drop showed a peak at a certain α while the plastic elongation increased monotonously with increasing α. The plastic elongation tended to saturate for large α especially at low strain. The stress drop for large α had a negative value and this was explained in terms of the plastic instability of local deformation of the specimen.