조선 해양 구조물용 강재의 변형률 속도 민감도

초록

This is the fifth of serial companion papers dealing with dynamic hardening properties of various marine structural steels at intermediate strain rates. Five steps of strain rate levels of 0.001, 1, 10, 100, 200/s and three steps of temperature levels of LT(-40℃), RT, and HT(200℃) are taken into account for the dynamic tensile tests of three types of marine structural steels of API 2W50 and Classification EH36 and DH36. The number of specimens is totally 180 pieces. It is witnessed that effects of dynamic hardening become clearer at LT than at RT. Dynamic strain aging accompanying serrated flow stress curves is also observed from high temperature tests for all kinds of steels. Dynamic hardening factors (DHFs) at two temperature levels of LT and RT are derived at the three plastic strain levels of 0.05, 0.10, 0.15 from dynamic tensile tests meanwhile any DHF is not assessed for the high temperature tests since slight negative strain rate dependency due to dynamic strain aging has occurred. A new formulation to determine material constant D in Cowper-Symonds constitutive equation is provided as function of plastic strain rate as well as plastic strain level. The proposed formula is verified by comparing with test flow stress curves not only at intermediate strain rate ranges but also at high strain rate ranges.