A novel model for evaluating the strain-controlled fracture behavior in strip martensite steel

Abstract

To address the challenges posed by the limitations of the Hall-Petch equation for strip martensite steel and the complex relationship between its macro-plasticity/toughness (MATP) and micro-plasticity/toughness (MITP), a composite parameter model (CPM) was developed, factoring in strain-controlled fracture (Di=di/nδC). The fracture behavior of strip martensite steel and fracture characteristics were investigated in detail. The fracture morphology results showed that the effective grain of plasticity/toughness from the lath martensite steel involved in the ductile fracture was martensite lath, while the brittle fracture was martensite packet. It was found that the Hall-Petch equation was not applicable due to different fracture modes of the same material under different heat treatment processes, as well as the equivalent size and the small angle orientation of martensite laths. However, the CPM effectively determined the effective grains of MATP and MITP as laths in the strain-controlled fracture, and the Di of MATP and MITP less than 1, revealing the consistent relationship between MATP and MITP. In addition, although Di was infinite in the stress-controlled fracture, which rendered CPM practically insignificant, it effectively revealed the relationship between MATP and MITP. When the effective grains of MATP and MITP were different, the Di value of MATP was greater than 1, and that of MITP was less than 1, reflecting the mixed fracture mode. The MATP and MITP showed an inverse relationship with the coarse grain size. When the Di values of MATP and MITP approached infinity, and the effective grains were the same, they demonstrated martensite packet sizes, showing stress-controlled fracture. MATP and MITP showed the same variation with grain coarsening and decreased in size simultaneously. Moreover, the relationship between CPM and the critical cavity expansion ratio revealed the microscopic characteristics of the macroscopic change during ductile fractures, providing a fresh perspective for studying the coupling relationship of mechanical properties.