Effects of Temperature Drop on Ultralow Cycle Fatigue of G20Mn5QT Cast Steel

Abstract

Cast steels are inherently suitable for fabricating complex joints in steel constructions. These joints may suffer from ductile fracture under ultralow cycle fatigue (ULCF) loadings during earthquakes. This problem could be even worse for cast steel joints working at subzero temperatures. In this investigation, ULCF tests on 48 smooth notched tensile specimens were carried out at 20°C, −20°C,−40°C, and −60°C for G20Mn5QT cast steel. Notches with three different radii were designed for different stress triaxialities. Two load patterns, cycle to fracture and cycle and pull to fracture, were applied for the tests. Ductile fracture caused by the ULCF loadings was observed and fatigue lives were obtained for all test specimens. The characteristic lengths were determined for G20Mn5QT cast steel at the four temperatures through scanning electron micrograph analysis of the fracture surfaces. The cyclic damage degradation parameters in the cyclic void growth model were then calibrated for G20Mn5QT cast steel at 20°C, −20°C, −40°C, and −60°C based on test results and complementary finite element (FE) analysis. The cyclic damage degradation parameter decreases with the drop of temperature, which indicates that the material’s resistance to ductile fracture degrades faster at lower temperatures under ULCF loadings, especially when the temperature drops below −40°C.