Investigation on cryogenic fatigue crack growth and fracture toughness of Q500qENH based on thermodynamic entropy analysis

Abstract

While low-temperature environments can induce brittle fracture in low-carbon steel, they concurrently enhance its strength and fatigue life. This study employs thermodynamic entropy as an evaluation criterion to investigate the low-temperature fatigue crack growth rate and fracture toughness of Q500qENH steel (A novel 500–550 MPa high-strength and high-toughness weathering steel plate with excellent weldability for bridges in harsh high-altitude environments.). The classical fatigue fracture entropy concept was augmented by incorporating the J-integral from fracture mechanics. The J-integral facilitates the rapid determination of plastic strain energy density during fatigue crack propagation. Low-temperature crack growth and fracture toughness tests were conducted on Q500qENH steel. The results demonstrate that low temperatures reduce the crack propagation rate and diminish the critical crack length in the specimens. Ultimately, the areal density of plastic strain entropy throughout the fatigue crack propagation process and total fatigue life was obtained using the J-integral approach. For equivalent crack extension lengths, specimens tested at low temperatures were found to expend a higher magnitude of plastic strain entropy areal density. When critical crack length is adopted as the failure criterion, the requisite plastic strain entropy areal density for specimen failure diminishes with decreasing temperature. Consequently, low temperatures detrimentally affect the ability of Q500qENH steel to resist low-temperature brittle fracture. Though this investigation focused specifically on Q500qENH steel, the findings can be extended to other low-carbon steels with similar compositions and comparable mechanical properties.