Fatigue of 8630 cast steel in the presence of porosity

Abstract

Fatigue and monotonic test specimens having porosity ranging from micro- to macroscopic levels were cast from 8630 steel. Monotonic and fatigue properties were obtained to determine the effect of porosity on the mechanical performance of the cast steel. Axial fatigue tests were conducted under fully reversed conditions in both strain and load control on specimens containing microporosity, and in load control for specimens containing macropores. Monotonic tests revealed that specimens containing microporosity had strength properties comparable to sound material, but with substantially reduced ductility (76% less reduction in area). At stress amplitudes of 126 MPa, microporosity specimens were found to have lives greater than 5 million cycles (run-out) whereas macroporosity specimens had fatigue lives in the 102–104 cycle range at the same stress level. Fatigue lives for macroporosity specimens were in a range from 104 to 106 cycles when tested at the lowest stress amplitude, 53 MPa. The measured specimen elastic modulus was found to vary with porosity volume. Specimens with higher measured modulus outperformed the lower modulus specimens. Fatigue lives of the cast steel specimens were calculated using conventional models of fatigue behaviour, the strain–life and linear elastic fracture mechanics (LEFM) approaches. Life calculations made using the strain–life approach gave good agreement with measurements for specimens having microporosity, but this approach gave non-conservative results for macroporosity. LEFM modelling gave non-conservative results for both micro- and macroporosity specimens. For specimens with macroporosity, the calculations are difficult because of the porosity's complex shape and large size relative to the specimen, and the inability to determine the specific macropores responsible for fatigue failure of the specimens which is necessary for direct model-measurement comparisons.