Ball Indentation Technique to Estimate Fracture Toughness of High-Strength Rail Steels

In this work, the non-destructive ball indentation technique is applied to estimate fracture toughness for three types of high-strength rail steels based on continuum damage mechanics. Damage parameter, in terms of the deterioration of elastic modulus, is measured for three rail steels using the loading-unloading smooth tensile test, based on which a ductile damage model is calibrated to determine the critical damage parameter at the onset of fracture. Meanwhile, an instrumented ball indentation test is conducted on the three rail steels to generate damage as a function of contact depth under indentation compression. The critical damage parameter from the smooth specimen is then applied to the indentation test to determine the critical contact depth for calculating the indentation fracture toughness based on the concept of indentation energy to fracture. Results show that although the magnitude of the so-determined indentation fracture toughness is greater than that of the corresponding mode I critical stress intensity factor (K_Ic) measured using the pre-cracked single-edge-notched bend (SENB) specimen, the former can well predict the ranking order of the K_Ic values among the three rail steels.