Crack growth path of high-strength steel with different toughness under monotonic and cyclic loadings

In this paper, the crack growth path of 4340 steel with different toughness was studied in quasi-in-situ testing by scanning electron microscopy and X-ray technology. The crack growth behavior under monotonic loading shows that the critical load and crack growth distance increase with the fracture toughness. The angles between the crack growth direction and loading direction are 43.07 degrees for 180 degrees C tempered steel, 28.36 degrees for 350 degrees C tempered steel and 34.69 degrees for 500 degrees C tempered steel, respectively. The crack grows synchronously in high fracture toughness steel, demonstrating high crack resistance and large energy consumption. The severe deformation zone width of 4340 steel after first unloading increases from 7.5 mu m to 28.1 mu m with fracture toughness. On the other hand, the crack growth behavior under cyclic loading displays that the surface roughness in high fracture toughness steel is higher than those in low fracture toughness steel. The fatigue cycle of high fracture toughness steel is nearly twice higher than that of low fracture toughness steel. Macro/micro crack bifurcation, crack growth uniformity, crack surface roughness and plastic deformation complexity play significant roles in the crack resistance of materials. These findings would provide an in-depth understanding for the damage tolerance design of high-strength steel.