Increasing the reliability of fracture toughness assessment results for ductile materials with heterogeneous multiscale structures
摘要
Fracture toughness, or the resistance of materials to crack propagation, is an important component of the structural strength of steels and alloys. The most widely used criterion in linear fracture mechanics is the critical stress intensity factor known as plane-strain fracture toughness (KIc). The significant scatter in KIc values observed in practice is typically caused by structural differences between specimens. However, obtaining accurate KIc values for evaluating ductile materials is possible given an acceptable level of constraint on plastic deformation at the fatigue crack tip. Traditionally, this is achieved by increasing specimen dimensions, which is often limited by the size of the rolled stock. In this regard, nonlinear fracture mechanics criteria have been developed, such as the energy criterion (Cherepanov-Rice integral (J-integral) or Jc) and the deformation criterion (critical crack tip opening displacement (CTOD) or δc). Variations in fracture toughness levels across different specimens (for a given structural state of the material) are generally due to the varying morphology of nominally identical structures. However, multiscale structures are often heterogeneous, even within a single specimen. In such cases, there is a high risk that fracture toughness criteria may become a misleading average. To refine approaches for determining nonlinear fracture mechanics criteria, the crack opening geometry and propagation patterns were analyzed for specimens made of heat-treatable structural steels 38KhN3MFA-Sh and 15Kh2NMFA-Sh with varying degrees of structural heterogeneity. By using a traditional fracture toughness testing scheme, the Jc and δc criteria were determined, while accounting for stable crack growth and the curvature of the crack front.