Variation of Cleavage Trigger Morphology in Low Carbon Steel, Depending on Stress Condition

Fracture toughness of low carbon steel is one of the important properties to assure the integrity of steel structure. However, it is well known that fracture toughness is sensitive to many factors; materials factors such as microstructure, loading conditions such as loading rate and temperature, mechanical factors such as plate thickness and plastic constraint. The local fracture criterion approach is helpful to estimate the effect of some factors above on fracture toughness. When we evaluate the temperature effect and/or the constraint effect on fracture toughness using the local fracture criterion approach, the same fracture mechanism should be assured in the variation of temperature and/or plastic constraint. Cleavage facet and river pattern on the fracture surface generally suggest the cleavage type of fracture. However, cleavage trigger mechanism is rather important because an onset of unstable crack propagation along cleavage plane is generally the bottleneck process of brittle fracture. It should be well examined whether cleavage trigger mechanism is kept constant independent of temperature and plastic constraint for the applicability of the local fracture criterion approach. In the present work, the morphologies of cleavage triggers, which appeared at different temperatures and constraint, were examined. The variation of cleavage trigger morphology was also discussed referring to the local stress and strain condition.