Investigation of cleavage fracture under dynamic loading conditions: Part I fractographic analysis

The fracture toughness of ferritic steels under dynamic loading conditions on the one hand shows decreasing values with elevated loading rates but on the other hand the shape of the temperature dependent fracture toughness curve has turned out to be different from the static Master Curve according to ASTM E 1921. This difference is often explained by adiabatic heating in the crack tip region, yet it is not clear if there are other additional mechanisms under dynamic loading conditions that contribute to these changes. This work is dedicated to systematically identifying and quantifying additional mechanisms regarding cleavage fracture under dynamic loading conditions. In part I of this study an extensive fractographic analysis of the fracture surfaces was conducted for various crack tip loading rates and testing temperatures. The primary fracture-inducing mechanism was found to be identical to the dominant one under quasi-static conditions (carbide cracking). Yet, the dynamic loading conditions appear to change the origin of fracture, promote local crack arrest, and cause multiple fracture initiation sites that lead to global failure. These results also question the reliability of current local approach concepts if used to assess fracture probability at elevated loading rates. The fractographic results are used in, and complemented by, part II of this study which deals with the numerical analysis of other additional mechanisms such as inertia.