Effect of loading rate on the fracture toughness of structural steel weld metal
Defect assessment against fracture initiated failure is carried out using fracture characterising parameters determined under quasi-static rates of loading. In practice, however, there are many instances where much higher loading rates prevail such as collision, blast and earthquake damage; and in transport. For these situations the rate sensitivity of the material to fracture should be considered. Fracture toughness tests (COD) have been conducted on C-Mn steel weld metal over a range of temperature and loading rates. The effect of increased loading rates is to reduce the crack-opening displacement whilst changing the fracture behaviour, such change being accompanied by an increase in the ductile-brittle transition temperature. Thermally activated flow is found to be the predominant mechanism governing plastic flow at intermediate strain-rates. It is then possible to evaluate the effect of strain-rate and temperature on the yield strength as a function of rate parameter kT 1n (A/ε), based on the Arrhenius equation. An attempt is presented to model the influence of temperature and loading rate on the fracture initiation toughness, COD. It may be fairly said that numerical results quite well describe the behaviour of the experimentally determined variation of COD with temperature and loading rate over the range 0.1 < K < 10⁶MPam1/2 s^-1 and O < T < 500^oK. Thus, it has been demonstrated that the constitutive surface δI (T, KI) can be produced numerically using the constitutive relation σy (ε,ε, T). Consideration of the defect sizes for engineering critical assessment showed that a significant reduction in maximum allowable defect size, bar amax can result as a consequence of increased loading rate.