Experimental investigations on the ‘shallow crack effect', on the 10 MnMoNi 5 5 steel, and computational analysis in the upper shelf by means of the global and local approaches

摘要

Shallow cracks have been of great interest for some years now, since it has been observed that standard fracture toughness parameters, determined on deep crack specimens, exhibit a geometrical dependence and that most of the structures containing cracks (in particular pressure vessels) exhibit crack depth to width ratios α/W much lower than fracture mechanics specimens tested in the laboratory. This study performed at MPA Stuttgart in collaboration with EDF, aimed first of all at investigating the ‘shallow crack effect' on the high ductile, high strength, low strain-hard- ening shape welded 10 MnMoNi 5 5 steel over the whole temperature range of the transition region of toughness. Deep crack (CT25 and TPB25 with α / W= 0.55) and shallow crack (TPB25 with α/ W= 0.066) specimens were tested, providing K_IC J_i and J_IC results. At the same time, computations were performed, in the upper shelf with, on the one hand, the usual elastic-plastic model (Von Mises model) and on the other hand, the local approach model of Rousselier. Experiments did not exhibit any shallow crack effect in the lower shelf (no increase in K_IC) and show a slight effect in the transition region and in the upper shelf (slight increase in J_i, larger increase in J_IC). Computations confirmed the ability of the ‘modified' Rousselier model to describe the macroscopical behaviour (F-CMOD curves) of the three types of specimens as well as to simulate their crack propagation behaviour (CMOD-Δα curves). The combination of the Rousselier model with the line contour integral method enables us to develop the