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

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 a/W much lower than those of the specimens tested in laboratory. This study performed at MPA Stuttgart with the collaboration of EDF, aimed first of all at investigating the 'shallow crack effect' on the high ductile, high strength, low strain-hardening shape welded 10 MnMoNi 5 5 steel over the whole temperature range of the transition region of toughness: Deep crack (CT25 and TPB25 with a/W=0.55) and shallow crack (TPB25 with a/W=0.066) specimens were tested, providing KIC, Ji and JIC results. At the same time, computations were performed at two temperatures, in the upper and the lower shelf with, on the one hand, the usual elastic-plastic model (Von Mises model) and on the other hand, two local approach models: The Rousselier model with a slight modification in the upper shelf (model simulating crack growth) and the Weibull model in the lower shelf (statistical model). This paper concentrates on the experimental results over the whole transition region and on the numerical results in the ductile region. Experiments did not exhibit any shallow crack effect in the lower shelf (no increase in KIC) and show a slight effect in the transition region and in the upper shelf (slight increase in Ji, larger increase in JIC). Computations confirmed the ability of the 'modified' Rousselier model to describe the macrosocpical behaviour (F-CMOD curves) of the three types of specimens and as well to simulate their crack propagation behaviour (CMOD-Δa curves). (orig./DG)