Issue 41

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 41 (2017) 191-196; DOI: 10.3221/IGF-ESIS.41.26 195 It can be seen from this figure that the shot blasting effect has nearly no effect on the crack resistance of the two materials. The same results have been obtained when comparing materials I and II. A more sophisticated analysis taking into account the 3D shape of the cracks for the calculation of the Stress Intensity Factors (SIF) with the Raju & Newman analytical formulas [8] shows again no difference between the four materials in terms of crack growth rates. Figure 4 : da/dN curves for materials III and IV. The SIF values are based on Murakami’s (area) 1/2 parameter. Figure 5 : Crack aspect ratio for the four materials studied (several samples per materials). One difference between the materials is observed when the crack shape is being considered. In case of material I, II and III the crack fronts acquire a semi-circular (penny) shape from a very early stage of propagation, conserving this geometry until final failure. In the case of material IV, however, the crack front acquires a semi-elliptical shape, a geometry which is maintained until final fracture. For materials I, II and III the crack intersects the surface with a 90 degree angle; this is not the case for material IV as observed on Fig. 3. Those differences are shown on Fig. 5 which gives a summary of the crack aspect ratio (depth/half surface length) for all materials: material IV is the only one which has a ratio below 1. A Raju & Newman analysis of the SIF values suggests that a penny shape correspond to a crack growing with an out of equilibrium shape that is to say a crack front along which K is not constant but higher at the surface. One explanation for this might be the “tunneling” effect of the crack which is well known for through cracks in CT samples (see for example [9]): a larger level of crack closure at the sample due to the larger plastic zone size “holds back” the crack front.