PSI - Issue 19

P. Cussac et al. / Procedia Structural Integrity 19 (2019) 463–471 P. Cussac / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Experimental results and discussion

3.1. Initiation and propagation of the main crack from surface imperfections

In fatigue tests conducted on specimens containing a surface imperfection, the initiation and the propagation of the main crack systematically occurred from the imperfection. More specifically, local observations (focused on the imperfection), carried out by means of a long focal length microscope, highlighted a systematic initiation in the center and at the bottom of the defects (that means in areas associated with maximum depth) regardless of the initial depth of imperfection. In some cases, multi-cracking with the formation of cracks in different planes is noticed. However, the coalescence of these micro cracks quickly occurs, followed by the propagation of a single crack encompassing the entire length of the initial imperfection. Such a systematic propagation of the main crack from the imperfection makes possible the implementation of the electrical potential monitoring which requires to know the crack initiation site in order to correctly place the potential probes. Fig. 3 shows an example of a main crack initiated on the surface imperfection. The weld points on both sides of the anomaly are also visible on this figure. Complementary analysis of the specimens by scanning electron microscopy has shown that damage (micro cracks) is mainly located in the vicinity of the plastic wake and at the tip of the main crack whereas the other part of the specimen surface exhibits only a small amount of secondary damage. However, for the highest value of the total strain amplitude (Δε t /2 = 0.6%), signs of plasticity are clearly visible with optical microscopy over the entire surface of the tested specimens.

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Fig. 3. Example of crack having initiated and then propagated from a surface imperfection; Δε t /2 = 0.6%; Initial notch depth = 15 0 μm

The crack propagation history from surface imperfections can be reconstructed from observations of periodic ink markings. The shape of the crack front very quickly tends towards a semi-elliptical shape that can be characterized by a ratio a/b between the crack depth a (semi minor axis of the ellipse) and b (semi major axis of the ellipse) (Fig. 4).