Issue 30

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 30 (2014) 454-461; DOI: 10.3221/IGF-ESIS.30.55 460 a) b) c) d) Figure 7 : Fatigue crack path at R=0.5: a) and b) two different values of crack extension, c) a zone in the front of conventional crack tip, d) damaged zone near the main crack path. C ONCLUSIONS n this work, the fatigue crack propagation in a Cu-Zn-Al shape memory alloy, obtained in laboratory, has been evaluated in correlation to main crack micromechanisms by using Scanning Electron Microscopy. Fatigue crack propagation has been carried out by means of traditional hydraulic fatigue crack machine tests and CT specimens, at ΔP=constant, R=0.1 and R=0.5 in lab conditions (ASTM E 647). For R=0.1, the ability for material to change its structure under load generates a particular fatigue crack propagation behavior characterized by five different stages. Corresponding to the third stage, microstructure transformation takes place. Considering the crack paths, the following conclusions can be summarized: 1) da/dN-  K results are strongly influenced by the R value, probably due to peculiar mechanical behavior of the investigated alloy, with a possible reversion of the stress induced martensite (obtained for K max ) to austenite, depending on the loading conditions (R and  K values) 2) main crack path is characterized by the presence of a “zig-zag” path, probably due to the presence of brittle inhomogeneities at boundary grains; the angle between main crack and boundary grains determine the initiation of secondary cracks; I