Issue 33

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 33 (2015) 335-344; DOI: 10.3221/IGF-ESIS.33.37 342 а) b) с) Figure 9 : Elastic (a) and plastic stress intensity ( b -B95, c -D16) factor distributions along crack front (1-initial, 2-3- intermediate , 4-final). E XPERIMENTAL RESULTS AND DISCUSSION he evolution of the crack growth rate of the elliptical-fronted edge cracks during the tests is determined using COD and the microscope. In order to study the crack growth under fatigue tension loading with superimposed cyclic torsion, several hollow specimens of both aluminum alloys B95 and D16 are tested with an initial notch depth equal to 3 mm. Fig. 10 shows plot of the break through point advances b and of COD against the number of cycles N under pure tension and combined loads, respectively. As shown in Fig. 10, in-phase cyclic torsion loading superimposed on cyclic tension leads to different effects on the relationship between crack length on the free surface of the specimen and crack opening displacement under combined cyclic loading which depend on the material properties. Nevertheless there is a strong correlation between these two parameters that can be very useful for automation of experimental studies of fatigue and fracture under multiaxial stress state. On the base of this experimental data, polynomial functions can be used to express the COD as a function of the superficial crack length. Fig. 11 represents the superficial crack growth rate db/dN versus COD on the hollow cylindrical specimens undergoing pure tension and combined loading. It is found that the crack growth rate along the external surface direction as a function of COD fit into a single curve with a small scatter band of the experimental results under different loading conditions for both tested aluminum alloys. However, looking at Figs.3,4 and considering changes in the general durability of the specimens in pure tension and combined loading, significant differences in the crack growth rate in the depth direction a and on the free surface b of hollow specimens under the above types of loading conditions are expected. Figure 10 : Relationship between COD and crack length on free surface of hollow specimen under different loading conditions. Figure 11 : Crack growth rate on free surface of hollow specimen versus COD under different loading conditions. T