Issue 35

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 35 (2016) 114-124; DOI: 10.3221/IGF-ESIS.35.14 121 а) b) Figure 9 : Elastic (a) and (b) plastic stress intensity factor distributions for bending plate 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-compression biaxial loading and bending, several flat specimens of aluminum alloy D16 are tested with an initial notch depth equal to 3 mm. Fig. 10 shows plot of the break through point advances c and of COD against the number of cycles N under different biaxial loading and bending, respectively. As shown in Fig. 10, in-phase cyclic tension-compression leads to different effects on the relationship between crack length on the free surface and crack opening displacement for the cruciform specimens and bending plate for the same main 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. Figure 10 : Relationship between COD and crack length on free surface of cruciform specimens and bending plate. Fig. 11a represents the superficial crack growth rate dc/dN versus COD on the cruciform specimens undergoing pure Mode I tension and compression 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 when (dc/dN)>10 -4 m/cycle and COD>0.4 mm. Thus, load biaxiality has a significant influence on the initial stage of surface flaw growth. The bending plate (Fig. 11b) has a smaller range of crack growth rates as compared to biaxially loaded CS samples. However, looking at Fig. 3 and Fig. 4b, significant differences in the crack growth rate in the depth direction a and on the free surface c are expected. T