Figure 2: 3D mesh representation of Eq. 4b applied to 6013 T6 DISCUSSION K-Validity Within the range of K-validity the opened-crack part of K may be a better driving force parameter than the nominal value of K. For the particular case of the aluminium alloy 6013 T6 at f=20 Hz no significant effect of a 3.5% NaCl solution on the amount of crack closure with respect to air was observed [1]. Furthermore, the plateau of the crack growth curves obtained for 6013 T6 at f=20 Hz are in the same K range for R=0.1 and R=0.5 [1]. Thus the use of K is justified at a constant value of the stress ratio, R. The modulus of elasticity, E, the shear modulus, μ, the yield stress, y, (and others) are interchangable material parameters from the viewpoint of dimensional analysis. Nevertheless, the values of the fitting constants are essentially affected. If E is used, the estimated value of D~10-13 m2/s is of the order of magnitude of the coefficient of diffusion of H in the Al lattice [7]. In this respect the alternative use of μ or y would cause deviations by many orders of magnitude. Another argument for the use of E instead of y is given by the similarity of the environmental effects for the alloys 6013 T6 and 6013 T4, which are represented by equal values of the elastic moduli but different values of the yield stress.
RkJQdWJsaXNoZXIy MjM0NDE=