the absolute values of the F/FY ratios depend on the chosen yield stress and may differ slightly, if σ0.1, σ0.2 or σ0.5 are used for the yield strength. Figure 3 Plastic limit load ratio F/FY versus load line displacement curves for the two foams, which are depicted in Fig. 2. Left image shows the foam with the density of 0.25 g/cm3 and the right image the foam with 0.40 g/cm3. Since valid fracture toughness values based on KIC could not be obtained in these tests, single specimen Jintegral determinations according to ASTM standard E813 and E1152 were performed. Fig. 4 shows Jintegral versus crack extension plots for different specimen sizes and different foam densities. Although in all J-integral tests the specimen size criterion was fulfilled, a certain size dependence of the initial J-value J0.2 and the J-curve can be observed. Furthermore, the initial part of the J-integral curves shows an atypical shape and a large scatter, which makes the determination of initial J-values, like Ji or J0.2, according to the standards difficult or impossible. A typical plateau in the J-integral curves that is associated with a steady state J-value JSS, as reported in [6], cannot be observed in all samples. For the foams with lower densities a decrease in the J-integral curve at large crack extensions is evident. This is in agreement with the previous observations in the K vs. ∆a and F/FY vs. vLLD curves. In general, the application of ASTM standards (E813, E1152), which are designed for solid metals, to metallic foams is problematic. Due to their special mechanical behaviour, metallic foams exhibit a different response in the fracture mechanic tests as assumed in common standards. Figure 4 J-integral versus crack extension plots for different specimen sizes and at a constant density of 0.25 g/cm3 (left image) and for different densities at constant specimen size (right image). The described methods calculate from the global mechanical response the fracture toughness values (stress intensity factor K, J-integral). A more direct method is provided by the crack opening displacement (COD) concept. In this investigation the COD5 value is used, which is measured 5 mm behind the initial crack tip or
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