Figure 1 Load and crack extension versus load line displacement curve (left image) and corresponding surface strain map at a crack extension of 5 mm (right image, ε=x100%) for an ALPORAS® aluminium foam CT-specimen with a density of 0.40 g/cm3, W=100 mm and B=30 mm. Because of the wide plastic regime in the load versus load line displacement curve no valid fracture toughness values in terms of the critical stress intensity factor KIC according to ASTM E399 could be obtained with the specimen sizes used in this investigation. The calculated KQ values are relatively low and vary between 0.35 to 1.0 MPa.m1/2 depending on the density of the foam. Fig. 2 shows K versus crack extension ∆a curves for two foams with different densities. Both foams reveal an increase of the fracture toughness with crack propagation up to a crack extension of about 5 mm (it looks like a R-curve behaviour). This toughness enhancement with increasing ∆a is caused by plasticity, but also toughening mechanisms like crack bridging [6] and the formation of a FPZ containing localised yielding and micro cracking may contribute. In the region of larger crack extensions a decrease of K can be observed, which is dramatically in the case of the lower density foam. The “softening mechanisms” that are causing this drop in K (and also in the plastic limit load ratio F/FY) are not clear until now. Figure 2 Stress intensity factor K versus crack extension ∆a curve for two foams with a density of 0.25 g/cm3 (left image) and 0.40 g/cm3 (right image). CT-specimens with W=290 mm and B=30 mm were used. Fig. 3 shows the load - plastic limit load ratio versus load line displacement plots for the same two foams as depicted in Fig. 2. Also in the load - plastic limit load ratio F/FY (load divided by the corresponding plastic limit load) curves a significant drop in the region of larger crack extensions (or larger load line displacements) can be observed. Especially the lower density foam (0.25 g/cm3) reveals a dramatically drop in the plastic limit load ratio. It seems that both foams do not reach the stage of full plastification (F/FY=1). But due to the very small linear elastic regime of these foams it is difficult to measure the yield stress σy. So
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