Issue 50

V. Kytopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 414-422; DOI: 10.3221/IGF-ESIS.50.35 419 0 100 200 300 400 ρ z [μm] 3 2 1 0 I [sec -1 ] Ĩ [sec -1 ] Figure 3 : Integral area principle for calculation of damage or integrity loss number q d . To further enlighten this fact a new parameter, the specific damage number δ=q d /ρ z , is introduced, describing the intensity of damaging processes. Thereafter it seems that the 2124 MMC with higher matrix ductility (Table 1), responds to loading with a larger specific damage number than the 8090 MMC, which has lower matrix ductility. A reasonable explanation for this interesting behavior could be given as follows: The more ductile metal matrix is associated with a higher strain hardening rate-induced dislocation motion activity [26]. This means that in this matrix (compared to less ductile ones) larger dislocation piling-up accumulations at metal matrix-SiC particle interface sites may occur at early stages of loading. The high level of dislocation accumulation, in turn, produces strong stress concentration fields at these sites, promoting premature and in- tensive interfacial degradation activity, reflected by rapid lowering of the interfacial strength [27]. This implies, in turn, strong matrix-particle debonding-decohesion and micro-voids formation effects, which on further loading induce linkage of de- bonded particles followed by rapid crack initiation and propagation and final fracture. In this case, the related elastic-plastic damage processes are associated with energy absorption, by which a significant portion of the total micro-fracture energy flows into the crack-tip region dissipated within a “restricted” and characteristic damage process zone-volume, screened from the outer undamaged region, leading to intensive micro-failure events reflected by a large specific damage number. Ιn Figs.4(a,b) the damage controlled integrity change distribution ahead of notch root in 2124 MMC and in 8090 MMC, respectively for a higher strain rate is presented. One can observe the dramatic reduction of the fracture process zone compared to the lower stain rate (see Figs.2(a,b)). In this context, in Table 2 one can observe the increased specific damage number for both MMC materials with increasing strain rate of deformation. From the above findings the obvious em- brittling influence of strain rate on the damage process can be deduced. This may be due to the dominating quasi-adiabatic process where high deformation rates limit the dissipation time required for the absorbed micro-failure energy to form new surfaces. Hence the corresponding effective-specific damage process volume is reduced, a fact that is experimentally observed by the reduction of the measured process zone length ρ z . Moreover the high deformation rate seems to exhibit a lower embrittling influence on 8090 MMC. This is reasonable, since at high deformation rates the dislocation motion activity in the less ductile matrix (compared to the more ductile one) is subjected to a larger fractional reduction, a fact that (as was explained earlier) should induce corresponding micro-damage activity with reduced intensity. 0 10 20 30 ρ z [μm] 1.5 1.0 0.5 0 I [sec -1 ] Ĩ [sec -1 ] 1.8 1.2 0.6 0 I [sec -1 ] 0 5 10 15 ρ z [μm] Ĩ [sec -1 ] (a) (b) Figure 4 : Reduced structural integrity change distribution ahead of notch root for material (a) 2124 MMC and (b) 8090 MMC. Note that the strain rate for both cases was equal to 10 -2 sec -1 .