Issue 50

V. Kytopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 414-422; DOI: 10.3221/IGF-ESIS.50.35 418 the pre-heated iron molds. The matrix was reinforced with 20 % volume fraction SiC particles of radius about 3μm. The composites were prepared by BP-International by mixing and blending of Al matrix alloy and SiC powders followed by canning, vacuum de-gassing and consolidation and then hot rolling to plate and sheet. The basic mechanical properties of these two materials are given in Table 1. The specimens were dogbane-shaped in ac- cordance with the requirements of the grips of the loading device (of load capacity equal to 2.5 kN) specially designed for in-situ observations by SEM. The edge- notch was machined very carefully by means of a rotating diamond disc-slow cutting apparatus (in order to avoid any parasitic damage). Type of MMC-material AL-Li 8090 Al-Cu 2124 Constituents Alloy MMC Change (%) Alloy MMC Change (%) Elastic Modulus (GPa) 92.5 114.0 23 80.0 109.9 37 Tensile Strength (MPa) 428 575 34 435 622 43 Ductility (%) 4.5 4.0 -11 6.7 4.9 -27 Table 1 : Mechanical properties of the materials tested. R ESULTS AND DISCUSSION n Figs.2(a,b) the data obtained from the measurements of micro-damage controlled micro-structural integrity dis- tribution ahead of the notch root for a 2124 MMC- and a 8090 MMC-specimen, respectively are presented. Taking into consideration the discussion concerning Eqs.(5), one can clearly observe that this distribution, measured by the specific-count accumulation rate Ĩ shows a distinct decrease towards notch root. Also this signal tends to equilibrate at certain distance ρ z away of notch root. At this distance the damage has a vanishing limit value. The parameter ρ z , called fracture process zone, is an important factor in the experimental fracture mechanics of materials [7,19,25]. The fracture process or cohesive zone is a small characteristic region surrounding the crack tip or notch where micro-fracture develops mainly through the successive stages of inhomogeneous void growth and coalescence and bound breaking on atomic scale. In this context, the increased size, of about 5 times, of the process zone of the 8090 MMC, compared to the 2124 MMC specimen is to be properly highlighted. 3 2 1 0 I [sec -1 ] 0 100 200 300 400 ρ z [μm] 2.4 1.6 0.8 0 I [sec -1 ] 0 30 60 90 ρ z [μm] Ĩ [sec -1 ] Ĩ [sec -1 ] (a) (b) Figure 2 : 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 -5 sec -1 . As shown in Fig.3 the proposed damage degree number q d =A d /A 0 was determined, where A 0 is the total sampled area and A d is the damage-controlled integrity loss area. In Table 2 one can see the larger damage degree number obtained for the 2124 MMC material compared to the 8090 MMC, for the same strain rate conditions (of 10 -5 /s). Comparing now the two parameters ρ z and q d of these materials one can deduce that a larger damage degree or integrity loss is developed in 2124 MMC “along” a smaller fracture process zone. I