Issue 50

V. Kytopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 414-422; DOI: 10.3221/IGF-ESIS.50.35 420 Material and strain rate Damage number q d =A d /A 0 Specific damage number δ= q d / ρ z MMC 8090 (Strain rate 10 -5 /sec) 0.27 9 x 10 -4 MMC 8090 (Strain rate 10 -2 /sec) 0.37 3.1 x 10 -2 MMC 2124 (Strain rate 10 -2 /sec) 0.58 3.8 x 10 -2 MMC 2124 (Strain rate 10 -5 /sec) 0.33 5.5 x 10 -3 Table 2 : Basic, final results of micro-damage evaluation measurements of the investigated materials, before thermal shock. Furthermore, in an isotropic body subjected to a non-uniform temperature distribution, the elements attempt to undergo dilation (or shrinkage) as a result of the temperature changes from an initially uniform temperature. However, the elements cannot dilate (or shrink) in an unrestricted manner. Since the body must remain continuous during the temperature change there will be partial constraint internally, even if the body externally is unconstrained, as to change in geometry. One can then introduce in this way a general stress field in the body, with a parameter called thermal stress [28]. High- speed aircraft and space vehicles are subjected to considerable thermal stress from aerodynamic or solar heating on the outside surfaces and from the heat originating in propulsion system. Furthermore, even without external constraint, thermal shock can occur due to the steep temperature gradients created because of a finite thermal conductivity. Again, such a thermal shock is the stress state resultant of combined thermal conductivity and steep temperature gradient effect. For example, rapid cooling of the surface of a high-temperature wall is accompanied by surface tensile stresses. The surface contracts more than the interior, which is still relatively hot. As a result, the surface “pulls” the interior, into compression and “pulls” itself into tension. With the inevitable presence of Griffith flaws at the surface, this surface tensile stress creates the clear potential for brittle fracture. The ability of a material to withstand a given temperature change depends on a complex combination of thermal expansion, thermal conductivity, overall geometry, and the inherent brittleness of the material. The presence of Griffith flaws in solids in form of cracks or crack-like defects are usually classified as micro- scopic, meso-scopic and macroscopic according to crack length [5]. Microscopic cracks in the sub-micrometer range often exist as a result of the material production processes. On thermal loading of materials, surface cracks propagate and additional cracks can be generated, which may finally lead to fracture. In other words, with thermal shock, stresses are caused by heterogeneous thermal expansion of the loaded material, which may be a result of both a heterogeneous material structure and a heterogeneous (local) energy load. Moreover, the above described thermal cracking damage, is very critical in many primary and secondary manufacturing processes of materials such as casting, forging and welding where due to the improper related parameter such a damage can develop [29]. In Fig.5 the measured X-Ray intensity before tensile loading ahead of edge notch for the heat treated specimens by sudden cooling in liquid hydrogen at -196 o C temperature is presented. Thus sudden cooling in this case produces the so- called thermal shock by which, due to high thermal stresses, the fracture of material may occur. The produced thermal stresses are proportional to ΔαΔΤ where ΔΤ is the temperature difference between cooling agent (- 196 o C ) and specimen (20 o C) and Δα the thermal expansion coefficient of the MMC material. In this case two dominant parallel processes may occur: a fracture mechanics - controlled macroscopic thermo-elastic stress concentrations around a cut [30] and a damage mechanics - controlled microscopic one around inclusions (SiC particle) [5] .The microscopic process is characterized by highly localized thermal shock stresses produced between SiC particle and matrix where due to large differences between thermal expansion coefficients, Δα, of particle and matrix these stresses may lead to intensive and rapid mechanical degra- dation of interphase microstructure. The combined effect of these two thermal stress concentration processes result in the interfacial damage - controlled structural integrity change shown in Fig.5. One can see in this figure that MMC-2124 material shows a distinct higher thermal damage response than the other one. Thus, one can argue that the composite material with large differences between the components (SiC and matrix) con- cerning the tensile moduli and ductility, are more prone to thermal shock damage. In Fig.6 the increased damage response is presented for the same specimens as in Fig.5 after tensile loading to ultimate stress. This was expected since damage accumulated by thermal shock, on tensile loading, acts as an initial – present damage in the material. The basic data for the specimens after thermal shock treating are given in Table 3. One can deduce that the initial presence of damage in the material, after thermal shock, exhibits a considerable influence on the subsequently tensile load – induced damage. In this aspect one can at first observe the increased proneness to thermal shock of the MMC2124 material compared to the other one. Moreover, the combined effect of tensile load and thermal shock induced damage is larger for the MMC2124 material . This can be seen first by the large shift of both curves to lower values of measured count accumulation intensity and