PSI - Issue 28

A.D. Evstifeev et al. / Procedia Structural Integrity 28 (2020) 2261–2266 / Structural Integrity Procedia 00 (2019) 000–000

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estimate dynamic strength materials properties. It should be also noted the whole complication of dynamic fracture process, first of all a strong strain-rate sensitivity of stress level at the fracture moment. For example, a material response to high-rate loading is drastically distinct to its behavior under quasistatic regimes (see Bragov and Lomunov, 1989; Kanel et al., 1996; Gruzdkov et al., 2003, 2009; Garkushin et al., 2010; Bragov et al., 2012). Nowadays a large number of various tests is provided in order to evaluate the strength of structural materials. The additional difficulty to the data analysis and the accuracy estimation is a choice of proper fracture criterion and material model. The developing method of experimental measuring is based on the structure-temporal approach to predict critical fracture condition under arbitrary dynamic load (Petrov and Utkin, 1989; Petrov and Morozov, 1994). An advantage of this approach is that considered in it fracture criterion contains only two parameters, they are the well-defined critical stress � characterizing fracture condition in statics and the incubation time responsible to material dynamic strength. The previous researches demonstrated high applicability of the incubation time approach to various problems of mechanics and physics (Petrov, 2004; Petrov et al, 2010). The value of � is determined as result of direct observation whereas the incubation time value is evaluated implicitly according to relatively new Sign-Perturbed Sums (SPS) method (Csaji et al., 2015). This randomized procedure evaluates a system parameter as some confident interval which contains its true value with a given probability. The enhancement of incubation time approach by implementation of SPS-algorithm alloys to offer a new methodology of material testing. The number of impact and quasistatic tests on Al-Mg specimens were performed in order to approve developing experimental technique and to study influence of magnesium percentage to strength properties of alloys. 2. Experimental study The quasistatic test were provided on a standard pull-test machine Shimadzu AG-50kNX. The drop tower equipment with additional accelerating system Instron CEAST 9350 was applied to dynamic tensile tests. All specimens had a similar flat shape with a gauge length 5 and width 2 . The same special holders for the drop tower and the pull-test machine were designed in order to decrease measuring accuracy between different experimental schemes. A dynamic tensile study of such small samples and the validity of this experimental scheme were considered in (Evstifeev et al., 2019). The dependence of ultimate stress at the fracture moment on corresponding stress rate of load was chosen as a main characteristic of material dynamic strength. Fig. 1 shows obtained experimental results.

Fig.1. Ultimate stress dependency on stress rate for various Al-Mg alloys AMg2, AMg3, AMg4.5, AMg5, AMg6. It is very difficult to conclude by analyzing present scattering whether one material has better dynamic strength properties or not, for example, AMg3 and AMg4.5 alloys. Moreover, it is impossible to estimate an accuracy of these results and an acceptable deviation of strength parameters. Data points that are suspected to be result of observations with extremely large random noise are circled by red color. But it should be noted that is only a probable choice based on the experience and intuition of authors and of course it cannot be recommended in determined methodological