PSI - Issue 20

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M.P. Lebedev et al. / Procedia Structural Integrity 20 (2019) 81–86 M.P. Lebedev et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Testing of PM and PCM under natural climatic conditions can improve the accuracy of forecasts if the test duration exceeds 10-15 years or more as proved by Kablov (2013). Climatic tests of PCM lasting 1-3 years in some cases do not solve the problem of assessing their climatic resistance. During this period, a transient process occurs in which Startsev (2009) superimposes relaxation processes of the initial structural and chemical nonequilibrium on long-term physicochemical transformations in the material. In addition, efforts are required to identify this transition process. The purpose of this work is to discuss the methodological features of measurements of mechanical indicators PM and PCM to increase the reliability of assessments of their performance in the early stages of aging. 2. Heterogeneity of climatic PM aging Goykhman B.D (1980) bases a common approach to the study of climatic resistance of PM based on polyolefins, organic glass, polyamides, polyvinyl chloride, polycarbonate, epoxy compounds on the concept of damage accumulation and superposition under the influence of environmental factors. From the standpoint of chemical kinetics, the degree of damage accumulation in the PM is measured by the ratio of the number of broken bonds to the total number of bonds per unit of a cross-sectional area of the sample. Therefore, when measuring the mechanical strength σ, the destruction of a PM sample occurs at a load depe nding on the degree of damage accumulation. Algorithms have been developed for finding quantitative relationships between the magnitude of a macroscopic indicator and the concentrations of chemical and physical bonds that are formed during aging of PM. However, the results, generalized, for example, by White (2017) and Goykhman (1980), did not allow creating the basis for predicting the mechanical strength of PM even with a detailed identification of the mechanisms and kinetics of physicochemical transformations. In a recent paper by Startsev (2018), attention was drawn to the fact that during the climate impact a thin surface layer is damaged, which is a few percent of the total thickness of PM plates, equivalent to a conventional defect such as a common crack. When measuring strength indicators, the application of mechanical loads causes crack growth, accompanied by plastic or viscoelastic deformation in the zone of its top. One of the criteria for fracture during flat deformation of the sample is the ratio. From (1) it follows that Startsev (2018) should use the traditional concept of modeling climatic aging of PM based on the analysis of hydrolysis, oxidation, degradation, plasticization, crystallization, disorientation and other molecular processes founded by Goyhman (1980) in conjunction with the fundamental principles of linear fracture mechanics. Combining the principles of molecular transformation kinetics and linear fracture mechanics is promising for modeling and predicting the mechanical properties of PM during operation under climatic conditions. The urgent task of climatic testing of PM is an in-depth study of the relationship of physico-chemical transformations and characteristics of fracture toughness in a wide range of temperatures depending on the speeds and loading patterns, the shape and size of samples, the nature of damage in the surface layer, the thickness of the aged layer.Improving the reliability of measurements of mechanical indicators of PCM at the stages of climate exposure We considered the climate impact on the change in the generalized index - the relative coefficient of persistence kR=Rt/Ro, where Rt is strength limits and tensile m oduli (σt, Et), compression (σc, Ec), bend (σb, Eb), interlayer shear (τ, G), measured after different exposure times; R0 are baseline values of relevant indicators. According to Kablov (2018), fiberglass, carbon and other PCM after exposure in open climatic conditions for 1-3 years in most cases, change the values of kR within ±10%. This is confirmed by the analysis of the histograms of the distribution of the relative persistence coefficient of mechanical parameters kR in Fig. 1. 2 2 0 (1 ) IC u k EG nl (1)