PSI - Issue 30

A.A. Vasilieva et al. / Procedia Structural Integrity 30 (2020) 186–192

187

Vasilieva A.A. et al. / Structural Integrity Procedia 00 (2020) 000–000

2

1. Introduction Polymer composite materials (PCM) belong to a new generation of materials. These materials can reduce weight and in initial state have high physical and mechanical properties compared to metallic materials. Among a wide variety of PCMs the materials whose strength properties during the 25 – 30 year period of operation are reduced by no more than 10 - 20% are of particular interest. Now, a rigorous scientific basis that produces reliable prediction of the physical and mechanical characteristics of PCM for the long-term operation, in world science does not yet exist. One of a generally recognized world practice of justifying the conditions for safe operation of PCM is the conducting of "climate qualification testing". Specifically, Filatov (1962); Chersky (1986); Startsev (1994); Kablov et al. (2010) and others have understood that prolonged exposure to open climatic conditions causes the aging of PCM - a complex of physicochemical and structural processes that occur in materials under the influence of the external environment. According to the works, in which the initial stage of PCM aging was investigated, the use of widespread deformation-strength measurement methods is not effective. For this purpose, indicators that have sensitivity to physicochemical structural transformations in the material are promising. One of such indicators is the coefficient of moisture diffusion, which characterizes the kinetics of “sorption-desorption” in PCM (Startsev et al. (2002)). On the other hand, the moisture damage phenomenon in such rebar is interested itself, since affects to aging degradation of PCMs after the temperature factor that is studied by Kirillov et al. (2010), Startsev et al. (2004). Than Startseva et al. (2014) found that in various climatic conditions PCM sorbs moisture from 0.5 to 2.5%. Recently, continuous fibers extruded from naturally fire-resistant basalt have been investigated as a replacement for asbestos fibers, in almost all of its applications. Therefore, basalt has emerged as a contender in the fiber reinforcement of composites and may offer manufacturers a less-expensive alternative to carbon fiber for products in which the latter represents over-engineering. Fick’s model is the most often used with fiber reinforced plastics, as shown by Crank (1975). Also models with the larger number of parameters are used: a model taking into account the two-phase state of absorbed moisture in the material (Langmuir model); a model taking into account the two-phase nature of the material (Jacob’s–Jones model); a model with a time-variable diffusivity; relaxation model; and convection model. These models give the better description of experimental data, that is, smaller values of absolute deviation. However, as the choice of parameters is ambiguous, the models with a larger number of parameters cannot be successfully used to predict long-term sorption curves. However, the problem of determining the diffusion characteristics of heterogeneous anisotropic materials is not a trivial one. There are two main approaches to evaluation of sorption characteristics in FRP: (i) structural, which is based on determining the sorption characteristics of the composite by the properties of its constituents, as found by Aniskevich (1986), Aniskevich and Yanson (1991), Aniskevich and Jansons (1998), Starkova and Aniskevich (2004), and (ii) direct experiment. Moreover, despite a large variety of existing models for determining the diffusivities of FRP (different number and composition of included parameters), the results obtained using different expressions normally differ less than 20%. As experimental data showed, a reasonable agreement between the experiment and calculated values of the diffusivities cannot be obtained in many cases. Thus, experimental evaluation of the diffusion coefficients is often the only way to obtain reliable data, as pointed by Aniskevich and Glaskova-Kuzmina (2019).

Nomenclature c

moisture concentration

t

exposure time

D R M

diffusion coefficient radius of specimen

total moisture uptake after time t

moisture saturation level, equilibrium moisture uptake

eq M