Technological watch

Abstract A technique based on the tensile testing is proposed for a quantitative characterization of materials with a wide strain range and nonlinear stress and strain dependencies which can be represented as two sequential processes (stages) described in Hollomon power-like approximation. The distinct and unambiguous criterion is developed to determine the transition between these processes associated with prevailing deformation events (process I) or following it process II with dominating failure events. The method is tested for ethylene vinyl acetate (EVA) copolymer used widely as the encapsulant material for photovoltaic (PV) modules. The values of activation energies and structural factors (as defined by Zhurkov equation relating the creep time to the tensile stress applied) were derived for each of the stages and two sets of the EVA samples with various levels of the cross-linking. The activation energies are determined to differ slightly for each stage regardless the sample treatment, whereas the structural factors have specific values indicating a higher stress concentration level for the initial stage as compared to that for the final process II. The research results are in the appropriate consistency with the existing concepts available for the polymer failure and with the data of the complementary techniques applied in this research: (1) broadband proton nuclear magnetic resonance (1H NMR); (2) attenuated reflection Fourier-transform infrared spectroscopy (ATR-FTIR); (3) fractographic and optical microscopy. The approach allows one to get the parameters applicable for prediction of a material behavior under the creep and aging.

Graphical Abstract