Fatigue life prediction via the time‐dependent evolution of linear and nonlinear mechanical parameters determined via Fourier transform of the stress

Abstract

ABSTRACTThis study proposes a new concept for the lifetime prediction of solid polymers under mechanical fatigue. The time evolution of linear and nonlinear mechanical parameters was studied using polystyrene (PS), polymethylmethacrylate (PMMA), and styrene‐acrylonitrile (SAN) samples in strain controlled (large amplitude) oscillatory shear (torsion). Higher harmonics of the stress were detected and quantified via Fourier transform (FT). Besides storage and loss modulus (G′, G″), the ratio of the third (I3) harmonic over the fundamental one, I1 (I3/1), was analyzed. After an initial transient period (regime I), the three parameters (G′, G″, and I3/1) linearly de/increase with the cycle number (N) (regime II) before failure onset (regime III) occurs. In regime II, the absolute values of the rates of change (derivative) of G′, G″, and I3/1 (dG′/dN, dG″/dN, and dI3/1/dN) were correlated with the lifetime (Nf). An inverse proportionality between the rate of change and lifetime was found allowing better prediction while testing. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46634.