Paper Number
SG17 

Session
Solids, Composites & Granular Materials

Title
Fatigue fingerprints via Fourier transform of the stress

Presentation Date and Time
October 18, 2018 (Thursday) 10:25

Track / Room
Track 4 / Post Oak

Authors (Click on name to view author profile)

1. Hirschberg, Valerian (Université Laval, Department of Chemical Engineering and CERMA)
2. Rodrigue, Denis (Université Laval, Department of Chemical Engineering and CERMA)
3. Wilhelm, Manfred (Karlsruhe Institute of Technology, Institute for Chemical Technology and Polymer Chemistry)

Author and Affiliation Lines (in printed abstract book)
Valerian Hirschberg¹, Denis Rodrigue¹, and Manfred Wilhelm^2
1Department of Chemical Engineering and CERMA, Université Laval, Quebec City, Quebec G1V 0A6, Canada; ²Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, Karlsruhe 76128, Germany

Speaker / Presenter 
Hirschberg, Valerian

Text of Abstract
In this work, the stress response of mechanical fatigue on solid polystyrene (PS), polymethylmethacrylate (PMMA) and styrene-acrylonitrile (SAN) samples in oscillatory shear tests was analyzed via Fourier transform (FT) to determine fingerprints of continuous fatigue. The tests were performed at room temperature using a torsion rectangular geometry (notched rectangular samples) and filmed by a video camera to visualize changes in the samples such as crack initiation or propagation. Large strain amplitudes were applied so the stress response was nonlinear and higher harmonics detectable in the FT spectra. The analysis includes the time evolution of the stress response via linear (storage (G’) and loss (G”) moduli) and nonlinear (higher harmonics) parameters, as well as their time derivatives. These parameters were used to better understand and further analyze the fatigue behavior of a material and to detect/follow specific events such as crack initiation and propagation leading to fatigue lifetime prediction. The results showed that during a test, the linear parameters (G’ and G’’) decreased monotonically, while the I_3/1 intensity (relative amplitude of the third harmonic to the fundamental one) increased steadily until failure. These three parameters were found to change linearly with time (number of cycles) over a specific time frame, shortly after the beginning of the test. The fatigue lifetime was found to follow a power-law function of the (time dependent) rates of change (slopes) of G’, G’’ and I_3/1 in this regime. For undamaged samples, the nonlinear parameter I_2/1 (relative amplitude of the second harmonic to the fundamental one) is within the noise level, but its intensity increased when (visible) defects are created (macroscopic cracks). The time evolution of G’, G’’, I_3/1 and I_2/1 are proposed as new criteria to predict failure and detect the onset of macroscopic cracks under the conditions tested, to better determine safety limits (partial damage).