Natural fiber composites are widely used in many non-structural applications. Expanding natural fiber composites into structural applications in industry requires further evaluation of their safety and reliability. For example little is known on the hygrothermal and fatigue behavior of natural fiber composites which are important factors contributing to deterioration of composites and their malfunction.
Researchers led by Professor Lotfi Toubal from University of Quebec in Canada investigated the tensile and fatigue behavior of quasi-isotropic laminated flax/epoxy composites after a long-term hygrothermal aging. They also used Hwang–Han’s model to predict the fatigue damage of the tested flax/epoxy composites. The research work is now published in journal of Materials Science.
The authors assembled combinations of [02/902/±45]S quasi-isotropic laminate configurations by hand lay-up. Hwang-Han’s model based on minimal strain was incorporated to determine the damage evolution of the laminated flax/epoxy composites samples, during and after hygrothermal aging coupled with the usage of principal component analysis to discover the factors behind the damage evolution of all samples. In addition, the study validated that the principal component analysis can be used to show the difference in the evolution of damage between specimens that at first glance had the same behavior.
When the water absorption test was conducted it showed that the selected samples absorbed water quickly, with a reduction in Young’s Modulus by 60% after two days. An increase in failure strain was also noticed, highlighting the effects of hygrothermal aging on laminated flax/epoxy composites samples.
The samples which possessed high cycle fatigue strength of more than 50% of ultimate tensile strength reduced to 40% with a lower fatigue life response as a result of hygrothermal aging. It was also discovered that the dissipation energy during the damage evolution depended largely on level of loading and no hysteretic heating effect was detected.
When the authors used Hwang-Han’s modeling they showed similar damage evolution for samples before and after hygrothermally aged quasi-isotropic laminated flax/epoxy composites due to the trivial increase in evolution damage of samples after hygrothermal aging.
Acoustic emission analysis indicated an extra cumulative energy growth in the course of the fatigue life of the samples after hygrothermal aging. Higher acoustic amplitude was noticed in the range of 50-60 dB when a loading level of 0.7 and 0.8 of ultimate tensile strength was applied. This was attributed to the fiber-matrix debonding, longitidunal cracking and fiber pullout as the main damage mechanism during the hygrothermal aging of the quasi-isotropic laminated flax/epoxy composites. Figure 1 shows the presence of new damage mechanisms for aged sample. This also demonstrates the capacity of AE to distinguish damage mechanisms better during fatigue tests. These outcomes, which were backed with scanning electron microscopy indicates the potency of the acoustic emission analysis in determining the damage evolution of the samples after hygrothermal aging.
This study showed that long-term hygrothermal effects on natural fiber composites need to be considered due to their increased use in industrial applications. Moreover it provided important understanding of hygrothermal effects on natural fiber composites in relation to the fatigue life of laminated flax/epoxy bio-composites.
Sodoke, F.K., Toubal, L., Laperriére, L. Hygrothermal Effects on Fatigue Behavior of Quasi-isotropic Flax/Epoxy Composites using Principal Component Analysis, Journal of Materials Science 51 (2016) 10793-10805.
Laboratory of Mechanics and Eco-Materials, Mechanical Engineering Department,University of Quebec at Trois-Rivières, Trois-Rivières, Canada.Go To Journal of Materials Science