Issue 46

Z. Lu et alii, Frattura ed Integrità Strutturale, 46 (2018) 150-157; DOI: 10.3221/IGF-ESIS.46.15 150 Developments in the fracture and fatigue assessment of materials and structures Fatigue behaviour and mean stress effect of thermoplastic polymers and composites Zongjin Lu, Bill Feng, Charlie Loh Jaguar Land Rover Limited, UK zlu5@jaguarlandrover.com , bfeng1@jaguarlandrover.com , cloh1@jaguar.com A BSTRACT . More and more polymers and polymer composite materials are used in automotive industry to reduce cost and weight of vehicles to meet the environmental requirement. However, the fatigue behaviour for these materials is less understanding than metallic materials. The current work is focussed on the fatigue behaviour for a range of thermoplastic polymer/composite materials. It reveals that the fatigue behaviour of these materials can be described by S-N curves using the Basquin Equation. All the materials exhibit significant mean stress effect. The most commonly used mean stress correction equations developed in metal fatigue were evaluated with the current test results. It reveals that Goodman, Gerber and Soderberg cannot be used as generic equations for the materials investigated, whereas Smith-Watson-Topper can correlate the test data reasonably well, but the best correlation is given by Walker with material constant γ = 0.4. K EYWORDS . Fatigue; Mean stress effect; Polymers; Polymer composites. Citation: Lu, Z., Feng, B., Loh, C., Fatigue behaviour and mean stress effect of thermoplastic polymers and composites, Frattura ed Integrità Strutturale, 46 (2018) 150-157. Received: 25.01.2018 Accepted: 22.04.2018 Published: 01.10.2018 Copyright: © 2018 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION ore and more thermoplastic polymers and polymer composite materials are used in automotive industry to reduce cost and weight of vehicles to meet the environmental requirement. However, the fatigue behaviour for the polymers and polymer composite materials is less understood than metallic materials, especially the effect of mean stress. It is well known that fatigue life of metals is influenced by mean stress significantly [1]. Various equations have been established to account for the mean stress effect. Among these, Goodman [2], Gerber [3], Soderberg [4], Smith- Watson-Topper (SWT) [5] and Walker [6] are the most commonly used. They are listed below: Goodman 1 a m ao u S S S S   (1) Gerber 2 1 a m ao u S S S S         (2) M