EXPERIMENTAL STUDY OF MULLINS EFFECT IN NATURAL RUBBER FOR DIFFERENT STRETCH CONDITIONS Elli Gkouti 1* , Burak Yenigun 1 , Krystof Jankowski 2 , Aleksander Czekanski 1** 1 Department of Mechanical Engineering, Lassonde School of Engineering, York University, Toronto, ON, Canada 2 Magna Closures of America Inc., Troy, MI, USA *gkoutiel@yorku.ca, **alex.czekanski@lassonde.yorku.ca ABSTRACT We subjected rubber coupons to cyclical uniaxial tension to investigate the softening effect, where the primary loading at its initial position was followed by additional unloading and reloading. Less stress was required upon reloading than that required in the previous loading for the same degree of stretch, reached on the first loading. This stress softening is significant when reloading follows virgin loading. The magnitude of stress softening is related to the maximum stretch elastomers can achieve in each cycle. To investigate this phenomenon, rubber coupons were subjected to four cycles of simple tension until the desired stretch was reached. We expected that several tests under the same conditions would provide almost identical results. However, we observed different stress requirements for different degrees of stretch when multiple cycles of the same stretch were performed. For three different experimental tests of the same amount of stretch, we saw huge differences in each cycle of loading–relaxation–reloading, a phenomenon that was more obvious during stress relaxation. Keywords: elastomers; natural rubber; Mullins effect; stress softening; stress relaxation 1. INTRODUCTION For over 60 years, the phenomenon of stress softening has remained a high priority for engineers intent on understanding and thus predicting and simulating elastomer mechanical behaviour. Stress softening occurs when elastomers subjected to repeated extension exhibit a significant change in their mechanical behaviour. This change is more obvious upon unloading that follows virgin loading. Though Bouasse and Carrière were the first to observe this phenomenon [1], Mullins expanded upon this work [2]. He concluded that this phenomenon affects filled natural rubbers only, as their softening is excited by their increasing stiffening ability. Later, Mullins realized that pure natural rubbers also experience stress softening and coined it the “Mullins effect” [3,4]. From that point forward, this phenomenon has stymied engineers; without a thorough understanding of softening, the mechanical behaviour of rubber cannot be predicted or simulated. The most common approach to describing the Mullins effect concerns damage continuum mechanics. Specifically, the material is treated as hyperelastic, and the strain energy density function is modified to include a damage parameter. Based on this idea, Ogden and Roxburgh [5] introduced a pseudo-elastic model to predict the stress softening of rubber parts. In the present work, we investigated the softening of rubber coupons in several repeated tests of uniaxial tension, where the magnitude of softening is related to the maximum stretch value reached in every cycle. After a few cycles—six cycles according to Dorfmann and Ogden [6]—the required stress for loading– reloading converges to almost the same value. Comparing the results between several stretch values, it becomes obvious that the magnitude of softening is proportional to the maximum stretch reached in every cycle. We hypothesize that for the same stretch values and conditions, the results of the softening effect would be the same. 2. MATERIALS AND METHODS 2.1 Softening We investigated the stress softening of natural rubbers when uniaxial tension was applied during experimental testing. To capture this effect, two different procedures were selected. First, the rubber material was subjected to continuous loading– unloading cycles of uniaxial tension. For every loading, the desired stretch was the same, and the subsequent unloading was programmed to create a residual amount of the maximum stretch reached during loading. This procedure was repeated in order to DETC2020-22565 Copyright © 2020 ASME Proceedings of the ASME 2020 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference IDETC/CIE2020 August 17-19, 2020, Virtual, Online V002T02A043-1 Downloaded from http://asmedigitalcollection.asme.org/IDETC-CIE/proceedings-pdf/IDETC-CIE2020/83914/V002T02A043/6586127/v002t02a043-detc2020-22565.pdf by York Univ user on 04 November 2020