1. Introduction Owing to their advantageous properties and low cost, polymers are used in a wide range and complex structural applications [1]. However, polymers can exhibit a complex nonlinear behavior due to external factors such as strain rate, temperature and hydro- static pressure (stress triaxiality). This behavior was the subject of various experimental studies [2–4]. High-density polyethylene (PE-HD) is one of the widely utilized polymers in a diversity of industrial applications due to its multiple advantages that it has over more conventional materials. In this regard, some studies on the fatigue characteristics of PE-HD have been published over the years [5–9]. According to those reports, molecular weight and the degree of crystallization or tie molecule concentration have been found to influence the fatigue characteristics of PE-HD. Additionally, the residual stresses iduced by the production process of the polymer pipe was found to have an effect on the service lifetime of PE-HD [10]. Moreover, it has also been reported that exter- nal factors such as the mean stress, frequency, shape and loading history can have a significant effect on the fatigue behavior of PE-HD. Zok and Shinozaki studied the effect of dilatational strain on the damage accumulation during fatigue of polypropylene [11]. They found that irreversibility on each cycle results in an accumulation of damage in the studied poly- mer, which would ultimately result in failure. How- ever, several studies on both, amorphous and semi- crystalline polymers indicated that cavities form during cyclic deformation and the volume fraction of these cavities increases with number of cycles [12–15]. Macroscopically, at room temperature, the fracture of semi-crystalline polymers has a form of ductile tearing caused by bridging between cavities 117 PE-HD fatigue damage accumulation under variable loading based on various damage models M. Bourchak 1* , A. Aid 2,3 1 Aeronautical Engineering Department, King Abdulaziz University, Jeddah, Saudi Arabia 2 Laboratoire LPQ3M, Université de Mascara, BP305 Mascara, Algeria 3 Laboratoire de Mécanique de Lille, Université de Lille 1, UMR CNRS 8107, 59650 Villeneuve d’Ascq, France Received 2 August 2016; accepted in revised form 30 September Abstract. Despite numerous studies on fatigue of polymer materials under variable loading, there is little work on high- density polyethylene (PE-HD). In this context, an experimental analysis for determining the fatigue strength of PE-100, under constant and variable amplitude loading is presented. Further, the cumulative fatigue damage behavior of PE-100 was experimentally investigated. First, the fatigue curve (S-N: stress vs. number of cycles) was obtained in order to establish the fatigue life of PE-100 subjected to constant stress amplitude. Secondly, Miner’s fatigue rule as well as stress-based and en- ergy-based fatigue damage models were used to estimate the cumulative variable amplitude fatigue damage. Comparison between predictions and experimental results showed different trends depending on the choice of prediction model used im- plying careful fatigue damage consideration when designing under variable amplitude loading. Keywords: mechanical properties, polyethylene, cumulative fatigue damage, viscoelastic behavior, block loading eXPRESS Polymer Letters Vol.11, No.2 (2017) 117–126 Available online at www.expresspolymlett.com https://doi.org/10.3144/expresspolymlett.2017.13 * Corresponding author, e-mail: mbourchak@kau.edu.sa © BME-PT