Model of fatigue damage in strain-rate-sensitive composite materials
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Tinh Nguyen Materials and Construction Research Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Tze-jer Chuang Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 7 June 2002; accepted 10 October 2002)
A fatigue damage accumulation model based on the Paris law is proposed for strain-rate-sensitive polymer composite materials. A pre-exponent factor c2/f and strain-rate-sensitive exponent n are introduced. Numerical analysis of the model was performed using experimental data obtained in the literature. Both factors were found to enhance fatigue damage accumulation. The analysis also revealed that the extent of damage increases with decreasing frequency and that the damage rate is more sensitive to the applied maximum stress than to the stiffness of the material.
I. INTRODUCTION
Advanced polymeric composites have increasingly gained applications as structural materials. Development of advanced methodologies for assessing the safety and reliability of these complex products has become urgent. Fatigue reliability under severe environments and extreme load is one of the most challenging issues facing the use of these materials for structural applications. Fatigue damage mechanisms in polymer composites have been studied extensively.1–10 The evolution of damage in composite materials consists of matrix cracking, interface debonding, delamination, splitting, and fiber breaking. Until now, the effect of various parameters on the stage of damage process has not been thoroughly investigated. For example, Ye divided the damage process into two stages.8 In the first stage, damage initially increases rapidly and then gradually slows down to reach a plateau. In the second stage, the damage increases rapidly until fracture. The duration of the second stage is very short, normally occupying about 20% of the total lifetime of the composite materials. Talreja also considered the damage degradation as a two-stage process separated by the characteristic damage state in which a stable crack pattern develops.2 However, he suggested that the first stage is generally 80% of the total lifetime of the composites.
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II. DAMAGE ACCUMULATION MODEL
Due to the complexity of fatigue mechanisms, it is very difficult to characterize micro-damage in polymer composites in terms of deformation and stress state. To tackle this problem, it is customary to introduce a damage variable to characterize the fatigue damage state. The
e-mail: [email protected] J. Mater. Res., Vol. 18, No. 1, Jan 2003
http://journals.cambridge.org
Because the fatigue behavior of advanced polymeric composites involves a very complicated process, it is very difficult, if not impossible, to thoroughly characterize the fatigue failure mechanisms of these materials simply based on stress analysis alone. However, several studies have used the Paris law, which can describe the fatigue of metallic alloys very well, to analyze fatigue behavior of polymer composites. For example, Wn
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