Effects of stress ratio on fatigue crack growth of thermoset epoxy resin C. Kanchanomai * , A. Thammaruechuc Department of Mechanical Engineering, Faculty of Engineering, Thammasat University, Klong-Luang, Pathumthani 12120, Thailand article info Article history: Received 8 April 2009 Received in revised form 10 June 2009 Accepted 12 June 2009 Available online 23 June 2009 Keywords: Epoxy resin Fatigue crack growth Cyclic dependence Time dependence Creep effect abstract The influences of stress ratio (R) on fatigue crack growth (FCG) of thermoset epoxy resin with polyamine hardener were investigated. The FCG growth rates (da/dN) have been correlated by the linear-elastic fracture mechanics parameters (DK and K max ), and nonlinear-elastic fracture mechanics parameter (DJ). The effects of R on FCG were observed when the DK and DJ were used as fracture mechanics parameters for FCG. However, the K max successfully characterized FCG under cyclic-dependent condition (FCGs at R ¼ 0.1 and 0.4); but it failed to characterize the FCG under time-dependent condition (FCG at R ¼ 0.7). As a time-dependent fracture mechanics parameter, C* was applied to correlate the time-dependent FCG rate (da/dt). A reasonable agreement was obtained between time-dependent FCG (R ¼ 0.7) and creep crack growth (CCG) results. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Epoxy resin generally has low shrinkage after curing, low mois- ture absorption, wide range of operating temperature (25 to 150  C); therefore it has been used as a matrix in various polymer- matrix composites [1–3]. During services, epoxy resins usually perform under cyclic loading at various stress amplitudes and mean stresses. For examples, sport equipment are subjected to low stress amplitudes and low mean stresses; while pressure vessels are sub- jected to high stress amplitudes and high mean stresses. As a process of defect accumulation, crack initiation, and crack propagation with number of load cycles, the fatigue cracks may initiate and propagate from the discontinuities within a part, i.e. defects during production, cracks during service, or complex geometry of product. The fatigue crack propagation rate depends on various factors, e.g. crack length, stress amplitude, mean stress, frequency, temperature [4]. As a relationship between fracture mechanics parameter and fatigue crack growth rate (da/dN), the fatigue crack growth (FCG) curve could be used in the life prediction of engineering parts under cyclic loading. For linear-elastic materials, the stress intensity factor range (DK) could be used to characterize FCG rate; while the range of strain energy released rate (DJ) could be used for nonlinear-elastic materials [4]. Since room temperature corresponds to a high homologous temperature for epoxy resin; the time-dependent deformation or creep could become a significant factor in determining the fatigue life. For time- dependent FCG behavior (creep–fatigue interaction), the DK and DJ are unlikely the appropriate fracture mechanics parameters to crack growth behavior. On the other hand, the time-dependent fracture mechanics parameter (C*), i.e. a path independent energy rate line integral [5–7], could be used for describing the crack growth rate (da/dt) in both creep-fatigue regime, and creep regime. Unfortunately, the majority of the FCG studies of polymers have used the linear-elastic fracture mechanics parameters (DK and K max ) to characterize FCG rates. Ramsteiner and Armbrust [8] studied the FCG of ABS (acrylonitrile-butadiene-styrene) and PMMA (polymethyl methacrylate) using compact-tension (CT) specimens and single edge notch bending (SENB) specimens under stress ratio (R ¼ K min /K max ) of 0.1. The FCG of both polymers could be characterized by DK, and have not influenced by the shape of specimen (CT and SENB) and frequency (1 and 10 Hz). Sadananda and Vasudevan [9] studied FCG of PMMA and polycast PMMA under various R-ratios. They found that crack growth became increasingly dominated by creep process with increasing K max . In contrast, polycast material showed more fatigue damage process (or less of creep-related process) because of its high density of cross-linked chains. Fang et al. [10] studied the FCG behavior of polycarbonate (PC) and acrylonitrile-butadiene-styrene (ABS). They found that the DK  da/dN from low to high crack growth rate could be described with Paris law. Coarse features of porous and dimples have been seen on the crack surfaces with higher crack growth rate; while * Corresponding author. Tel.: þ66 2 564 3001; fax: þ66 2 564 3010. E-mail address: kchao@engr.tu.ac.th (C. Kanchanomai). Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab 0141-3910/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2009.06.012 Polymer Degradation and Stability 94 (2009) 1772–1778