13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Fatigue delamination growth of composite laminates with fiber bridging: Theory and simulation Lei Peng1, Jifeng Xu1,* 1 Beijing Aeronautical Science and Technology Research Institute, Beijing 102211, China * Corresponding author: xujifeng@comac.cc Abstract Mode I fatigue delamination growth rates and thresholds of composite laminates have been experimentally studied. Due to the fiber bridging generated across fracture interface, additional fracture resistance was found rising with crack growth, which make the traditional Paris law and threshold model unsuitable. Therefore, novel models taking the normalized strain energy release rate as fracture governing parameter were developed. The delamination resistance during fatigue crack growth caused by interface fracture and fiber bridging was totally evaluated by a new parameter namely fatigue delamination resistance Gcf. Excellent agreement with the experimental data was achieved by the normalized fatigue delamination growth rate and threshold models. Numerical simulation method for fatigue delamination was subsequently investigated. The fracture constitutive behavior with fiber bridging was defined by a tri-linear cohesive zone law, in which fatigue damage was introduced by a parameter available for both fracture and bridging zones. Development of the degradation law of the damage parameter is based on the normalized fatigue delamination models, enabling a direct link with experimental data. The simulation was implemented using user defined interface elements within finite element software ABAQUS. Keywords Delamination, Fatigue crack growth, Cohesive zone, Damage degradation law, Composites 1. Introduction Carbon fiber reinforced polymer composites are widely used in aircraft structures due to property advantages. Meanwhile, it brings design and analysis challenges caused by the ply-by-ply formulation of composites which is totally different from the traditional metal structures [1]. The failures in composite structures are mainly due to the defect, environment and out-of-plane sensitivities of the materials. Delamination is one of the key factors for composites from damage initiation to final failures. The delamination growth behaviors have gained significant attention in the research communities in the past decade [2-7]. However, the delamination behavior of composites has not been completely understood under complex conditions, such as multidirectional interfaces, fatigue loading and fiber bridging case [8]. Linear elastic fracture mechanics is commonly utilized to study the interlaminar fracture of composites. Strain energy release rate (SERR) is accepted as the fracture governing parameter to evaluate interlaminar fracture toughness for composites rather than the stress intensity factor (SIF) for metals due to the simplicity of the calculation. Experimental studies and test methods for delamination resistance have been reviewed by Davies et al. [9] and Brunner et al. [10], numerical studies reviewed by Tay [11], respectively. Multidirectional interface and fiber bridging are two important factors in real engineering applications, which bring significant influence on the interlaminar fracture of composite structures. As a summarized result from reports in literature [6, 12-14], multidirectional laminates always exhibit higher interlaminar fracture toughness, which is assumed to be caused by extrinsic toughening mechanisms such as blunted crack tips or deviation of the crack from the main crack plane to the adjacent layers and some in-ply energy absorption [15]. Fiber bridging could also bring considerable delamination resistance due to the energy absorbed in the bridging zone behind the
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