Fatigue Life Prediction of Adhesively-Bonded Fiber-Reinforced Polymer Structural Joints under Spectrum Loading Patterns

New load-bearing structures made of fiber-reinforced polymer (FRP) composites comprise adhesively-bonded joints, which are components vulnerable to fatigue failure. These structural components are frequently subjected to complex cyclic loading histories during their service life and the development of reliable methodologies for prediction of their fatigue life under variable amplitude loading patterns is therefore essential. Experimental investigations on FRP laminates showed significant effects of spectrum loading on the fatigue life. However, scientific efforts to study the fatigue behavior of adhesively-bonded FRP joints are mainly focused on constant amplitude fatigue loading and many loading parameters involved in the variable amplitude spectrums have not yet been investigated. The aim of this research is to understand the fatigue behavior of adhesively-bonded FRP joints under different loading patterns and establish a reliable methodology for the fatigue life prediction of these structural components. The fatigue response of a typical adhesivelybonded structural joint, a double-lap joint, was experimentally investigated under different loading patterns including constant amplitude, block and variable amplitude loading. The development of fatigue cracks during the lifetime and their correlation with the observed failure modes and applied cyclic load were analyzed. The experimental investigations revealed the loading parameters that significantly influence fatigue behavior and that therefore must be considered in the fatigue life prediction methodology. A new semi-empirical S-N formulation was developed to characterize the constant amplitude fatigue life and overcome the deficiencies of the fatigue models commonly used for composite materials. Based on the experimental investigation results, two phenomenological formulations were proposed in order to model the loading parameters that affect fatigue life. A new constant life diagram was developed to model the effect of mean stress on fatigue life and its accuracy was assessed using the experimental data. Also, a method was proposed to take into account the load interaction effects under variable amplitude loading. A fatigue life prediction methodology was established using the newly developed models and implemented in the form of a computational tool to predict the fatigue life of adhesivelybonded FRP joints. The variable amplitude fatigue life predictions obtained using this methodology correlated fairly well with the experimental results and proved its effectiveness in real applications.