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.
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