Abstract

It is essential to understand the behavior of composite laminates against fatigue crack initiation and growth in order to accurately predict component service life and to establish safe maintenance periods.

In this work, an extrapolation procedure to characterize the mode II fatigue behavior based on the J_0i-C_0i and Δ_0i-C_0i master curves is presented and validated.

The extrapolation procedure, based on the compliance variation, assumes that all effects associated with damage are included in the equivalent crack length. In this method, new factored expressions for flexibility (C_0i), Integral-J (J_0i) and crack tip displacement (Δ_0i) are defined and according to their polynomial expressions with respect to the equivalent crack length, invariant relationships between J_0i-C_0i and Δ_0i-C_0i are obtained for a given material system and test configuration.

Once the master curves have been calibrated, the extrapolation procedure allows characterizing the fatigue behavior of a material system by determining the Paris law during the fatigue test, monitoring only the test flexibility and the maximum load with respect to the number of cycles.

Abstract

This study experimentally analyses the phenomenon of delamination under static and fatigue loading in mode II using the standardized End-Notched Flexure (ENF) test. Adhesive joints made of two epoxy matrix laminates reinforced with unidirectional carbon fiber are studied, bonded with an epoxy adhesive, and subjected to environmental degradation in a saline environment for exposure periods of 1, 2, 4, and 12 weeks.

By conducting initial static tests on the joints, we identified crucial values for the rate at which energy is released under mode II conditions. These values served as a benchmark to establish the approach for dynamic fatigue testing. Our objective was to generate characteristic curves that illustrate how these adhesive joints behave when fatigue delamination begins, while also assessing the impact of varying exposure periods on their performance. To enhance result interpretation, we employed a probabilistic model rooted in the Weibull distribution to analyse the experimental data.

The results show that adhesive joints experience a decrease in their delamination resistance when exposed to a saline environment.

Abstract

In this work, the authors propose an experimental study on the possible improvement of the mechanical behaviour produced by ultra-thin plies in laminates subjected to cyclic loading after impact. More specifically, this work introduces the use of these plies in carbon fibre quasi-isotropic laminates, with the aim of studying the evolution of impact damage under tensile cyclic loads. To this aim, the behaviour of quasi-isotropic laminates consisting of conventional thickness plies is compared with laminates with 90° ultra-thin plies, keeping the same stacking sequence. When considering the same loading level, the damage evolution shows a lower progress and a later failure for the laminates containing ultra-thin plies.