Abstract

This study analyzes delamination in CFRP adhesive joints under mode II loading in both static and dynamic regimes. The ENF test was used to evaluate the effect of exposure to salt spray and a climatic chamber over various periods—one, two, four, and twelve weeks—as well as for unaged specimens. Based on the experimental data, fatigue initiation curves (ΔG-N) and fatigue crack growth curves (G-da/dN) were constructed to analyze both degradation processes. In the fatigue initiation phase, the data were analyzed using a probabilistic model based on a Weibull distribution. The most relevant findings of this study are as follows: regarding the fatigue limits obtained for the adhesive joint under mode II fracture, a decrease in load-bearing capacity was observed due to degradation processes—around 20% under static loading conditions for salt spray exposure, and 25% for hygrothermal degradation. As for the fatigue crack growth phase, the crack propagation rates were found to depend on the specific environmental degradation process to which the tested specimens were subjected.

Abstract

This research addresses the challenge of the extended duration and high number of tests required to predict the fatigue life stress of materials. To tackle this issue, a methodology is proposed based on the intrinsic viscoelastic effects of thermoplastic and thermoset-based materials and composites. The approach utilizes thermographic techniques to monitor the temperature generated during the application of a test protocol aimed at reducing the time required to predict material fatigue life. This has key applications in sectors such as automotive, aerospace, and wind energy, enhancing both testing efficiency and quality control. Furthermore, it aligns with sustainability and circular economy strategies, contributing to advances in innovative materials and industrial processes.

Abstract

The study of weld lines in polymers is far from settled, especially in predicting how materials perform over long periods under stress. While previous research has explored the behavior of isotactic polypropylene (iPP) in both its unfilled and glass fiber-reinforced forms, the focus has been largely on short-term behavior. However, this study shifts the lens to long-term performance under complex conditions, specifically looking at creep and fatigue. We take a novel approach by considering iPP with 30% glass fiber reinforcement, using tensile samples that induce weld line formation. Creep behavior is measured across a broad range of strain rates and temperatures, aiming to understand the underlying mechanisms that govern material failure. These short-term tests are then linked to more comprehensive long-term evaluations, including cyclic loading and creep-to-rupture tests. Crack growth is assessed using CT specimens, enabling us to capture failure modes that are otherwise difficult to quantify. The study goes further by proposing a new way to model deformation. Instead of relying on traditional methods, we turn to the Eyring equation for a more accurate prediction of failure times under cyclic stress, especially as materials transition to brittle fracture at higher temperatures. These predictions match experimental results, demonstrating the potential of linear elastic fracture mechanics (LEFM) in assessing long-term material performance. Ultimately, this research challenges conventional models and provides a pathway for more accurate long-term predictions, a crucial step for industries relying on polymer materials in demanding environments.

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.

Abstract

This study analyzes the delamination behavior in mode I, under static and fatigue loading, in adhesive joints on a composite material with an epoxy matrix and unidirectional carbon fiber reinforcement (CFRP). The samples were exposed in a climate chamber at 60°C and 70% relative humidity for different periods (no exposure, 1, 2, and 4 weeks). Subsequently, standardized DCB tests were performed to evaluate the effect of environmental aging on interlaminar fracture toughness and adhesive strength. After an initial static characterization, reference parameters for fatigue tests were defined, obtaining initiation (ΔG–N) and crack growth (G–da/dN) curves. The initiation data were analyzed using a Weibull probabilistic model. The results show a change in the epoxy adhesive behavior with exposure time, evidenced by a reduction in the fatigue limit and an increase in crack propagation rates.