Abstract

Abstract

En este trabajo se presenta y valida numéricamente un novedoso procedimiento de extrapolación para predecir las leyes cohesivas de uniones adhesivas en modo I y II.

En primer lugar, partiendo de los métodos de reducción de datos basados en la variación de la flexibilidad recientemente propuestos para extraer las leyes cohesivas, se definen nuevas expresiones factorizadas con respecto a la carga aplicada y a los espesores de adhesivo y adherente de la flexibilidad (C0i), la Integral–J (J0i) y el desplazamiento de la punta de la grieta (Δ0i) para cada modo de fractura.

Suponiendo que todos los efectos asociados al daño se incluyen en la longitud de grieta equivalente, y de acuerdo con las expresiones polinómicas de J0i, C0i y Δ0i con respecto a la misma, se obtienen relaciones invariantes entre J0i-C0i y Δ0i-C0i para cualquier relación de espesores adhesivo-adherente de un sistema material y configuración de ensayo determinados, tanto en modo I como en modo II.

Por último, se presenta un procedimiento de extrapolación basado en las curvas calibradas J0i-C0i y Δ0i-C0i, que permite estimar las leyes cohesivas para un amplio rango de relación de espesores adhesivo-adherente de un sistema material dado procesando únicamente la curva carga-desplazamiento del ensayo.

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.

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

Finite element modelling of adhesive joints has been analysed in this work, applying the results and conclusions of the design process of a multi-material base of an elevator. The main approach of the study has been the modelling of the adhesive joint, different parameters and its application in complex models. For that purpose, a case study has been carried out in a simply supported plate model. Several modelling techniques have been analysed and design recommendations have been proposed. Characterization of composite materials and adhesives has also been studied, given the fact that the anisotropy of the composites is an additional difficulty in the Finite Element Analysis. Having the real properties of these materials is a very important factor for efectiveness in terms of economical and computational cost. The correlation of simulation and experimental results has been analysed in the practical case study and the contribution of the adhesive modelling has been studied. To conclude, the importance of the material characterization and the sensitiveness to the variation of their properties are to be mentioned. The work offers the designer several recommendations on adhesive joint modelling as well as parameters that might affect the analysis of a structure that contains adhesive bonding.