Abstract

The failure of adhesion between two polymers is a complex phenomenon involving bulk dissipation (i.e., shear yielding, cavitation and crazing), surface adhesion (through Van der Waals interactions) and connector chain pull-out. This article is concerned with a cohesive model for polymer surface separation, which incorporates connector pull-out since it is attempted to adjust this model on coarse-grained molecular dynamics (MD) simulations. This model is made of a viscoplastic constitutive equation taking inspiration from the works by Anand and Gurtin (2003). The MD simulations performed and here analysed were done in close collaboration with Prof. Erik van der Giessen (MuMec, Zernike Institute for Advanced Materials, University of Groningen, Groningen, the Netherlands).

Abstract

A numerical approach has been developed for modelling the behaviour of a carbon fibre woven laminate under low-level energy impact. The model results have been compared with dart drop tests under sub and supercritical impact energies in fibre carbon textile laminates of different thickness.The models were developed using Ls-Dyna explicit finite element code. The validation approach was based in increasingly complexity approach. First models were developed and compared with subcritical impact energy test. Afterwards, supercritical impact energy has been applied in the laminates and delamination failure has been considered. In order to model delamination damage, cohesive law has been applied using Ls-Dyna cohesive and contact elements. The validation of the model has been carried out by comparing force and displacements.

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

Carbon/epoxy pre-impregnated has become one of the most demanded materials for some aeronautics structures. Boeing 787 and Airbus A350 are pioneers in incorporating carbon fiber-reinforced composite materials in primary structures as wings and airframe. Tailoring the laminate ply sequence involves a difficult challenge owing to the rigorous design and operational criteria. Given that composite materials present a high sensitivity to impact loads, it has been indispensable to examine the influence of different factors in the failure mechanisms after an impact event. In this work, the effect of impact energy, stacking sequence and thickness on the impact damage resistance of CFRP composites was investigated. Damage response parameters as peak force, absorbed energy and delamination threshold were obtained from the impact curves. Then, a C-Scan imagen showed the damage extension using the non-destructive inspection Phased Array.

Abstract

A novel method is presented for the determination of mode II cohesive law for the characterization of delamination in thin adhesive joints The proposed method is based on the three point end notched flexure test. The data reduction method used is the Beam Theory including Bending Rotation effects method (BTBR), which includes the effect of local deformations, shear and bending rotations. By this method, the initial crack length, the apparent crack length during the crack advance, and the mode II resistance curve are determined. The relative shear displacement at the crack tip is determined as a function of the apparent crack advance that occurs when the generation of the fracture zone progresses. In order to determine the shear displacement an analytical analytical model based on the classical theory of beams is used. Finally, the fracture toughness versus the shear relative displacement is represented and the cohesive law is determined by numerical derivation.

Abstract

This work presents an experimental method to determine the cohesive law of thin adhesive joints in mixed mode. The proposed method is based on the MMB (Mixed Mode Bending test). The Energy release Rate and Crack Tip Relative Displacement are determined based on a compliance variation data reduction method by processing the global load displacement curve, without monitoring the crack length and the crack tip relative displacement during the test. Representing the energy release rate versus relative displacement the cohesive law is determined by numerical differentiation. The method is validated numerically by virtual tests including the cohesive zone model. The results reveal that the proposed data reduction scheme is suitable to obtain the mixed mode cohesive law using only the load and displacement data obtained from the testing machine, without any external displacement measurement technique and without any assumption of the form of the cohesive law.