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Abstract Models for simulating interlaminar crack growth in composites under static and fatigue loads are rarely validated with experimental data different from simple standard tests where the crack front propagates with a uniform shape. The underlying reason for not making use of more realistic experimental data, where the crack front shape changes during the delamination growth, is the lack of well-controlled benchmark cases. This work introduces a comprehensive benchmark case consisting of a double cantilever beam (DCB)-like specimen with partially reinforced arms. A complete set of elastic and fracture properties for the material and a detailed description of load-displacement curves and crack front geometries, monitored with X-ray radiography, is provided. The benchmark case generates a rich phenomenology of crack advance in terms of varying crack growth rate and front shape, while being geometrically simple enough to be simulated. [ABSTRACT FROM AUTHOR]

Crack growth rate curves provide information about the delamination resistance of composite materials under cyclic loading. The existing methodologies for mode II fatigue testing using three-point bending end-notched flexure (3-ENF) under constant cyclic displacement conditions yield discontinuous delamination growth rate curves, therefore requiring a batch of several specimens to be tested under different severity conditions in order to fully characterize the crack growth. This work describes a variable cyclic displacement test procedure that, in combination with the real time monitoring of the specimen’s compliance, allows the crack growth rate to be measured for the desired range of severities with a single specimen, thus avoiding any human intervention during the test. [ABSTRACT FROM AUTHOR]

This work introduces a procedure based on the real time monitoring of the specimen’s compliance for the experimental characterization of the fatigue behaviour of interlaminar cracks in composites. The methodology was applied to a mode II loading in a three-point end notch flexure test. The onset point for fatigue damage was determined with a precision of one cycle by establishing a threshold on the compliance variation from its initial value. On the other hand, the crack growth rate curve was derived from the continuous monitoring of the compliance and an experimental calibration of said compliance. The precision of this methodology allowed the effect of commonly neglected phenomena such as the thermal expansion of the testing system or the friction between the specimen and the fixture, to be unveiled. [ABSTRACT FROM AUTHOR]

Expressions for the energy release rate and compliance are defined for some asymmetric beam like specimens. The expressions are derived taking into account the Timoshenko beam theory with elastic foundation. The energy release rate is expressed by the applied loads or by a combination of loads and rotation angles, the latter allows to characterize bonded interfaces without taking into account the elastic foundation properties. The condition of stability under displacement controlled test is analysed for typical specimens. The results presented are useful for the characterization of fracture toughness of bimaterial interfaces. • Expressions for the ERR and compliance are defined for asymmetric beam specimens. • ERR is expressed by applied loads or a combination of loads and rotation angles. • The condition of stability is analyzed for typical test. • The results are useful for the characterization of bimaterial interfaces. [ABSTRACT FROM AUTHOR]

• A Wedge Driven Test to obtain the mode I fracture energy. • The test does not need to measure the crack length, it is automated and objective. • Friction between the wedge and the specimen is measured in the data reduction. • Same specimens than in DCB test. There is no need for specimen preparation. The wedge driven test is a valuable tool to determine the mode I fracture energy ( G Ic ) of adhesives or interfaces in composites. At first glance, the test is simple enough (introduction of a wedge into the specimen midplane to progagate a crack) to be a good alternative to the standard method, the DCB test. However, friction between the wedge and the specimen's arms is not properly dealt in existing studies and, consequently, there is still a need for a reliable test procedure. In this work, we present a test methodology and its associated data reduction procedure to obtain G Ic . The test does not need to measure the crack length and the friction coefficient is determined as the wedge is introduced in between the specimen's arms. The new testing methodology is validated experimentally comparing the G Ic obtained from the wedge driven and the DCB tests. A very close agreement is obtained between them. [ABSTRACT FROM AUTHOR]

This work presents a novel procedure to characterize the crack growth rate curve under fatigue mixed mode I + II loading conditions with only one specimen. The procedure is based on applying a variable cyclic displacement during a Mixed Mode Bending (MMB) test together with a real time monitoring of the specimen’s compliance. The method introduces a dynamic compliance calibration for the crack length monitoring that relates the dynamic compliance to the visually obtained crack length using digital recording of the crack tip region during the fatigue test. The testing procedure is validated to be efficient in terms of testing time and required material. [ABSTRACT FROM AUTHOR]

The complex failure mechanisms involved in failure of interfaces requires the use of an accurate description of the cohesive law. In recent years, there have been many developments to determine the full shape of the cohesive law. However, most of the existing cohesive zone models assume a simplified shape, such as bilinear, trapezoidal or exponential, which are usually simple to model. Their accuracy is found to be rather limited, especially in the presence of a large fracture process zone due to either plastic deformation or fibre bridging. In this work, a new cohesive element description is proposed to formulate a general cohesive zone model to overcome these limitations. The benefit of the new approach is that it allows for convenient implementation of any arbitrary shape of the cohesive law obtained experimentally. The authors present a new procedure based on the superposition of n -bilinear cohesive zones to obtain an equivalent multilinear cohesive law that fits any experimental measurement. The new element formulation has been implemented in the commercial finite element software ABAQUS, using user element subroutine. Verification of the methodology is performed at the single element level and the approach is validated for different material systems (adhesives and composites) using the double cantilever beam, end-notched flexure and mixed-mode bending tests. Excellent correlation between all numerical predictions and experimental results is obtained, demonstrating the robustness of the proposed methodology. • A general cohesive zone model for interlaminar failure involving large process zones. • A simple but powerful method to construct mixed mode multilinear cohesive laws. • Validation with different material systems (adhesive joints and composites). • Excellent numerical-experimental correlation under different loading conditions. [ABSTRACT FROM AUTHOR]

Abstract: The uncertainty of the crack tip position inside composite coupons during a delamination test under variable fracture mode conditions reduces the accuracy of the experimental results. In this work a method is presented where the crack tip position is located using long embedded Fibre Bragg Grating (FBG) sensors together with the Optical Low Coherence Reflectometry (OLCR) technique. With this technique the local Bragg wavelength is measured which then enables to calculate the axial strain profile within the grating. Carbon/epoxy samples are tested under different mode ratios through the standard Mixed-Mode Bending (MMB) test. Embedded long FBG sensors are used to measure the axial strain profile along the whole grating at different stages of the test. The crack tip can be precisely identified with the long embedded FBGs and then checked by means of a visual inspection after complete delamination of the sample. The data indicate that the long FBG sensor detects the crack tip inside the sample at least as precisely as the traditionally performed visual inspections carried out during the test by means of the lateral markings. Numerical simulations using cohesive elements are in good agreement with the experimental data. [Copyright &y& Elsevier]

Abstract: A methodology for predicting accurately the propagation of delamination under mixed-mode fracture with cohesive elements is proposed. It is shown that changes in the local mode ratio during the evolution of damage under mixed-mode loading can cause errors in the determination of the energy dissipation and result in inaccurate predictions of the global load–displacement response – even under conditions where, according to Linear Elastic Fracture Mechanics, the global mode ratio is constant. To address this difficulty, relations between the interlaminar strengths and the penalty stiffness are proposed which ensure a correct energy dissipation when delamination propagates. The validity of the proposed methodology is demonstrated for different mode ratios by comparison with the corresponding analytical solutions. [Copyright &y& Elsevier]