P. Robinson

A Modified DCB Specimen for Mode I Testing of Multidirectional Laminates

A detailed study of the Mode I interlaminar fracture toughness measure ment of multidirectional laminates is presented. The use of a novel DCB specimen is pro posed in which the conventional DCB specimen is delaminated along its edges by the in corporation of inserts in addition to the usual delamination at the loaded end. Investigations using this specimen to examine the fracture toughness at +45°/+45° and +45°/-45° interfaces indicate that the appropriate width of edge delamination can sup press the crack jumping and fibre bridging which occur in the conventional DCB speci men. Preliminary tests show that GIC for +45°/+45° and +45°/-45° interfaces in multidirectional laminates are significantly higher than the GIc measured for a 0°/0° inter face in a unidirectional laminate.

Material and structural response of polymer-matrix fibre-reinforced composites: Part B

This article showcases the authors’ predictions for Part B of the second World Wide Failure Exercise. Predictions are made using the failure criteria published in the submission for Part A. In several cases, the original predictions are found to match the experimental data well and no revisions are made. A novel constitutive model for unidirectional composite materials is used to improve predictions for cases involving multidirectional laminates.

Mode I DCB testing of composite laminates reinforced with z-direction pins: a simple model for the investigation of data reduction strategies

Abstract The applicability of standard composite mode I interlaminar toughness data reduction schemes is investigated for composites reinforced by inserting straight pins in the z-direction. A simple model of a z-pinned mode I test is used to generate load, displacement and crack length data. Data reduction schemes are applied to the model data and the resulting toughness is compared to the true toughness. These schemes are shown to be inaccurate for the initial stage of crack growth, during which the zone bridged by z-pins is developing and an improved scheme is proposed. Beyond this stage, the toughness is most accurately predicted if the ‘developing’ data is omitted.

The development of an improved mode III delamination test for composites

Abstract A new test method for measuring the mode III interlaminar fracture toughness of composites has been devised. The method uses a specimen similar to the split cantilever beam method but the loading pattern is modified to ensure the unwanted mode II component is eliminated. A test jig has been manufactured and a series of tests to evaluate the new method has been conducted. Extensive finite element analyses have also been performed to investigate the performance of the test method. The results of these investigations confirmed that a mode III fracture was successfully achieved in the new test. Test results for an XAS/913C carbon-fibre/epoxy composite indicate that the critical mode III energy release rate is approximately six times the mode I value and two and a half times the mode II value.

A FINITE ELEMENT MODEL FOR DELAMINATION PROPAGATION IN COMPOSITES

Abstract In this paper the theory and application of a modelling technique for three-dimensional planar delamination growth in laminated composites has been presented. The method is based on linear elastic fracture mechanics assumptions for delamination cracks and uses a strain energy release rate criterion. For a given component, strain energy release rate is considered to be a non-linear function of the location of the delamination front. Hence, satisfaction of the growth criterion reduces to the solution of a non-linear system of equations. A generalized secant method, by the Broydens update method, is used to solve the system of non-linear equations. Several examples of the application of the technique are presented.

Shear driven delamination propagation in two dimensions

This paper investigates, theoretically and experimentally, the propagation of delamination in shear mode II between internal ply surfaces in a laminated composite plate. By considering two special cases the problem is approximated as an axisymmetric model amenable to analytical solutions. In one case the circular crack front expands and in the other it contracts. It is shown clearly that only a (linear) fracture mechanics approach will explain the mode and threshold of failure, but that a strength-based criterion can be used to initiate propagation without the need for an initial flaw. Apart from explaining some curious phenomena, the simple axisymmetric analytical solutions may be used as benchmarks for testing the ability of finite element models to predict plate delamination when the crack front is curved (at present only simple beam models are available for the standard DCB and ENF tests, but even these tests do not really develop straight crack fronts).

Finite element modelling of delamination growth in the DCB and edge delaminated DCB specimens

This paper presents the results of a finite element investigation of delamination growth in a conventional Mode I double cantilever beam (DCB) specimen and in an edge-delaminated version of this specimen. The investigation was performed using a recently developed FE model for delamination growth prediction to which an approximate contact area detection method has been added. The results of the FE analyses are used to evaluate the existing data reduction techniques for calculation of interlaminar fracture toughness. It is shown that the conventional DCB data reduction schemes can be applied to the edge delaminated specimen but the results are dependent upon the difference between the measured apparent delamination length and the actual length being constant. An alternative data reduction method is presented in which the critical energy release rate is determined by comparison of the experimental data with finite element results and does not require measurement of the delamination length.

High performance composites with active stiffness control.

High performance carbon fiber reinforced composites with controllable stiffness could revolutionize the use of composite materials in structural applications. Here we describe a structural material, which has a stiffness that can be actively controlled on demand. Such a material could have applications in morphing wings or deployable structures. A carbon fiber reinforced-epoxy composite is described that can undergo an 88% reduction in flexural stiffness at elevated temperatures and fully recover when cooled, with no discernible damage or loss in properties. Once the stiffness has been reduced, the required deformations can be achieved at much lower actuation forces. For this proof-of-concept study a thin polyacrylamide (PAAm) layer was electrocoated onto carbon fibers that were then embedded into an epoxy matrix via resin infusion. Heating the PAAm coating above its glass transition temperature caused it to soften and allowed the fibers to slide within the matrix. To produce the stiffness change the carbon fibers were used as resistance heating elements by passing a current through them. When the PAAm coating had softened, the ability of the interphase to transfer load to the fibers was significantly reduced, greatly lowering the flexural stiffness of the composite. By changing the moisture content in PAAm fiber coating, the temperature at which the PAAm softens and the composites undergo a reduction in stiffness can be tuned.

On longitudinal compressive failure of carbon-fibre-reinforced polymer: from unidirectional to woven, and from virgin to recycled

Modelling the longitudinal compressive failure of carbon-fibre-reinforced composites has been attempted for decades. Despite many developments, no single model has surfaced to provide simultaneously a definitive explanation for the micromechanics of failure as well as validated predictions for a generic stress state. This paper explores the reasons for this, by presenting experimental data (including scanning electron microscopic observations of loaded kink bands during propagation, and brittle shear fracture at 45° to the fibres) and reviewing previously proposed micromechanical analytical and numerical models. The paper focuses mainly on virgin unidirectional (UD) composites, but studies for woven and recycled composites are also presented, highlighting similarities and differences between these cases. It is found that, while kink-band formation (also referred to in the literature as microbuckling) is predominant in UD composites under longitudinal compression, another failure mode related to the failure of the fibres can be observed experimentally. It is also shown that the micromechanics of the failure process observed in UD composites is similar to that in other fibre architectures, hence encouraging the adaptation and application of models developed for the former to the latter.

Response and damage propagation of polymer-matrix fibre-reinforced composites: Predictions for WWFE-III Part A

This paper showcases the authors’ predictions for the 13 challenging test cases of the third World Wide Failure Exercise. The cases involve the prediction of lamina biaxial stress–strain curves, matrix cracking and delamination in various cross-ply and quasi-isotropic laminates under uniaxial loading, variation of thermal expansion coefficient of a laminate with matrix cracking, bending of a general laminate, loading-unloading behaviour and the strength of various thin and thick laminates containing an open hole. The laminates were made of various glass and carbon fibre/epoxy materials. The constitutive model is based on plasticity theory, includes hydrostatic pressure effects and accounts for multiaxial load combination effects. The failure criteria distinguish between matrix failure, fibre kinking and fibre tensile failure. In-situ strengths are used for matrix failure. Propagation of failure takes into consideration the fracture energy associated with each failure mode and, for matrix failure, the accumulation of cracks in the plies. The model is used to make blind predictions of all test cases from the third World-Wide Failure Exercise.