Plasticity induced crack closure in adhesively bonded joints under fatigue loading

The mean load of a cyclic loading has a large effect on fatigue crack growth rates in metallic materials and bonded joints. In metallic structures, this effect has been attributed to plasticity-induced crack closure, but little is known about the mechanism responsible for this mean load effect on fatigue crack growth in adhesively bonded joints. This paper presents a computational investigation of the plasticity-induced crack closure mechanism affecting disbond growth in adhesively bonded joints under fatigue loading. The results show that the ratios of crack-opening and crack-closure are approximately independent of the level of plastic constraint, indicated by the ratio between the plastic zone size and the adhesive thickness. An effective strain-energy release rate parameter, which accounts for the crack closure behaviour, has been developed as a new correlating parameter for disbond growth. Comparisons with the experimental results pertinent to four different adhesive bonded joints reveal that this new correlating parameter is capable of unifying the fatigue growth rates by eliminating the effect of mean loads

Scaling parameter for fatigue delamination growth in composites under varying load ratios

Fatigue delamination growth in composite laminates is strongly influenced by mean loads or load ratios. Description of this behaviour currently relies on empirical curve fitting, which renders it difficult to predict fatigue lives of composite structures subjected to variable amplitude fatigue loading. This paper presents a new scaling parameter that is consistent with the similitude concept and incorporates the crack-tip shielding effects of fibre bridging under fatigue loading. Static and fatigue experiments were carried out on IM7/977-3 composite laminates under mode I and mode II. Large-scale fibre bridging was observed as a major toughening mechanism under both static and fatigue loading. To correctly account for the effect of fibre bridging, an inverse method was developed to determine the traction stresses acting in the crack wake. The new scaling parameter, accounting for the effect of bridging by cross-over fibres, is shown to unify the fatigue growth rates under different load ratios obtained in this study

Critical assessment of failure criteria for adhesively bonded composite repair design

Due to the high stress concentration and non-linear deformation in bonded joints, accurate strength prediction remains challenge. The aim of this paper is to evaluate the accuracies of different failure criteria and computational methodologies for bonded composite joints and their suitability as an engineering design tool. A total of four analytical and four numerical predictive models were evaluated against experimental results obtained from single lap and skin-doubler joints. Experimental observations reveal two main failure modes: cohesive and first-ply fracture. Strain-based models based on cohesive properties were found to be applicable only to joints exhibiting cohesive failure. Fracture mechanics-based models, on the other hand, can predict both cohesive and composite ply failure

On the design methodology of scarf repairs to composite laminates

Scarf repairs are the preferred method of repairing composite structures, especially when externally bonded patches can no longer meet the stiffness, strength, and flushness requirements. Present designs of scarf repairs are based on two-dimensional analyses of scarf joints, assuming a uniform stress distribution along the scarf. The purpose of this investigation is to first critically assess the adequacy of the current design method, and then to develop an improved design methodology. Strain concentrations in the adhesive and the composite laminates are determined using the elastic-plastic finite element method, taking into account of the influence of stacking sequence, laminate thickness, and adhesive yielding. Three series of experiments have been carried out, involving joints between metallic and composite adherends, with matched and un-matched lay-ups. Based on the experimental findings of joints and the computational analyses of joints and repairs, an improved method has been proposed that accounts for local peak plastic strains due to non-uniform shear strain distribution. Comparison with experimental results confirmed that the new approach provides an improved first-order prediction of repair efficiency of scarf joints

Optimum shapes for minimising bond stress in scarf repairs

Bonded scarf repairs are used in composite structures when high strength recovery is needed or when there is a requirement for a flush surface to satisfy aerodynamic or stealth requirements. However, scarf repairs are complex to design and require the removal of significant parent structure, particularly for thick skins. In this investigation, analytical and numerical approaches have been developed to investigate whether an optimum repair shape for a known biaxial load can be determined. The results clearly demonstrate that the strength of a repaired panel can be improved by optimising both the initial damage cut-out shape and the scarf angle distribution

Evaluation of Impact Assessment Methodologies. Part II: Experimental Validation

The present paper is the second part of an evaluation study of the impact simulation tools CODAC and IDAT. While in Part I the methodologies for stress analysis, failure detection, material degradation and time integration were presented and discussed, Part II focuses on the experimental validation of the applied methodologies. Therefore, computational results in terms of impact damage and residual compression strength are compared to the results of an impact test program with monolithic composite panels. The evaluation of the tools is based on the accuracy and efficiency of the impact and residual strength analyses

Evaluation of Impact Assessment Methodologies. Part I: Applied Methods

The present paper is the first part of a comparative evaluation of two methodologies for the analysis of damage in composites. The subject of the investigation is low-velocity impact and residual compression strength of monolithic composite panels. One of the methodologies is implemented in the tool CODAC, which is a stand-alone software that aims to be a fast tool and works with specialized finite element (FE) and material models. The other methodology IDAT parametrically generates the FE models by using MSC.Patran and performs the analysis by the FE code ABAQUS/Standard. The evaluation in regard to accuracy and efficiency is performed by comparing and judging the models and techniques, which are applied for stress analysis, failure detection, material degradation and time integration. Part I of the paper describes the applied methodologies. In Part II, the methodologies are validated by comparing computational results against experimental results

Detection of laminar damage in composite beams using nonlinear ultrasonic modulation techniques

This paper presents the findings of a simulation study of nonlinear ultrasonic modulation in composite laminate beams with delamination damage. Nonlinear modulation of a low-frequency pumping wave and a high-frequency probing wave due to structural damage results in the generation of higher-order harmonics and sideband frequency components. The influence of different system parameters on the magnitude of nonlinear modulations was investigated numerically using 3D explicit finite element analysis. These parameters were the size of the damage relative to the wavelengths of the low-frequency and high-frequency waves; the magnitudes of the low-frequency and high-frequency excitation waves; and the location of the damage relative to the mode shape of the low-frequency pumping wave. The results show that these system characteristics have a strong influence on the magnitude of generated sideband frequencies, and that the development of strategies to optimise the nonlinear interaction of stress waves with localised damage are essential to maximise detectability and damage characterisation in composite structures

Engineering solutions for complex composite material behaviour spanning time and temperature scales

The issue of time and temperature dependencies is considered in the behaviour of advanced fibre-reinforced polymer composite materials. Currently, for the vast majority of analyses of composite structures, time and temperature are considered invariant. In an effort to further improve the design of composite structures, more advanced analyses are now being developed to accurately capture the behaviour under a range of conditions. Various events and load cases in which time and temperature are critical are described in general terms. The specific cases of viscoelastic distortion under mechanical and thermal loading, the behaviour of adhesive joints and the structural response of composites to fire are discussed in detail. The key material response, characterisation methods and analysis approaches developed are described. It is observed that key challenges in the development of improved predictive models are measurement of time- and temperature-dependent material properties and the implementation of efficient multidisciplinary analysis methods