J.F. Durodola

Performance of bi-adhesive bonded aluminium lap joints

The stress concentration towards the ends of a bonded lap joint depends to some extent on the relative stiffnesses of the adherend and the adhesive used. For a given adherend, the lower the stiffness of the adhesive used in the bondline, the lower the stress concentration, giving rise to potentially higher joint strength. The paper presents the results of a study of the application of two adhesives with different stiffnesses along the overlap length in single lap joints. A stiff adhesive was applied in the middle portion of the overlap, while a low modulus adhesive was applied towards the edges prone to stress concentrations. The results show measurable increase in strength of the bi-adhesive bonded joints compared with those in which single adhesives were used over the full length of the bondline.

Deformation and stresses in functionally graded rotating disks

Functionally graded materials are attractive because of the additional possibilities they offer for optimising the design of components in terms of material usage and performance. This paper explores, in part, the potential benefits of using fibre-reinforced, functionally graded materials for rotating hollow and solid disks. Several forms of gradation with the same nominal volume fraction of reinforcement were considered. This established a basis for comparison of results obtained for the different cases. The functionally graded material was modelled as a non-homogeneous orthotropic material. The finite-element method and direct numerical integration of the governing differential equations were used to predict stress and deformation distribution in the disks.

Property gradation for modification of response of rotating MMC discs

Abstract Aerospace and industrial gas turbine engine components such as discs, blades, and propeller shafts for which long fibre reinforced metal matrix composites are being made, operate under complex mechanical and thermal loading conditions. A major aim of functionally graded materials application is to optimise component response through appropriately tailored microstructures. This paper explores the influences of property gradation, centrifugal body force loading, and thermal loading on stresses in rotating discs. The discs were modelled as non-homogeneous orthotropic materials such as those obtained through non-uniform reinforcement of a metal matrix by long fibres. The results show how different temperature change distribution patterns and property gradation types correlate with hoop stresses developed in the disc.

Effect of strain hardening on residual stress distribution in beams determined using the crack compliance method

This work used the crack compliance method for the determination of residual stresses in beams subjected to prior straining before the introduction of residual stresses through bending. The paper also introduces a support system that allows free movement of specimens during cutting by electric discharge machining. The experimental testing and verification procedure considered factors such as different materials and strain hardening levels. The results obtained provide a quantitative demonstration of the effect of prior strain hardening on residual stress distribution in beams.

Analysis of stiffness of adhesive joints in car bodies

Abstract Adhesive bonding of joints in car bodies is known to contribute significantly to increased body stiffness. Although analytical techniques are able to give some prediction of this enhancement, the accuracy of the models is limited by the relatively low resolution of the joint details. Numerical models based on finite element analysis have been used to model different joint configurations and their predicted stiffness characteristics have been validated by experimental testing. It is shown that the joint stiffness of coach (T-peel) joints is critically dependent on the presence of fillets and flange bend radii. The application of stiffness data from ‘micro-models’ of joints into the prediction of full body behaviour presents computational problems due to the large number of elements in the FE analysis and the associated processing time. Various substitution element methods have been considered and a novel approach has been developed based on an undercut element concept. This technique enables the designer/modeller to easily include corrections for different joint parameters in the analysis of structural details in full body behaviour without the need for extended computing requirements.

A method for the simultaneous derivation of tensile and compressive behaviour of materials under Bauschinger effect using bend tests

Abstract Some materials exhibit Bauschinger effect as a consequence of strain hardening. The effect leads to asymmetric tensile and compressive stress-strain behaviour. If the hardening behaviour in either tension or compression is known, combined isotropic/kinematic hardening rules can be used to estimate the hardening behaviour in the other. These rules are, however, only approximate empirical relationships that are derived from the analysis of separate tensile and compressive test results. This article presents a method for the simultaneous derivation of tensile and compressive stress-strain behaviour from bending tests only. The information required is strains at the top and bottom surfaces of beams and moment as load is incrementally applied. The derivation of the method is based on the application of tensile and moment equilibrium conditions. The proposed method is tested on theoretical data obtained from finite-element analysis and as well as on data from actual experimental testing. The agreement between the results obtained is very good.

Failure prediction in carbon composites subjected to bearing versus bypass loading

To lighten structures, many metallic components, such as aircraft wings, are being replaced by composite components. To join these components with the rest of the structure, various joining techniques are used. When using multiple bolted joints, bypass vs. bearing loading is developed around each joint. The ratio of bearing to bypass loading is known to affect the level of load at which failure occurs. There have been many models created to predict failure within composites but very little work has been carried out to investigate how well numerical models predict failure within bolted joints subjected to bearing and bypass loading. In addition, few models have been developed that account for the through thickness stresses that are developed underneath the bearing load. This paper compares a range of failure criteria and degradation models utilizing a three-dimensional model and compares how well they predict failure for bearing vs. bypass loading for a supported-pin-loaded joint.

Locating Defects Using Dynamic Strain Analysis and Artificial Neural Networks

An inverse artificial neural network (ANN) assessment for locating defects in bars with or without notches is presented in the paper. Postulated void defects of 1mm x 1mm were introduced into bars that were impacted with an impulse step load; the resultant elastic waves propagate impinging on the defects. The resultant transient strain field was analyzed using the finite element method. Transient strain data was collected at nodal points or sensors locations on the boundary of the bars and used to train and assess ANNs. The paper demonstrates quantitatively, the effects of features such as the design of ANN, sensing parameters such as number of data collection points, and the effect of geometric features such as notches in the bars.

Effects of design and adhesive modulus on the torsional stiffness of automotive structures

Abstract The torsional stiffness of automotive structures is one of the major factors that affect vehicle handling, safety, and passenger comfort. Several factors such as material property, geometry, and joining technique contribute to the stiffness of a structure. This paper investigates the effect of adhesive modulus on the torsional stiffness of idealized box beams and plenum chambers for a luxury car. The effect of the adhesive modulus was investigated by considering four different adhesives ranging in modulus from 6MPa to 3GPa. To assess the influence of the apertures, some tests were carried out on box beams containing a single aperture on one side. Spot-welded plenum chambers and beams were also tested to provide a comparison of adhesive performance against conventional methods of joining. The effect of the adhesive modulus and the apertures in the plenum chambers on the results are discussed in the light of the results obtained, as well as the suitability of using adhesive bonding to reduce the vehicle weight.

Prediction of Brittle Failure of Notched Graphite and Silicon Nitride Bars

High stress gradients occur at metal-to-ceramic joints due to the different thermal and mechanical properties of the materials. In some cases, the magnitude of the highly localized stresses lead to failure thus compromising the structural integrity of such joints. The study of notched ceramic bars with high stress gradients can assist with the prediction of failure of metal ceramic joints. Experiments and fracture mechanics analysis were performed on notched and un-notched POCO E.D.M 3 graphite and AS800 Silicon Nitride bars with different notch parameters. The twoparameter, multi-axial Weibull statistics method and a brittle fracture criterion based on the average stress over an area approach were used to predict the failure of the bars and the results obtained were compared with experimental results. The brittle failure criterion appears to give much better correlation with experimental results than the multi-axial Weibull statistics approach. The findings also appear to highlight the limitations of the Weibull’s statistics method in cases involving very high stress gradients.