N.A. Fellows

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.

Development of impact model for ceramic-faced semi-infinite armour

A model is presented which predicts the penetration of projectiles into ceramic faced semi-infinite armour. This model facilitates the study of material property and armour configuration changes on penetration. The model results correlate well with previously published experimental data for long-rod penetrator impacts with similar armours. Tests with spherical projectiles against ceramic faced semi-infinite armour were carried out, for which the model results correlated reasonably well.

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.

Localization of plastic deformation during high strain rate torsion testing of rolled homogeneous armour

A recently developed technique which allows a study of the localization of plastic flow during high-speed torsion testing is briefly described. High-speed photographs showing successive stages in the deformation of three rolled homogeneous armour (RHA) steel specimens are presented. Three stages are identified corresponding to uniform shear, localized shear and the propagation of ductile failure. The results are analysed to estimate the conditions under which localization and subsequent ductile fracture are initiated. The results are compared with a previous study on a soft iron.

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.

Determination of Tensile and Compressive Stress Strain Curves from Bend Tests

A new inverse method has been developed for the simultaneous derivation of tensile and compressive stress strain behaviour from bending tests only. This new procedure can be applied to materials having asymmetric tensile and compressive stress strain behaviour and also materials that have been previously strain hardened (Bauschinger Effect). This paper presents results obtained using the new method and compares them with experimentally obtained tensile and compressive stress strain curves. The agreement of the derived stress strain data in tension and compression is encouraging.

A Neural Network Approach for Locating Multiple Defects

A method is presented to demonstrate the use of artificial neural networks (ANNs) in providing additional information regarding defects or flaws when used in conjunction with the ultrasonic A-scan method. ANNs were employed both as pattern classifiers and as function approximators to maximise the amount of data available from the temporal A-scan signal. A steel bar was modelled in 2D using ABAQUS finite element analysis (FEA) software. A single defect was introduced to the bar, modelled as a void, and parametric studies conducted to record data with the defect at various locations. An ultrasonic Lamb wave was introduced at the top of the bar. The longitudinal wave propagated along the length of the bar and was partially reflected by the defect. Multiple cases were simulated, modelling voids between 1mm and 6mm in width in various locations. Mean displacement of all the nodes at the top of the bar was recorded throughout the simulation, and features extracted from this waveform to create the data set for the ANNs. The ANNs were trained with a percentage of the data collected, selected at random, and assessed with the remaining data. The target data for the ANNs were the depth and size of the defect. The case of two separate defects was also investigated. The procedure was carried out in the same manner as for one defect, but in this case the target data for the ANNs were the depth of the first defect and the distance between the defects. A separate ANN was employed as a pattern classifier, to determine if the reflected A-scan signal represented one or two defects. The final system was tested using previously unseen data, and provided very good results both in determining the number of defects and the size and location of the defects, even with data to which noise had been added.