Showing papers in "Journal of Strain Analysis for Engineering Design in 1998"

Development and applications of thermoelastic stress analysis

Abstract: The theory of thermoelastic stress analysis is reviewed and the implications of some theoretical developments are assessed. Available instrumentation is described and techniques available for separation of individual stress values are summarized. The scope of the technique is illustrated with reference to a number of applications covering crack-tip stress studies, stress analysis and damage assessment in composite materials, and ‘field’ work on a traffic-loaded road bridge.

Automated fringe pattern analysis in experimental mechanics: A review

Abstract: The paper reviews the main numerical techniques that have been developed to carry out fully automated analysis of fringe patterns resulting from solid mechanics experiments. These include temporal and spatial phase shifting interferometry, temporal and spatial phase unwrapping, and calculation of strain fields from the phase maps. Systematic and random errors associated with the various procedures are also analysed. A unified treatment for both speckle and smooth-wavefront interferograms is presented, and the common features underlying many of the algorithms are emphasized. The paper is illustrated with applications that include ball impact (moire photography), bending waves in orthotropic plates (double-pulsed dual-reference wave holography) and finite strains in propellant grains (fine grid technique).

A review of automated methods for the collection and analysis of photoelastic data

Abstract: Photoelasticity is one of the most widely used full-field methods for experimental stress analysis. However, the collection of photoelastic parameters can be a long and tedious process. The advent of automated photoelastic systems has allowed the experimentalists to speed up the rate of analysis and to perform more complex investigations.This paper provides a survey of recent methods of automated photoelasticity developed in the last 20 years, i.e. methods of the fringe centres, half-fringe photoelasticity, phase-stepping photoelasticity, methods based on the Fourier transform, spectral content analysis (SCA) and RGB (red, green, blue) photoelasticity.

Simultaneous observation of phase-stepped images for automated photoelasticity

Abstract: A novel instrument is described for the simultaneous observation and capture of four phase-stepped photoelastic images. A theoretical description of the optics of the instrument is presented for the first time. Three examples are given of the use of the instrument in reflection photoelasticity to generate full-field maps of isochromatic and isoclinic parameters. The results from these experiments show close correlation to results from both theoretical analyses and manual measurements. The instrument can be used in either reflection or transmission mode and it is concluded that the new instrument significantly enhances the potential for real-time studies using reflection photoelasticity.

A method for reducing the influence of quarter-wave plate errors in phase stepping photoelasticity

Abstract: The phase stepping technique has recently been applied to the automated analysis of photoelastic fringes to determine the isoclinic parameter and the relative retardation. Generally, in the...

Recent developments and applications in electronic speckle pattern interferometry

Abstract: Electronic speckle pattern interferometry (ESPI) is a wholefield non-contact optical metrology technique for displacement measurement, based on the optical physics of surface-generated laser speckle. Since its inception during the early 1970s ESPI has gradually evolved into different optical designs and has been applied to a range of engineering and non-engineering applications. Development of ESPI has continued during the 1990s with the introduction of new laser and optical technology into the interferometers, allowing further optimization and extending the potential applicability of the technique. This review considers the most notable developments of interferometer design and application that have occurred and been widely published during the 1990s, and examines the current and near-future direction of research into the technique.

The development of intergranular strains in a high-strength steel

Abstract: In situ neutron diffraction measurements have been made on a high-strength low-alloy steel to study the generation of intergranular strains under uniaxial load. Intergranular strains were found to be actively developed at small plastic deformation (< 4.5 per cent) and stay unchanged in subsequent loadings. Large tensile strains of an average 7 × 10−4 were observed for the (002) reflections transverse to the stress axis whereas intergranular effects are comparatively insignificant for other reflections. Experimental results were found to be in excellent agreement with the predictions based on the elastoplastic self-consistent model.

Strain analysis by means of digital shearography: Potential, limitations and demonstration

Abstract: Shearography, also called speckle pattern shearing interferometry, is a coherent optical method which measures displacement derivatives directly. It is suited well for localization of strain concentrations and has been used as an industrial tool for non-destructive testing (NDT) in the last few years. However, its application for strain measurement has not been widely adopted in industry, because, in general, shearography can measure only out-of-plane displacement derivatives ∂w/∂x and ∂w/∂y. This paper presents recent developments of shearography for strain measurement. With the support of digital image processing the automatic and quantitative evaluation of the shearogram becomes possible. Not only flexural strains [∂ 2 w/∂x 2 , ∂ 2 w/∂y 2 and ∂ 2 w/(∂x∂y)] but also in-plane strains (∂u/∂x, ∂v/∂y, ∂u/∂y and ∂v/∂x) can be determined by the shearographic measuring method. The potentials and limitations for strain measurement are discussed. Some applications are shown.

Polymer coating as a strain witness in thermoelasticity

Abstract: The work described in this paper offers the possibility of using a polymer coating as a strain witness in thermoelasticity. In particular, the efficacy of a polymer coating for making thermoelastic measurements is investigated by experiment and the supporting theory is presented. It was found that the thermoelastic response is greatest with thick coatings at high frequencies. However, thicknesses of more than 0.5 mm and frequencies greater than 5 Hz provide adequate results.

A review of recent developments and applications in the field of X-ray diffraction for residual stress studies

Abstract: Much research effort has been conducted recently in the field of residual stresses. This presentation is a review of different studies carried out during the last few years on the X-ray diffraction technique. The first part of this paper presents an introduction to the residual stresses and the different techniques of measurement. The second part of this paper is focused on the X-ray diffraction technique. Then the different factors that affect the measurement results are discussed and some examples shown for different applications of the X-ray method. The last part of the work deals with the residual stress measurement techniques and the overall necessity to combine destructive (incremental hole drilling) and non-destructive (X-ray and neutron diffraction) methods in order to evaluate precisely the residual stress distribution.

Finite element modelling of the interaction of residual stress with mechanical load for a crack emanating from a cold worked fastener hole

Abstract: A three-dimensional finite element simulation has been conducted for the cold working of a fastener hole in an aluminium plate. The simulation predicts the residual stress distribution resu...

Study of through-thickness residual stress by numerical and experimental techniques

Abstract: The quenching process of aluminium alloys is modelled using the finite element method. The study of residual stress field induced by quenching is divided into two: the thermal and mechanica...

An assessment of the Sachs method for measuring residual stresses in cold worked fastener holes

Abstract: Measurements of residual stresses in 6 mm thick aluminium alloy 2024 plates containing 4 per cent cold worked fastener are made using the Sachs method. The measurements are made on discs extracted from the plates. The measured tangential residual stress distribution adjacent to the hole edge are found to be affected by the disc diameter. The measured residual stresses are also in good agreement with averaged through-thickness predictions of residual stresses from an axisymmetric finite element (FE) model of the cold working process. A finite element analysis is also conducted to simulate disc extraction and then the Sachs method. The measured FE residual stresses from the Sachs simulation are found to be in good agreement with the averaged through-thickness predicted residual stresses. The Sachs simulation was not able to reproduce the detailed near-surface residual stresses found from the finite element model of the cold working process.

Substrate stress concentrations in bonded lap joints

Abstract: A method based on the successive boundary stress correction approach is presented for the determination of the stress concentration in substrates of adhesively bonded joints with square edges or spew fillets at the ends of the overlap. The emphasis is given to developing an estimate of the stress elevation at the end of a bonded joint while the issue of corner singularity at the substrate-adhesive interface is not addressed in detail. It is shown that the adhesive shear stress which acts on the substrates is the main cause of the stress concentration; the adhesive peel stress has little effect. To circumvent the deficiencies of existing bonded joint theories which generally predict a maximum adhesive shear stress at the ends of the adhesive layer, an eigenfunction solution has been derived for the shear stress distribution near the ends of the overlap. Based on the improved adhesive shear stress solution presented here, the stress concentration determined from the present theory is found to be in ...

Finite element solutions comparing the normal contact of an elastic—plastic layered medium under loading by (a) a rigid and (b) a deformable indenter

Abstract: The main purpose of this paper is to assess the influence of a deformable indenter by comparing the finite element results for a deformable indenter with those of a rigid indenter. Elastic-plastic solutions for the case of a stiff hard layer rigidly bonded to a softer substrate are given and the effect of the indenters (i.e. rigid; deformable, E = 600 GPa; deformable, E = 1000 GPa) on the contact pressures and stresses and strains are critically examined. The results showed that, when the indenter is assumed to be rigid, the contact pressures in the elastic regime are higher than those given by the deformable indenters. However, as deformation proceeds and plasticity becomes dominant, the differences between the three sets of results reduce. The rigid indenter results were also shown to give the lowest peak radial tensile stress which is responsible for cracking of the film normal to the surface. Finally, comparison of the von Mises stress contours revealed that varying the indenter properties affects the point at which yielding commences. Results showed that the rigid indenter caused the coating and substrate to yield prematurely, i.e. at a lower load than the deformable indenters.

On the relaxation of interface stresses during creep of ferritic steel weldments

Abstract: This paper examines the interaction between materials joined together that individually have high and low degrees of creep resistance. A specific example considered is an overtempered layer of the heat-affected zone of a ferritic steel weldment which lies between stronger materials. Experimental evidence is reviewed which shows that adjacent strong and weak layers of material creep relatively independently of each other when a tensile stress is applied perpendicular to the plane of the layer.Finite element analyses are presented of a simplified model of a narrow and weak layer of material sandwiched within much stronger material. A constant load is applied perpendicular to the plane of the weak layer and the stress and strain distributions are considered after the accumulation of a significant degree of creep deformation. If the finite element mesh is constructed in a conventional manner it is shown that significant constraint of the narrow, weak layer is to be expected. However, if the mesh is mo...

Moiré strain analysis : an introduction, review and critique, including related techniques and future potential

Abstract: The moire fringe method is described and reviewed with particular reference to its application in strain analysis. The theory is briefly dealt with, followed by a discussion of the common m...

A new calculation procedure for non-uniform residual stress analysis by the hole-drilling method

Abstract: The hole-drilling method is one of the most used semi-destructive techniques for residual stress analysis in mechanical parts. In the presence of non-uniform residual stress, the stress field can be determined from the measured relaxed strains using several calculation methods, but the most used one is the so-called integral method. This method is characterized by some simplifications that lead to approximate results, especially when the residual stress varies abruptly. In this paper a new calculation procedure called the spline methods is proposed, which allows these drawbacks to be overcome. Numerical simulations and an experimental test have corroborated the best performance of the proposed technique with respect to the calculation procedures used at present.

Designing against fretting fatigue: Crack self-arrest

Abstract: Issues concerned with self-arrest of fretting fatigue cracks at small initial lengths are discussed for a range of well-defined contact geometries. The conditions under which self-arrest will occur are given, together with the maximum tolerable initial flaw size, beyond which self-arrest may not be anticipated.

An alternative finite difference method for post-processing thermoelastic data using compatibility

Abstract: A finite difference technique for processing thermoelastic data for biaxial problems is presented. The method enables thermoelastic data to be smoothed at interior points and boundary information to be calculated from measured non-boundary values by an iterative procedure based on the compatibility equation. The technique has been successfully applied to quantify stress concentrations and to perform stress separation.

A theoretical and experimental study of alternating current stress measurement under different loading modes

Abstract: This paper presents some experimental studies on the inverse magnetostriction effect that were recently conducted at University College London (UCL) by the authors. A form of alternating current field measurement (ACFM) instrumentation, which was developed to detect and size surface defects in ferromagnetic components, was used in the present study to induce a low alternating magnetic field near the surface of a mild steel specimen and to pick up the influences of applied stresses on that field. The mild steel specimen was tested both in four-point bending and uniaxial tension to investigate the response under different stress modes within the elastic region. Consistent and repeatable results have been obtained under both bending and tension loading modes when different probe types with different inducing frequencies are used. In the present paper, the mathematical model of half-space electromagnetic induction which was recently developed at UCL has been combined by the authors with the previously...

Stress concentration factors of edge-notched orthotropic plates

Abstract: For an edge-notched semi-infinite orthotropic plate, the stress concentration factor has been shown to be 1 + FmFs, where Fm and Fs are material factor and shape factor respectively. Since Fs is independent of material properties, a simple formula for Fs is proposed by interpolating two theoretical values of the isotropic material. The accuracy of the formula is assured by finding that its predictions are in excellent agreement with available solutions. An example of the computation of the stress concentration factors of edge-notched orthotropic plates is presented to illustrate its simplicity and accuracy.

Approximate method for the analysis of components undergoing ratchetting and failure

Abstract: An approximate method has been presented for the design analysis of engineering components subjected to combined cyclic thermal and mechanical loading. The method is based on the discretization of components using multibar modelling which enables the effects of stress redistribution to be included as creep and cyclic plasticity damage evolves. Cycle jumping methods have also been presented which extend previous methods to handle problems in which incremental plastic straining (ratchetting) occurs. Cycle jumping leads to considerable reductions in computer CPU (central processing unit) resources, and this has been shown for a range of loading conditions. The cycle jumping technique has been utilized to analyse the ratchetting behaviour of a multibar structure selected to model geometrical and thermomechanical effects typically encountered in practical design situations. The method has been used to predict the behaviour of a component when subjected to cyclic thermal loading, and the results compared with those obtained from detailed finite element analysis. The method is also used to analyse the same component when subjected to constant mechanical loading, in addition to cyclic thermal loading leading to ratchetting. The important features of the two analyses are then compared. In this way, the multibar modelling is shown to enable the computationally efficient analysis of engineering components.

Model for the variation of the residual stress state during plastic deformation under uniaxial tension

Abstract: A theoretical model has been developed to explain the variation of surface residual stress introduced by shot-peening with external plastic deformation, during a uniaxial tensile test. The model is based on the difference of yield stress values of the shot-peened surface layer and the remaining bulk material. It has been shown that the model fits well with experimental results obtained for the base metal and heat-affected zone of a 5.0Cr-0.5Mo steel.

Characterization of residual stresses generated during inhomogeneous plastic deformation

Abstract: Residual stresses generated by macroscopic inhomogeneous plastic deformation are predicted by an explicit finite element (FE) technique. The numerical predictions are evaluated by character...

The ratchetting of plain carbon steel pressurized cylinders subjected to simulated seismic bending: The effect of the D/t ratio and comparison with finite element predictions

Abstract: A series of simulated seismic bending tests have been undertaken on pressurized cylinders having diameter-thickness ratios in the range 10 ≤ Dm/t ≤ 28. Ratchetting of the cylinder wall has been observed and recorded in the hoop direction. Finite element analysis which models the experimental set-up and the cylinder material has been undertaken. The results obtained from the finite element work and from a previously available analytical solution both overestimate the experimentally obtained ratchetting rates considerably. The dynamic bending moment necessary to initiate ratchetting is, however, well predicted.

The finite element analysis of a strain hardening layered medium under normal loading by (a) a rigid and (b) a deformable indenter

Abstract: The finite element method (FEM) is used to investigate the normal indentation problem of a deformable indenter in contact with a strain hardening substrate coated with an elastic-perfectly plastic layer. In order to assess the influence of the deformable indenter, the solution is compared with results of a rigid sphere indenting the same system. The comparison shows that, when the indenter is assumed to be rigid, the contact pressures are significantly higher than those for the deformable indenter during elastic deformation. When plasticity is more pronounced, the peak pressures at just inside the contact edge are also higher for the rigid indenter. Similarly, the results assuming a rigid indenter give a lower value of maximum radial tensile stress along the coating surface when compared with those using a deformable indenter, which is responsible for the cracking of the film. In order to examine the effects of strain hardening, the above solutions are compared with a system having an elastic-perfectly plastic substrate. Comparison between the two sets of results shows that the strain hardening medium develops a smaller contact area and higher central and peak contact pressures inside the contact edge during plastic deformation. It also shows that the use of a strain hardening substrate alleviates the maximum radial tensile stress just outside the contact edge.

Analysis of large deflections for pressure loaded plates with rigid inclusions

Abstract: In the present paper, the non-linear Berger equation for large deflection problems of isotropic plates and the point-matching method are utilized to obtain solutions for out-of-plane deformations in rectangular isotropic plates with a circular rigid body at the centre. Numerical solutions are presented in order to illustrate the influence of the rigid body size on the deflection distribution in rectangular plates subjected to uniform static load. The numerical procedure is easier than other procedures based on the von Karman theory, and reasonable results are obtained.

Ultimate strength of a double-tee tubular joint subjected to combined chord compression and brace out-of-plane bending

Abstract: The effects of chord axial compression on the ultimate strength of a double-tee (DT) tubular joint subjected to brace out-of-plane bending have been studied both experimentally and numerically. The results from four experimental tests with different levels of chord compression are presented, together with the results of a parametric study using non-linear finite element procedures. The results are compared with the American Petroleum Institute's design rules for DT joints subjected to combined brace and chord loading.

Modelling plasticity in finite bodies containing stress concentrations by a distributed dislocation method

Abstract: An efficient method for the analysis of limited plasticity at stress raising features such as notches and holes in finite bodies has been developed. A network of stationary dislocations is used to simulate the plasticity and simple constant displacement boundary elements form the borders of the geometries. The notch or hole itself is implicitly included in the formulation by using specialized kernels for these features in an infinite plane, thereby improving the numerical efficiency. The cyclic plastic behaviour of an edge-notch in an infinite plane and a finite rectangular plate are analysed under elastic-perfectly plastic, plane strain conditions. Examples of the resulting stress state after stress redistribution and plastic shakedown are displayed which aid in the reliable prediction of component life.