J.M. Dulieu-Barton

An Overview of Intelligent Fault Detection in Systems and Structures

This paper describes a coherent strategy for intelligent fault detection. All of the features of the strategy are discussed in detail. These encompass: (i) a taxonomy for the relevant concepts, i.e. a precise definition of what constitutes a fault etc., (ii) a specification for operational evaluation which makes use of a hierarchical damage identification scheme, (iii) an approach to sensor prescription and optimisation and (iv) a data processing methodology based on a data fusion model.

A temperature correction methodology for quantitative thermoelastic stress analysis and damage assessment

In thermoelastic stress analysis, an infrared detector is used to obtain the small temperature change resulting from the thermoelastic effect. The output from the detector, known as the thermoelastic signal, is dependent on both the surface stresses and the surface temperature of the component under investigation. For quantitative thermoelastic stress analysis, it is important that the response resulting from changes in the surface temperature is decoupled from the response resulting from the stress changes. In this paper, a means of decoupling the response is presented that involves making corrections for increases in surface temperature so that the thermoelastic signal is dependent only on the stresses. The underlying theory is presented and a correction factor is developed using an experimental approach. A methodology for applying the correction factor to full-field data is provided. The methodology is validated through a number of case studies and applied to a composite component subject to fatigue damage initiated at a central hole.

Paint coating characterization for thermoelastic stress analysis of metallic materials

In thermoelastic stress analysis (TSA) it is normal practice to coat metallic specimens with black paint to enhance and standardize the surface emissivity. It is assumed that the paint coating has no effect on the thermal emission from the specimen, but it is well known that the response is sensitive to paint coating thickness, particularly at higher frequencies. In this paper the effects of loading frequency and paint coating thickness on the thermoelastic response are investigated. The thermoelastic response is compared to theory, and optimum test conditions and coating characteristics are suggested. The motivation for the work is to develop a TSA-based means of residual stress assessment, where the measurement of much smaller temperature changes than those that are resolved in standard TSA is required; therefore the analysis is much more sensitive to the effects of the paint coating. However, the work presented in this paper is relevant to a wide range of TSA investigations and presents data that will be of interest to all practitioners of TSA.

Identification of the sources of non-adiabatic behaviour for practical thermoelastic stress analysis

In this paper the sources of non-adiabatic behaviour in standard thermoelastic stress analysis are identified. Previous work on non-adiabatic effects is critically reviewed in the context of the practitioner of thermoelastic stress analysis, and from this a hierarchical relationship between stress gradients, material and loading frequencies is developed. These three effects are studied in turn, and the use of paint coatings is identified as the major cause of non-adiabatic behaviour.

On the thermoelastic response of woven composite materials

The thermoelastic response from a woven composite component is complicated because of the variation in material properties across its surface. In this paper the response is interpreted by treating the woven structure as a ‘patchwork’ of unidirectional cells and considering each in isolation. To make matters as simple as possible, single-ply specimens loaded in uniaxial tension are studied. The experiments show that, even for this simple tensile case, the strain field is non-uniform and the approach fails to produce any correlation with predictions based on treating each cell in isolation. In deriving the predicted response a detailed knowledge of the material properties required for thermoelastic stress analysis (TSA) is necessary. It is demonstrated that the response is sensitive to small variations in certain quantities and this is discussed in detail in the paper. The TSA is carried out using a new system and a description of this together with the motion compensation techniques used in the analysis of the high-resolution data is provided.

Evaluation of Sub-Surface Stresses Using Thermoelastic Stress Analysis

Thermoelastic stress analysis (TSA) is a well established technique for stress analysis. Recent studies have revealed that the technique can be used to detect sub-surface defect effectively. In this study, the technique has been used to examine the thermoelastic response to sub-surface damage in simple bar specimens. The non-adiabatic thermoelastic response from areas close to the damage has been studied. The study shows that the phase of the response along with the thermal diffusion can provide a parameter that will help reveal subsurface stresses.

Deformation and strain measurement techniques for the inspection of damage in works of art

Abstract The engineering techniques used for inspecting structural damage are not widely known in the conservation sector. Techniques are available based on deformation or strain measurement that have the ability to provide quantitative data. This paper reviews currently available techniques, covering point-strain measurements using resistance strain gauges and fibre-optic sensors, as well as full-field optical measurement approaches such as holography, electronic speckle pattern interferometry, photoelastic stress analysis and photogrammetry. The underlying technology of each of the techniques is described for the non-specialist. The relevance of each technique is established from a conservation perspective through accounts of usage. The application of the techniques to a wide range of artwork, including panel paintings, statues, murals and mosaics is described and the results critically reviewed. The paper also provides an insight into possible future applications of the techniques and identifies areas for further investigation.

Determination of hygrothermal ageing effects in sandwich construction joints using thermoelastic stress analysis

Typical marine sandwich construction composite tee joints have been exposed to a hygrothermal ageing environment and were simultaneously subjected to one of three loading conditions: free standing, static compression and cyclic compression. The joints were removed from their environment for experimental evaluation after 60 and 144 days of ageing. The experimental approach adopted for this work was thermoelastic stress analysis (TSA), using the DeltaTherm equipment to obtain full-field thermoelastic data from the region of interest in the structural specimens. Readings from the aged joints have been obtained and compared to reference readings taken from an unaged joint. The implications of the changes in thermoelastic signal in specific regions of the joints have been discussed. The work shows the potential for using TSA to detect material changes due to hygrothermal ageing and for establishing damage severity levels resulting from mechanical loading during hygrothermal exposure.

Determination of the stress distribution in foam-cored sandwich construction composite tee joints

The stresses in sandwich construction tee joints are determined using both experimental and numerical techniques. The features of interest in the tee joint are the use of a boundary angle laminate which reinforces the connection between the two component parts of the tee joint, the use of a fillet at the connection and the effect of any gap at the join. The experimental approach is thermoelastic stress analysis (TSA) based on the use of the SPATE (stress pattern analysis by thermal emissions) equipment. Techniques for calibrating the constituent parts of the joint are described so that quantitative stress values can be obtained from the TSA. A detailed finite element study of the tee joints is also carried out, the results of which are compared with the experimental data.

Design and commission of an experimental test rig to apply a full-scale pressure load on composite sandwich panels representative of an aircraft secondary structure

This paper describes the design of a test rig, which is used to apply a representative pressure load to a full-scale composite sandwich secondary aircraft structure. A generic panel was designed with features to represent those in the composite sandwich secondary aircraft structure. To provide full-field strain data from the panels, the test rig was designed for use with optical measurement techniques such as thermoelastic stress analysis (TSA) and digital image correlation (DIC). TSA requires a cyclic load to be applied to a structure for the measurement of the strain state; therefore, the test rig has been designed to be mounted on a standard servo-hydraulic test machine. As both TSA and DIC require an uninterrupted view of the surface of the test panel, an important consideration in the design is facilitating the optical access for the two techniques. To aid the test rig design a finite element (FE) model was produced. The model provides information on the deflections that must be accommodated by the test rig, and ensures that the stress and strain levels developed in the panel when loaded in the test rig would be sufficient for measurement using TSA and DIC. Finally, initial tests using the test rig have shown it to be capable of achieving the required pressure and maintaining a cyclic load. It was also demonstrated that both TSA and DIC data can be collected from the panels under load, which are used to validate the stress and deflection derived from the FE model.