Modelling and evaluation of pulsed and pulse phase thermography through application of composite and metallic case studies

A transient thermal finite element model has been created of the pulsed thermography (PT) and pulse phase thermography (PPT) experimental procedure. The model has been experimentally validated through the application of four case studies of varying geometries and materials. Materials used include aluminium, carbon fibre reinforced plastic (CFRP) and adhesively bonded joints. The same four case studies have also formed a basis for comparison between three experimental techniques: PT, PPT and the more established ultrasonic (UT) c-scan.Results show PPT to be advantageous over PT due to its deeper probing as it is less influenced by surface features. Whilst UT is able to reveal all the defects in these case studies, the time consuming nature of the process is a significant disadvantage compared to the full field thermography methods.Overall, the model has achieved good correlation for the case studies considered and it was found that the main limiting factor of the PT model accuracy was knowledge of thermal material properties such as conductivity and specific heat. Where these properties were accurately known the model performed very well in comparison with experimental results. PPT modelling performed less well due to the method of processing the PT data which aims to emphasise small differences. Hence inaccuracies in inputted values such as material properties have a much greater influence on the modelled PPT data. The model enables a better understanding of PT and PPT and provides a means of establishing the experimental set-up parameters required for different components, allowing the experimental technique to be appropriately tailored to more complex situations including bonded joints or structures where several materials are present.The paper ends with a section on defect detectability based on thermal diffusivity contrast between the defect and the bulk material. It shows that in aluminium, because of its higher conductivity, greater thermal contrast is achieved for small differences in diffusivity. Regions where the diffusivity ratio between defect and bulk materials was insufficient to provide thermal contrast for defect identification were found. PPT phase data is shown to reduce the extent of such regions increasing the detectability of defects. Effusivity is introduced as a means of determining the thermal contrast between the defect and non-defective areas and hence establishing the defect detectability

Material heterogeneity or stress concentration: the thermoelastic response from woven composite materials subjected to cyclic fatigue

A study of the growth of fatigue damage in 2 x 2 twill woven composite materials, subjected to cyclic tensile loading are described. Thermoelastic stress analysis (TSA) is used to monitor the stress field. As a result of the damage, a net reduction in the thermoelastic signal is observed. Laminates are found to be more resistant to fatigue

Derivation of the stress concentrations at holes in orthotropic plates using thermoelastic stress analysis

An experimental study of the stress distribution around holes in orthotropic composite laminates has been conducted using thermoelastic stress analysis (TSA). Quantitative thermoelastic studies of stress concentrations in metallic plates is a straightforward matter, all that is required is the ratio of the response from the hole and a far-field reading. For orthotropic materials the situation is more complex as the response is not simply proportional to the sum of the principal stresses. In general the thermoelastic response of an orthotropic laminate is a function of the stresses in the principal surface material directions and the associated coefficient of thermal expansion. The approach in this paper is to obtain ‘stress factors’ at the hole and identify the maxima in the plot. Specimens manufactured from a variety of different laminate lay-ups (unidirectional (UD), cross-ply (CP), angle-ply (AP) and quasi-isotropic (QI)) are considered. In all these cases the principal stress directions at the hole are not coincident with the principal material directions and it is a challenging proposition to derive meaningful stress data from these configurations. To validate the approach the experimental data are compared to analytical models. To better understand the nature of the response finite element models are produced that mimic the thermoelastic response

Identification of the source of the thermoelastic response from orthotropic laminated composites

In previous work, a series of theoretical considerations have been made aimed at identifying the source and assessing prominent factors influencing the thermoelastic response from laminated composites. In this paper four different methods of interpreting the data are investigated and the theoretical thermoelastic response is compared to experimental data to identify the source of the thermoelastic response

Development of thermoelastic stress analysis as a non-destructive evaluation tool

A modified methodology is proposed in which only a single transient load is used for the TSA measurement. Specimens with different damage severities are tested and it is shown that the modified TSA method has the potential to be applied in the field as a non-destructive evaluation tool

Thermoelastic investigation of residual stress: plastic deformation and the change in thermoelastic constant

Plastic deformation causes very small changes in the thermoelastic response of metallic materials; this variation of the thermoelastic constant has the potential to form the basis of a new non-destructive, non-contact, full-field technique for residual stress assessment that is quicker and cheaper than existing methods. The effect of plastic strain on the thermoelastic constant is presented as a potential basis for a calibration methodology that reveals areas of a component that have experienced plastic strain. Establishing this basis provides the initial step in identifying a new approach to residual stress analysis using the thermoelastic response. An evaluation of initial calibration results is presented and the feasibility of applying the methodology to actual components is assessed. As the response to plastic strain is likely to be small it is necessary to identify the effects of the paint coating; experimental work is presented that highlights the importance of repeatable coating approaches

The Failure of Private Ordering and the Financial Crisis of 2008

This Article analyzes the Financial Crisis of 2008 in the context of failures by market participants to engage in private ordering thus leading to opportunistic behavior at the expense of market stability. The Financial Crisis of 2008 offers a decidedly negative verdict on a decades-long project to deregulate financial markets and rely on private ordering mechanisms, including securitization and default swaps, to mitigate opportunistic behavior and improve market efficiency. Although the regulatory approach of the past two decades, which relied in great measure on private parties fending for themselves, helped to generate a number of innovations and positive developments in finance, it ultimately failed to bring about more resilient financial markets. The market for mortgage securitizations found itself subject to adverse selection biases leading to a lemons market for asset-backed securities. At the same time, developments in derivative markets made it possible for central actors there to engage in more risk (moral hazard) than was optimal. Ultimately, parties that should have engaged in private ordering did not. As a consequence, we are left struggling for a new regulatory path forward that recognizes that market participants are human agents subject to the frailties of cognitive limitations, euphoria and perhaps even the occasional self-delusion. What is required is a close examination of the institutional and micro incentives (including incentives of agents) in order to strike a balance between market-based regulation and a more interventionist approach to regulating markets. A more pragmatic approach to market regulation recognizes that the earlier hands-off approach to regulation resulted in over-reliance on weak heuristics and little by way of robust private ordering. A new more pragmatic vision of market regulation will likely stop short of legislating against bubbles, but could, and should, result in less systemic risk and a more sustainable growth trajectory going forward

Asset Specificity and Transaction Structures: A Case Study of @Home Corporation

This is a case study of asset specific investments, a class of transactions that is well understood in the context of economic theory but that is under-analyzed empirically. Because specific investments are particular to a single location, use or customer, their next best use is of much lower value than the use for which they are initially intended. Consequently, asset specific investments face the threat of ex post opportunism and allocative inefficiency. This contracting problem is particularly difficult when firms that are otherwise rivals must coordinate individual investments to create a shared resource. In such cases, generating credible expectations of cooperation among rivals is critical to coordinating these investments. The case of @Home Corporation is an example of how rival cable companies were able to employ “hybrid” structures including contractual safeguards like joint ownership, specialized governance devices and economic lock-in to overcome the problem of asset specificity and then build out a nationwide cable-based online service network during the 1990s. As the market subsequently develope alternatives to @Home, the economic lock-in required to induce cooperation failed to materialize, and @Home collapsed. The ultimate failure of @Home points out that those strategies that provide the proper ex ante incentives many not always be durable, leaving contracting parties with less than perfect options