Sub-surface damage location and identification using infra-red techniques

The paper presents a new methodology for identifying sub-surface damage in composite components using a combination of Pulse Phase Thermography (PPT) and Thermoelastic Stress Analysis (TSA)

Thermoelastic investigations for fatigue life assessment

An investigation is presented on the suitability and accuracy of a thermoelastic technique for the analysis of fatigue cracks. The stress intensity factor ranges ΔKI and ΔKII are determined from thermoelastic data recorded from around the tip of a sharp slot in a steel specimen under biaxial load, in order to assess the accuracy of the technique. ΔKI and ΔK II are determined to within 4% and 9% of a theoretical prediction, respectively. The results from a similar test on a fatigue crack under biaxial load are also presented. These show that thermoelastic stress analysis is a rapid and accurate way of analyzing mixed-mode fatigue cracks. A discussion is given on the potential of thermoelastic stress analysis of propagating cracks

Assessment of foam fracture in sandwich beams using thermoelastic stress analysis

Thermoelastic Stress Analysis (TSA) has been well established for determining crack-tip parameters in metallic materials. This paper examines its ability to determine accurately the crack-tip parameters for PVC foam used in sandwich structures

T-stress determination using thermoelastic stress analysis

T-stress and mixed-mode stress intensity factors have been determined experimentally using thermoelastic stress analysis and using a finite element method. Pure mode I, strong mixed-mode I and II, and interacting cracks have been used as the case studies. A new technique has been proposed to identify the crack tip from thermoelastic images. It has also been shown that using three terms of Williams's stress field formulation to determine the T-stress, yields a more accurate solution than using only the first two terms of the expansion

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

A review of residual stress analysis using thermoelastic techniques

Thermoelastic Stress Analysis (TSA) is a full-field technique for experimental stress analysis that is based on infra-red thermography. The technique has proved to be extremely effective for studying elastic stress fields and is now well established. It is based on the measurement of the temperature change that occurs as a result of a stress change. As residual stress is essentially a mean stress it is accepted that the linear form of the TSA relationship cannot be used to evaluate residual stresses. However, there are situations where this linear relationship is not valid or departures in material properties due to manufacturing procedures have enabled evaluations of residual stresses. The purpose of this paper is to review the current status of using a TSA based approach for the evaluation of residual stresses and to provide some examples of where promising results have been obtained

Full-scale performance assessment of aircraft secondary sandwich structure using thermoelastic stress analysis

The use of resin film infusion (RFI) has been proven to reduce the cost of production of aircraft secondary sandwich structure. In this paper thermoelastic stress analysis (TSA) is used to assess the performance of full scale aircraft sandwich structure panels produced using both the conventional autoclave process and RFI. Finite element (FE) models of both panel types are developed and TSA is used to validate the models

A stress free model for residual stress assessment using thermoelastic stress analysis

Thermoelastic Stress Analysis (TSA) has been proposed as a method of obtaining residual stresses. The results of a preliminary study demonstrated that when Al-2024 plate containing holes that were plastically deformed by cold expansion process to 2% and 4% strain the thermoelastic response in the material around the hole was different to that obtained from a plate that had not experienced any plastic cold expansion (i.e. a reference specimen). This observation provides an opportunity for obtaining residual stresses based on TSA data. In many applications a reference specimen (i.e. residual stress free specimen) may not be available for comparison, so a synthetic, digital bitmap has been proposed as an alternative. An elastic finite element model is created using commercially available software Abaqus/Standard and the resultant stress field is extracted. The simulated stress field from the model is mapped onto a grid that matches the TSA pixel data from a physical reference specimen. This stress field is then converted to a ?T/T field that can be compared to the full-field TSA data. When the reference experimental data is subtracted from the, bitmap dataset the resultant ?T/T field is approximately zero. Further work proposes replacing the experimental reference data with that from specimens that have undergone cold expansion with the aim of revealing the regions affected by residual stress through a departure from zero in the resultant stress field. The paper demonstrates the first steps necessary for deriving the residual stresses from a general specimen using TSA

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

SIF Determination with Thermoelastic Stress Analysis

AbstractThis work is focused on the experimental determination of the stress intensity factor (SIF) using thermoelastic stress analysis (TSA) for a compact tension specimen during a fatigue crack growth test. A comparison of the stress field obtained with computational modelling, finite element method, against the experimental data obtained with the thermoelastic stress analysis under mode I loading in a fatigue test is presented. The stress field in front of the crack tip obtained with TSA, was used in William’s expansion, together with an overdetermined algorithm to calculate the SIF under mode I loading. The proposed methodology has a hybrid experimental-numerical nature where the stress intensity factor determination depends on a stress field obtained with an optical technique, TSA. The soundness of the experimentally obtained SIF solution was validated through finite element method computations