Papers by Keyword: Thermoelastic Stress Analysis (TSA)

Abstract: Thermoelastic Stress Analysis (TSA) is a well-established full-field technique for experimental stress analysis that has proved to be extremely effective for studying stress fields in the vicinity of cracks. Recently, work has focused on the observation that the stress-sum contours (isopachics) obtained from TSA take the form of a cardioid. Genetic Algorithms (GAs) and Differential Evolution (DE) have proved successful for accurate parameter estimation of the cardioids, thus allowing the SIFs to be calculated. Originally, some curve-fits indicated that a pure cardioid form is inappropriate for the base model, especially for mixed-mode cracks. The deviation from the cardioid form has been shown to be due to higher-order terms within the stress function. The objective of the current paper is to use a modified version of the original methodology (that fitted parameters to a single isopachic) to find the higher-order parameters from the entire data field obtained from the TSA.

Abstract: The paper describes initial work on using 2D digital image correlation (DIC) and thermoelastic stress analysis (TSA) to obtain data from edge cracks in cross-ply laminates. It is demonstrated that detailed data related to the crack tip stresses can be obtained using TSA. The work reveals some of the limitations experienced when using DIC in applications where high spatial resolution is required. A detailed discussion is provided along with an outline for future work.

Abstract: The thermoelastic response obtained from an infra-red (IR) detector contains two components: the magnitude of the small stress induced temperature change caused by the thermoelastic effect and the phase angle of the temperature change relative to a reference signal generated by an application of a stress change. The phase angle is related to nonlinearity in the thermoelastic response and departures from the simple linear relationship that underpins thermoelastic stress analysis (TSA). The phase data could be used to make an assessment of temperature evolutions caused by viscoelastic behaviour resulting from damage and provide a basis for its evaluation. In the current paper the physics of other infra-red techniques used for non-destructive evaluation is used to better understand the nature of the thermoelastic response. The objective is to provide better exploitation of TSA by alternative processing of the IR measurements. Three case studies are presented that demonstrate the potential of the alternative processing for evaluating damage.

Abstract: The material assumptions made to facilitate Thermoelastic Stress Analysis (TSA) are linear elasticity, material homogeneity and isotropy, and mechanical properties that are independent of temperature. The unusual shape memory and superelastic properties of near equiatomic NiTi alloys complicate the application of any experimental stress analysis technique, and in the case of TSA, make these assumptions invalid. This paper describes a detailed analysis conducted to characterise the material properties of NiTi shape memory alloys and to identify loading conditions suitable for quantitative stress analysis using TSA. The mechanical behaviour of the material in three distinct regions is considered and the suitability of each region for TSA is discussed. It is shown that the thermoelastic response is dependent on the mean stress when tested at room temperature in the pre-martensitic phase, due the presence of an intermediate R-phase. Theoretical calculations are used to confirm that this effect is related to the high temperature dependence of the material’s Young’s modulus.

Abstract: The sensitivity of the thermoelastic response to variations in the fibre volume fraction, resin material and manufacturing route is assessed. To quantify any effects a comprehensive materials testing programme has been conducted to obtain coefficients of thermal expansion, specific heat, density and the elastic properties, which is described in detail in the paper. The work is focused on attempting to ascertain if the source of the response is from the isotropic resin rich layer or from the orthotropic substrate. It is also demonstrated that small variations in material properties can have a significant effect on the calculated thermoelastic response.

Abstract: Thermolastic Stress Analysis (TSA) has been recently developed as a direct investigating method for the study of the stress field around the crack tip of a cyclically loaded structure. The advantage of using measurement techniques based on the thermoelastic effect lays in the fact that stress intensity factors may be determined based on the effective stress distribution around the crack tip rather than calculated from the crack length and amplitude of cyclic loads. This paper reports results related to fatigue tests on Friction Stir Welded alluminium alloys sheets. Fatigue crack propagation experiments were performed by employing single-edge notched specimens, in tensiontension condition with R=0.1, up to failure. The application of TSA allowed the monitoring of crack formation and growth in real time, providing the actual stress distribution around the crack tip for the different technological parameters used in the welding process. Stress intensity factors were determined based on the TSA data and compared to those calculated using an ABAQUS FE model.

Abstract: 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.