Thermoelastic Stress Analysis (TSA)

Video 1 – Measurent Procedure using the ThermoESA sensor & MiTE Suite software

Thermoelastic Stress Analysis (TSA) is an optical, non-contact measurement technique, meaning it requires no mechanical connection to the test object surface. TSA employs a thermal imaging sensor with an advanced image processing algorithm to convert thermo-elastically induced temperature changes into an image of surface stress.

Figure 1 – Thermoelastic Stress Analysis (TSA) measurement of the stress concentration of a strut joint under compression (left) and aircraft wing skin under a bending load, whereby the isolated stress concentrations are presented in white/red.
Figure 1 – Thermoelastic Stress Analysis (TSA) measurement of the stress concentration of a strut joint under compression (left) and aircraft wing skin under a bending load, whereby the isolated stress concentrations are presented in white/red.

TSA has four major advantages compared to other Experimental Measurement Techniques, including;

  1. High Sensitivity In-Situ Measurement Capability
  2. Optical, Full-Field Measurements Technqiue
  3. Minor Surface Preparation
  4. Easy Finite Element Model Validation
Figure 2 – Comparison of Experimental Measurement Technqiues for Thermoelastic Stress Analysis (TSA)
Figure 2 – Comparison of Experimental Measurement Technqiues

TSA provides in-situ (semi, real-time) measurements of (progressively improving) stress images. Typically, after several minutes the sensor has achieved a thermal sensitivity sufficient to generate a high-fidelity stress field image (down to 1 millikelvin), akin to a Finite Element model simulation (depending on the load amplitude). Generally, the image quality improves as a function of the square-root of the observation time, e.g. if the processing time is quadrupled, the signal-to-noise (SNR) ratio of the measurement improves by a factor of two.

TSA- Effect of Time Analysis
Figure 3 – Effect of analysis time on measurement quality for a uniaxially loaded aluminium alloy plate (hole diameter is 20 mm) inspected under 5Hz sinusoidal loading using a microbolometer. Scale normalised to peak tensile thermoelastic response. Far field stress amplitude is 17 MPa.
Video 2 – Measurement Procedure using the ThermoESA sensor & MiTE Suite software showing the visual overlay functionality.

TSA requires a non-reflective (high-emissivity) surface for the generation of measurements, since high-emissivity improves thermal exitance and reduces reflection. Some objects require no supplementary coating, e.g. primed aircraft components, and 3D printed plastic (i.e. Nylon PA12) Compared to DIC, which requires a stochastic speckle pattern to be applied to the surface, TSA only requires minor surface preparation (material-surface dependant).

Primed airframe structure can be inspected without additional coating
Figure 4 – Primed airframe structure can be inspected without additional coating
FEM numerical vs. TSA experimental model validation showing near indistinguishable results
Figure 5 – FEM numerical vs. TSA experimental model validation showing near indistinguishable results

Since TSA only measures temperature change using one IR-camera, it can be applied to structural features of almost arbitrary complexity. This gives it a unique advantage compared to DIC, which requires the projection to be calibrated in two or more cameras, in order to retrieve 3D-strain. For TSA, the measured equivalent change in surface principle stress that allow FEA models to be easily refined and validated. Conversely to other experimental measurement techniques, that typically provide displacement and strain information, TSA provides stress information, which is better reflected in FEA modelling.  TSA sensors can produce stress images that can be easily mistaken for a numerical simulation (but without all the uncertainty!)

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