Thermoelastic stress analysis with a compact low-cost microbolometer system

Short carbon fiber composite materials are receiving a lot of attentions because of their excellent moldability and productivity, however they show complicated behaviors in fatigue fracture due to the random fibers orientation. In this study, thermoelastic stress analysis (TSA) using an infrared thermography was applied to the evaluation of fatigue damage in short carbon fiber composites. Second harmonic component of thermoelastic temperature change that is obtained by lock-in processing based on double-frequency against loading frequency was conducted to identify the turbulence in thermoelastic waveform due to fatigue damage evolution. It was found that the portions showing high second harmonic component values coincided with the portions where delamination damages were detected.KeywordsNondestructive evaluationThermoelastic stress analysisSecond harmonic analysisInfrared cameraShort carbon fiber reinforced plastics

Bolted connections are prevalently used to fasten separate mechanical or structural members together. Lack of knowledge of the contact stresses at the bolt-plate interface can make it difficult to stress analyze bolted connections theoretically or numerically. Thermoelastic stress analysis (TSA) is therefore utilized to stress analyze a plane-stressed pin-loaded plate. The approach involves processing the recorded temperatures gradients throughout the plate with an Airy stress function, plus imposing the traction-free conditions discretely on the non-contacting edge of the hole and on the external boundaries of the plate. In addition to agreeing with the frequently assumed interface contact stresses in mechanical connections having zero bolt-hole clearance, the TSA stresses satisfy force equilibrium and are correlated with FEM results. TSA is convenient in that the technique provides the individual components of stress full-field without necessitating any photoelastic model or surface pattern or ruling. Unlike with moiré, holography, speckle or digital image correlation, TSA necessitates neither knowing the elastic constitutive properties nor differentiating the recorded information.KeywordsThermoelastic Stress Analysis (TSA)Bolt-hole ClearanceTraction-free Boundary ConditionsDigital Image CorrelationRadial Contact StressThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

This study focused on the application of the Thermoelastic Stress Analysis (TSA) technique as a tool for assessing the residual stress state of structures. TSA is based on the fact that materials experience small temperature changes when compressed or expanded. When a structure is cyclically loaded, a surface temperature profile results which correlates to the surface stresses. The cyclic surface temperature is measured with an infrared camera. Traditionally, the amplitude of a TSA signal was theoretically defined to be linearly dependent on the cyclic stress amplitude. Recent studies have established that the temperature response is also dependent on the cyclic mean stress (i.e., the static stress state of the structure). In a previous study by the authors, it was shown that mean stresses significantly influenced the TSA results for titanium and nickel based alloys. This study continued the effort of accurate direct measurements of the mean stress effect by implementing newly developed temperature correction curves. In addition, a more in-depth analysis was conducted which involved analyzing the second harmonic of the temperature response. By obtaining the amplitudes of the first and second harmonics, the stress amplitude and the mean stress at a given point on a structure subjected to a cyclic load can be simultaneously obtained. The experimental results showed good agreement with the theoretical predictions for both the first and second harmonics of the temperature response. As a result, confidence was achieved concerning the ability to simultaneously obtain values for the static stress state as well as the cyclic stress amplitude of structures subjected to cyclic loads using the TSA technique. With continued research, it is now feasible to establish a protocol that would enable the monitoring of residual stresses in structures utilizing the TSA technique.

Thermoelastic Stress Analysis (TSA) is one of the very few methods allowing the determination of a continuous stress distribution on the object’s surface under variable loading conditions. Such results provide a lot of valuable information in the field of technical condition assessment and residual life prediction. In order to improve the accuracy of the TSA, the Lock-In signal processing method is implemented. This research is aimed at verifying the effectiveness of this improvement and determining the TSA stress detection threshold, as it is important information in terms of the applicability of this method in the low-stress conditions encountered in considerations of fatigue of load-carrying structures. A steel sample with a centrally located hole was subjected to cyclic loads to determine the threshold of stress detection and accuracy of TSA. As a result of the research, the relationship between the magnitude of stress excitations and the underestimation of the measured stresses was developed. Based on the conducted investigations, it was concluded that reasonable TSA results can be acquired for excitations that induce a temperature response above 10 mK (0.5 NEDT). The presented field test example proves that in industrial applications reasonable results can be acquired for thermal responses below the NEDT of the IR camera. It was concluded that it is possible to successfully implement TSA in low-stress applications (temperature response below NEDT).

The emergence recently of a thermoelastic stress analysis (TSA) capability exploiting low-cost, compact and rugged microbolometer detector technology provides a significant opportunity topromote a broader use of this powerful non-contact full-field stress analysis technique. An area whereit has considerable and hitherto unexplored potential is in in-situ structural health monitoring (SHM).The present paper outlines the case for a nexus between SHMand TSA in this new form. It is proposedthat the approach should yield diagnostic and prognostic capabilities surpassing those of some existingSHM modalities. An F/A-18 centre-fuselage full-scale structural fatigue test is employed as a casestudy to illustrate the practical feasibility of the approach and to underscore some of its potential.Although the case study focuses on an aircraft structure, the concept has potential application to awide variety of different engineering assets across the aerospace, civil and maritime sectors.

Thermoelastic stress analysis (TSA) is a non-contact and full-field type method of experimental stress analysis. Since TSA provides only the sum of the principal stresses, many studies have been carried out for developing techniques of stress separation. Unfortunately, most of the techniques developed hitherto are rather complicated and require much labor in practical application. Recently, the authors have developed a simple technique by utilizing the nonlinearity of the thermoelastic effect. In this technique, principal stresses are determined from two sets of temperature amplitude data measured under sinusoidal loadings of same amplitude and different mean values. However, the difference between temperature amplitudes obtained at different mean loadings is very small and, therefore, the result of stress separation becomes very noisy. In this paper, several smoothing techniques are applied to the temperature amplitude data. It is found that some simple smoothing techniques are effective for improving the accuracy of the stress separation.

Editorial

Self-expanding stents are small medical devices used to treat vascular disease and are typically fabricated from a super-elastic, shape memory alloy known as Nitinol and have a fine mesh structure. This paper describes preliminary work on the application of Thermoelastic Stress Analysis (TSA) to Nitinol stents. Uniaxial tensile tests were conducted on thin tubes of Nitinol to characterise the material mechanical properties. TSA calibration exercises were conducted, which showed that Nitinol exhibits a non-uniform thermoelastic response through its elastic region that corresponded to the superelastic behaviour. Initial TSA demonstrated that a viable thermoelastic signal could be obtained from the stents. In high resolution tests the effect of motion and noise were considerable but it was still possible to obtain a readable thermoelastic signal.

Residual stress can be related to plastic strain experienced by a component; therefore the measurement of plastic strain presents the potential for residual stress to be investigated. Previous work has shown that Thermoelastic Stress Analysis (TSA) can identify regions that have undergone plastic strain. To do this, it was necessary to manufacture a test specimen of identical geometry containing zero plastic strain known as a reference specimen. Identifying the regions that have undergone plastic strain is then a simple matter of subtracting two data sets. This approach assumes there is always a reference specimen available, which in an industrial context is not the case. To make the work applicable to in-service components it is necessary to create a simulated reference specimen. The paper presents a means of establishing the simulated TSA reference specimen using Finite Element (FE) modelling through building a linear, elastic model and then including effects of nonlinear plasticity. To validate the idea a welded mock-up is used that contains a known level of plastic strain alongside a strain free specimen of identical material and geometry to the mock-up that has been manufactured to shape using water jet cutting. The work in the paper describes the outcome of using both the experimental and simulated reference specimens and considers the effect of varying material properties.

The performance of any component to a great extent is determined by residual stress, resulting from virtually all manufacturing processes. Therefore the problem of residual stress detection and measurement is urgent in modern engineering, mechanics and material physics. A good deal of effort has been undertaken in developing different methods for the residual stress detection. Optical, ultrasonic, magnetic methods, Raman spectroscopy, X-ray and neutron diffraction are effectively applied for detection of residual stresses in many cases. Thermoelastic stress analysis (TSA) is also now a well-known experimental technique (Pitarresi and Patterson [1]) providing information on the surface stress fields in structures. Two common systems based on TSA are developed at present. Stress pattern analysis by measurement of thermal emission (SPATE) has a single detector, while DELTATHERM® systems have a staring array of detectors. These techniques have been already implemented effectively for the residual stress detection. It was experimentally demonstrated that mechanical stresses modify the thermal emission signal. Hole drilling and compliance methods are also actively investigated at present for residual stress detection. Recently holographic or speckle interferometry in conjunction the hole-drilling method has attracted serious attention for solution of this problem. Unfortunately, as a rule many of these methods have limited fields of application. For example, Raman spectroscopy is mainly used in science and technology of semiconductors for which the phonon lines have a relatively simple structure and their shifts with stress are well known. SPATE, DELTATHERM® and holographic interferometry are of more general applicability, but have comparatively low spatial resolution.

ABSTRACTQuantitative damage detection in composite single lap joints during fatigue is difficult and not practical with many traditional experimental methods. The objective of this study is to combine a new experimental analysis method with existing stochastic models that is capable of taking the scatter inherent in composite fatigue data into account for better representation of damage evolution in single‐lap joints. This study presents a new quantitative approach where thermoelastic stress analysis (TSA), a remote sensing and non‐destructive evaluation technique, is used for in situ continuous monitoring for fatigue damage detection of Fibre Reinfoced Polymeric (FRP) S2‐glass/E733FR single‐lap joints. Image analysis algorithms are developed to capture the debonding fronts, and Markov Chain models (MCMs) are developed to capture the fatigue damage evolution in composite lap‐joints based on the experimental analysis. The TSA damage index at the last fatigue cycle is used to calibrate the MCMs, and damage predictions using the MCMs are then examined at different fatigue cycles. Methods to construct a predictive stochastic S–N curve utilizing the MCMs are discussed. The proposed IR‐TSA with MCMs is shown to be very effective in predicting the damage evolution and allowed constructing S–N curves that take the variability into account for damage in composite single‐lap joints.

A new approach for evaluating stress intensity factor based on thermoelastic stress analysis

Experimental evaluation of effective stress intensity factor using thermoelastic stress analysis and digital image correlation

Recent innovations in thermal-detector technology are shown to foster the development of an in situ structural health monitoring capability based on thermoelastic stress analysis, a powerful full-field stress-measurement technique that offers an attractive set of diagnostic and prognostic capabilities, some of which are not found in any of the established structural health monitoring modalities. After reviewing these capabilities, the article examines an experimental case study involving the monitoring of an F/A-18 airframe structure subject to full-scale fatigue testing under representative flight spectrum loading spanning nearly one aircraft life time. The case study confirms the viability of the concept and also highlights some of its primary limitations.

Crack growth analysis in welded and non-welded T-joints based on lock-in digital image correlation and thermoelastic stress analysis