Thermoelastic Stress Analysis Research Articles

Towards automated characterisation of fatigue damage in composites using thermoelastic stress analysis

Composite materials demonstrate complicated fatigue behaviour due to their microstructure and the varied types of defects that can occur during loading. This necessitates experimentation to determine their performance under loading. In this study an algorithm is introduced for identifying and categorising different defects forming during fatigue tests. Thermoelastic stress analysis was used to obtain high spatial and temporal resolution stress information from the surface of notched composite specimens. Specimens with three different geometries were loaded in tension–tension fatigue to failure. An algorithm was used to identify when and where matrix cracking and delaminations formed within the specimens as well as quantify how this changed over time. By improving how damage events are identified and characterised, the algorithm reduces the amount of time needed to process experimental fatigue data and helps to provide greater understanding of fatigue processes in new materials from early small-scale cracking all the way to final failure.

TSA and FEM Analysis Applied to CAD/CAM Titanium All-on-Four Prosthesis

Patient with fully edentulous maxillaries often suffer of several disease as an alteration of facial appearance, difficulties in eating, speech, and chews. Fixed and removable prosthesis can be a solution to rehabilitate this issue. The All-on-Four protocol is one of the possible treatment modalities for implant-supported prosthetics. It consists of placing four implants in an intermorainal position, two perpendicular to the occlusal plane and two tilted distally at a 45° angle. This paper aims to assess the stress distribution on an All-on-Four titanium prosthesis by means of a Thermoelastic Stress Analysis (TSA). This full-field non-contact measurement technique allows, through a thermal imaging camera, to observe how the surface temperature of an object changes under the effect of cyclic load. The principle on which thermoelastic analysis is based is the existence of a relationship between deformation, hence applied stress, and temperature change, called the thermoelastic effect. To perform the tests, the prosthesis was fixed to a resin jaw cast that has similar characteristics to the human bone, so that the same damping that would be present under normal working conditions could be accounted for. The distal artificial molar tooth was kept in contact to a narrow screw, which was attached to a Medium-Force (M-Series) Air-Cooled Shakers made by Santek. This shaker generates a sinusoidal load of 800N with a load cell at 10 Hz frequency, which can maintain the temperature variation of the prosthesis constant. A pre-load was applied before starting the tests. To see the change in temperature a Flir A6751SC thermal camera was used, and videos were acquired with a frame rate of 125 Hz and a resolution of 640x512 pixels. From the experimental TSA tests, the trend of the experimental stress concentration on the titanium specimen was detected. The outcomes of the experimental tests were compared to a 3D prosthesis model by Finite Element Analysis (FEM).

Fiducial marker and blob detection-based motion compensation algorithm for Thermoelastic Stress Analysis measurements

Non-contact techniques for measuring stress and strain on structures offer advantages including preservation of structural integrity, remote and continuous monitoring, full-field measurement capability, non-destructive testing, and adaptability to various materials. Among them, thermoelastic stress analysis is one of the most established non-contact techniques to measures stress distributions in structures by analysing temperature-induced deformations. This research proposes a methodology for the compensation of the edge effect, a undesirable phenomenon resulting from rigid displacements in the analysed structure when using thermoelasticity approach. These displacements give rise to virtual stress distributions along the sample’s edges. For this purpose, a fiducial marker was used to track rigid displacements in thermal images, that were then processed using a blob detection algorithm, for compensating for edge effects under different loading conditions. The algorithm was capable of measuring the displacement of a sample at different frequencies (5-25Hz) and distances from the target (300-600 mm), with the lowest uncertainty of 0.2 mm. The impact of compensation was found The impact of the compensation was found to be significant, especially at frequencies lower than 10 Hz, where the displacement was the highest.

Industrial application of a low-cost structural health monitoring system in large-scale airframe tests

A small, novel, integrated SHM system has been deployed during full-scale testing of a wing fatigue test for several weeks and a fuselage pressurisation test for several months. Complementary NDE measurement techniques were combined, with inputs from visible and infrared optical sensors, as well as resistance strain gauges. Sensor units were deployed at regions of interest and integrated board computers permitted near real-time data processing. The outputs were full-field measurement datasets from digital image correlation and thermoelastic stress analysis systems. Changes in these datasets in the regions of interest were successfully quantified using orthogonal decomposition and were indicative of changes in the condition of the structure. The results from these case studies demonstrate that this system can be successfully deployed in spatially restricted areas within airframe structures to monitor crack growth. The low cost and small footprint of the system presents the opportunity for installation of arrays of similar sensors for both test and in-service data collection. Near real-time data processing would allow timely reporting to service engineers, informing maintenance or operational decisions.

Stress ratio effect on fatigue crack growth assessment via thermoelastic stress analysis

The fatigue performance of welded joint is strongly dependent on welding residual stresses as well as the local geometry of the weld toes. To ensure the structural integrity over time of such components, fine predictive models must be proposed together with efficient experimental methods for their assessment.This study introduces a model based on linear elastic fracture mechanics to forecast fatigue life in service of high strength steel welded joints. This model considers the effect of the local stress ratio (dependent on stabilized residual stresses) on the crack growth rate through the definition of an effective stress range. The use of Thermoelastic Stress Analysis (TSA) is used to perform in-situ monitoring of fatigue cracks on welded joints to experimentally characterize the crack-closure phenomenon at different load ratios. Indeed, under adiabatic conditions, the temperature's first harmonic at the surface of a structure submitted to cyclic loading is proportional to the stress tensor's first invariant's amplitude. As a result, the presence of a surface crack and the non-linearity induced by the alterative opening and closing within one loading cycle can be observed in the immediate temperature response. This phenomenon is used to evaluate a crack-opening rate that is related to the fatigue life of the welded joints.

An improved crack tip location algorithm using the principles of thermoelastic stress analysis

AbstractA novel method of determining the crack tip location from a thermoelastic quadrature signal is presented. The method is utilised for crack tip locations within complex stress fields, namely within fastened aircraft lap joints. Coupled structural-thermal finite element modelling was undertaken to investigate the thermal response field around the crack tip location and develop the algorithmic principles. Experimental validation of the crack tip location was conducted using established crack mouth compliance techniques and optical measurements. The crack tip finding algorithm used the location of the maximum spatial gradient of the thermal field in the direction of crack growth. The method showed good accuracy when compared to traditional methods. Resultant crack growth rates were further verified using quantitative fractography.

Thermoelastic Stress Analysis in the Presence of Biaxial Stresses in Titanium: Effect of the Mean Stress on Errors in Stress Evaluation

BackgroundThermoelastic Stress Analysis (TSA) is a contactless technique capable of estimating superficial stresses on components subjected to dynamic loads. Surface stresses can be obtained by means of calibration methods that require the assessment of the thermoelastic constants. However, these methods can lead to errors in stress evaluation in those materials where the effect of the mean stress on the thermoelastic signal cannot be neglected (e.g., titanium).ObjectiveThe aim is the development of an analytic formulation of error made in first stress invariant amplitude evaluation for a biaxial stress state, when neglecting mean stress effect.MethodsBy considering the general theory of thermoelastic stress analysis accounting for the mean stress effect, the formulation of the thermoelastic effect in the presence of a biaxial stress state was obtained. The results were compared to those obtained by a numerical simulation and the proposed formulation has been validated for titanium by means of experimental tests.ResultsFirstly, the new formulation of thermoelastic temperature variations accounting for mean stress effect in presence of a biaxial stress state was provided. Secondly, an error analysis provided an analytical formulation for the error made in case mean stress effect is neglected for different case studies.ConclusionsThe error in stress evaluation can be considered as the error originating from the use of an incorrect calibration formula (traditional one)”. The new analytical formulation accounting for the general theory of thermoelastic stress analysis allows to account for the mean stress on titanium in the presence of a uniaxial and biaxial stress states and to evaluate the error made in neglecting such a second order effect when using TSA.

Study of Effective Stress Intensity Factor through the CJP Model Using Full-Field Experimental Data.

In this work, the Christopher-James-Patterson crack tip field model is used to infer and assess the effective stress intensity factor ranges measured from thermoelastic and digital image correlation data. The effective stress intensity factor range obtained via the Christopher-James-Patterson model, which provides an effective rationalization of fatigue crack growth rates, is separated into two components representing the elastic and retardation components to assess shielding phenomena on growing fatigue cracks. For this analysis, fatigue crack growth tests were performed on Compact-Tension specimens manufactured in pure grade 2 titanium for different stress ratio levels, and digital image correlation and thermoelastic measurements were made for different crack lengths. A good agreement (~2% average deviation) was found between the results obtained via thermoelastic stress analysis and digital image correlation indicating the validity of the Christopher-James-Patterson model to investigate phenomena in fracture mechanics where plasticity plays an important role. The results show the importance of considering crack-shielding effects using the Christopher-James-Patterson model beyond considering an exclusive crack closure influence.

Quantitative Full-Field Data Fusion for Evaluation of Complex Structures

BackgroundValidation of models using full-field experimental techniques traditionally rely on local data comparisons. At present, typically selected data fields are used such as local maxima or selected line plots. Here a new approach is proposed called full-field data fusion (FFDF) that utilises the entire image, ensuring the fidelity of the techniques are fully exploited. FFDF has the potential to provide a direct means of assessing design modifications and material choices.ObjectiveA FFDF methodology is defined that has the ability to combine data from a variety of experimental and numerical sources to enable quantitative comparisons and validations as well as create new parameters to assess material and structural performance. A section of a wind turbine blade (WTB) substructure of complex composite construction is used as a demonstrator for the methodology.MethodsThe experimental data are obtained using the full-field experimental techniques of Digital Image Correlation (DIC) and Thermoelastic Stress Analysis (TSA), which are then fused with each other, and with predictions made using Finite Element Analysis (FEA). In addition, the FFDF method enables a new high-fidelity validation technique for FEA utilising a precise full-field point by point similarity assessment with the experimental data, based on the fused data sets and metrics.ResultsIt is shown that inaccuracies introduced because of estimation of comparable locations in the data sets are eliminated, The FFDF also enables inaccuracies in the experimental data to be mutually assessed at the same scale regardless of differences in camera sensors. For example, the effect of processing parameters in DIC such as subset size and strain window can be assessed through similarity assessment with the TSA.ConclusionsThe FFDF methodology offers a means for comparing different design configurations and material choices for complex composite substructures, as well as quantitative validation of numerical models, which may ultimately reduce dependence on expensive and time-consuming full-scale tests.

Towards assessment of fatigue damage in composite laminates using thermoelastic stress analysis

A new approach that utilizes Thermoelastic Stress Analysis (TSA) is proposed to investigate fatigue-induced material degradation in laminated fiber-reinforced polymer composites (FRP). The proposed model accounts for non-adiabatic conditions, the effects of the material temperature on the material properties, and the effects of stiffness material degradation due to damage. Experimental data from the literature is used to validate the part of the model that simulates the heat transfer, which results in a non-adiabatic contribution to the thermoelastic response. Specimens made from E-glass FRP representative of those used in wind turbine blade manufacture are used in the study, which make a challenging proposition for TSA. The evolution of tunneling cracks caused by cyclic loading causes stiffness degradation and changes in the thermoelastic response. The added features of the proposed model are shown to be necessary to interpret the thermoelastic response. The model improves correspondence with experimental data compared to previous TSA methods. Hence a generalized framework is proposed for incorporating the mechanisms that affect the thermoelastic response as materials degrade due to fatigue loading.

Temperature patterns obtained in thermoelastic stress test at different frequencies, a FEM approach

PurposeThe goal of this work is to create a computational finite element model to perform thermoelastic stress analysis (TSA) with the usage of a non-ideal load frequency, containing the effects of the material thermal properties.Design/methodology/approachThroughout this document, the methodology of the model is presented first, followed by the procedure and results. The last part is reserved to results, discussion and conclusions.FindingsThis work had the main goal to create a model to perform TSA with the usage of non-ideal loading frequencies, considering the materials’ thermal properties. Loading frequencies out of the ideal range were applied and the model showed capable of good results. The created model reproduced acceptably the TSA, with the desired conditions.Originality/valueThis work creates a model to perform TSA with the usage of non-ideal loading frequencies, considering the materials’ thermal properties.

A modified model for thermoelastic stress analysis under low frequency fatigue loading

Thermoelastic stress analysis (TSA) can be used to determine the stress field of structures or materials under cyclic loadings through temperature under adiabatic conditions. So high-frequency cyclic loadings are considered as a fundamental condition. However, loading frequencies in many practical applications do not meet the adiabatic conditions. In order to improve the accuracy of TSA under low-frequency fatigue loadings, the non-adiabatic response in TSA was derived by combining the classical thermoelastic equation and heat conduction equation. This response was used to create a modified model that accounted for heat transfer and temperature distribution under low-frequency loading conditions. Otherwise, fatigue test and finite element method were used to evaluate the correctness of the modified model using epoxy resin material. The results showed that the temperature change obtained from the modified model were in good agreement with the experiment results. The modified model can effectively evaluate the structural stress under low-frequency fatigue loadings.

A developed hybrid experimental–analytical method for thermal stress analysis of a deep U-notched plate

A general framework of hybridizing experimentally-recorded load-induced thermal information by means of thermoelastic stress analysis (TSA) of a loaded structure with the Michell solution of Airy stress function in isotropic linear elasticity is proposed for in-plane stresses determination. The capability of the proposed hybrid method in separating the TSA signals into individual stresses is demonstrated by stress-analyzing a deep U-notched aluminum plate without neither knowing the entire geometry and distant loading/boundary conditions nor using supplementary experimental information. Even though no experimental data were processed at, and adjacent to, the edges of the U-notched plate, the individual stresses are determined throughout the surface of the plate. Prior processing actual experimental TSA data, the accuracy and the stability of the proposed hybrid method through adding artificial noise scatters in the processed simulated data with the number of retained terms in the generalized Airy stress expressions are firstly investigated. The effects of the superimposed noise scatters as well as the number of employed data in the evaluated stresses are also assessed. Finally, the reliability of the determined hybrid-experimental stresses is supported through a comparison with finite-element predictions and strain gauge measurements. The ease of implementation, the accuracy and the precision of the evaluated stress, and the versatility to the type of employed data make the proposed hybrid method more attractive and superior than other hybrid methods.

Use of infrared thermography to model the effective stress ratio effect on fatigue crack growth in welded T-joints

Welded joints constitute critical components in the high-cycle fatigue dimensioning of naval structures. In particular, the local geometry of the weld toe as well as the welding process induced residual stress strongly impact the fatigue properties. In that regard, such factors must be considered in a dedicated fatigue life forecast model, identified via an adapted experimental protocol. On the basis of X-ray stress analysis and in-situ crack growth monitoring by thermoelastic stress analysis, the present study introduces a linear elastic fracture mechanics model that considers the initial residual stress field in the vicinity of the weld toe.

Temperature Patterns in TSA for Different Frequencies and Material Properties: A FEM Approach

Thermography techniques are gaining popularity in structural integrity monitoring and analysis of mechanical systems’ behavior because they are contactless, non-intrusive, rapidly deployable, applicable to structures under harsh environments, and can be performed on-site. More so, the use of image optical techniques has grown quickly over the past several decades due to the progress in the digital camera, infrared camera, and computational power. This work focuses on thermoelastic stress analysis (TSA), and its main goal was to create a computational model based on the finite element method that simulates this technique, to evaluate and quantify how the changes in material properties, including orthotropic, affect the results of the stresses obtained with TSA. The numeric simulations were performed for two samples, compact and single lap joints. when comparing the numeric model developed with previous laboratory tests, the results showed a good representation of the stress test for both samples. The created model is applicable to various materials, including fiber-reinforced composites. This work also highlights the need to perform laboratory tests using anisotropic materials to better understand the TSA potential and improve the developed models.

Inspection of Wind Turbine Blades Based on Modified Thermoelastic Stress Analysis Using Thermal Detection of Infrared Radiation

Inspection of Wind Turbine Blades Based on Modified Thermoelastic Stress Analysis Using Thermal Detection of Infrared Radiation

Thermoelastic Stress Measurement Using SVD Thermo-Component Analysis

BackgroundIn self-correlation lock-in thermography for thermoelastic stress analysis (TSA), the acquisition position of the reference signal affects the accuracy of the obtained stress amplitude distribution. When the reference signal is not large enough compared to the noise, the stress amplitude distribution may be incorrect.ObjectiveThis study proposes a method that does not require a reference signal and frequency analysis to obtain the stress amplitude distribution with comparable or higher accuracy than that obtained using self-correlation lock-in thermography.MethodsAn observation matrix is generated from the temporal variation across all thermographic pixels to describe the thermal fluctuations due to stress. Thereafter, stress amplitude distribution and the original load signal are extracted from the observation matrix using singular value decomposition (SVD). The proposed method is called SVD thermo-component analysis. To investigate the effectiveness of the proposed method, the reconstructed load signal and stress distribution are obtained from the captured thermal images for the specimen under a sinusoidal load.ResultsThe stress amplitude distribution obtained using the proposed method is equivalent to that obtained using conventional lock-in thermography with the original load signal as the reference signal. In addition, the reconstructed load signal obtained using the proposed method successfully represents the original load signal.ConclusionsSVD thermo-component analysis does not require prior knowlege of the evaluated mechanical structure to select a suitable reference-signal acquisition position as in self-correlation lock-in thermography. Therefore, the proposed TSA method reduce analysis failures compared to the conventional method.

Thermometric investigations for the characterization of fatigue crack initiation and propagation in Wire and Arc Additively Manufactured parts with as‐built surfaces

AbstractMetal Additive Manufacturing (AM) allows for the fabrication of complex shapes with high added value at low costs. Indeed, as‐built structures are near net shape: they require few to no finishing operations. However, as‐built AM parts present significant roughness caused by the layer discretization. In the case of the Wire and Arc Additive Manufacturing (WAAM) process, used for large‐scale structures, the as‐built roughness is estimated to several hundreds of micrometers. For complex geometries, a complete machining of the surfaces is not necessarily possible. In this study, an experimental method is proposed, relying on thermoelastic stress analysis, to characterize the effect of as‐built WAAM surface roughness on high‐cycle fatigue properties. Using an infrared camera, multiple cracks can be detected and monitored over a large surface on rough WAAM samples under cyclic bending. The collected data constitutes valuable information for the identification of a fatigue model dedicated to as‐built WAAM structures.

A thermal emissions-based real-time monitoring system for in situ detection of fatigue cracks

The advent of packaged infrared (IR) bolometers has led to thermography-based techniques becoming popular for non-destructive evaluation of aerospace structures. In this work, a real-time monitoring system for in situ crack detection has been presented which uses an original equipment manufacturer microbolometer. The system costs one-tenth the price of a packaged bolometer and has the potential to transform the use of IR imaging for condition and structural health monitoring in the aerospace industry and elsewhere. A computer, consisting of a single circuit board with dimensions comparable to a credit card, has been integrated into the system for real-time, on-board data processing. Crack detection has been performed based on the principles of thermoelastic stress analysis (TSA). Proof-of-concept laboratory tests were performed on open-hole aluminium specimens to compare the performance of the proposed system against a state-of-the-art cooled IR photovoltaic effect detector. It was demonstrated that cracks as small as 1 mm in length can be detected with loading frequencies as low as 0.3 Hz. This represents a significant advance in the viability of TSA-based crack detection in large-scale structural tests where loading frequencies are usually lower than 1 Hz.

Quantitative microbolometer-based thermoelastic stress analysis

The use of microbolometers for Thermoelastic Stress Analysis (TSA) has the potential of substantially reducing deployment costs, as well as providing more compact and lightweight solutions in comparison to more costly, bulky and less rugged photon detector based infrared cameras. However, microbolometer performance in terms of response time and sensitivity is less than what can be achieved with photon detectors. As the use of microbolometers for TSA is becoming more widespread, it is timely to devise a means of assessing microbolometer performance and identifying the limitations of using such a system for TSA. A simulation approach is devised that enables the effect of the microbolometer response time on transient signals (such as that from TSA) to be investigated. As the TSA involves measuring a small transient temperature change, usually induced by sinusoidal cyclic loading, the simulation approach also includes the effect of a noisy non-sinusoidal signal. To identify any deleterious effects cause by a cyclic load, the transient temperature change is also generated using an optical chopper and a black body source. The variables studied are the Signal-to-Noise Ratio (SNR), the impact of different waveforms, variation in the signal mean, as well as in-built system features such as the noise reduction. A new experimentally validated calibration approach is presented that accounts for the microbolometer response time and rolling averages introduced by inbuilt noise reduction features. It is shown the calibration approach is independent of specimen material and camera frame rates, and it is demonstrated on a CFRP test specimen containing a hidden crack.