The Thermoelastic Stress Analysis (TSA) is a contactless technique based on the thermoelastic effect that consists of the generation of small temperature variations caused by the volume variations induced by stresses applied in the linearelastic range. Recent works demonstrated the capability of the TSA for the characterization of materials behaviour in presence of residual stresses. The use of a general TSA analytical expression allows the researchers to find a relationship between the amplitude of the thermal signal varying at the same frequency as the applied load and the characteristics of the residual stress tensor in terms of principal stresses and their direction. The just said relationship, under certain conditions, can be also affected by the uncertainty in the knowledge of the thermo-physical properties of the material which can enhance or blur the presence of residual stresses. In this work, the effect of the main variables, such as the material properties and the presence of residual stress on the TSA were investigated by applying a sensitivity analysis to the analytical general model. The analytical results were then verified and compared with TSA experimental measurements performed on AA2024 samples affected by biaxial residual stresses and the residual stresses measured with a standard test method.
Adhesively bonded joints represent an interesting alternative to mechanical joints due to the advantages over conventional mechanical fasteners: continuity of the structure, high strength-to-weight ratio, design flexibility. The aim of this work is to assess and predict the quality of aeronautical adhesive bonded CFRP T-joints made by the automated fibre placement process by means of the Thermoelastic Stress analysis (TSA) technique used as non-destructive technique. The results provided by TSA technique, in terms of debonded area, were compared to the well-established lock-in thermography technique showing the capability of TSA to evaluate the quality of T-joints. The approach allows to perform a cost-efficient characterisation process by means of non-destructive evaluations.
The present research is focused on the use of different non-destructive techniques for detecting damage in CFRP composite structures obtained by an innovative technological process: Automated Fiber Placement. The component was a T-joint stringer adhesively bonded to a skin panel. The aim of the present work is to show the capability of these techniques to provide complementary information for detecting the damage in composites. Automated Fibre Placement consists in an automatic deposing of prepeg or dry plies on a specific mould. The innovation lies in the possibility to reduce the time of the manufacturing process of large and complex structures by using a robotic arm that contemporary deposes fibre tows and pre-polymerizes them. The resulting products present higher quality in terms of surface finish, internal flaws absent and higher mechanical properties. The T-joint component tested in the present research was addressed to both static and cyclic tests. After the damage was induced in the material it was performed a qualitative and quantitative study of the damage by using different nondestructive techniques: Thermoelastic stress analysis (TSA), Ultrasound tests (UT) and displacement/strain measurements provided by strain gages. Processing and post-processing procedures were developed to analyze the data from each tests and finally the comparison of the results allowed a complete characterization and an overview of damage in the component by observing specifically where and when it occurred and how many regions of the component were interested. Finally, dimension, shape and depth where assessed.
Infrared Thermography has been successfully used as an experimental, non-destructive, real-time and non-contact technique both to perform non-destructive evaluations and to study the fatigue behaviour of materials. However, the temperature is a very sensitive parameter to the environment conditions such as the thermal heat exchanges. It follows that, heavy and time expensive algorithms have to be setup to accurately filter out the overall ‘disturbing’ heat sources. Otherwise, an in-depth analysis of thermal signal allows the assessment of different indexes related to physical processes of fatigue damage and failure. Theoretical and experimental framework becomes complicated in case the material is a composite due to the layups of lamina or due to the viscous properties of the bulk of the matrix, or due to the pattern described by the yarns or fibers making difficult any quantitative and qualitative analysis. In fact, anisotropy and heterogeneity of composites influences unavoidably the mechanical response of the material to external excitation and the failure mechanisms. In all the cases, the study of thermal heat sources related to dissipative phenomena becomes complicated. Thermal signal analysis provides a localised analysis for assessing qualitatively and quantitatively the state of degradation of material in terms of stiffness or in term of damage detection, by extracting temperature components related to the appearance of plastic zones or cracks or in general to dissipative heat sources. The focus of the present research is to provide an innovative method and algorithm for processing the signal from innovative composites obtained by Automated Fiber Placement process in order to assess the fatigue behaviour and damaged regions qualitatively and quantitatively.
KEYWORDS: Titanium, Signal detection, Metals, Stress analysis, Aluminum, Signal analyzers, Temperature metrology, Infrared cameras, Numerical simulations, Detection theory
The Thermoelastic Stress Analysis (TSA) is a contactless technique able to determine the superficial stress of a component subjected to a dynamic load in a linear elastic field. In these conditions the thermoelastic effect shows the generation of small reversible temperature variations. In this work, a general equation was obtained for the evaluation of the thermoelastic signal. The proposed equation is valid under adiabatic and isentropic conditions, for generalized homogeneous and anisotropic materials in any load condition. By using the proposed generalized equation, TSA sensitivity to the variation of the physical and mechanical material characteristics and TSA sensitivity in the determination of residual stresses were studied. The case studies for performing numerical simulations were represented by AA6082 and Ti6Al4V non-ferrous metals. The results were then compared with the data obtained from experimental tests performed on AA 6082 samples.
Aluminium alloys present some criticalities in terms of fatigue life characterisation due to the absence of a point representing the ‘fatigue limit’, the topic becomes complicated when the material is welded. In this case, the fatigue characterisation lies on design specifications which have to clearly explain the guidelines for the performing the tests and for evaluating the failures, in order to design tailored welded joints. However, the fatigue of welded joints is a difficult subject since the welding process makes the material different, introducing residual tensions, defect, etc. Also, the standard test methods provide only the estimation of the strength at fixed loading cycles but no information on the damage processes occurring in the material. Prompted by these issues researchers deal with the study of other approaches to achieve not only information on fatigue resistance but also damage information. In particular, the thermography can be used for thermal signal analysis of dissipative heat sources involved in fatigue of material undergoing cyclic test. In this paper, this approach is adopted to study the fatigue behavior of friction stir welded joints of AA5754-H111 during specific loading conditions. The component of the temperature related to intrinsic dissipations is assessed and the fatigue strength is evaluated together with a graphical study of the location of damaged areas.
The Friction Stir Welding (FSW) is an innovative solid-state welding method based on frictional and stirring phenomena, discovered and patented by TWI Ltd in 1991, providing high quality components for aerospace, marine and automotive industrial fields. In this process, a rotating non-consumable tool that plunges into the work piece and moves forward produces the heat necessary to weld the parts together. The much lower temperatures compared with those achieved in traditional welding processes by melting, determine the following main advantages of FSW: minimal mechanical distortion, excellent surface finish, absence of splash, no crack formation and porosity after welding, thanks to the low input of total heat. This work deals with the use of thermographic techniques for monitoring the friction stir welding process applied on AA 5754-H111 plates, in order to evaluate the quality of the produced joints in terms of presence of defects and Mechanical strength. The adopted experimental approach was addressed to study and optimizing the FSW process by analyzing the thermographic sequences and extracting several indexes related to the heating involved in the process. Such the indexes, the maximum temperature, the heating and cooling rate of the material, correlated to the frictional power input and the presence of defects respectively, have been investigated for different process parameters (the travel and rotation tool speeds) configurations. The results of the research have been quantitatively supported and characterized by destructive and non-destructive techniques.
One of the most important advantages of using high-diffusivity alloys like aluminium, in industry, is to reduce the weight without renouncing to high strength components. To accelerate the time of the mechanical characterisation, frequently experimental methods based on temperature measurements are adopted, even if in this case, these methods could involve in wrong estimations. In particular, the study of energy dissipations could produce some assessment errors of fatigue limit due to the fact that the fraction of the detected energy dissipated could be lower if compared to the effective energy intrinsically dissipated in the material due to damage. Furthermore, the fatigue life assessment of Aluminium alloys is problematic due to a non-distinct ‘knee’ in the S-N curve. To take into account these issues and to estimate the fatigue strength in rapid and accurate way, in this work, a method providing a specific thermal signal analysis is presented applied to an aluminium alloy 5754 H-111. Firstly, the well-known methods based on direct temperature measurements for estimating fatigue strength of metals, were applied on an aluminium alloy 5754 H111 in order to demonstrate their problematic application for high-diffusivity materials. Furtherly, a specific thermal signal analysis was adopted for extracting first and second order temperature variations as better parameter for fatigue strength assessment. This work questions the use of direct temperature evaluation in high diffusivity materials and fully replaces it in favor of an approach based on in-depth analysis of thermal signal by using thermoelastic and dissipative temperature variations.
In this work, the Thermographic technique (IRT) was used to characterize the fracture mechanics behaviour of stainless steels. In particular, IRT is proposed for evaluating the dissipated energy and the plastic area around the crack tip in order to study the fatigue crack growth.
Experimental approaches used for the measure of dissipated energy require an accurate equipment and suitable techniques that may restrict the applications just to laboratory tests. The proposed approach is based on thermal signal investigation in the frequency domain in order to separate the two heat sources related to the material behaviour during fracture mechanics test: thermoelastic sources and intrinsic dissipations. These latter are directly related to the plastic phenomena around the crack tip and occur at the twice of the loading frequency. Both amplitude and phase signals at the twice of the loading frequency can be used for evaluating the crack growth rate. In particular, the first index through an estimation of the heat dissipated while the second due to the effects occurring at the crack tip.
It was also demonstrated as the proposed approach is capable of monitoring the crack growth over time and in automatic way by means of such the contactless and full field technique.
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