Thermoelastic Source Research Articles

Full-field investigation of dissipative mechanisms and thermoelastic inversion effects within glass-fiber reinforced polyamides subjected to low-cycle fatigue

A comprehensive thermomechanical investigation has been conducted to study the local kinematic and calorimetric responses of wet glass-fiber reinforced polyamide 6.6 subjected to tensile-tensile low-cycle fatigue tests. Digital image correlation (DIC) and quantitative infrared thermography (QIRT) were combined simultaneously to monitor the onset and development of spatial heterogeneities. Fields of strain, strain rates, intrinsic dissipation and resulting thermoelastic sources were successively observed and thoroughly analyzed with regards to the main fiber orientation and water content. Areas of precocious heterogeneities were detected from the very beginning of the loading, roughly propagating along the main fiber orientation. A ratcheting effect induced by the positive mean stress was observed and accompanied by a slight cyclic creep of the specimens. At high water content, the thermoelastic inversion phenomenon appeared and progressively spread along the gage part of the specimen, highlighting both glassy and rubbery behavior of the samples investigated.

Monitoring method of phase transition interface for natural gas hydrate based on thermoacoustic effect

In the drilling and exploitation for natural gas hydrate, the decomposition of combustible ices creates the interface between their solid state and liquid state, which would play an important role in the dynamic monitoring while exploitation. In this paper, a monitoring method based on the thermoacoustic effect is proposed to detect the position of the interface. The mechanism of the thermoacoustic coupling is introduced into the natural gas hydrate, that is, the physical model of thermoacoustic effect is established and detailed when the hydrate is excited by a strong electrical pulse. Two kinds of acoustic sources, the thermoelastic source and the phase transition acoustic source, are proposed to explain the obviously enhanced acoustic signals. By modeling and simulating, it is verified that the acoustic signals can be generated by the phase transition around the interface in hydrate. At the same time, many influencing factors on the thermoacoustic signals are researched based on simulations, such as the size of hydrate, the latent heat absorption coefficient and the amplitude of the transient electrical pulse. Finally, the experimental system both for hydrate generation and for thermoacoustic monitoring is built. We prepared carbon dioxide hydrate in a high-pressure reactor at an initial pressure of 5 MPa and temperature of −5 °C. Then take the hydrate into the environment of room temperature and standard pressure, apply the transient electrical pulse to the hydrate and use the ultrasonic probe to detect the acoustic signals. The results show that acoustic signals emitted from the position of the solid-liquid interface can be employed to monitor the hydrate decomposition experimentally and actually.

Determining longitudinal and transverse elastic wave attenuation from zero-group-velocity Lamb waves in a pair of plates

A method for the determination of longitudinal and transverse bulk acoustic wave attenuation from measurements of the decay-rate of two independent zero-group-velocity resonances in a couple of matched plates is presented. A linear relation is derived, which links the bulk-wave attenuation coefficients to the decay-rate of plate-resonances. The relation is used to determine the acoustic loss of tungsten at GHz frequencies from noncontact laser-ultrasonic measurements in plates with thicknesses of about 1 µm. The longitudinal and transverse attenuation was found to amount to 1918 m-1 and 7828 m-1 at 2.16 GHz and 3265 m-1 and 12181 m-1 at 2.46 GHz. The presented approach is validated with calculated responses to a thermoelastic source, and the accuracy of the obtained attenuation values is estimated to be in the range of 10%.

Thermal and energy analysis of DMTA tests

This paper investigates the suitability of the isothermal linear viscoelastic framework to describe the behavior of polymers observed during DMTA tests. A good interpretation of these tests is important because, in practice, they are used to construct master curves using the time-temperature superposition principle at small strain. These curves are then considered to predict the material behavior under experimentally unreachable thermal and/or loading frequency conditions. Currently, the DMTA protocol neglects the temperature variations induced by the deformation of polymers. We wonder if these temperature variations can have an influence on the measurement of dynamic moduli. To answer this question, quantitative infrared techniques were developed and used to assess small temperature variations of samples undergoing cyclic loadings during mechanical spectrometry tests. Thermal and mechanical data were used to quantify the viscous dissipated and the thermoelastic coupling energies that can be both associated with the hysteretic stress-strain response of polymers. Energy balances were then performed to quantify the relative importance of dissipative and thermoelastic coupling heat sources. From the energy standpoint, it is found that the thermoelastic energy rate was dozens of times higher than the dissipation. Especially at low frequencies, thermoelastic effects can have a greater influence on the loss modulus value than viscosity.

Assessment of the heterogeneous microstructure in the vicinity of a weld using thermographic measurements of the full‐field dissipative heat source

AbstractDuring a material deformation process, part of the mechanical energy is dissipated as heat due to thermodynamically irreversible processes occurring at the microscale of the material. In particular, part of the plastic deformation energy is transformed into heat and is referred to as ‘intrinsic dissipation’ as it is intrinsic to the material behaviour. The intrinsic dissipation is a heat source that is sensitive to microstructural states which can be used to identify different microstructural regions resulting from material processing such as welding. To determine the heat source in a full‐field manner, it is necessary to use an infrared camera to measure any temperature rise in a specimen undergoing elastic cyclic loading. Unlike the intrinsic dissipative heat source, the temperature change is sensitive to thermal exchanges with the surroundings. Hence, the thermomechanical heat diffusion equation is used to determine the full‐field dissipative heat from the thermographic temperature measurement by implementing an image processing procedure based on least squares fitting enabled by specially devised experimental approach. The procedure is verified by deriving both the thermoelastic and dissipative heat sources from a ‘hole‐in‐plate’ specimen manufactured from 316L stainless steel, that is, a specimen with a known stress distribution. The approach is then applied to a 316L laser welded specimen, and it is demonstrated that the different microstructures resulting from the welding process can be identified with the procedure. The heterogeneous microstructure is confirmed using micrographs and further verified by the different stress–strain behaviour obtained for each microstructural region using digital image correlation (DIC).

A spherical cavity problem with nonlocal elastic effect considering memory-dependent thermoelastic diffusion and laser pulse heat source

With expanding demand of electronic gadgets at miniature/nano-scales, mathematical modelling of the related problems calls for the study of various properties of nanostructures. Also, memory dependence phenomenon plays significant role to make models more rational. Considering this frame, thermoelasticity with diffusion based on memory-dependent derivatives and nonlocal elastic effect is studied under unified model of four theories of thermoelasticity. Infinite body containing a spherical cavity whose inner surface is acted upon by chemical and thermal shock in stress-free state is considered. Initially the medium is assumed to be quiescent. In heat conduction equation, laser pulse heat source which is decaying exponentially with time is present. Laplace transformation technique is adopted to obtain the solution by direct approach. The consideration of spherical cavity in infinite medium and combined model of thermoelastic theories with coupling of elastic, thermal, diffusion processes augments the novelty of present work. The study analyzes the effect of memory-dependent and nonlocal parameters on the various field quantities involved in the model which are calculated numerically using MATLAB software & presented graphically. Memory dependence of heat conduction and diffusion with nonlocal elasticity is observed to have significant impact on the variations of considered field quantities.

About the heat sources generated during fatigue crack growth: What consequences on the stress intensity factor?

During cyclic loading of a cracked metallic alloy at room temperature, heat sources are generated and produce a heterogeneous temperature field around the crack tip. Those heat sources are: (i) the thermo-elastic coupling source, (ii) the intrinsic dissipation due to microplasticity in the material, and (iii) the cyclic plasticity dissipated into heat in the reverse cyclic plastic zone (RCPZ) ahead of the crack tip. The thermoelastic source is computed by finite element analysis in agreement with classic linear thermoelasticity theory. The intrinsic dissipation due to microplasticity is experimentally estimated by carrying out self-heating fatigue tests on uncracked specimens, and then approximating its values in the cracked specimens by using self-heating curves. The cyclic plastic strain energy dissipated into heat in the RCPZ is also experimentally quantified by carrying out fatigue crack growth tests and using infrared measurements. The temperature fields, generated by the three types of heat sources, are separately computed by using the linearity of the heat diffusion equation. Afterward, the stress fields, associated with each thermal effect and induced by the material thermal expansion, are computed by considering the hypothesis of the linear elastic fracture mechanics (LEFM). Thus, the mode I stress intensity factor is calculated by taking into account the thermal effect associated with each heat source. The consequences on K,ΔK and RK=Kmin/Kmax are discussed. It is shown that the heat sources do not modify significantly ΔK, but the modification of RK can be significant since the effects are proportional to the loading frequency.

Generation of negative group velocity Lamb waves by a moving laser source

Analytical existence conditions for negative group velocity (NGV) of an arbitrary mode Lamb waves in elastic plate was obtained. Experimentally, NGV waves were excited by means of a thermoelastic laser source moving across the sample plate with a controllable velocity. Such source is coupled with modes which phase velocity component parallel to the surface coincides with the source speed. Additional mode selection was performed by choosing a quasiperiodic shape of the laser spot, which specifies the range of the wavenumber k. This technique was successfully applied for generation of backward (NGV) waves. By tuning the speed of the source the propagation direction was switched from forward to reversal.

Strength Assessment of a Titanium Alloys under Tensile Tests Based on the Analysis of Heat Sources

IR thermography is very useful non-destructive tools for investigate fracture mechanics behavior of materials. The dissipative effects in metals during plastic deformation and change in the internal structure leads to intensive heat generation in the areas in the strain localization zones. The infrared thermography provides a direct estimate of heat sources, especially intrinsic dissipation and thermoelastic sources, using the local heat equation. This work deals with the investigation of heat sources of a titanium alloys (Grade 2 and Ti-1Al-1Mn) during monotonic tensile tests and monitoring the state of material based on the heat source value. Experiments performed on flat specimens show that the value of the stored energy could be correlated with the mechanical behavior of the material.

Study of damage evolution in composite materials based on the Thermoelastic Phase Analysis (TPA) method

Abstract Standards and conventional procedures used for analysing fatigue damage in composite materials involve high experimental campaign costs due to time-consuming tests. This aspect becomes relevant for large structures where the cost of experimental setup tends to rise according to structure dimensions. In this regard, in recent years, efforts to produce fatigue characterisation of materials have made use of several experimental techniques, i.e. thermographic techniques. Most of these, however, refer to Standard specimens and laboratory equipment and set-up. Through the use of Thermography, in this work a new procedure has been developed which is capable of monitoring damage in GFRP composite material. The analysis of thermal signal in the frequency domain allows for the isolation of indexes which are related to the thermoelastic and dissipative heat sources. In particular, the phase of thermoelastic signal, associated with intrinsic dissipation processes occurring in the material, has been used to localize and assess the damaged areas in a quantitative manner. Moreover, the thermoelastic phase analysis leads to an evaluation of the endurance limit of composites. In fact, by comparing the results with those provided by the standard test methods, the potential has been shown of the proposed procedure firstly as a non-destructive technique for continuous monitoring of damage in composite structures undergoing fatigue loadings, and secondly, as a fatigue limit index.

Subsurface, thermoelastic line source excitation of a transversely isotropic half space

In this work, mathematical descriptions of material displacements in the wavefield surrounding a subsurface, thermoelastic line source in a transversely isotropic half space are given. These are derived from continuum representations that yield wave equations describing material motion that are solved using transform techniques. These solutions provide insight into the overall character of these sources in anisotropic materials and can also be used to model more generalized source distributions that might occur in a range of physical systems. Analytical, closed form solutions for epicentral displacements are derived and are graphically represented for particular source locations below the surface of a titanium half space. These results illustrate general aspects of the wavefield including the nature of various component modes of the displacement waveform. Wavefront characteristics associated with these modes are considered along with the nature of direct shear wave emission by this type of source. Mathematical results for displacements are used to illustrate how the surface-borne thermoelastic line source can be represented as a surface, shear stress dipole and are also used to represent more complicated subsurface source distributions that might occur in physical systems.

Nonlinear laser ultrasound formation in silicon

In this paper, the mechanisms of laser ultrasound response formation in monocrystalline silicon are discussed. The ultrasound waves in the test specimen were generated with laser pulses of two different wavelengths and registered with a piezoelectric transducer. The amplitude of the measured signal was found to be a nonlinear function of the laser radiation intensity. It was shown, that the observed nonlinearity is related to the features of optical absorption and thermoelastic sources formation in the material. A simple model taking into account temperature dependencies of the thermal conductivity and thermal expansion coefficient was developed. An excellent agreement between experimental and simulation for different wavelengths was demonstrated.

Laser-ultrasound-based non-destructive testing—optimization of the thermoelastic source

Laser-ultrasound-based non-destructive testing (NDT) applications usually use circular or linear thermoelastic sources. This paper studies two more complex sources and presents their specific advantages. The first thermoelastic source is a regular line array, whereas the second one is a chirped line array. The surface acoustic waves generated on an aluminum sample by these two thermoelastic sources are detected by an interferometric optical probe, the measurements showing a good agreement with theoretical expectations. Finally, the potentialities of these two sources in NDT applications are described.

Influence of relative humidity and loading frequency on the PA6.6 cyclic thermomechanical behavior: Part I. mechanical and thermal aspects

Using quantitative infrared techniques, this study investigated the influence of relative humidity (RH) and loading rate on the mechanical and thermal responses of PA6.6 matrix specimens. The specimens were subjected to tensile and oligocyclic tensile-tensile tests at 3 different RHs. For tensile experiments, the studied samples were loaded at 4 displacement rates. For oligocyclic tensile-tensile fatigue tests, they were subjected to 2 distinct loading frequencies with a 0.1 stress ratio. The overall aim of this study was to analyze the mechanical and thermal responses for each cyclic loading case. The influence of the loading frequency on the hysteresis loops and self-heating was particularly highlighted. The plasticizing role of water and its influence on the glass transition temperature were also discussed relative to the nature of temperature variations accompanying the deformation processes. These thermal responses were associated with dissipation, standard and/or entropic thermoelastic coupling sources. These different heat sources and the complete energy rate balances will be analyzed in the second part of this paper.

Photoacoustic Signal Formation in Heterogeneous Multilayer Systems with Piezoelectric Detection

A new efficient model describing photoacoustic (PA) signal formation with piezoelectric detection is reported. Multilayer sandwich-like systems: heterogeneous studied structure—buffer layer—piezoelectric transducers are considered. In these systems, the buffer layer is used for spatial redistribution of thermoelastic force moments generated in the investigated structure. Thus, mechanical properties of this layer play a crucial role to ensure perfect control of the detected voltage formed on a piezoelectric transducer by contribution of different regions of the studied structure. In particular, formation of the voltage signal strongly depends on the point at which the thermoelastic source is applied. Therefore, use of relatively simple linear Green’s functions introduced in frames of the Kirchhoff–Love theory is chosen as an efficient approach for the PA signal description. Moreover, excellent agreement between the theoretical model and measured results obtained on a heterogeneous “porous silicon-bulk Si substrate” structure is stated. Furthermore, resolving of the inverse problem with fitting of the experimental curves by the developed model allows reliable evaluation of the thermal conductivity of the nanostructured porous silicon layer.

Non-contacting transfer of elastic energy into explosive simulants for dynamic property estimation

Non-contacting acoustical methods can be used to extract various material properties of liquid or solid samples without disturbing the sample. These methods are useful even in the lab since they do not involve coupling anything to the sample, which might change its properties. A forteriori, when dealing with potentially dangerous materials, non-contacting methods may be the only safe solutions to mechanical characterization. Here, we show examples of using laser ultrasound to remotely insonify and monitor the elastic properties of several granular explosive simulants. The relatively short near-infrared laser pulse length (a few hundred nanoseconds) provides a broad-band thermoelastic source of ultrasound; we intentionally stay in the thermoelastic regime to avoid damaging the material. Then, we use a scanning laser Doppler vibrometer to measure the ultrasonic response of the sample. LDV technology is well established and very sensitive at ultrasonic frequencies; atomic level motions can be measured with modest averaging. The resulting impulse response of the explosive simulant can be analyzed to determine decay rates and wave speeds, with stiffer samples showing faster wave speeds and lower decay rates. On the other hand, at the low-frequency end of the acoustic spectrum, we use an electronically phased array to couple into a freely suspended sample's normal modes. This allows us to gently heat up the sample (3 °C in just under 5 min, as shown with a thermal IR camera). In addition to the practical interest in making the sample more chemically visible through heat, these two measurements (low-frequency resonant excitation vs high-frequency wave propagation) bracket the frequency range of acoustic non-destructive evaluation methods available.

Kinetics of stored and dissipated energies associated with cyclic loadings of dry polyamide 6.6 specimens

An experimental protocol was developed to achieve complete energy balances associated with the low cycle fatigue (LCF) of dry polyamide 6.6 (PA6.6) matrix. The protocol uses two quantitative imaging techniques, infrared thermography (IRT) and digital image correlation (DIC). The first technique provides a direct estimate of heat sources, especially intrinsic dissipation and thermoelastic sources, using the local heat diffusion equation. The second technique gives access to the deformation energy by means of strain and stress assessments. Stresses were derived from strain measurements using a simplified local form of equilibrium equations. Both techniques were then successfully combined with the aim of quantifying various energies involved in the energy balance (e.g. deformation, dissipated and stored energies) and then to obtain an estimate of the Taylor-Quinney ratio.From a thermomechanical modeling standpoint, the experimental results exhibit some interesting findings during the first few cycles. It was found that there was neither mechanical nor thermodynamic cyclic stability. From a mechanical standpoint, a significant ratcheting phenomenon characterized by accumulation of cyclic strain is classically observed. From a thermodynamic viewpoint, it was shown that the dissipated energy per cycle was always less than the mechanical energy that could be associated with the area of the hysteresis loop. This energy difference reflects the significant contribution of the stored energy associated, cycle by cycle, with the microstructural changes. Moreover, a 2d full-field measurement analysis highlighted hot spots occurring in the dissipation fields. The surface detection of these spots was thus correlated with those of the thermoelastic source with the aim of monitoring the fatigue damage accumulation in the region where the crack finally occurred.

Acoustic beam steering by light refraction: illustration with directivity patterns of a tilted volume photoacoustic source.

The symmetry of a thermoelastic source resulting from laser absorption can be broken when the direction of light propagation in an elastic half-space is inclined relatively to the surface. This leads to an asymmetry of the directivity patterns of both compressional and shear acoustic waves. In contrast to classical surface acoustic sources, the tunable volume source allows one to take advantage of the mode conversion at the surface to control the directivity of specific modes. Physical interpretations of the evolution of the directivity patterns with the increasing light angle of incidence and of the relations between the preferential directions of compressional- and shear-wave emission are proposed. In order to compare calculated directivity patterns with measurements of normal displacement amplitudes performed on plates, a procedure is proposed to transform the directivity patterns into pseudo-directivity patterns representative of the experimental conditions. The comparison of the theoretical with measured pseudo-directivity patterns demonstrates the ability to enhance bulk-wave amplitudes and to steer specific bulk acoustic modes by adequately tuning light refraction.

Numerical study for a new methodology of flaws detection in train axles

Train loads and travel speeds have increased over time, requiring more efficient non-destructive inspection methods. Railway axles are critical elements; despite being designed to last more than 20years several cases of premature failure have been recorded. Train axles are inspected regularly, but the limits associated to the traditional inspection technologies create a growing interest towards new solutions. Here a novel non-destructive inspection method of in-service axles based on non-contact data collection is presented. The propagation of surface waves, generated by a thermo-elastic laser source, is investigated using a finite element method based on dynamic explicit integration. Coupled thermo-mechanical simulations allow visualization of the ultrasonic field guiding the definition of the optimal NDT setup. The geometry of the axle and of the elements mounted on it is accurately reproduced; moreover the press fit effect caused by the wheel and the bearing rings is implemented. The current NDT techniques for railway axles require removing wheels and other components from the axle. The presented scheme uses non-contact ultrasonic generation and detection allowing non-contact in-service inspection of railway axles at trackside station. The numerical results are promising and encourage us to test the new approach experimentally.

Influence of transparent coating hardness on laser-generated ultrasonic waves

Numerical models are established to investigate the influence of transparent coating hardness on the laser-generated thermoelastic force source and ultrasonic waves in coating-substrate systems by using finite element method. With the increase of coating hardness, the characteristic of longitudinal wave in substrate is more obvious due to the gradual increase of reactive force produced by coating constraint; the directivity patterns of longitudinal wave show that the energy concentration area transfers from bilateral area to the axial direction area gradually. Therefore, the directivity pattern can be regulated to obtain the better ultrasonic signals by coating different hardness materials. It is significant for further development of the experiment in composite evaluation and in extreme condition.