Thermomechanical Strain Research Articles

A LN/Si-Based SAW Pressure Sensor.

Surface Acoustic Wave (SAW) sensors are small, passive and wireless devices. We present here the latest results obtained in a project aimed at developing a SAW-based implantable pressure sensor, equipped with a well-defined, 30 μm-thick, 4.7 mm-in-diameter, Lithium Niobate (LN) membrane. A novel fabrication process was used to solve the issue of accurate membrane etching in LN. LN/Si wafers were fabricated first, using wafer-bonding techniques. Grinding/polishing operations followed, to reduce the LN thickness to 30 μm. 2.45 GHz SAW Reflective Delay-Lines (R-DL) were then deposited on LN, using a combination of e-beam and optical lithography. The R-DL was designed in such a way as to allow for easy temperature compensation. Eventually, the membranes were etched in Si. A dedicated set-up was implemented, to characterize the sensors versus pressure and temperature. The achieved pressure accuracy is satisfactory (±0.56 mbar). However, discontinuities in the response curve and residual temperature sensitivity were observed. Further experiments, modeling and simulations were used to analyze the observed phenomena. They were shown to arise essentially from the presence of growing thermo-mechanical strain and stress fields, generated in the bimorph-like LN/Si structure, when the temperature changes. In particular, buckling effects explain the discontinuities, observed around 43 °C, in the response curves. Possible solutions are suggested and discussed.

Numerical modeling of saturation type and lateral pressure influences on thermo-mechanical stresses caused by laser drilling in granite and limestone

Numerical methods for study effect of parameters on rock behaviour have increased in recent decades. Simulation is an important tool for analysis which the possibility of doing it is not feasible under experimental conditions and the possibility of analysis of thermo-mechanical stress and strain caused by the laser. In this study, the finite element method has been used to study thermal and mechanical stresses caused by drilling with laser ND: YAG in rock samples of limestone and granite. For this purpose, the software ABAQUS was used for thermal and mechanical analyses. After applying drilling rate and modeling laser drilling based on experimental tests, the effect of type of saturation and high lateral pressures on the rock laser drilling was studied. Results showed that the numerical model used is in good agreement with the experimental data. The thermo-mechanical stress developed in the rock has a direct relationship with the drilling specific energy. Also, in the limestone and granite, the maximum stress caused by laser drilling in the water-saturated rocks is higher than the oil-saturated and dry state.

3D Path independent integral for thermoelastic and magnetostriction problem

In this work, a universal path independent integral Ju that represents the energy release rate or flux during crack extension in a homogeneous and isotropic material, has been derived for straight crack subjected to multiple loads. This version of integral includes the effect of elastic strain, Eigen strain, thermal strain, body forces and magnetic strain. Particularly focus is made on thermo-mechanical and magnetic strains. The 3D J-integral expression represents the energy release rate in the combined thermoelastic field with magnetostriction characteristics. This integral is applicable to the real field problems concerning medium in an electric field where the multiphysics of heat generation and magnetic field are involved. 3D finite element analysis of edge cracked magnetostrictive specimen has been carried out to demonstrate the efficacy of the integral expressions.

Modeling the strain impact on refractive index and optical transmission rate

We propose a new and simple modeling approach for strain impact on the transmission and reflection rate of semiconductor devices. The model is applied to graphene or carbon nanotubes deposited on substrates. Any change in transmission rate by strain can directly impact on the short-circuit current density of an electronic device. The nanolayers of graphene and nanotubes are often used as the excellent replacement for the conventional metallic contacts. However, these nanolayers are sensitive to in-plain and out-plain strain. It is shown that the transmission rate is significantly reduced by the strain. We have also calculated the change in the refractive index under in-plain strain and the consequent change in reflection rate. The modeling can be extended to calculate the change in the refractive index under out-plain strain. Furthermore, one can calculate the change in short-circuit current density of the full device (i.e. solar cell) under in-plain or out-plain strains. A practical outcome of our modeling approach is to optimize the thickness or concentration of graphene and carbon nanotube to en extent which is less sensitive to any thermo-mechanical strain. This leads the reader to strain tuning techniques which are rarely applied to sensors, solar cells or photodetector devices through fabrication and characterization process.

A Time-Dependent Finite Element Formulation for Thick Shape Memory Polymer Beams Considering Shear Effects

In this paper, employing a thermomechanical small strain constitutive model for shape memory polymers (SMP), a beam element made of SMPs is presented based on the kinematic assumptions of Timoshenko beam theory. Considering the low stiffness of SMPs, the necessity for developing a Timoshenko beam element becomes more prominent. This is due to the fact that relatively thicker beams are required in the design procedure of smart structures. Furthermore, in the design and optimization process of these structures which involves a large number of simulations, we cannot rely only on the time consuming 3D finite element analyses. In order to properly validate the developed formulations, the numeric results of the present work are compared with those of 3D finite element results of the authors, previously available in the literature. The parametric study on the material parameters, e.g., hard segment volume fracture, viscosity coefficient of different phases, and the external force applied on the structure (during the recovery stage) are conducted on the thermomechanical response of a short I-shape SMP beam. For instance, the maximum beam deflection error in one of the studied examples for the Euler–Bernoulli beam theory is 7.3%, while for the Timoshenko beam theory, is 1.5% with respect to the 3D FE solution. It is noted that for thicker or shorter beams, the error of the Euler–Bernoulli beam theory even more increases. The proposed beam element in this work could be a fast and reliable alternative tool for modeling 3D computationally expensive simulations.

A two-dimensional finite element model of the grain boundary based on thermo-mechanical strain gradient plasticity

In this work, a two-dimensional finite element model for the grain boundary flow rule is de- veloped based on the thermo-mechanical gradient-enhanced plasticity theory. The proposed model is temperature-dependent. A special attention is given to physical and micromechani- cal nature of dislocation interactions in combination with thermal activation on stored and dissipated energy. Thermodynamic conjugate microforces are decomposed into energetic and dissipative components. Correspondingly, two different grain boundary material length sca- les are present in the proposed model. Finally, numerical examples are solved in order to explore characteristics of the proposed grain boundary flow rule.

Influence of oscillation frequency and focal diameter on weld pool geometry and temperature field in laser beam welding of high strength steels

As weld pool geometry and thermomechanical strains are known to affect formation of solidification cracks, the influence of a superimposed beam oscillation on these characteristics is investigated for this paper. In this context the effects of the oscillation frequency and focal diameter on the weld pool and temperature field are determined by means of infrared thermography. As a result an increase of the weld pool size for larger focal diameters and a more even shape of the seam edges for higher frequencies can be identified.

On the formation of fold-type oscillation marks in the continuous casting of steel.

Asymptotic methods are employed to revisit an earlier model for oscillation-mark formation in the continuous casting of steel. A systematic non-dimensionalization of the governing equations, which was not carried out previously, leads to a model with 12 dimensionless parameters. Analysis is provided in the same parameter regime as for the earlier model, and surprisingly simple analytical solutions are found for the oscillation-mark profiles; these are found to agree reasonably well with the numerical solution in the earlier model and very well with fold-type oscillation marks that have been obtained in more recent experimental work. The benefits of this approach, when compared with time-consuming numerical simulations, are discussed in the context of auxiliary models for macrosegregation and thermomechanical stresses and strains.

Micromorphic approach for gradient-extended thermo-elastic–plastic solids in the logarithmic strain space

The coupled thermo-mechanical strain gradient plasticity theory that accounts for microstructure-based size effects is outlined within this work. It extends the recent work of Miehe et al. (Comput Methods Appl Mech Eng 268:704–734, 2014) to account for thermal effects at finite strains. From the computational viewpoint, the finite element design of the coupled problem is not straightforward and requires additional strategies due to the difficulties near the elastic–plastic boundaries. To simplify the finite element formulation, we extend it toward the micromorphic approach to gradient thermo-plasticity model in the logarithmic strain space. The key point is the introduction of dual local–global field variables via a penalty method, where only the global fields are restricted by boundary conditions. Hence, the problem of restricting the gradient variable to the plastic domain is relaxed, which makes the formulation very attractive for finite element implementation as discussed in Forest (J Eng Mech 135:117–131, 2009) and Miehe et al. (Philos Trans R Soc A Math Phys Eng Sci 374:20150170, 2016).

High Temperature Fatigue of Welded Joints—Experimental Investigation and Local Analysis of Butt Welded Flat and Cruciform Specimens

Austenitic stainless steel of type X6CrNiNb18-10 exhibits advantageous mechanical and chemical properties and is a common material for numerous applications in the nuclear power plant and chemical industries. Besides the mechanical strain induced by high pressure, the fatigue life in welded pipelines is affected by additional thermomechanical strains due to thermal loading. The welding process mainly determines the geometry and metallurgical constitution of the welded joint. Therefore, the butt welds additionally influence the strain gradient along the component and reduce its lifetime. While the base and weld material are similar, they show different softening and hardening behavior, especially at ambient temperature. Cyclic hardening occurs in the base material, whereas cyclic softening can be observed in the weld material. The hardness distribution along the welded joint reveals no clear differentiation of the base material, the heat affected zone, and the weld material. The attributes of the individual materials cannot be transferred to the welded joint automatically. Thus, the analysis of the interaction between the materials along the welded joint is a main topic of this research. To this end, digital image correlation (DIC) is used for different kinds of specimens and load conditions. The position along the testing area at which fatigue failure occurs depends on the specimen type and the load condition but not on the temperature. Further, isothermal and anisothermal fatigue tests on welded cruciform specimens are presented. The common practice of the effective strain is discussed for the analyzed conditions.

Coupled thermomechanical response of gradient plasticity

The coupled thermo-mechanical strain gradient plasticity theory that accounts for micro-structure based size effects is outlined within this work. This incorporates spatial gradients of selected micro-structural fields based on length-scales that describe the evolving dissipative mechanisms. In the mechanical part, the model problem of von Mises plasticity with gradient-extended hardening/softening response is considered as discussed in Miehe et al. (2013, 2014a). In the thermal part, we follow the investigations of Simó and Miehe (1992) that demonstrate the effect of temperature on the mechanical fields resulting in a thermal expansion. To this end, two classes of solution schemes for the coupled problem are considered: (i) Global product formula algorithm arising from operator split which leads to a two step solution procedure, and (ii) an implicit coupled algorithm which employs simultaneous solution of the coupled system of equations. In the product formula algorithm, the mechanical and thermal problems are solved separately, resulting in a symmetric problem. However, in the implicit coupled algorithm, a simultaneous solution of the coupled system of equations for gradient thermo-plasticity is employed. A noteworthy drawback of this solution scheme arises from the high computational efforts in comparison with the product formula algorithm. From the computational viewpoint, the standard Galerkin finite element method fails in the context of isochoric plastic flow due to the over-constrained pressure field. To circumvent these difficulties, we extend the well-known Q1P0-type and MINI-type mixed finite elements design of gradient plasticity to account for thermal effects. The performance of the formulation is demonstrated by means of some representative examples.

Cyclic thermo-mechanical stress, strain and continuum damage behaviors in light alloys during fatigue lifetime considering heat treatment effect

In this article, thermo-mechanical fatigue behaviors in light alloys have been investigated to find the effect of heat treatments. For this objective, thermo-mechanical fatigue tests were performed on the A356.0 aluminum alloy and the AZE911 magnesium alloy, with and without typical T6 heat treatments. Obtained results demonstrated no significant difference in thermo-mechanical fatigue lifetime of the A356.0 alloy between non-heat-treated and heat-treated test specimens at 250°C of the maximum temperature, which was attributed to the over-ageing phenomenon. As a consequence, this low effect showed that the heat treatment could be eliminated for cylinder heads. However, the thermo-mechanical fatigue lifetime of the AZE911 alloy was significantly affected by the heat treatment. The explanation for the mentioned behavior could be found in the material micro-structure, which was affected by dissolving the brittle intermetallic phase in the matrix of the AZE911 alloy. However, the magnesium alloy still requires more improvements in the fatigue lifetime for a possible substitution in cylinder heads. Continuum damage behaviors showed that higher damage values occurred in the aluminum alloy, in comparison to the magnesium alloy, both for heat-treated and non-heat-treated specimens.

A multiaxial load-induced thermal strain constitutive model for concrete

The paper presents a novel thermomechanical 3D Load-Induced Thermal Strain (LITS) model that captures the experimentally demonstrated behavior of concrete in the case of heating under multiaxial mechanical load, for temperatures up to 250 °C. In contrast to the models available in the literature, the new model takes into account the observed dependency of LITS on stress confinement. Such a dependency is introduced through a confinement coefficient which makes LITS directly proportional to the confinement of the stress state. Also, a new practical bilinear LITS model is proposed and proved to be suitable for fitting the general trend of the curves experimentally obtained for different loading conditions. The presented model is embedded in a thermoelastic material constitutive law, and then verified and validated against experiments performed on concrete specimens subjected to transient temperatures up to 250 °C under uniaxial, biaxial and triaxial compressive stress states. Once calibrated and validated, the constitutive model is used to evaluate the effects of LITS on the structural behavior of a Prestressed Concrete Pressure Vessel (PCPV) of a typical Advanced Gas cooled Reactor (AGR) subjected to a heating-cooling cycle simulating a temporary fault in its cooling system. The results of this study indicate that the development of LITS significantly influences the stress redistribution in the structure. Moreover, it is shown that in the case of PCPVs (and by extension similar structures) it is crucial to consider the LITS dependence on the stress confinement.

Performance Enhancement of Flip-Chip Packaged AlGaN/GaN HEMTs by Strain Engineering Design

The piezoelectric polarization of the AlGaN/GaN high-electron-mobility transistors (HEMTs) is strongly related to the strain state in the active area. Therefore, understanding the strain behavior inside the channel is crucial to the device electrical performance improvement of devices. This paper, for the first time, reveals the potential of optimizing flip-chip structures with active-region bumps to modulate the strain state of the AlGaN/GaN HEMT for enhancing the piezoelectric effect. The thermo-mechanical strain is observed to be affected by the physical dimensions and the material properties of the package. Thus, incorporating device strain engineering into packaging design will be very important for GaN devices due to their strong piezoelectric effects.

Novel failure mode of chip corrosion at automotive HALL sensor devices under multiple stress conditions

Semiconductor devices used in automotive applications undergo numerous stress situations depending on their particular application. Corrosion, as one main crucial failure mechanisms, can affect the lifetime of electronic components on system, device or even die level. In this paper, a novel corrosion mechanism on HALL sensor devices is investigated and clarified. This corrosion is only occurring under complex conditions like layout aspects, ionic impurities combined with humidity penetration and thermo-mechanical strain due to packaging and additional mechanical load from further over moulding. It is shown how advanced physical and chemical analysis can be combined with finite element simulation to ascertain a chemical degradation running on silicon, silicon dioxide and metallisation level to derive the complete chemical reaction mechanism for the observed corrosion defects. To verify the new failure mode, experiments to recreate this type of corrosion were carried out. Finally, conclusions are drawn on how failure modes can be prevented and how the robustness of the HALL devices under harsh environments can be increased.

Experimental study of the effect of simultaneous mechanical and high-temperature loadings on the behaviour of textile-reinforced concrete (TRC)

Textile-reinforced mortar (or textile-reinforced concrete, TRC) may be used to repair and/or strengthen structural elements of existing construction works (buildings, bridges, etc.) or may be used as constituent elements of new structures. When structures are subjected to fire, they are simultaneously submitted to high temperatures (potentially up to 1200°C) and mechanical loadings. The elevated temperature thermomechanical behaviour of TRC remains both poorly understood and poorly studied. This study describes the experimental identification, at the macroscale level, of the thermomechanical behaviour of two TRC composites. It shows that the thermomechanical behaviour of two TRC composites is nonlinear and progressively evolves with the increase of the temperature level. The evolution of certain characteristics (the ultimate stress, the stress corresponding to the loss of linearity of the first zone of the stress/strain curve, the stress corresponding to the start of the post cracking stage, the maximum thermomechanical strain, the initial stiffness, the stiffness of the post cracked composite) of the studied materials as a function of the temperature levels will be identified. When temperature levels vary from 100°C to 300°C, the main parameters (the ultimate stress, the stress corresponding to the loss of linearity of the first zone of the stress/strain curve, the stress corresponding to the start of the post cracking stage, the initial stiffness, the stiffness of the post cracked composite) of the studied materials are in general gradually decreased with the increase of the temperature level. The influence of a grid and chopped strand textile reinforcement on the cement based composite under thermomechanical loading is analysed and discussed. The experiments show a clear difference between the thermomechanical responses of two TRC composites. Additionally, the cracked specimens are examined.

Investigation of thermomechanical couplings, strain localization and shape memory properties in a shape memory polymer subjected to loading at various strain rates

This paper presents experimental and modeling results of the effects of thermomechanical couplings occurring in a polyurethane shape memory polymer (SMP) subjected to tension at various strain rates within large strains. The SMP mechanical curves, recorded using a testing machine, and the related temperature changes, measured in a contactless manner using an IR camera, were used to investigate the polymer deformation process at various loading stages. The effects of thermomechanical couplings allowed the determination of the material yield point in the initial loading stage, the investigation of nucleation and development of the strain localization at larger strains and the estimation of the effects of thermoelastic behavior during the unloading process. The obtained stress–strain and thermal characteristics, the results of the dynamic mechanical analysis and estimated values of the shape fixity and shape recovery parameters confirmed that the shape memory polymer (Tg = 45 °C) is characterized by good mechanical and shape memory properties, as well as high sensitivity to the strain rate. The mechanical response of the SMP subjected to tension was simulated using the finite element method and applying the large strain, two-phase model. Strain localization observed in the experiment was well reproduced in simulations and the temperature spots were correlated with the accumulated viscoplastic deformation of the SMP glassy phase.

Elastic and thermal expansion asymmetry in dense molecular materials.

The elastic modulus and coefficient of thermal expansion are fundamental properties of elastically stiff molecular materials and are assumed to be the same (symmetric) under both tension and compression loading. We show that molecular materials can have a marked asymmetric elastic modulus and coefficient of thermal expansion that are inherently related to terminal chemical groups that limit molecular network connectivity. In compression, terminal groups sterically interact to stiffen the network, whereas in tension they interact less and disconnect the network. The existence of asymmetric elastic and thermal expansion behaviour has fundamental implications for computational approaches to molecular materials modelling and practical implications on the thermomechanical strains and associated elastic stresses. We develop a design space to control the degree of elastic asymmetry in molecular materials, a vital step towards understanding their integration into devicetechnologies.

High-Pressure Thermal Isotropic Cell for Evaluation of Thermal Volume Change of Soils

This paper describes a new high-pressure, temperature-controlled isotropic cell used for evaluation of the thermal volume change mechanisms of saturated and unsaturated soils under isotropic stress states. Specifically, details of the experimental setup, instrumentation, thermo-mechanical calibration of the device, experimental procedures, and typical results are presented in this paper. The thermal isotropic cell includes suction control using the axis translation technique, saturation control/monitoring using a pore water pressure flow pump, cell pressure control using a high-pressure flow pump, and a stainless steel cell to permit application of isotropic net mean stresses up to 10 MPa. The cell fluid temperature is regulated by circulating heated water through a copper heating coil within the cell, and an internal circulating fan is used to promote homogenous temperature throughout the cell chamber. Non-contact proximity transducers are used to directly measure soil deformation in the radial and axial directions, permitting assessment of thermo-mechanical anisotropic strains during changes in mean effective stress or temperature while also avoiding the need to consider complex thermo-mechanical cell deformations. The high-pressure flow pump and thermal control system are designed to apply changes in net mean stress and temperature at slow rates to characterize the full soil compression and thermal volume change curves, respectively. Along with the thermo-mechanical calibration of the cell, the results from two tests on compacted silt specimen having different initial degrees of saturation are presented that show how the cell can be used to characterize changes in volume and degree of saturation during thermo-mechanical loading. Both normally consolidated soil specimens were contracted during heating, although the specimen with a lower degree of saturation showed slightly greater thermal volume change.

A Thermomechanically Consistent Constitutive Theory for Modeling Micro-Void and/or Micro-Crack Driven Failure in Metals at Finite Strains

Within a continuum approximation, we present a thermomechanical finite strain plasticity model which incorporates the blended effects of micro-heterogeneities in the form of micro-cracks and micro-voids. The former accounts for cleavage-type of damage without any volume change whereas the latter is a consequence of plastic void growth. Limiting ourselves to isotropy, for cleavage damage a scalar damage variable [Formula: see text] is incorporated. Its conjugate variable, the elastic energy release rate, and evolution law follow the formal steps of thermodynamics of internal variables requiring postulation of an appropriate damage dissipation potential. The growth of void volume fraction [Formula: see text] is incorporated using a Gurson-type porous plastic potential postulated at the effective stress space following continuum damage mechanics principles. Since the growth of micro-voids is driven by dislocation motion around voids the dissipative effects corresponding to the void growth are encapsulated in the plastic flow. Thus, the void volume fraction is used as a dependent variable using the conservation of mass. The predictive capability of the model is tested through uniaxial tensile tests at various temperatures [Formula: see text]C, [Formula: see text]C]. It is shown, via fracture energy plots, that temperature driven ductile-brittle transition in fracture mode is well captured. With an observed ductile-brittle transition temperature around [Formula: see text]C, at lower temperatures fracture is brittle dominated by [Formula: see text] whereas at higher temperatures it is ductile dominated by [Formula: see text].