Thermomechanical Behavior Of Materials Research Articles

From the material behaviour to the thermo-mechanical long-term response of asphalt pavements and the alteration of surface drainage due to rutting: a sensitivity study

ABSTRACT In this contribution, a simulation chain for the thermo-mechanical and hydromechanical analysis (surface drainage) of a pavement structure during its whole service life is applied to a typical asphalt pavement (Bk100) used for motorways in Germany. Transient load signals for each tyre position of a truck-trailer combination (vehicle) are generated from a multibody analysis of the vehicle driving on a motorway with rough pavement surface. Material model parameters are derived from experimental identification tests of asphalt materials. The model parameters are used together with a continuum-mechanical description of the asphalt material representing the thermo-mechanical material behaviour at the material scale. The material response of the pavement layers is integrated on the structural scale by using the finite element method (FEM) in combination with an arbitrary Lagrangian-Eulerian (ALE) formulation and equivalent tyre loads of a representative, finite element (FE) discretised truck tyre rolling on an FE discretised pavement section. Different scenarios of rut formation are computed by varying different influence factors (climate temperature, vertical tyre force, type of asphalt material of the surface layers etc.). During the subsequent hydromechanical analysis of the pavement, the simulated deformed geometry of the pavement surface is used to numerically compute surface drainage characteristics.

Plasma–Wall Interaction Model for Erosion of Materials Under Ion Bombardment

Understanding the evolution and behavior of materials exposed to plasma is critical for the design of future electric propulsion devices. As ions are ejected from the device generating thrust, they also impact the ceramic walls. This induces wall erosion, ultimately exposing the magnetic circuit, leading to malfunction and failure of the device. There are several models that try to predict this effect by accounting for material sputtering. However, they cannot predict the millimeter-scale surface features that develop after prolonged exposure. In this work, we address this issue by introducing a plasma–material interaction model able to capture the evolution of surface features at the macroscopic scale on materials exposed to plasma over a long period of time. Results show that the model is able to reproduce not only the mean erosion rate but also the macroscopic anomalous ridges that appear after long exposure. Furthermore, it highlights the need to account for complex thermomechanical material behavior to be able to explain such features.

Influence of the Reaction Injection Moulding Process on the Thermomechanical Behaviour of Fast Curing Polyurethane

In this contribution, the influence of the reaction injection moulding process on the thermomechanical material behaviour of aliphatic hexamethylene diisocyanate (HDI) based fast curing polyurethane is demonstrated. Uniaxial tensile tests, temperature-frequency dependent dynamic mechanical thermal analysis (DMTA) and Differential Scanning Calorimetry (DSC) are used to show the differences in properties for ten different sets of process parameters. The mould and resin components temperature, the mass flow during the filling process and the residence time during the reaction process of the polyurethane are varied in several stages. Further experiments to determine the molar mass of the molecular chain between two crosslinking points of the polyurethane are used to explain the process influences on the thermomechanical properties. Thus, a direct correlation between manufacturing and material properties is shown. In addition, the mutual effect of the different parameters and their overall influence on the material behaviour is presented.

Mechanical Consequences of Dynamically Loaded NiTi Wires under Typical Actuator Conditions in Rehabilitation and Neuroscience.

In the field of rehabilitation and neuroscience, shape memory alloys play a crucial role as lightweight actuators. Devices are exploiting the shape memory effect by transforming heat into mechanical work. In rehabilitation applications, dynamic loading of the respective device occurs, which in turn influences the mechanical consequences of the phase transforming alloy. Hence in this work, dynamic thermomechanical material behavior of temperature-triggered phase transforming NiTi shape memory alloy (SMA) wires with different chemical compositions and geometries was experimentally investigated. Storage modulus and mechanical loss factor of NiTi alloys at different temperatures and loading frequencies were analyzed under force-controlled conditions. Counterintuitive storage modulus- and loss factor-dependent trends regarding the loading frequency dependency of the mechanical properties on the materials’ composition and geometry were, hence, obtained. It was revealed that loss factors showed a pronounced loading frequency dependency, whereas the storage modulus was not affected. It was shown that force-controlled conditions led to a lower storage modulus than expected. Furthermore, it turned out that a simple empirical relation could capture the characteristic temperature dependency of the storage modulus, which is an important input relation for modeling the rehabilitation device behavior under different dynamic and temperature loading conditions, taking directly into account the material behavior of the shape memory alloy.

A Study on Measuring Distribution of Temperature for Instrumented Taylor Impact Test

It is a challenge to apply the infrared detector to a test at higher strain rate because lots of articles report that the detector has an extremelyhigh responsiveness. However, all the research works about the measurement of temperature is focused on 103 s-1 of the strain rate achieved by the split Hopkinson pressure bar test. To evaluate the thermo-mechanical behaviour of materials in higher strain rate range, a method for measuring the temperature in Taylor impact test is required to establish. Additionally, a use of the fiber should be considered to protect the detector from any damages by the impact of the specimen and realize the measurement in the point-like area. In this study, an optically-measuring system of temperature with the infrared detector and PIR fiber is designed. Then, the designed system is introduced into the apparatus based on the instrumented Taylor impact test proposed recently. The temperature rise at a local point on the surface of pure aluminium is measured during the test. Then, a calculation for distribution of temperature from a heat conduction equation is proposed.

Reassessment of the Dynamic Thermomechanical Conversion in Metals.

Heat dissipation still remains an unsolved problem in dynamic plasticity, where nearly adiabatic conditions prevail during high-rate loading scenarios. It is well known that the mechanical energy that is not dissipated as heat during material straining remains stored in the lattice as microstructural defects, and thus, a one-to-one relationship can be expected between the stored energy, the materials microstructure, and its mechanical characteristics. This work demonstrates that this is not so straightforward. High-rate experiments on a Kolsky bar, combined with insitu thermal measurements, were performed on two well-studied materials: pure nickel and aluminum. A dislocation-based constitutive model was used to estimate the mechanical and thermomechanical material behavior. For both materials, the thermal response was observed to be strongly strain rate sensitive, while the mechanical flow, and microstructural characteristics (as characterized by transmission electron microscopy at similar strains), were not. This apparent discrepancy between mechanical and microstructural vs thermal results is discussed, and the concept of thermomechanical conversion is reassessed.

Developing a model for the microscopic material behaviour of a tailored formed joining zone of an aluminium‐steel hybrid solid component

AbstractTailored Forming is a complex process joining two materials before forming. Especially the joining zone of the hybrid solid component is a possible weakness. Hence, our goal is to predict and adjust the thermomechanical material behaviour of the joining zone during and after the tailored forming process. The hybrid solid components studied in this work are composed of the aluminium alloy EN AW‐6082 and the steel 20MnCr5. Metallurgical investigations on both materials show a polycrystalline microstructure. In the steel component, the two constituents ferrite and pearlite can be found with nearly the same volume fraction and randomly distributed grains. Furthermore, between the two bulk materials aluminium and steel, a thin layer of brittle intermetallic phases can be observed. In this work, the intermetallic phases are considered to behave purely thermoelastic, whereas the material model for aluminium, ferrite and pearlite represents thermoplastic material behaviour. We present the dislocation density based models for the thermoplastic materials formulated in a continuum mechanics framework. Thereby, different mechanisms of annihilation contributing to the evolution equation for the internal variables will be on focus.

Experimental and modelling techniques for hot stamping applications

Hot stamping techniques have been developed for the production of complex-shaped components since the 1970s, increasingly used for the automotive industry. The application of these techniques includes hot stamping of boron steel for critical automobile safety components, and solution heat treatment, forming and cold die quenching (HFQ®) for forming complex-shaped high strength aluminium panels of automobile bodies and chassis structures. The developed forming techniques need dedicated experimental testing methods to be improved for characterising the thermomechanical behaviour of materials at the hot stamping conditions, and advanced materials modelling techniques to be developed for hot stamping applications. In this paper, requirements for thermomechanical tests and difficulties for hot stamping applications are introduced and analysed. The viscoplastic modelling techniques have been developed for hot stamping applications. Improved experimental methods have been proposed and used in order to obtain accurate thermomechanical uniaxial tensile test data and determine forming limits of metallic materials under hot stamping conditions.

Modelling the thermoplastic material behaviour of a tailored formed joining zone on a microscopic length scale with focus on the steel component

AbstractSince Tailored Forming is a complex process and the joining zone of the hybrid solid component is a possible weakness, our goal is to simulate the thermomechanical material behaviour of the joining zone during the Tailored Forming process. Investigations on the steel component of the joining zone show a polycrystalline microstructure with the two constituents pearlite and ferrite. In this work, the pearlitic phase is considered to be purely thermoelastic whereas the material model for the ferritic phase represents thermoplastic material behaviour. Besides reasons for the choice of the mentioned material models, we present the thermoplastic material model developed by Zeller et al. [1]. Based on the experimental observation that the persistent deformation is a result of slip of dislocations, state variables are defined to formulate a necessary shear stress to move dislocations as well as temperature and deformation dependent evolution equations for the introduced state variables. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermomechanical characterization of stress localization in glass: An experimental and numerical study

AbstractThermoelastic stress analysis and quantitative calorimetry are full‐field noncontact techniques widely used to study the thermomechanical behaviour of materials. The first one linearly relates the sum of the principal stresses to the temperature variation, and the second one can be used to measure the mechanical dissipation. However, brittle materials such as glass are a priori bad candidates for these techniques. Indeed, their low‐temperature variations under loading lead to very noisy infrared images, and their brittle mechanical behaviour does not allow to deform them significantly. In the present paper, the thermomechanical characterization of a holed glass sample under cyclic loading is performed. A preliminary new filtering methodology has been applied to the thermal movie to remove the noise. The stress field obtained from the thermoelastic stress analysis is well correlated to the finite element model showing that this technique is adapted to study the thermoelastic response of brittle materials. Finally, the corresponding calorimetric response has been determined by using a simplified formulation of the heat diffusion equation. This permits to quantify heat sources and to carry out energy balances.

High temperature and dynamic testing of AHSS for an analytical description of the adiabatic cutting process

In the automotive industry, advanced high strength steels (AHSS) are widely used as sheet part components to reduce weight, even though this leads to several challenges. The demand for high-quality shear cutting surfaces that do not require reworking can be fulfilled by adiabatic shear cutting: High strain rates and local temperatures lead to the formation of adiabatic shear bands (ASB). While this process is well suited to produce AHSS parts with excellent cutting surface quality, a fundamental understanding of the process is still missing today. In this study, compression tests in a Split-Hopkinson Pressure Bar with an initial strain rate of 1000 s-1 were performed in a temperature range between 200 °C and 1000 °C. The experimental results show that high strength steels with nearly the same mechanical properties at RT may possess a considerably different behavior at higher temperatures. The resulting microstructures after testing at different temperatures were analyzed by optical microscopy. The thermo-mechanical material behavior was then considered in an analytical model. To predict the local temperature increase that occurs during the adiabatic blanking process, experimentally determined flow curves were used. Furthermore, the influence of temperature evolution with respect to phase transformation is discussed. This study contributes to a more complete understanding of the relevant microstructural and thermo-mechanical mechanisms leading to the evolution of ASB during cutting of AHSS.

Influence of substrate roughness on bonding mechanism in cold spray

Abstract In this study, optimization of substrate roughness for enhancing interface bonding state in cold spray for different material combinations is performed. Material combinations are classified into four different particle/substrate impact pairs (i.e., soft/soft, soft/hard, hard/soft and hard/hard) based on their thermos-mechanical and physical properties. Finite Element Analysis (FEA) was employed to simulate the models in relation with constant powder size (25 μm), for estimating impact interface parameters. Experimentally, particles were deposited on different roughened substrates in order to compare the simulation results. Microstructures, deposition efficiencies and bond strengths were obtained for the coatings. On the basis of simulation and experimental results, optimum substrate roughnesses for different cases are suggested for better bonding. The optimum roughness for different material combinations was obtained and found to be the fraction of particle size based on the thermo-mechanical behavior of materials upon impact.

On Thermodynamic Consistency of a Stochastic Constitutive Model for Glassy Polymers

A full understanding of the thermomechanical behavior of materials in the glassy state remains one of the outstanding challenges in condensed matter physics. A recently developed stochastic constitutive model (SCM) that explicitly acknowledges nanoscale dynamic heterogeneity has had success in describing a number of nonlinear mechanical and relaxation experiments. The model employs stochastic differential equations to describe evolution of the local thermodynamic variables of entropy and the stress tensor, where the magnitude of the fluctuations is itself a strong nonlinear function of these variables. Notwithstanding the successes of the model, there is a legitimate question whether the formulation is thermodynamically consistent because, unlike the SCM, the stochastic systems treated in the literature satisfy the fluctuation dissipation theorem and assume constant magnitude of fluctuations. We propose expression for the free energy that results in fulfilment of the H-theorem statement and develop the second law in the Clausius–Duhem form for the SCM, where the expression for the power of the external driving forces is modified in the stochastic case as compared to that in standard continuum mechanics.

Infra-red techniques for thermomechanical characterisation of materials and structures

The application of infra-red (IR) imaging techniques to the mechanics of materials and structures has grown considerably over past decades. The expansion is marked by the increased spatial and temporal resolution of the infra-red detectors, faster processing times and much greater temperature resolution. The improved sensitivity and more reliable temperature calibrations of the devices have meant that more accurate data can be obtained than were previously available. The purpose of the special issue is to bring together novel work on all aspects of thermomechanics with the focus on the application of IR imaging approaches. The main thrust is on the analysis of thermomechanical behavior of materials and using this behavior to elicit information on material characteristics, stress patterns, deformation mechanisms and failure modes. Of particular interest are strong thermomechanical couplings that result in complex, often nonlinear behaviour such as visco(thermos)elasticity, anisotropic heat diffusion mechanisms, strain and damage localisation and solid state phase changes. An objective is to share experience on how datarich experimental mechanics can help scientists and engineers to better understand and simulate the behavior of materials and structures. An important aspect is the use of other imaging techniques in conjunction with IR imaging, demonstrating how imaging is the route to a full thermomechanical characterization of both material behaviour and structural response. The special issue starts with a paper that uses a technique known as thermoelastic stress analysis (TSA), which exploits the thermoelastic coupling that exists in materials under load. The thermoelastic coupling was first observed in the 1830s and the mathematical formulation of the thermoelastic effect was derived in the 1850s by Lord Kelvin. Essentially when a material experiences a strain change in the elastic range a small temperature change occurs that can be related to the trace of the stress tensor. In the 1980s single cell scanning IR detection systems were developed to obtain the ‘thermoelastic response’, essentially the output from system was a digital level. The first commercially available system was called SPATE ‘Stress Pattern Analysis by Thermal Emissions’, which was developed in the UK by and organization called SIRA ‘Scientific Instruments Research Associations. The review by Lin, Saman, Khaja, and Rowlands, covers developments from the early days of SPATE to the present, where array based calibrated detection systems are used. One clear shortcoming of TSA is that can only deliver the sum of the principal stresses. The hybrid TSA approach described provides a route for determining the individual stress components by combining the measured thermal data with Airy stress functions. In developing the formulation for TSA it is assumed that the small temperature change occurs isentropically. This is achieved by cyclically loading the specimen in the elastic range (reversible) and at such a rate that the thermal diffusion length is small (adiabatic). A key component of TSA is the use of a lock-in algorithm, which uses a reference signal (usually from a test machine signal generator), to process the data. The IR detection systems used for TSA are costly as they are cryogenically cooled photon detectors with fast framing rates that enable sufficient data to be captured to reconstruct the cyclic thermal response with great accuracy. The second paper in the issue is by Pitarresi and proposes an off-line lock-in correlation algorithm enabling TSA and lock-in thermography A. Chrysochoos University of Montpellier, Montpellier, France

Development of a Pneumatic Bulge Test for High Temperatures and Controlled Strain Rates

Due to new material concepts (e.g. boron-manganese steels), hot stamping of sheet metal parts has emerged in order to produce high strength components. Thereby, the design of hot stamping processes by means of finite element simulations requires information about the thermo-mechanical material behaviour up to high strain levels at various temperatures as simulation input. It is known that hot tensile tests are only evaluable until low strain levels. Therefore, a hot gas bulge test for temperatures in the range of 600 °C to 900 °C and strain rates up to 1/s is being developed. In order to design such a hot gas bulge test, the requirements (e.g. forming pressure) are estimated by finite element simulations. The result is a test bench, which already enables a pneumatic forming of specimens at room temperature and pressures up to 200 bar without any unexpected side effects.

POD Preprocessing of IR Thermal Data to Assess Heat Source Distributions

Infrared thermography is a useful imaging technique for analyzing the thermomechanical behaviour of materials. It allows, under certain conditions, surface temperature monitoring and, via a diffusion model, estimation of heat sources induced by dissipative and/or thermally coupled deformation mechanisms. However, the noisy and discrete character of thermal data, the regularizing effect of heat diffusion and heat exchanges with the surroundings complicate the passage from temperature to heat source. The aim of this paper is to show that the prior use of reduced-basis projection of thermal data improves the signal-to-noise ratio before estimating the heat source distributions. The reduced basis is generated by proper orthogonal decomposition (POD) of physically-admissible thermal fields. These fields are solutions of ideal diffusion problems related to a set of putative heat sources. preprocessing is applied to different direct methods (finite differences, spectral solution, local least-squares fitting) already used in the past. The gain of this preprocessing is determined using a numerical penalizing benchmark test. The methods are finally compared using data extracted from a dynamic cyclic test on a pure copper specimen.

Method for the evaluation of stretch blow molding simulations with free blow trials

Finite-Element (FE) simulations are a valuable tool to support the analysis and optimization of production processes. In order to achieve realistic simulation results, a consistent simulation set-up followed by an evaluation through experiments is crucial. Stretch Blow Molding (SBM) is a commonly applied forming method to produce thin walled bottles. Polyethylene terephthalate (PET) preforms are biaxially stretched into a closed cavity to form a bottle. In this process the thermo-mechanical material behavior during forming greatly influences the performance of the end product and consequently plays a key role for a reliable process simulation. To ensure a realistic material representation in the simulation model, an adequate material model is calibrated with stress-strain curves from biaxial tests. Thin PET-sheets are stretched under defined temperatures and strain rates. These representative experiments include process simplifications regarding geometry, heating and deformation parameters. Therefore, an evaluation step subsequent to the simulation set-up is inevitable. This paper presents a method for extracting temperature dependent stress-strain-curves from experiments close to the production process which enables the crucial evaluation of a process simulation. In the SBM process, the wall thickness distribution of the bottle refers to the preform deformation over time but does not fully define the thermo-mechanical material behavior. In the presented method, PET-preforms receive thermal treatment with Infrared (IR)-heaters from an SBM-machine and are subsequently inflated into free air (free-blow-trial). An IR-camera is used to obtain the temperature distribution on the preform immediately before blowing. Two high speed cameras are synchronized with a pressure sensor to consequently calculate reliable stress-strain curves at any point on the preform surface. These data is finally compared to results from FE-simulations of the free blow trials.

Mechanical damage assessment of Glass Fiber-Reinforced Polymer composites using passive infrared thermography

This study deals with characterization of the damage and thermomechanical behavior of the Glass Fiber-Reinforced Polymer composite materials (GFRP), submitted to static tensile loadings, using a passive infrared thermography technique. During mechanical testing, thermal measurements are performed by means of an IR camera. The thermal data post-processing involves the analysis of both the thermal maps and the thermomechanical behavior of the material. The thermal maps analysis allows qualitative evaluation of the created material damage at high stress levels. While the thermomechanical analysis gave us a quantitative evaluation of the material damage, for both low and high stress levels, through definition of a new thermoelastic damage variable.

From Orthogonal Cutting Experiments towards Easy-to-Implement and Accurate Flow Stress Data

Despite recent advances in numerical modeling of machining processes showing a significant potential for shortening product and process design activities, the broader application of the Finite Element Method (FEM)–based modeling approaches in the manufacturing industry is still limited by the expensive and time-consuming techniques involved in obtaining accurate material flow stress data. This study proposes a new efficient approach consisting of a combined numerical-empirical methodology for inversely identifying the thermomechanical material behavior of AISI 316L stainless steel from machining experiments. In order to establish the work material's flow stress properties under the extreme conditions of machining, the Johnson–Cook (JC) constitutive equation is chosen to consider the impacts of strain, strain-rate, and temperature. Due to the unstable material behavior of AISI 316L stainless steel under certain thermomechanical conditions in the primary shear zone, the calibration of a damage model is integrated into the process for evaluating the flow stress data. The methodology approximates the material constants of the JC relationship by systematically comparing experimentally measured machining results with those predicted equivalents of a 2D implicit FEM process model. The methodology is experimentally verified on AISI 316L stainless steel with respect to cutting forces, chip geometries and temperatures in the tool-chip interface.

High-temperature strain measurement using active imaging digital image correlation and infrared radiation heating

A technique for non-contact and full-field high-temperature strain measurement of a sample subjected to radiation heating using active imaging digital image correlation is described in this work. A high-performance quartz lamp heater system was designed to reproduce transient thermal environments experienced by hypersonic vehicles. The digital images of the test sample surface at various temperatures are captured using a novel active imaging optical system based on a combination of monochromatic light illumination and bandpass filter imaging. Subsequently, the captured images are processed by a robust reliability-guided displacement tracking algorithm with an automatic reference image updating scheme to extract full-field thermal deformation. With the improvements made in both the imaging system and correlation algorithm, the de-correlation problem of speckle patterns caused by the thermal radiation and surface oxidation of the heated test object are effectively addressed, enabling reliable deformation measurement in extremely high temperature environments. The performance of the proposed active imaging digital image correlation technique is verified by two experiments: (1) measurement of the uniform thermal strains of a chromium–nickel austenite stainless steel sample which is heated from room temperature to 1300 °C, and (2) measurement of the non-uniform thermal strain fields of a woven C/SiC composite at 1550 °C. The test results show that the active imaging digital image correlation is an easy-to-implement yet effective optical technique for high-temperature strain measurement, and has great potential in characterizing thermo-mechanical behaviour of materials and structures for hypersonic vehicles. Limitations and potential improvements of the present technique are also discussed.