Inelastic Heat Research Articles

Experimental Investigation of Strain Rate Influence on Anisotropy of Uniaxial Tensile Mechanical Properties of CuFe2P Alloy Sheet.

Wire crimping, a process commonly used in the automotive industry, is a solderless method for establishing electrical and mechanical connections between wire strands and terminals. The complexity of predicting the final shape of a crimped terminal and the imperative to minimize production costs indicate the use of advanced numerical methods. Such an approach requires a reliable phenomenological elasto-plastic constitutive model in which material behavior during the forming process is described. Copper alloy sheets, known for their ductility and strength, are commonly selected as terminal materials. Generally, sheet metals exhibit significant anisotropy in mechanical properties, and this phenomenon has not been sufficiently investigated experimentally for copper alloy sheets. Furthermore, the wire crimping process is conducted at higher velocities; therefore, the influence of the strain rate on the terminal material behavior has to be known. In this paper, the influence of the strain rate on the anisotropic elasto-plastic behavior of the copper alloy sheet CuFe2P is experimentally investigated. Tensile tests with strain rates of 0.0002 s-1, 0.2 s-1, 1 s-1, and 5.65 s-1 were conducted on sheet specimens with orientations of 0°, 45°, and 90° to the rolling direction. The influence of the strain rate on the orientation dependences of the stress-strain curve, elastic modulus, tensile strength, elongation, and Lankford coefficient was determined. Furthermore, the breaking angle at fracture and the inelastic heat fraction were determined for each considered specimen orientation. The considered experimental data were obtained by capturing the loading process using infrared thermography and digital image correlation techniques.

Estimation of inelastic heat fraction of Ti-6Al-4V alloy using inverse analysis

This work aims to determine the inelastic heat fraction of Ti-6Al-4V alloy by coupling the experimental and Finite Element Method (FEM) results by an inverse analysis approach. The inelastic heat fraction is a crucial factor in revealing information on the deformation mechanism and forms the basis of the microstructural evolution of a material. Hot compression tests on Ti-6Al-4V alloy at various temperatures and strain rates were carried out using the Gleeble thermo-mechanical simulator. An axisymmetric thermo-mechanical modelling was carried out using the FEM package i.e., ABAQUS. The work reveals that the inelastic heat fraction increases with an increase in strain rate and decreases with an increase in deformation temperature. The proposed method is capable of predicting the inelastic heat fraction of a material for a wide range of temperatures and strain rates. The FEM modelling and experimental results exhibit good agreement. The findings of this study can aid in more precise material modelling and contribute to the formation of a thermo-mechanical material database.

Coherence-enhanced thermodynamic performance in a periodically driven inelastic heat engine

Coherence-enhanced thermodynamic performance in a periodically driven inelastic heat engine

Reversible inter-particle bonding in SPH for improved simulation of friction stir welding

Friction stir welding (FSW) is a complex joining process which is governed by multiple intertwined physical phenomena. Besides friction, inelastic heat generation, and heat conduction, it involves high plastic deformations, resulting in a need for a numerical method being able to handle all these. Such a scheme is smoothed particle hydrodynamics (SPH), which is a mesh-free computational technique. Absence of a fixed mesh results in the ability of the method to deal with another challenge of friction stir welding, a coalescence of initially separate workpieces into one due to bonding mechanisms. The background of this phenomenon is a transition from contact between two pieces to one continuum due to enormous changes in several material condition, such as temperature, pressure, strain, and strain rate. This work deals with a new development related to bonding, which will provide deeper understanding about the physical weld formation during FSW. The SPH framework must be extended to consider this bonding mechanism. This involves the bonding criterion definition, the interaction type change, and the SPH–SPH contact formulation. Then, the implementation is tested for two different examples, a compression test and FSW.

A parametric numerical study of ASB-assisted chip formation in high-speed machining of Ti-6Al-4V

ABSTRACT The present study aims at studying the influence of some parameters on the numerical results obtained from the simulation of the high-speed machining process using a commercial finite element computation code, namely Abaqus-Exp. Starting from models available by default in the computation code, comparisons are made between full and weak thermo-mechanical coupling, and the influence of the fracture strain and inelastic heat fraction values is studied. The high-strength Ti-6Al-4 V titanium alloy is taken as an example. Continuous, serrated and segmented chips are reproduced at low, moderate and high cutting speeds and the differences observed between experimental and numerical results are discussed.

Understanding Deformation Behavior in Uniaxial Tensile Tests of Steel Specimens at Varying Strain Rates.

Uniaxial tensile tests are routinely conducted to obtain stress-strain data for forming applications. It is important to understand the deformation behavior of test specimens at plastic strains, temperatures, and strain rates typically encountered in metal forming processes. In this study, the Johnson-Cook (J-C) flow stress model was used to describe the constitutive behavior of ASTM International (ASTM) A 1008 steel specimens during uniaxial tensile tests at three different average strain rates (10-⁵ s-¹, 10-³ s-¹, and 10-¹ s-¹). The digital image correlation (DIC) technique was used for displacement and strain measurement, and two-dimensional (2D) infrared (IR) imaging was employed for temperature measurement. Separate optimization studies involving relevant finite element (FE) modeling with appropriate measured data yielded optimum values of convective heat transfer coefficients, J-C parameters, and inelastic heat fraction variables. FE modeling employing these optimum parameter values was then used to study the mechanical behavior. While FE predictions matched measured strain localization and thermal field very well in the intermediate- and low-rate experiments, the high-rate test showed narrower strain localization and a sharper temperature peak in the experiment. Possible use of a higher steel thermal conductivity value and/or exclusion of material inhomogeneities may have resulted in discrepancies between computed and measured temperature and strain fields. The study shows that an optimized set of parameters obtained with a controlled test could be reasonably applied for other tests conducted at very different strain rates.

Measurement and Prediction of Temperature Increase during Isothermal Forging of Titanium–6 Aluminum–4 Vanadium

Abstract Titanium–6 aluminum–4 vanadium is isothermally forged at 1,023–1,223 K (750°C–950°C) at 0.05, 0.1, and 1.0/s using a 1,500-ton forging press simulator. The temperature increase of a workpiece (100 mm in diameter, 50 mm in height) during forging is measured by inserting thermocouples into the workpiece, and this increase is predicted using finite element analysis (FEA). The physical properties necessary for the FEA are determined using the same material as the forged one in order to improve the prediction accuracy. The true stress–true strain curves and heat transfer coefficient between the workpiece and dies are inversely obtained. The temperature change predicted by FEA using those parameters and curves agreed well with the experimental results. In the FEA, the inelastic heat fraction, which is a conversion factor for plastic deformation energy to heat energy, did not depend on the strain rate, but it remained the same at strain rates of 0.05–1.0/s. This result suggests that the large heat capacity of the workpiece in this study suppressed the effect of heat transfer between the workpiece and dies/air on the temperature of the workpiece. It is also shown that the prediction accuracy of the workpiece’s temperature could be degraded unless the appropriate specific heat value is used, especially near the β transus temperature, which can be affected by the composition of light elements.

Fully Coupled Thermo-Mechanical Modelling of the Initial Phase of the Friction Stir Welding Process Using Finite Element Analysis

In the present study a finite element model of the FSW process is built and a FSW butt weld of two aluminium alloy 2024-T3 plates is simulated using a fully coupled thermo-mechanical analysis. Besides the welded panels, the model includes the backing plate and the welding tool as physical bodies, which makes the simulations more realistic. The model also includes conductive heat transfer between the contact surfaces of the FSW tool, the aluminium plates and the backing plates, heat generation due to friction between the tool and the welded plates and heat loss to the ambient air due to convection.The simulated model makes it possible to analyse and check several aspects of this welding technique. It is proved that tool geometry has a vital importance in the FSW process. The influence of using instantaneous or ramped velocity at the beginning of the simulations is also studied. Moreover, it is seen that the mesh used in the finite element analysis and the adjustment of the inelastic heat fraction have a great influence on the obtained results.

Computational prediction of probabilistic ignition threshold of pressed granular Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) under shock loading

The probabilistic ignition thresholds of pressed granular Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine explosives with average grain sizes between 70 μm and 220 μm are computationally predicted. The prediction uses material microstructure and basic constituent properties and does not involve curve fitting with respect to or prior knowledge of the attributes being predicted. The specific thresholds predicted are James-type relations between the energy flux and energy fluence for given probabilities of ignition. Statistically similar microstructure sample sets are computationally generated and used based on the features of micrographs of materials used in actual experiments. The predicted thresholds are in general agreement with measurements from shock experiments in terms of trends. In particular, it is found that grain size significantly affects the ignition sensitivity of the materials, with smaller sizes leading to lower energy thresholds required for ignition. For example, 50% ignition threshold of the material with an average grain size of 220 μm is approximately 1.4–1.6 times that of the material with an average grain size of 70 μm in terms of energy fluence. The simulations account for the controlled loading of thin-flyer shock experiments with flyer velocities between 1.5 and 4.0 km/s, constituent elasto-viscoplasticity, fracture, post-fracture contact and friction along interfaces, bulk inelastic heating, interfacial frictional heating, and heat conduction. The constitutive behavior of the materials is described using a finite deformation elasto-viscoplastic formulation and the Birch-Murnaghan equation of state. The ignition thresholds are determined via an explicit analysis of the size and temperature states of hotspots in the materials and a hotspot-based ignition criterion. The overall ignition threshold analysis and the microstructure-level hotspot analysis also lead to the definition of a macroscopic ignition parameter (J) and a microscopic ignition risk parameter (R) which are statistically related. The relationships between these parameters are established and delineated.

An Experimental-Numerical Method to Determine the Plastic Work Converted into Heat Applied on AHSS

In this Study a Heat Transfer Model in Combination with Experimental Tests is Used to Determine the Portion of Plastic Work that is Converted into Heat (also Known as the Taylor-Quinney Coefficient, Inelastic Heat Fraction or IHF and Generally Noted β) during the Deformation of Two Modern Automotive Advanced High Strength Steels (AHSS) DP600 and DP1000. Therefore, Uniaxial Tension Tests were Performed under Vacuum in a Deformation-Dilatometer and the Temperature was Captured by Fine-Wire Thermocouples on Three Different Points on the Surface of the Tensile-Specimen during the Plastic Deformation. Afterwards, a Heat Transfer Model was Used to Calculate the Heat Loss at the Points of the Temperature Measurements and they were Accounted in the Final Energy Balance to Determine the Fraction of Plastic Work Converted to Heat. the Results Show that the Fraction of Plastic Work Converted into Heat is Decreasing from 1 to 0.21 over a Tensile Strain Range of 0 to 0.18. Finally, a Finite Element Model of the Tensile Test was Used to Show the Improvement of the Determined Factor in the Calculation of the Temperature Field Compared to the Classical Assumption that β Equals to 0.9.

Influence of temperature and strain rate on the longitudinal compressive crashworthiness of 3D braided composite tubes and finite element analysis

This article reports the longitudinal compressive crashworthiness of three-dimensional four-step circular braided carbon/epoxy composite tubes at temperatures of 23, −50, and −100℃ under strain rate ranging from 340 to 760/s both experimentally and finite element analysis. The experimental results showed that the compression strength, stiffness, and specific energy absorption increased with the decrease in temperature and with the increase in strain rate. It also showed that, the compressive damage morphologies were sensitive to the change in temperature and strain rate. A coupled thermal-mechanical numerical analysis was conducted to find the thermo/mechanical coupling effect on the compressive crashworthiness of the three-dimensional composite tube. The temperature distributions in the braided preform and the resin during the impact compression were also calculated through finite element analysis. From the finite element analysis results, the inelastic heat generation was seen to be more in the preform than the matrix and its distribution and accumulation led to the damage progress along the loading direction.

Transient heat generation and thermo-mechanical response of epoxy resin under adiabatic impact compressions

This investigation examines transient heat generation and the thermal-mechanical response of epoxy resin subjected to quasi-static and dynamic compression. High-speed optical and infrared imaging systems are used to obtain visual and thermo-graphic images during dynamic tests. It is found that the post-yield response of epoxy resin depends on whether the loading is quasi-static or dynamic. Results from quasi-static compression show that yield is followed by post-yield softening, plastic flow and final hardening. An obvious difference for high-rate compression is that post-yield softening persists without a final hardening phase. From the high-speed infrared images, localizations of adiabatic heating and temperature distribution were identified morphologically in high strain rate compressions. Dynamic mechanical analysis (DMA) and quasi-static data for elevated temperatures confirm the temperature sensitivity of epoxy resin. By calculating the different pixel numbers in infrared images at the different temperature values, the inelastic heat fraction is estimated to be about 0.45 for the epoxy. A simple constitutive relationship that describes the mechanical behavior of the polymer, such as elasticity–plasticity, post-yield softening and final strain hardening for quasi-static loading, as well as adiabatic-heating softening for dynamic loading, is proposed.

Thermo-mechanical behaviors of 3-D braided composite material subject to high strain rate compressions under different temperatures

ABSTRACTThis article reports the compressive behaviors of 3-D braided basalt fiber tows/epoxy composite materials under the temperature range of 23–210°C with the strain-rate range of 1300–2300 s−1. A split Hopkinson pressure bar apparatus with a heating device was designed to conduct the out-of-plane compression tests. It was found that compression modulus, specific energy absorption, and peak stress decreased with the elevated temperatures, while failure strain gradually increased with the elevated temperatures. Compression modulus and peak stress were more sensitive to the temperature effect, whereas failure strain and specific energy absorption were more easily affected by the strain rate effect. The plasticity can be divided into two types: (a) the platform-shape plasticity; or (b) the slope-shape plasticity. The experimental condition of 150°C with 1827 s–1 was a dividing threshold to differentiate the compression-failure mode and the shear-failure mode. The authentic microstructural finite element analysis results revealed that the distribution and accumulation of the inelastic heat led to the development of shear bands. Braided reinforcement had an important influence on the damage characteristics. When the temperature was below Tg, the material underwent a significant temperature rise during failure. But above Tg, the temperature rise was relatively steady.

A thermo-mechanically coupled finite strain model considering inelastic heat generation

The procedure for reuse of finite element method (FEM) programs for heat transfer and structure analysis to solve advanced thermo-mechanical problems is presented as powerful algorithm applicable for coupling of other physical fields (magnetic, fluid flow, etc.). In this case, nonlinear Block-Gauss–Seidel partitioned algorithm strongly couples the heat transfer and structural FEM programs by a component-based software engineering. Component template library provides possibility to exchange the data between the components which solve the corresponding subproblems. The structural component evaluates the dissipative energy induced by inelastic strain. The heat transfer component computes the temperature change due to the dissipation. The convergence is guaranteed by posing the global convergence criterion on the previously locally converged coupled variables. This enables reuse of software and allows the numerical simulation of thermo-sensitive problems.

Interfacial effects on fracture nucleation and propagation in crystalline–amorphous energetic material systems

Local failure modes, such as the nucleation and propagation of a pre-existing crack, have been investigated for energetic materials with a viscoelastic binder and crystalline grains subjected to dynamic thermal and mechanical loading conditions. A crystalline plasticity with dislocation density, finite viscoelasticity, dynamic fracture nucleation and propagation methods, and finite element methods were used to study crack nucleation and propagation due to dynamic thermal and mechanical loading conditions. The interrelated effects of dislocation densities, grain boundary (GB) misorientations, polymer binder relaxation, and interactions between crystal and binder were coupled to material thermal decomposition, adiabatic inelastic heating, viscous dissipation heating, and thermal conduction to analyze interfacial fracture behavior in RDX–estane systems. The predictions indicate that cracks propagated toward the binder and were arrested due to the viscous nature of the polymer binder and plasticity buildup. For low angle misorientations, the pre-existing crack propagated toward the binder with increasing crack tip speed until it reaches the binder, at which point the crack was arrested. For high angle misorientations, the crack propagated toward the binder and was arrested, adjacent to the binder, due to plastic deformation and lattice rotations. A secondary crack eventually nucleated and propagated to the interface, where it was arrested.

Determination of the fraction of plastic work converted into heat in metals

A new approach for measuring the fraction of plastic work that is converted into heat (also known as the Inelastic Heat Fraction, IHF or the Taylor–Quinney coefficient) during the plastic deformation of metals is proposed in this paper. The method is based on the uniaxial tension of a slender metal rod, with the lateral surface of the rod being under well-defined thermal boundary conditions. The corresponding one-dimensional model of heat transfer during the process is considered in the form that accounts for all major effects. The current approach is based on the fact that the unknown IHF enters this heat equation linearly and therefore can be found explicitly if all the other terms are accounted for experimentally. The experimental procedure developed is based on local measurements of temperature and its gradients and of strain and velocity with the aid of an infrared (IR) camera and a Digital Image Correlation (DIC) system, respectively. A vacuum tube that encloses the specimen was designed to provide control over the heat losses due to convection during the test. A fitting method based on Hermite splines allows us to deal with smooth functions instead of noisy data sets, improving the overall accuracy of the procedure. The IHF is obtained as a function of plastic strain for a set of strain rates for four materials: 303 and 316 stainless steels, commercially-pure titanium (CP Ti) and the alloy Ti–6Al–4V. The findings indicate that the IHF is not constant with the plastic strain and furthermore, it is also sensitive to the strain-rate. Our measurements revealed that depending on the material the IHF was generally between 0.55 and 0.8, but it could also be as low as 0.3.

Inelastic heat fraction estimation from two successive mechanical and thermal analyses and full-field measurements

A new method to identify inelastic heat fraction (β factor) evolution models is proposed in this paper. It is based on: (i) simultaneous kinematic and thermal field measurements on heterogeneous tensile tests and (ii) Finite Element Updating (FEU) inverse method. Two inverse calculations, that involve a Levenberg–Marquardt optimization algorithm, are successively used to identify on the one hand the material parameters of a mechanical constitutive model (anisotropic plasticity coefficients and hardening parameters) and on the other hand the plastic power ratio converted into heat. The power balance of the thermomechanical problem is calculated and presented, and the assessment of thermal heat sources is detailed. Finally, six mechanical parameters and four evolution models of β are successively identified for commercially pure titanium. Results confirm a strain dependency of β.

Theory of local heating in single molecular bridge junctions

Inelastic scattering of conduction electrons due to vibronic coupling gives arise local heating of the single molecular junction. We have discussed the problem based on a self-consistent theory treating inelastic heat generation and heat diffusion/transport to thermalized electrodes on equal footing, including both electron-phonon coupling and electron correlation effects. We found that the local heating is suppressed by reducing phonon damping either via a large electronic energy gap or electron correlation. Theoretical results were compared with available experimental results in literature, especially the case of alkanedithiol single-molecular scanning tunneling microscope (STM) break junctions.

Influence of Dissipated Energy on Shear Band Spacing in HY-100 Steel

To analyze the formation of multiple shear bands in HY-100 steel, we consider an infinitely extended layer of finite thickness subjected to shear loading. The perturbation approach, associated with numerical methods, is used to determine the instability modes. The criteria of Wright–Ockendon and Molinari are used to determine the shear band spacing. The hypothesis consisting in considering the proportion of plastic work dissipated as heat (quantified by the Taylor–Quinney coefficient β) as independent of the loading path is now recognized as highly simplistic. The present study attempts to provide a systematic approach to the inelastic heat fraction evolution for a general loading within the framework of thermoviscoplastic standard modeling, including a number of material parameters as strain hardening, strain rate sensitivity, and thermal softening. The effect of each material parameter on the shear band spacing is determined by using a power law constitutive relation. The Johnson–Cook and power law models are used to illustrate the influence of the constitutive relation on the results for the adiabatic shear band spacing by studying the response of HY-100 steel. We have compared our results with available experimental results in the literature over a very wide range of strain rates (103–105 s−1). In this study, we show that the variation in the Taylor–Quinney parameter β(γ) as a function of shear strain is an important parameter that plays a significant role in the calculation of the shear band spacing.

Influence of material parameters on serrated chip prediction in finite element modeling of chip formation Process

A 2D Finite Element Model set up using the Arbitrary Lagrangian Eulerian (A.L.E) formulation proposed in Abaqus/Explicit© (v6.8) is employed to analyze the sensitivity to material input parameters and cutting speed when predicting serrated chip in cutting process. No artificial criterion is employed to create the chip or to initiate serrated chip formation. Results showed significant influence of the thermal parameters (thermal conductivity, specific heat and inelastic heat fraction) and material behaviour law (Johnson-Cook law coefficients) on the serrated chip phenomena, especially in the case of the thermal conductivity and specific heat, but as well for parameters no so mentioned in previous research like the strain hardening or the yield stress.