Thermo-mechanical Finite Element Analysis Research Articles

Welding residual stress analysis of 347H austenitic stainless steel boiler tubes using experimental and numerical approaches

A three-dimensional finite element model that can simulate multiple start/stop welding process of 347H austenitic stainless steel boiler tubes was developed to estimate welding residual stress distributions. Two-dimensional axisymmetric finite element model and threedimensional finite element model that exhibit half symmetry along circumferential direction are also employed for verification purposes. Sequentially coupled thermo-mechanical finite element analyses were carried out using three different models to obtain and compare temperature fields and resulting residual stress fields. In addition, node-based temperature heat input method and element-based body flux heat input method were applied to three-dimensional half symmetry model to review the adequacy of heat input model that can best estimate welding residual stress distribution in the tubes. Finally, welding residual stresses obtained using different models were compared with measured data, and it was observed that results using three-dimensional finite element model that can take into account welding start/stop effects are in good agreement with those of measured data obtained by the X-ray diffraction (XRD) method.

Damage Manipulation and In Situ Repair of Composite T-Joints

A novel method for the in situ repair of subcritical damage in complex composite structures (T-joints) is demonstrated. In situ repair is achieved through the use of embedded microvascular networks within the structure. Once failure occurs, the embedded vascules are ruptured, providing a route for the injection of a repair agent directly into the damage site. The efficacy of these networks at infiltrating damage is assessed for a number of configurations of vascularized T-joints. The effect of vasculature on the mechanical performance of the component is assessed both numerically and experimentally. Two- and three-dimensional thermomechanical finite element analyses are performed to determine the influence of vasculature on thermal residual stresses and joint strength under 90 deg tensile (pulloff) loading. Failure mechanisms observed during 90 deg tensile pulloff testing agree well with the prediction from finite element analyses. In all cases, successful interaction of vascules with the damage is achieved, giving promise for the future applicability of these channels for in situ repair applications. Further optimization is required, but the vascularized deltoid configuration shows considerable promise for future industrial adaptation.

3D numerical study on microwave induced stresses in inhomogeneous hard rocks

The aim of this research is to present a novel 3D simulation procedure to assess microwave induced stresses in inhomogeneous hard rocks at a microstructure level. For a realistic rock model a two-component 3D microstructure is generated by a Voronoi tessellation algorithm. The two components are microwave absorbing (phase A) and transparent (phase T), respectively. In order to calculate the electric field inside the inhomogeneous rock, a 3D finite difference time domain (FDTD) simulation is performed. The absorbed heat is computed and applied as temperature distribution in a subsequent thermo-mechanical finite element (FE) analysis in order to calculate the thermally induced stresses. Two irradiation times (15s and 25s) and a microwave power of 25kW at 2.45GHz as well as three different morphologies are analyzed. Moreover, the phase transformation of quartz at 573°C is considered in the FE model. The influence of the anisotropic nature of the quartz grains is assessed by comparing the stress formation in the isotropic with those of the anisotropic case. A comparative analysis with a homogeneous model is performed in order to draw conclusions on the influence of the microstructure on the microwave induced stress formation. High maximum principal stresses on the boundaries of the microwave absorbing phase (phase A) exceeding the tensile strength are observed in the 15s irradiation model. After 25s of microwave irradiation even higher stresses as a consequence of phase transformation of quartz are determined. In the anisotropic case significantly more areas with high maximum principal stresses especially in phase T are observed. Microwave irradiation experiments on granite samples are performed in order to correlate the numerical results with experiments.

Analysis and Design of Thermally Actuated Micro-Cantilevers for High Frequency Vibrations Using Finite Element Method

Vibrational behavior of thermally actuated cantilever micro-beams and their mechanical response at moderately high frequency under a non-harmonic periodic loading is studied in this paper. Two different configurations are considered: 1) a straight beam with two actuation layers on top and bottom which utilizes the bimorph effect to induce bending; 2) a uniform beam with base excitation, where the beam is mounted on an actuator which moves it periodically at its base perpendicular to its axis. Generally, vibrating micro-cantilevers are required to oscillate at a specified frequency. In order to increase the efficiency of the system, and achieve deflections with low power consumption, geometrical features of the beams can be quantified so that the required vibrating frequency matches the natural frequencies of the beam. A parametric modal analysis is conducted on two configurations of micro-cantilever and the first natural frequency of the cantilevers as a function of geometrical parameters is extracted. To evaluate vibrational behavior and thermo-mechanical efficiency of micro-cantilevers as a function of their geometrical parameters and input power, a case study with a specified vibrating frequency is considered. Due to significant complexities in the loading conditions and thermo-mechanical behavior, this task can only be tackled via numerical methods. Selecting the geometrical parameters in order to induce resonance at the nominal frequency, non-linear time-history (transient) thermo-mechanical finite element analysis (using ANSYS) is run on each configuration to study its response to the periodic heating input. Approaches to improve the effectiveness of actuators in each configuration based on their implementation are investigated.

Effect of process control mode on weld quality of friction stir welded plates

Friction stir welding (FSW) is a solid state welding process which requires no filler material where the heat input is generated by frictional energy between the tool and workpiece. The objective of the present work is to conduct a fully coupled thermomechanical finite element analysis based on Arbitrary Lagrangian Eulerian (ALE) formulation for both “Force-Controlled” and “Displacement-Controlled” FSW process to provide more detailed insight of their effect on the resulting joint quality. The developed finite element models use Johnson- Cook material model and temperature dependent physical properties for the welded plates. Efforts on proper modeling of the underlying process physics are done focusing on the heat generation of the tool/workpiece interface to overcome the shortcomings of previous investigations. Finite elements results show that “Force-Controlled” FSW process provides better joint quality especially at higher traveling speed of the tool which comes to an agreement with published experimental results.

Dynamic Analysis of Carbon Fiber-Reinforced Wood Composites Based on Finite Element Model

Carbon fibers were pretreated by ultrasonic agitation and then were used to manufacture carbon fiber-reinforced wood composites (CFRWCs). The modulus of rupture (MOR) and the modulus of elasticity (MOE) for the CFRWCs were measured. The effects of the compounding pattern and proportion of carbon fiber on the dynamic performance were investigated by dynamic thermomechanical analysis (DMA) and finite element analysis (FEA). Finite element modeling was performed to simulate composite dynamic analysis using ANSYS commercial software. DMA results showed that the critical parameters for the mixed boards appeared to initially increase and then decrease as the content of carbon fiber increased. ANSYS simulations showed that the first-order natural frequency increased for single-sided and two-sided boards as the composite’s carbon fiber content increased, whereas it initially increased and then decreased for mixed boards. In general, the mixed board with 20 wt.% carbon fiber had maximum dynamic performances. ANSYS simulation results were very close to those of the static test, which demonstrated that the finite element model was accurate.

Finite element analysis of residual stress distribution in a thick plate joined using two-pole tandem electro-gas welding

A computational approach considering moving heat sources was introduced to predict the residual stress distribution produced by electro-gas welding (EGW) joints. Considering the two-pole tandem of EGW, a finite element analysis (FEA) was suggested to evaluate the thermal behavior of EGW applied to a joint of ultra-thick plates. Based on a thermo-mechanical FEA, the profiles of the residual stresses are investigated for EH40 thermo-mechanical control process (TMCP) steel plates with a thickness of 80mm. In order to simulate the weaving motion of two weldment poles, a quasi-steady heat flux model is introduced based on Goldak’s double ellipsoidal heat source model. The X-ray diffraction methods were used to measure the residual stress field on the surface treated by chemical polishing. The residual stress profiles determined by the FEA and measurement showed quite good agreement, as regards the values both peaks and of the profile. Residual stresses relieved by post-weld treatments, particularly by ultrasonic peening and toe grinding, is also presented.

Comparative study of ductile fracture prediction of 22MnB5 steel in hot stamping process

Damage growth and ductile fracture prediction is an urgent question for hot stamping operations. Numerous models for ductile fracture prediction in cold forming processes have been extensively developed. There is a real need to compare them to choose the best suitable one for hot stamping applications. In the present study, several ductile fracture criteria under the category of “uncoupled phenomenological criterion” and the “fully coupled damage criterion,” i.e., the continuum damage mechanics (CDM)-based Lemaitre model, were employed to compare their prediction capability on ductile failure prediction. These two categories of criteria were coded into an explicit thermo-mechanical finite element code dedicated to hot stamping simulation. Both hot forming limit and hot tensile tests of 22MnB5 steel were conducted in order to provide suitable data for calibrating these models. Numerical results of the applications of these models to the hot stamping process simulation of an automotive B-pillar were compared with the experimental ones. It is concluded that thermo-mechanical finite element analysis in conjunction with CDM-based Lemaitre model can be used as a reliable tool to predict ductile damage and fracture of 22MnB5 steel in hot stamping process.

Influence of ring growth rate on damage development in hot ring rolling

As an incremental forming process of bulk metal, ring rolling provides a cost effective process route to manufacture seamless rings. In the production of hot rolled rings, defects such as porosity can sometimes be found in high alloyed steel, manufactured from ingots having macro-segregation. For the reduction of the waste of material and improvement of product quality, a better understanding of the relations between parameters in the hot ring rolling process and the occurrence of porosity is needed.In this study round bars were used to manufacture rings on an industrial ring rolling mill. Different ring growth rates were applied to investigate the influence on the occurrence of porosity in the final rings. The hot rolled rings were inspected by ultrasonic testing, of which the results were also validated by metallographic investigation.In addition to the experimental investigations, coupled thermo-mechanical multi-stage finite element (FE) analysis was performed with integrated adaptive motion control of the rolls. A damage indicator was implemented in a user-defined elasto-viscoplastic material model. The deformations, stresses as well as temperature history from preform forging were included as initial conditions for the rolling stage. Damage indication from the numerical model matches the experimental result in the considered process conditions.In spite of the suggestion of a more careful process when a low ring growth rate is used in hot ring rolling, experimental and numerical studies demonstrate that with a low ring growth rate there is an increased susceptibility to damage as compared to application of a high ring growth rate.

Thermal stresses and kinetics of phase transformation on the run-out table after hot strip rolling of low-carbon steels

A thermo-elastic analysis coupled with a phase change model is employed to evaluate thermal stresses as well as progress of austenite decomposition after hot strip rolling operations. The thermo-mechanical finite element analysis is utilized to assess the distributions of temperature and thermal stresses during cooling while at the same time, a model based on the additivity rule is used to determine the rate of austenite to ferrite decomposition. The proposed model is applicable to multi-pass rolling schedules while the effects of different process parameters including the rolling speed and cooling configuration of the run-out table can be taken into account. In order to verify the predictions, the measured and predicted temperature-time diagrams are compared and a reasonable agreement is observed. The results show that significant variations occur in the thermal stresses in the period of austenite to ferrite transformation owing to changes in the thermo-physical properties such as the thermal expansion and Young modulus during continuous cooling.

Prediction of the Grain-Microstructure Evolution Within a Friction Stir Welding (FSW) Joint via the Use of the Monte Carlo Simulation Method

A thermo-mechanical finite element analysis of the friction stir welding (FSW) process is carried out and the evolution of the material state (e.g., temperature, the extent of plastic deformation, etc.) monitored. Subsequently, the finite-element results are used as input to a Monte-Carlo simulation algorithm in order to predict the evolution of the grain microstructure within different weld zones, during the FSW process and the subsequent cooling of the material within the weld to room temperature. To help delineate different weld zones, (a) temperature and deformation fields during the welding process, and during the subsequent cooling, are monitored; and (b) competition between the grain growth (driven by the reduction in the total grain-boundary surface area) and dynamic-recrystallization grain refinement (driven by the replacement of highly deformed material with an effectively “dislocation-free” material) is simulated. The results obtained clearly revealed that different weld zones form as a result of different outcomes of the competition between the grain growth and grain refinement processes.

A Thermo Fluid and Thermo Mechanical Modelling of a Microtubular Solid Oxide Fuel Cell Stack for Unmanned Aerial Vehicles

This work presents a three-dimensional thermo fluid and thermo mechanical computational analysis of a microtubular solid oxide fuel cell (SOFC) stack as part of a microtubular SOFC generator. The latter powers a lithium-ion battery as a microtubular SOFC hybrid power system to extend the flight endurance of a mini unmanned aerial vehicle. The microtubular SOFC stack and balance of plant components, comprising microtubular SOFC generator, were designed based on computational fluid dynamics (CFD) assumptions. The stack design process has been further upgraded with thermo mechanical finite element analysis (FEA) simulation to capture deformations and stresses in the stack due to thermal expansion of microtubular SOFCs and manifolds. The developed microtubular SOFC stack will be built based on CFD and FEA predictions and tested to determine the suitability of the microtubular SOFC hybrid power system for this application.

A Thermo Fluid and Thermo Mechanical Modelling of a Microtubular Solid Oxide Fuel Cell Stack for Unmanned Aerial Vehicles

This work presents a three-dimensional thermo fluid and thermo mechanical computational analysis of a microtubular solid oxide fuel cell (SOFC) stack as part of a microtubular SOFC generator. The latter powers a lithium-ion battery as a microtubular SOFC hybrid power system to extend the flight endurance of a mini unmanned aerial vehicle.The microtubular SOFC stack and balance of plant components, comprising microtubular SOFC generator, were designed based on computational fluid dynamics (CFD) assumptions. The stack design process has been further upgraded with thermo mechanical finite element analysis (FEA) simulation to capture deformations and stresses in the stack due to thermal expansion of microtubular SOFCs and manifolds. The developed microtubular SOFC stack will be built based on CFD and FEA predictions and tested to determine the suitability of the microtubular SOFC hybrid power system for this application.

Influence of the clamps configuration on residual stresses field in friction stir welding process

Friction stir welding is a joining process developed in 1991 by The Welding Institute. This welding technique is a solid-state joining process leading to joints with good mechanical performance and low residual stresses. In all welding techniques, the clamping systems have an important role in determining the quality of the welds and mechanical characteristics. Even more in friction stir welding, the position of the clamps plays a critical role because it is mainly a mechanical process with high forces involved. In this article, the correlation between the residual stress field and configurations of clamps has been established numerically. For this purpose an uncoupled thermo-mechanical finite element analysis has been carried out. The mechanical loads due to the tool have been also implemented into the model. The thermal and mechanical models have been validated on temperature field recorded by an infrared camera and residual stress field measured by X-ray diffraction analysis. The friction stir welding test was conducted on 6-mm-thick 5754 H111 aluminium alloy plates.

Langmuir probes design for the actively cooled divertor baffle in WEST

The WEST project (W-Environment in Steady-State Tokamak) aims to transform the Tore Supra limiter configuration to an x-point divertor, providing a test bed for ITER-like plasma-facing components (actively cooled W monoblocs) under high heat flux, steady-state plasma irradiation. The lower divertor includes an actively cooled, W-coated CuCrZr baffle to provide neutral compression and improve particle exhaust. As part of the new diagnostic equipment of Tore Supra within WEST project, a set of Langmuir probes will find place on the baffle in order to provide plasma flux and electron temperature measurements for physics studies and real-time machine protection functions during steady-state discharges. On the baffle top surface, irradiation coming from the plasma, energetic ripple-ions losses, photons and energetic neutrals from charge exchange reactions produce power fluxes up to 3MW/m2, representing a challenge for the Langmuir probes operating conditions. In this paper Copper–Chrome–Zirconium (CuCrZr) cylindrical probe concept design is proposed. Finite element thermo-mechanical analysis (FEA) confirmed the consistency of this solution under the steady-state plasma condition in the worst case (highest thermal load).

Microwave propagation and absorption and its thermo-mechanical consequences in heterogeneous rocks

A numerical analysis in a two-component model rock is presented including the propagation and absorption of a microwave beam as well as the microwave-induced temperature and stress distributions in a consistent way. The analyses are two-dimensional and consider absorbing inclusions (discs) in a non-absorbing matrix representing the model of a heterogeneous rock. The microwave analysis (finite difference time domain — FDTD) is performed with values of the dielectric permittivity typical for hard rocks. Reflections at the discs/matrix interfaces and absorption in the discs lead to diffuse scattering with up to 20% changes of the intensity in the main beam compared to a homogeneous model rock. The subsequent thermo-mechanical finite element (FE) analysis indicates that the stresses become large enough to initiate damage. The results are supported by preliminary experiments on hard rock performed at 2.45GHz.

Finite Element Modelling of Welding Residual Stress and Its Influence on Creep Behavior of a 2.25Cr-1Mo-0.25V Steel Cylinder

Thermo-mechanical finite element analysis was performed to predict the residual stress distribution in a single-U butt weld of a 2.25Cr-1Mo-0.25V steel cylinder using ANSYS. An axisymmetric model of the cylinder with a wall thickness of 150mm, having a 55-pass weld, was adopted in the study to reduce computation complexity. The influence of welding residual stress on creep behavior of the welded joint subjected to internal pressure under operation condition was investigated using the continuum damage mechanics (CDM) theory. The results showed that the distribution of welding residual stress in the multi-pass weld was quite complex, and the residual stress can be relaxed to a lower level in a short time during creep, which can accelerate the creep damage of the welded joint.

Analysis of new Gleeble tensile specimen design for hot stamping application

Hot tensile testing is useful to understand the material behavior at elevated temperatures. Hence it is of utmost importance that the test condition is accurate enough to derive stress-strain data in fully austenitic state and to ensure homogeneous deformation throughout the gauge length of the specimen. But present limitation of standard Gleeble hot tensile sample geometry could not be used to achieve a uniform temperature distribution along the gauge section, thus creating errors of experimental data. In order to understand the effect of sample geometry on temperature gradient within the gauge section coupled electrical-thermal and thermo-mechanical finite element analysis has been carried out using Abaqus, with the use of viscoplastic damage constitutive equations presented by Li (1). Based on the experimental study and numerical analysis, it was observed that the new sample geometry introduced by Abspoel (2), is able to achieve a better uniformity in temperature distribution along the gauge length; The temperature deviation along the gauge length within 25 ◦ C during soaking and 5 ◦ C after cooling and onset of deformation); also the strain deformation is found to be almost homogeneous.

Thermal Borehole Shear Device

This paper presents the details of a new modified borehole shear device that has the capability of measuring the impact of temperature on the in situ shear stress–displacement curves for soil–concrete interfaces. The thermal borehole shear device incorporates concrete shoes with embedded heaters, a pneumatic loading device for application of horizontal normal stresses, and an automated loading system with local vertical displacement and load measurement systems that permits either displacement–control or load–control testing. A methodology for measurement of the soil–concrete shear stress–displacement curves and for evaluation of the drained interface shear strength failure envelopes at different temperatures is presented in this study. Typical results from proof-of-concept tests performed in a clay layer compacted in a laboratory tank in a borehole in a silty sand deposit in the field are presented in this paper. The results are synthesized to show how the impacts of temperature and normal stress on the normalized shear stress–displacement curves can be evaluated. These normalized curves can be measured on a site-specific basis for the calibration of thermo-mechanical load transfer analyses or finite element analyses, which are often used to design and evaluate soil–structure interaction in drilled shaft foundations with geothermal heat exchangers (energy foundations).

Investigation of Clamping Effect on the Welding Residual Stress and Deformation of Monel Plates by Using the Ultrasonic Stress Measurement and Finite Element Method

Welding of nickel-based alloys is increasingly used in the industry to manufacture many important components of the marine industries, chemical processing, etc. In this study, a 3D thermomechanical finite element (FE) analysis is employed to evaluate residual stresses and deformations caused by the tungsten inert gas (TIG) welding of Monel 400 (Nickel-Copper alloy) plates. The FE results related to the residual stresses and deformations have been verified by using the hole-drilling stress measurement and common dimensional measurement tools, respectively. Residual stresses analyzed by the FE simulation are then compared with those obtained from ultrasonic stress measurement. The ultrasonic stress measurement is based on acoustoelasticity law, which presents the relation between the acoustic waves and the stress of material. The ultrasonic stress measurement is carried out by using longitudinal critically refracted (LCR) waves which are longitudinal ultrasonic waves propagated parallel to the surface inside the tested material. Two welded plates are experimentally prepared (with and without using clamp) to investigate the clamping effect on the welding residual stress and deformations. By utilizing the FE analysis along with the LCR method, the distribution of longitudinal residual stress could be achieved. It has been concluded that the applied methodologies are enough accurate to distinguish the clamping effect on the welding residual stresses and deformations of Monel plates.