Large Thermal Deformation Research Articles

Concurrent topology optimization of parts and their supports for additive manufacturing with thermal deformation constraint

A typical manufacturing failure in powder bed additive manufacturing (AM) is caused by the collision between the already printed structure and the powder recoater due to large thermal deformation. This paper presents a density-based topology optimization (TO) approach to concurrently design parts and their supports that are not susceptible to such a manufacturing failure. The thermal deformation during the layer-by-layer AM process is predicted based on an inherent strain method, and the top surfaces' upward deformations are constrained below the powder layer thickness to avoid recoater collision. To increase the design freedom of TO, the parts and their supports are composed of lattice unit cells with different volume fractions, whose effective stiffness matrixes are predicted by the numerical homogenization and surrogate models. Besides, an overhang constraint and a two-field-based length scale control formulation are also utilized, ensuring overall manufacturability. With the proposed approach, lightweight structures with optimized mechanical properties and controllable manufacturing qualities can be obtained. Numerical examples are given to demonstrate the effectiveness of the proposed approach.

Achieving local overheating and length scale control in density-based topology optimization

The main defect of additive manufacturing technology based on laser melting is that large thermal deformation is easily generated inside the part during the forming process. The traditional method is to adjust the process parameters, stress relief annealing and other hardware methods. From the point of view of design, this paper proposed a method to reduce the deformation by changing the product structure and configuration without changing the process form and design goals. The layer-by-layer thermal process model is considered as a constraint in the SIMP topology method, and the MMA is applied as the optimization solver to take care of the multi-constraints topology optimization. Then, the Heaviside based morphological filter is applied to control the minimum length of the part in the optimization results. The validity of the proposed method is tested by the MBB beam and cantilever compliance minimization problems.

Shape Adjustment and Experimental Study of a Shape Memory Cable (SMC) Structure.

Large deployable cable net antennas have attracted extensive attention worldwide because of their simple structure and high storage ratio. The cable net structure is affected by long exposure in a harsh space environment during satellite operation, resulting in large thermal deformation and stress relaxation, which leads to a degradation of antenna performance. To address the thermal deformation of the cable net structure, a shape memory cable (SMC) net structure model was proposed with surface accuracy as the research objective. Specifically, we aimed to utilize its phase transition characteristics to adjust the thermal deformation of cable net structure and improve its surface accuracy. A shape memory cable net structure model with a diameter of 2.2 m was built, and a normal temperature experiment and high- and low-temperature experiments were carried out. High- and low-temperature test refers to environmental simulation testing of shape memory cable net structures under high- and low-temperature conditions. This was done to determine whether the adjustment method for surface accuracy meets the requirements. The results showed that the shape memory alloy wire has a relatively stable ability to adjust the surface accuracy of the cable net structure at room temperature. During temperature cycling, the thermal deformation of the shape memory cable net structure is slight, and the surface accuracy is good. Compared with ordinary cable net structures, the shape memory cable net structure has improved surface accuracy by 44.4% and 45.2% at high and low temperatures, respectively. This proved the effectiveness of the method for adjusting surface accuracy. These experimental results offer guiding significance for engineering applications.

Thermal performance of CRTS Ⅱ slab track-bridge structure under extreme temperatures: Numerical simulation

Large thermal deformation of CRTS Ⅱ slab track due to thermal effect is harmful to the operational safety of the high-speed railway. In order to explore the thermal deformation of the CRTS Ⅱ slab track under extreme ambient temperatures, the parameters of two bilinear cohesive zone models (CZMs) of vertical interlayer and longitudinal slab interlayer are firstly deduced by the splitting and shearing modelling tests, respectively. Secondly, the transient heat transfer models of CRTS Ⅱ slab track-bridge structure incorporatedwith two CZMs are verified by our previous thermal experimental results. Finally, thermal responses (e.g. temperature distribution, stress, and displacement) of the track structure are calculated under two extreme high temperatures (i.e.50℃ and 60℃) and two low temperatures (i.e.-20℃ and −40℃), respectively. Results show that the heat transfer model could effectively reflect thermal performance of the CRTS II track-bridge structure under extreme high and low temperatures. The internal temperatures and stresses of the track slab rapidly change in two hours and then have a slow change rate, an obvious mutation is found in the stress and displacement response under extreme high temperatures. Furthermore, the maximum arching displacement change from the ends of the track slab under the 50℃ to the middle of the track slab under the 60℃, while both the maximum warping displacement occur at the ends of the track slab under the −20℃ and −40℃, respectively.

Control of Welding Residual Stress in Large Storage Tank by Finite Element Method

T-joint welding is a key manufacturing process of large storage tanks. However, complex residual stresses are generated and have a great effect on the structural integrity of storage tanks. The high residual stress caused by welding and the discontinuous structure may result in tank cracking and failure. In this work, the residual stress distributions on the inner surface, outer surface, and thickness direction of the T-joint were investigated by using the finite element method and indentation test method. The effect of local PWHT with different heating temperatures, heating rates, and heating widths on the residual stress distribution was also discussed. Results show that the residual stress of the T-shaped joint is high due to the serious structure discontinuity, multi-layer welding, and high strength. Among all the stresses, the circumferential residual stress is the highest and most concentrated in the outer weld connected with the annular plate. The residual stress gradually decreases with the increase in the heat treatment temperature. When the heating rate is less than 106 °C/h, the residual stress gradually decreases with the decrease in the heating rate. The large thermal deformation caused by heat treatment can be simultaneously avoided by heating the inside and outside of the T-joint. The residual stress decreases with the decrease in the width of the heating zone. The residual stress can be regulated by using a smaller width in the heating zone. An optimized heat treatment scheme with a heating temperature of 700 °C, heating rate of 56 °C/h, and heating width of 200 mm was proposed, which has a good ability to control residual stresses and improve the quality of the T-joint. It also has a good application in engineering.

Active Adjustment Method for Space Deployable Structure Considering Parameter Uncertainties

Affected by the alternating temperature field in space, the space deployable structure produces large thermal deformation, which leads to the deterioration of surface accuracy and performance. To achieve high surface accuracy of the deployable structure in the presence of model parameter uncertainties caused by manufacturing and assembly errors as well as complicated space environments, an active surface adjustment method for a space deployable structure based on interval model updating and the kriging model is proposed. First, the initial finite element model is modified by the interval model updating based on the universal gray number to reveal the inherent relationship between the adjustment amount and surface accuracy. Second, based on the updated model, the sensitivity analysis of the adjusting rod is carried out to reveal the influence degree of the adjusting rod on the displacement of the surface measuring point so as to determine the position of the adjusting rod quickly. Then, the interval optimization model of the surface accuracy adjustment is established. Next, the optimal adjustment amount is determined through optimization iteration combined with the kriging model. Finally, a numerical example is presented to demonstrate the feasibility and validity of the proposed active adjustment method.

Optimization design of surface accuracy of SMC structure driven by space thermal radiation

The cable-net structure of large peripheral truss deployable antenna is radiated by the sun and planets for a long period of time, resulting in large thermal deformation and mechanical relaxation, which degrades the service performance of the antenna. In this paper, an optimal control strategy for surface accuracy of shape memory cable-net (SMC) structure under space thermal radiation is proposed. First, the shape memory alloy (SMA) is introduced and integrated with the cable-net structure. The SMC structure with shape memory effect (SME) has the capability to transform the space thermal radiation into a favorable factor for on-orbit adjustment of surface accuracy, in order to realize the on-orbit shape preservation of cable-net antenna in space environment. Then, the space temperature field of cable-net structure is calculated. Finally, the cable-net structure with a diameter of 2 m is simulated, and the influence of the diameter of SMA wire, slope for the relation between the critical stress of phase transformation and inverse phase transformation and temperature on surface accuracy are analyzed. It shows that the implantation of SMA can greatly improve the surface accuracy of cable-net structure in a single thermal cycle.

Experimental study on heat transfer attenuation due to thermal deformation of horizontal GHEs

In soil thermal energy storage system with high temperature (exceeds 50 °C), the pipe thermal deformation (buckling) can lead to air gap between pipe and soil. During the pipe-soil heat transfer, the gap is actually the thermal resistance. The aim of this study is to investigate the gap between pipe and clay soil, the corresponding heat transfer attenuation and the influence of the initial pipe bending degree. However, the pipe is buried in the soil, the gap is hard to measure. So, the novelty of this work is to put forward the measuring methods of the pipe-soil gap width and the heat transfer attenuation rate from pipe to soil during the heating process. By experiment, the thermal deformation of pipe, the gap width and the heat transfer attenuation rate are analysed. The results indicate that the pipe displacement and the gap width increase with the temperature rise. Correspondingly, the heat transfer attenuation can be found in the gap-side soil area. During the heating process, the heat transfer attenuation rate will decrease with the temperature rise. In comparison, larger pipe bending could get larger thermal deformation, larger gap width and larger heat transfer attenuation rate.

Design of thermal diodes using asymmetric thermal deformation of a Kirigami structure

Driving heat flow in one direction and blocking in the other direction can provide a potential advantage in thermal engineering – e.g., thermal management and energy harvesting. Pushing the idea toward a passive operation of the thermal diode can save more thermal energy, yet extremely challenging. In this work, we design a Kirigami structure that can function as a thermal diode providing a symmetry-breaking of heat flux activated by an asymmetric thermal deformation. Large thermal deformation of the metamaterial by an asymmetric arrangement of bilayer-hinges in a symmetric Kirigami geometry is the fundamental principle for the design of thermal diodes. Analytical modeling of thermomechanical deformation, thermal resistance, and thermal rectification together with finite element simulations can help select two base materials and geometric modifications, finally generating a phase-map for the design of thermal diodes with the Kirigami structure. The asymmetric heat flow powered by a large directional thermal deformation can maximize a thermal rectification ratio, which is the essential requirement for the design of thermal diodes. The designed thermal diodes with the Kirigami structure produce a record-high thermal rectification ratio of ~240, implying that we open a new avenue of mechanical metamaterials in thermal engineering. The developed innovation can potentially be applied for the next generation thermal devices such as heat engines, refrigeration, thermal regulations of buildings, and thermal logic devices.

Thermoelastic analysis of a geological repository with distributed decay heat sources by the image method in combination with a numerical integration scheme

The disposal of nuclear waste involves thermo-mechanical reaction of the host rock to the buried waste – a distributed heat source that decays. To solve the problem within the half infinite space confined by the ground surface, an image method is developed. Specifically, a negative image of the heat source with the ground surface as the mirror, i.e. a mirrored heat sink is utilized so that the normal traction generated by the heat source can be counterbalanced, and a numerical scheme of integration of the classical Cerruti solution is developed to include the effect of tangential shear traction on the ground surface caused by the heat sources and their mirrored sinks. For a conceptual repository model, large thermal shear stress, tensile stress, and deformation occur at the corner, between adjacent drifts, and at the boundary of the repository area, respectively. For a prescribed thermal loading, it is more efficient to mitigate the thermo-mechanical effects through enlarging the pit spacing than increasing the drift spacing.

Experimental testing of fly ash containing recycled aggregate concrete exposed to high temperatures

ABSTRACT This paper presents the results of experimental studies which were carried out to investigate residual properties of heat-treated concrete mixes made with recycled aggregates by adding fly ash. Mixes made with natural aggregates were used as control mixes. Concrete cylinders made of both types of mixes were employed in the experimental programme. The cylinders were heat treated at temperature varying from 100°C to 1000°C in increments of 100°C and were tested in compression and tension to determine their respective residual strengths. The residual strengths of both types of mixes followed a similar pattern of reduction at higher temperature exposure. The residual compressive stress and elastic modulus became nearly 15% at 900°C and 700°C, respectively. A slightly higher mass loss was observed in RAC mixes due to the use of recycled aggregates which contained more free water and sustained larger thermal deformation on heating at high temperatures.

Thermo-Mechanical Coupling Modeling of Active Phased Array Antennas

The large thermal deformation of the panel in active phased array antennas (APAAs) caused by the thermal power consumption of the internal devices and the change in the ambient temperature in actual operating conditions will result in a considerable decline in the electromagnetic (EM) performance of radar. Experimental research indicates that the thermal deformation of the panel is affected not only by temperature but also by the constraints caused by the installation. Thus, this paper proposes a thermo-mechanical coupling modeling method for an APAA considering the panel temperature and constraints. The coupling model can accurately predict the panel thermal deformation, thereby providing a basis for the compensation of the EM performance of radar. The thermal deformation of the panel is decomposed into a thermal expansion and an offset. Models of the thermal expansion with respect to temperature and the offset with respect to the constraint force are established. The final thermo-mechanical coupling model is obtained by coupling the two models. Experiments verify that the prediction accuracy of the coupling model is 86.3%, which is 21.0% higher than that of a finite element software simulation. The content of this research has practical engineering importance for improving the overall performance of APAAs.

A Passive Thermal Switch with Kirigami‐Inspired Mesostructures

The objective of this work is to design a passive thermal switch with Kirigami‐inspired architected materials composed of bilayer hinges and to analyze thermomechanical and thermal performance at the ON/OFF states based on a composition of materials and geometry of metamaterials. Analytical models are constructed to determine the thermal deformation and heat transport of the metamaterial, followed by finite element‐based simulations. A two‐step design approach − i) selection of multi‐materials and ii) geometric modification of mesostructures provides a robust method for the design of a passive thermal switch with Kirigami‐inspired metamaterials. The mesostructures designed have potential as a passive thermal switch enabling to engineer a large thermal deformation, a small thermal resistance, and a sizable thermal switch ratio. With a systematic approach, this work demonstrates that Kirigami structures with multi‐materials can be potentially used for a tool of thermal transport.

A feasible method of support slimming based on the different thresholds of polar angles in selective laser melting

The large thermal deformation induced by the high temperature gradient severely restricts the threshold level of polar angle in selective laser melting (SLM). As such, different types of support structures are usually added to improve the manufacturing capacity and broaden the application of SLM technology. However, enormous support structures will result in large time consumption, material waste, high labor intensity and inferior surface quality of part. In this research, a feasible method of slimming support structures was presented. Firstly, a series of experiments for exploring the thresholds of polar angles at different azimuth angles relative to the recoating direction of powders were conducted and the effect of powder recoating by scraper on the building of inclined surfaces was analyzed. Then, a generation algorithm of slimmed support structures was proposed based on the varying thresholds of polar angles. In addition, several typical cases demonstrated the usefulness of this method by eliminating the quantities of their support structures by 35% in average.

Thermoelastic-tunable magnetic response of BiFeO3thin film on colloidal photonic crystal substrate fabricated by pulsed laser deposition

Investigation of the thermoelastic-tunable magnetic response of polycrystalline bismuth ferrite thin film is herein reported. The polycrystalline bismuth ferrite thin films were grown by room-temperature pulsed laser deposition on the designed polymer colloidal photonic crystal substrates. The evolution of these films was assessed by X-ray diffraction testing and scanning electron microscopy images. The thermal-dependent magnetic measurements and magneto-optical properties were also investigated as a function of deposition substrate and target–substrate distances via the Faraday rotation method. Due to the large anisotropic thermal deformation of the polymer colloidal interface, the huge rotation downshift was observed versus a slight thermal change. Furthermore, we found that this returnable reduction had the same behavior during the heating and cooling processes. This reversible thermal property of the polymer colloidal crystal in combination with the magnetic response of the multiferroic compounds can possibly lead to innovations in sensors and switches based on thermomagneto-optical coupling and facilitate the nanoscale thermal characterization.

A molecular dynamics investigation into the mechanisms of material removal and subsurface damage of nanoscale high speed laser-assisted machining

Molecular dynamics is employed to study the mechanism of material removal and subsurface damage of monocrystalline silicon when it is under a nanoscale high-speed laser-assisted grinding of a diamond tip. Laser-assisted machining (LAM) is that the workpiece is locally heated by an intense laser beam before material removal. The effects of laser moving speed, laser pulse intensity and laser spot radius are thoroughly investigated in terms of atomic trajectories, phase transformation, temperature distribution, grinding temperature, grinding force and friction coefficient. The investigation shows that a higher laser moving speed reduces the subsurface damage and improves the material remove rate because of fewer atoms with five and six coordination atoms and more chips. Besides, both tangential grinding force (Fx) and normal grinding force (Fy) decrease as the laser moving speed increases. The distribution of high-temperature zone strongly depends upon the effect of laser pulse intensity and laser spot radius. Larger laser pulse intensity can make the material more fully softened before being removed. Moreover, as the laser pulse intensity becomes larger, the friction coefficients became smaller, the material remove rate improves and the depth of grinding increases. However, larger laser pulse intensity may result in a larger thermal deformation of workpiece. A larger laser spot radius reduces the grinding depth but increases the width of laser irradiation zone on machined surface. Thus, a suitable laser spot radius can improve the material removal rate. These results indicate that it is possible to control and adjust the laser parameters according to laser moving speed, laser pulse intensity and laser spot radius, and it provides a potential technology to improve a surface integrity and a smoothness of ground surface.

Experimental studies on nonuniform heat transfer and deformation performances for trough solar receiver

The nonuniform heat transfer and deformation performances of trough solar receiver are measured and analyzed. The temperature field of glass envelope for trough solar receiver is directly measured by Flir thermal infrared imager. On the positive region face to the collector, the glass temperature is remarkably higher than ambient temperature for concentrated solar radiation, and it approaches to a minimum near the center region positively face to the collector. On the opposite region face to the collector, the glass temperature remarkably decreases, and its minimum may be lower than ambient temperature because of infrared radiation to the sky. The experimental thermal efficiency is 57.8–65.6% with fluid temperature 473K and effective solar flux of 183–842W/m2, and the maximum glass temperature difference in the same cross section can reach as high as 44.2K. The temperature distribution and thermal efficiency of trough solar receiver are further calculated by 1-D uniform model and 2-D nonuniform model, and the calculated results fit with experimental data. According to experimental observation results, the absorber tube remarkably bends because of nonuniform heat transfer, and the glass envelope will probably break as the absorber tube contacts with the glass envelope for large thermal deformation. In order to avoid the glass envelope breakage, the deformation of receiver can be reduced by increasing the flow rate.

Effects of water and temperature variations on deformation of limestone aggregates, cement paste, mortar and High Performance Concrete (HPC)

The present paper deals with the dimensional variations of limestone aggregates, cement paste, mortar, and High Performance Concrete (HPC) made with these constituents, when subjected to temperature changes in drying and sealed conditions. Variable water contents are studied for each material. The overall experimental results were obtained using strain gauges and Digital Image Correlation technique (DIC) is also applied for one configuration test. A significant difference between coefficients of Thermal Expansion (CTE) of cement paste and aggregate is observed, which leads to a large differential thermal deformation, able to explain thermal damage for temperatures between 50 °C and 100 °C. Water content has a great influence on material thermal dilation, causing delayed deformations especially for temperatures above 60 °C and for initially saturated samples. These experimental data may allow Thermo-Hydro-Mechanical models to be improved so that the in-situ long term behaviour of concrete can finally be predicted. The results confirm the dilation coefficient contrast between paste and aggregate, and also show that this contrast is markedly affected by the drying conditions and the initial state of saturation.

Thermally assisted electric field control of magnetism in flexible multiferroic heterostructures.

Thermal and electrical control of magnetic anisotropy were investigated in flexible Fe81Ga19 (FeGa)/Polyvinylidene fluoride (PVDF) multiferroic heterostructures. Due to the large anisotropic thermal deformation of PVDF (α1 = −13 × 10−6 K−1 and α2 = −145 × 10−6 K−1), the in-plane uniaxial magnetic anisotropy (UMA) of FeGa can be reoriented 90° by changing the temperature across 295 K where the films are magnetically isotropic. Thus, the magnetization of FeGa can be reversed by the thermal cycling between 280 and 320 K under a constant magnetic field lower than coercivity. Moreover, under the assistance of thermal deformation with slightly heating the samples to the critical temperature, the electric field of ± 267 kV cm−1 can well align the UMA along the two orthogonal directions. The new route of combining thermal and electrical control of magnetic properties realized in PVDF-based flexible multiferroic materials shows good prospects in application of flexible thermal spintronic devices and flexible microwave magnetic materials.

Influence of Strengthening Rib Location on Temperature Distribution of Heavy Vertical Lathe Worktable

During heavy vertical lathe working, heat caused by oil film friction and self-generated is transmitted to worktable through contact surface between oil film and worktable .That makes temperature distribution of worktable uneven and lead large thermal deformation which can affect the machining accuracy. In this article, the temperature field of worktable with reinforcing plate in different locations is simulated. Based on heat transfer theory and the actual working conditions, convective heat transfer coefficient of worktable and the initial boundary conditions are calculated. The simulation reveals the influence law of reinforcing plate arrangement on temperature distribution of worktable and the optimum position of reinforcing plate is obtained. These results provide a theoretical basis for the structural design of worktable.