TiNi形状记忆合金丝在约束态热循环后的力学特性

研究了TiNi形状记忆合金丝在约束态热循环后的应力-应变特性.结果表明,如果停止加热并加载,或者在母相状态下冷却但是尚未发生正相变时加载,加载应力-应变曲线首先呈线弹性关系,然后才出现应力平台.如果在冷却过程中发生正相变的时候停止冷却并加载,则加载应力-应变曲线只出现应力平台.与之相似,如果在约束态冷却过程中停止冷却并卸载,卸载应力-应变曲线首先呈线弹性关系,然后才出现应力平台.如果在加热过程中停止加热并卸载,则卸载应力-应变曲线立即出现应力平台.     

A new viscoelastic method of calculation of low-temperature thermal stresses in asphalt layers of pavements

The paper presents a new method of calculation of thermal stresses in asphalt layers of pavements induced by cooling or heating. The method, developed by the author, is based on the analytical solution for the linear viscoelastic Burgers model extended numerically for the case of asphalt layers whose rheological parameters are strongly dependent on temperature. The coefficient of thermal expansion and Poisson’s ratio may optionally be taken either as constant or varying with temperature. Two equivalent numerical methods have been developed, named as ‘the incremental method’ and ‘the stress increase and relaxation method’. The computer program was developed for calculation of thermal stresses in several different cases of temperature impact, included constant rate cooling and heating, change of temperature and relaxation and a cycle of first cooling, relaxation and second cooling. What the most important practically, the solution for any change of temperature, modelling real winter changes in long period of time, was also developed. Several examples of calculations are presented. The results obtained with use of this method are promising and appear to be acceptable. The advantages of the new method in comparison with existing methods are that it takes fully into account the viscoelastic properties of a layer and the method is correct if the material does not meet the time–temperature superposition principle.     

Optimisation of Ni–Ti shape memory alloy response time by transient heat transfer analysis

Nickel–Titanium (Ni–Ti) shape memory alloys (SMAs) are commonly applied in commercial actuator design due to the high associated fatigue and tensile strengths, low cost and high activation temperature. Consequently, Ni–Ti SMAs provide an opportunity for the development of novel electromechanical actuators. However, the cooling response time is typically of significantly larger duration than the associated heating response time. The applicability of SMA actuators would be significantly greater if the cooling response time was reduced to allow a symmetric, high speed activation profile. This work provides insight into the opportunities associated with enhancing thermal heat transfer efficiency to achieve this objective. An explicit model of the temperature of Ni–Ti SMA wire is developed to estimate the temperature–time profile during resistive heating. A finite-difference equation is developed to predict the associated temperature during cooling. These models are used to confirm that for a typical scenario, the cooling stage dominates the total response time, and that lagging with a highly conductive media can be used to dramatically reduce the cooling response time. The finite-difference equation is validated against steady state data, and extended to provide insight into the effects of SMA lagging, including the effects of periodic excitation on cooling rate and the minimum observed SMA temperature during a heating cycle. The outcomes of this work are generally applicable to any axisymmetric transient heat transfer optimisation problem.     

Influence of thermoviscoelastic properties and loading conditions on the recovery performance of shape memory polymers

This work investigated the influence of material properties and loading conditions on the recovery performance of amorphous shape memory polymers using a recently developed thermoviscoelastic model. The model incorporated the time-dependent mechanisms of stress and structural relaxation and viscoplastic flow to describe the glass transition of the material from a soft viscoelastic rubber to a hard viscoplastic glass. The model captured many important features of the unconstrained strain recovery response and of the stress hysteresis observed in the constrained recovery response. A parameter study was developed that varied the model and loading parameters one-by-one to compare their effects on the start and end temperatures and recovery rate of the unconstrained recovery response and on the stress hysteresis of the constrained recovery response. The loading parameters included the cooling rate, the annealing time, and the high and low temperatures of the programming stage and the heating rate of the recovery stage. The results confirmed experimental observations that viscoelasticity is the underlying mechanism of the unconstrained recovery response. In contrast, the constrained recovery response was influenced by the interaction of many different mechanisms, including thermal expansion and structural and stress relaxation. For the loading parameters, the cooling rate of the programming stage and the heating rate of the recovery stage had the largest influence on both the constrained and unconstrained recovery response.     

Reproducibility of thermal effects within the glass transition region

The sensitivity of DSC-traces to controlled ageing in successive heating/cooling cycles within the glass transition region was studied. The effects of cooling rate both of the melt and of the supercooled liquid, as well as of the heating rate, were considered. It was concluded that within the transformation region the cooling rate of supercooled liquid exerts the predominant effect on the reproducibility of the DSC-curves. It was demonstrated that the thermal history of a sample during glass formation may be erased by only one heating/cooling cycle within the transformation region.     

Effects of thermal rates on the thermomechanical behaviors of amorphous shape memory polymers

Shape memory polymers (SMPs) are polymers that can recover a large pre-deformed shape in response to environmental stimuli, such as temperature, light, etc. For a thermally triggered (or activated) amorphous SMP, the pre-deformation and recovery of the shape require the temperature of the material to traverse the glass transition temperature T _g under constrained or free conditions. In this paper, effects of thermal rates on the thermomechanical behaviors of amorphous SMPs are investigated. Under uniaxial compression, during a temperature cycle (cooling followed by heating), the stress decreases to zero as the temperature decreases to below the glass transition temperature, and increases to a value larger than the initial stress (termed stress overshoot) as the temperature is raised above the glass transition temperature. These observations are examined by a thermoviscoelasticity model that couples the nonequilibrium structural relaxation and temperature dependent viscoelastic behavior of the material. In addition, using this model, stress-temperature behaviors during temperature cycles with various thermal rate conditions and tensile loading conditions are studied.     

Internal stress and solid solubility effects on the thermal expansivity of Al-Si eutectic alloys

Thermal expansion measurements are reported for a number of as-cast Al-Si eutectic alloys including a Sr-modified alloy which gives nearly spherical Si particles. The measurements were obtained by heating and cooling over repeated temperature cycles between room temperature and 500°C. In general, lower expansivity values were measured on the cooling cycle as compared with the heating cycle, resulting in a net positive permanent deformation at room temperature. Analytical solutions are described for the thermal expansivity of a concentric-spheres model for a Si particle contained within an Al matrix. The effect of plastic flow in the Al is included. Overall, the predictions show reasonable agreement with the measured expansivities. The observed differences between heating and cooling are of the same order as that which is predicted. At high temperatures, the measured increase in expansivities is smaller than calculated. The latter effect is explained by the decrease in expansivity which results from an increasing solid solubility of silicon in aluminum with increasing temperature.Paper presented at the Ninth International Thermal Expansion Symposium, December 8–10, 1986, Pittsburgh, Pennsylvania, U.S.A.     

A Method for Cure Process Design of Thick Composite Components Manufactured by Closed Die Technology

During the manufacture of polymer-matrix composite components the cure degree must be uniform to have a good quality of the product. For thick composite components this condition is not often respected in fact the cure degree trend between the core and the external surface is different causing structural and geometrical/dimensional unconformities. In most cases, these problems are caused by a wrong design of cure process in terms of thermal cycle and tooling, therefore the cure cycle must be designed and optimized. The optimization of cure thermal cycle should include several performance criteria for the production system such as the targeted cure degree, the targeted maximum temperature of the part and the duration of the cure cycle as well as the production system limitations such as the maximum allowable heating rate, the maximum allowable cooling rate etc. This work aims to define by thermochemical phenomena a first step toward the definition of a method to optimize the cure degree of a thick composite components by focusing particular attention to the aspects of thermal degradations and residual stress.     

Heating/cooling channels design for an automotive interior part and its evaluation in rapid heat cycle molding

Rapid heat cycle molding (RHCM) is a recently developed innovative injection molding technology. Rapid heating and cooling of the injection mold is the most crucial technique in RHCM because it not only has a significant effect on part quality but also has direct influence on productivity and cost-efficiency. Accordingly, Heating and cooling system design plays a very important role in RHCM mold design. This study focuses on the heating/cooling system design for a three-dimensional complex-shaped automotive interior part. Heat transfer simulation based on finite element analysis (FEA) was conducted to evaluate the thermal response of the injection mold and thereby improve heating/cooling channels design. Baffles were introduced for heating/cooling channels to improve heating/cooling efficiency and uniformity of the mold. A series of thermal response experiments based on full factorial experimental design were conducted to verify the effectiveness of the improved heating/cooling channels design with baffles. A mathematical model was developed by regression analysis to predict the thermal response of the injection mold. The effects of the cavity surface temperature on weld mark and surface gloss of the part were investigated by experiments. The results show that the developed baffle-based heating/cooling channels can greatly improve thermal response efficiency and uniformity of the mold. The developed mathematical model supplies an efficient approach for precise predication of mold thermal response. As the cavity surface temperature raises to a high enough level, automotive interior parts with high gloss and non-weld mark surface can be obtained.     

Temperature and Time Dependence on Recovery Stress Relaxation in Nickel Titanium Wire during Isothermal Holding at High Temperatures

In this study, the influence of kinematically constrained thermal cycling (heating, isothermal holding and cooling) on the recovery stress in annealed nickel titanium wire was investigated. A 4% pre‐strained nickel titanium wire was heated to temperatures (150, 200, 250 and 300 °C) much higher than the austenite finish temperature. It was observed that the maximum recovery stress obtained at different conditions decreases significantly after the first thermal cycle and reduces gradually with further increasing the number of thermal cycles. It was also seen that the recovery stress increases with time during isothermal holding at 150 °C. During isothermal holding at other temperatures, the recovery stress shows an exponential decrease, and the decrease rate of the recovery stress depends on the isothermal holding temperature. The higher isothermal holding temperature is the more the recovery stress decreases. The decrease rate reduces with increasing the number of thermal cycles.     

Thermal shock in an edge-cracked plate subjected to uniform surface heating

Thermal shock due to sudden surface heating of an edge-cracked plate is examined and compared with the opposite thermal shock condition that is associated with surface cooling. The plate is assumed to be insulated on one face with convective thermal boundary conditions existing on the side of the plate containing the crack. It is shown that surface heating results in compressive transient thermal stresses close to the plate surface which force the crack surfaces together over a certain contact length. The resulting nonlinear crack contact problem is formulated in terms of a singular integral equation and solved numerically. Calculated results include the transient stress intensity factors for various crack lengths at different values of the Biot number. A result of particular interest is the crack length at which the maximum stress intensity factor during heating exceeds the maximum stress intensity factor for cooling with otherwise identical heat transfer conditions.     

A Two-network thermomechanical model of a shape memory polymer

The aim of this work is to demonstrate a Helmholtz potential based approach for the development of the constitutive equations for a shape memory polymer undergoing a thermomechanical cycle. The model is able to simulate the response of the material during heating and cooling cycles and the sensitive dependence of the response on thermal expansion. We notice that the yield-stress of the material controls the gross features of the response of the model, and suggests that the material yields differently depending on not just the current value of the temperature but also on whether the temperature of the material dropped or increased from the previous time-step somewhat similar to the Bauschinger effect in plasticity, except that here the controlling parameter is the rate of temperature change rather than rate of plastic strain. The results of the simulation are in qualitative and quantitative agreement with experiments performed on two different shape memory polymer samples: polyurethane and epoxy resin. We find that modeling the hysteresis of the yield stress of the material during temperature changes is the key to the results.     

10CrNiCu钢激光-MAG复合焊热循环及组织特性研究

采用热电偶测温技术测量了激光-MAG复合焊及常规MAG焊接头粗晶区热循环曲线,并通过金相显微镜对比分析了两种焊接接头粗晶区组织特征。研究结果表明:与常规MAG焊相比,HLAW热循环加热及冷却速度较大,高温停留时间短,呈陡升陡降特征;粗晶区主要由粒状贝氏体、贝氏体、贝氏体基体上析出粒状碳化物组织及细小板条状马氏体组成;M-A岛含量较少,以细小颗粒状弥散分布在铁素体基体上。     

Precise determination of isomorphous and eutectoid transformation temperatures in binary and ternary Zr alloys

Differential thermal analysis (DTA) was used to accurately determine the phase boundary temperatures in the binary and ternary phase diagrams of Zr alloys containing Fe, Cr and O. The effects of heating and cooling rates on the onset temperatures of allotropic, isomorphous and eutectoid phase transformations were investigated both experimentally and theoretically. The phase boundary temperatures were determined by extrapolating the heating or cooling rate to 0 K min^–1 using a predictive equation derived in this work. Comparison of theoretical calculation with experimental data showed good agreement.     

Evolution of thermal stress in a coating/substrate system during the cooling process of fabrication

An analytical model is established for the thermal stress evolution in a film/substrate system during the cooling process of fabrication. Herein, heat transfer characteristics are incorporated which is critical for thermal spray coatings. The in situ temperature field solution is used to derive the instantaneous thermal stress field. Since the loss of thermal energy is account for, the new model may provide the basis for a more realistic prediction of the in situ thermal stress the fabrication process. The magnitude of thermal stress derived from the present model is lower than that of the classic one. The thermal stress is generated quickly and significantly during the initial seconds of the cooling process, and stabilizes later. The effects of several spray factors, such as the pre-heat temperature and the thicknesses of coating and substrate, are discussed and compared with a parallel experiment.     

Fatigue-crack advance mechanisms in polymers

The room-temperature fatigue-crack propagation behaviour of poly(butylene terephthalate) is strongly influenced by hysteretic heating near the crack tip since the glass transition temperature is just above room temperature. At low frequencies or stress intensities, the crack tip damage zone consists of several layers of crazes. At high frequencies or stress intensities, hysteretic heating causes a drop in yield stress and a large increase in the depth of the crack tip damage layer. At the same time, the increase in the plane stress plastic zones near the free surfaces produces large shear lips which flank the interior craze zone. This transformation results in a crack growth rate transition which appears as a crack deceleration followed by rapid crack acceleration. This thermal transition can be suppressed or delayed by immersion in water or silicone oil to reduce heat build-up in the sample during testing.     

Thermo-elasto-plastic stresses in functionally graded materials subjected to thermal loading taking residual stresses of the fabrication process into consideration

Functionally graded materials (FGMs) have recently been received with considerable interest, primarily as high temperature resistant materials for space vehicles subjected to high temperature environment. FGMs are one of the composite materials and consist of continuous change of composition of different material components from one surface to the other. FGMs usually fabricated at high temperature at which the FGMs have stress free condition. After the FGMs cooled from the fabrication temperature to the room temperature residual thermal stresses produced. In this paper, elasto-plastic thermal stresses in a rectangular plate (FGP) of a particle reinforced composite FGM are treated by finite element method due to the microscopic combination law when the FGP is subjected to three kinds of temperature conditions, first is cooling from the fabricated temperature to the room temperature, second is heating and last is heating after cooling from the fabricated temperature. In the analysis, the thermal stress constitutive equation of a particle-reinforced composite taking temperature change and damage process into consideration is used. The effects of the particle volume fraction and the three kinds of temperature conditions on the stresses in the matrix, stresses in the particle and macroscopic stress are discussed.     

Numerical simulation of transient and residual stresses caused by laser hardening of slender elements

A mathematical and numerical model of progressive laser hardening with regard to influence of thermal field and phase transformation under transient and residual axial stresses field is presented. The thermal field was determined on the basis of the diffusion-convection equation which was solved by a suitable superposition of Greens function. TIT diagrams were chosen as a basis for describing phase transformations during the heating and cooling process. The stresses were determined on the base of model of plastic flow with isotropic hardening. The changes in thermomechanical parameters of material as a function of phase contents and thermal field were considered in the stress calculations.     

Thermal radiation of nanoparticles occurring at a heated flat surface in vacuum

The most general expression for the rate of radiative heating (cooling) of an electrically neutral nanoparticle occurring in vacuum near a flat surface of a homogeneous polarizable medium is obtained for the first time. The magnetic polarizability of a conducting nanoparticle radically influences the rate of heat exchange between this particle and a metal surface. The rate of radiative cooling is several orders of magnitude higher than the power density of thermal radiation from a blackbody of the same size. This ratio is retained for micron size particles occurring at a distance of several hundred microns from the surface.     

Morphology Change in the Optical Microscopic Microstructure of Titanium by Nitriding Reaction and Combustion Synthesis Under Normal and Microgravity

Experiments on heating titanium wire specimens 0.5 mm in diameter by electric current in 0.5 absolute atmospheric pressure (0.5 kg/cm²) of a pure nitrogen gas environment under normal gravity (g = 9.8 m/s² = 1 G) and microgravity (g = G) using a drop shaft that enables 10 s of a microgravitational condition were carried out to investigate fundamentally the influence of microgravity on the reaction between Ti and nitrogen gas. The influence of gravitational acceleration on the morphological changes in optical microscopic microstructures of the specimens was studied. Under normal gravity, the thickness of the TiN layer resulting from the reaction of Ti and nitrogen gas at the outermost surface of the specimen increased with the heating time, but the tendency for the increase was reduced. A Widmanstätten structure, which is made by rapid heating and cooling of the specimen, was observed. However, the typical characteristics of the Widmanstätten structure vanished as the heating time proceeded. The progress of the nitriding reaction of the specimen was reduced, because heating by the electrical current and cooling by the convection of nitrogen gas balance each other out, and the temperature of the specimen became constant. Under microgravity, on the other hand, the thickness of the TiN layer of the outermost surface of the specimen increased with the heating time and was always thicker than that under normal gravity. The influence of microgravity, which suppresses the cooling effect of the thermal convection of nitrogen gas, on the nitriding reaction between titanium and nitrogen gas was significant. A dendritic structure caused by electrical heating was observed. Since the combustion synthesis of TiN was initiated at an elevated temperature after a certain heating time, huge dendritic structures were observed. The TiN tube was made on a final stage by leaching the titanium from the specimen. The influence of microgravity on the microstructure of the specimens that underwent nitriding was notable. Various results of the present study are explained by gravitational acceleration effects. The nitriding reaction became active by a temperature increase in the specimens with heating time due to the suppression of thermal convection and the cooling effect of the nitrogen gas on the specimens was reduced.