Finite element analysis of small-scale hot compression testing

This paper models hot compression testing using a dilatometer in loading mode.These small-scale tests provide a high throughput at low cost,but are susceptible to inhomogeneity due to friction and temper-ature gradients.A novel method is presented for correcting the true stress-strain constitutive response over the full range of temperatures,strain-rates and strain.The nominal response from the tests is used to predict the offset in the stress-strain curves due to inhomogeneity,and this stress offset Δσ is applied piecewise to the data,correcting the constitutive response in one iteration.A key new feature is the smoothing and fitting of the flow stress data as a function of temperature and strain-rate,at multiple discrete strains.The corrected model then provides quantitative prediction of the spatial and tempo-ral variation in strain-rate and strain throughout the sample,needed to correlate the local deformation conditions with the microstructure and texture evolution.The study uses a detailed series of 144 hot compression tests of a Zr-Nb alloy.While this is an important wrought nuclear alloy in its own right,it also serves here as a test case for modelling the dilatometer for hot testing of high temperature alloys,particularly those with dual α-β phase microstructures(such as titanium alloys).     

Numerical investigations of the stepped double torsion fracture toughness specimen

This study aims at investigating the fracture behaviour of double torsion specimens using the finite element method. Typical double torsion tests encompass a series of constant-thickness specimens to evaluate the material plane strain fracture toughness. In contrast, the concept of using a novel variable thickness stepped specimen aims at deducing the fracture toughness using a single specimen. In this work, the feasibility of this approach is examined and the effect of the number of steps and fracture thickness in a specimen upon the resulting conditional stress intensity factor is evaluated. The finite element models employed experimentally determined values of the fracture load to evaluate the conditional stress intensity factor of the specimen. Finite element predictions were compared with earlier experimental results using both cast aluminium silicon alloy and gray cast iron specimens and good matching was observed between experimental results and numerical predictions.     

Thermal stability of a nanostructured aluminium alloy

The microstructural changes taking place in a hydrostatically extrusion-processed nanostructured aluminium alloy during annealing were evaluated quantitatively, by measuring the size and shape of the grains. It was found that the grain size is stable up to an annealing temperature of 300 °C. Within this temperature range, the microstructural evolution proceeds through the annihilation of dislocations in the interior of the grains. At higher annealing temperatures, the recovered grains begin to grow and the microstructure becomes more homogeneous in terms of the grain size. The possibilities of an improvement of alloy thermal stability are discussed.     

Thermal fatigue analysis of small-satellite structure

The small-satellite thermal subsystem main function is to control temperature ranges on equipments, and payload for the orbit specified. Structure subsystem has to ensure the satellite structure integrity. Structure integrity should meet two constraints; first constraint is accepted fatigue damage due to cyclic temperature, and second one is tolerable mounting accuracy at payload and Attitude Determination and Control Subsystem (ADCS) equipments’ seats. First, thermal analysis is executed by applying finite-difference method (IDEAS) and temperature profile for satellite components case is evaluated. Then, thermal fatigue analysis is performed applying finite-element analysis (ANSYS) to calculate the resultant damage due to on-orbit cyclic stresses, and structure deformations at the payload and ADCS equipments seats.     

界面形貌对热障涂层残余应力分布的影响

用ABAQUS有限元软件建立了热障涂层模型,计算得出,在热载荷作用下,界面形貌对热障涂层材料残余应力分布影响很大,σ22主要集中在热生长氧化层和过渡层波峰处,随着热生长氧化层变厚、波长变小、振幅变大,σ22变大,且最大值产生在热生长氧化层/过渡层界面的波峰处.     

Thermal shock and thermal fatigue study of ceramic materials on a newly developed ascending thermal shock test equipment

A test equipment was designed to study thermal shock and thermal fatigue of ceramic materials subjected to fast heating (ascending). The equipment was designed to generate thermal stress in a test specimen by heating one surface of it by an oxy-hydrogen flame while cooling the opposite surface. The sample cracked when thermal stress exceeded its mechanical strength. The in situ crack formation was detected by an acoustic emission system coupled to the set up. The hot zone temperature was measured by an infra red pyrometer. The equipment was also designed to run thermal fatigue test cycles in automatic mode between two selected temperatures. The temperature and thermal stress distribution in the test specimen were modelled using finite element software. The effect of temperature distribution of the top and bottom surfaces on thermal stresses was studied. It was observed that the thermal stress is very sensitive to the temperature distribution on the top surface and maximum near the periphery of the top surface. This was in agreement with the experimental results in which the cracks were originated from the periphery of top surface. It was also observed that the failure temperature was higher for thicker samples.     

Thermal stability of pure bcc Zr fabricated by high pressure torsion

Recently pure omega plus bcc Zr was fabricated for the first time through the simultaneous application of compression and shear to pure alpha Zr by high pressure torsion. This phase was found to be stable under ambient conditions after processing. Here the thermal stability of the pure bcc Zr thus fabricated is analyzed using differential scanning calorimetry (DSC), in-situ X-ray diffraction at high temperature and transmission electron microscopy (TEM). Our results show that the temperature of the reverse transformation of the bcc phase is close to that of the omega phase. The presence of a mixed structure formed by alternating nanolaminates of the omega and the bcc phases might play a key role in the retention of these two phases at ambient pressure and temperature.     

Prediction of crack growth in a nickel-based superalloy under fatigue-oxidation conditions

Prediction of oxidation-assisted crack growth has been carried out for a nickel-based superalloy at elevated temperature based on finite element analyses of oxygen diffusion, coupled with viscoplastic deformation, near a fatigue crack tip. The material constitutive behaviour, implemented in the finite element code ABAQUS via a user-defined material subroutine (UMAT), was described by a unified viscoplastic model with non-linear kinematic and isotropic hardening rules. Diffusion of oxygen was assumed to be controlled by two parameters, the oxygen diffusivity and deformation-assisted oxygen mobility. Low frequencies and superimposed hold periods at peak loads significantly enhanced oxygen concentration near the crack tip. Evaluations of near-tip deformation and oxygen concentration were performed, which led to the construction of a failure envelop for crack growth based on the consideration of both oxygen concentration and accumulated inelastic strain near the crack tip. The failure envelop was then utilised to predict crack growth rates in a compact tension (CT) specimen under fatigue-oxidation conditions for selected loading ranges, frequencies and dwell periods. The predictions from the fatigue-oxidation failure envelop compared well with the experimental results for triangular and dwell loading waveforms, with marked improvements achieved over those predicted from the viscoplastic model alone. The fatigue-oxidation predictions also agree well with the experimental results for slow-fast loading waveforms, but not for fast-slow waveforms where the effect of oxidation is much reduced.     

Thermal conductivity of AZ91 magnesium integral foams measured by the Transient Plane Source method

The thermal conductivity of a collection of magnesium integral foams has been measured by using the Transient Plane Source (TPS) method. The results have shown a power-trend dependency with bulk density as the existing models predict. Additionally, micro-computed tomography (μCT) studies have been carried out on selected samples whose thermal conductivity values slightly deviate from the fitted curve to inspect the density distribution. Differences have been explained in terms of the local average density obtained by μCT in the volume covered by the heat flux. These results have revealed the high accuracy of the TPS method when it is combined with micro-tomographic techniques.     

Microstructure development and hardening during high pressure torsion of commercially pure aluminium: Strain reversal experiments and a dislocation based model

The effect of strain reversal on hardening due to high pressure torsion (HPT) was investigated using commercially pure aluminium. Hardening is lower for cyclic HPT (c-HPT) as compared to monotonic HPT (m-HPT). When using a cycle consisting of a rotation of 90° per half cycle, there is only a small increase in hardness if the total amount of turns is increased from 1 to 16. Single reversal HPT (sr-HPT) processing involves torsion in one direction followed by a (smaller) torsion in the opposite direction. It is shown that a small reversal of 0.25 turn (90°) reduces hardness drastically, and that decrease is most marked for the centre region. These behaviours and other effects are interpreted in terms of the average density of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs). A model is presented that describes the experimental results well. A key element of the model is the assumption that at the very high strains developed in severe plastic deformation processes such as HPT, the dislocation density reaches a saturation value. The model indicates that the strength/hardness is predominantly due to GNDs and SSDs.     

Evaluation of the dynamic properties of PVC foams under flexural vibrations

The paper presents an investigation of the damping of PVC foams under flexural vibrations of clamped-free beams. The PVC foams are constrained by two aluminium beams and different densities of the PVC foams are studied. An experimental investigation is implemented using an impulse technique. The natural frequencies and the damping of the beams are modelled by using a finite element analysis based on the sandwich theory. Next, the numerical and experimental results are used to derive the shear modulus and the damping of PVC foams as functions of the frequency. Finally, the experimental investigation and the developed modelling show how the damping of aluminium–foam beams must be corrected to estimate the damping of PVC foams.     

Thermal fatigue analysis of solar panel structure for micro-satellite applications

Thermal fatigue analysis based on 2D finite difference and 3D finite element methods is carried out to study the performance of solar panel structure during micro-satellite life time. Solar panel primary structure consists of honeycomb structure and composite laminates. The 2D finite difference (I-DEAS) model yields predictions of the temperature profile during one orbit. Then, 3D finite element analysis (ANSYS) is applied to predict thermal fatigue damage of solar panel structure. Meshing the whole structure with 2D multi-layer shell elements with sandwich option is not efficient, as it misses thermal response of the honeycomb structure. So we applied a mixed approach between 3D solid and 2D shell elements to model the solar panel structure without the sandwich option.     

Thermal fracture analysis of orthotropic functionally graded materials using an equivalent domain integral approach

A new computational method based on the equivalent domain integral (EDI) is developed for mode I fracture analysis of orthotropic functionally graded materials (FGMs) subjected to thermal stresses. By using the constitutive relations of plane orthotropic thermoelasticity, generalized definition of the J-integral is converted to an equivalent domain integral to calculate the thermal stress intensity factor. In the formulation of the EDI approach, all the required thermomechanical properties are assumed to have continuous spatial variations through the functionally graded medium. Developed methodology is integrated into a fracture mechanics research finite element code FRAC2D using graded finite elements that possess cubic interpolation. Steady-state and transient temperature distribution profiles in orthotropic FGMs are computed using the finite elements based heat transfer analysis software HEAT2D. EDI method is validated and domain independence is demonstrated by comparing the numerical results obtained using EDI to those calculated by an enriched finite element method and to those available in the literature. Single and periodic edge crack problems in orthotropic FGMs are examined in order to study the influences of principal thermal expansion coefficient and thermal conductivity components, relative crack length and crack periodicity on the thermal stress intensity factors. Numerical results show that among the three principal thermal expansion coefficient components, the in-plane component perpendicular to the crack axis has the most significant influence on the mode I stress intensity factor. Gradation profile of the thermal expansion coefficient parallel to the crack axis is shown to have no effect on the outcome of the fracture analysis.     

Mechanical testing of titanium/aluminium-silicon interface: Effect of T6 heat treatment

A previous paper reported on the mechanical behaviour of insert-moulded Ti/Al-7Si bimetallic test pieces as studied by a classical push-out test as well as a variant: the circular bending test. When a chemical bond was formed at the Ti/Al-7Si interface, promising results were obtained in terms of joint strength and damage mechanism (Dezellus et al., 2008 [1]). As a continuation, the aim of the present work was to examine the influence on this mechanical behaviour of a T6 heat treatment (re-heating for 10 h at 540 °C, quenching in cold water and ageing for 6 h at 170 °C) applied to the as-moulded Ti/Al-7Si test pieces. For that purpose, push-out and circular bending tests were performed on heat-treated samples, and the results were correlated with a characterization of the morphology, the constitution and composition of both transverse sections through the metal/metal reaction zone and fracture surfaces, as revealed after removal of the Ti insert. As expected, applying the T6 heat treatment to chemically bonded Ti/Al-7Si bimetallic assemblies resulted in an improvement of the mechanical properties of the Al-7Si matrix itself. Moreover, a significant increase of the load level for the onset of joint damage in push-out mode was observed. Concerning the damage mechanism, the presence of angular Si particles in the eutectic region of the Al-7Si matrix near the interface had a weakening effect. After T6 solution heat-treatment, the shape of the Si particles changed from angular to globular. Moreover, due to the formation of Si-rich compounds at the Al-7Si/Ti interface, Si diffuses from the alloy towards the Ti rod and the size and number of Si particles became significantly decreased near the insert/alloy interface. These two features explained the favourable influence of the T6 heat-treatment on the mechanical properties of the Ti/Al-7Si assemblies.     

Using finite element modeling to examine the flow processes in quasi-constrained high-pressure torsion

Finite element modeling was used to examine the flow processes in high-pressure torsion (HPT) when using quasi-constrained conditions where disks are contained within depressions on the inner surfaces of the upper and lower anvils. Separate simulations were performed using applied pressures from 0.5 to 2.0 GPa, rotations up to 1.5 turns and friction coefficients from 0 to 1.0 outside of the depressions. The simulations demonstrate the distribution of effective strain within the depressions is comparable to the prediction by ideal torsion, and the applied pressure and the friction coefficient outside the depressions play only a minor role in the distribution of effective strain. The mean stresses during processing vary linearly with the distance from the center of the disk such that there are higher compressive stresses in the disk centers and lower stresses at the edges. The torque required for rotation of the anvil is strongly dependent upon the friction coefficient between the sample and the anvil outside the depressions.     

Finite Element Simulation on Thermal Fatigue of a Turbine Blade with Thermal Barrier Coatings

In this paper, a finite element model was developed for a turbine blade with thermal barrier coatings to investigate its failure behavior under cyclic thermal loading. Based on temperature and stress fields obtained from finite element simulations, dangerous regions in ceramic coating were determined in terms of the maximum principal stress criterion. The results show that damage preferentially occurs in the chamfer and rabbet of a turbine blade with thermal barrier coatings and its thermal fatigue life decreases with the increase of thermal stress induced by high service temperature.     

Thermal Stress in Free-standing Diamond Films with Cr Interlayer Destroyed

Thermal stress in large area free-standing diamond films was remarkable during the post-deposition cooling of direct current (DC) arc plasma jet chemical vapor deposition (CVD) process.In this research,the stress release caused by delamination of Cr interlayer was of great importance to ensure the integrity of free-standing diamond film.The effects of Cr interlayer on Mo substrate,namely composite substrate,on thermal stress were investigated.Thermo-mechanical coupling analysis of the thermal stress was app...     

平面絷原法测量均质固体的蓄热系数

本建立了平面热源法实验条件下的非稳态导热的数学模型,应用拉普拉斯变换和热四端网络法,求解了实验测量系统的解析模型和各种相应的简化模型,根据参数估计的基本原理,对模型中的待估计做了灵敏度分析。建立了实验测量条纹,可以满足20－200℃环境下固体材料蓄热系数测量的需要。测量了常温下几种材料的蓄热系数,测量结果与热线法实验结果相吻合。     

Training effect on damping capacity in Fe–20 mass% Mn binary alloy

Two-dimensional finite element analysis together with stream function and neural network models are employed to determine thermo-mechanical behavior during hot strip rolling of AA5083. An appropriate velocity field and stream function is first determined using the rule of volume constancy and upper bound theorem and then temperature field within the metal is predicted by means of a two-dimensional conduction–convection model. In order to consider the effect of flow stress and its dependence on temperature, strain and strain rate, a neural network model is also employed in the analysis. Based on the performed tensile tests, two different neural network models are constructed one for smooth yielding and the other one for the serrated flow. Then, the ANN models are coupled with the thermo-mechanical analysis. In the next step, by combination of the predicted temperature, strain and strain rates fields and the experimental data achieved from the tensile tests, the occurence of dynamic strain ageing during hot rolling is predicted. The model predictions are then compared with the experimental data and good agreement is observed between the two sets of results that shows the validity of the proposed model.     

Two-dimensional and axisymmetric unit cell models in the analysis of composite materials

Unit cell models have been widely used for investigating fracture mechanisms and mechanical properties of composite materials assuming periodically arrangement of inclusions in matrix. It is desirable to clarify the geometrical parameters controlling the mechanical properties of composites because they usually contain randomly distributed particulate. To begin with a tractable problem this paper focuses on the effective Young’s modulus E of heterogeneous materials. Then, the effect of shape and arrangement of inclusions on E is considered by the application of FEM through examining three types of unit cell models assuming 2D and 3D arrays of inclusions. It is found that the projected area fraction and volume fraction of inclusions are two major parameters controlling effective elastic modulus of inclusions.