An analytical model to determine the stress singularity and critical bonding angle for an elastic–viscoelastic bonded joint

Measurements of interface bonding strengths are necessary for predicting the failure behavior of structures and materials with bi-material interfaces. However, it is well known that due to the discontinuity of material properties, stress singularity may exist at the edges of the interface. For accurate determination of the bonding strength of bi-material interface, the elimination of the stress singularity is necessary. This paper presents an analytical solution for the determination of the stress singularity and the critical bonding angle of a bonded joint between elastic and viscoelastic materials. This solution is based on the analytical solution available for an elastic?Celastic bonded joint via the elastic?Cviscoelastic corresponding principle. For the viscoelastic material, both time-independent and time-dependent Poisson??s ratios are considered to find its effect on the stress singularity. As an example, the developed solution is applied to a simulated aluminum-epoxy bonded joint with a spherical interface. It is found that the critical bonding angle and the order of the stress singularity are different for assuming a time-independent or time-dependent Poisson??s ratio of the idealized viscoelastic epoxy. With the analytical solution developed, it is possible to design an optimal interface geometry that can eliminate the stress singularity from the interface corner.     

Application of pyrolysis–gas chromatography/mass spectrometry for the identification of polymeric materials in failure analysis in the automotive industry

The application of analytical pyrolysis–gas chromatography mass spectrometry (Py–GC/MS) in the failure analysis of two hydraulic cylinders and their rubber membranes from the automotive industry were presented.     

An iterative procedure for determining effective stress–strain curves of sheet metals

An iterative correction procedure using 3D finite element analysis (FEA) was carried out to determine more accurately the effective true stress–true strain curves of aluminum, copper, steel, and titanium sheet metals with various gage section geometries up to very large strains just prior to the final tearing fracture. Based on the local surface strain mapping measurements within the diffuse and localized necking region of a rectangular cross-section tension coupon in uniaxial tension using digital image correlation (DIC), both average axial true strain and the average axial stress without correction of the triaxiality of the stress state within the neck have been obtained experimentally. The measured stress–strain curve was then used as an initial guess of the effective true stress–strain curve in the finite element analysis. The input effective true stress–strain curve was corrected iteratively after each analysis session until the difference between the experimentally measured and FE-computed average axial true stress–true strain curves inside a neck becomes acceptably small. As each test coupon was analyzed by a full-scale finite element model and no specific analytical model of strain-hardening was assumed, the method used in this study is shown to be rather general and can be applied to sheet metals with various strain hardening behaviors and tension coupon geometries.     

Critical fluid shear stress analysis for cell–polymer adhesion

A simple methodology to assess cell adhesion on materials was developed. We demonstrated that the cell adhesion strength could be quantified. Using this method, we were able to compare the NIH/3T3 Swiss mouse fibroblasts adhesion strength to poly(methyl methacrylate) and polycarbonate. A controlled fluid shear stress was applied to cells using a parallel plate rotational system. Cells detached from the surface in the radial direction. Results showed that there was a critical radius where the shear stress experienced by the cells equaled the cell adhesion strength. The cells outside this radius were removed while those inside maintained initial confluency. The quantitative evaluation of cell adhesion is beneficial for development of biomaterials.     

A systematic AMF–FEM coupled method for the thermo-elasto-plastic contact analysis of the plasma sprayed HA-coated biocomposite

Hydroxyapatite (HA) coated Ti-6Al-4V alloy biocomposite has been accepted as one of the most promising implant materials for orthopaedic and dental applications because of its favorable biocompatibility and mechanical properties. After the plasma sprayed HA composite coating on titanium alloy substrate biocomposite was prepared, a novel meshless numerical analysis method of the coupled adaptive meshfree method and finite element method (AMF–FEM) is developed for the simulation of the thermo-elasto-plastic contact problems of the biocomposites in this paper. The adaptive meshfree method based on strain energy gradient is used in the concerned contact domain, and FEM is used in the non-contact domain to overcome the difficulties of the meshfree method and improve the calculation efficiency. The thermo-elasto-plastic contact model using the incremental-initial stiffness method, error estimation and the local adaptive refinement strategy for the AMF–FEM method are combined. The AMF–FEM thermo-elasto-plastic model takes into account the temperature variation, micro plastic flow, the thermo-elasto-plastic coupling behavior and the strain-hardening property of the materials. The examples of the elastic/thermal-elastic contact of real HA-coated rough surfaces using the AMF–FEM is studied for two biomaterial models, respectively. The results all show that the AMF–FEM solutions are accurate, efficient, and can be widely applied to different thermo-elasto-plastic contact multi-layer biomaterial models considering different geometric parameter, material parameter, thermal and friction properties.     

Numerical analysis on the heat and work transfer due to shear in a hot cascade Ranque–Hilsch vortex tube

Cascading of vortex tubes is a possible implementation to extract significantly larger amount of useful work. A hot cascade-type RHVT makes use of the cold gas for cooling purposes while improving the heating capacity of the hot gas. In a vortex tube inflow pressure is the only source of energy which converts into thermal energy. The conversion of pressure energy into thermal energy is associated with the heat and work transfer due to shear along the radial, axial and tangential directions. In this paper, the physics of fluid flow and thermal separation are studied based on the heat and work transfer due to shear along all three directions. The work transfer due to the action of tangential shear is always from the cold to hot fluid layers and is the most dominant factor in the thermal separation process. The contribution increases considerably with hot cascading. However, the process of thermal separation degrades due to the effect of sensible heat transfer.     

Mechanical property of magnesium alloy sheet with hardening deterioration at warm temperatures and its application for failure analysis: Part II – failure analysis

The mechanical properties of a thin AZ31B Mg alloy sheet (with the thickness of 0.5 mm) were characterized for its anisotropy, temperature-dependent hardening (including its deterioration) and strain rate sensitivity based on simple tension test data measured at 100 °C, 150 °C, 200 °C, 250 °C, respectively, in Part I. As for anisotropy, simple tension tests were performed along three (rolling, transverse and in-between) directions to calibrate the Hill1948 yield function. As for temperature-dependent hardening, hardening as well as its deterioration (or softening) behavior observed beyond the uniform elongation limit was numerically characterized based on the inverse calibration method, in which strain rate sensitivity was also considered. The mechanical properties were confirmed to properly predict failure by strain localization for all the simple tension tests involved in the characterization procedure. Ultimately, the mechanical properties characterized in Part I were applied in Part II to analyze the failure by strain localization in the cross-shaped cup drawing tests developed as the benchmark problem for the NUMISHEET2011 conference [1]. The results showed that the mechanical properties with hardening deterioration properly predicted failure, while hardening without deterioration (obtained following the common practice) did not, confirming the importance of including the hardening deterioration in tensile property characterization, especially to predict forming failure by strain localization.     

Method to determine the strain-rate sensitivity of a superplastic material from the initial slopes of its stress–strain curves

New method to estimate quantitatively the strain-rate sensitivity of a superplastic material is suggested. It is based upon measurements of the initial slopes of the stress–strain curves. In contrast with known techniques the method suggested is appropriate to apply to small deformations, so that the result obtained are to be assigned to the initial structural state of the material under consideration. The experimental verification of the method suggested is fulfilled in practice for the example of Wood's alloy. It is shown that the strain-rate sensitivity of this alloy decreases monotonically with increasing strain, ; the value of m determined by means of the method suggested are in good agreement with that determined independently using standard techniques. An important feature of the method suggested is that in contrast with standard techniques its accuracy improves as the value of strain, , becomes smaller, since the tilts of – curves have a maximum when 0. © 1998 Chapman & Hall     

Disassembly methodology for conducting failure analysis on lithium–ion batteries

To facilitate construction analysis, failure analysis, and research in lithium–ion battery technology, a high quality methodology for battery disassembly is needed. This paper presents a methodology for battery disassembly that considers key factors based on the nature and purpose of post-disassembly analysis. The methodology involves upfront consideration of analysis paths that will be conducted on the exposed internal components to preserve the state (operational or failed) of the battery. The disassembly processes and exposures must not alter the battery materials once they are removed from their hermetically sealed containers. Because the process of battery disassembly can involve exposure to potentially hazardous compounds or lead to thermal run-away, a brief review concerning the safety hazards of disassembly is also given.     

A combined BEM–FEM model for the velocity–vorticity formulation of the Navier–Stokes equations in three dimensions

This paper describes a combined boundary element and finite element model for the solution of velocity–vorticity formulation of the Navier–Stokes equations in three dimensions. In the velocity–vorticity formulation of the Navier–Stokes equations, the Poisson type velocity equations are solved using the boundary element method (BEM) and the vorticity transport equations are solved using the finite element method (FEM) and both are combined to form an iterative scheme. The vorticity boundary conditions for the solution of vorticity transport equations are exactly obtained directly from the BEM solution of the velocity Poisson equations. Here the results of medium Reynolds number of up to 1000, in a typical cubic cavity flow are presented and compared with other numerical models. The combined BEM–FEM model are generally in fairly close agreement with the results of other numerical models, even for a coarse mesh.     

Residual stress field analysis of Al/steel butt joint using laser welding–brazing

The objective of this study is to analyse residual stress of the Al/steel butt joint using the laser welding–brazing. The welding parameters were a laser power of 1200?W, a welding speed of 600?mm min^?1, thickness of 150×150×2?mm and 150×150×1?mm, respectively. The residual stress was measured by the hole-drilling methods. Then, a finite-element model of the welding process was established, and it was verified by experiments. The results show that the calculated results are in conformity with the experimental results. The longitudinal residual stress on the galvanised steel (329?MPa) is larger than that on the aluminium alloy (293?MPa). At the location of the fixture, the longitudinal residual stress is substantially zero.     

Ductile fracture in thin sheet metals: a FEM study of the Sandia fracture challenge problem based on the Gurson–Tvergaard–Needleman fracture model

In this paper, the Gurson–Tvergaard–Needleman (GTN) fracture model has been implemented in a 3D nonlinear finite element framework to investigate ductile failure. The simulation consists of both model parameter calibrations and predictions based on the experimental configurations that were described in the Sandia fracture challenge in 2012. The goal is to test the robustness of the GTN model in predicting crack initiation and propagation in a ductile structural stainless steel (15-5 PH). It has been observed that the predicted load drops at crack initiation agree well with the experimental data. Both the simulation and experiments reveal that cracks always initiate in the subsurface. The observed crack path in the experiments, however, is not unique and is strongly influenced by the geometric variations in the specimen introduced in the fabrication process. The crack path is quite sensitive to the relative location of pre-existing features (notch and holes) in this challenge problem, and a very small variation can potentially lead to different path, hence affecting the ductile failure pattern significantly. Variations in the crack path are also observed in the simulation with different choices of GTN model parameters that control the void nucleation and growth behavior.     

Stress and failure analysis of crimped metal–composite joints used in electrical insulators subjected to bending

Experimental and numerical investigations are carried out on metal/fibreglass-reinforced-plastic joints integrated in electrical insulators subject to bending. Numerical stress and strain distributions through the bond are calculated with a solid 3D finite element model and the damage initiation in the composite is highlighted. The simulations are compared to experimental data obtained from several joint specimens tested under bending on an experimental setup equipped with strain gauges and a six-channel acoustic emission system. Good correlation between the finite element predictions and the test results is found. The investigations have identified the stress concentrations in the rod, the onset of damage when the load–displacement curve characterizing the bending test deviates from linearity, and the different failure mechanisms.     

Thermo-metallurgical and thermo-mechanical computations for laser welded joint in 9Cr–1Mo(V,Nb) ferritic/martensitic steel

Transient thermo-metallurgical and thermo-mechanical computations for laser welded joint, in 9 mm thick 9Cr–1Mo(V, Nb) ferritic/martensitic steel plate, in square-butt configuration, have been carried out by simulating the laser welding process on the 3-dimensional (3D) solid model using a finite element based welding and heat treatment simulation solution package – SYSWELD. The heat source has been modeled as a combination of a 3D Gaussian and a double ellipsoid profiles for realistic representation of fusion zone morphology. Phase and temperature-dependent physical and mechanical properties of this material were used in these computations. The results show very short residence time (<0.5 s) for the material in the heat affected zone (HAZ). The results clearly delineate the effects of different thermo-metallurgical processes like heating, softening, cooling and solid state phase transformation (SSPT) on temporal evolution of the stress-field resulting from laser welding. Longitudinal component of the residual stress is the most significant followed by the normal component and the transverse component is the least significant. Cross-weld residual stress profiles show a trough in the fusion zone and the heat affected zone (HAZ) with a peak in the parent metal region bordering the metallurgical HAZ. Also, the longitudinal and the normal components of the residual stress show nearly similar profile with different magnitudes. The computed residual stress profiles show reasonably good agreement with that measured by neutron diffraction. The results also show significant plastic deformation and strain-hardening of the austenitic phase-field prior to its transformation into martensite.     

Microstructural and thermo-mechanical analysis of quench cracking during the production of bainitic–martensitic railway wheels

Quench cracking during the production of newly developed low carbon bainitic–martensitic (LCBM) rail wheels was investigated using a microstructural and thermo-mechanical Finite Element (FE) model. The stresses associated with quench cracking during martensite phase transformation were predicted under various quenching conditions for two different grades of LCBM steels with different kinetics of martensite phase transformation. The FE analyses showed that the likelihood of quench cracking can be reduced by using a low coolant spray intensity since the internal stresses generated during the martensitic phase transformation were found to be below the steel’s flow stress. The internal stresses were predicted to be even lower with a low carbon grade LCBM steel. The microstructural and thermo-mechanical model has been used to determine favourable quenching conditions that have the potential to reduce the propensity of quench cracking during the production of LCBM railway wheels.     

Phase transformations in Fe–Ni–Cr heat-resistant alloys for reformer tube applications

Fe–35Ni–25Cr–0.4C alloys with different compositions are aged between 750 and 1150°C up to ～10,000?h. As-cast microstructure contains interdendritic carbides of type M₇C₃ (‘Cr₇C₃’) and MC (‘NbC’). At service temperatures, M₇C₃ transform into M₂₃C₆ (‘Cr₂₃C₆’) within hours. Then, a hardening precipitation of secondary intragranular M₂₃C₆ occurs over hundreds of hours, the nose of the ‘temperature-time-hardening’ curve being around 1000°C. G phase forms after long aging; its solvus temperature and formation kinetics depend on silicon content. Z phase is observed after long aging at 950°C or above. G and Z phases form at the expense of MC. Very long aging causes nitridation under air, with first a transformation of M₂₃C₆ into chromium-rich M₂X carbonitrides (X?=?C,N), then of MC into chromium-rich MX carbonitrides.     

Using particle trajectory for determining the fluidization regime in gas–solid fluidized beds

Transition from bubbling to turbulent in a conventional gas–solid fluidized bed was evaluated from trajectory of particles in fluidized bed. A series of experiments were carried out in a lab-scale fluidization bed using radioactive particle tracking (RPT) technique for recording the position of a tracer in the bed. Statistical parameters, such as standard deviation and skewness of the time–position data, were utilized to determine the transition velocity from bubbling to turbulent regime. The results showed that the data obtained by the RPT technique can predict transition velocity. It was shown that the standard deviation of position fluctuations reach a maximum with increasing superficial gas velocity corresponding to regime transition. It was shown that transition from bubbling to turbulent can be determined using skewness and kurtosis of time–position data. The velocities obtained in this work are in good agreement with the available correlations.     

FE analysis of coupled thermo-mechanical behaviors in radial–axial rolling of alloy steel large ring

Radial–axial rolling of alloy steel large ring (ASLR) is an advanced plastic forming process with complex coupled thermo-mechanical deformation behaviors which have significant influences on the microstructure and properties of the product. In this paper, the stable forming conditions and ranges of key forming parameters for the radial–axial rolling process of ASLR are determined reasonably. Then a 3D elastic–plastic and coupled thermo-mechanical FE model of radial–axial ring rolling is explored using the dynamic explicit code ABAQUS/Explicit, and its reliability is verified theoretically and experimentally. Using FE simulation and analysis, the effects of key forming parameters on the uniformity of deformation and temperature distribution of ASLR are investigated. The main results show that: (1) The deformation and temperature distribution of the ASLR are nonuniform in radial–axial ring rolling. The PEEQ gradually decreases from the surface region to the central region of the ASLR while the temperature distribution is reverse. The largest PEEQ and smallest temperature appear in the edge region of the ASLR. (2) With increasing the feed speeds of rolls, initial temperature of blank or decreasing the rotation speeds of rolls, the deformation of the ASLR becomes more uniform. (3) With increasing the feed speeds of rolls, rotation speeds of rolls or decreasing the initial temperature of blank, the temperature distribution of the ASLR becomes more homogeneous. (4) The friction coefficient has a slight effect on the uniformity of deformation and temperature distribution of the ASLR. The results provide an important basis for improving the microstructure and forming quality of ASLR through optimization of forming parameters.     

Optical methods for complete stress determination — An experimental alternative to finite-element analysis

Experimental optics provides an interesting alternative to finite-element analysis (FEA) for estimating stress distribution in bodies subjected to load. Complete determination of the state of stress in a two-dimensional transparent model of the body was performed, using modern interferometry in combination with conventional photoelasticity. Stress concentrations and singularities may be evaluated in the same experimental set-up by means of the methods of caustics.The present approach eliminates the effect of limited optical quality of the model material, which is an advantage in engineering applications of the methods proposed. In contrast to most testing procedures using interferometry, this technique provides particularly simple handling of equipment and ease in evaluation. The paper describes an application to restorative dentistry of these methods of experimental stress analysis.