Thermal stresses in WC-Co hard alloy after sintering

The stressed-strained state of individual elements of th emicrostructure of tungsten hard alloys as a result cooling the material to room temperature after sintering has been estimated by the method of numerical modelling. It is shown that both tensile and compressive stresses may form in individual tungsten carbide grains and certain interlayers of the cobalt phase. The stressed state of the carbide phase is analyzed which makes it possible to predict the possibility of failure for individual grains and also sliding along intergranular boundaries during alloy cooling after sintering. It has been established that the binder phase deforms plastically during the whole cooling process thus creating at room temperature a very inhomogeneous distribution for the intensity of plastic deformation through interlayers.Translated from Problemy Prochnosti, No. 12, pp. 87–93, December, 1990.     

Assessment of conventional elastic limit for a hard WC-Co alloy in tension

This paper presents an algorithm for the calculation of a conventional elastic limit for WC-Co alloy in tension considering residual thermal stresses in its phases. The algorithm is based on equations of thermoelasticity for two-phase composite materials. Comparison of theoretical and experimental results revealed their fair agreement. Residual thermal stresses were found to have an appreciable effect on the elastic limit of a hard alloy in tension. Institute of Superhard Materials, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 6, pp. 116–122, November–December, 1999.     

Assessment of conventional elastic limit for a hard WC-Co alloy in compression

This paper presents an algorithm for the calculation of a conventional elastic limit for the alloy WC-Co in compression considering residual thermal stresses in its phases. The algorithm is based on equations of thermoelasticity for two-phase composite materials. Comparison of the theoretical and experimental results show their fair agreement. Institute of Superhard Materials, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 111–119, January–February, 2000.     

Modeling development of residual stresses in thermal spray coatings

The diameter, velocity and temperature of stainless steel and tungsten carbide cobalt particles applied onto stainless steel substrates using a high velocity oxy-fuel (HVOF) torch were measured. The microstructure of the coatings produced was examined using a scanning electron microscope and coating thickness, porosity and roughness measured. Using the experimental spray parameters as inputs to a 3-D stochastic model we simulated coating formation. Measured values of coating thickness and porosity agreed well with predicted values while calculated surface roughness was somewhat higher than that observed in experiments. An object oriented finite element code (OOF) developed at the National Institute of Standards and Technology was used to calculate residual stresses in the coating. The model uses an adaptive meshing technique to discretize the coating microstructure into a mesh suitable for finite element analysis. To define the coating geometry we used either micrographs of coating cross-sections or computer generated images of coatings. Similar values of residual stress are obtained in either case. High stresses are present at the interface between the coating and substrate. The magnitude of stresses increases significantly with coating thickness. Stresses are relieved by voids such as pores or cracks in the coating. Residual stresses increase with coating temperature and can be decreased by preheating the substrate.     

Furnace-melted surface layers from carbide hard alloy powder compounds

No technological difficulties were encountered in the processing of pseudo-hard alloys in the form of powder compounds of conventional nickel-based hard alloys with carbides. The alloy greatly influences the resulting structures of the surface layers. Under some processing conditions tungsten carbide is completely dissolved by the molten alloy matrix. Hard phases based on chromium carbide form on cooling. The induced chromium carbide, Cr₃C₂, retains its structure while absorbing large amounts of iron into its matrix. It can be concluded that not only alloying properties but also to a great extent structural criteria determine the stability of the applied supplementary hard phases.     

Modeling of residual stresses variation with thermal cycling in thermal barrier coatings

Thermal barrier coatings (TBCs) are commonly used as protective coatings for engine metal components to improve performance. Many investigations have shown that residual stresses in TBCs applications play an important role, but the residual stresses are mainly obtained by simulation method. As we know, there are a few analytical solutions of residual stress in TBCs system. In this paper, a new two-dimensional analytical solution has been obtained under the condition of non-linear coupled effects of temperature gradient, thermal fatigue, deposited residual stress, thermally grown oxide (TGO) thickening, elasto-plasticity deformation and creep deformation of TBC. Moreover, the influences of bending moment and curvature on stress variation in TBCs are considered during thermal cycling. The calculated results are in agreement with the prior experimental results.     

On the relationship between the length of Palmqvist cracks and residual surface stresses in WC-Co

WC-Co cylinders were subjected to cyclic compressive stresses for varying numbers of cycles and maximum compressive stresses. The fractional changes in the length of Palmqvist cracks and in the residual surface stresses were measured after precompression. It is found that the relationship between Palmqvist crack length and residual surface stresses depends on the conditions of precompression, and thus Palmqvist crack lengths cannot provide quantitative information on residual surface stresses.     

The indentation of hard metals: the role of residual stresses

Experimental evidence to support Johnson's [6] analysis of the residual surface stress distribution produced in a flat surface by a spherical indenter is presented. The theory suggests that high residual stresses develop just outside the contact area as a result of the superposition of elastic unloading stresses onto the stresses at maximum load when the specimen has deformed plastically. The experiments involved the use of a semi-brittle steel, sufficiently hardened so that, while tensile stresses in the surface produced cracks, the substrate deformed plastically under the triaxial stress system beneath the indenter. Radial cracks produced by the indenter frequently extended after load removal, implying the presence of the high tensile circumferential residual stresses predicted by the theory. This work and recent studies of indentation loading of glasses show that there are important situations where residual stresses can contribute to their failure and wear.     

Non-destructive identification of residual stresses in steel under thermal loadings

We discuss computational aspects of the inverse and ill-posed problem of identifying residual stresses in steel structures under thermal loading. This corresponds to an inverse source problem in linear thermo-elasticity. The studies aim in investigating whether thermal loadings for the excitation of structures are sufficient in order to detect reliably inherent residual stresses. These stresses may result from the construction process or later thermal or mechanical treatment of the structure-like welding. By answering the raised question positively, our method provides an important basis for successful thermal straightenings. The quality of the solution of the inverse problem depends on a series of parameters, like material parameters, noise in the measurements, and the experimental setup. We numerically study the effects of these parameters and quantify the uncertainties in the results of the inverse problems by means of Sobol indices.     

Aspects of residual thermal stresses in continuous-fibre-reinforced ceramic matrix composites

An analytical model is presented that predicts the thermal stresses which arise from mismatch in coefficients of thermal expansion between a fibre and the surrounding matrix in a continuous fibre composite. The model consists of two coaxial isotropic cylinders. Stress transfer between the fibre and the matrix near an unstressed free surface has been modelled by means of a shear-lag analysis. Away from the free surface the theoretical approach satisfies exactly the conditions for equilibrium and continuity of stress at the fibre-matrix interface. Application of the model to a composite consisting of a glass-ceramic calcium alumino-silicate (CAS) matrix containing unidirectional Nicalon fibres points to a strong dependence of stress on fibre volume fraction. Surface effects are significant for depths of the order of one fibre diameter. Near-surface shear stresses resulting from cooling from the stress-free temperature are sufficiently high to suggest that a portion of fibre close to the surface is debonded at room temperature. Experimental results acquired with a scanning electron microscope (SEM) equipped with a heating stage are consistent with this prediction. Consequently, the model has been modified in a simple way to incorporate frictional slip at the interface, according to the Coulomb friction law. Although detailed measurements are limited by the resolution of the technique, experimental evidence suggests that the transfer length is within an order of magnitude of the model prediction.     

The treatment of thermal and residual stresses in fracture assessments

The behaviour of a crack under combined mechanical and thermal/residual loads is described in terms of a reference stress or equivalent mechanical loading. The reference stress can be readily established from a knowledge of the stress intensity factor for the thermal/residual load, the magnitude of the mechanical load, and the material stress-strain curve. The result enables the J-integral to be evaluated and is also used to define a procedure for the inclusion of thermal/residual stresses in the CEGB defect assessment method (R6). In particular, a detailed procedure is given for treating thermal/residual stresses in recent revisions of R6 which cater for structures made of materials which strongly work-harden.     

Matrix solidification and the resulting residual thermal stresses in composites

The disparate thermal expansion properties of the fibres and matrices in high-performance composites lead to an inevitable build up of residual thermal stresses during fabrication. We first discuss the thermal expansion behaviour of thermoplastic and thermoset polymers that may be used as high-performance composite matrices. The three classes of polymers considered are epoxies, amorphous thermoplastics, and semicrystalline thermoplastics. The relevant thermal expansion data for prediction of the magnitude of the residual stresses in composites is the zero (atmospheric)-pressure thermal expansion data; these data are plotted for a range of thermoplastics and a typical epoxy. Using the technique of photoelasticity, we have measured the magnitude of the residual stresses in unidirectional graphite composites with an amorphous thermoplastic matrix (polysulfone) and with an epoxy matrix (BP907). The temperature dependence of the residual stress build up and the resulting magnitude of the residual stresses correlate well with the thermal and physical properties of the matrix resin.     

Apparent coefficient of thermal expansion and residual stresses in multilayer capacitors

The thermal expansion behavior and residual stresses in multilayer capacitor (MLC) systems are analyzed in the present study. An MLC consists of a laminate of multiple alternating electrode layers and dielectric layers sandwiched between two ceramic cover layers. An analytical model is developed to derive simple closed-form solutions for the apparent coefficients of thermal expansion (CTEs) of the laminate. Plasticity of electrodes is included in the analysis. The predicted apparent CTEs are compared with measurements of some laminated ceramic composites. The effects of plasticity on apparent CTEs and residual stresses in MLC systems are discussed.     

Influence of thermal residual stresses on the composite macroscopic behavior

The influence of the thermal residual stress on the deformation behavior of a composite has been analyzed with a new micromechanical method. The method is based on secant moduli approximation and a new homogenized effective stress to characterize the plastic state of the matrix. It is found that the generated thermal residual stresses after cooling and their influence on the subsequent deformation behavior depends significantly on the aspect ratio of the inclusions. With prolate inclusions, the presence of thermal residual stresses generate a higher compressive hardening curves of the composite, but it is reversed with oblate inclusions. For particle reinforced composite, thermal residual stresses induce a tensile hardening curve higher than the compressive one and this is in agreement with experimental observations.     

Analytical modelling of thermal residual stresses in exponential functionally graded material system

An analytical model is developed for prediction of thermal residual stresses, arising from the fabrication of exponential functionally graded material (simply called E-FGM) systems. The thermomechanical properties of functionally graded layers are assumed to vary exponentially through the thickness. Residual stresses were found to increase when fully ceramic and/or fully metal regions are included in the structure, adjoining the graded zone. The effects of temperature dependent elastic and thermal expansion characteristics of constituents on residual stress were found to be small.