Local Thermal Stress Research Articles

Minimization of elastic thermal stresses during laser machining of ceramics using a dual-beam arrangement

Lasers are emerging as a valuable tool for shaping and cutting hard and brittle ceramics. Unfortunately, the large, concentrated heat flux rates that allow the laser to efficiently cut and shape the ceramic also result in large localized thermal stresses in a small heat-affected zone. These notable thermal stresses can lead to microcracks, a decrease in strength and fatigue life, and possibly catastrophic failure. In order to assess where, when, and what stresses occur during laser scribing, an elastic stress model has been incorporated into a two-dimensional drilling as well as a three-dimensional scribing and cutting code. The results of the analysis for a single Gaussian laser source show that substantial tensile stresses develop over a thick layer below and parallel to the surface, which may be the cause of experimentally observed subsurface cracks. In this work the effect of splitting the beam into two parts is discussed: a highly focused beam that does the actual drilling or scribing, and a partially defocused beam, used to heat the material ahead of, around, or behind the laser beam. Different secondary beam scenarios are investigated, and their effect on reducing the damaging tensile stresses is evaluated.

Fiber alignment shift formation mechanisms of fiber-solder-ferrule joints in laser module packaging

The fiber alignment shifts of fiber-solder-ferrule (FSF) joints in laser module packaging under temperature cycle testing using PbSn and AuSn solders are studied experimentally and numerically. The measured results showed that the fiber shifts of FSP joints with the hard AuSn solder exhibited shifts two times less than that with the soft PbSn solder. This suggests that the hard solder may be more suitable for FSF assembly than the soft solder. The results also showed that fiber shifts increased as the temperature cycle number and the initial fiber eccentric offset increased. The experimental measurements of fiber shifts were in good agreement with the numerical calculations of the finite-element method analysis. The major fiber shift formation mechanisms of FSF joints in temperature cycling may come from the localized plastic solder yielding introduced by the local thermal stress variation, the redistribution of the residual stresses, and the stress relaxation of the creep deformation within the solder. Furthermore, the stress relaxation of creep deformation in solder with either 21% (PbSn solder) or 5% (AuSn solder) may have significant influence on the fiber shifts. This study has provided an optimum approach for reduction of the fiber alignment shift of FSF joints in laser module packaging under temperature cycle testing, which is to solder the fiber near to the center of the ferrule and to select the AuSn hard solder.

Transient Elastic Thermal Stress Development During Laser Scribing of Ceramics

Lsaers are emerging as a valuable tool for shaping and cutting hard and brittle ceramics. Unfortunately, the large, concentrated heat flux rates that allow the laser to efficiently cut and shape the ceramic also result in large localized thermal stresses in a small heat-affected zone. These notable thermal stresses can lead to micro-cracks, a decrease in strength and fatigue life, and possibly catastrophic failure. In order to assess where, when, and what stresses occur during laser scribing, an elastic stress model has been incorporated into a three-dimensional scribing and cutting code. First, the code predicts the temporal temperature fields and the receding surface of the ceramic. Then, using the scribed geometry and temperature field, the elastic stress fields are calculated as they develop and decay during the laser scribing process. The analysis allows the prediction of stresses during continuous wave and pulsed laser operation, a variety of cutting speeds and directions, and various shapes and types of ceramic material. The results of the analysis show substantial tensile stresses develop over a thick layer below and parallel to the surface, which may be the cause of experimentally observed subsurface cracks.

Processing, Microstructure, and Mechanical Properties of B4C ― TiB2 Particulate Sintered Composites. Part II. Fracture and Mechanical Properties

The fracture response of pressureless sintered boron carbide ceramics containing 5-25 vol.% TiB2 phase produced via the in-situ chemical reaction between B4C, TiO2 and elemental carbon was studied. Both strength and fracture toughness depend on TiB2 volume fraction, reaching their maximum values of 500 MPa and 4.6 MPa·m1/2, respectively, at 15 vol.% TiB2. The observed increase in strength and fracture toughness was ascribed to the interaction between the propagating crack front and local thermal mismatch stress associated with TiB2 particles. Induced circumferencial microcracking and crack impedance are discussed as the major toughening mechanisms. Spontaneous circumferencial microcracking due to thermal mismatch stress in TiB2 particles was found to occur when the particle size exceeds its critical value. The theoretical interpretation of spontaneous circumferencial microcracking, toughening via induced microcracking, and crack impedance was justified experimentally.

Optical and structural analysis of degraded high power InGaAlAs/AlGaAs lasers

The failure of high power InGaAlAs/AlGaAs double quantum well (DQW) lasers has been studied by plan-view photoluminescence (PL), cross sectional (002) dark field TEM and X-ray microanalysis. The lasers were operated at elevated driving currents beyond rollover in order to stimulate and analyse degradation processes encountered during accelerated lifetime testing. The damage consists of elliptical areas within which the original DQW structure was destroyed due to outdiffusion of Al into the confinement layers. The damaged areas started from the output mirror indicating that the device temperature was somewhat higher near the facet. The Ga vacancies governing the Al diffusion were likely produced by a mechanism of recombination enhanced defect reaction (REDR). The dislocations always present in the damaged areas are expected to be generated by the point defects produced by REDR as well as by localized thermal stresses.

Transient Elastic and Viscoelastic Thermal Stresses During Laser Drilling of Ceramics

Lasers appear to be particularly well suited to drill and shape hard and brittle ceramics, which are almost impossible to netshape to tight tolerances, and are presently machined in industry only by diamond grinding. Unfortunately, the large, focussed heat fluxes that allow the ready melting and ablation of material, also result in large localized thermal stresses within the narrow heat-affected zone, which can lead to microcracks, significant decrease in bending strength, and even catastrophic failure. In order to assess the where, when, and what stresses occur during laser drilling, that are responsible for cracks and decrease in strength, elastic and viscoelastic stress models have been incorporated into our two-dimensional drilling code. The code is able to predict temporal temperature fields as well as the receding solid surface during CW or pulsed laser drilling. Using the resulting drill geometry and temperature fields as well as the receding solid surface during CW of pulsed laser drilling. Using the resulting drill geometry and temperature field, elastic stresses as well as viscoelastic stresses are calculated as they develop and decay during the drilling process. The viscosity of the ceramic is treated as temperature-dependent, limiting viscoelastic effects to a thin layer near the ablation front where the ceramic has softened.

High heat load properties of tungsten coated carbon materials

Tungsten coatings of 0.5 and 1.0 mm thickness were successfully deposited by the vacuum plasma spraying technique (VPS) on carbon/carbon fiber composite, CX-2002U, and isotropic fine grained graphite, IG-430U. High heat flux experiments were performed on the coated and non-coated samples in order to prove the suitability and load limit of such coating materials. The electron beam irradiation experiments showed that there was little difference in temperature increases among CX-2002U and the coated materials below surface temperature of 2200°C. These results indicated that thermal and adhesion properties between the substrate and coatings were good under high heat flux. A few cracks with a width of 15 μm were formed from the surface to the bottom side of the all coated samples, but plastic deformation and microcracks due to grain growth by recrycrallzation were not observed below a surface temperature of about 2200°C. The cracks are expected to be formed due to local thermal stress produced by spot-like beams.

Localized swelling of Fe-Cr-Ni alloy rollers in high-temperature applications

Localized swelling has been observed in 24Cr-24Ni-Nb steel transportation rollers used in the normalizing furnace of a plate mill after prolonged service at high temperature. Due to high localized thermal and mechanical stresses, the chromia layer formed on the roller surface ruptures, exposing the roller substrate to furnace oxygen. Oxidation of second-phase carbides results in the formation of carbon monoxide at very high partial pressure. This leads to formation of voids, leading in turn to localized swelling of the roller material.

Residual stress and its consequences on both sides of the wheel-rail interface

The contact pressures from service loads gradually change the residual stresses in the heads of rails and the rims of wheels from their initial (as manufactured) distributions as permanent plastic deformation accumulates. The resultant state has a near-surface compressive component, aligned with the major live stress, which protects the contact surface against fatigue propagation of shallow surface cracks unit wear removes them. If the contact pressures are high enough, internal residual tension may develop and promote subsurface fatigue cracking. The fatigue environment can also be exacerbated by high traction/ braking shear loads or by local heating and thermal stress if the interface is subjected to high power dissipation. The Federal Railroad Administration sponsors research on problems involving rail and wheel cracks in order to foster safer railroad operations. The goal is to establish rational criteria for load or life limits. The research program development history and major findings are summarized.

Particle emissions from graphite during disruption heat load

It is regarded that the life time of a graphite divertor wall is determined by erosion during the disruptions. In the disruption heat load experiments, the erosion of graphite due to particle emissions has been observed to be extremely serious. The particle emission process is considered based on localized thermal stress at the surface. There is a critical heat flux for the particle emission to take place. Erosion depth observed in the experiment is also discussed using the present simple model.

Failure mechanism of GTO devices and optimization for minimum current crowding during turn-off

The maximum controllable anode current is influenced by the current crowding phenomenon observed in GTO thyristors during turn-ofT. This phenomenon initiates (1) the formation of the current filament with a density much higher than its surrounding the leads to localized thermal instability; and (2) the conductivity modulation at the conduction channel causing a non-uniform electric field distribution within the depletion layer, and a resulting an early punch-through breakdown. In this paper, the failure mechanism is discussed. Internal parameters are optimized qualitatively through detailed two-dimensional device simulations to minimize the current crowding and, at the same time, to maintain a low forward voltage drop and short turn-off time for a GTO model-cell. It is found that a lower localized thermal stress and a more uniform electric field distribution can be obtained.

Thermally Induced Interfacial Microcracking in Polymer Matrix Composites

Novel experiments on a cluster of fibers combined with finite element analysis was utilized to investigate the influence of both inter-fiber spacing and interphase properties on thermally induced microcracking. Local thermal stresses were predicted and microcracking observed as fiber spacing was systematically decreased and interphase properties were varied. Both the computational and experimental results demonstrated that interphase properties and fiber spacing alter the location of the maximum equivalent stress and the initiation of microcracks. Microcracks were predicted and observed to initiate first (at the lowest thermal load) in the case of a higher modulus interphase. The cracks ini tiated at the fiber/interphase interface at a lower thermal load than if no interphase were present. In contrast, computations for a low modulus interphase predicted the maximum equivalent stress to be lower than the case of no interphase and to occur in the matrix. Ex perimentally, microcracks were observed to initiate in the matrix at the interphase/matrix interface with higher thermal loads. The presence of the low modulus coating prevented cracks from reaching the fiber surface. Overall, the investigation demonstrated the ability of the interphase to enhance or hinder microcracking in a cluster of fibers. The interphase can be tailored to reduce the local stress state and reduce the initiation of microcracks in the composite.

Hot Clamp Design for LMFBR Piping Systems

Thin-wall, large-diameter piping for liquid metal fast breeder reactor (LMFBR) plants can be subjected to significant thermal transients during reactor scrams. To reduce local thermal stresses, an insulated “cold” clamp was designed for the fast flux test facility and was also applied to some prototype reactors thereafter. However, the cost minimization of LMFBR requires much simpler designs. This paper presents a “hot” clamp design concept, which uses standard clamp halves directly attached to the pipe surface leaving an initial gap. Combinations of flexible pipe and rigid clamp achieved a self-control effect on clamp-induced pipe stresses due to the initial gap. A 3-D contact and inelastic history analysis were performed to verify the hot clamp concept. Considerations to reduce the initial stress at installation, to mitigate the clamp restraint on the pipe expansion during thermal shocks, and to maintain the pipe-clamp stiffness desired during a seismic event were discussed.

A model for the formation and solidification of grain boundary liquid in the heat-affected zone (HAZ) of welds

The rapid thermal cycle experienced in most welding operations can promote the constitutional liquation of precipitates in certain alloy systems. Grain boundary liquid films form in the subsolidus portion of the heat-affected zone (HAZ) as a consequence of constitutional liquation. Rapid cooling limits the extent of solute diffusion into the matrix from the grain boundary liquid and hence extends its solidification temperature range. Liquation cracking can occur in the HAZ if the grain boundary film exists at a time when the local thermal stresses become tensile. Hence, in order to predict the liquation cracking susceptibility of an alloy under a given welding condition, both the microstructural evolution which centers around grain boundary liquation and the stress generation have to be modeled. This article addresses the issue of microstructural evolution and attempts to present a model for the formation of grain boundary liquid during the heating cycle and its solidifiction during the cooling cycle. The variables which increase the life of the transient grain boundary liquid during the thermal cycle are identified. The onedimensional (1-D) model presented here is an important first step toward the ability to predict liquation cracking susceptibility of an alloy during welding.

Residual Stresses in a Composite with Continuously Varying Young's Modulus in the Fiber/Matrix Interphase

The incorporation of a realistic interphasial region into the micromechani cal analyses of composite systems is critical to the understanding of composite behavior. The interphase is usually modeled as a homogeneous region, despite the fact that it may have spatial property variations. A representative volume element with three cylinders (concentric cylinder assemblage) is considered here for the determination of local thermal stresses in a composite. Three different expressions are used to simulate the Young's modulus variations in the interphase, while the Poisson's ratio and coefficient of thermal expansion of the interphase region are chosen to be constant. The governing field equa tions in terms of displacements are solved in closed form. It is found that, though the solu tion is "dilute," the Young's modulus variations have a distinct effect on the local thermal stresses.

Semi-empirical investigation of thermal expansion behavior of components in a two-phase particle-reinforced metal matrix composite

The paper describes a semi-empirical method to obtain the average coefficient of thermal expansion (CTE) of components in a composite environment with any particulate geometry. An analysis was performed to examine the CTE behavior of components in a two-phase particle-reinforced metal matrix composite. This analysis was performed with the assumption that the CTE of the composite is known through measurement and the material is elastic. The present analysis is therefore semi-empirical. The results obtained by the analysis can be used to predict the effect of particulate geometry on the CTE of components in these composites. The analysis may also be useful for investigating local thermal stress effects in the area surrounding a particle. Experimental measurements were performed for an AlSiC composite material to support the CTE obtained by previous investigators. Numerical calculations were performed for an AlSiC system suggesting that the CTE behavior of each composite component differs in the composite environment from that in monolithic form. The results of the analysis and experimental measurements are discussed.

An Investigation of Thermal Stress Ranges Under Stratification Loadings

This paper deals with a unique and severe stratification phenomena in the pressurizer surge line piping system. The methodology used in the calculation of total stress state includes pressure stresses from internal pressure, global thermal stresses from the constraining of the system by supports, local thermal stresses from hot/cold stratified condition, and transient thermal stresses in the pipe and at structural discontinuities. Special discussion on the interpolation to obtain moment loadings from one state of thermal condition to the next with nonlinear gapped support system is presented. Finally, the thermal cycling associated with operating transients in the surge line system is also addressed.

Thermal stresses due to thermal expansion anisotropy in materials with preferred orientation

Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Thermal stresses result from thermal expansion incompatibility in materials during the temperature change., Sources of thermal stresses include thermo- mechanical mismatch between two materials [1, 2], non-uniform temperature distribution in a material [3] and thermal expansion anisotropy in a single- phase material [4, 5]. The first two types of thermal stress have been studied extensively. The third type of thermal stress can be classified into stresses at the microscopic and the macroscopic levels. Examples of stresses at the microscopic level exist in single- phase polycrystalline materials, where localized thermal stresses arise from thermal expansion mis- match among randomly orientated grains [4]. Stres- ses at the macroscopic level can exist in materials with preferred crystallographic orientation [5]. A brief discussion of these macroscopic stresses is the topic of the present letter. Thermal expansion anisotropy is introduced in materials (especially non-cubic materials) with pre- ferred crystallographic orientation. This texture can occur during deposition of a coating on a substrate, a process that has been used extensively for fabrica- ting electronic and optical components. A particular crystal plane of the coating is often found to be parallel to the deposition surface. Hence, a large degree of anisotropy in properties, such as thermal expansion, can exist between the directions parallel and perpendicular to the deposition surface [6, 7]. For example, boron nitride crucibles can be synthes- ized by chemical vapour deposition on a mandrel, which results in the basal planes of the boron nitride structure being oriented parallel to the deposition surface. As a result, the textured boron nitride crucible has a thermal expansion coefficient in the radial direction (i.e. the direction perpendicular to the deposition surface) about 14 times of that in the tangential direction (i.e. the direction parallel to the deposition surface) [8]. Highly oriented supercon- ducting YBa2Cu30 x coatings on fibres with the basal planes of the coating material parallel to the fibre surface have been produced recently, and the thermal expansion coefficient of the coatings in the radial direction is found to be about four times of that in the tangential direction [9, 10]. This preferred orientation in the YBa2Cu30 x coating provides a very important contribution to increasing the current density in the direction of the fibre axis. Experimental evidence of macroscopic thermal (~ us Government. Contract No. DE-ACO5-84OR21400. stresses due to preferred orientation-related thermal expansion anisotropy has been found. For example, delamination of boron nitride crucibles is a frequent problem during cooling from the fabrication tem- perature [5], a failure mode that is consistent with the existence of the residual tensile stress in the radial direction. Extensive cracking due to the residual stress is also observed in the textured superconducting YBa2Cu3Ox coating during cooling which, in turn, results in low current density and low strength of the superconducting coating [9, 10]. The macroscopic thermal stresses due to thermal expan- sion anisotropy have been analysed for a two-dimen- sional case [5]. The results showed that radial tension, and combined tangential tension and com- pression, are induced in boron nitride crucibles during cooling. The purpose of the present letter is to visualize these thermal stresses schematically. Stresses due to thermal expansion anisotropy arise because of different thermal expansion coefficients in the radial and tangential directions in a cylindrical geometry during a temperature change. To facilitate visualizing the origin of these stresses, stresses are considered for a two-dimensional solid circle as shown in Fig. 1, where the radial thermal expansion coefficient, o#, is higher than the tangential thermal expansion coefficient, ol0, and b is the radius of the circle. Before cooling, an element that is originally at position

Ductile fracture estimation of reactor pressure vessel under thermal shock.

This paper presents a new scheme for the estimation of unstable ductile fracture of a reactor pressure vessel under thermal shock conditions. First, it is shown that the bending moment applied to the cracked section can be evaluated by considering the plastic deformation of the cracked section and the thermal deformation of the shell. As the contribution of the local thermal stress to the J-value is negligible, the J-value under thermal shock can be easily evaluated by using fully plastic solutions for the cracked part. Next, the phenomena of ductile fracture under thermal shock are expressed on the load-versus-displacement diagram which enables us to grasp the transient phenomena visually. In addition, several parametrical surveys are performed on the above diagram concerning the variation of (1) thermal shock conditions, (2) initial crack length, and (3) J-resistance curve (i.e. embrittlement by neutron irradiation).

Local elastic thermal stresses in a column (wall) with concave or convex surfaces.

Elastic thermal stresses in a column of wall, such as a boier wall, with concave or convex surfaces are analyzed by the indirect fictitious boundary integral method. As a example, we treat a wall, which has a cncavity (notch) or convexity (protrusion) on the one side of the wall and is heated on either side. As the results the temperature distributions and the thermal stresses were obtained and the influenced of shape parameters such as a depth of notch where observed. Also the availability of this method to this problem we shwon.