Thermally-induced Stresses Research Articles

RESIDUAL STRESS MEASUREMENTS AND STRUCTURAL INTEGRITY IMPLICATIONS FOR SELECTIVE LASER MELTED TI-6AL-4V

Selective laser melting (SLM) of Ti-6Al-4V has significant potential in the aerospace and biotechnology industries. SLM employs a focused laser beam to melt successive layers of metallic powder into complex components. This process can result in the generation of high thermally-induced residual stresses. These residual stresses, together with micro-flaws/ pores from the inherent fabrication process, may lead to premature fatigue crack initiation and propagation at relatively low cyclic stresses. The hole-drilling strain gauge method was used to evaluate residual stresses within SLM Ti-6Al-4V specimens, with the intention of understanding the associated mechanisms for the successful application of SLM Ti-6Al-4V in industry.

Stability analysis of solid‐state lasers regarding thermal lensing effect

AbstractThe thermal lensing effect in the crystal influences the stability of solid‐state lasers. The deformation of the end faces of the crystal and the temperature dependence of the refraction index causes this effect. The photoelastic effect produced by thermal induced stress in the crystal has also an influence. For high power lasers and for lasers with a radially polarized laser beam the analysis of the photoelastic effect is important. We use FE model to calculate the deformation, heat and stress distribution in the laser crystal with high accuracy in this work. Using the calculated stress distribution we also simulated the refraction index anisotropy with respect to the crystal orientation. We can study stability of laser resonators for both radially and azimuthally polarized laser beam, as a result of our simulation. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Coupling Geomechanics and Transport in Naturally Fractured Reservoirs

Large amounts of hydrocarbon reserves are trapped in naturally fractured reservoirs which arechallenging in terms of accurate recovery prediction because of their joint fabric complexity andlithological heterogeneity. Canada, for example, has over 400 billion barrels of crude oil in fracturedcarbonates in Alberta, most of this being bitumen of viscosity greater than 106 cP in the GrosmontFormation, which has an average porosity of about 13-15%. Thermal methods are the most commonexploitation approaches in such viscous oil reservoirs which, in the case of steam injection, are associatedwith up to 275-300°C temperature changes, leading to considerable thermoelastic expansion. Thistemperature change, combined with pore pressure changes from injection and production processes, leadsto massive effective stress variations in the reservoir and surrounding rocks. The thermally-induced(thermoelastic) stress changes can easily be an order of magnitude greater than the pore pressure effectsbecause of the high intrinsic stiffness of the low porosity limestone and bounding strata. Study of thesestress-pressure-temperature effects requires a thermo-hydro-mechanical (THM) coupling approach whichconsiders the simultaneous variation of effective stress, pore pressure, and temperature and theirinteractions. For example, thermal expansion can lead to significant joint dilation, increasing themacroscopic, joint-dominated transmissivity by an order of magnitude in front of and normal to thethermal front, while reducing it in the direction tangential to the heating front. This leads to stronginduced anisotropy of transport processes, which in turn affects the spatial distribution of the heatingarising from advective heat transfer.

Thermal instabilities in a yield stress fluid: Existence and morphology

We present new laboratory experiments on the development of thermal plumes out of a localized heat source in Carbopol, a yield-stress and shear-thinning fluid. Depending on the yield number Y0, which compares the thermally-induced stress to the yield stress, three different regimes obtain. For low Y0 (<120), no convection develops; while for intermediate values, a small-scale convection cell appears and remains confined around the heater. For high Y0 (>260), thermal plumes develop. Their morphology differs from the mushroom-shape typically encountered in Newtonian fluids. Combined temperature and velocity field measurements show that a plug flow develops within the plume thermal anomaly, therefore producing a rising finger-shape. Moreover, light scattering highlights the development of a damaged zone prior to the plume onset, and the peculiar structure of the gel around the plume as it rises. This brings new insights into the solid–liquid transition of soft gels.

Numerical Analysis on Cooling Process of Monocrystal Silicon Rod Manufactured with Czochralski Method

A numerical analysis was performed to investigate the temperature distribution and thermal stress field in monocrystal silicon rod in the cooling process of manufactured with Czochralski (CZ) method. The thermally-induced residual stress fields of silicon rod under different length of cool-down time conditions were obtained as well as temperature fields, respectively. All simulations were finished by using ANSYS finite element code. It showed that, maximum thermal stress was mainly appeared on rod surface, the influence of length of cool-down time on it was not remarkable, the magnitude of it was far below the critical strength of silicon throughout.

A three-dimensional analytical model to predict the thermo-metallurgical effects within the surface layer during grinding and grind-hardening

The high amount of energy input during machining processes with geometrically undefined cutting edges, such as grinding, requires the knowledge of the transient temperature field within the machined surface to avoid thermal damages of the workpiece due to thermal-induced residual tensile stresses or softening for instance. An approach to use the heat generated during grinding is the grind-hardening process, which allows for the process-integrated martensitic surface layer hardening of the workpiece. Because of the used simplifications, for example two-dimensionality, existing approaches to calculate the temperature fields during wet grinding cannot be successfully utilized to predict the phase transformations and the resulting hardened surface layer due to grind-hardening.This paper presents an analytical model, which enables to calculate a three-dimensional temperature field due to grinding and grind-hardening using a triangular heat source and considering the effect of the grinding fluid. Based on the transient temperature distribution, the phase transformation within the surface layer and the resulting hardened layer thickness are determined also using analytical models. The analytical method is validated and analyzed comparing the results with measured ones and with those calculated using the finite element analysis. Thereby, the presented approach allows for the efficient prediction of the thermo-metallurgical effects within the surface layer during grinding and grind-hardening.

LED용 실리콘 봉지재의 경화방법이 신뢰성에 미치는 영향

Encapsulant curing in terms of convection oven leads to thermal induced stress due to nonuniform thermal conductivity in LED package. We have adopted infrared (IR) light for silicone curing in order to release the stress. The light uniformity irradiated on an encapsulant surface is confirmed to be uniform by optical simulation. Shear strength of die paste using IR compared to convection oven is increased 19.2% at the same curing time, which indicates curing time can be shortened. The indentation depth difference between center and edge of silicone encapsulant in terms of convection oven and IR are 14.8% and 3.4%, respectively. Curing by IR also shows 2.3% better radiant flux persistency rate of LED at <TEX>$85^{\circ}C$</TEX> after 1,000 h reliability test compared to convection curing.

The influence of adhesive materials on chip-on-board packing of MEMS microphone

Adhesive material is commonly used for attaching die onto the printed wiring board (PWB) in the Chip-on-Board (COB) packaging of MEMS devices. However, the polymer-based adhesive usually possesses large difference in the coefficient of thermal expansion (CTE) between silicon chip and PWB. Therefore, the mismatch of CTE could lead to the thermally-induced stress and coupling deformation of multilayer structure in the reflow process as surface-mount technology (SMT) on printed circuit board (PCB). In this study, three different adhesive materials, namely 2025D, 3140RTV and SDA6501, and two different cap materials, namely liquid crystal polymer (LCP) and nickel (Ni), were evaluated the influences on the thermally-induced stress in the ploy-silicon diaphragm of MEMS microphone based on Finite Element Analysis (FEA). According to the results, we obtained the following two findings: (1) The CTE mismatch of LCP cap and the metal (Ni) cap caused different type of thermal deformations of PWB and lower thermally-induced stress and deformation were found in the case of LCP cap, however, different cap materials less affected the thermal stress in the diaphragm. (2) Soft adhesive materials (3140RTV and SDA6501) have better mechanical isolation of PWB thermal deformation due to the buffer layer effects. On the contrary, hard adhesive material (2025D) could be affected by PCB thermal deformation when the thickness of adhesive was less than 30µm, thus, a lower stress in the diaphragm existed due to the stress compensation by PWB thermal deformation. In general, present study provides the basis of selection of adhesive material for COB MEMS packaging.

Residual stress in radiation-cured acrylate coatings

Radiation-cured acrylate-based materials are used in a wealth of applications. For coatings in particular, adhesion is a key property and remains challenging for some substrates such as plastics and metals. Stress build-up during and after resin polymerization is a handicapping issue that may lead to defect formation or debonding which results in bad coating performances. Using the substrate deflection method, stress development has been investigated in ultraviolet(UV)-cured acrylate coatings based on an epoxy acrylate combined with mono-, di- and triacrylate diluents. The coatings were prepared in conditions relevant to industrial practice using high UV intensity and short processing times. Stress measurements were conducted according to the prescriptions of a recent ASTM standard. A new set of data is provided which shows that photo-cured epoxy acrylates develop low stress as compared to other dilution acrylates (monomers). When formulated with most of the reactive diluents, the stress does not exceed 4MPa as long as the dilution factor stays below 50%. This is fairly low given the high conversion rates. For vitrified coatings, it appears that in addition to polymerization stress, thermally-induced stress represents a non-negligible contribution to the overall stress. In an industrial UV-curing process however, the latter contribution is estimated to be less than 2–3MPa. Lastly, an estimate of the interfacial fracture toughness between trimethylolpropane triacrylate and carbon steel is inferred from the stress data.

Measurement and Simulation of Thermal-Induced Stress in C2W 3D Integration with Template Alignment

This paper presents the experimental and theoretical investigation of the thermal stress development in the C2W 3D integration with template alignment and the impact of fabrication parameters on the thermal stress. The experimental approach is accompanied with an optical profilometer, while the theoretical study is based on COMSOL FEM analysis. It is found that the thermal stress is primarily induced in the polymer hard curing process, and is accumulated within the cooling process due to the mismatch of coefficients of thermal expansion (CTE) with substrate. The thermal stress highly depends on the polymer thickness and the treatment temperature, and could be partially released by polymer etching. The measurement and simulation results indicate that the template alignment technology is compatible with semiconductor fabrication by optimizing the fabrication parameters. The feasibility of C2W 3D integration with template alignment is furthered demonstrated.

Microlevel manufacturing process simulation of carbon fiber/epoxy composites to analyze the effect of chemical and thermal induced residual stresses

A micromechanical implicit finite element analysis using Samcef/Mecano was performed to analyze the effect of process-induced deformations and stresses on a unidirectional carbon fiber–epoxy composite material (G1157/RTM6). During the manufacturing process, chemical and thermal shrinkage deformations occur and lead to internal stresses. In this article, several effects on the level of stress on the microscale will be discussed, which address the nonlinear behavior of the polymer matrix material. A parametric study was done on the influence of these effects on residual stresses including temperature dependency of the young’s modulus, thermal expansion coefficient, nonlinear thermomechanical stress–strain behavior, microyielding, microdegradation, and viscoelasticity. Several experiments have been carried out to investigate the thermomechanical behavior of the matrix material and to derivate constitutive equations. The derived equations and the discussed effects are integrated into an analysis model of a squared unit cell and used to perform a coupled curing thermomechanical simulation. As a result, the development of process-induced stresses is presented with the integration of nonlinear local material effects. Assessment of these effects is one of the key aspects for interpretation of process-induced stresses on the macroscale.

Numerical Analysis of Thermally-Induced Residual Stresses in Beryllium Caused by BeO

A numerical analysis was performed to investigate the thermally-induced residual stress caused by the thermal expansion mismatch between Be and BeO. Models were constructed according to the real situation that the BeO particles embedded in grains or distributed at the grain boundaries, where the temperature-dependent materials properties, particle volume fraction and size and interparticle distance were taken into account. In simulation, it supposes particles inside grain no interacting with others, particles along the grain boundary being a period distribution. Be is modeled as elastic and elastic-plastic material with bilinear kinematic hardening model. BeO is modeled as isotropic and linear elastic. All simulations are performed by using ANSYS finite element code. Results show that the size, location and the volume fraction of particles influence on the residual stress (strain) to different extent during cooling process.

Structural and Thermal Properties of Single Crystalline Epitaxial Gd2O3 and Er2O3 Grown on Si(111)

Lattice mismatch with a substrate and difference in coefficient of thermal expansion to the substrate induces stress and deformations in crystalline structure of epitaxially grown layers. In this work, we present results of structural study of gadolinium oxide and erbium oxide grown on silicon (111) substrate applying X-ray diffraction at room temperature and during in-situ heating up to 1273 K. The layers are almost fully relaxed at room temperature. Only minor tetragonal distortion of the crystal lattice was indirectly detected. No thermal induced stress relaxation occurs in the oxide layer during the in-situ heating and cooling procedure though strong tetragonal distortion of the oxide lattice due to thermal expansion difference with silicon. The time-domain thermo-reflectance measurements reveal that thermal conductivity of the rare earth oxides is approximately five times higher than that of silicon dioxide.

Modeling the temperature and stress distributions in rigid pavements: Impact of Solar Radiation absorption and heat history development

This study describes the impact of the absorption of Solar Radiation (SR) on temperatures and subsequent thermal stress development in Jointed Plain Concrete Pavements (JPCP). A one-dimensional heat transfer model is proposed to predict the temperature distribution through the slab depth. In this model, the inputted SR has features with both deterministic and probabilistic features. The deterministic factor is the monthly mean observed SR. The probabilistic component is computed by assuming that the monthly SR’s variation between the maximum and minimum observed radiations follow a random normal distribution, whose standard deviation depends on the uncertainties of the observed monthly SR. Considering that the amount of SR absorption may be inherited from both within and under the slab, we study the impact of the SR history on the temperature and thermal-induced stresses through the slab. The results show that when the local weather shifts from sunny day to cloudy day or vice versa, the maximum temperature and JPCP’s thermal stresses shift by up to 5°C and 0.3 MPa, respectively. Other than the SR, we also estimate the influences of the sunrise/sunset times and of the pavement surface absorptivity on rigid pavements.

Moisture absorption of unidirectional hybrid composites

Unidirectional hybrid composite rods were conditioned in humid air to investigate the sorption kinetics and the effects of moisture on mechanical and physical properties. Sorption curves were obtained for both hybrid and non-hybrid composite rods to determine characteristic parameters, including the diffusion coefficient ( D) and the maximum moisture uptake ( M ∞). The moisture uptake for the hybrid composites generally exhibited Fickian behavior (no hybridization effects), behaving much like non-hybrid composites. A two-dimensional diffusion model was employed to calculate moisture diffusivities in the longitudinal direction. Interfaces and thermally-induced residual stresses affected the moisture diffusion. In addition, the effect of hygrothermal aging on glass transition temperature ( T g ), short beam shear strength (SBS), and tensile strength was determined for hygrothermal exposure at 60 °C and 85% relative humidity (RH). Property retention and reversibility of property degradation were also measured. Microscopic inspection revealed no evidence of damage.

Laser-induced radial birefringence and spin-to-orbital optical angular momentum conversion in silver-doped glasses

Samples of Ag+/Na+ ion-exchanged glass that have been subject to intense laser irradiation may develop novel optical properties, as a consequence of the formation of patterns of silver nanoparticles and other structures. Here, we report the observation of a laser-induced permanent transverse birefringence, with the optical axis forming a radial pattern, as revealed by the spin-to-orbital angular momentum conversion occurring in a probe light beam. The birefringence pattern can be modeled well as resulting from thermally-induced stresses arising in the silver-doped glass during laser exposure, although the actual mechanism leading to the permanent anisotropy is probably more complex.

Neutral Beam Armor for NSTX Upgrade

The National Spherical Torus Experiment (NSTX) is a low aspect ratio, spherical torus (ST) configuration device which is located at Princeton Plasma Physics Laboratory (PPPL). This device is presently being upgraded to enhance its operations by adding a second Neutral Beamline (NBL). This change will nearly double the power available to the plasma but necessitate improvements to other design aspects of NSTX. Included in these upgrades are the relocation and upgrade of the NSTX Neutral Beam Armor to capture both sets of beamline source profiles while maintaining the same level of vacuum vessel wall protection.In order to minimize the space required to accomplish this, it has been proposed to relocate and reuse the existing armor array, improving the design so that two overlapping sets of beam profiles both fit completely. This beamline overlap could possibly cause the armor tiles to experience higher heat fluxes which translate into higher internal mechanical stresses. This would be mitigated by changing the isotropic graphite (ATJ) tiles in the overlap areas to a rugged 3D carbon-fiber composite (CFC) material, capable of handling thermally-induced stresses. Additional benefits to this recycling design proposal include opportunities to reduce project cost, increase diagnostic port access, and improve an awkward and difficult mounting scheme.

The Thermal Induced Temperature and Stress Damage

During the Canister Retrieval Test (CRT), the temperature around the deposition hole DD0092G01 was elevated and the stress in the rock was consequently increased. This report concerns the study to investigate if the increased stress around the deposition hole has led to micro fracturing in the surrounding rock and if the temperature and possible heat induced damage can be predicted by numerical simulation.The temperature field from thermal analysis generally matches the measured results well, whereas the calculated temperature is much higher than measured.The damage analysis might be a quite reasonable simplification for the area near the middle height of the hole. For the other part, the influence due to tunnel excavation or the end effect should have more important influence.

Comparative study of Laser induce damage of HfO_2/SiO_2 and TiO_2/SiO_2 mirrors at 1064 nm

A comparative study of laser induced damage of HfO2/SiO2 and TiO2/SiO2 mirrors at 1064 nm has been carried out. One TiO2/SiO2 mirror with absorption of 300 ppm and two HfO2/SiO2 mirrors with absorption of 40 and 4.5 ppm were fabricated using electron beam evaporation method. For r-on-1 test, all HfO2/SiO2 mirrors with low average absorption are above 150 J/cm2 at 10 ns. However, the TiO2/SiO2 mirrors with high average absorption are just 9.5 J/cm2, which are probably due to the rather high absorption and rather low band gap energy. Meanwhile, all the samples were irradiated from front and back side respectively using the raster scan test mode. In case of front side irradiation, it is found that: for TiO2/SiO2 high reflectors, the representative damage morphologies are shallow pits that were probably caused by absorbing centers. However, for HfO2/SiO2 high reflectors, the dominant damage morphologies are micrometer-sized nodules ejected pits and the delamination initiating from the pits. The absorption of HfO2/SiO2 coatings is low enough to have minor influence on the laser damage resistance. In case of backside irradiation, the morphology of TiO2/SiO2 mirrors is mainly center melted pits that are thermal melting induced damage. Meanwhile, HfO2/SiO2 mirrors with isometrical fracture rings damage morphology are thermal induced stress damage.

Numerical Analysis of Thermally-Induced Residual Stresses in Plasma Facing Components for Fusion Reactor

The realization of a fusion reactor is critically dependent on the successful development of high performance materials. Especially, the plasma facing components (PFCs) which basically consist of a direct plasma facing armor material and a heat sinking material. Tungsten (W) and Copper-alloy (CuCrZr) have been considered as the potential candidates for armor materials and heat sinking materials, respectively, due to their attractive nuclear and physical properties. However, due to the incompatibility of the coefficient of thermal expansion and the elastic properties between the W and the Cu-alloy as well as the non-homogeneous temperature distribution in PFCs, one of the crucial issues is the generation of thermally-induced residual stresses in W/CuCrZr PFC on cooling either during fabrication or during operation of fusion reactor. Therefore, the thermo-mechanical response of PFCs under high heat flux from the fusion reactor is a critical issue for the development of fusion technology. In the present work, in order to optimize the thermal and mechanical integrity of PFCs, thermally-induced residual stresses in W/CuCrZr PFCs with a compliant interlayer (OFHC-Cu: Oxygen Free High Conductivity Copper) are analyzed numerically by means of finite element method. Result indicated that the use of interlayer in PFCs could significantly reduce the magnitude and the concentration of thermally-induced stresses in comparison to the PFCs without interlayer. And also the optimum thickness for interlayer was suggested based on the current analysis conditions.