Papers by Keyword: Residual Thermal Stress

Abstract: This paper discussed the residual thermal stress on single lap joint of various materials. Finite element method (FEM) was adopted to simulate the experimental phenomena. The adherend and adhesive were the main research objects. Mismatch of adherend, adhesive thickness, temperature variation which were the three key factors on the residual thermal stress were analyzed. While, four kinds of materials that include C/SiC, SiC, high temperature nickel based alloy GH1035 (GH1035) and high temperature boron fiber reinforced epoxy composite (composite) formed seven approaches for numerical analysis. The results showed that the adhesive with the C/SiC has the best performance and the best thickness of every approach was determined. Moreover, the ladder temperature is better than other temperature styles.

Abstract: Electrical Discharge Machining (EDM) is the machining technique of discharging corrosion of the material surface. The effect of the EDM pulse current on the micro structure and properties of the P20 specimen surface were investigated. Three layers were observed on the specimen surface after discharging: ablation layer, molten layer and recrystallized layer. The experimental results show that with the increment of the pulse current, the thickness of the molten layer and recrystallized layer increased. The contents of the oxygen and carbon on the outside of the molten layer were higher than the base metal for the diffusion during the machining. The micro hardness reduced from the out layer to the base metal after the heat treatment, indicating the mechanical properties changing of the machining surface. In the molten layer, micro fracture is caused by the residual thermal stress during the machining.

Abstract: Residual thermal stress, which has a strong effect on the coating performance, is generated during the fabrication of TBC. In this paper, the residual thermal stress in TBC (92%ZrO2-8%Y2O3) with two different thicknesses (0.38 mm and 0.90 mm) is analyzed with a bi-material model. TBC specimens were sprayed by an Air Plasma Spray (APS). In addition, a chemical material removal method was used to obtain TBC specimens after it cooling down to the room temperature. Before and after the coating removed from the substrate, the deformation of the TBC was measured. Then the bi-material model was used to calculate the residual thermal stress in the TBC. The experiment result shows that, the residual thermal stress in thin TBC is smaller than that in thick TBC (the average residual thermal stress in thin and thick specimens are 145.5 MPa and 225.13 MPa respectively). The micro-structures in thin and thick TBC are also observed and discussed.

Abstract: One of the most important problems in both the fabrication and exploitation of ceramicmetal composites are residual thermal stresses. The paper presents the results of a numerical analysis (by the Finite Elements Method) of the stress state induced in the NiAl matrix composites reinforced with spherical particles of a ceramic phase (Al2O3, ZrO2, TiC), including examinations of the dependence of this stress state on the volumetric fraction of the ceramics (20 to 40vol.%). The stress state prevailing in this composite appeared to be complex. In all the samples, the stresses active in the ceramic regions were compressive whereas those active in the metal matrix were tensile in the circumferential direction and compressive in the radial direction. An increase of the ceramic volumetric fraction resulted in an increase of the tensile stresses in the NiAl matrix and a decrease of the compressive stresses in the ceramic particles. These theoretical results were verified experimentally by examining the properties of the NiAl-Al2O3, NiAl-ZrO2 and NiAl-TiC (20 and 30 vol.% fraction of the ceramics) composites produced by hot-pressing. The microstructure, density, and bending strength of these composites were examined, and the results are discussed in the paper.

Abstract: Non-homogenous cooling rates and solidification conditions during DC-casting of high strength aluminum alloys result in the formation and accumulation of residual thermal stresses with different signs and magnitudes in different locations of the billet. Rapid propagation of micro-cracks in the presence of thermal stresses can lead to catastrophic failure in the solid state, which is called cold cracking. Numerical models can simulate the thermomechanical behavior of an ingot during casting and after solidification and reveal the critical cooling conditions that result in catastrophic failure, provided that the constitutive parameters of the material represent genuine as-cast properties. Simulation of residual thermal stresses of an AA7050 alloy during DC-casting by means of ALSIM5 showed that in the steady-state conditions large compressive stresses formed near the surface of the billet in the circumferential direction. Stresses changed sign on moving towards the centre of the billet and became tensile with high magnitudes in radial and transverse directions, which made the alloy prone to hot and cold cracking.

Abstract: In the present paper, singular stress field at the vertex on the interface in three-dimensional bonded joints is analyzed using BEM and eigen analysis. The order of stress singularity is determined solving an eigen equation based on FEM formulation and the stress distribution is expressed using the result of the eigen-value analysis. A relationship between the thickness of interlayer and residual thermal stresses is presented. Then, a three-dimensional intensity of singularity is determined.

Abstract: To combine the merits of both metals and ceramics into one material, many researchers have been studying the deposition of alumina coating using plasma spray on metal substrates. However, as the coatings are deposited at a high temperature, residual thermal stresses develop due to the mismatch of thermal expansion coefficients of the coating and substrate and these are responsible for the initiation and expansion of cracks, which induce the possible failure of the entire material. In this paper, the residual thermal-structural analysis of a Fe3Al/Al2O3 gradient coating on carbon steel substrate is performed using finite element modelling to simulate the plasma spray. The residual thermal stress fields are obtained and analyzed on the basis of temperature fields in gradient coatings during fabrication. The distribution of residual thermal stresses including radial, axial and shear stresses shows stress concentration at the interface between the coatings and substrate. The mismatch between steel substrate and composite coating is still the dominant factor for the residual stresses.

Abstract: Effects of the body centered cubic distribution of the dispersed SiC and (W,Ti)C particles on the residual thermal stresses of (W,Ti)C/SiC/Al2O3 ceramic composite were analyzed with the finite element method, especially with the comparison with that of the face centered cubic distribution. Results show that the residual thermal stress is affected not only by each material compositions but also by the distribution forms of the dispersed SiC and (W,Ti)C particulates. Both distribution and magnitude of residual thermal stress differs with the difference in the particle distribution forms. The calculation results of different particle distribution forms, however, are just the reflection of different cases of the residual thermal stresses inside the ceramic composite from different points of view. Both reflections can research agreement in trends.

Abstract: Layered structure can greatly improve the toughness of the ceramics. Manufacture techniques, including matrixes and interlayer materials choosing and matching, structure and interface design, parameter deciding, processing of layered materials making were introduced in this paper.

Abstract: The intent of this article was to study the failure mechanism of thermal barrier coatings (TBCs) induced by buckling. The main content included the following two parts. The first part investigated the thermal residual stresses fields in TBCs with thermal cycles, which induced by the non-linear coupled effect of temperature gradient, thermal fatigue and creep strain of TBCs. One found that the residual stresses in ceramic coating were compressive and accumulated with thermal cycles, which may be high enough to induce the buckling failure of ceramic coating. The second part studied the critical buckling failure loading of the ceramic coating in TBCs under the condition of the compressive loading by use a theoretical model. Finally, a buckling plane, i.e. s T n − plane, was obtained by combined the above sections and applied to predict the buckling failure of the TBCs system. In this plane, it was divided into the two parts, i.e., non-buckling region and buckling region.