Effect of the Porosity on Thermal Stress within TBCs Using Finite Element Method

Thermal barrier coatings (TBCs) with NiCoCrAlY bond-coats (BC) and ceramic top coats (TBC) were prepared on IN738 superalloy by plasma spraying technology. Effect of porosity on the maximum stress under thermal cycling conditions was carried out by using finite element software ANSYS. The results showed that maximum axial thermal tensile stress in TBCs decreases from 868MPa to 498MPa with the increase of bond-coats porosity near the TGO/BC interface, and the maximum axial thermal compressive stress decreases from 1060MPa to 711MPa. The stress distribution changes with increasing bond-coats porosity near the TGO/BC interface. The simulation results are in agreement with the experiments, exhibiting a clearly evidence that the porous TBCs have a longer lifetime.

Thermal Stress Analysis of a Tubesheet with a Welding Clad

In this paper, numerical simulation was carried out for the tube bundle of a slurry oil steam generator with concentration on the thermal stresses at the tubesheet with or without a welding clad on the tubesheet surface. It is found that as having a larger heat expansion coefficient, thermal expansion of the welding clad is constrained and most areas are in compressive state. But the tensile stresses in the clad are also notable especially at the interface and could break the clad if added by the tensile stresses produced by pressure loadings. Presence of the welding clad causes significant tensile stresses in the base tubesheet. It is possible that the maximum tensile stress comprised by the thermal tensile stress and pressure induced tensile stress will exceed the tensile strength of the material and cause initiation of cracks in the tubesheet.

Effect of Peclet Number in Thermo-Mechanical Cracking Due to High-Speed Friction Load

The present paper discussed the critical depth, i.e., the depth at which the thermal tensile stress reaches a maximum, caused by the frictional excitation of a fast moving asperity. In the study, the critical depth was computed directly by maximizing the thermal tensile stress with respect to positions under the asperity inside the material. The relationship between critical depth and Peclet number for all materials in the two-dimensional formulation may be simplified to satisfy the exponential form R(ηcr)2.275=20.4368. Stellite III was chosen as the indicator material. Other parametric effects including mechanical properties and thermal properties were tested with materials having diverse property values. These tests confirmed that for the two-dimensional formulation, the Peclet number is the only one which dominates the critical depth.

Thermomechanical Cracking in the Vicinity of a Near-Surface Void Due to High-Speed Friction Load

This paper discusses the temperature distribution and the stress state in the vicinity of a near-surface rectangular cavity. They occur when the solid is subjected to the Coulomb frictional loading of an asperity moving at moderately high speed. The finite difference method is employed to calculate both the temperature and stress fields. The energy balance method is applied at the corners of the rectangular cavity to resolve the problem of singularities in the temperature field there. The stress singularity at each corner is represented by a special element that is introduced representing the behavior of the known stress singularity at the corner and its surrounding vicinity. Results show that the thermal stress effect dominates the stress field and eventually leads to failure. When a defect, such as a cavity, exists, the stress state that determines the failure phenomenon is more severe and can be quantified depending on the location of the cavity. These results were determined through a numerical computation based on the material properties of Stellite III. However, the parametric effect of material variations including changes in both thermal and mechanical properties were also considered. The study of the cavity location also established the existence of a critical cavity location. This location is defined by the critical ligament thickness (thickness between the wear surface and the top edge of the cavity), at which the cavity-influenced thermal tensile stress reaches a maximum. This thickness is important to designers when cavities at coating/substrate interface are either unavoidable or are too expensive to control in fabrication.