Parametric studies of failure mechanisms in thermal barrier coatings during thermal cycling using FEM

Thermal barrier coatings (TBCs) are widely used on different hot components of gas turbine engines such as blades and vanes. Although, several mechanisms for the failure of the TBCs have been suggested, it is largely accepted that the durability of these coatings is primarily determined by the residual stresses that are developed during the thermal cycling. In the present study, the residual stress build-up in an electron beam physical vapour deposition (EB-PVD) based TBCs on a coupon during thermal cycling has been studied by varying three parameters such as the cooling rate, TBC thickness and substrate thickness. A two-dimensional thermomechanical generalized plane strain finite element simulations have been performed for thousand cycles. It was observed that these variations change the stress profile significantly and the stress severity factor increases non-linearly. Overall, the predictions of the model agree with reported experimental results and help in predicting the failure mechanisms

An analytical model for explosive compaction of powder to cylindrical billets through axial detonation.

An analytical model, describing an explosive compaction process performed axially on a powder assembly of cylindrical geometry, is discussed. The powder is encapsulated in a cylindrical metal container surrounded by an explosive pad, which is detonated parallel to the major axis of the compact. The pressure generated in the powder is a function of the nature and the thickness of the explosive material as well as the powder characteristics. The model is based on the principle of shock propagation in powder aggregate and, the detonation as well as the refraction wave characteristics of the explosives. For the purpose of validation and illustration, this investigation considers the explosive compaction of aluminium powder particles for different explosive pad thicknesses. The model brings-out a closed-form solution for densification of powders. The density of the final powder compact depends on the pad thickness. Inadequate pad thickness leads to under compacted core, while higher pad thickness leads to melting at the core leading to over all low density. The optimum pad thickness of the explosive to produce the highest densification is thus determined using the model. The densification depends on the size of the powder particles also, since; the heat generated by the high pressure shock wave melts the surface of the powder particles depending on the specific heat, thermal conductivity and the latent heat of the powder material. The study essentially covers the effect of the explosive pad thickness and the particle size of the powder on densification. The analytical results are compared with a few experimental data and the comparison is found to be satisfactory

Review on 3D printing techniques for cutting tools with cooling channels

This review paper critically emphasizes the possibilities and recent developments for producing high-performance conventional cutting tools that are in demand in the traditional machining industry. These cutting tools are considered for use in traditional machining of materials that provide a high strength-to-weight ratio for various applications with high precision. This review focuses on the machinability of turning, milling, drilling, and other special tools manufactured using various additive manufacturing methods. The materials and methods used are also studied, enabling us to understand the vast requirements of tool materials and the additive manufacturing methods available for production. The critical suggestions discussed would establish a platform for the selection of printing methods and printing strategies to develop cutting tools with complex internal geometries

Prediction of Creep Curves of High Temperature Alloys using θ -Projection Concept

AbstractA computer program based on “MATLAB” was developed to predict creep curves of two high temperature alloys viz. a near α titanium alloy and Fe-Ni-Cr-Al alloy using θ-projection concept. Experimental creep curves generated at 600°C at different stress levels were used. This technique involves fitting of experimental creep curves according to eq εc=εt−ε0=θ1(1−e−θ2t)+θ3(eθ4t−1) to obtain θi parameters for different stress levels. Different θ-values obtained from fitting are then fitted according to eq logθi=ai+biσ+ciT+diσT and experimental strain to fracture values are fitted according to eq logεf=a+bσ+cT+dσT to obtain a set of twenty material constants (ai, bi, ci, dit), and εi), which are further used along with eq εc=εt=ε0=θ1(1−e−θ2t)+θ3(eθ4t−1) to determine four θ-values and fracture strain for each desired combination of stress and temperature in order to generate new creep strain curve. The predicted creep curves for both the alloys have been validated by generating experimental curves. Further, in order to simulate creep behaviour of components of complex geometry such as gas turbine blades and vanes this user defined material model i.e, θ-projection concept has been interfaced with ANSYS. Simulations have been carried-out and validation of the simulation results will be presented in this paper

Microtensile testing of a free-standing Pt-aluminide bond coat

A finite element method (FEM)-based study has been carried out for the design of flat microtensile samples to evaluate tensile properties of Pt-aluminide (PtAl) bond coats. The critical dimensions of the sample have been determined using a two-dimensional elastic stress analysis. In the present testing scheme, the ratio of the dimensions of the holding length to the fillet radius of the sample was found important to achieve failure within the gage length. The effect of gage length and grip head length also has been examined. The simulation predictions have been experimentally verified by conducting microtensile test of an actual PtAl bond coat at room temperature. The sample design and testing scheme suggested in this study have also been found suitable for evaluation of tensile properties at high temperature. (C) 2010 Elsevier Ltd. All rights reserved

Study of Brittle-to-ductile-transition in Pt-aluminide bond coat using micro-tensile testing method

The Brittle-to-ductile-transition-temperature (BDTT) of free-standing Pt-aluminide (PtAl) coating specimens, i.e. stand-alone coating specimens without any substrate, was determined by micro-tensile testing technique. The effect of Pt content, expressed in terms of the thickness of initial electro-deposited Pt layer, on the BDTT of the coating has been evaluated and an empirical correlation drawn. Increase in the electrodeposited Pt layer thickness from nil to 10 mu m was found to cause an increase in the BDTT of the coating by about 100 degrees C

Study of brittle-to-ductile-transition in Pt-aluminide bond coat using micro-tensile testing method

The Brittle-to-ductile-transition-temperature (BDTT) of free-standing Pt-aluminide (PtAl) coating specimens, i.e. stand-alone coating specimens without any substrate, was determined by micro-tensile testing technique. The effect of Pt content, expressed in terms of the thickness of initial electro-deposited Pt layer, on the BDTT of the coating has been evaluated and an empirical correlation drawn. Increase in the electrodeposited Pt layer thickness from nil to 10 μm was found to cause an increase in the BDTT of the coating by about 100°C

Implementing hardware Trojans: Experiences from a hardware Trojan challenge

Abstract—Hardware Trojans have become a growing concern in the design of secure integrated circuits. In this work, we present a set of novel hardware Trojans aimed at evading detection methods, designed as part of the CSAW Embedded System Challenge 2010. We introduced and implemented unique Trojans based on side-channel analysis that leak the secret key in the reference encryption algorithm. These side-channel-based Trojans do not impact the functionality of the design to minimize the possibility of detection. We have demonstrated the statistical analysis approach to attack such Trojans. Besides, we introduced Trojans that modify either the functional behavior or the electrical characteristics of the reference design. Novel techniques such as a Trojan draining the battery of a device do not have an immediate impact and hence avoid detection, but affect the long term reliability of the system. Keywords- Hardware Trojan, side-channel, hardware security I

Microtensile testing of a free-standing Pt-aluminide bond coat

A finite element method (FEM)-based study has been carried out for the design of flat microtensile samples to evaluate tensile properties of Pt-aluminide (PtAl) bond coats. The critical dimensions of the sample have been determined using a two-dimensional elastic stress analysis. In the present testing scheme, the ratio of the dimensions of the holding length to the fillet radius of the sample was found important to achieve failure within the gage length. The effect of gage length and grip head length also has been examined. The simulation predictions have been experimentally verified by conducting microtensile test of an actual PtAl bond coat at room temperature. The sample design and testing scheme suggested in this study have also been found suitable for evaluation of tensile properties at high temperature