We propose a local reinforcement technique for lattices in the vicinity of a stress-raiser such as a notch, in order to elevate the macroscopic strength and ductility. A spatially non-uniform waviness distribution of sinusoidally-shaped struts is assumed in the vicinity of the notch, and the sensitivity of macroscopic tensile response to strut waviness distribution is studied by finite element analysis. Optimized lattice structures are determined in order to maximise the macroscopic tensile strength or ductility from these various strut waviness distributions. Both hexagonal and triangular lattices are studied as these geometries are representative of bending-dominated and stretching-dominated lattices, respectively.

Recent advances in additive manufacturing methods make it possible, for the first time, to manufacture complex micro-architectured solids that achieve desired stress versus strain responses. Here, we report experimental measurements and associated finite element (FE) calculations on the effect of strut shape upon the tensile response of two-dimensional (2D) lattices made from low-carbon steel sheets. Two lattice topologies are considered: (i) a stretching-dominated triangular lattice and (ii) a bending-dominated hexagonal lattice. It is found that strut waviness can enhance the ductility of each lattice, particularly for bending-dominated hexagonal lattices. Manufacturing imperfections such as undercuts have a small effect on the ductility of the lattices but can significantly reduce the ultimate tensile strength. FE simulations provide additional insight into these observations and are used to construct design maps to aid the design of lattices with specified strength and ductility.

An assessment is made of the J-integral test procedure for initial crack growth in an open-cell aluminium alloy foam by combining finite element (FE) simulations with experiment. It is found experimentally that a zone of randomly failed struts develops ahead of the primary crack tip, and is comparable in size to that of the plastic zone. Hence, a crack tip J-field is absent at the initiation of crack growth from the primary crack tip. This implies that the measured JIC value and the J versus crack extension Δa curve cannot be treated as material properties despite the fact that the specimen size meets the usual criteria for J validity. The toughness tests were performed on a single-edge notched bend specimen, and crack extension was measured by the direct current potential drop method, by digital image correlation and by X-ray computed tomography. The crack growth resistance of the foam is associated with two distinct zones of plastic dissipation: (i) a bulk plastic zone emanating from the crack tip (containing a cluster of randomly failed struts), and (ii) a crack bridging zone behind the advancing crack tip. The applicability of a cohesive zone model to predict the fracture response is explored for the observed case of large scale bridging. To do so, FE simulations are performed by replacing the discrete lattice of the open-cell metallic foam by a compressible, elastic-plastic hardening solid while the fracture process zone in the foam is represented by a cohesive zone, as characterised by a tensile traction versus separation law. A detailed comparison of the cohesive zone model with experimental observations reveals that it is possible to capture the load versus displacement response but not the details of the fracture process zone using a single set of process zone parameters.

Rapid prototyping is an emerging technology for the fast make of engineering components. A common technique is to laser cut a two-dimensional (2D) part from polymethyl methacrylate (PMMA) sheet. However, both manufacturing defects and design defects (such as stress raisers) exist in the part, and these degrade its strength. In the present study, a combination of experiment and finite element analysis is used to determine the sensitivity of the tensile strength of PMMA hexagonal lattices to both as-manufactured and as-designed defects. The as-manufactured defects include variations in strut thickness and in Plateau border radius. The knockdown in lattice tensile strength is measured for lattice relative density in the range of 0.07 to 0.19. A systematic finite element (FE) study is performed to assess the explicit role of each type of as-manufactured defect on the lattice strength. As-designed defects such as randomly perturbed joints, missing cells, and solid inclusions are introduced within a regular hexagonal lattice. The notion of a transition flaw size is used to quantify the sensitivity of lattice strength to defect size.

Additive manufacture and rapid prototyping are versatile methods for the generation of lattice materials for applications in the creep regime. However, these techniques introduce defects that can degrade the macro-scopic creep strength. In the present study, the uniaxial tensile response of two-dimensional PMMA lattices is measured in the visco-plastic regime: tests are performed at 100C which is slightly below the glass transition temperature T g of PMMA. Both as-manufactured defects (Plateau borders and strut thickness variation) and as-designed defects (missing cell walls, solid inclusions, and randomly perturbed joints) are introduced. The dispersion in macroscopic strength is measured for relative densities in the range of 0.07 to 0.19. It is observed that initial failure of the lattice is diffuse in nature: struts fail at a number of uncorrelated locations, followed by the development of a single macroscopic crack transverse to the loading direction. In contrast, the same PMMA lattice fails in a correlated, brittle manner at room temperature. An FE study is performed to gain insight into the diffuse failure mode and the role played by as-manufactured defects, including the dispersion in tensile strength of individual struts of the lattice. A high damage tolerance to as-designed defects is observed experimentally: there is negligible knock-down in strength due to the removal of cell walls or to the presence of solid inclusions. These findings aid the design and manufacture of damage tolerant lattices in the creep regime.

To understand the stress evolution and failure mechanisms of thermal barrier coatings (TBCs), finite element simulations are an invaluable tool. Simulations are especially useful to unwrap complex interactions of different phenomena at high temperature, including creep, sintering, diffusion, and oxidation. However, the correct setup and evaluation of a finite element model for this problem are difficult. This article reviews critical issues in modelling TBC systems. Some of the most important aspects are as follows: (a) stresses in 3D simulations may differ considerably from 2D models; (b) the interface shape strongly affects the stresses and using an idealized geometry may underestimate stresses; (c) crack propagation requires simulating sufficiently large regions to correctly capture stress redistribution; (d) a correct description of the material behaviour (visco-plasticity, TGO growth, sintering) is crucial in determining the stress state. The article discusses these and other issues in detail and provides guidelines on the choice of model parameters, boundary conditions, etc. The paper also points out open questions in modelling TBC systems and discusses aspects of verification and validation.

Inconel 617 (IN-617) mainly contains nickel (Ni), chromium (Cr), cobalt (Co) and molybdenum (Mo). IN-617 is widely used in applications that require high temperature operation due to its high temperature stability and strength as well as its strong resistance to oxidation and carburization. The current work focuses on the measurement of temperature dependent mechanical properties of IN-617 from room temperature (around 25 degrees C) up to 800 degrees C. The properties measured are reduced modulus, elastic modulus, hardness, indentation creep rate, indentation creep exponent, and thermal activation volume. The indentation size effect is analyzed as a function of temperature. Using a combination of optical microscopy and scanning electron microscopy (SEM) imaging, the effect of precipitate distribution and oxidation on the measured properties is found to be negligible beyond a critical indentation depth. The mean hardness value ranged from 3.1 GPa at room temperature to 1.6 GPa at 800 degrees C. A relation between indentation depth and hardness as a function of temperature change was used to extract strain gradient plasticity associated length scales with values changing from 1.0 mu m at room temperature to 1.8 mu m at 400 degrees C and to 1.6 mu m at 800 degrees C. (C) 2017 Elsevier Ltd. All rights reserved.

A first order visco-plastic strain gradient constitutive model and a cohesive zone formulation are embedded within a modified boundary layer (MBL) model for the analysis of crack tip fields and crack growth. The MBL model is loaded using a mode I asymptotic crack tip solution, with the boundary displacement calculated from the stress intensity factor. The influence of the rate-dependent constitutive parameters and the intrinsic material length on fracture relevant quantities are investigated in parametric study. Two scenarios are considered: (1) a stationary crack under constant loading and (2) a crack advance under monotonic loading. Finite element model analyzes are performed. For stationary cracks it was found that the effects of the intrinsic lengthscale of the strain gradient plasticity model are more prominent for large visco-plastic power exponents, and increase with hold time. The results of computations of crack advances under monotonic loading suggest that plastic strain gradients reduce crack growth resistance and crack initiation toughness, especially for a large visco-plastic power exponent. For small values of intrinsic material length the dependence of the initiation toughness and tearing modulus on the intrinsic length is strong, but then saturates for large values of intrinsic material length. Loading rate effects are found to be more pronounced for cases with a small value of the intrinsic lengthscale.

In the present work the failure mechanisms of thermal barrier coating systems are examined. These layer systems are used on highly stressed components in gas turbines to protect the substrate materials. The failure mechanisms of the thermal barrier coating systems could not be fully elucidated due to the complex interactions of different phenomena (e.g. creep, sintering, differential thermal expansion, diffusion and oxidation processes). For this reason, a simplified model system is presented, which offers the possibility to investigate two essential aspects of layer failure, the influence of the creep properties and the interface roughness. In the present work finite element simulations of the model system are varied on the micro and mesoscale according to experimental test series. The crack initiation and the crack propagation are analyzed, from which a failure scenario of the model system is derived

Thermal barrier coating systems are well-established on top of present gas turbine components. They consist of different coating layers: the bond-coat (BC), the thermal barrier coating (TBC), and the thermally grown oxide (TGO), which is formed during operation. The complex failure mechanisms of the coating systems are not fully understood, yet. Therefore, a simplified model system is analysed to focus on the influence of bond-coat creep and segmentation cracks on the formation and propagation of delaminations.Two finite element (FE) models are discussed in this paper and compared to experimental results: (i) a micro-scale model describes the formation of initial delaminations and (ii) a segmentation crack model analyses the interaction between delaminations and segmentation cracks on the meso-scale.It was found that initial delaminations develop mainly in the valley of asperities at the TGO/TBC interface, both in fast and slow creeping bond-coat materials. Segmentation cracks can be found in the coating system from the first thermal cycle. They promote the development of delaminations between two segmentation cracks. The propagation of delaminations is not stopped in the TBC and the delaminations grow together with the segmentation cracks. In conclusion, a possible scenario of TBC failure in the analysed model system is presented. (C) 2013 Elsevier B. V. All rights reserved.

The lifetime of thermal barrier coating systems is limited by cracks close to the interfaces, causing delamination. To study the failure mechanisms, a simplified model system is analysed which consists of a bond-coat bulk material, a thermally grown oxide, and an yttria-stabilised zirconia topcoat. The stresses in the model system are calculated using a finite element model which covers the simulation of full thermal cycles, creep in all layers, and the anisotropic oxidation during dwelling. Creep in the oxide and the thermal barrier coating is varied with the use of different creep parameter sets. The influence of creep in the bond-coat is analysed by using two different bond-coat materials: fast creeping Fecralloy and slow creeping oxide dispersion strengthened MA956. It is shown that creep in the bond-coat influences the lifetime of the coatings. Furthermore, a fast creeping thermally grown oxide benefits the lifetime of the coating system.

A typical thermal barrier coating system consist of the bond-coat (BC) the thermal barrier coating (TBC) and a thermally grown oxide (TGO) between the bond-coat and TBC. A simplified coating system is introduced and simulated which consists of a MCrAlY bond-coat material as the substrate, an TGO, and a TBC on top. The influence of the nickel-based substrate can be neglected, which reduces the influencing parameters. The failure mechanism can be analyzed by varying the creep properties of the bond-coat material (fast creeping Fecralloy and slow creeping ODS strengthened MA956). The influence of the interfacial roughness can be examined varying the amplitude and wavelength of the interface. It is shown that a fast creeping bond-coat benefits the lifetime of the coating system. Different FEM simulations of the coatings support this assumption.

Abstract Thermal barrier coating systems are used to enhance the temperature resistance of hot section components in gas turbines. The coatings protect the underlying nickel based components and consist of the bond coat (BC) the thermal barrier coating (TBC) and a thermally grown oxide (TGO) between the BC and TBC. The coating systems fail in service at or near the TBC/TGO interface. To study the failure mechanisms a simplified coating system is introduced which consists of a MCrAlY bond-coat material as the substrate, a TGO, and a yttria-stabilised zirconia TBC as a topcoat. The TBC is applied by atmospheric plasma spraying on top of specimens with defined roughness profiles, manufactured by a micromachining process. The main advantage of micro-machining is a defined interfacial roughness between the TBC and the BC in contrast to sandblasted specimens. Furthermore, a FEM simulation of the coating system was developed which approximates the interface by sinusoidal functions. This simplified model system and additional FEM calculations show the influence of varying the interfacial roughness between the BC and the TBC.

Abstract Presently, highly stressed components in gas turbines are mainly made of single crystal nickel based alloys and the maximum application temperature (without coatings) is typically limited to 1100°C. Superalloys are now reaching limits posed by their melting temperatures. Increasing the substrate temperature beyond 1200°C will increase the efficiency of the turbine significantly. A new generation high temperature Co-Re alloys are aimed for use at +100°C above present single crystal nickel-superalloys. The substrates will be protected against the higher gas temperatures by thermal barrier coatings. For Co-Re alloy substrates CoReCrSi is a promising bond-coat material. CoReCrSi is thermo-chemically compatible to Co-Re due to the very similar mechanical and chemical properties. The oxide formation and the adhesion of the top coat are being investigated by studying a simplified coating system. The coating system consists of a CoReCrSi bond coat bulk material, and an yttria-stabilised zirconia top coat. The system was tested under cyclic conditions at 1200°C. This study provides a first insight into the TGO growth, the basic failure mechanism of the top coat, and the diffusion processes at the top coat/bond coat interface. It is shown that CoReCrSi with 2 at.% silicon promotes a good adhesion of the top coat by forming a dense chromium oxide layer. The critical TGO thickness beyond which the TGO fails by spallation was determined to be 25 microns and is roughly 2.5 times the critical thickness in MCrAlY based system in nickel-alloys.

Plasma sprayed thermal barrier coating systems are used on top of highly stressed components e.g. on gas turbine blades to protect the underlying substrate from the high surrounding temperatures. A typical coating system consists of the bond-coat (BC the thermal barrier coating (TBC. The thermally grown oxide (TGObetween the BC and the TBC develops in service as a third layer which is caused by the diffusion of oxygen through the TBC. To study the behaviour of the complex failure mechanisms in thermal barrier coatings, a simplified model system is used to reduce the number of the system parameters. The artificial system consists of a bond-coat material (fast creeping Fecralloy or slow creeping MA956as the substrate with a Y2O3 partially stabilised plasma sprayed zirconium dioxide TBC on top and a TGO between the two layers. Alongside the experimental studies a FEM model was developed to calculate the stress distribution inside the system [1]. The simulation permits the identification of compression and tension areas which are established by the growth of the oxide layer and the stresses which occur during the heating and cooling processes. Furthermore, a 2-dimensional finite element model of crack propagation in the model system was developed in which the crack direction is calculated by using short test cracks in different directions. The direction of the crack in the coating system is defined as the crack direction with the maximum energy release rate [2, 3]. The simulated stress distributions and the obtained crack path provide an insight into the possible failure mechanisms in the coating and allow to draw conclusions for optimising real thermal barrier coating systems.

Plasma sprayed thermal barrier coating systems are used on top of highly stressed components, e. g. on gas turbine blades, to protect the underlying substrate from the high surrounding temperatures. A typical coating system consists of the bond-coat (BC), the thermal barrier coating (TBC), and the thermally grown oxide (TGO) between the BC and the TBC. This study examines the failure mechanisms which are caused by the diffusion of oxygen through the TBC and the resulting growth of the TGO.To study the behaviour of the complex failure mechanisms in thermal barrier coatings, a simplified model system is used to reduce the number of system parameters. The model system consists of a bond-coat material (fast creeping Fecralloy or slow creeping MA956) as the substrate with a Y2O3 partially stabilised plasma sprayed zircon oxide TBC on top and a TGO between the two layers. Alongside the experimental studies a FEM simulation was developed to calculate the stress distribution inside the simplified coating system [1]. The simulation permits the identification of compression and tension areas which are established by the growth of the oxide layer. Furthermore a 2-dimensional finite element model of crack propagation was developed in which the crack direction is calculated by using short trial cracks in different directions. The direction of the crack in the model system is defined as the crack direction with the maximum energy release rate [2, 3].The simulated stress distributions and the obtained crack path provide an insight into the possible failure mechanisms in the coating and allow to draw conclusions for optimising real thermal barrier coating systems. The simulated growth stresses of the TGO show that a slow creeping BC may reduce lifetime. This is caused by stress concentration and cracks under the TGO. A slow creeping BC on the other hand reduces the stresses in the TBC.The different failure mechanisms emphasise the existence of a lifetime optimum which depends on the creep properties of the used bond-coat material. Experimental results show a good agreement with the predicted failure mechanisms.

In order to study the behavior of the complex failure mechanisms in thermal barrier coatings on turbine blades, a simplified model system is used to reduce the number of system parameters. The artificial system consists of a bond-coat material (fast creeping Fecralloy or slow creeping MA956) as the substrate with a Y2O3 partially stabilized plasma sprayed zircon oxide TBC on top and a TGO between the two layers. A 2-dimensional FEM simulation was developed to calculate the growth stress inside the simplified coating system. The simulation permits the study of failure mechanisms by identifying compression and tension areas which are established by the growth of the oxide layer. This provides an insight into the possible crack paths in the coating and it allows to draw conclusions for optimizing real thermal barrier coating systems.

AbstractTo study the behaviour of the complex failure mechanisms in thermal barrier coatings, a simplified model system is used to reduce the number of system parameters. The artificial system consists of a bond‐coat material (fast creeping Fecralloy or slow creeping MA956) as the substrate with a Y2O3 partially stabilized plasma sprayed zircon oxide TBC on top and a TGO between the two layers. A FEM simulation was developed to calculate the stress distribution inside the coating system. The simulation permits the identification of compression and tension areas which are established by the growth of the oxide layer and the stresses which occur during the heating and cooling processes. Full thermal cycles, including heating from RT to 1000°C, dwelling for 2 hours and cooling to RT, were simulated.The stress distributions provide an insight into the possible failure mechanisms in the coating and allow to draw conclusions for optimizing real thermal barrier coating systems. It is shown that the failure mechanism depends on the creep properties of the bond‐coat. Therefore, optimizing the creep properties may adjust lifetime of the whole coating system. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)