Numerical Investigation into the Effect of Splats and Pores on the Thermal Fracture of Air Plasma-Sprayed Thermal Barrier Coatings

The effect of splat interfaces on the fracture behavior of air plasma-sprayed thermal barrier coatings (APS-TBC) is analyzed using finite element modeling involving cohesive elements. A multiscale approach is adopted in which the explicitly resolved top coat microstructural features are embedded in a larger domain. Within the computational cell, splat interfaces are modeled as being located on a sinusoidal interface in combination with a random distribution of pores. Parametric studies are conducted for different splat interface waviness, spacing, pore volume fraction and fracture properties of the splat interface. The results are quantified in terms of crack nucleation temperature and total microcrack length. It is found that the amount of cracking in TBCs actually decreases with increased porosity up to a critical volume fraction. In contrast, the presence of splats is always detrimental to the TBC performance. This detrimental effect is reduced for the splat interfaces with high waviness and spacing compared to those with low waviness and spacing. The crack initiation temperature was found to be linearly dependent on the normal fracture properties of the splat interface. Insights derived from the numerical results aid in engineering the microstructure of practical TBC systems for improved resistance against thermal fracture

Dynamics of phase transformations in thermoelastic solids

The dynamical aspects of solid-solid phase transformations are studied within the framework of the theory of thermoelasticity. The main purpose is to analyze the role of temperature in the theory of phase transitions. This investigation consists of two parts: first, it is shown that by imposing a kinetic relation and a nucleation criterion it is possible to single out a unique solution to the Riemann problem for an adiabatic process. This extends to the thermomechanical case results previously found in a purely mechanical context. Secondly, based on an admissibility criterion for traveling wave solutions within the context of an augmented theory that includes viscosity, strain gradient and heat conduction effects, a special kinetic relation is derived using singular perturbation techniques

On the fracture of Al/NiTi composite manufactured by friction stir processing

Damage controlling involves a challenging issue in metal matrix composites due to the dominant failure at the interface between matrix and reinforcement agents. Shape memory alloys (SMAs) are considered good candidates to overcome this problem since they allow the creation of local compressive residual stresses that may delay fracture initiation. The present work investigates the fracture properties in friction stir processed Al1050/NiTip composite. Shape memory effect (SME) of the integrated NiTi particles was triggered by cold rolling and heat treatment to introduce local residual stresses. In-situ tensile tests were performed in a scanning electron microscope to study the fracture behaviors in both Al/NiTip and pure Al (also friction stir processed and cold rolled) samples. It is found that the Al/NiTip composite involves higher ultimate tensile strength than the pure Al. When looking at the crack propagation in the pre-cracked samples, we notice that the NiTi particles lead to fracture path deviation in the composite, conversely to the straight crack growth in the pure Al. Moreover, according to the fractographic analysis, a flat to slant fracture mode transition is found in Al/NiTip, in contrast to the fracture of pure Al in which only the so called “flip-flap” feature is observed. We propose that the strengthening effect comes from not only the shape memory effect of NiTi particles, but also some local stresses induced by the cold rolling in the heterogeneous Al/NiTip composite, as highlighted by finite element simulations

Analysis of banded morphology in multiphase steels based on a discrete dislocation–transformation model

The influence of the austenitic microstructural morphology on the mechanical response of a multiphase steel is analyzed by comparing two relevant configurations, namely (i) uniformly distributed grains of retained austenite embedded in a ferritic matrix and (ii) a banded morphology of the two phases. The analysis is carried out numerically using a discrete dislocation–transformation model that captures processes occurring at sub-grain length scales connected to nucleation and evolution of individual dislocations and martensitic platelets inside the austenitic grains. The simulations indicate that a microstructure composed of uniformly distributed grains of austenite is optimal in terms of strength since it delays the onset of plastic localization compared with banded microstructures.

Simulation of moving solid-phase boundaries using the partition of unity concept

International audienceIn this paper, numerical techniques are proposed to model phase transformation mechanisms. The evolution of interfaces is governed by field equations and jump conditions, and by a kinetic relation. Jumps in displacement gradients are incorporated by means of the partition of unity concept. The interface is described by a level set function.Dans cet article, de nouvelles approches numériques pour modéliser les mécanismes de transformations de phases sont présentées. L'évolution de l'interface est prise en compte dans l'écriture des équations de champs associées à une relation cinématique. La discontinuité du champ de déformation à l'interface est modélisée par l'intermédiaire de la méthode de la partition de l'unité. Pour la description de l'interface, l'approche fonction de niveau est mise œuvre

An enhanced curvature-constrained design method for manufacturable variable stiffness composite laminates

In this paper, design strategies are developed to explore better approaches of enforcing local layer-wise curvature constraints in the optimization of variable stiffness laminates in order to ensure the manufacturability of optimized designs based on the limitations of automated fiber placement. The methods developed here aim to improve an existing approach of imposing the curvature constraint directly on the fiber angles (i.e., direct control method) and are suitable for a design framework that uses lamination parameters as primary design variables. One approach developed here, termed the indirect control method, enforces the curvature constraint indirectly with better computational efficiency through the spatial gradient of the lamination parameters. It is shown that the curvature constraint on the actual fiber angles can also be satisfied with a sufficiently stringent upper bound albeit it produces overly conservative designs. Alternatively, an enhanced approach, termed the hybrid control method, is developed by combining the direct method and a relaxed version of the indirect control method. The case studies of minimum compliance design indicate that it provides the best manufacturable design among the three methods in the context of variable stiffness laminates using lamination parameters

Simulation of moving solid-phase boundaries using the partition of unity concept

International audienceIn this paper, numerical techniques are proposed to model phase transformation mechanisms. The evolution of interfaces is governed by field equations and jump conditions, and by a kinetic relation. Jumps in displacement gradients are incorporated by means of the partition of unity concept. The interface is described by a level set function.Dans cet article, de nouvelles approches numériques pour modéliser les mécanismes de transformations de phases sont présentées. L'évolution de l'interface est prise en compte dans l'écriture des équations de champs associées à une relation cinématique. La discontinuité du champ de déformation à l'interface est modélisée par l'intermédiaire de la méthode de la partition de l'unité. Pour la description de l'interface, l'approche fonction de niveau est mise œuvre

Fracture investigation in Al/NiTip composite manufactured by friction stir processing

Fracture strength involves one of the key parameters in the design of metal-based structures in automobile and aerospace applications. Over the past few years, increased attention has been paid on metal matrix composites (MMCs) because of the flexibility of tailoring their mechanical properties. However, controlling damage remains a challenging issue in these composites due to the prevailing failure at the interface between matrix and reinforcement agents [1, 2]. Shape memory alloys (SMAs) are considered good candidates to overcome this problem since they permit to introduce local compressive residual stresses that may delay fracture initiation. This concept, which has been studied numerically [3], requires further experimental validation. The present work investigates the fracture behavior in friction stir processed Al1050/NiTip composite. In order to trigger the shape memory effect (SME) of the integrated NiTi particles, cold rolling and heat treatment were consecutively performed following the composite manufacturing. In-situ tensile tests were performed in a scanning electron microscope to monitor the crack paths in both Al/NiTip and pure Al (also friction stir processed and cold rolled) samples. Fracture initiation and propagation were separately investigated in crack-free and pre-cracked samples. It is found that the Al/NiTip composite involves higher yield stress and ultimate tensile strength than the pure Al. The real time monitoring on the pre-cracked samples shows that the NiTi particles lead to fracture path deviation in the composite, while the crack growth is quite straight in the pure Al. Moreover, when looking at the fracture surface, a flat to slant transition is observed in Al/NiTip, in contrast to the fracture of pure Al in which only the so called “flip-flap” feature is involved. The strength improvement is found to be dependent of the volume fraction of NiTi particles. The largest increase of the fracture strength achieved in the present work is nearly 35%, which is much higher than that reported in previous works on Al/NiTip composites [4, 5]. The strengthening effect is proposed to stem from not only the shape memory effect of NiTi particles, but also some local stresses induced by the cold rolling in the heterogeneous Al/NiTip composite, according to finite element simulations