Homogenization of heat diffusion in a cracked medium

We develop in this note a homogenization method to tackle the problem of a diffusion process through a cracked medium. We show that the cracked surface of the domain induces a source term in the homogenized equation. We assume that the cracks are orthogonal to the surface of the material, where an incoming heat flux is applied. The cracks are supposed to be of depth 1, of small width, and periodically arranged.Comment: 28 pages, 10 figure

Development of ductile cladding for TD-nickel turbine vane applications Final report, 5 Jul. 1967 - 31 Dec. 1968

Slurry technique for applying NI-CR-AL-SI coatings to TD-N

Dual microcapsule system for autonomous self-healing coatings

Polymer coatings are vulnerable to external and internal damage. Formation of microcracks can occur because of an impact event or through oscillatory stresses such as thermal expansion and contraction. Self-healing offers a solution to respond to internal damage and repair the polymeric structure. This work utilizes a dual microcapsule system as the autonomous self-healing mechanism for use in an epoxy coating. The system is comprised of an epoxy resin microcapsule and an amine adduct capsule embedded in an epoxy matrix. Encapsulation of the epoxy resin was achieved, however encapsulation of the amine adduct is very challenging, and was the main focus of this work. The amine adduct has been successfully encapsulated, resulting in a satisfactory microcapsule payload and size, but reproducibility has proven to be difficult. Though reproducibility is an issue, the adhesive properties of amine adduct and epoxy capsules have been successful by adhering two pieces of epoxy together

Models for Metal Hydride Particle Shape, Packing, and Heat Transfer

A multiphysics modeling approach for heat conduction in metal hydride powders is presented, including particle shape distribution, size distribution, granular packing structure, and effective thermal conductivity. A statistical geometric model is presented that replicates features of particle size and shape distributions observed experimentally that result from cyclic hydride decreptitation. The quasi-static dense packing of a sample set of these particles is simulated via energy-based structural optimization methods. These particles jam (i.e., solidify) at a density (solid volume fraction) of 0.665+/-0.015 - higher than prior experimental estimates. Effective thermal conductivity of the jammed system is simulated and found to follow the behavior predicted by granular effective medium theory. Finally, a theory is presented that links the properties of bi-porous cohesive powders to the present systems based on recent experimental observations of jammed packings of fine powder. This theory produces quantitative experimental agreement with metal hydride powders of various compositions.Comment: 12 pages, 12 figures, 2 table

Literature study report of plasticity induced anisotropic damage modeling for forming processes

A literature study report covering the topics; micromechanics of damage, continuum damage mechanics (gurson model and effective variable concept) and the dependence of damage on strain rate and temperature

SOLID-SHELL FINITE ELEMENT MODELS FOR EXPLICIT SIMULATIONS OF CRACK PROPAGATION IN THIN STRUCTURES

Crack propagation in thin shell structures due to cutting is conveniently simulated using explicit finite element approaches, in view of the high nonlinearity of the problem. Solidshell elements are usually preferred for the discretization in the presence of complex material behavior and degradation phenomena such as delamination, since they allow for a correct representation of the thickness geometry. However, in solid-shell elements the small thickness leads to a very high maximum eigenfrequency, which imply very small stable time-steps. A new selective mass scaling technique is proposed to increase the time-step size without affecting accuracy. New ”directional” cohesive interface elements are used in conjunction with selective mass scaling to account for the interaction with a sharp blade in cutting processes of thin ductile shells

Sol–gel thermal barrier coatings: Optimization of the manufacturing route and durability under cyclic oxidation

A new promising and versatile process based on the sol–gel transformation has been developed to deposit yttria-stabilised thermal barrier coatings. The non-oriented microstructure with randomly structured pore network, resulting from the soft chemical process, is expected to show satisfactory thermo-mechanical behaviour when the TBC is cyclically oxidized. First stage of the research consists of optimizing the processing route to generate homogeneous microstructure and controlled surface roughness. The objective is to reduce, as much as possible, the size and depth of the surface cracks network inherent to the process. Indeed, the durability of the TBC when cyclically oxidized strongly depends on the sharpness of those cracks that concentrate thermo-mechanical stresses and generate detrimental propagation resulting in spallation. Cyclic oxidation tests are performed using a cyclic oxidation rig instrumented with CCD cameras to monitor in a real time basis the mechanism of crack propagation and spallation. The impact of various parameters either directly related to the processing route, e.g. the intimate microstructure of the TBC and the TBC thickness, or to the thermal loading, e.g. the oxidation temperature and the cumulated hot time, on the durability of the TBC is investigate

Downscaling of fracture energy during brittle creep experiments

We present mode 1 brittle creep fracture experiments along fracture surfaces that contain strength heterogeneities. Our observations provide a link between smooth macroscopic time-dependent failure and intermittent microscopic stress-dependent processes. We find the large-scale response of slow-propagating subcritical cracks to be well described by an Arrhenius law that relates the fracture speed to the energy release rate. At the microscopic scale, high-resolution optical imaging of the transparent material used (PMMA) allows detailed description of the fracture front. This reveals a local competition between subcritical and critical propagation (pseudo stick-slip front advances) independently of loading rates. Moreover, we show that the local geometry of the crack front is self-affine and the local crack front velocity is power law distributed. We estimate the local fracture energy distribution by combining high-resolution measurements of the crack front geometry and an elastic line fracture model. We show that the average local fracture energy is significantly larger than the value derived from a macroscopic energy balance. This suggests that homogenization of the fracture energy is not straightforward and should be taken cautiously. Finally, we discuss the implications of our results in the context of fault mechanics