Key Engineering Materials Vols. 525-526

Abstract: A 3D grain boundary formulation is presented for the analysis of polycrystalline microstructures. The formulation is expressed in terms of intergranular displacements and tractions, that play an important role in polycrystalline micromechanics, micro-damage and micro-cracking. The artificial morphology is generated by Hardcore Voronoi tessellation, which embodies the main statistical features of polycrystalline microstructures. Each crystal is modeled as an anisotropic elastic region and the integrity of the aggregate is restored by enforcing interface continuity and equilibrium between contiguous grains. The developed technique has been applied to the numerical homogenization of cubic polycrystals and the obtained results agree well with available data.

Abstract: PHE (Process Heat Exchanger) is a key component for transferring the high-temperature heat generated from a VHTR (Very High Temperature Reactor) to a chemical reaction for massive production of hydrogen. Recently, Korea Atomic Energy Research Institute (KAERI) has manufactured a medium-scale PHE prototype made of Hastelloy-X of high-temperature alloy and a performance test on the PHE prototype is scheduled in a small-scale nitrogen gas loop established at KAERI. In this study, in order to evaluate the high-temperature structural integrity of the PHE prototype under the steady-state and trip conditions of the gas loop before the performance test on the PHE prototype, elastic and elastic-plastic structural analyses on the PHE prototype were carried out and the analyses results were compared each other.

Abstract: The niobium film is prepared by magnetron sputtering on the surface of the AZ91D magnesium alloy. The morphology, phase structure, roughness, nanohardness and elastic modulus of the niobium films were studied by filed emission scanning electron microscope, X-ray diffraction, atomic force microscope and nanoindentation respectively. The influences of film deposition parameters, such as substrate temperature, negative bias and power on the properties of films were investigated. The corrosion resistance of niobium films on magnesium alloy was investigated by electrochemical system. Results show that the microstructure, phase structure, roughness, nanohardness and elastic modulus of the niobium films are determined by power, negative bias and substrate temperature. And the corrosion resistance of magnesium alloy improved obviously when coated with the niobium films.

Abstract: Micro-scale cantilever beam specimens have been created in air plasma sprayed thermal barrier coatings (APS-TBC) by focus-ion beam milling and tested in-situ using a force measurement technique. The elastic modulus, fracture toughness and the flexural strength of the specimens are calculated from the loading-deflection curve. In addition, the failure modes of the tested TBC are analysed.

Abstract: This paper presents a new fatigue crack growth prediction by using the dimensional reduction methods including the dual boundary element method (DBEM) and element-free Galerkin method (EFGM) for two dimensional elastostatic problems. One crack extension segment, i.e. a segment of arc, is introduced to model crack growth path. Based on the maximum principle stress criterion, this new prediction procedure ensures that the crack growth is smooth everywhere except the initial growth and the stress intensity factor of mode II is zero for each crack extension. It is found that the analyses of crack paths using coarse/large size of crack extension are in excellent agreement with analyses of the crack paths by the tangential method with very small increments of crack extension.

Abstract: The aim of this work is to investigate the potential of transient thermography in the nondestructive evaluation of structural defects of NECURON 1001 using the FLIR thermographic system. Necuron is a polyurethane material used for applications like: fixture and gauges, master and copy models, models with high mechanical stress, etc. Transient thermography is a thermographic method which implies the investigation of materials that are of a different (often higher) temperature than the ambient. The heat flow into the sample is altered in the presence of a subsurface defect or feature, creating a temperature contrast at the surface that is recorded by the infrared system. Results show that this method of evaluation can indicate, in necuron, defects of small sizes that can be overlooked in the manufacturing process. Also, it was shown that the transient thermography method presented can be an important tool in evaluating structural defects of materials.

Abstract: The main aim of the paper is an estimation of the macroscopic mechanical properties of particulate composites using numerical methods. Matrix of the considered composite was cross-linked polymethyl methacrylate - PMMA in a rubbery state, which exhibits hyperelastic behaviour. The three parameter Mooney Rivlin material model, which is based on the strain energy density function, was chosen for description of the matrix behaviour. Alumina based particles (Al₂O₃) were used as a filler. Numerical modelling based on the finite element method (FEM) was performed to determine stress-strain curve of the considered particulate composite. Representative volume element (RVE) model was chosen for FE analyses as a modelling approach of a composite microstructure. Various geometry arrangements of particles and various directions of loading have been considered and composite anisotropy has been investigated. A good agreement between numerical calculations with damage model and experimental data has been found and the described method may have a great potential for numerical modelling of composite behaviour and design of new particulate composite materials.

Abstract: Rigid low-density closed-cell polyisocyanurate (PIR) foams are used primarily as a thermal insulation material. Traditionally, they are manufactured from constituents produced by petrochemical industry. Introducing renewable materials in PIR formulation brings definite economical and environmental benefits. Fracture toughness of PIR foams obtained from renewable resources (with the polyol system comprising up to 80% of rapeseed oil esters) and petrochemical PIR foams has been characterized experimentally, by compact tension tests, for mode I crack propagation along the rise direction of the foams.

Abstract: Deformation process and strength of rock environment is significantly influenced by a presence of discontinuity planes. The paper deals with experimental modeling of underground structures in such a rock environment. Parametric analysis presents results from twin circular tunnels simulated in a scale model. The cases with and without zone of dislocations in between the tunnels were observed. Another variable factor studied was the distance between tunnels. The relation between model surface displacement and excavated length of the tunnel, which was monitored during the simulations, was used for mutual comparison of individual cases.