Ki-Ju Kang

A method for optimal material selection aided with decision making theory

Abstract As a CAE (Computer Aided Engineering) tool to help design engineers, the procedure of material selection should be objective so as to minimize personal bias and it should be able to be coded into the software of ‘expert system’. In this work, we have utilized the theories of decision-making. One of them is the concept of entropy; to evaluate the weight factor for each material property or performance index, and the other is TOPIS; to rank the candidate materials, for which several requirements are considered simultaneously. As an example, the procedure to select the optimal material for a flywheel has been developed. We can grade the candidate materials for given subjective weight. In the cases for which fatigue strength or fracture toughness is weighted, the chosen materials coincide with the ones utilized for a commercial flywheel in support of our approach.

A New Type of Low Density Material: Shellular

A new type of cellular material named Shellular, in which cells are composed of a continuous, smooth-curved shell according to the minimal surface theory, is proposed. Shellular specimens are fabricated using 3D lithography with negative templates and hard coating, and exhibit superb strength and stiffness at densities lower than 10(-2) Mg m(-3), incorporating benefits from hierarchical structures and constituent materials with nanosized grains.

Effect of Geometry and Materials on Residual Stress Measurement in Thin Films by Using the Focused Ion Beam

Recently, a new method for residual stress measurement in thin films by using the focused ion beam (FIB) has been proposed by the authors. It is based on the combined capability of the FIB imaging system and of high-resolution strain mapping software (VIC-2D). A simple equation based on two-dimensional elasticity is used to evaluate the residual stress from the displacements due to introducing a slot. The slot length is assumed to be much larger than the slot width or depth. And the effect of the slot width was neglected. However, it is often hard, depending on film materials, to introduce a narrow and deep slot by FIB. In this work some practical issues regarding the slot geometry are addressed. Through two- and three-dimensional finite element analyses, it is explored how the slot length, width and measurement location affect the displacements which are the basic data for residual stress evaluation. As a result, the validity and limit of the equations based on two-dimensional elasticity are evaluated. Also, the effect of material dissimilarity between film and substrate is explored. Finally, examples for a diamond-like carbon film on glass substrate and an aluminum oxide film thermally grown upon an alloy are presented.

Overview no. 112: The cyclic properties of engineering materials

Abstract The basic fatigue properties of materials (endurance limit, fatigue threshold and Paris law constants) are surveyed, inter-related and compared with static properties such as yield strength and modulus. The properties are presented in the form of Material Property Charts. The charts identify fundamental relationships between properties and, when combined with performance indices (which capture the performance-limiting grouping of material properties) provide a systematic basis for the optimal selection of materials in fatigue-limited design.

Plastic zone size near the crack tip in a constrained ductile layer under mixed mode loading

Abstract Two models are derived to predict the plastic zone size near a crack in a ductile layer sandwiched by two rigid substrates when the size is larger than the layer thickness. To examine the validity, the results are compared with the one’s of finite element analysis. As a result, it is found that for the mode mixity of φ

Effects of mode mix upon fracture behavior of a solder joint

Results of fracture experiments of brass/solder/brass sandwich CTS (Compact Tension-Shear) specimens are presented together with observations of the crack propagation behavior and the fractographs. The fracture behaviors of the interface crack are analyzed by the finite element method with a modified boundary layer formulation. Several fracture mechanisms and the corresponding criteria are examined. And the crack growth behavior and fracture toughness are predicted. As the results various crack growth procedures such as the crack jump to another interface on the opposite side, the nucleation of a new crack far from the initial crack front, and the asymmetric relation of fracture toughness versus mode mix J∞c−φ can be successfully explained. The fractographs, the crack growth behaviors, and stress-strain distribution along the interface are inter-related.

Criteria for kinking out of interface crack

Abstract Three kinds of criteria for kinking out of the interface crack between dissimilar materials are discussed. Their validity and applicability are examined by comparison with the experimental results from two types of specimens. As the results G max criterion based on energy release rate shows best prediction on both the kink initiation and the kink angle while σ 00 max criterion based on hoop stress around the crack tip and zero K II criterion based on stress intensity factor shows limited success.

Measurement of the strains induced upon thermal oxidation of an alumina-forming alloy

Abstract The mechanisms that govern the formation of a thermally grown alumina on Al-containing super-alloys has been the subject of much recent speculation, because of the key influence of this oxide on the durability of thermal barrier systems for gas turbines. Among the important issues to be resolved are the location of the new oxide growth (interface, surface or internal grain boundary), the strains induced by the growth on non-planar surfaces and the associated stresses. To address some of the uncertainties, an experimental probe is designed. The experiments are based on the curvature changes, Δ κ , that occur on curved thin foils, caused by differences in strain accommodation on convex and concave surfaces. Experiments performed on thin FeCrAlY foils provide a vivid illustration of the strains induced upon growth, demonstrating a large increase in the radius of curvature upon oxidation. The sign and magnitude of the changes affirm that most of the new oxide forms at the interface.

Experimental Studies on Friction Factor and Heat Transfer Characteristics Through Wire-Woven Bulk Kagome Structure

Periodic cellular metals with open, periodic cell topologies have received much attention owing to their potential for multi-functionality such as load bearing, thermal dissipation, and actuation. Recently, a new technique, known as wire-woven bulk Kagome, has been introduced, which is used for fabricating multi-layered Kagome with truss periodic cellular metal. The fabrication of the wire-woven bulk Kagome is based on a concept where continuous helical wires are systematically assembled in six directions. Besides its excellent load-bearing capability with light weight, the wire-woven bulk Kagome has potential for a heat dissipation media because of the high ratio of surface area to volume and low flow resistance. This article presents the experimental results of the fluid flow and heat transfer characteristics of the multi-layered wire-woven bulk Kagome composed of aluminum 1100 helix wires. Under forced-air convection conditions, the friction factor and heat transfer rate of the wire-woven bulk Kagome specimen were investigated for two specimen orientations. The results showed that the friction factor of the wire-woven bulk Kagome was mainly affected by the flow blockage area, and the heat transfer characteristics depended on the open-area ratio. In addition, the results were compared with other heat dissipation media (e.g., open-celled foams, woven screens, lattice-frame materials, and cast Kagome structures). The results showed that the heat transfer performance of the multi-layered aluminum wire-woven bulk Kagome competed favorably with the best heat dissipation media currently available.

Measurement of residual stresses in a circular ring using the successive cracking method

A method which the authors describe as the successive cracking method for measuring residual stresses in a circular ring is presented. In this method, the residual stresses are evaluated using a fracture mechanics approach. The strains measured at a point on the outer edge of the ring as a crack is introduced and extended from the edge are used to deduce the residual stress distribution in the uncracked ring. Finite element analysis is carried out to examine the validity of the theoretical derivation. Experiments to measure the residual stresses in a steel ring specimen are done by the successive cracking method. For comparison purposes, the experimental results using the sectioning method are presented as well. The successive cracking method is shown to be valid, simple, and effective for measuring the two-dimensional residual stress distribution in an axisymmetric member.