Kenji Oguni

Tensor field tomography based on 3D photoelasticity

A new method for non-destructive measurement of arbitrary 3D stress state using photoelasticity has been developed. The new approach, namely, load incremental approach, is an attempt to solve the non-linear inverse problem of 3D photoelasticity by considering change in stress state for a load increment and linearizing the non-linear governing equation. As long as the applied load increment is small and photoelastic images are taken with high resolution in load increment, this method works and can reconstruct arbitrary 3D stress state. Numerical simulations clarify the effect of (i) the material sensitivity, (ii) the number of the observations and (iii) the constraint on the observation directions on the performance of the proposed method. These results provide the fundamental data for designing the experimental setup for 3D photoelasticity.

Analysis on evolution pattern of periodically distributed defects

A similar pattern is formed in various materials, when periodically distributed defects evolve. Mathematically, this pattern formation is understood as the consequence of symmetry breaking, while physically it is caused by interaction effect which vary depending on materials or defects. In examining the nature of the interaction effects, this paper analyzes the bifurcation induced growth of a periodic array of defects. With the aid of group-theoretic bifurcation analysis, it is clearly shown that when the uniform pattern (the evolution of all defects) is broken, only the alternate pattern (the evolution of every second defect) can take place for smaller defects, as often observed in nature. Therefore, two defects should be considered to examine a possible bifurcation of periodic defects. Furthermore, the conclusion obtained can be extended to explain the phenomena whereby every second, fourth, and then eighth defect continue to evolve, and whereby alternate bifurcation is repeated successively until the evolution is localized.

3D dynamic simulation of crack propagation in extracorporeal shock wave lithotripsy

Some experimental observations of Shock Wave Lithotripsy(SWL), which include 3D dynamic crack propagation, are simulated with the aim of reproducing fragmentation of kidney stones with SWL. Extracorporeal shock wave lithotripsy (ESWL) is the fragmentation of kidney stones by focusing an ultrasonic pressure pulse onto the stones. 3D models with fine discretization are used to accurately capture the high amplitude shear shock waves. For solving the resulting large scale dynamic crack propagation problem, PDS-FEM is used; it provides numerically efficient failure treatments. With a distributed memory parallel code of PDS-FEM, experimentally observed 3D photoelastic images of transient stress waves and crack patterns in cylindrical samples are successfully reproduced. The numerical crack patterns are in good agreement with the experimental ones, quantitatively. The results shows that the high amplitude shear waves induced in solid, by the lithotriptor generated shock wave, play a dominant role in stone fragmentation.

Image analysis of measuring building configuration for seismic damage estimation

A novel methodology of image analysis was designed to assess more quantitative seismic damage estimation at the building level for a city area in which 10-30% of the entire building stock was damaged. The methodology is capable of accurate configuration determination, which forms the kernel part toward seismic damage estimation based on displacement detection. This provides the following advantages. (1) A narrow stripe is extracted around an approximate building configuration in a virtual image generated based on high-resolution geographical information system (GIS) data. Analysis of building edges incorporated by the narrow stripe reduces computation cost and helps better distinguish building edges from background edges, such as trees, shadows, and windows. (2) An edge detection technique combined with a cumulative Gaussian distribution method and a gradient descent searching algorithm is validated. It offers subpixel location accuracy of a configured structure in a two-dimensional (2D) digital image. (3) Given the approximate position, orientation, resolution, view angle, screen distance, and upward pointing vector in the camera coordinate system, a three-dimensional (3D) wireframe city can be presented pre- and postearthquake.

Crack propagation imaging by the ISIS camera and a video trigger system

An ultra-high speed camera of 1Mfps was applied to visualize the crack propagation. Change of stress field around the propagating crack tip was captured as a change of the fringe pattern by means of the photo-elastic imaging technique. Newly developed video trigger system is employed to detect the occurrence of the crack propagation as a trigger in the experiment. The trigger successfully perceived the initiation of the crack propagation stably. Also its response time was fast enough even for the image capturing with 1Mfps. As a result, it is revealed that the elastic wave, propagating in the continuous body, has a significant effect on the velocity and kinking behavior of the propagating crack.

Inverse Analysis Method for Photoelastic Measurement of 3D Stress State

A new nondestructive method for identifying boundary conditions applied on a 3D linear elastic body is developed based on the load incremental approach, which linearizes the nonlinear governing equation of photoelasticity by considering small increments in applied load. Direct stress identification based on load incremental approach is highly sensitive to measurement errors and involves considerable amount of computations. On the other hand, identification of boundary conditions based on load incremental approach and thereby the state of stress is not only less sensitive to measurement errors but also involves less computation. This boundary conditions identification can be considered as an introduction of equilibrium condition and the property of linear elasticity to overcome the shortcomings of direct stress identification.

INTEGRATED SIMULATION FOR EARTHQUAKE HAZARD AND DISASTER PREDICTION

This paper presents the integrated simulation for earthquake hazard and disaster prediction. The earthquake hazard simulation takes advantage of the macro-micro analysis method which estimates strong ground motion with high spatial and temporal resolution. The earthquake disaster simulation calculates seismic responses for all structures in a target area by inputting synthesized strong ground motion to a structure analysis method which is plugged into the system; a suitable analysis method, linear or non-linear, is chosen depending on the type of the structure. The results of all simulations are visualized so that government officials and residents can share common recognition of possible earthquake hazard and disaster. Two examples of this integrated earthquake simulations are presented; one is made by plugging nonlinear structure analysis methods into the system, and the other is made for an actual city, the computer model of which is constructed with the help of available geographical information systems.

Bifurcating initiation and evolution of periodic micro-defects

Abstract The following two phenomena observed in torsional shear experiments are strudied in this paper: (1) micro-defects are periodically initiated under uniform loading; and (2) the alternative evolution of periodic micro-defects occurs when they uniformly grow to some extent. Micro-defects are modeled as concentrated strain and suitable boundary value problems are posed for these initiation and evolution processes. With the aid of the equivalent inclusion method, the existence of bifurcated solutions is presented in a transparent manner, and the stability of these solutions are examined. It is shown that such unstable bifurcated solutions correspond to the phenomena observed in the experiments. Variational problems corresponding to the boundary value problems are formulated and the bifurcation and stability analyses used for the boundary value problems are presented in a more general manner.

Inverse Analysis Method for Identification of Local Elastic Properties by Using Displacement Data

ABSTRACT This paper presents a new inverse analysis method of finding local elastic moduli from displacement data for a heterogeneous and anisotropic elastic body in state of plane stress or strain. The method chooses a small block by connecting points at which displacement is measured and analyzes its deformation to determine elastic moduli separately from the rest of the body. The procedures of the elasticity inversion are developed, with some techniques to exclude measurement errors from the displacement data. It is demonstrated in numerical simulation that the elasticity inversion is able to determine local elastic moduli accurately when displacement data of sufficient quantity and quality are given.

3D stress field tomography based on photoelasticity

Publisher Summary A new method for nondestructive measurement of arbitrary 3D stress state, using photoelasticity, has been developed. The new approach, load incremental approach, is an attempt to solve the nonlinear inverse problem of 3D photoelasticity by considering change in stress state for a load increment and linearizing the nonlinear governing equation. As long as the applied load increment is small and photoelastic images are taken with high resolution in load increment, this method works and can reconstruct arbitrary 3D stress state. A new approach for nondestructive measurement system for arbitrary 3D stress state, using photoelasticity, is studied. As a next step, a new experiment should be developed for the measurement of 3D stress state based on the proposed load incremental approach. The conclusions from the simulations are used as basic data for designing this measurement system.