Eisaku Umezaki

Improvement in separation of SMA from matrix in SMA embedded smart structures

Crack closure in epoxy resin plates with an embedded spiral shape memory alloy (SMA) wire as an actuator in smart structural members was investigated as a function of the duration of the supply of electric current to the spiral SMA wire under three-point bending using photoelastic experiments. The effect of heat generation due to the spiral SMA wire in the matrix was also investigated. Crack closure was evaluated by the stress intensity factor obtained from the photoelastic fringes generated near the crack tip. The results showed that the closure function in the plate with the spiral SMA wire was slightly inferior to that in the plate with a linear SMA wire. In addition, the spiral SMA wire was observed to greatly improve separation between the matrix and the SMA wire due to the heat generation compared with that of the linear SMA wire.

Extraction of isotropic points using simulated isoclinics obtained by photoelasticity-assisted finite element analysis

A method for extracting isotropic points in structures using simulated isoclinics obtained from a combination of photoelastic experiments and finite element analysis (FEA) is presented. This method is divided into two parts. The first part involves the confirmation of the boundary condition necessary for FEA using isochromatics obtained by photoelastic experiments. The second part involves the determination of isotropic points using simulated isoclinics obtained by FEA under the boundary condition confirmed by photoelastic experiments. This method is applied to a ring and T-shaped plates subjected to a compressive load. The results show that the isotropic points in the ring and T-shaped plates could be accurately and easily extracted by this method.

Temperature Distributions of SMA Wires Embedded in Epoxy Resin Plates and Heated by Supplying Electric Current

As a fundamental study of a smart structure having the functions of crack prevention and crack closure, the temperature distributions of multiple shape memory alloy (SMA) wires embedded in epoxy resin plates and heated by supplying electric current are investigated using an infrared thermography technique and the finite element method (FEM). The temperature distributions of SMA in the air and of the plate surface obtained using FEM are in reasonable agreement with those obtained using the thermography technique. The numerical results show that the temperature of the SMA wire surface embedded in the plates depends on the length between SMA wires and the plate thickness, and is less than that in air.

Fatigue crack-tip deformation, crack growth and life prediction in polycarbonate

Low-cycle fatigue fracture of polycarbonate is investigated. Local strain in the vicinity of the notch root and crack tip has been measured in real time by using a fine-grid method. The relationships among the local strain, crack initiation from the notch root and crack propagation of the crack tip are studied, and a method for more precise life estimation is suggested.

Whole-Field Measurement Of Principal Stress Directions From Photoelastic Experiment Using Image Processing System

A method for measuring automatically isoclinic parameters (i.e. principal stress directions) in the whole field of a model from photoelastic experiment has been developed. A basic idea for obtaining the directions of principal stresses was to utilize that a time series curve of light intensity changes sinusoidally at each point in the model using a plane polari-scope in which a polarizer and analyzer were rotated, and the minimum location on the curve corresponds to the direction of principal stresses. The minimum location was detected by using the Fourier-series expansion of the curve. The above method was realized on an automatic polariscope with a TV camera, a personal computer, an image processing equipment etc. Using this system, the directions of principal stresses in the whole field could be automatically determined with a high accuracy and without any interpolation.

Evaluation of mechanical properties of esthetic brackets.

Plastic brackets, as well as ceramic brackets, are used in various cases since they have excellent esthetics. However, their mechanical properties remain uncertain. The purpose of this study was to determine how deformation and stress distribution in esthetic brackets differ among materials under the same wire load. Using the digital image correlation method, we discovered the following: (1) the strain of the wings of plastic brackets is within 0.2% and that of ceramic and metal brackets is negligible, (2) polycarbonate brackets having a stainless steel slot show significantly smaller displacement than other plastic brackets, and (3) there is a significant difference between plastic brackets and ceramic and stainless steel brackets in terms of the displacement of the bracket wing.

Application of isotropic points extracted using computerized photoelastic experiment to design structural members

This study investigates a method for optimally designing structural members with holes, which are made to decrease the weight of and to pass pipes and wires through these members. In this method, first the isotropic points, where the principal stresses are equal, are automatically extracted on an image with the directions of principal stresses obtained from the computerized photoelastic experiment. The finite element method is then used for analysing the models with holes differing in size at an isotropic point extracted experimentally. Finally, the hole size is determined, comparing with stress distributions around those holes. The present method is applied to a T-shaped plate subjected to a compressive load. As a result, the stresses in models with holes at locations different from the isotropic point are higher than those with the same-size hole at the isotropic point. Therefore the proposed method is effective for optimally designing structural members with holes.

Determination of Principal-Stress Directions by Three-Step Color Phase Shifting Technique

A three-step phase shifting approach for automatic determination of the isoclinic parameter in photoelasticity is presented. Unreliable isoclinic values affected by the isochromatic parameter are solved using the white light. The method uses three fringe images digitally captured by a digital camera for three different configurations of the dark-field plane polariscope. For a circular disk under compression, results show the method permits the isoclinic parameter in the range -π/2 to π/2 with almost free of the isochromatic parameter and comparable with theory.

Development of System for Simultaneous Measurement of Stress and Temperature

A system was developed for simultaneously measuring stress and temperature in structures in time series. The stress and temperature were measured using the photoelastic technique in the form of phase stepping and the thermographic technique, respectively. Four phase-stepped photoelastic images were captured simultaneously using beam-splitting optics. A beam splitter was used for transmitting light in the visible range while simultaneously reflecting light in the infrared range. The system was applied to ultraviolet curing resin with a step part illuminated with ultraviolet rays, and the stress and temperature in the curing process were measured. Results showed that the stress and temperature in the curing process can be measured simultaneously in time series using the system. The step part of the resin affected the distribution of the stress and temperature

Evaluating the stress intensity factor using white light photoelastic experiment

In this study, a method is proposed to evaluate the stress intensity factor using isochromatics multiplied and extracted from original isochromatic images obtained from a white light photoelastic experiment. This method utilizes R, G and B isochromatics which are captured using a color CCD camera and an image processor in dark- and light-field circular polariscopes. First, for each color, the dark-field intensities are subtracted from the light-field intensities so that each point of the resultant image has a positive, zero or negative value. The isochromatics are then obtained as a series of points whose value is zero, which are extracted using image processing. The fringe orders are assigned to the extracted isochromatics using calibration curves. The stress intensity factor is determined using the Irwin method. This method is applied to an epoxy resin plate with a crack formed at one side under 3-point bending. The result shows that the stress intensity factor is accurately evaluated using R, G and B isochromatics near the crack tip.