Terry Yuan Fang Chen

A contact-type piezoresistive micro-shear stress sensor for above-knee prosthesis application

A prototype contact-type micro piezoresistive shear-stress sensor that can be utilized to measure the shear stress between skin of stump and socket of above-knee (AK) prosthesis was designed, fabricated and tested. Micro-electro-mechanical system (MEMS) technology has been chosen for the design because of the low cost, small size and adaptability to this application. In this paper, the finite element method (FEM) package ANSYS has been employed for the stress analysis of the micro shear-stress sensors. The sensors contain two transducers that will transform the stresses into an output voltage. In the developed sensor, a 3000/spl times/3000/spl times/300 /spl mu/m/sup 3/ square membrane is formed by bulk micromachining of an n-type [100] monolithic silicon. The piezoresistive strain gauges were implanted with boron ions with a dose of 10/sup 15/ atoms/cm/sup 2/. Static characteristics of the shear sensor were determined through a series of calibration tests. The fabricated sensor exhibits a sensitivity of 0.13 mV/mA-MPa for a 1.4 N full scales shear force range and the overall mean hysteresis error is than 3.5%. In addition, the results simulated by FEM are validated by comparison with experimental investigations.

Spatially resolved assessments of composite shrinkage in MOD restorations using a digital-image-correlation technique

OBJECTIVES To analyze the influence of cavity geometry and lining materials in MOD composite restorations by characterizing the polymerization shrinkage and cusp deflection. METHODS Eighty intact molars with similar sizes were collected and randomly divided into eight groups. MOD cavities with various widths and depths were prepared on these teeth: Group I, 2((W)) × 2((D)) mm; Group II, 4((W)) × 2((D)) mm; Groups III, IIIf, IIIg, 2((W)) × 4((D)) mm; and Groups IV, IVf, IVg, 4((W)) × 4((D)) mm. In Groups IIIf and IVf, flowable composite liner was placed prior to composite restoration, while glass-ionomer liner was used in Groups IIIg and IVg. Deformations of restorations resulted from composite shrinkage were recorded for 30 min following light irradiation using a digital-image-correlation (DIC) method to subpixel level. The displacements at the boundaries of the restorations were analyzed using one-way ANOVA and the post hoc test at a 5% significance level. The correlation between the geometric factors and the displacements was also analyzed. RESULTS The inward displacements on free surfaces were greater than those on the bonded surfaces. Groups with flowable composite linings showed greater amount of displacements on free and bonded surfaces compared to the unlined and glass ionomer lining groups. The correlation analysis showed that the free surface shrinkage was related with the cavity width and C-factor, while cusp deflections were correlated with the cavity depth and the cusp compliance. SIGNIFICANCE The DIC technique measures composite shrinkage on different boundaries of restorations to facilitate the investigation of polymerization kinetics. Using flowable composite lining and increased cusp depth may aggravate the cusp flexure.

Computerized fringe analysis in photomechanics

A generalized method is presented for analyzing the fringe patterns frequently encountered in experimental mechanics. By utilizing digital-image-processing and computer-graphics techniques, a set of menu-driven software is developed for interactively implementing the fringe processing. Tests of this software on the images obtained experimentally by photoelasticity, holographic interferometry and speckle interferometry demonstrate its usefulness. Good agreement between the experimental and theoretical results is established.

Digital determination of photoelastic birefringence using two wavelengths

A new approach for digital determination of photoelastic birefringence is proposed. The relationships between the intensity values and the fringe orders of two wavelengths are derived. This scheme allows for automatic determination of fringe orders of a full-field photoelastic fringe pattern without using zero-order fringes in the fringe pattern. The usefulness of this method was demonstrated on two experimental fringe patterns with different wavelengths.

Contraction behaviors of dental composite restorations — Finite element investigation with DIC validation

The objective of this study was to examine the effects of cavity configuration on the polymerization shrinkage and stress of light-cured composite restorations by combining local strain measurement and a finite element analysis (FEA). Dental mesio-occluso-distal cavities of various widths and depths (each for 2 vs. 4 mm), representing different configuration factors, were prepared on extracted molars. The displacements of the bonded tooth cusps and cavity floors, caused by polymerization shrinkage of composite restorations, were assessed utilizing a digital-image-correlation (DIC) technique. The cervical marginal microleakage was investigated by examining the resin replicas of these restorations under SEM. The local material properties of the polymerized composite along the curing depth were defined by the nanoindentation test and applied in the subsequent FEA. In the FEA, four models were generated to correspond with the experimental restorations. In the DIC measurement results, the 4(w)×4(D) mm cavity presented the greatest values of inward displacements at the cusp and floor. The cavity depth, rather than the cavity width, was found to significantly correlate to the floor deformation, the location of shrinkage centers, and also the cervical microleakage ratio. The FEA simulation results showed that the 2(w)×4(D) mm cavity presented the maximal von Mises and principal stress located respectively on the cervical margins and cavity floor. Additional safety factor analysis showed a high risk of bond failure over the cavity floor in the 4-mm deep cavity. With the experimental validation, the simulation revealed that the cavity depth was significant to the formation of contraction stress and the incidence of interfacial debonding.

Whole-field digital measurement of principal stress directions in photoelasticity

Abstract A method that uses three light-field isoclinic images and the associated light-field unloaded model (or white) images for whole-field digital determination of the principal stress directions in photoelasticity is presented. Relevant theory is derived and explicit conditions for directly determining the directions to the range 0– π /2 are given. Tests of this method on a directly loaded two-dimensional disc and a stress frozen photoelastic slice are demonstrated. The results agree well with the values obtained from the manual method and/or the theory.

Measurement of the ballscrew contact angle by using the photoelastic effect and image processing

The contact angle of ball and screw affects the ballscrews life. A novel method for measurement of the contact angle of ballscrew is developed by using the photoelastic effect and digital image processing techniques. Substituting the steel ball by a photoelastic disk, the contact angles of the disk at contact with a 90° groove and a real screw are measured by processing the digitized photoelastic disk images. Acceptable results have been achieved for consideration of the accuracy and reliability of the measurement. The method is cost-effective and is able to substitute the commonly used profiler for practical inspection of the screw.

A simple method for the digital determination of the photoelastic fringe order

A new and simple approach to the digital determination of a photoelastic fringe order using two different loads is proposed. The relationships between the intensity values of light and the isochromatic fringe orders generated from two different loads are derived. The scheme used for the automated determination of the total fringe orders of a full-field photoelastic fringe pattern is described. The usefulness of this method is demonstrated using two isochromatic fringe patterns under two different loads. Extra filters are not needed in the proposed method as in the case of the two-wavelength method.

Digital fringe multiplication in three-dimensional photoelasticity

Abstract Digital fringe multiplication of three-dimensional photoelastic images using the image-division technique and multiple-angle relations of the cosine function is described and demonstrated. Fringe multiplication of two slices from a stress-freezing model to a factor of 9 is shown. The fractional fringe orders counted by this method agree well with that determined by the Tardy method and a null-balance compensator. The multiplication process is very simple and effective for practical use.

Development of a microelectromechanical system pressure sensor for rehabilitation engineering applications

Based on computer finite-element analysis ANSYS 5.3 and microelectromechanical systems (MEMS) technologies, a micropressure sensor was designed and fabricated. The sensor can be used to measure the distribution of normal stress between soft tissues on an above-knee amputees skin and the contacting surface of a rehabilitation device. A square membrane with dimensions 2400 µm × 2400 µm × 80 µm is formed by backside photolithography and wet etching of an n-type ⟨100⟩ monolithic silicon wafer. On the middle of the membrane edge, an X-shaped silicon wafer was implanted with boron ions and then enhanced by diffusion to form a piezoresistive strain gauge. In the design process, a finite-element method is used to analyse the effects of pressure sensitivity and its temperature coefficients. The developed micropressure sensors, which have smaller weight and volume than a conventional machine type, perform well and fit our design specifications.