Jubing Chen

The evaluation of Young's modulus and residual stress of nickel films by microbridge testings

The variation of deflection and maximum stress of a nickel film microbridge with load was investigated over a wide range of length, thickness, elastic modulus and residual stress by both large and small deflection theories of microbridge testings. The limits of deflection and load for a microbridge to be deformed elastically were determined by the assumption that the maximum stress could not be more than the yield strength. Furthermore, the deflection and load limits of small deflection theory were decided by setting a threshold beforehand for the normalized deflection and maximum stress difference between large and small deflection theories. Based on the results above, the dimensions of the nickel microbridge samples were chosen and the deflection range suitable for the small deflection theory was calculated. The nickel film microbridge samples electroplated on a Si(100) substrate were fabricated by MEMS and the microbridge testings were conducted with a load and displacement sensing nanoindenter system. From small deflection theory, the Youngs modulus and residual stress for the electroplated nickel films were calculated and the results were 190.5 GPa and 86.6 MPa, respectively.

Calibration of fringe projection system for surface profile measurement

Abstract. We propose a calibration method of a fringe projection system for surface profile measurement. The calibration method is divided into two parts: (1) phase to z calibration based on measuring the absolute phase distributions of a flat calibration board placed at several different known z positions. (2) Camera calibration based on Zhang’s method of imaging a checkerboard with different poses. Experiments of a flat plane, a sphere, and a Gorky sculpture demonstrate the validity and accuracy of the proposed technique.

A Phase Compensation Method for Phase Extraction of Abrupt Surfaces in Projection Moiré

Projection moiré method is a high-resolution, non-contacting and full-field optical measurement technique. But the 2π-ambiguity problem in phase unwrapping limits the phase difference between neighbouring pixels under π which makes projection moiré method ineffective at measuring abrupt surfaces or steps. This paper aims at putting forward a phase compensation method to solve this 2π-ambiguity problem in projection moiré. Since the phase of moiré pattern is the variance between the phase of specimen grating on object’s surface and the phase of the reference grating, our method is to calculate the phases of specimen grating and the reference grating respectively, regard their difference as the phase of the moiré pattern in theory and correct the wrong unwrapped moiré fringe phase with integral multiple of 2π. In order to acquire the phase of specimen grating which also has the 2π-ambiguity problem, using multiple fringe pictures with different pitches. The experiments successfully reconstruct two objects with discontinuities in their surfaces. This phase compensation technique no longer requires us to count fringe orders or find a reference point to mark the zero-order moiré fringe during experiments.

Three-dimensional shape reconstruction via an objective function optimization-based point cloud registration method

Abstract. This paper proposes an approach for the full-field surface reconstruction of three-dimensional (3-D) objects. The method uses a fringe projection profilometry system to obtain a 3-D point cloud with a multiview and proposes an objective function that involves all of the parameters of rotation and translation. The feature points of the surface of the object are extracted to determine the initial value of the objective function, which is then optimized to achieve accurate registration of the point clouds. The experimental results demonstrate that the proposed method is accurate and effective.

High-resolution projection moiré and its application in out-of-plane displacement test on large area

This paper develops a set of projection Moire system. It projects the grating onto the surface of specimen and forms grid lines of which the pitch is adjustable on large area through light source and projection grating. At the same time, the imaging system images the grid lines precisely onto the reference grating and then the Moire fringes are formed. The modulation of out-of-plane displacement to grid lines transform into the distortion of Moire fringes. The greatest advantage of this projection Moire system is that what the CCD camera captures is the Moire fringes, not the gird lines. The Moire fringes can magnify the variation of out-of-plane displacement of the specimen with no distortion and the measuring resolution is improved a lot. Besides, this system is appropriate for large area measurement by using a wide-angle lens in imaging system. The full-field out-of-plane displacement can be acquired through image processing. The measuring resolution of this system can reach 10μm on a large area of 3m*3m.

Experimental and simulation research on residual stress depth by hole-drilling method and grating rosette

Hole-drilling is one of the most common methods for test residual stress; it is widely used to measure the residual stresses in various materials. This method provides access to the residual stress profile in the through specimen thickness. In this paper, experimental and numerical simulation are serviced in hole-drilling method, by means of measuring the relieved strains on specimen surface when drilling each depth step by step, to find some relationship between the drilling depth and the surface strain, and comparing with an identified stresses specimen to determine the residual stresses in different depth.