Residual-stress analysis by local laser heating and speckle-correlation interferometry

The uneven reflective metal surface has great influence on the high-precision measurement of the structured light 3-D shape. By analyzing the laser scattering and reflection models, a set of light stripe center’s subpixel extraction methods which has strong robustness is proposed. The method first cuts off the influence of the uneven fixed background by using the difference image method to process the bright field and the dark field. Then, the regional growth statistics method is used to eliminate the influence of random laser speckle noise, and then the gray-gravity method is used to obtain the coarse center of the laser stripe. The Sobel operator is used to obtain the gradient vector of the stripe pixel point, and then the normal direction field of the light stripe is obtained. The normal direction field vector is taken as the direction to find the $5\times5$ neighborhood of the coarse light stripe’s center, and then the gray-gravity method is reused to determine the center position of the laser stripe in the normal line direction; the pixel coordinates of the subpixel level are obtained by using bilinear interpolation. The experimental results show that the method can effectively eliminate the interference of flash point noise and has strong robustness. The detection error of the stripe is less than 0.1 pixels, which ensures the system measurement accuracy. Compared with other methods, the system’s resolution can reach 0.02 mm.

Laser Stripe Center Detection Under the Condition of Uneven Scattering Metal Surface for Geometric Measurement

Fatigue damage detection and fatigue life prediction are important technological issues in both academic and industrial fields. Numerous techniques have been proposed and developed for the detection of early fatigue damage in structural members. This paper provides a brief review of the current state-of-the-art on fatigue damage sensors. Eight types of existing fatigue damage sensors based on different detecting principles are introduced and discussed, with the emphasize on the two commercially available fatigue damage sensors, the fatigue fuse with a slit and the electrical resistance fatigue gauge. Moreover, a novel concept of utilizing carbon films as a fatigue monitoring sensor is proposed. The preliminary experimental results suggest that metal-containing diamond-like carbon (Me-DLC) films with optimum composition and micro-structure will be good candidate materials for fatigue monitoring sensors.

Early fatigue damage detecting sensors—A review and prospects

Effect of laser surface hardening on fatigue crack growth rate in AISI-4130 steel

The paper contains a proposal of a new model for the stress-life curve. The new model reflects the nature of the stress curve occurring under current conditions, which means its shape of an inclined letter S. It can be used for both low and high cycle fatigue. Verification of the proposed model was performed on experimental results of ten various materials from three main types that are low-alloy, high-alloy and non-alloy steels.

/pdf/stress-life-curve-for-high-and-low-cycle-fatigue-2aozkptdpp.pdf

Stress-life curve for high and low cycle fatigue

We investigated a method to evaluate fatigue damage of steels without contact using a laser speckle sensor. In the earlier stage of fatigue in steels, slipbands are produced on the surface and the slipband density increases as fatigue damage progresses. When a laser illuminates the surface of the fatigued steel, light intensity distribution of the laser speckle pattern caused by the reflected light changes depending on the change of surface properties caused by slipbands. Width of the speckle pattern broadens correspondingly to spatial frequency distribution of the surface and thus it is presumed that speckle pattern broadens with increase of slipband density. The method presented in this paper is based on this phenomenon. In this study, we observed change of the speckle pattern during fatigue loading. We propose a method to estimate fatigue life by observing change in the speckle pattern depending on number of loading cycles in the earlier stage of fatigue before crack initiation.

Fatigue life estimation of steel using laser speckle sensor

An image-processing system using a personal computer was applied to stress measurement by copper electroplating. The microscopic TV image of the plating structure of an electroplated specimen after cyclic loading was directly analyzed by the image-processing system. The density of grains grown in the plated layer was measured automatically. The surface stress of the specimen can be estimated based on the grain density. The stress-concentration factors of grooved shafts under torsion were obtained as an example of application. The result was in good agreement with the numerical values published previously.

Stress measurement by copper electroplating aided by a personal computer

Detection of fatigue damage in steel using laser speckle

In this study, we investigated a noncontact method of measuring plastic strain of metals using laser speckle. This method is based on observation of the change in laser speckle pattern with surface profile change due to plastic deformation. Analysis of the laser speckle pattern is performed on a computer-based image processing system. The relationships between speckle pattern and surface roughness and frequency characteristics of the surface profile were investigated. Surface roughness and surface profile were observed for steel specimens polished with emery paper or plastically deformed. The surface profile diagram was analyzed by means of the fast Fourier transform with a computer and distribution of the spatial frequencies was obtained. The results showed that distribution of light intensity of the speckle is related to frequency distribution of the surface profile, but it is not always related to surface roughness. It was also clarified that the laser speckle pattern is closely related to the magnitude of plastic strain.

Damage Monitoring of Metal Materials by Laser Speckle Assisted by Image Processing Techniques : Relationship between Distribution of Laser Speckle and Surface Properties

This paper presents a stress measurement method using the grain-growth phenomenon in the copper-electroplated layer by cyclic loading. The growth rate of grains by cyclic loading is related to the stress amplitude. The relation between grain area and number of stress cycles was observed under various stress amplitudes and the relation between grain growth rate and the stress amplitude was obtained. The grain area was measured by an image-processing system using a personal computer. Because grain-growth rate depends on the amplitude of stress, surface stress can be estimated from the grain-growth rate based on this relation. For grain size in the range from 0.01 to 0.1 mm, stress in a very small region can be measured based on the growth of grains. Stress measurement was actually carried out using this method. The result showed that the error was less than 1.0 percent, confirming that this method is sufficiently accurate.

Akira Kato

Papers

Fatigue life estimation of steel using laser speckle sensor

Stress measurement by copper electroplating aided by a personal computer

Detection of fatigue damage in steel using laser speckle

Damage Monitoring of Metal Materials by Laser Speckle Assisted by Image Processing Techniques : Relationship between Distribution of Laser Speckle and Surface Properties

Stress measurement by copper electroplating based on grain growth