Chunguang Xu

Characteristic Evaluation on Bolt Stress by Ultrasonic Nondestructive Methods

Based on the acoustoelasticity theory, a certain relationship exists between ultrasonic velocity and stress. By combining shear and longitudinal waves, this paper provides a nondestructive method of evaluating axial stress in a tightened bolt. For measuring the bolt axial stress in different situations, such as under low or high loads, this paper provides guidelines for calculating the stress for a given load factor. Experimental and calculated results were compared for three bolt test samples: an austenitic stainless steel bolt (A2-70) and low-carbon steel 4.8 and 8.8 bolts. On average, the experimental results were in good agreement with those obtained through calculations, thus providing a nondestructive method for bolt stress measurements.

Nondestructive testing and characterization of residual stress field using an ultrasonic method

To address the difficulty in testing and calibrating the stress gradient in the depth direction of mechanical components, a new technology of nondestructive testing and characterization of the residual stress gradient field by ultrasonic method is proposed based on acoustoelasticity theory. By carrying out theoretical analysis, the sensitivity coefficients of different types of ultrasonic are obtained by taking the low carbon steel(12%C) as a research object. By fixing the interval distance between sending and receiving transducers, the mathematical expressions of the change of stress and the variation of time are established. To design one sending-one receiving and oblique incidence ultrasonic detection probes, according to Snell law, the critically refracted longitudinal wave (LCR wave) is excited at a certain depth of the fixed distance of the tested components. Then, the relationship between the depth of LCR wave detection and the center frequency of the probe in Q235 steel is obtained through experimental study. To detect the stress gradient in the depth direction, a stress gradient LCR wave detection model is established, through which the stress gradient formula is derived by the relationship between center frequency and detecting depth. A C-shaped stress specimen of Q235 steel is designed to conduct stress loading tests, and the stress is measured with the five group probes at different center frequencies. The accuracy of ultrasonic testing is verified by X-ray stress analyzer. The stress value of each specific depth is calculated using the stress gradient formula. Accordingly, the ultrasonic characterization of residual stress field is realized. Characterization results show that the stress gradient distribution is consistent with the simulation in ANSYS. The new technology can be widely applied in the detection of the residual stress gradient field caused by mechanical processing, such as welding and shot peening.

Benchmark of residual stress for ultrasonic nondestructive testing

Based on acoustoelasticsticity theory, critically refracted longitudinal (LCR) wave was used to detect the residual stress in X70 steel. The method of fabrication and calibration for residual stress reference block was introduced and the detecting error brought by environmental temperature changes was corrected. The theoretical equation about the effect of temperature on residual stress detecting was analyzed. Low/high temperature test chambers and ultrasonic residual stress detecting equipment was used to detect the change of ultrasonic transit time difference, which caused by the temperature change. MATLAB second order polynomial was used to fit the experimental detecting results and fitted equation was obtained. The fitted equation was compensated to the system by software programming. The change of residual stress in reference block in two situations was compared. The results showed that temperature change had a large impact on the accuracy of ultrasonic residual stress detecting. The error of residual stress detecting can be effectively reduced by the temperature compensation modules. As result, the absolute precision of detection reaches ±10MPa.

Automatic Method for Synchronizing Workpiece Frames in Twin-robot Nondestructive Testing System

The workpiece frames relative to each robot base frame should be known in advance for the proper operation of twin-robot nondestructive testing system. However, when two robots are separated from the workpieces, the twin robots cannot reach the same point to complete the process of workpiece frame positioning. Thus, a new method is proposed to solve the problem of coincidence between workpiece frames. Transformation between two robot base frames is initiated by measuring the coordinate values of three non-collinear calibration points. The relationship between the workpiece frame and that of the slave robot base frame is then determined according to the known transformation of two robot base frames, as well as the relationship between the workpiece frame and that of the master robot base frame. Only one robot is required to actually measure the coordinate values of the calibration points on the workpiece. This requirement is beneficial when one of the robots cannot reach and measure the calibration points. The coordinate values of the calibration points are derived by driving the robot hand to the points and recording the values of top center point(TCP) coordinates. The translation and rotation matrices relate either the two robot base frames or the workpiece and master robot. The coordinated are solved using the measured values of the calibration points according to the Cartesian transformation principle. An optimal method is developed based on exponential mapping of Lie algebra to ensure that the rotation matrix is orthogonal. Experimental results show that this method involves fewer steps, offers significant advantages in terms of operation and time-saving. A method used to synchronize workpiece frames in twin-robot system automatically is presented.

Kinematic constraint analysis in a twin-robot system for curved-surface nondestructive testing

Purpose – Nondestructive testing based on cooperative twin-robot technology is a significant issue for curved-surface inspection. To achieve this purpose, this paper aims to present a kinematic constraint relation method relative to two cooperative robots. Design/methodology/approach – The transformation relation of the twin-robot base frame can be determined by driving the two robots for a series of handclasp operations on three points that are noncollinear in space. The transformation relation is used to solve the cooperative motion problem of the twin-robot system. Cooperative motions are divided into coupled and combined synchronous motions on the basis of the testing tasks. The position and orientation constraints for the two motion modes are also explored. Findings – Representative experiments between two industrial robots are conducted to validate the theoretical developments in kinematic constraint analysis. Artificial defects are clearly visible in the C-scan results, thereby verifying the validi...

Inverse kinematic analysis and evaluation of a robot for nondestructive testing application

The robot system has been utilized in the nondestructive testing field in recent years. However, only a few studies have focused on the application of ultrasonic testing for complex work pieces with the robot system. The inverse kinematics problem of the 6-DOF robot should be resolved before the ultrasonic testing task. A new effective solution for curved-surface scanning with a 6-DOF robot system is proposed in this study. A new arm-wrist separateness method is adopted to solve the inverse problem of the robot system. Eight solutions of the joint angles can be acquired with the proposed inverse kinematics method. The shortest distance rule is adopted to optimize the inverse kinematics solutions. The best joint-angle solution is identified. Furthermore, a 3D-application software is developed to simulate ultrasonic trajectory planning for complex-shape work pieces with a 6-DOF robot. Finally, the validity of the scanning method is verified based on the C-scan results of a work piece with a curved surface. The developed robot ultrasonic testing system is validated. The proposed method provides an effective solution to this problem and would greatly benefit the development of industrial nondestructive testing.

Inspecting integrity and residual stress of plate by ultrasonic wave

Ultrasonic tomography and critically refracted longitudinal (LCR) wave method is studied for inspecting the integrity and residual stress of plate. A transducer array based on sixteen elements is employed to imaging the corrosion defect for evaluating the size of defect quantitatively. The central frequency of element is chosen as 1MHz to excite low mode lamb wave. The algebraic reconstruction technique (ART) and simultaneous iterative reconstruction technique (SIRT) by attenuation of Lamb wave signal are used to get the projection data for tomographic reconstruction. The reconstructed image indicates that ultrasonic tomography is efficient to evaluate the integrity of plate. The residual stresses inspecting is based on the effect of stresses on the propagation velocity of elastic waves. The LCR wave mode was employed and accurate travel time data were measured for plate of steel. According to the results of experiments, the active LCR wave will effectively interact with the internal stress within a defined depth range and generate a measurable change on the travel time.

AN ULTRASONIC MODEL FOR REVEALING ELECTRONIC PACKAGE’S INNER STRUCTURE USING ACOUSTIC IMPEDANCE

The reflected ultrasonic echo’s amplitude is relative to the sample material’s acoustic impedance change. So the distribution of acoustic impedance is correlated with the inner structure. In this paper, an ultrasonic model is suggested to reveal electronic package’s inner structure using acoustic impedance. The model first reconstructs the impedance profile of layered medium through an incidence point, then gets other position’s impedance by moving the transducer regularly. Finally, the inner structure of the package can be revealed by the 3D image formed with the impedance matrix.

Magnetic field analysis and optimal design of magnetic bearing

Magnetic field distribution of a radial magnetic bearing with sixteen-pole was analyzed by using finite element method. It was verified by magnetic field measurement. Magnetic bearing structure was optimized based on finite element analysis (FEA) and magnetic circuit method. Optimization was done in object of maximum magnetic force. Two optimizations had similar results. Analysis showed that FEA-based optimization is more accurate and efficient, which had been performed with FEA software-ANSYS 10.0.

Ultrasonic Adaptive Detection for Aerospace Components with Varying Thickness

Automatic inspection of microdefects located in the aerospace components is difficult due to the imprecise scanning trajectory, especially for those specimens with varying thickness. In this paper, a new ultrasonic nondestructive testing (NDT) system using the robotic scanning trajectory is constructed for inspecting turbine blades. Additionally, an approach based on the analysis of ultrasonic signals is proposed to calibrate the trajectory; the ultrasonic image based on the threshold function represents the distribution of inner defects when the following gate is used to track the flaw echo. Therefore, the characteristic parameters of the flaw echo signals are easy to be discriminated if the reflection waves are stable in the time domain. Experimental result verified the effective and feasibility of the proposed approach; the distribution of inner defects can be shown with a higher resolution than other NDT methods when robotic orientation is correct at each point of scanning trajectory. Furthermore, the feature signals can be tracked more accurately during the ultrasonic signal processing if the ultrasonic distance was considered as a calibration coefficient of positional matrix. The proposed ultrasonic adaptive detection is adapted to complex geometric structure with a minimum resolution of equivalent diameter of the inner flaw being 0.15 millimeters.