Xinjun Wu

Enhancement of the excitation efficiency of a torsional wave PPM EMAT array for pipe inspection by optimizing the element number of the array based on 3-D FEM.

Electromagnetic acoustic transducers (EMATs) can generate non-dispersive T(0,1) mode guided waves in a metallic pipe for nondestructive testing (NDT) by using a periodic permanent magnet (PPM) EMAT circular array. In order to enhance the excitation efficiency of the sensor, the effects of varying the number of elements of the array on the excitation efficiency is studied in this paper. The transduction process of the PPM EMAT array is studied based on 3-D finite element method (FEM). The passing signal amplitude of the torsional wave is obtained to represent the excitation efficiency of the sensor. Models with different numbers of elements are established and the results are compared to obtain an optimal element number. The simulation result is verified by experiments. It is shown that after optimization, the amplitudes of both the passing signal and defect signal with the optimal element number are increased by 29%, which verifies the feasibility of this optimal method. The essence of the optimization is to find the best match between the static magnetic field and the eddy current field in a limited circumferential space to obtain the maximum circumferential Lorentz force.

A Magnetic Flux Leakage and Magnetostrictive Guided Wave Hybrid Transducer for Detecting Bridge Cables

Condition assessment of cables has gained considerable attention for the bridge safety. A magnetic flux leakage and magnetostrictive guided wave hybrid transducer is provided to inspect bridge cables. The similarities and differences between the two methods are investigated. The hybrid transducer for bridge cables consists of an aluminum framework, climbing modules, embedded magnetizers and a ribbon coil. The static axial magnetic field provided by the magnetizers meets the needs of the magnetic flux leakage testing and the magnetostrictive guided wave testing. The magnetizers also provide the attraction for the climbing modules. In the magnetic flux leakage testing for the free length of cable, the coil induces the axial leakage magnetic field. In the magnetostrictive guided wave testing for the anchorage zone, the coil provides a pulse high power variational magnetic field for generating guided waves; the coil induces the magnetic field variation for receiving guided waves. The experimental results show that the transducer with the corresponding inspection system could be applied to detect the broken wires in the free length and in the anchorage zone of bridge cables.

Detecting broken-wire flaws at multiple locations in the same wire of prestressing strands using guided waves.

Broken wires often occur at multiple locations in the same wire of a strand due to the recovery length, which is defined as the length of the wire taking up its full share of the axial load from the break point. The detection of broken-wire flaws at multiple locations along the same wire is investigated using guided waves below 400kHz. Herein, a sample with three broken-wire flaws in the same wire is analyzed using magnetostrictive guided waves. Our data show that three flaws are found using the low-frequency guided waves (50kHz) but only one flaw is found using the high-frequency guided waves (320kHz). By analyzing the reflection and transmission coefficients at the three different flaws, we observe that the energy exchange decreases as the frequency increases along the same propagating distance. Hence, the recovery length for elastic waves, the length of the wire taking up its full share of elastic-wave energy from the break point, is observed. The recovery length for elastic waves in prestressing strands increases with the frequency. To detect prestressing strands using magnetostrictive guided waves, several one-broken-wire flaws at different locations can be distinguished from in different wires or the same wire by employing both low-frequency waves and high-frequency waves. Nevertheless, we cannot identify in which wire the flaws are located because the magnetostrictive sensor analyzes the whole strand.

Research on reconfigurable robot technology for cable maintenance of cable-stayed bridges in-service

In this paper, according to the technical characteristics of the maintenance of cable of cable-stayed bridges, a new robot for cable maintenance is developed. The climbing equipment, the surface inspection equipment and the internal defects inspection equipment, the cleaning equipment, the spraying equipment for cable maintenance are all designed as reconfigurable module. According to different cable diameters, different numbers of robot modules are reconstructed to form corresponding maintenance robot. The magnetic mechanisms that are used in the climbing equipment of robot keep the robot cling to the cable. The robot can climb cable along by wheels which installed on it. The robot has many advantages, such as simple structure, easy manipulative, fast moving, the less damage to the cable and the good adaptability to the shape of the cable.

Enhancement of the Excitation Efficiency of the Non-Contact Magnetostrictive Sensor for Pipe Inspection by Adjusting the Alternating Magnetic Field Axial Length

The non-contact magnetostrictive sensor (MsS) has been widely used in the guided wave testing of pipes, cables, and so on. However, it has a disadvantage of low excitation efficiency. A new method for enhancing the excitation efficiency of the non-contact MsS for pipe inspection using guided waves, by adjusting the axial length of the excitation magnetic field, is proposed. A special transmitter structure, in which two copper rings are added beside the transmitter coil, is used to adjust the axial length at the expense of weakening the excitation magnetic field. An equivalent vibration model is presented to analyze the influence of the axial length variation. The final result is investigated by experiments. Results show that the excitation efficiency of the non-contact MsS is enhanced in the whole inspection frequency range of the L(0,2) mode if the axial length is adjusted to a certain value. Moreover that certain axial length is the same for pipes of different sizes but made of the same material.

A Guided Wave Sensor Based on the Inverse Magnetostrictive Effect for Distinguishing Symmetric from Asymmetric Features in Pipes

The magnetostrictive guided wave sensor with a single induced winding cannot distinguish axially symmetric from non-axially symmetric features in a pipe, because it is impossible for the sensor to detect the non-axially symmetric mode waves. When we study the effect of the change of the magnetic field in the air zone for receiving the longitudinal guided wave mode, we find that the change of the magnetic flux in the air zone is almost equivalent to the change of the flux in the pipe wall, but in opposite directions. Based on this phenomenon, we present a sensor that can detect the flexural-mode waves in pipes based on the inverse magnetostrictive effect. The sensor is composed of several coils that are arranged evenly on the outside of pipes. The coils induce a change in magnetic flux in the air to detect the flexural-mode waves. The waves can be determined by adding a phase delay to the induced signals. The symmetric and asymmetric features of a pipe can be distinguished using the sensor. A prototype sensor that can detect F(1,3) and F(2,3) mode waves is presented. The function of the sensor is verified by experiments.

Bolt axial stress measurement based on a mode-converted ultrasound method using an electromagnetic acoustic transducer.

A method is proposed to measure the stress on a tightened bolt using an electromagnetic acoustic transducer (EMAT). A shear wave is generated by the EMAT, and a longitudinal wave is obtained from the reflection of the shear wave due to the mode conversion. The ray paths of the longitudinal and the shear wave are analyzed, and the relationship between the bolt axial stress and the ratio of time of flight between two mode waves is then formulated. Based on the above outcomes, an EMAT is developed to measure the bolt axial stress without loosening the bolt, which is required in the conventional EMAT test method. The experimental results from the measurement of the bolt tension show that the shear and the mode-converted longitudinal waves can be received successfully, and the ratio of the times of flight of the shear and the mode-converted longitudinal waves is linearly proportional to the bolt axial tension. The non-contact characteristic of EMAT eliminates the effect of the couplant and also makes the measurement more convenient than the measurement performed using the piezoelectric transducer. This method provides a promising way to measure the stress on tightened bolts.

Signal processing for the guided wave test based on the empirical mode decomposition

In order to detect the small defect, the empirical mode decomposition (EMD) method is employed to process the magnetostrictive guided wave testing signal. Firstly, the principle of the EMD is introduced. Then, the experimental setup is developed to obtain guided wave testing signals. Both the prestressing strand and the parallel wire cable have been manufactured grooves in the wires. It is difficult to find the defect signal from the original signal. By means of the EMD, the test signal is decomposed into series of the intrinsic mode functions (IMFs). The small defect signal is enhanced by analyzing the different order of the IMF. It is easy to find the small defect signal in the first mode wave for the prestressing strand. For the parallel wire cables, the second and third mode can be used to find the small defect signal. These test results indicate that the EMD approach can suppress noise effectively and improve the ability of guided wave testing of the small defect.

Development of an inspection robot for long‐distance transmission pipeline on‐site overhaul

Purpose – Damage of oil and gas long‐distance transmission pipelines caused by corrosion, stresses, geological change, and other factors can result in catastrophic failures. The purpose of this paper is to describe a mobile out‐pipe inspection robot for the pipeline inspection.Design/methodology/approach – This paper adapts to the requirements of long‐distance transmission pipelines on‐site overhaul, the robot developed in this paper utilizes a local magnetic flux leakage testing method for the pipeline inspection. Detection, walking, attachment, steering, and control units are designed for the robot.Findings – The detection apparatus of the robot plays dual roles as detection and attachment unit. So, the structure of the robot is relatively simple and it is relatively small in size and relatively light.Practical implications – A prototype of the robot has been manufactured according to the design. Results of the test in lab show the robot can be used to inspect pipe.Originality/value – The robot is able ...

An improved longitudinal wave EMAT based on the shielding effect

Electromagnetic acoustic transducer (EMAT) has been used in many industrial areas due to its non-contact nature. However, the longitudinal wave EMAT is rarely employed because it is difficult to be generated. An improved longitudinal wave EMAT is proposed in this paper. By inserting the copper sheet between the coil and the test specimen, the eddy currents induced in the test specimen are changed and the components of the eddy currents for generating the shear wave are effectively suppressed. The performance of the EMAT is tested by the experiment.