Jingpin Jiao

Low-frequency vibration modulation of guided waves to image nonlinear scatterers for structural health monitoring

Guided wave structural health monitoring offers the prospect of continuous interrogation of large plate-like structures with a sparse network of permanently attached sensors. Currently, the most common approach is to monitor changes in the received signals by subtraction from a reference signal obtained when the structure was known to be defect-free. In this paper a comparison is made between this defect-free subtraction approach and a technique in which low-frequency vibration modulation of guided wave signals is used to detect nonlinear scatterers. The modulation technique potentially overcomes the need for the defect-free reference measurement as the subtraction is now made between different parts of an externally applied low-frequency vibration. Linear defects were simulated by masses bonded onto a plate and nonlinear scatterers were simulated by loading a similar mass against the plate. The experimental results show that the defect-free subtraction technique performs well in detecting the bonded mass whereas the modulation technique is able to discriminate between the bonded and loaded masses. Furthermore, because the modulation technique does not require a defect-free reference, it is shown to be relatively independent of temperature effects, a significant problem for reference based subtraction techniques.

Excitation and detection of shear horizontal waves with electromagnetic acoustic transducers for nondestructive testing of plates

The fundamental shear horizontal(SH0) wave has several unique features that are attractive for long-range nondestructive testing(NDT). By a careful design of the geometric configuration, electromagnetic acoustic transducers(EMATs) have the capability to generate a wide range of guided wave modes, such as Lamb waves and shear-horizontal(SH) waves in plates. However, the performance of EMATs is influenced by their parameters. To evaluate the performance of periodic permanent magnet(PPM) EMATs, a distributed-line-source model is developed to calculate the angular acoustic field cross-section in the far-field. Numerical analysis is conducted to investigate the performance of such EMATs with different geometric parameters, such as period and number of magnet arrays, and inner and outer coil widths. Such parameters have a great influence on the directivity of the generated SH0 waves that arises mainly in the amplitude and width of both main and side lobes. According to the numerical analysis, these parameters are optimized to obtain better directivity. Optimized PPM EMATs are designed and used for NDT of strip plates. Experimental results show that the lateral boundary of the strip plate has no perceivable influence on SH0-wave propagation, thus validating their used in NDT. The proposed model predicts the radiation pattern of PPM EMATs, and can be used for their parameter optimization.

Vibro-acoustic modulation technique for micro-crack detection in pipeline

For the poor sensitivity of traditional ultrasonic technique for micro crack detection, a kind of nonlinear ultrasonic technique, vibro-acoustic modulation technique, is used for micro crack detection in pipes. The influence of the frequency and amplitude of vibration on the modulation index (MI) is experimentally investigated. The experiment results prove that the vibro-acoustic modulation technique can be used for crack detection in pipes. The modulation index is influenced by the frequency and amplitude of vibration, therefore it is important to choose the appropriate parameters in order to get higher sensitivity in crack detection.

Vibration modulated Lamb waves for structure health monitoring

In this paper, Lamb wave health monitoring in plate structure for detection of nonlinear contact scatter is conducted using vibration modulation technique. The low frequency vibration varies the interface area within the contact defect; therefore the amplitude and phase of high frequency Lamb waves reflected from the contact defect will be modulated. By time-frequency analysis of modulated Lamb wave series, the nonlinear responses at frequency of vibration are extracted out and used for defect imaging. It was shown that the nonlinear Lamb wave imaging method based on vibration modulation can effectively characterize and locate the nonlinear contact defects.

A Pulse Compression Technique for Improving the Temporal Resolution of Ultrasonic Testing

It is difficult to interpret ultrasonic signals when the echoes that are reflected from the various target features overlap. In this paper, a pulse compression technique based on a combination of Wiener filtering and autoregressive (AR) spectral extrapolation is used to process ultrasonic signals to improve their temporal resolution. The influence of the AR order and the width of the frequency window in AR spectral extrapolation are discussed in detail, and optimal parameters are obtained. The technique is then applied to measure the size of the defects in a plate and discern two adjacent defects in a block using a phased array. It is shown that the widths of the defects in the plate were measured accurately and the two adjacent defects were distinguished well by the proposed technique.

Application of Ultrasonic Methods for Early Detection of Intergranular Corrosion in Austenitic Stainless Steel

ABSTRACT Intergranular corrosion (IGC) is one of the major localized corrosion problems faced by austenitic stainless steel components in ultra-supercritical power plants, which occurs along the grain boundaries of material and results in loss of engineering properties. Ultrasonic technique was applied to the early detection of IGC in austenitic stainless steel super304H. Acoustic attenuation and nonlinear parameter were measured at multiple excitation frequencies, and an equivalent attenuation factor and an equivalent nonlinear factor were defined to evaluate the IGC damage. Results indicated that both factors increased monotonously as the degree of IGC developed, and the equivalent nonlinear factor is more sensitive to IGC at early stage. Thus they can be utilized for early detection of IGC in austenitic stainless steel.

Measurement of lubricant film thickness using normal incidence ultrasound

The study of the reflection of ultrasonic waves from a solid-solid interface can be used to measure the thickness between two solid interfaces. This paper describes the theoretical analysis and experimental research of the lubricant film thickness measurement through using normal incidence ultrasonic reflection. The proportion of the incident wave which is reflected at the interface was measured as a function of frequency with a single wideband ultrasonic transducer. With the thickness of the lubricant film increasing, the measured reflection coefficient decreases. According to the thickness of the lubricant film, the resonant model or the spring model could be used in the measurement. When the wavelength of the ultrasound is large compared to the thickness of the lubricant film, it is generally not practical to test the frequency, then, the film thickness measurement need to use a spring model.

The Pipeline Leak Location Techniques Based on Modal Acoustic Emission

Due to the multiple modes and dispersive nature of acoustic leak signal propagation in pipeline, the traditional AE cross correlation method was ineffective to location the leak in pipeline. Based on the theory of modal acoustic emission (MAE), two different pipeline leak location techniques are brought forward. One is based on two modes with one sensor; the other is based on one mode with two sensors. The general process is narrated as following three steps. Firstly, based on the theory of guided waves propagation in cylinder, the acoustic leak signals in positions nearby the sensor(s) can be predicted using the information of dispersion characteristics of guided waves in pipeline and the acoustic signal received by the sensor(s). Secondly, by transformed the predicted signals in time and space, two (one) individual modes (mode) of the acoustic signal at the position of the sensor(s) are isolated. By correlated the two modes of the same sensor (one mode of the two sensors), the leak position can be located accurately. The leak location experiments have been carried out to prove the validity of the one sensor and two sensors leak location techniques; it is indicated that the location errors of the two leak location techniques are usually less than 5%, and the techniques can be used to location the leak of pipeline filled with liquid or gas.Copyright