Zhanjun Wu

Guided waves based diagnostic imaging of circumferential cracks in small-diameter pipe

To improve the safety and reliability of pipeline structures, much work has been done using ultrasonic guided waves methods for pipe inspection. Though good for evaluating the defects in the pipes, most of the methods lack the capability to precisely identify the defects in the pipe features like welds or supports. Therefore, a novel guided wave based cross-sectional diagnostic imaging algorithm was developed to improve the ability of circumferential cracks identification in the pipe features. To ensure the accuracy of the imaging, an angular profile-based frequency selection method is presented. As validation, the approach was employed to identify the presence and location of a small circumferential crack with 1.13% cross sectional area (CSA) in the welding zone of a 48 mm diameter type 304 stainless steel pipe. Accurate identification results have demonstrated the effectiveness of the developed approach.

Damage size characterization algorithm for active structural health monitoring using the A0 mode of Lamb waves

In this study, a damage size characterization algorithm has been developed to continuously obtain the extent of damage, which is vital for further investigations into the remaining life or residual strength of damaged structures. This technique uses an active PZT network with pulse-echo and pitch-catch configurations. In order to facilitate the identification of scattered wave components, a dual-PZT actuation scheme was applied to generate a comparatively pure A0 mode with an enhanced energy. The damage size characterization algorithm starts by identifying the damage location. To this end, relying on temporal information of the scattered signal, a diagnostic image was constructed to highlight the most probable location of damage. Then, as wave scattering occurs from the edges of damage sites, for each sensing path the most probable location of the wave scattering source was estimated and considered as one point on the damage boundary. As a result, the location of some points on the damage boundary are estimated. Since, in practice, the captured signals are usually polluted with noise, a data processing scheme was used to separate points correctly located on the damage boundary from those related to noise. Finally, a convex hull of selected points gives the approximate shape and size of the damage. The approach was validated by defining the location, size and shape of corrosion at its earliest stage of existence. Corrosion severity was also evaluated by obtaining reflection and transmission coefficients, subject to corrosion with different depths. The obtained experimental results demonstrated the potential of the algorithm in providing detailed information about the damage, such as its location, size, shape and severity.

A novel probability-based diagnostic imaging with weight compensation for damage localization using guided waves

To avoid direct interpretation of the complicated ultrasonic guided wave signal and the effect of dispersion, probability-based diagnostic imaging is proposed to identify damages for guided wave–based structural health monitoring. However, the probability-based diagnostic imaging algorithm usually requires a relatively large number of actuator–sensor paths to cover the monitoring area. Therefore, a weight-compensated probability-based diagnostic imaging approach is developed to improve the ability of damage localization with higher precision under the same sensor configuration. The weight-compensated probability-based diagnostic imaging algorithm compensates for the weight difference from sensing paths with the unit weight of various positions, inhibited by the path weight difference effect on diagnostic image. The validity of the approach is assessed by identifying damages at different locations with different groups of sensing paths on a stiffened composite panel. Accurate localization results have demonstrated the effectiveness of the developed probability-based diagnostic imaging approach.

Guide waves-based multi-damage identification using a local probability-based diagnostic imaging method

Multi-damage identification is an important and challenging task in the research of guide waves-based structural health monitoring. In this paper, a multi-damage identification method is presented using a guide waves-based local probability-based diagnostic imaging (PDI) method. The method includes a path damage judgment stage, a multi-damage judgment stage and a multi-damage imaging stage. First, damage imaging was performed by partition. The damage imaging regions are divided into beside damage signal paths. The difference in guide waves propagation characteristics between cross and beside damage paths is proposed by theoretical analysis of the guide wave signal feature. The time-of-flight difference of paths is used as a factor to distinguish between cross and beside damage paths. Then, a global PDI method (damage identification using all paths in the sensor network) is performed using the beside damage path network. If the global PDI damage zone crosses the beside damage path, it means that the discrete multi-damage model (such as a group of holes or cracks) has been misjudged as a continuum single-damage model (such as a single hole or crack) by the global PDI method. Subsequently, damage imaging regions are separated by beside damage path and local PDI (damage identification using paths in the damage imaging regions) is performed in each damage imaging region. Finally, multi-damage identification results are obtained by superimposing the local damage imaging results and the marked cross damage paths. The method is employed to inspect the multi-damage in an aluminum plate with a surface-mounted piezoelectric ceramic sensors network. The results show that the guide waves-based multi-damage identification method is capable of visualizing the presence, quantity and location of structural damage.

Integrated impedance and Lamb wave–based structural health monitoring strategy for long-term cycle-loaded composite structure:

Impedance of a sensor is sensitivity to small structural damage which surrounds the sensor. Lamb wave propagation provides higher damage detection efficiency in the range of large area. Both methods have been widely developed for structural health monitoring. This article presents integrated impedance and Lamb wave–based structural health monitoring strategy for composite pressure vessels. The output of the presented structural health monitoring strategy includes distribution and classification of damage and health condition of sensor network under varying internal pressure loading environments. In the strategy, a novel damage index adjusting method for Lamb wave damage detection is developed based on the signal features of real and imaginary parts of the sensor impedance. First, the potential structural damage is pre-warned by monitoring the impedance variation of the piezoelectric transducer sensor network. Then, the health condition of sensor network under the working condition is assessed by impedance-based self-diagnosis method; subsequently, the Lamb wave damage index is adjusted based on the result of sensor self-diagnosis. Finally, on the basis of sensor self-diagnosis and damage index adjustment result, damage identification and classification are performed. Practical efficacy of the structural health monitoring approach is tested by the damage monitoring experiment on loaded composite structure.

In-service structural health monitoring of a full-scale composite horizontal tail

In-service structural health monitoring (SHM) technologies are critical for the utilization of composite aircraft structures. We developed a Lamb wave-based in-service SHM technology using built-in piezoelectric actuator/sensor networks to monitor delamination extension in a full-scale composite horizontal tail. The in-service SHM technology combine of damage rapid monitoring (DRM) stage and damage imaging diagnosis (DID) stage allows for real-time monitoring and long term tracking of the structural integrity of composite aircraft structures. DRM stage using spearman rank correlation coefficient was introduced to generate a damage index which can be used to monitor the trend of damage extension. The DID stage based on canonical correlation analysis aimed at intuitively highlighting structural damage regions in two-dimensional images. The DRM and DID stages were trialed by an in-service SHM experiment of CFRP T-joint. Finally, the detection capability of the in-service SHM technology was verified in the SHM experiment of a full-scale composite horizontal tail. Experimental results show that the rapid monitoring method effectively monitors the damage occurrence and extension tendency in real time; damage imaging diagnosis results are consistent with those from the failure model of the composite horizontal tail structure.

A Novel Complementary Method for the Point-Scan Nondestructive Tests Based on Lamb Waves

This study presents a novel area-scan damage identification method based on Lamb waves which can be used as a complementary method for point-scan nondestructive techniques. The proposed technique is able to identify the most probable locations of damages prior to point-scan test which lead to decreasing the time and cost of inspection. The test-piece surface was partitioned with some smaller areas and the damage probability presence of each area was evaluated. mode of Lamb wave was generated and collected using a mobile handmade transducer set at each area. Subsequently, a damage presence probability index (DPPI) based on the energy of captured responses was defined for each area. The area with the highest DPPI value highlights the most probable locations of damages in test-piece. Point-scan nondestructive methods can then be used once these areas are found to identify the damage in detail. The approach was validated by predicting the most probable locations of representative damages including through-thickness hole and crack in aluminum plates. The obtained experimental results demonstrated the high potential of developed method in defining the most probable locations of damages in structures.

Lamb waves and electro-mechanical impedance based damage detection using a mobile PZT transducer set

HIGHLIGHTSA novel‐joint damage detection algorithm based on Lamb waves and electro‐mechanical impedance methods is developed.A mobile transducer set contains PZT patches is designed.A baseline‐free damage characterizing algorithm is presented. ABSTRACT Lamb waves and electro‐mechanical impedance (EMI) based methods are increasingly used in damage detection owing to their high sensitivity to small structural defects. Lamb wave based methods are effective in detecting damages in a large area and electro‐impedance based methods are suitable for characterizing the identified damage. Based on these two methods, a novel combined damage detection method is presented in this research. To achieve this, first, a mobile transducer set is developed, which can be used for both the Lamb waves and EMI based methods. Then, a baseline‐free damage detection strategy that combines the Lamb waves and EMI methods is presented. Finally, a laboratory‐sized test piece is used to validate the effectiveness of the proposed approach. The results achieved with the application of the presented combined method for characterizing an L‐shape crack in an aluminum plate show better location accuracy and detection efficiency than those obtained by applying only one method.

High Strain Survivability of Piezoceramics by Optimal Bonding Adhesive Design

As one of the most common transducers used in structural health monitoring (SHM), piezoceramic sensors can play an important role in both damage detection and impact monitoring. However, the low tensile strain survivability of piezoceramics resulting from the material nature significantly limits their application on SHM in the aerospace industry. This paper proposes a novel approach to greatly improve the strain survivability of piezoceramics by optimal design of the adhesive used to bond them to the host structure. Theoretical model for determining the strain transfer coefficient through bonded adhesive from the host structure to piezoceramic is first established. Finite element analysis is then utilized to study the parameters of adhesive, including thickness and shear modulus. Experiments are finally conducted to validate the proposed method, and results show the piezoceramic sensors still work well when they are bonded on the host structures with tensile strain up to 4000 με by using the optimal adhesive.

The Performance of a Surface-bonded PZT Sensor Based Rocket Tanks SHM System Under the Cryogenic Temperature Operating Environment

A series of tests have been conducted to determine the survivability and functionality of a PZT sensor-based guide wave structural health monitoring (GWSHM) technology under the operating conditions of typical liquid rocket tanks such as cryogenic temperature. The performance of a typical piezoelectric sensor (PZT-5A) and different low temperature adhesives under cryogenic temperature was first investigated. The GWSHM sensor system for liquid rocket tanks was exposed to flight various environments from room temperature (10℃) to cryogenic temperature (- 180℃) to evaluate the physical robustness of the sensor- adhesives system as well as operational survivability and functionality. Test results demonstrated that the developed sensors system can withstand operational levels of vibration and shock energy on a representative rocket tanks duct assembly, and is functional under the combined cryogenic temperature and vibration environment. doi: 10.12783/SHM2015/365