Huidong Gao

Active health monitoring of an aircraft wing with embedded piezoelectric sensor/actuator network: I. Defect detection, localization and growth monitoring

This work focuses on an ultrasonic guided wave structural health monitoring (SHM) system development for aircraft wing inspection. In part I of the study, a detailed description of a real aluminum wing specimen and some preliminary wave propagation tests on the wing panel are presented. Unfortunately, strong attenuation and scattering impede guided waves for large-area inspection. Nevertheless, small, low-cost and light-weight piezoelectric (PZT) discs were bonded to various parts of the aircraft wing, in a form of relatively sparse arrays, for simulated cracks and corrosion monitoring. The PZT discs take turns generating and receiving ultrasonic guided waves. Pair-wise through-transmission waveforms collected at normal conditions served as baselines, and subsequent signals collected at defected conditions such as rivet cracks or corrosion detected the presence of a defect and its location with a novel correlation analysis based technique called RAPID (reconstruction algorithm for probabilistic inspection of defects). The effectiveness of the algorithm was tested with several case studies in a laboratory environment. It showed good performance for defect detection, size estimation and localization in complex aircraft wing structures.

A comparison of embedded sensor Lamb wave ultrasonic tomography approaches for material loss detection

Computerized tomography (CT) algorithms have been used mainly in the medical field but their powerful capabilities are being exploited more and more in industrial applications. This paper demonstrates that the technology is capable of detecting material loss on real aircraft components using embedded piezoelectric sensors on hidden surfaces. The work is novel in more than one respect. Firstly, it demonstrates that Lamb wave ultrasonic tomography can be used to accurately map material loss on an exposed aircraft surface with sensors embedded on the structures hidden surface. Hidden, in this case, refers to the surface that is not exposed to the atmosphere—the underneath of an aircraft wing, for example. Secondly, it compares tomographic images generated by fan-beam back projection and the signal difference coefficient methods, showing clearly that the latter are more sensitive to material loss.

Guided Wave Tomography on an Aircraft Wing with Leave in Place Sensors

Leave in place sensor and guided wave tomographic techniques are combined together in structural health monitoring aging aircraft. In a sample problem, eight PZT transducers are mounted on the surface of the wing of an E2 aircraft. An area within a circle of 240 mm diameter is monitored with through transmission guided waves. The occurrence and progression of an artificial defect beside one rivet is clearly detected. Different computed tomography (CT) algorithms such as the back projection method, algebraic reconstruction method (ART) and multi resolution algebraic reconstruction technology (MART) are used in the defect image reconstruction with the feature of signal difference coefficient (SDC).

Ultrasonic Sensor Placement Optimization in Structural Health Monitoring Using Evolutionary Strategy

In structural health monitoring (SHM), sensor network scale and sensor distribution decisions are critical since sensor network performance and system cost are greatly affected. A quantitative sensor placement optimization method with covariance matrix adaptation evolutionary strategy (CMAES) is presented in this paper. A damage detection probability model is developed for ultrasonic guided wave sensor networks. Two sample problems are presented in this paper. One is for structure with irregular damage distribution probability, and the other is for an E2 aircraft wing section. The reliability of this genetic and evolutionary optimization method is proved in this study. Sensor network configurations with minimum missed‐detection probability are obtained from the results of evolutionary optimization. The tradeoff relationship between optimized sensor network performance and the number of sensors is also presented in this paper.

Sensor placement optimization in structural health monitoring using genetic and evolutionary algorithms

An optimized sensor design and sensor placement strategy will be extremely beneficial to both safety ensuring and cost reduction considerations of structural health monitoring systems. A new framework for structural health monitoring sensor placement optimization was recently developed at Pennsylvania State University based on genetic and evolutionary computation. The formulation of the optimization problem is to minimize the damage misdetection rate as well as to minimize the number of sensors by searching the optimized patterns of sensor placement topology on the feasible region of the structure being monitored. Two types of SHM sensors are considered. One is a single sensor scenario; the other is an actuator-damage-sensor scenario. The program was applied to a sample sensor placement problem of an aging aircraft wing. Optimized sensor placement designs are obtained. The tradeoff relationship between the sensor performance, sensor number, and the overall sensor network performance are also presented in this paper.

Goodness dispersion curves for ultrasonic guided wave based SHM: a sample problem in corrosion monitoring

Ultrasonic guided wave techniques have great potential for structural health monitoring applications. Appropriate mode and frequency selection is the basis for achieving optimised damage monitoring performance. In this paper, several important guided wave mode attributes are introduced in addition to the commonly used phase velocity and group velocity dispersion curves while using the general corrosion problem as an example. We first derive a simple and generic wave excitability function based on the theory of normal mode expansion and the reciprocity theorem. A sensitivity dispersion curve is formulated based on the group velocity dispersion curve. Both excitability and sensitivity dispersion curves are verified with finite element simulations. Finally, a goodness dispersion curve concept is introduced to evaluate the tradeoffs between multiple mode selection objectives based on the wave velocity, excitability and sensitivity.

Ultrasonic Guided Wave Annular Array Transducers for Structural Health Monitoring

The work presented in this paper utilizes the physics of guided wave propagation for structural health monitoring (SHM) transducer designs. Both the theoretical and experimental studies illustrated the importance of guided wave mode selection for SHM applications. Guided wave mode control is realized with an annular array transducer design on a PVDF polymer piezoelectric film. A sample problem on a 1mm thick aluminum plate is presented. Numerical calculations of the wave structures and guided wave power flow distribution inside the plate provide quick guidelines for the wave mode selection in structural health monitoring. Experimental study illustrates the importance of mode control with the comparison of PVDF annular array transducers and PZT ceramic disc transducers. The characteristics of wave mode reflections to defect depth and the defect sizing effect are also discussed in this paper.

Determination of density distribution in powder compacts using ultrasonic tomography

Laser-induced ultrasonic bulk wave tomography is used for density variation determination of powder metal compacts. A laser beam is used to excite ultrasonic energy, and the signals passing through the specimen are received by an air-coupled transducer. The density variations of powder metal compacts can be determined directly by the cross-sectional tomographic images of slowness obtained by using a filtered, backprojection algorithm based on measured time of flights. Interpolations with respect to sample and projection angles are used to generate the input data required for displaying a well-balanced, reconstructed image to reduce the aliasing distortions caused by insufficient input data. Results of presintered cylindrical ferrous powdered samples show that this novel approach makes the reconstruction process more cost effective than the very tedious, time-consuming, and inaccurate metallographic methods, thus making it a potentially powerful tool for studying manufacturing processes through significant parameters to obtain a more uniform density distribution.

Flexible ultrasonic guided wave sensor development for structural health monitoring

Sensor development and signal processing are two key issues in structural health monitoring (SHM). A process of PVDF annular array sensor design via guided ultrasonic wave propagation, excitation, and wave damage interaction modeling is presented in this paper. A sample problem to monitor the occurrence and progression of simulated corrosion damage in an aluminum plate is studied. An effective guided wave mode for easy corrosion depth assessment was selected based on guided wave propagation analysis. Three dimensional finite element method (FEM) analyses were performed to study the wave field excited from PVDF annular arrays and sectioned annular arrays in the aluminum plate. Annular arrays with enhanced mode selection capabilities were permanently bonded to a 1mm aluminum plate. Simulated corrosion damage with progressive corrosion depths was successfully detected in the structure using wave mode based signal analysis and feature extraction. The relation between the damage depth and the reflection wave amplitude from the signal were studied with both numerical simulation and experiments.

Impact of ultrasonic guided wave transducer design on health monitoring of composite structures

Structural health monitoring of composite materials will lead to a significant safety and economic impact on the aircraft and aerospace industries. Ultrasonic guided wave based methods are becoming popular because of an excellent compromise between coverage area and sensitivity for localized damage detection. The transducers currently used in composite health monitoring are designed mostly in an empirical manner. The work presented in this paper provides an analytical procedure to study the wave excitation phenomenon in composite laminates. A hybrid semi-analytical finite element method and global matrix method is used to obtained the guided wave modal solutions. A normal mode expansion technique is then used to simulate the guided waves excited from a surface mounted piezoelectric transducer with transient loading. Parametric studies are performed to obtain the guided wave mode tuning characteristics and to study the influence of piezoelectric wafer geometry on wave excitation. In an inverse problem, an appropriate loading pattern can be designed to achieve selective guided wave mode excitation for improved sensitivity and/or penetration power in the health monitoring of composites. A wave field reconstruction algorithm based on normal mode expansion is also introduced in this paper. This method is also very computationally efficient compared with the commonly used finite element method in wave field excitation simulation.