Salvatore Salamone

Guided-wave Health Monitoring of Aircraft Composite Panels under Changing Temperature

This study deals with the health monitoring of fiber-reinforced composite panels using ultrasonic guided waves and flexible piezocomposite transducer patches in a changing temperature environment corresponding to normal aircraft operations (—40°C to +60°C). The wave propagation problem is first studied analytically by a model that accounts for temperature effects on the transducer piezo-mechanical properties, the transducer-panel interaction, and the panel wave dispersion properties. Experiments are also conducted on a Carbon-fiber Reinforced Plastic (CFRP) [0/±45/0]S laminate subjected to the —40°C to +60°C temperature excursion. Both model and experiment indicate substantial changes in the detected guided wave amplitude solely due to the temperature excursion. The second part of the study presents an application to bond defect detection in a simulated CFRP skin-to-spar joint of Unmanned Aerial Vehicle wings. It is shown that a statistical outlier analysis based on multiple guided-wave amplitude features and on a baseline partition is effective in detecting bond defects (poorly cured adhesive and two sizes of disbonds) despite the —40°C to +60°C temperature change. The results encourage the development of a continuous health monitoring system for composite aircraft wings during their normal operations.

High-velocity Impact Location on Aircraft panels Using Macro-fiber Composite piezoelectric Rosettes

In this article, an approach based on an array of macro-fiber composite (MFC) transducers arranged as rosettes is proposed for high-velocity impact location on isotropic and composite aircraft panels. Each rosette, using the directivity behavior of three MFC sensors, provides the direction of an incoming wave generated by the impact source as a principal strain angle. A minimum of two rosettes is sufficient to determine the impact location by intersecting the wave directions. The piezoelectric rosette approach is easier to implement than the well-known time-of-flight-based triangulation of acoustic emissions because it does not require knowledge of the wave speed in the material. Hence, the technique does not have the drawbacks of time-of-flight triangulation associated to anisotropic materials or tapered sections. The experiments reported herein show the applicability of the technique to high-velocity impacts created with a gas-gun firing spherical ice projectiles.

Nonlinear ultrasonic guided waves for stress monitoring in prestressing tendons for post-tensioned concrete structures

Many bridges, including 90% of the California inventory, are post-tensioned box-girders concrete structures. Prestressing tendons are the main load-carrying components of these and other post-tensioned structures. Despite their criticality, much research is needed to develop and deploy techniques able to provide real-time information on the level of prestress in order to detect dangerous stress losses. In collaboration with Caltrans, UCSD is investigating the combination of ultrasonic guided waves and embedded sensors to provide both prestress level monitoring and defect detection capabilities in concrete-embedded PS tendons. This paper presents a technique based on nonlinear ultrasonic guided waves in the 100 kHz - 2 MHz range for monitoring prestress levels in 7-wire PS tendons. The technique relies on the fact that an axial stress on the tendon generates a proportional radial stress between adjacent wires (interwire stress). In turn, the interwire stress modulates nonlinear effects in ultrasonic wave propagation through both the presence of finite strains and the interwire contact. The nonlinear ultrasonic behavior of the tendon under changing levels of prestress is monitored by tracking higher-order harmonics at (nω) arising under a fundamental guided-wave excitation at (ω). Experimental results will be presented to identify (a) ranges of fundamental excitations at (ω) producing maximum nonlinear response, and (b) optimum lay-out of the transmitting and the receiving transducers within the test tendons. Compared to alternative methods based on linear ultrasonic features, the proposed nonlinear ultrasonic technique appears more sensitive to prestress levels and more robust against changing excitation power at the transmitting transducer or changing transducer/tendon bond conditions.

Load monitoring in multiwire strands by interwire ultrasonic measurements

Nearly 90% of the bridges in California are post-tensioned box-girders. Prestressing (PS) tendons are the main load-carrying components of these and other post-tensioned structures. Despite their criticality, much research is needed to develop and deploy techniques able to provide real-time information on the level of prestress and on the presence of structural defects (e.g. corrosion and broken wires) in the PS tendons. In collaboration with Caltrans, UCSD is investigating the combination of ultrasonic guided waves and embedded sensors as an approach to provide both prestress level monitoring and defect detection capabilities in concrete-embedded PS tendons. This paper will focus on the prestress level monitoring by first discussing the behavior of ultrasonic guided waves propagating in seven-wire, 0.6-in diameter twisted strands typically used in post-tensioned concrete structures. A semi-analytical finite element analysis is used to predict modal and forced wave solutions as a function of the applied prestress level. This analysis accounts for the changing inter-wire contact as a function of applied loads. A feature shown sensitive to load levels is the inter-wire energy leakage. In order to monitor such feature, the method uses low-profile piezoelectric sensors able to probe the individual, 0.2-in wires comprising the strand. Results of load monitoring in free and embedded strands during laboratory tests will be presented.

Noncontact Ultrasonic Guided-Wave System for Rail Inspection: Update on Project at University of California, San Diego

The University of California, San Diego (UCSD), with an FRA Office of Research and Development grant, is developing a system for high-speed and noncontact rail defect detection. A prototype was designed and field tested with the support of Volpe National Transportation Systems Center and ENSCO, Inc. The goal of this project was to develop a rail defect detection system that provided (a) better defect detection reliability (including internal transverse head defects under shelling and vertical split heads) and (b) higher inspection speed than achievable by current rail inspection systems. This effort was also in direct response to safety recommendations issued by the National Transportation Safety Board after the disastrous train derailments at Superior, Wisconsin, in 1992 and Oneida, New York, in 2007, among others. The UCSD prototype used noncontact ultrasonic probing of the rail head (laser and air-coupled sensors), ultrasonic guided waves, and a proprietary real-time statistical analysis algorithm that maximized the sensitivity to defects while it minimized false positives. The design allowed potential inspection speeds up to 40 mph, although to date all field tests were conducted up to 15 mph. This paper (a) summarizes the latest technology development test conducted at the rail defect farm of Herzog, Inc., in Saint Joseph, Missouri, in June 2010 and (b) describes the completion of the new rail defect farm facility at the UCSD Camp Elliott Field Station with partial in-kind donations from the Burlington Northern Santa Fe Railway.

Stress dependence of guided waves in rails

This paper presents numerical results on the dynamic behavior of continuously welded rails (CWR) subjected to a static axial stress. The results quantify the sensitivity of guided waves to stress variations and could be potentially used to estimate the stress level in CWR or alternatively the rail Neutral Temperature (stress free rail temperature). This work represents the initial concept phase of a research and development study funded by the Federal Railroad Administration. The ultimate objective of this study is to develop and test a prototype system that uses non-contact dynamic sensing to measure in-situ rail stress in motion, to determine rail Neutral Temperatures (NT) and the related Incipient Buckling Risks in CWR.

Noncontact Ultrasonic Guided Wave Detection of Rail Defects

Recent train accidents, increasing tonnage, and aging rail transportation infrastructure have reaffirmed the need to improve current rail inspection technologies, consisting primarily of ultrasonic wheel testing. A recent development in rail inspection is the use of ultrasonic guided waves in the 20 kHz to 1 MHz range and noncontact probing techniques. This paper first reports on theoretical studies of ultrasonic guided wave propagation in rails based on a semianalytical finite element approach. The paper then describes the latest version of the University of California, San Diego, and FRA rail defect detection prototype, which is based on noncontact guided wave testing and real-time statistical pattern recognition for defect detection and classification. The system specifically targets transverse head cracks such as transverse fissures and detail fractures. It is also expected to be sensitive to longitudinal head cracks such as vertical split heads and mixed-mode cracks such as compound fractures. The system was field tested in March 2008 at speeds of up to 10 mph with excellent results under changing environmental conditions. Plans are in place for further improvements, including higher test speeds of up to 40 mph and installation of the system in an FRA research car for technology demonstration.

Monitoring Prestress Level in Seven Wire Prestressing Tendons by Inter Wire Ultrasonic Wave Propagation

Researchers at UCSD are investigating, in collaboration with Caltrans, the combination of ultrasonic guided waves and embedded sensors as an approach to provide both prestress level monitoring and defect detection capabilities in concrete-embedded PS tendons. This paper will focus on the prestress level monitoring by first discussing the behavior of ultrasonic guided waves propagating in seven-wire, 15.2-mm diameter twisted strands typically used in post-tensioned concrete structures. A semianalytical finite element analysis is used to predict forced wave solutions as a function of the applied prestress level. A feature shown sensitive to load levels is the inter-wire energy leakage. In order to monitor such feature, piezoelectric sensors were experimentally employed to probe the individual, 5-mm wires comprising the strand. Results of load monitoring in embedded strands during laboratory tests will be presented and a statistical approach will be used to enhance the evaluation of prestress loss in the strands.

Impact force identification on isotropic and composite panels

Many studies have demonstrated for composite structures such as aerospace components the peak force of impacts can be correlated to damage. Hence, techniques able to estimate the impact forces are being investigated. The present paper deals specifically with the inverse problem of identification of impact forces on isotropic and composite panels given the dynamic response of Macro Fiber Composite (MFC) sensors bonded to the components. First a Semi Analytical Finite Element (SAFE) approach is employed to predict the frequency response and time history response of the MFC sensors assuming an impulse force excitation. Subsequently, the impact force is estimated updating the force time history assumed in the SAFE by minimizing the difference between numerical and experimental signals from MFC sensors. Such procedure has been used in isotropic and composite plates. Impact forces generated using impact hammers and different ice projectiles launched with gas cannon on panels have been identified.

Temperature effects in Lamb-wave structural health monitoring systems

There is a need to better understand the effect of temperature changes on the response of ultrasonic guided-wave pitchcatch systems used for Structural Health Monitoring. A model is proposed to account for all relevant temperaturedependent parameters of a pitch-catch system on an isotropic plate and a fiber-reinforced composite laminate, including the actuator-plate and plate-sensor interactions through shear-lag behavior, the piezoelectric and dielectric permittivity properties of the transducers, and the Lamb wave dispersion properties of the substrate plate. The model is used to predict the S0 response spectra in for the temperature range of -40°C to +60°C which accounts for normal aircraft operations. The transducers examined are flexible Macro-Fiber Composite type P1 patches. The study shows substantial changes in Lamb wave amplitude response caused solely by temperature excursions. It is also shown that, for the transducers considered, the response amplitude changes follow two opposite trends below and above ambient temperature (20°C), respectively. These results can provide a basis for the compensation of temperature effects in guided-wave damage detection systems.