Claudio Nucera

Monitoring load levels in multi-wire strands by nonlinear ultrasonic waves

Monitoring load levels in multi-wire steel strands is relevant to ensuring the proper structural performance of post-tensioned concrete structures, suspension bridges, and cable-stayed bridges. This article investigates the use of ultrasonic nonlinearity as a means to determine the level of load applied to the strands. Since an axial load on a multi-wire strand generates proportional contact stresses between adjacent wires, ultrasonic nonlinearity from the inter-wire contact must be related to the level of axial load. This article shows that the higher harmonic generation of ultrasonic guided waves propagating in individual wires of the strand indeed changes monotonically with the applied load, with smaller higher harmonic amplitudes with increasing load levels. This trend is consistent with known studies on higher harmonic generation from ultrasonic plane waves incident on a contact interface under a changing contact pressure. The article presents experimental studies on free strands and embedded strands, and numerical studies (nonlinear Finite Element Analysis) on free strands.

Nondestructive measurement of neutral temperature in continuous welded rails by nonlinear ultrasonic guided waves

Modern rail construction uses continuous-welded rail (CWR). The presence of very few joints leads to an increasing concern due to the large longitudinal loads caused by restrained thermal expansion and contraction, following seasonal temperature variations. The knowledge of the current state of thermal stress in the rail or, equivalently, the rail neutral temperature (corresponding to zero net longitudinal force) is a key need within the railroad transportation community in order to properly schedule slow-order mandates and prevent derailments. This paper presents a nondestructive diagnostic system for measurement of the neutral temperature in CWR based on nonlinear ultrasonic guided waves. The theoretical part of the study involved the development of a constitutive model in order to explain the origin of nonlinear effects arising in complex waveguides under constrained thermal expansion. A numerical framework has been implemented to predict internal resonance conditions of nonlinear waves in complex waveguides. This theoretical/numerical phase has led to the development of an experimental prototype (Rail-NT) that was tested both in the laboratory and in the field. The results of these experimental tests are also summarized.

Health Monitoring of Prestressing Tendons in Posttensioned Concrete Bridges

Currently 90% of bridges built in California are posttensioned box girder. In such structures, the steel tendons are the main load-carrying components. The loss of prestress, as well as the presence of defects or tendon breakage, can be catastrophic for the entire structure. No well-established method monitors prestressing tendons that can provide simultaneous information related to the presence of defects and the level of prestress in a continuous, real-time manner. If such a monitoring system were available, considerable savings would be achieved in bridge maintenance because repairs would be implemented in a timely manner without traffic disruptions. This paper presents a health-monitoring system for prestressing tendons in posttensioned structures of interest to the California Department of Transportation. Such a system uses guided ultrasonic waves and embedded sensors to measure the level of applied prestress and to detect defects at early growth stages simultaneously and in real time. The results of tests on a large-scale, single-tendon posttensioned joint specimen, subjected to multiple load cycles, are presented to validate the monitoring of prestress loads (through nonlinear ultrasonic probing) and the monitoring of damage progression and location (through acoustic emission techniques).

Nonlinear Semianalytical Finite-Element Algorithm for the Analysis of Internal Resonance Conditions in Complex Waveguides

AbstractResearch efforts on nonlinear guided wave propagation have increased dramatically in the last few decades because of the high sensitivity of nonlinear waves to structural conditions (defects, quasi-static loads, instability conditions, and so on). However, the mathematical framework governing the nonlinear guided wave phenomena becomes extremely challenging in waveguides that are complex in either materials (damping, anisotropy, heterogeneous, etc.) or geometry (multilayers, geometric periodicity, etc.). The present work develops predictions of nonlinear second-harmonic generation in complex waveguides by implementing a semianalytical finite-element formulation that accounts for material nonlinearities into a highly flexible, yet very powerful, commercial finite-element code. Once formulated correctly, the proposed analysis can easily take into account damping effects, anisotropic multilayered properties, periodic geometries, and other complex waveguide properties in a computational efficient and ...

Higher-Harmonic Generation Analysis in Complex Waveguides via a Nonlinear Semianalytical Finite Element Algorithm

Propagation of nonlinear guided waves is a very attracting phenomenon for structural health monitoring applications that has received a lot of attention in the last decades. They exhibit very large sensitivity to structural conditions when compared to traditional approaches based on linear wave features. On the other hand, the applicability of this technology is still limited because of the lack of a solid understanding of the complex phenomena involved when dealing with real structures. In fact the mathematical framework governing the nonlinear guided wave propagation becomes extremely challenging in the case of waveguides that are complex in either materials (damping, anisotropy, heterogeneous, etc.) or geometry (multilayers, geometric periodicity, etc.). The present work focuses on the analysis of nonlinear second-harmonic generation in complex waveguides by extending the classical Semianalytical Finite Element formulation to the nonlinear regime, and implementing it into a powerful commercial Finite Element package. Results are presented for the following cases: a railroad track and a viscoelastic plate. For these case-studies optimum combinations of primary wave modes and resonant double-harmonic nonlinear wave modes are identified. Knowledge of such combinations is critical to the implementation of structural monitoring systems for these structures based on higher-harmonic wave generation.

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.

Impact monitoring in stiffened composite aerospace panels by wave propagation

The technique of piezoelectric rosettes has been developed for the location of acoustic emission sources. Since the rosette-based acoustic emission source location algorithm is not dependent on the wave velocity, the method lends itself to monitoring complex composite structures where the velocity can change with wave propagation direction (due to anisotropy) or along the wave propagation path (due to tapered and multilayered geometries). This article presents the acoustic emission source location results from “blunt” impact tests conducted on a curved carbon fiber–reinforced plastic composite panel with co-cured stiffeners and bolted shear ties of overall dimensions of 0.9 m × 0.9 m. The results of these tests are of interest to follow the evolution of damage during the impact event. In addition, an impact force identification procedure is presented. The procedure is a model-based approach that minimizes the difference between predicted and experimental acoustic emission waves generated by the impact. The numerical procedure is tested favorably against the output of a hammer impact on a stiffened carbon fiber–reinforced plastic panel.

Ultrasonic Guided Wave Monitoring of Railroad Tracks

Among structural concerns for the safety of rail transportation are internal flaws and thermal stresses, both of which can cause disruption of service and even derailments. Ultrasonic guided waves lend themselves to addressing both of these problems. This paper reports on two inspection systems for rails being developed at UCSD under the auspices of the US Federal Railroad Administration. Both systems utilize ultrasonic guided waves as the main probing mechanism, for the two different applications of flaw detection and thermal stress detection.

System for In Situ Measurement of Neutral Temperature in Continuous-Welded Rail: Results from Laboratory and Field Tests

Research has been focused on developing a system for in situ measurement of stress in continuous-welded rail to use as a basis for the determination of neutral temperature (NT). Continuous-welded rail is known to break in cold weather and buckle in hot weather because of thermally induced stresses. The need for a reliable technique for determination of NT (rail temperature with zero thermal stress) has been an ongoing challenge for railroads since the advent of continuous-welded rail more than 40 years ago. Railroads need to know the level of stress in the rail to schedule slow-order mandates properly and prevent derailments. A prototype (Rail-NT) was developed for wayside rail NT measurement based on nonlinear ultrasonic guided waves. Numerical models were first developed to identify optimum guided wave modes and frequencies for maximum sensitivity to the thermal stresses in the rail web, with minimal influence of the rail head and rail foot. Experiments indicated a rail NT measurement accuracy of a few degrees. The first field tests of the Rail-NT prototype were performed in June 2012 at the Transportation Technology Center in Pueblo, Colorado, in collaboration with the Burlington Northern Santa Fe Railway. The results of these field tests were encouraging; the tests indicated an accuracy for NT measurement of 5°F at worst on both wood and concrete ties. One of the issues that remains to be investigated is the effect of passing trains on the rail NT measurements.

Nonlinear ultrasonic guided waves for prestress level monitoring in prestressing strands for post-tensioned concrete structures

Monitoring load levels in multi-wire steel strands is crucial to ensuring the proper structural performance of post-tensioned concrete structures, suspension bridges and cable-stayed bridges. The post-tensioned box-girder structural scheme is widely used in several bridges, including 90% of the California inventory. In this structural typology, prestressing tendons are the main load-carrying components. Therefore loss of prestress as well as the presence of structural defects (e.g. corrosion and broken wires) affecting these elements are critical for the performance of the entire structure and may conduct to catastrophic failures. Unfortunately, at present there is no well-established methodology for the monitoring of prestressing (PS) tendons able to provide simultaneous and continuous information about the presence of defects as well as prestress levels. In this paper the authors develop a methodology to assess the level of load applied to the strands through the use of ultrasonic nonlinearity. Since an axial load on a multi-wire strand generates proportional contact stresses between adjacent wires, ultrasonic nonlinearity from the inter-wire contact must be related to the level of axial load. The work presented shows that the higher-harmonic generation of ultrasonic guided waves propagating in individual wires of the strand varies monotonically with the applied load, with smaller higher-harmonic amplitudes with increasing load levels. This trend is consistent with previous studies on higher-harmonic generation from ultrasonic plane waves incident on a contact interface under a changing contact pressure. The paper presents the results of experimental researches on free strands and embedded strands, and numerical studies (nonlinear Finite Element Analysis) on free strands.