Craig S. Long

Long range guided wave defect monitoring in rail track

A guided wave ultrasound system was previously developed for monitoring rail track used on heavy duty freight lines. This system operates by transmitting guided waves between permanently installed transmit and receive transducers spaced approximately 1km apart. The system has been proven to reliably detect rail breaks without false alarms. While cracks are sometimes detected there is a trade - off between detecting cracks and the possibility of false alarms. Adding a pulse-echo mode of operation to the system could provide increased functionality by detecting, locating and possibly monitoring cracks. This would require an array of transducers to control the direction and mode of propagation and it would be necessary to detect cracks up to a range of approximately 500 m in either direction along the rail. A four transducer array was designed and full matrix capture was used for field measurements. Post processing of the signals showed that a thermite weld could be detected at a range of 790m from the trans...

A piezoelectric deformable mirror for intra-cavity laser adaptive optics

This paper describes the development of a deformable mirror to be used in conjunction with diffractive optical elements inside a laser cavity. A prototype piezoelectric unimorph adaptive mirror was developed to correct for time dependent phase aberrations to the laser beam, such as those caused by thermal expansion of materials. The unimorph consists of a piezoelectric disc bonded to the back surface of a copper reflective mirror. The rear electrode of the piezoelectric ceramic disc is divided into segments so that a number of different control voltages can be applied to deform the mirror in a desired displacement distribution. The mirror is required to be able to deform in the shape of each of the lower order Zernike polynomials, which describe aberrations in optical systems. A numerical model of the device was used to determine a suitable electrode configuration. Finally, the device was constructed and the deformed shapes measured using a laser vibrometer.

Prediction of guided wave scattering by defects in rails using numerical modelling

A guided wave based monitoring system for welded freight rail, has previously been developed. The current arrangement consists of alternating transmit and receive stations positioned roughly 1 km apart, and is designed to reliably detect complete breaks in a rail. Current research efforts are focused on extending this system to include a pulse-echo mode of operation in order to detect, locate, monitor and possibly characterize damage, before a complete break occurs. For monitoring and inspection applications, it is beneficial to be able to distinguish between scattering defects which do not represent damage (such as welds) and cracks which could result in rail breaks. In this paper we investigate the complex interaction between selected propagating modes and various weld and crack geometries in an attempt to relate scattering behaviour to defect geometry. An efficient hybrid method is employed which models the volume containing the defect with conventional solid finite elements, while the semi-infinite in...

Time domain simulation of piezoelectric excitation of guided waves in rails using waveguide finite elements

Piezoelectric transducers are commonly used to excite waves in elastic waveguides such as pipes, rock bolts and rails. While it is possible to simulate the operation of these transducers attached to the waveguide, in the time domain, using conventional finite element methods available in commercial software, these models tend to be very large. An alternative method is to use specially formulated waveguide finite elements (sometimes called Semi-Analytical Finite Elements). Models using these elements require only a two-dimensional finite element mesh of the cross-section of the waveguide. The waveguide finite element model was combined with a conventional 3-D finite element model of the piezoelectric transducer to compute the frequency response of the waveguide. However, it is difficult to experimentally verify such a frequency domain model. Experiments are usually conducted by exciting a transducer, attached to the waveguide, with a short time signal such as a tone-burst and measuring the response at a position along the waveguide before reflections from the ends of the waveguide are encountered. The measured signals are a combination of all the modes that are excited in the waveguide and separating the individual modes of wave propagation is difficult if there are numerous modes present. Instead of converting the measured signals to the frequency domain we transform the modeled frequency responses to time domain signals in order to verify the models against experiment. The frequency response was computed at many frequency points and multiplied by the frequency spectrum of the excitation signal, before an inverse Fourier transform was used to transform from the frequency domain to the time domain. The time response of a rail, excited by a rectangular piezoelectric ceramic patch, was computed and found to compare favorably with measurements performed using a laser vibrometer. By using this approach it is possible to determine which modes of propagation dominate the response and to predict the signals that would be obtained at large distances, which cannot be measured in the lab, and would be computationally infeasible using conventional finite element modeling.

A thermo-hydraulic wax actuation system for high force and large displacement applications

An actuation system, making use of paraffin wax as a smart material, has been developed for high force, large displacement applications. Wax actuators exploit the significant volumetric expansion (typically between 10 and 15%) experienced during the solid to liquid phase change of paraffin wax. When contained, this expansion results in considerable hydrostatic pressure. Traditionally, wax actuators are designed such that the wax acts directly, via a compliant seal, on an output device such as a piston. We propose using an additional intermediate (passive) fluid to transmit pressure to a separate remote actuator. In essence, we propose a solid-state pump for hydraulic actuation, with no moving parts and which requires no maintenance. The pump makes use of paraffin wax pellets, submerged in hydraulic fluid. The pellets are encapsulated in silicone rubber to prevent contamination of the hydraulic fluid. Upon melting, the volumetric expansion is used to displace the hydraulic working fluid, which is in turn used to drive a conventional hydraulic actuator. Making use of only 65g of paraffin wax, heated from room temperature to 80ºC, the pump generated a blocked pressure of 45MPa and displaced 15.7ml of hydraulic fluid. The pump was used to drive a commercial actuator, and achieved a free stroke of 24.4mm and a blocked force of approximately 29kN.

SAFE-3D analysis of a piezoelectric transducer to excite guided waves in a rail web

Our existing Ultrasonic Broken Rail Detection system detects complete breaks and primarily uses a propagating mode with energy concentrated in the head of the rail. Previous experimental studies have demonstrated that a mode with energy concentrated in the head of the rail, is capable of detecting weld reflections at long distances. Exploiting a mode with energy concentrated in the web of the rail would allow us to effectively detect defects in the web of the rail and could also help to distinguish between reflections from welds and cracks. In this paper, we will demonstrate the analysis of a piezoelectric transducer attached to the rail web. The forced response at different frequencies is computed by the Semi-Analytical Finite Element (SAFE) method and compared to a full three-dimensional finite element method using ABAQUS. The SAFE method only requires the rail track cross-section to be meshed using two-dimensional elements. The ABAQUS model in turn requires a full three-dimensional discretisation of th...

Laser vibrometer measurement of guided wave modes in rail track

The ability to measure the individual modes of propagation is very beneficial during the development of guided wave ultrasound based rail monitoring systems. Scanning laser vibrometers can measure the displacement at a number of measurement points on the surface of the rail track. A technique for estimating the amplitude of the individual modes of propagation from these measurements is presented and applied to laboratory and field measurements. The method uses modal data from a semi-analytical finite element model of the rail and has been applied at frequencies where more than twenty propagating modes exist. It was possible to measure individual modes of propagation at a distance of 400 m from an ultrasonic transducer excited at 30 kHz on operational rail track and to identify the modes that are capable of propagating large distances.

Variable flattened Gaussian beam order selection by dynamic control of an intracavity diffractive mirror

In this paper we present the design of an optical resonator that produces as the stable transverse mode a flattened Gaussian laser beam by making use of an intra-cavity diffractive mirror. We consider the modal build-up in such a resonator and propose the required dynamic changes to an intra-cavity piezoelectric unimorph mirror for selecting the flattened Gaussian beam order of the stable mode. The feasibility of using a deformable diffractive mirror is demonstrated numerically. An optimization approach is employed to determine the optimal voltage distribution required to deform the mirror into a prescribed shape for the selection of the flattened Gaussian beam order. Good agreement between an ideal static diffractive mirror and the proposed adaptive mirror is achieved.

Large scale implementation of guided wave based broken rail monitoring

A guided wave ultrasound system has been developed over the past 17 years to detect breaks in continuously welded rail track. Installation of the version 4 system on an 840 km long heavy duty freight line was conducted between January 2013 and June 2014. The system operates in pitch – catch mode with alternate transmit and receive transducers spaced approximately 1km apart. If the acoustic signal is not received at the receive station an alarm is triggered to indicate a break in the rail between the transmit station and the receive station. The system is permanently installed, powered by solar panels and issues broken rail alarms using the GSM network where available, and digital radio technology in other areas. A total of 931 stations were installed and the entire length of rail is interrogated every fifteen minutes. The system operates reliably although some problems involving unreliable GSM communication and theft of solar panels have been experienced. In the first two months of operation four broken r...

Semi-Analytical Finite Element Analysis of the Influence of Axial Loads on Elastic Waveguides

Guided wave ultrasound is a promising technology for non-destructive inspection and monitoring of long slender structures such as pipes and rails. These structures are effectively one-dimensional waveguides and therefore a large length of the structure can be inspected from a single transducer location. A good introduction to guided wave inspection is available in (Rose, 2002). During the design of guided wave inspection systems it is advantageous to understand the modes of propagation, to predict how these modes interact with the damage to be detected and to be able to transmit and receive these modes independently (Lowe et al., 1998). Analytical solutions for wave propagation in circular cylinders are well known (Gazis, 1959) and may be used when developing pipe monitoring systems. When the cross-sectional geometry is more complex, such as in the case of rails, it becomes necessary to employ numerical solutions.