Gareth Pierce

Non-contact measurement of the mechanical properties of materials using an all-optical technique

Describes an optically-based measurement mechanism which realizes a totally noncontact assessment of the most important mechanical properties of structural materials - namely effective stiffness and Poisson ratio. These parameters are sensitive indicators of material integrity. The technique uses laser generated broadband ultrasound as a probe and interferometric optical detection as the detector again exploiting the broadband capability of optics in both space and time. Both detection and excitation systems are most conveniently realized in practical systems through optical fiber linkages. Observing the coupled waveforms between source and detector as a function of source: detector separation after a space : time Fourier transform yields a set of dispersion curves for the ultrasonic (typically Lamb wave) transfer function of the sample. This, in turn, can be inverted using curve fitting routines to obtain effective values of modulus and stiffness. An initial assessment of this inversion process is presented and demonstrates that the effective modulus can be extracted with a confidence level of better than a few percent with slightly larger errors in the Poisson ratio.

Golay code modulation in low-power laser-ultrasound ☆

The current work presents a correlation-based detection technique with application in modulated laser-ultrasonics. In standard use of coded sequences the impulse response of a system is recovered in the time domain with improved signal to noise ratio (SNR). The presented method is an extension of this technique, where the response to a chirped waveform is restored with improved SNR; hence, the response is in a well-defined frequency range. To achieve this goal the chirped waveforms are modulated by Golay codes. It will be shown that the response to this bandlimited carrier waveform can be recovered in the time domain with improved signal to noise ratio using a cross-correlation technique. Improvement in the SNR is discussed analytically and it is shown that this improvement is proportional to the square root of the length of the applied sequences. Experimental applications in laser-ultrasound are shown using modulated laser diodes as excitation sources with an output power of ∼1W. In the experiments a plate with a thickness of 50μm is investigated using Lamb waves in the MHz range to confirm the predicted improvement in the SNR. Golay codes with three different lengths were used with 7, 9 and 11 bits resulting in 2(7)=128, 2(9)=512, and 2(11)=2048 repetitions in an individual signal, respectively. The predicted improvements of 2 in the SNR between the 7 and 9 bits, and between the 9 and 11 bits waveforms, respectively, were well approximated by the experimentally obtained values of 1.83 and 2.17. As Lamb wave dispersion curves can be used for the characterization of plates or layered samples by inverse problems, it is also shown that by using multiple measurement points the recovered waveforms can be utilized in the evaluation of the dispersion relation.

RELIABLE CRACK DETECTION IN TURBINE BLADES USING THERMOSONICS: AN EMPIRICAL STUDY

Excitation generated by ultrasonic horns typically used in thermosonics (or Sonic IR) is non‐reproducible, raising concerns that cracks in some locations can be missed. This paper presents an empirical study on the thermosonic inspection of turbine blades. The objective is to assess the reliability of thermosonics as an NDT screening method for findings crack in turbine blades. First, a study was carried out to establish the operating parameters that generated the highest possible temperature rise from a crack. Next, a repeatability study was conducted to measure consistency of results in 300 tests, which showed 100% repeatability. Finally, 60 cracked blades were inspected, with the known cracks in 57 blades detected. These results show the potential of thermosonics as a reliable NDT screening method for finding cracks in turbine blades.

Features in geometric receiver shapes modelling bat-like directivity patterns

The directional properties of bat ears as receivers is a current area of interest in ultrasound research. This paper presents a new approach to analyse the relationship between morphological features and acoustical properties of the external ear of bat species. The beam pattern of Rousettus leschenaultiis right ear is measured and compared to that of receiver structures whose design is inspired by the bat ear itself and made of appropriate geometric shapes. The regular shape of these receivers makes it possible to control the key reception parameters and thus to understand the effect on the associated beam pattern of the parameters themselves. Measurements show one receiver structure has a beam pattern very similar to that of R. leschenaultiis ear, thus explaining the function of individual parts constituting its ear. As it is applicable to all bat species, this approach can provide a useful tool to investigate acoustics in bats, and possibly other mammals.

A Method for Acoustic Emission Source Identification Based on Optimisation

When a metal or composite structure begins to fail, for example due to high cycle fatigue, acoustic emissions caused by the propagation of cracks give rise to bursts of ultrasonic waves travelling through the structure. The health of a structure can be monitored by means of sensors which detect these waves. Acoustic emissions are often generated in experiments by breaking a pencil lead against the surface of the structure in a standardised way but the forces that this imparts are not well understood at present. A Local Interaction Simulation Approach (LISA) algorithm has been implemented to simulate the propagation of ultrasonic waves. This code has been validated against experiments in previous work and has been shown to accurately reproduce the propagation of Lamb waves (including reflections and dispersion etc.) within thin-plate like structures. This paper deals with the use of the LISA code to characterise the forces associated with standard pencil lead breaks. The displacement due to waves emanating from a break is measured and a Differential Evolution (DE) optimisation scheme is used to find the optimal profile of forcing to match the simulation with experiment.

Automatic ultrasonic robotic array

A novel, autonomous reconfigurable ultrasonic phased array inspection robot for non-destructive evaluation (NDE) is presented. The robotic system significantly will reduce manual labor over current inspection regimes, as well as enabling inspection of inaccessible/hazardous areas such as those found in the nuclear and petrochemical industries. It will offer three quantitative benefits: improved inspection accuracy, improved safety and reduced inspection costs. The current major innovation is in embedding ultrasonic phased array technology into a small form-factor robotic vehicle, overcoming issues in ultrasonic coupling, miniaturized electronics and robot positioning. This paper presents an overview of the robot specification and system architecture along with details of a specific inspection scenario where the robot is required to inspect a saddle weld found in reheat bifurcation. This weld is formed from the intersection of two 60 mm thick steel pipes with diameters 500 and 300 mm. The robot will be capable of tracking the weld from either pipe, projecting an ultrasonic beam normal to the direction of travel. The design of a 2 MHz, 16 element embedded phased array controller is presented. A timing model of the controller details the throughput required to enable the robot to perform ultrasonic inspection while tracking the weld at 20 mm/s. The paper also considers robot positional estimation. The nature of the inspection prohibits the use of external positioning systems so the system is limited to on-board sensors, namely wheels encoders, a six axis inertial sensor and a surface feature tracking camera. The results section focuses on the characterization of inspection performance, driven in part by the ultrasonic phased array controller and robot positional estimation. A-Scans are presented to show the SNR of each array channel which was approximately 24 dB when measuring the back wall echo. It is shown that ultrasonic scan rate is limited by 802.11g wireless transmission from the robot to the host computer.

3D model generation using an airborne swarm

Using an artificial kinematic field to provide co-ordination between multiple inspection UAVs, the authors herein demonstrate full 3D modelling capability based on a photogrammetric system. The operation of the system is demonstrated by generating a full 3D surface model of an intermediate level nuclear waste storage drum. Such drums require periodic inspection to ensure that drum distortion or corrosion is carefully monitored. Performing this inspection with multiple airborne platforms enables rapid inspection of structures that are inaccessible to on-surface remote vehicles and are in human-hazardous environments. A three-dimensional surface-meshed model of the target can then be constructed in post-processing through photogrammetry analysis of the visual inspection data. The inspection environment uses a tracking system to precisely monitor the position of each aerial vehicle within the enclosure. The vehicles used are commercially available Parrot AR. Drone quadcopters, controlled through a computer i...

Conformable eddy current array delivery

The external surface of stainless steel containers used for the interim storage of nuclear material may be subject to Atmospherically Induced Stress Corrosion Cracking (AISCC). The inspection of such containers poses a significant challenge due to the large quantities involved; therefore, automating the inspection process is of considerable interest. This paper reports upon a proof-of-concept project concerning the automated NDT of a set of test containers containing artificially generated AISCCs. An Eddy current array probe with a conformable padded surface from Eddyfi was used as the NDT sensor and end effector on a KUKA KR5 arc HW robot. A kinematically valid cylindrical raster scan path was designed using the KUKA|PRC path planning software. Custom software was then written to interface measurement acquisition from the Eddyfi hardware with the motion control of the robot. Preliminary results and analysis are presented from scanning two canisters.

An automated miniature robotic vehicle inspection system

A novel, autonomous reconfigurable robotic inspection system for quantitative NDE mapping is presented. The system consists of a fleet of wireless (802.11g) miniature robotic vehicles, each approximately 175 × 125 × 85 mm with magnetic wheels that enable them to inspect industrial structures such as storage tanks, chimneys and large diameter pipe work. The robots carry one of a number of payloads including a two channel MFL sensor, a 5 MHz dry coupled UT thickness wheel probe and a machine vision camera that images the surface. The system creates an NDE map of the structure overlaying results onto a 3D model in real time. The authors provide an overview of the robot design, data fusion algorithms (positioning and NDE) and visualization software.

The application of finite element analysis to investigate the vibrational response of a turbine blade under thermosonic excitation

This paper presents a methodology, using a combination of experimental vibration measurements and finite element analysis (FEA), to model the vibrational energy within a turbine blade corresponding to a typical thermosonic inspection scenario. Laser vibrometry measurements were used to determine the steady-state vibration response at several locations on a blade and used to identify the prominent spectral components. These were then used to generate an excitation function for the FEA approach. After validation of the FEA model, the vibration response across the whole blade was simulated. Finally, the predicted displacement field was used to determine the vibrational energy at every point on the blade which was mapped onto a CAD representation of the blade, thereby highlighting areas on the blade that may be below the defect detection threshold.