Pierre Belanger

Guided wave diffraction tomography within the born approximation

Detection and sizing of corrosion in pipelines and pressure vessels over large, partially accessible areas is of growing interest in the petrochemical industry. Low-frequency guided wave diffraction tomography is a potentially attractive technique to rapidly evaluate the thickness of large sections of partially accessible structures. Finite element simulations of a 64-element circular array on a plate show that when the scattering mechanism of the object to be reconstructed satisfies the Born approximation, the reconstruction of the thickness is accurate. However, the practical implementation is more challenging because the incident field is not known. This paper describes the baseline subtraction approach commonly used in structural health monitoring applications and proposes a new approach in which the measurement of the incident field is not required when using a circular array of transducers. Experimental results demonstrate that ultimately the scattering from the array of transducers is a major source of error in the tomographic reconstruction, but when there is no scattering from the array of transducers the reconstructions are very similar to the finite element simulations.

Development of a low frequency omnidirectional piezoelectric shear horizontal wave transducer

Structural health monitoring (SHM) may offer an alternative to time based maintenance of safety critical components. Ultrasonic guided waves have recently emerged as a prominent option because their propagation carries information regarding the location, severity and types of damage. The fundamental shear horizontal ultrasonic guided wave mode has recently attracted interest in SHM because of its unique properties. This mode is not dispersive and has no attenuation due to fluid loading. In order to cover large areas using an SHM system, omnidirectional transduction is desired. Omnidirectional transduction of SH0 is challenging because of the required torsional surface stress. This paper presents a concept based on the discretisation of a torsional surface stress source using shear piezoelectric trapezoidal elements. Finite element simulation and experimental results are used to demonstrate the performance of this concept. The experimental modal selectivity is 17 dB and the transducer has a true omnidirectional behaviour.

FEASIBILITY OF LOW‐FREQUENCY STRAIGHT‐RAY GUIDED WAVE TOMOGRAPHY

Many aging pipelines and aircraft are suffering from corrosion and the corrosion patches are often inaccessible. There is therefore a need for a rapid, accurate, long range inspection technique to measure the remaining thickness in corrosion patches. Low‐frequency guided wave tomography is a potentially attractive technique to rapidly evaluate the thickness of large sections of partially accessible structures. This paper demonstrates that in the low‐frequency regime the ray theory may not be valid which compromises the use of any straight‐ray tomography algorithm. This paper also shows, in simulations and experimentally, that the same frequency regime can be used to successfully reconstruct thickness reduction in plates with diffraction tomography.

LAMB WAVE TOMOGRAPHY TO EVALUATE THE MAXIMUM DEPTH OF CORROSION PATCHES

The evaluation of the maximum depth of corrosion patches is a major issue in the oil and aerospace industries. In this context, time‐of‐flight Lamb wave tomography is a potentially attractive technique. The accuracy of the reconstruction depends on the guided modes and frequencies employed. The higher the dispersion of a mode the more sensitive it is to thickness changes. However for practical implementation it is very important to take the excitability, the detectability and the leakage attenuation into account. In this paper the most suitable point of operation for time‐of‐flight tomography is identified.

High order shear horizontal modes for minimum remnant thickness.

Thickness mapping in aging structures suffering from corrosion is challenging especially when the structure is only partially accessible. In plates the high order shear horizontal guided wave modes all have a cutoff frequency thickness product below which they cannot propagate. This property is potentially attractive to estimate the minimum remnant thickness between two transducers. When using a source and a sensor array it is possible to control the number of modes being excited and the size of the region interrogated by the technique. Finite element simulations were used to show that by exciting multiple guided wave modes simultaneously and identifying which modes are received by a sensor array it is possible to estimate the minimum remaining thickness along the propagation path. Initial experimental results showed excellent agreement with the finite element simulations when the plate is uniform and with a thickness reduction between the source and the sensor arrays the minimum remnant thickness was underestimated by approximately 20%.

Development of a low frequency shear horizontal piezoelectric transducer for the generation of plane SH waves

The shear horizontal guided wave fundamental mode (SH0) has the particularity of being the only non-dispersive plate guided wave mode. This characteristic makes this ultrasonic guided wave mode very attractive in non-destructive testing, facilitating signal processing for long range inspections. It is, however, difficult to generate only a single guided wave mode when using piezoelectric transduction. This work aims to develop a piezoelectric transducer capable of generating a virtually pure plane zeroth order shear horizontal wave. The chosen material was the PZT-5H for its dominant d15 piezoelectric constant, which makes it a perfect candidate for SH-wave generation. The transducer dimensions were optimised using an analytical model based on the Huygens’ principle of superposition and the dipole pattern of a shear point source. A 3D multiphysics finite element model was then used to validate the analytical model results. Experimental validation was finally conducted with a laser Doppler vibrometer (LDV) system. Excellent agreement between the analytical model, finite element model and experimental validation was seen.

The Feasibility of Structural Health Monitoring Using the Fundamental Shear Horizontal Guided Wave in a Thin Aluminum Plate

Structural health monitoring (SHM) is emerging as an essential tool for constant monitoring of safety-critical engineering components. Ultrasonic guided waves stand out because of their ability to propagate over long distances and because they can offer good estimates of location, severity, and type of damage. The unique properties of the fundamental shear horizontal guided wave (SH0) mode have recently generated great interest among the SHM community. The aim of this paper is to demonstrate the feasibility of omnidirectional SH0 SHM in a thin aluminum plate using a three-transducer sparse array. Descriptions of the transducer, the finite element model, and the imaging algorithm are presented. The image localization maps show a good agreement between the simulations and experimental results. The SH0 SHM method proposed in this paper is shown to have a high resolution and to be able to locate defects within 5% of the true location. The short input signal as well the non-dispersive nature of SH0 leads to high resolution in the reconstructed images. The defect diameter estimated using the full width at half maximum was 10 mm or twice the size of the true diameter.

Simulation of acoustic guided wave propagation in cortical bone using a semi-analytical finite element method

Axial transmission techniques have been extensively studied for cortical bone quality assessment. However, the modeling of ultrasonic guided waves propagation in such a complex medium remains challenging. The aim of this paper is to develop a semi-analytical finite element method to simulate the propagation of guided waves in an irregular, multi-layer, and heterogeneous bone cross-section modeled with anisotropic and viscoelastic material properties. The accuracy of the simulations was verified against conventional time-domain three-dimensional finite element. The method was applied in the context of axial transmission in bone to investigate the feasibility of first arrival signal (FAS) to monitor degradation of intracortical properties at low frequency. Different physiopathological conditions for the intracortical region, varying from healthy to osteoporotic, were monitored through FAS velocity using a 10-cycle tone burst excitation centered at 32.5 kHz. The results show that the variation in FAS velocity is mainly associated with four of the eight modes supported by the waveguide, varying with velocity values between 550 and 700 m/s along the different scenarios. Furthermore, the FAS velocity is shown to be associated with the group velocity of the mode with the highest relative amplitude contribution at each studied scenario. However, because of the evolution of the mode with the highest contribution, the FAS velocity is shown to be limited to discriminate intracortical bone properties at low frequency.

Semi-analytical finite-element modeling approach for guided wave assessment of mechanical degradation in bones

Abstract Numerical models based on the Semi Analytical Finite-Element method are used to study the characteristics of guided wave modes supported by bone-like multi-layered tubular structures. The method is first validated using previous literature and experimental studies on phantoms mimicking healthy and osteoporotic conditions of cortical bone, and later used to study a trilayer marrow–bone–tissue system at varying mechanical degradation levels. The results show that bone condition strongly affects the modal properties of axially propagating guided waves and indicates that L(0,3) and F(1,6) are suitable modes for assessing the mechanical condition of the bone. The work here reports suitable modal selection and their dispersion properties which would the aid in development of a transduction mechanism for mechanical assessment of bones.

Influence of local mechanical properties of high strength steel from large size forged ingot on ultrasonic wave velocities

In the metallurgical industry, ultrasonic inspection is routinely used for the detection of defects. For the non-destructive inspection of small high strength steel parts, the material can be considered isotropic. However, when the size of the parts under inspection is large, the isotropic material hypothesis does not necessarily hold. The aim of this study is to investigate the effect of the variation in mechanical properties such as grain size, Young’s modulus, Poissons ratio, chemical composition on longitudinal and transversal ultrasonic wave velocities. A 2 cm thick slice cut from a 40-ton bainitic steel ingot that was forged and heat treated was divided into 875 parallelepiped samples of 2x4x7 cm3. A metallurgical study has been performed to identify the phase and measure the grain size. Ultrasonic velocity measurements at 2.25 MHz for longitudinal and transversal waves were performed. The original location of the parallelepiped samples in the large forged ingot, and the measured velocities were used to produce an ultrasonic velocity map. Using a local isotropy assumption as well as the local density of the parallelepiped samples calculated from the chemical composition of the ingot provided by a previously published study, Youngs modulus and Poissons ratio were calculated from the longitudinal and transversal wave velocities. Micro-tensile test was used to validate Youngs modulus obtained by the ultrasonic wave velocity and an excellent agreement was observed.