Brennan Dubuc

Damage localization in metallic plate structures using edge-reflected lamb waves

This paper presents a model-based guided ultrasonic waves imaging algorithm, in which multiple ultrasonic echoes caused by reflections from the plates boundaries are leveraged to enhance imaging performance. An analytical model is proposed to estimate the envelope of scattered waves. Correlation between the estimated and experimental data is used to generate images. The proposed method is validated through experimental tests on an aluminum plate instrumented with three low profile piezoelectric transducers. Different damage conditions are simulated including through-thickness holes. Results are compared with two other imaging localization methods, that is, delay and sum and minimum variance.

Effect of pressurization on helical guided wave energy velocity in fluid-filled pipes

HighlightsTheory of acoustoelastic leak Lamb waves presented.Energy velocity of acoustoelastic leaky Lamb waves derived.Energy velocity direction‐dependent and varies linearly with respect to pressurization (stress).Theory compared to experimental helical guided wave measurements in pressurized pipe. ABSTRACT The effect of pressurization stresses on helical guided waves in a thin‐walled fluid‐filled pipe is studied by modeling leaky Lamb waves in a stressed plate bordered by fluid. Fluid pressurization produces hoop and longitudinal stresses in a thin‐walled pipe, which corresponds to biaxial in‐plane stress in a plate waveguide model. The effect of stress on guided wave propagation is accounted for through nonlinear elasticity and finite deformation theory. Emphasis is placed on the stress dependence of the energy velocity of the guided wave modes. For this purpose, an expression for the energy velocity of leaky Lamb waves in a stressed plate is derived. Theoretical results are presented for the mode, frequency, and directional dependent variations in energy velocity with respect to stress. An experimental setup is designed for measuring variations in helical wave energy velocity in a thin‐walled water‐filled steel pipe at different levels of pressure. Good agreement is achieved between the experimental variations in energy velocity for the helical guided waves and the theoretical leaky Lamb wave solutions.

A guided ultrasonic imaging approach in isotropic plate structures using edge reflections

This paper presents an imaging technique to locate damage in plate-like structures by permanently attached piezoelectric transducers (PZT) capable to generate and receive guided ultrasonic waves. The technique is based on a model capable of predicting envelope of scattered waves. Correlations between the estimated scattered waves and experimental data are used for image reconstruction. The approach is validated on an aluminum plate and results are compared with two common imaging algorithms, that is, Delay and Sum (DS) and Minimum Variance (MV). Damage is simulated by placing two magnets on sides of the plate. It is shown that the inclusion of Lamb wave reflections improves the localization accuracy while making use of fewer number of sensors possible.

Helical guided waves in liquid-filled cylindrical shells subjected to static pressurization stress

Helical guided waves in pipelines are studied under the effects of pressurization stresses from a contained liquid. The pipeline is approximated by an “unwrapped” plate waveguide, and a transfer matrix method is used to solve for guided wave velocity and attenuation dispersion curves in a multilayered plate waveguide subject to an arbitrary triaxial state of initial stress. The matrix-based model is able to incorporate both elastic and viscoelastic solid materials, as well as approximate non-uniform distributions in initial stress through the thickness of a waveguide. Experiments on a steel pipe filled with pressurized water are carried out to validate the modeling approach.

Damage Localization in Plate-like Structures Using Guided Ultrasonic Waves Edge Reflections

This paper proposes a novel idea for ultrasonic imaging of plate structures with a small number of permanently attached ultrasonic sensors. Guided ultrasonic waves excited and captured by a single actuator-sensor pair suffice as input for the proposed imaging technique. Taking advantage of several reflections from boundaries and geometric features of plates, the number of inspection lines is artificially increased. Only the first antisymmetric mode (A0) is used for imaging, because it has higher excitability than the first symmetric mode (S0) and simpler reflection behavior. Mode conversion from reflections is avoided by exciting below the first cut-off frequency of higher order propagating antisymmetric modes. The path and distance traveled by each echo is calculated using a ray tracking model, and a drop in energy of each echo compared to its baseline is associated with the presence of damage. Dispersion curves are used to calculate wave velocity and expected arrival times of the reflected echoes. Based on the expected time of arrival, a gate is defined for each echo, and an energy comparison is conducted for the high energy gated portion of the echo. A probabilistic method for image reconstruction is also proposed to locate damage. The proposed imaging method combines information from several inspection paths using a Bayesian probabilistic framework. To validate the approach, experiments have been carried out on an aluminum plate, instrumented with only two permanently attached low profile circular piezoelectric sensors. A tone burst packet is used an input excitation, and multiple echoed packets have been recorded at a receiving sensor. A large C-clamp is used to locally scatter the waves and simulate damage. Damage is simulated in nine locations, and the proposed method achieves notable success in localizing the applied damage with only two sensors. doi: 10.12783/SHM2015/313

Higher order longitudinal guided wave modes in axially stressed seven-wire strands

HighlightsStudy higher order modes in axially stressed strands using acoustoelastic rod theories.Adapt exact acoustoelastic rod theory under small deformations and derive mode shapes.Propose theoretical and measurement approximations for stress effect, with roughly 2% error from exact theory.Stable stress effect across mode and frequency found in experiments.Based on a representative steel, a 15% mismatch between theory and experiment was observed. ABSTRACT This paper investigates the effect of axial stress on higher order longitudinal guided modes propagating in individual wires of seven‐wire strands. Specifically, an acoustoelastic theory for a rod is used to predict the effect of stress on the phase velocity of guided modes in a strand. To this end, the exact acoustoelastic theory for an axially stressed rod is adapted for small deformations. Aside from the exact theory, approximate phase velocity changes (derived from both theory and experiment) are proposed, without the need to solve for dispersion curves. To validate the use of rod theories for strands, a custom‐built prestressing bed was designed to apply axial load (up to 50% of yield) to a strand while conducting guided wave measurements. Higher order modes were excited in individual wires, and their phase velocity change under stress is compared to the exact acoustoelastic theory. Furthermore, it is shown that the proposed approximate phase velocity changes derived from theory and experiment only differ by roughly 2% from their exact counterparts. Higher order modes are shown to have stable stress dependence near their peak group velocity, which is beneficial for stress measurement. Additionally, linear stress dependence is observed, which is predicted by rod theories. Due to the unavailability of third order elastic constants for the steel strand, constants for a steel with similar Carbon content (0.6% C Hecla 17) were used as representative values in the theory. Using the Hecla 17 constants, roughly 15% mismatch in the slope of the linear stress dependence was observed when compared to the measurements on a steel strand.

The effect of applied stress on the phase and group velocity of guided waves in anisotropic plates

This paper presents an analytical formulation for the phase and group velocity of acoustoelastic guided waves in anisotropic plates. Uniform in-plane applied stress is considered, with both arbitrary propagation and stress directions. An expression for the energy velocity in a stressed anisotropic plate is derived, from which the group velocity is computed. Since the wavefront and group velocity directions generally differ, the deviation angle between the two is also studied. A method is proposed for verifying the consistency of the formulation, based on the correspondence between a direct and an indirect formulation. Analytical results are presented for a unidirectional fiber-reinforced graphite/epoxy composite plate. The plate is considered homogeneous for large wavelength to fiber diameter ratios. Results for the phase velocity, group velocity, and deviation angle are presented for two uniaxial applied loading cases. These are used to study the effect of stress for various propagation and stress directions. The linearity of the deviation angle with respect to stress is also demonstrated. Exact correspondence between the direct and indirect formulations is observed, which verifies consistency. The importance of accounting for shear strain in the indirect formulation is also demonstrated, which has not been noted in previous guided wave studies.

Stress Measurement in Seven-Wire Strands using Higher Order Guided Ultrasonic Wave Modes

This paper investigates the use of higher order longitudinal guided modes for stress measurement within individual wires of a steel strand. The effect of stress on the phase velocity of higher order modes is studied using an approximate theory, which does not require the solution of dispersion curves. To validate the proposed approach, a prestressing bed was designed to apply a tensile load to a strand up to 25% ultimate tensile strength while recording guided wave signals. Guided waves were excited within individual wires of a strand, and the stress sensitivity of their phase velocity was used for stress measurement. Stress measurement was performed with higher order modes using the approximate theory with parameters for a steel of similar carbon content (Hecla 17), as well as with calibrated parameters. Using the Hecla 17 parameters, roughly 15% mismatch in stress was observed, whereas roughly 5% error was observed using calibrated parameters. Stress measurement was also performed using the fundamental mode, in order to compare the accuracy of higher order modes with the mode used previously in the literature. The greater stability of higher order modes across mode and frequency yielded significantly increased stress measurement accuracy, using both Hecla 17 and calibrated parameters.

Computation of propagating and non-propagating guided modes in nonuniformly stressed plates using spectral methods

This paper presents a numerical approach based on spectral methods for the computation of guided ultrasonic wave modes (i.e., Lamb and shear horizontal) in nonuniformly stressed plates. In particular, anisotropic elastic plates subjected to a normal stress profile, which varies nonuniformly over their thickness, are considered. The proposed approach computes the modeshapes and the full three-dimensional dispersion spectrum (i.e., real frequency, complex wavenumber). It therefore includes both propagating (real wavenumber) and non-propagating (complex wavenumber) modes. Furthermore, an approach for robustly post-processing the dispersion spectra in order to compute the group velocity of propagating modes is presented, which is based on a spectral quadrature method. Numerical results are presented for two case studies: (1) a bending profile in a fiber-reinforced graphite/epoxy plate, and (2) an exponential profile in a silver plate. The results show the computational efficiency (i.e., spectral convergence) of the proposed method compared to other existing approaches such as the sublayering and finite element methods.

A spectral method for computing the effect of stress on guided modes in plates and rods

This paper presents a numerical approach based on spectral methods for the computation of guided ultrasonic wave modes in stressed elastic plates and rods. The approach is applicable to Lamb modes in anisotropic plates and longitudinal modes in isotropic rods under uniform stress. The proposed approach computes the modeshapes and the full complex dispersion spectrum (real frequency, complex wavenumber), accommodating both propagating (real wavenumber) and non-propagating (complex wavenumber) modes. Numerical results are presented for plates composed of fiber-reinforced graphite/epoxy (GREP) and plates and rods composed of Hecla 17 steel. The results are used to investigate and compare the effect of stress on the dispersion curves for plates and rods, while demonstrating the computational efficiency of spectral methods. The convergence rate is demonstrated, showing spectral convergence.