Christine Valle

Crack characterization using guided circumferential waves

This paper examines the propagation of guided circumferential waves in a hollow isotropic cylinder that contains a crack, with the goal of using these guided waves to both locate and size the crack. The crack is sized using a modified Aulds formula, which relates the cracks length to a reflected energy coefficient. The crack is then located by operating on the backscattered signal with a time-frequency digital signal processing (DSP) technique, and then comparing these results to those obtained if the cylinder is perfect. The guided circumferential waves are generated with a commercial finite element method (FEM) code. One objective of this work is to demonstrate the effectiveness of using sophisticated DSP techniques to describe the effect of scattering on dispersive waves, showing it is possible to characterize cracks systematically and accurately by quantifying this scattering effect. The results show that the need for high frequency signals to detect small cracks is significantly decreased by using these techniques.

Guided circumferential waves in layered cylinders

This paper examines the propagation of time harmonic circumferential waves in a two-dimensional (infinitely long) hollow cylinder with an inner shaft. The boundary condition at the interface (between the inner shaft and the outer cylinder) is assumed as free-sliding; that is, the interfacial shear stresses are assumed to be zero, and the normal stress and radial displacements are continuous. The dispersion relationship is obtained from linear elasticity theory, and numerical results for the dispersion curves of the first several modes are presented. In addition, displacement profiles along the radial directions are provided. These results show that, at higher frequencies, the first mode asymptotically tends to a Rayleigh wave on a flat surface, while the second mode also approaches a Rayleigh wave propagating along the free-sliding interface.

Flaw localization using the reassigned spectrogram on laser-generated and detected Lamb modes

Rapid, accurate inspection of metallic plates using broadband guided waves has traditionally been limited by the multi-mode, highly dispersive nature of these waves; current practice typically restricts either the type of mode generated or detected, and/or its frequency range. The current study presents an effective alternative procedure by combining a novel digital signal processing technique, the reassigned spectrogram, with laser generated and detected Lamb waves. The reassigned spectrogram is used to characterize the modal and frequency content of a single ultrasonic signal as a function of time, enabling a procedure to locate flaws in an aluminum plate specimen.

DEVELOPMENT OF DISPERSION CURVES FOR TWO-LAYERED CYLINDERS USING LASER ULTRASONICS

In this paper, laser-ultrasonic techniques are employed to develop a quantitative understanding of the underlying principles of the propagation of guided circumferential waves in two-layered cylindrical components. The high-fidelity, broad-bandwidth, point source/receiver and noncontact nature of these optical techniques are critical elements to the success of this work. The experimental procedure consists of measuring a series of transient, circumferentially propagating waves in a cylindrical waveguide and then operating on these transient waveforms with signal-processing techniques to develop the dispersion relationship for that waveguide; this procedure extracts the steady-state behavior from a series of transient measurements. These dispersion curves are compared to theoretical values. There is good agreement between the experimental and theoretical results, thus demonstrating the accuracy and effectiveness of using laser-ultrasonic techniques to study the propagation of guided circumferential waves.

Automated methodology to locate notches with Lamb waves

This study develops an automated method capable of detecting notches in isotropic plates. Laser ultrasonic techniques are used to generate and detect Lamb waves in perfect and notched plates. These signals are first transformed into the time-frequency domain using a short time Fourier transform (STFT) and subsequently into the group velocity-frequency domain. Finally, the notch is located with an autocorrelation in the group velocity-frequency domain. A verification of the proposed methodology shows excellent agreement with the actual location of the notch.

Scattering of circumferential waves in a cracked annulus

This paper considers guided waves propagating in the circumferential direction of an annulus with a radial crack, with the objective of developing an ultrasonic technique that can detect and characterize these cracks. Specifically, the finite element method is used to simulate the propagation and scattering of guided circumferential waves in a cracked annulus. This method fosters a better understanding of the wave fields, so that a transducer configuration used in the field can be optimized for crack detection/characterization. Both a point source (simulating laser generated ultrasound) and a distributed source (simulating a PZT transducer) are modeled and compared to corresponding experimental results. Animations (snapshots at different instants in time) of the strain energy field in the annulus are given for various combinations of load profiles, incident angles, and incident frequencies. Results of this paper provide the necessary design guidelines for developing nondestructive ultrasonic techniques fo...

Panel session - improving learning and retention in introductory statics

Statics is the first engineering class mechanics-based majors encounter. It tends to operate as a “weeder” course, due to most students’ difficulties in learning the engineering approach to problem solving, which is characterized by model-based reasoning. Difficulty in model-building can cause a lack of confidence and a diminished sense of self-efficacy that is particularly problematic when amplified by gender and under represented minority (URM) issues. Many valuable attempts have been made in the past at developing new tools to help students learn modeling, and develop engineering intuition. The goal of this panel is a two-pronged discussion: on the one end, present and discuss the most pertinent and intractable challenges associated with Statics learning that students encounter; on the other, present and discuss various interventions, technological or otherwise, that the panelists have attempted and/or developed to remedy the situation. Ideally the panel outcome will be to define pathways for developing and disseminating cognitively-based interventions that better supports learning and retention of all engineering students.

Computational Characterization of Adhesive Bond Properties Using Guided Waves in Bonded Plates

This research develops a guided waves technique to nondestructively characterize the stiffness properties of bonded engineering components. This study first quantifies the influence of the relevant adhesive layer properties—Young’s modulus, Poisson’s ratio and bond thickness—on the dispersion curves of a two-layer bonded system, an aluminum plate with an adhesive tape layer bonded to its lower surface. Both a commercial finite element (FE) code (ABAQUS/Explicit) and the global matrix method (GMM) are used to determine the dispersion relationships of this bonded plate system in the form of frequency-wavenumber and slowness-frequency relations. These dispersion curves are then used to determine a set of adhesive tape parameter sensitive points, whose frequency coordinates represent the solution criteria for a proposed inversion procedure. This inversion is based on the GMM and assumes the three adhesive tape properties are unknown. The performance of this inversion procedure depends on the number of input time-domain signals; it is possible to solve the inverse problem for all three of the unknown adhesive tape properties if multiple input signals are known.

Development of Dispersion Relationships for Layered Cylinders Using Laser Ultrasonics

There currently exists a widespread need for the development of quantitative methods for the nondestructive evaluation of cylindrically shaped components such as a helicopter rotor hub. Of particular interest is a geometry, which consists of an outer hollow cylinder and an outer hollow cylinder with a solid inner shaft. Unfortunately, conventional ultrasonic methodologies (such as pulse-echo) often fail to detect these cracks, mainly because of a lack of accessibility and the inherent complexity of the waveforms generated.

Sizing notches in anisotropic plates

The objective of this work is to use broadband, multi-mode signals to size flaws in anisotropic plates. The 2DFFT is applied to synthetic Lamb waves propagating in a graphite-epoxy unidirectional plate. After agreement is obtained with the analytical dispersion curves, a notch is introduced in the model. 2DFFTs are calculated for varying notch depths and an energy coefficient is obtained. It is shown that a quasi linear relationship exists between this energy coefficient and the notch depth.