Stephen D. Holland

Air-coupled acoustic imaging with zero-group-velocity Lamb modes

A Lamb wave resonance has been found that allows unusually efficient transmission of airborne sound waves through plates. This occurs at the zero-group-velocity point at the frequency minimum of the first-order symmetric (S1) Lamb mode. At this frequency, plane waves with a range of incident angles can couple between the air and the Lamb mode in the solid plate, dominating the spectrum of transmitted focused sound beams by 10 dB or more. We use this frequency for C-scan imaging, and demonstrate the detection of both a 3.2-mm-diameter buried flaw and a subwavelength thickness changes of .005λ (1%).

Electric string instrument

The guitar has a novel feedback arrangement for sustaining the sound generated from the strings and including a pickup associated with each string and a corresponding driver intercoupled with the pickup in a feedback loop that also includes a gain-controlled amplifier. The amplifiers are controlled in common from a variable control means. The pickups, drivers and associated electronics are all commonly mounted from the bridge. In an alternate embodiment of the invention there is provided an improved sustain technique that provides a more natural sustain characteristic, employing a parametric type of excitation. This embodiment may comprise a pickup, amplifier and tensioning member responsive to the output of the amplifier for sustaining string vibration by causing longitudinal string displacement by means of the tensioning member. In a similar embodiment the string tension may be held constant and the string length varied instead.

Measurement of crack opening stresses and crack closure stress profiles from heat generation in vibrating cracks

A method is described to measure crack opening stresses and closure stress profiles of a surface-breaking crack. Vibration is used to generate frictional heat by rubbing crack face asperities. Heat is generated at regions of contacting crack asperities under low, but nonzero, closure stress. Increasing force is applied to incrementally open the crack and measure the locations of crack heating as a function of applied load. Surface crack closure stresses are approximated from the heating locations as the load is varied and the crack opening stress is measured from the load required to fully open the crack and terminate heat generation.

First Measurements from a New Broadband Vibrothermography Measurement System

We report on the construction and development of a broad‐spectrum vibrothermography (Sonic IR) measurement system at the Center for NDE. The new system uses a broadband actuator instead of an ultrasonic welder to generate vibration and induce heating of cracks. A high‐resolution infrared camera captures the IR signature of a crack, and a reconfigurable data acquisition software system acquires and processes the IR images and vibrometry waveforms in real time. We present and discuss results from initial experiments with this system, including the frequency dependence of vibrothermographic heating of flaws in a jet turbine stator vane and an analysis of the correlation of heating with vibration frequency in a cracked test specimen.

Air-coupled, focused ultrasonic dispersion spectrum reconstruction in plates

This paper presents and demonstrates a noncontact method for measuring the Lamb wave dispersion spectrum of a plate. Noncontact air-coupled source and receive transducers are used with line-focus mirrors and 50–700 kHz broadband apparatus for simultaneous measurement over a broad spectrum of refractive angles and multiple guided modes. Broadband, wide-angle wave forms are measured as a function of position. The Fourier transform of these wave forms from the t–x domain to the ω–k domain gives an approximate spectrum of the dispersion relation. We measure the dispersion spectra of Lucite™, aluminum, balsa wood, and a carbon fiber epoxy laminate, and show that the measured spectra agree well with the dispersion relation calculated from Lamb wave theory.

Toward a Viable Strategy for Estimating Vibrothermographic Probability of Detection

Vibrothermography is a technique for finding cracks and delaminations through infrared imaging of vibration‐induced heating. While vibrothermography has shown remarkable promise, it has been plagued by persistent questions about its reproducibility and reliability. Fundamentally, the crack heating is caused by the vibration, and therefore to understand the heating process we must first understand the vibration process. We lay out the problem and begin the first steps toward relating detectability to the local motion around a crack as well as the crack size. A particular mode, the third‐order free‐free flexural resonance, turns out to be particularly insensitive to the presence of clamping and transducer contact. When this mode is excited in a simple bar geometry the motions of the part follow theoretical calculations quite closely, and a single point laser vibrometer measurement is sufficient to evaluate the motion everywhere. Simple calculations estimate stress and strain anywhere in the bar, and these can then be related to observed crack heating.

Leak detection in spacecraft using structure-borne noise with distributed sensors

We have developed and tested in the laboratory a method for in-orbit detection and location of air leaks in manned spacecraft that uses only a small number of sensors distributed arbitrarily on the inner surface of the spacecraft skin. Then, structure-borne ultrasound in the range of 300–600 kHz is monitored from each of the sensors. When cross correlations between measured sensor waveforms indicate the presence of a leak, these correlations are compared with a large dynamically generated database of simulated correlations to locate the the leak on the pressure vessel. A series of experimental tests were performed and at worst the method identified some false locations, but the true location of the leak always appeared.

Two-sensor ultrasonic spacecraft leak detection using structure-borne noise

Micrometeorite hits can create air leaks in manned spacecraft. Leak-generated-guided ultrasonic waves can be monitored within the platelike spacecraft skin to detect and locate leaks. Cross-correlation techniques allow measurement of the deterministic behavior of the leak-generated noise. Measured leak-into-vacuum cross-correlations of noise signals from two adjacent transducers are recorded as the transducer pair is rotated to determine the relative phase delay as a function of rotation angle. The direction to the leak is found from the variation of phase with angle or from synthetic aperture analysis. The leak is then located through triangulation from two or more sensor-pair locations.

Locating air leaks in manned spacecraft using structure-borne noise.

All manned spacecraft are vulnerable to leaks generated by micrometeorite or debris impacts. Methods for locating such leaks using leak-generated, structure-borne ultrasonic noise are discussed and demonstrated. Cross-correlations of ultrasonic noise waveforms from a leak into vacuum are used to find the location of the leak. Four methods for sensing and processing leak noise have been developed and tested and each of these can be used to reveal the leak location. The methods, based on phased-array, distributed sensor, and dual sensor approaches, utilize the propagation patterns of guided ultrasonic Lamb waves in the spacecraft skin structure to find the source or direction of the leak noise. It is shown that each method can be used to successfully locate the leak to within a few millimeters on a 0.6-m2 aluminum plate. The relative merits of the four methods are discussed.

Vibrothermographic Crack Heating: A Function of Vibration and Crack Size

Vibrothermography is an inspection technique that detects cracks by observing vibration induced crack heating. Frictional crack heating in a vibrating specimen is directly linked to the resonant vibrational stress on the crack. In simple geometries we can measure the vibrational mode structure and intuit the dynamic vibrational stress field on the crack. This is used to establish a relationship between crack heating and vibration. Such a relationship will be critical for vibrothermography to be accepted as a viable inspection technology. We correlate stress to heating by exciting specimens in a well understood and repeatable resonant vibration mode. Our sample set consists of 65 Titanium and 63 Inconel specimens with low cycle fatigue cracks. Through knowledge of the mode shape, a single point surface velocity measurement is sufficient to calculate the deformed shape of the entire specimen. The loads and stresses within the specimen are calculated from the deformed shape and used to identify the relationship between crack heating and vibration. The observed relationship between normal stress, crack size, and crack heating is presented. This relationship may eventually prove viable for quantifying crack detectability in vibrothermography.Vibrothermography is an inspection technique that detects cracks by observing vibration induced crack heating. Frictional crack heating in a vibrating specimen is directly linked to the resonant vibrational stress on the crack. In simple geometries we can measure the vibrational mode structure and intuit the dynamic vibrational stress field on the crack. This is used to establish a relationship between crack heating and vibration. Such a relationship will be critical for vibrothermography to be accepted as a viable inspection technology. We correlate stress to heating by exciting specimens in a well understood and repeatable resonant vibration mode. Our sample set consists of 65 Titanium and 63 Inconel specimens with low cycle fatigue cracks. Through knowledge of the mode shape, a single point surface velocity measurement is sufficient to calculate the deformed shape of the entire specimen. The loads and stresses within the specimen are calculated from the deformed shape and used to identify the relations...