Longitudinal Wave Precursor Signal from an Optically Penetrating Thermoelastic Laser Source

The thermoelastic laser ultrasonic source depends on the optical absorption of energy at the sample surface to produce a volumetric expansion. This paper presents the results of calculations and measurements on the effects of optical penetration of the laser beam into the sample and the elastic waveforms produced. A central result is prediction of a sharp longitudinal waveform that precedes the main waveform and is very similar to that observed with an ablative source (normal point force). The shape of this precursor signal is strongly dependent on the optical penetration depth of the material. A basic explanation of the origin of the precursor signal is given in terms of a one-dimensional model using point sources imbedded within the material. Experimental measurements on a material with a substantial optical penetration depth directly confirm calculations using 2-D integral transform techniques by taking into account the temperature variation with depth

Microstructure Characterization with a Pulsed Laser Ultrasonic Source

Localized heating produced by absorption from a pulsed laser provides an efficient noncontacting source of ultrasonic waves in materials. This paper describes the results of experiments conducted to illustrate the feasibility of this type of source fot microstructure characterization in metal and ceramic materials. Piezoelectric and capacitive wide bandwidth detection transducers have been used to record attenuation and scattering in these materials for comparison with the conventional pulse echo technique. The laser source was found to be art efficient, versatile, and wide bandwidth noncontacting source

A Fiber-Based Laser Ultrasonic System for Remote Inspection of Limited Access Components

Surface and plate waves are commonly used to nondestructively inspect the near-surface region of a solid component for cracks and other defects due to, for example, structural fatigue. One particularly attractive method of generating and detecting such ultrasonic signals is laser based ultrasonics (LBU) [1]. In particular, because it is non-contact (i.e., does not require couplant), LBU can be implemented for inspection of limited access components using optical fibers, requiring only a small cross-sectional area for access. An example can be found in the inspection of internal surfaces of an aircraft wing as shown in Figure 1 where a contact method would obviously be difficult to apply. Furthermore, in cases where extremely high sensitivity is required, bandwidth reduction can be employed by concentrating the laser generated signal into a narrow frequency band

Point-Source/Point-Receiver Materials Testing

Conventional measurements in the ultrasonic testing of materials, when used as the basis of a materials characterization procedure, typically rely on one or two piezoelectric transducers operating as source and receiver, attached to a specimen to launch and detect ultrasonic waves in the object to be characterized. Measurements of signal arrival time (or velocity) and amplitude (or attenuation), possibly as a function of frequency, are then correlated with the composition and the macro- and micro-structure of the material, which may include voids, flaws and inclusions distributed through a region of the material. While relative measurements of the time-of-flight and ultrasonic amplitudes do not! present extraordinary measurement challenges, absolute measurements do. It is unfortunate that absolute quantities are often required since they are difficult to obtain reliably with a conventional piezoelectric transducer-based ultrasonic system. For this reason, a considerable effort over the past decade has been undertaken to develop and improve non-contact methods for generating and detecting ultrasonic signals in materials. However, a limiting factor of all the existing non-contact measurement systems is the care required for their use and their reduced sensitivity in comparison to-those utilizing piezoelectric transducers

Controlling Restricted Random Testing: An Examination of the Exclusion Ratio Parameter

In Restricted Random Testing (RRT), the main control parameter is the Target Exclusion Ratio (R), the proportion of the input domain to be excluded from test case generation at each iteration. Empirical investigations have consistently indicated that best failure-finding performance is achieved when the value for the Target Exclusion Ratio is maximised, i.e. close to 100%. This paper explains an algorithm to calculate the Actual Exclusion Ratio for RRT, and applies the algorithm to several simulations, confirming that previous empirically determined values for the Maximum Target Exclusion Ratio do give Actual Exclusion Ratios close to 100%. Previously observed trends of improvement in failure-finding efficiency of RRT corresponding to increases in Target Exclusion Ratios are also identified for Actual Exclusion Ratios.published_or_final_versio

Process Monitoring Using Optical Ultrasonic Wave Detection

Certain microstructural features of materials, such as grain size in metals, porosity in ceramics, and structural phase compositions, are important for determining mechanical properties. Many of these microstructural features have been characterized by ultrasonic wave propagation measurements, such as wave velocity and attenuation. Real-time monitoring of ultrasonic wave propagation during the processing stage would be valuable for following the evolution of these features. This paper describes the application of laser ultrasonic techniques to the monitoring of ceramic sintering. Prior to this work, ultrasonic wave measurements of the sintering of ceramics have been made only through direct contact with the material with a buffer rod [1,2]. Recently, several advances have been made using lasers for both generation and detection of ultrasonic waves in a totally noncontacting manner for material microstructure evaluation [3–5]. Application of laser ultrasonic techniques now opens the possibility for real-time monitoring of materials in very hostile environments as are encountered during processing [6]

Laser Ultrasonic Thermoelastic/Ablation Generation with Laser Interferometric Detection in Graphite/Polymer Composites

Ultrasonic signals have been generated and detected in graphite/polymer composites by optical methods. A Doppler interferometric technique was used for detection. The output voltage of this type of interferometer is proportional to the surface velocity of a sample area which is illuminated by cw laser light. Ultrasonic signals were generated by thermoelastic and ablation processes which occur as a consequence of laser pulses incident on the opposite surface of the sample. The evolution of the magnitude and shape of the detected signals was measured as a function of the pulse energy of the generating laser. Low-energy laser pulses generated ultrasound without causing obvious surface damage. At higher energies surface damage was observable in post inspection but could also be detected by observing (through protective goggles) bright flashes near the illuminated area. The energy at which these processes first occur is qualitatively referred to as the ablation threshold. Changes in the observed waveform were evident at energies above the ablation threshold. The higher-energy waveforms were found to consist of a superposition of a thermoelastic component and an ablatic component, whose relative magnitudes changed with laser power. A delay in the initiation of the ablatic wave relative to the thermoelastic wave was observed to be of the order of 0.3 μs, consistent with observations in pure polymer. [1] Photoelectric detection measurements of the ablation plume also showed a clear threshold and a time scale for growth of the ablation products with a characteristic time scale on the order of 0.3 μs

A Linear Systems Approach to Laser Generation of Ultrasound in Composites

Laser ultrasonic generation and detection systems have been shown to be effective in the inspection and evaluation of both metals and composite materials [1–3]. Advantages of these noncontact systems include rapid scanning capability, the inspection of parts with complex geometries, and the ability for use in hostile environments. Unfortunately, laser ultrasonic systems are somewhat less sensitive than conventional contact piezoelectric systems. In order to increase the sensitivity, careful consideration must be paid to the choice of both generation and detection laser systems. Although the sensitivity of current laser ultrasonic systems has been shown to be sufficient for several applications, small improvements may allow for a more wide-spread use.</p

Experimental Characterization of Ultrasonic Phenomena by a Neural-Like Learning System

This paper describes a novel approach for analyzing ultrasonic signals to permit an experimental determination of the relations between elastic wave phenomena and the properties of a source of sound in a material. It is demonstrated that an adaptive learning system comprising an associative memory can be used to map source and waveform data and vice versa with the auto- and cross-correlation portions of the associative memory. Experiments are described which utilize such an adaptive system, running on a laboratory minicomputer, to process the data from a transient ultrasonic pulse in a plate specimen. In the learning procedure, the system learns from experimental pattern vectors, which are formed from the ultrasonic waveforms and, in this paper, encoded information about the source. The source characteristics are recovered by the recall procedure from detected ultrasonic signals and vice versa. Furthermore, from the discrepancy between the presented and the learned signals, the changes in the wave phenomenon, corresponding, for example, to changes in the boundary conditions of a specimen, can be determined

Frequency-Shifted Low-Noise Sagnac Sensor for Ultrasonic Measurements

Laser generation of ultrasound and the subsequent detection of the ultrasonic waves using laser interferometry are areas of active research [1–6]. In earlier papers, the present authors have discussed an LBU system which employs a diffraction grating for illumination of a line-array to generate narrow-band surface waves and Lamb waves [4], and a fiberized heterodyne dual-probe laser interferometer to measure signals [3]. This paper reports progress towards the development of a robust low cost fiberized Sagnac laser interferometer suitable for field applications. Bowers first reported [7] the use of a Sagnac-type interferometer for surface acoustic wave detection, and the present authors have previously reported [8 QNDE 95] a variant of that scheme. In this paper, we present an alternative lower noise system that uses low cost, long coherence He-Ne lasers that have better intensity noise characteristics than typically used laser diodes. A scheme for elimination of a parasitic interference utilizing a frequency shifting technique has been developed. The primary advantage of the Sagnac interferometer is that it is exactly path matched and as such requires no heterodyning or static path compensation for sensor stabilization. The Sagnac interferometer described below is suitable for the measurement of ultrasonic surface waves arising from laser- or PZT-generated sources or from acoustic emissions. The laser-based ultrasonics (LBU) system can be used to detect and characterize discrete defects such as cracks.</p