Laser/Materials Interaction Studies for Enhanced Sensitivity of Laser Ultrasonic Systems

The current state-of-the-art for laser based ultrasonic measurement and inspection is such that detection sensitivity continues to be at the margin of utility for many applications. In earlier research, investigators at Hopkins have developed specialized laser sources capable of generating narrowband ultrasound or alternatively, phased array sources, which serve to provide enhanced signal-to-noise ratio for laser based ultrasonic systems. The purpose of the current program, however, is to go beyond narrowband and phased array sources to consider the fundamental issues of laser/materials interactions. The intent is to be able to predict and adjust source parameters such as pulse duration and shape in order to optimize the efficiency with which laser energy can be converted into useful ultrasonic signals. Although the principles investigated through this program will apply to a variety of materials, of particular interest to the research sponsor in this case is the interaction of laser sources with polymer-based composite materials.

Studies begun during the past year have pursued both theoretical and experimental aspects of the source efficiency problem. Models are under development to predict elastic displacements arising from laser sources absorbed both near the surface of the material and also for conditions when laser light is able to penetrate appreciably into the bulk of the material. Model development is also under way for initial propagation of elastic waves generated by a laser source in a highly anisotropic material such as a unidirectional fiber reinforced composite material.

Experimentally, work has begun to catalog the energy partitioning and frequency transfer characteristics as a function of propagation angle between the laser source and interferometer receiver in composite specimens including the Air Force's high temperature composite material AFR 700. Important to the experimental results, especially when being compared with model predictions, is the need to insure that the nature of the laser source is thermoelastic and nondamaging to the material. To better understand the nature of the transition from a nondamaging thermoelastic source to an ablative laser source, a brief study was performed this year to correlate signature changes in a laser generated ultrasonic signal with the appearance of damage as determined by SEM examination of the laser source region. Assuming that a close correlation can be established, it is hoped that acoustic signature information may be used for broader classes of materials, including composites, to determine when the laser source exceeds a damage threshold for the material.

Practical Application

By tailoring the laser source to optimize the efficiency with which sound is generated in a material and combining that optimized pulse shape with laser array methods for narrow banding or phased array signal enhancement, further improvements in detection sensitivity of laser based ultrasonic systems are anticipated, perhaps even with reduced costs, thus broadening the range of applications and general utility of this exciting non-contact ultrasonic testing modality.