Progress Towards a Fiber-Based Laser-Ultrasonics System for Rapid NDE of Large-Area Composites

Abstract

State-of-the-art integrally stiffened composite materials, manufactured for use in the next generation of commercial and military aircraft, are being increasingly used for structural components such as wings and fuselages. However, due to the complexity of the manufacturing process, small variations in the shape of integrally stiffened composite structures often occur. Thus, a prioriknowledge of the part shape often does not provide sufficient tolerance to allow an automated conventional ultrasonic inspection. Many of the advantages of laser-based ultrasonics, including its noncontacting nature and applicability to rapid scanning of contoured and integrally stiffened structures, have been described previously [1–5]. To further extend the utility of laser-based ultrasonics, enable limited access inspections and also provide an upgrade/retrofit path for existing ultrasonic scanning systems, it is desirable to reduce the size of current laser-based ultrasound (LBU) system scan heads and provide both generation and detection laser beam delivery via optical fibers. A promising approach is the use of a scanning head based on a Cassegrain optical collection system. This approach minimizes the load carrying requirements of the scanning assembly and is also well-suited for integration with fiber optics to allow the delivery and reception of the ultrasonic generation and detection laser beams via long lengths of optical fiber. This provides increased mobility of the LBU scan head and allows the ultrasonic generation and detection lasers and other sensitive equipment to be housed in a clean environment which potentially can be located hundreds of meters from the inspection area. The use of a pulsed CO₂ laser has been reported previously for generation of ultrasonic waves in composite materials [4]. However, the CO₂ laser wavelength (λ = 10.6 μm) and the high peak power laser pulses precludes the use of fiber-optic beam delivery over all but very short lengths (< 1.5 m) of specialized optical fiber. Consequently an alternative generating laser has been sought that can be transmitted efficiently over standard quartz optical fiber. An alexandrite laser, which is tunable over the 720–800 nm wavelength range, is being investigated for this application. Progress towards the implementation of a fiber-based LBU system for rapid NDE of large-area composites, and the use of an alexandrite laser for ultrasonic generation in composite materials are described below.