Guided Wave Non-contact Ultrasonic for Nde

Non-contact ultrasonic, enables rapid testing of structures using ultrasonic tests that are not possible via conventional contact transducers. This paper reports on laser ultrasonic and air/gas coupled ultrasonic measurements that are performed remotely, do not require direct access to test area, do not need traditional ultrasonic coupling and do not have traditional C-scan fixture requirements [1-4]. This hybrid non-contact ultrasonic transduction system extends ultrasonic measurements to test configurations and ultrasonic wave-modes that are difficult to perform using conventional technology. A Lamb wave or surface acoustical wave can be directed to reach regions of structure not accessible using conventional ultrasonic transducers. Analysis of recorded signals from a single test allows assessments of the defects or material condition. The experimental results demonstrate that such testing techniques can perform the inspection from only one side of the structure. Because there is no requirement for a coupling medium to transmit the ultrasound, and traveling stress waves cover larger area, the material or structure can be inspected remotely and at higher test speed then by conventional contact ultrasonic methods. These non-contact ultrasonic methods have potential for in-situ evaluation of components and structures for presence of corrosion, cracks and fatigue damage in metals and composites. Experimental tests reported below show feasibility of the technology to support periodic life service inspections. Introduction: Figure 1 and 2 illustrate ultrasonic wave generation via laser light. High-energy, nanosecond pulsed laser illumination of the material surface generates ultrasonic stress waves. The shape, frequency and propagation direction of laser generated ultrasonic waves is controlled by illumination pattern of the laser light. Such acoustical sources are very flexible and enable generation of plate waves (Lamb waves) or surface acoustical waves (Rayleigh waves). Stress waves propagation is effected by material properties, presence of the defects or corrosion. In the hybrid ultrasonic test configuration the stress waves are detected by an air-coupled capacitance transducer [5-6].