Laser Ultrasonic Inspection of Graphite Epoxy Laminates
Superior mechanical properties and reduced weight of fiber reinforced polymer matrix composite laminates (e.g., made of graphite epoxy) are leading to their increased use in aeronautic and aerospace structures. These materials are found more and more in load bearing components, which in turn, requires their integrity to be fully evaluated by nondestructive inspection. This applies to newly manufactured parts which can be flawed following improper manufacturing procedures and to parts which have been in service on an aircraft as well, since additional flaws could have occurred and old existing flaws could have grown and become more severe. Flaws which are found in these materials include porosity and foreign inclusions, which are produced during manufacturing and delaminations between plies, which can be produced at manufacturing or can be caused by the impact of foreign objects on the structure. Ultrasonics has been recognized to be a superior technique for detecting delaminations and can be used to detect foreign inclusions and assess porosity, as well [1,2]. The ultrasonic waves are usually generated and detected by piezoelectric transducers and coupled to the inspected part by direct contact or water. Although operation in transmission is widely used and easily implemented for curved parts, the pulse-echo mode is preferred since it requires only single side access and provides flaw depth information. In this case, the transducer should be properly aligned with respect to the surface of the inspected part (within a few degrees), since it is a phase sensitive device emitting and its whole surface.