ULTRASONIC TESTING OF SOLID FIBER-REINFORCED COMPOSITE PLATE

The pulse echo method of detecting defects in composite materials, usi ng a broadband ultrasonic probe, together with a commercial flaw detec tor, is probably the simplest of all ultrasonic methods. An ultrasonic probe produces an ultrasonic beam pattern which can be divided into t wo regions called the near field (or Fresnel zone) and the far field ( or Fraunhofer zone). In the near field, the amplitude of the beam norm ally varies as an interference pattern, while, in the far field, the a mplitude follows the normal inverse square distance law. Therefore, te sting in the near field is usually avoided. In this paper, experiments , using a commercial broadband probe and a digital ultrasonic flaw det ector, in a normal contact pulse echo mode, have shown that delaminati ons could be detected and sized, in thin carbon fiber composite plates , within the near field of the beam of the probe. Sizing of delaminati ons is possible so long as the reflected echoes can be separated from each other and from the transmission pulse. The use of a perspex delay does not improve the detectability of delaminations in the specimen. For defects where the reflected echoes cannot be separated from each o ther and from the transmission pulse, the Lamb wave technique shows po tential in detecting and sizing delaminations a few layers deep in the composite.