Cure monitoring of composite materials using optical techniques

Abstract

A fully remote all optical technique for the generation and detection of ultrasonics in epoxy materials has been developed at Victoria University. This requires a high power pulsed laser to induce ultrasound in the sample and an optical fibre interferometer for detection. The output of the interferometer can be recorded using a digital storage oscilloscope and subsequently processed by a personal computer. This Thesis presents the use of this arrangement for further studies of cure monitoring of composite materials. Samples of 3.0 cm diameter and 1, 2, 4, 6, 8, 10 and 12 cm thickness were made and the interferometer waveforms were recorded in 10 min time intervals after the curing agent was mixed with the epoxy. Attenuation coefficients and travel times were calculated for each waveform. During the analysis phase of this research an algorithm was developed to find the exact onset of ultrasonic signals. Different thickness samples were made and the interferometric waveforms were obtained and analysed. The relative travel time and wave attenuations were calculated and close correlations were found between these two factors and the state of cure. In this work, optically-generated ultrasonic waves, when reflecting from the rear surface of an object under test, cause a small surface motion (typically in the range of one tenth of a nanometre to few nanometres). A second diagnostic laser is used to illuminate the rear surface of the test object. The ultrasonic surface motion produces a small phase shift or frequency shift in the reflected light, which is detected by an interferometric system. Signals from the interferometer were recorded using a digital storage oscilloscope (Tektronix DSA 602) and this stored signal is subsequently read and processed using a personal computer. In these experiments, epoxy was poured into moulds and measurements were taken every few minutes thereafter.