Advances in thermosonics for detecting impact damage in CFRP composites

Thermosonics, or ultrasound stimulated thermography, has been shown to reveal structural defects such as impact damage in CFRP that are not detectable by conventional optically stimulated thermography. However, there are concerns that the large amounts of ultrasonic excitation energy employed to make thermosonic inspections may cause damage, particularly by heating at the attachment point of the exciter. The objectives of this study were to investigate methods of reducing the electrical power required to detect defects and thus to reduce surface damage during testing. The magnitude of the heating at the surface over ultrasonically excited defects was modelled, to determine the heating requirements for different materials. Thermosonic images were collected whilst measuring test piece vibrations, using strain gauges. A long pulse, low power excitation method has been found to produce satisfactory impact damage images whilst eliminating damage to the test piece caused, in other methods, by heating of the ultrasonic excitation horn. Images of impact damage in 4 mm, 8 mm and 16 mm CFRP plates have been obtained using only 1 W of electrical power applied to an ultrasonic excitation horn.