Impact damage detection in a stiffened composite wing panel using digital shearography and thermosonics

There has been a growing interest in the use of composites especially in structural application ranging from aerospace to automotive and marine sectors. However, their performances under impact loading represent one of the major concerns as impacts may occur during manufacture, normal operations and maintenance. This paper presents two novel NDT techniques, thermosonics and digital shearography (DISH) to detect and assess barely visible impact damage (BVID) produced on a stiffened composite wing panel by unknown low energy impacts. Thermosonics is based on synchronized infrared imaging and ultrasonic excitation. Despite the apparent simplicity of the experimental setup, thermosonics involves a number of factors, e.g. acoustic horn location, horn crack proximity, horn-sample coupling etc., that significantly tend to influence both the degree and the period of the excitation. Then, a numerical- experimental procedure for the assessment of the size and depth of delamination by digital shearography (DISH) is proposed. The flaw detection capabilities of DISH have been evaluated by measuring the dynamic response of the delaminated area to applied stresses. The shearographic methodology is based on the recognition of the (0 1) resonance mode per defect. A simplified model of thin circular plate, idealized above each impacted area, is used to calculate the natural frequency of vibrating delamination. The numerical difference between experimental resonance frequencies and those computationally obtained is minimized using an unconstrained optimization algorithm in order to calculate the delamination depth. The results showed that thermosonics is a quick and effective method to detect and localize BVID damage while the combined shearography and optimization methodology was able to size and localize delamination due to low velocity impacts.