Papers by Author: D.P. Almond

Abstract: 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.

Abstract: The objective of this work was to demonstrate the feasibility of nonlinear vibration/acoustic/ultrasonic diagnostic tools to be implemented in a structural health monitoring system for imaging damage. In particular, the sensitivity a second harmonic imaging technique (SHIT) based on material nonlinear elastic effect known as second harmonic generation (SHG) was investigated. Examples of the capability and limitations of the proposed damage detection process to detect and image barely visible impact damage (BVID) due to low velocity impact (<12J) are presented for various composite laminated. The presence of microcracks, debonding, delamination, etc… could induce the material to behave in a nonlinear elastic fashion and it is highlighted by the presence and amplitude of harmonics in the spectrum of the received signal. The results showed that the proposed SNIT methods appear to be highly accurate in assessing the presence and magnitude of damage with very promising future NDT and structural health monitoring applications. Moreover the technique was validated with two conventional NDT techniques: pulse thermography and thermosonic. The first failed in detecting the damage on the impact face, but delamination on back surface was localized. The second technique was capable of localising and quantifying the damage on the impacted surface agreeing well the results obtained using non linear method.