Vibration based structural health monitoring of composite skin-stiffener structures

Abstract

Composite materials combine a high strength and stiffness with a relatively low density. These materials can, however, exhibit complex types of damage, like transverse cracks and delaminations. These damage scenarios can severely influence the structural performance of a component. Periodic inspections are required to ensure the integrity of a component during its life. The current inspection methods are often time-consuming, costly and require the components to be readily accessible. Vibration based structural health monitoring (SHM) technologies propose a promising alternative and involve the continuous monitoring of a structure by employing an integrated sensor system. These methods are based on the concept that the dynamic behavior of a structure can change if damage occurs. Although many damage identification methods have been proposed in the literature, there are still numerous difficulties in the practical application of these approaches, especially to complex structures. The performance of a vibration based damage identification approach is highly dependent on the actual design of the structure and the damage scenario that is considered. This thesis focuses on the identification of damage in advanced composite skin-stiffener structures. The principle objective is to develop guidelines for the detection, localization and characterization of damage in composite skin-stiffener structures based on changes in the dynamic behavior. A literature study supported by an analytical model showed that mode shape curvatures combined with the modal strain energy damage index (MSE-DI) algorithm are a potentially powerful damage feature and classifier for the identification of damage in several advanced composite skin-stiffener structures. A experimental set-up, including a shaker and laser-vibrometer, was used to measure the dynamic responses. A linear dynamic system description is obtained by applying experimental modal analysis. The vibration experiments demonstrated the feasibility of the MSE-DI algorithm to detect, localize and roughly estimate the size of barely visible impact damage (BVID) in advanced composite skin-stiffener structures. It is concluded that the method is particularly effective for health monitoring of skin-stiffener connections. The method remained inconclusive in the case of pure skin related damage. Experiments showed that damage at the skin-stiffener interface can introduce clear nonlinear effects in the dynamic behavior of the structure. These nonlinear effects are attributed to the interaction between the skin and stiffener that occurs during opening and closing motion of the damage. It is shown that linear damage identification methods (e.g. modal domain methods) are feasible for low excitation amplitudes, but the presence of nonlinear dynamic effects cannot remain silent for higher amplitudes. The nonlinear dynamic effects can act as strong indicator of damage, but can also be useful for characterization purposes. The nonlinear dynamic effects introduced by the skin-stiffener damage urges the development of nonlinear damage identification methods. A study on the understanding and feasibility of using nonlinear vibro-acoustic modulations for the detection, localization and characterization of impact damage in a composite T-beam is presented. A time domain analysis at multiple spatial locations is used to detect and localize impact damage in a skin-stiffener connection, based on locally increased amplitude modulation effects. Analysis of the characteristics of the nonlinear modulations opens the ability to characterize the nonlinear dynamic behavior introduced by the damage at the skin-stiffener interface. The work presented in this thesis showed that the relations between the characteristics of the structure, the potential damage scenarios and the damage identification method together define the performance of the vibration based damage identification strategy. Therefore, it is concluded that the design of a vibration based damage identification strategy is made-to-measure work and requires a thorough physical understanding of the potential failure mechanisms, the critical damage locations and their effect on the dynamic behavior. To aid in this process, a scenario based procedure for the design of a damage identification strategy is proposed. All findings presented in this thesis contribute to the development of a design tool for research engineers, to assist the implementation of structural health monitoring technology in safety-critical composite structures.