This thesis aims to develop a damage detection method based on Coupled Response Measurements. The coupled response refers to the phenomenon that a structural member with a cross-sectional crack exhibits composite vibration modes (axial and bending) when the excitation is purely lateral. These composite modes are not present when the structure is intact. One method to observe the coupled modes is through the emergence of extra new peaks on a frequency response function plot that can be obtained by using standard vibration testing tools.
This technique is inspired by the attempt to address some practical issues in current vibration-based damage detection methods. These methods are based on the fact that changes of physical properties (stiffness, mass and damping) due to damage will manifest themselves as changes in the structural modal parameters (natural frequencies, mode shapes and modal damping). The task is then to monitor the selected indicators derived from modal parameters to distinguish between undamaged and damaged states. However, the quantitative changes of global modal parameters are not sufficiently sensitive to a local damage. The proposed Coupled Response Measurements approach, on the other hand, interprets the dynamic changes caused by damage in a different way. This approach focuses on distinguishable behaviour changes (namely modes coupling) rather than the quantitative variations of modal parameters. One of the outcomes by using this approach is the feasibility of detecting fatigue cracks. A typical structural type discussed in the thesis is frame-like welded structures that consist of thin-walled tubular members.
The thesis demonstrates this methodology through both analytical simulations and experimental validations. The key development in analytical derivation is to model the cracked structural member by using a local flexibility matrix, which describes the relationship between the displacements and the forces on the cross section. This matrix can be formulated from the stress intensity factors that are available from fracture mechanics analysis. The expression of local flexibility matrix is related to the crack location and its severity. The matrix contains off-diagonal terms that essentially cause the coupled responses.
For the experimental part, the thesis presents the following validating tests: a free-free circular hollow section beam with an artificial crack, a frame structure with an artificial crack and the frame structure with a fatigue crack. For each structure, the test is conducted under three scenarios: intact and damaged with two levels of crack severities. Both driving point FRFs and cross point FRFs are obtained for each case. During the fabrication of the frame, similarity to real engineering structures has been taken into account. All structural members are with hollow section profiles and connected by welded joints. The fatigue crack is generated by a cyclic loading mechanism applied on the frame structure. The fatigue crack originates from a welded joint on one end of a branch member, and then propagates along the weld seam. This observation represents a typical scenario as found in many engineering structures. The outcomes of the experiment provide valuable insights to applications on real structures. In each case the proposed coupling indicator of damage has been successfully observed in the test.
A few recommendations are made for the future study on damage detection methods.