Application of System Identification using ERA/NExT for Damage Assessment in Composite Laminates

Polymer matrix composites are widely used in aircraft structures due to their superior specific stiffness and strength. However, laminated composites are susceptible to delaminations due to the low interlaminar fracture toughness of the matrix. Traditional NDI techniques for damage detection involve significant costs due to the requirement for the grounding of aircraft, shutdown time, and labour involved in inspections. Vibration monitoring for damage assessment in aircraft has several advantages such as being applicable in situ, not requiring external excitation and the potential for application as a continuous structural health monitoring system. Frequency monitoring is employed in modern aircraft for engine diagnostics and identification of major faults; however, when applied to structural components, monitoring frequency shifts only identifies the presence of damage but not its exact location and severity. Eigensystem Realization Algorithm (ERA) has been proposed for application to many structural components but not for assessment of delamination in composites.This thesis proposes the application of ERA coupled with Natural Excitation Technique (NExT) for delamination assessment in laminated composite structures, including stiffened composite panels. NExT provides the advantages that any type of excitation of the structure can be employed and the location of the excitation point need not be known. The algorithm for delamination assessment using ERA coupled with NExT was developed using Matlab code. The proposed method is first investigated and validated using transient vibration response generated by finite element simulation of composite laminates with delaminations. The simulated response is fed into the ERA/NExT algorithm to identify changes in the system parameters to estimate the location and reductions in bending stiffness in the damaged areas. The use of different types of excitation modes and excitations provided at different locations have been examined. Sensitivity analyses have been performed conducted study the influence of spacing of measurement points on the accuracy of delamination size and severity estimated using responses by numerical modelling of composite beams and laminated plates. The proposed method is validated experimentally with vibration measurements conducted on laminated carbon/epoxy beams, plates and stiffened panels with embedded delaminations and estimating the location, size and severity of the delaminations in the samples from the measured dynamic responses