Lamb wave characterization of impact damage in composite plates

Hidden localized impact damage in composite structural components can seriously affect the safety and integrity of the structure. If left undetected, the damage can spread and lead to catastrophic failure. Therefore, real-time assessment of the impact event is highly desirable. It is toward this goal that the dissertation research is undertaken. The objectives of the dissertation are threefold: (1) to develop a reliable nondestructive method to accurately determine the local stiffness properties of composite plates, (2) to construct a theoretical model that accurately predicts the far-field response of the plate, and (3) inversion of the far-field data to determine whether an impact event and damage have occurred in the plate though waveform modeling and analysis. For the dissertation research, impact experiments are carried out on aluminum specimens and AS4/3501-6 [0/90]_8s graphite epoxy crossply composite plates. An instrumented impact testing system is used after appropriate modification to subject relatively large composite plates (30 x 60 cm) to low velocity impact with weighted hemispherical tups. The time history of the contact force and the surface motion caused by the impact load is recorded on the plate surface at several locations away from the impact location. The response of the plate due to localized impact is calculated using a modified laminate theory with shear correction for laminates of a general stacking sequence. Sources by both impact surface load and internal delamination are modeled. The solutions are applied to isotropic and orthotropic plates, where the far-field response is dominated by plate guided Lamb (flexural) waves. Signal analysis using wavelet transform method is applied to the experimentally acquired acoustic waveforms to improve the signal-to-noise ratio. Post-impact nondestructive evaluations are also preformed to correlate delamination and acoustic emission waveforms. It is determined that delamination can be successfully detected from the Lamb waves generated from damaging impacts.