Impact damage in graphite-epoxy laminates: Damage characterization, acoustic emission, and finite element simulation. (Volumes I and II)

This thesis is concerned with the characterization and detection of impact damage in graphite/epoxy quasi-isotropic composite laminates. Particular emphasis is placed on non-visual impact damage. It is well-established that such damage can cause a significant degradation in performance. Therefore, both experimental and analytical investigations were conducted, addressing some of the more critical issues related to the general subject of impact damage in composite laminates.;In the analytical phase, two major issues were addressed. First, the state-of-damage during the impact event has been predicted using a numerical simulation procedure. A dynamic finite element stress analysis was performed from which the stress distributions across the laminate thickness were calculated for cross-ply and quasi-isotropic laminates. The maximum stress criterion was incorporated into the numerical procedure, based on which failure maps could be constructed. Based on these failure maps the damage initiation and progression during the impact event itself could then be predicted. These predictions were compared for quasi-isotropic laminates with the experimental observations based on nondestructive and destructive examinations. An excellent qualitative agreement between predictions and experiments has been established. Second, the initiation and progression of matrix damage (i.e., matrix splitting and delamination) in model specimens have been investigated employing a numerical simulation procedure. Strain energy release rates (for Mode-I and Mode-II fracture) were calculated using a finite element stress analysis and employing the crack closure technique. Predictions were compared with experiments and a good agreement has been established.;In the experimental phase the impact-damaged zone was fully characterized employing a variety of nondestructive and destructive testing techniques. The effect of the damage on degradation in mechanical performance is discussed. Primary emphasis in this research was given to the acoustic emission technique. During the course of this research it has been determined that a significant amount of emission is generated by the grating among existing fracture surfaces. Therefore, emphasis was placed on analyzing and identifying this friction emission. Based on this analysis it was demonstrated that by using acoustic emission the initiation and progression of matrix splitting can be tracked. Non-visual impact damage can be detected and located in real-time during cyclic loading. During quasi-static loading to failure, however, internal impact damage can not be detected by acoustic emission. When friction emission is separated, a better correspondence between acoustic emission event amplitude and the different modes of damage can be established. Other important issues such as the effect of loading frequency and load levels on acoustic emission results were investigated in detail.