Analysis of drilling of composite laminates

This dissertation deals with the characterization, modeling, and monitoring of drilling process of composite materials through various experimental and analytical investigations. Analytical models were developed which predicts critical thrust force and feed rate above which the delamination crack begins to propagate in the drilling of multi-directional laminated composites. The delamination zone was modeled as a circular plate, with clamped edge and subjected to different load profiles. Based on fracture mechanics, classical laminate theory and orthogonal cutting mechanics, expressions were obtained for critical thrusts and feed rates at different ply locations. The proposed models have been verified by experiments and compared with the existing models. It was found that the new developed models provide more accurate and rigorous results than the formers. Quality of holes and drilling-induced damage when drilling fiber reinforced composite laminates were experimentally studied. Several quality responses were measured as indices of drilling performance, including thrust force, delamination size, residual compression strength, and flexural strength. Effects of key drilling parameters on these responses were statistically analyzed, and optimal drilling conditions for high performance and free-damage drilling were identified. Experimental results revealed that the choice of drilling conditions are critical to hole performance especially when these materials are subjected to structural loads. An experimental study of acoustic emission as a tool for in-process monitoring and nondestructive evaluation of drilling of composites was conducted. Acoustic emission was used to examine the relationship between signal response and drilling induced damages. A procedure for discrimination and identification of different damage mechanisms was presented utilizing different signal analysis tools. Based on the results, frequency distribution and energy percentage of most important damage mechanisms occurring during drilling were determined. It was concluded that acoustic emission has a great potential for the application of online monitoring and damage characterization in the drilling of composite structures.