Process monitoring on drilling fiber-reinforced plastics and aluminum stacks using acoustic emissions

Abstract

In order to meet the lightweight demands in aviation industry, more and more multi-material composites are processed in aircrafts’ structural parts. A common connection to join these stack materials to other parts of the components is to use rivets. For that type of connection it is necessary to drill precise holes in the stack material. Due to the fact of the multi-layer design with different material properties and therefore a different machinability of the components, high demands on tool performance and process stability are set. When drilling carbon fiber reinforced plastics (CFRP), flaws, such as delamination and fringes as well as a poor bore hole quality, have to be avoided regarding the demands of the manufactured workpiece. In contrast, when processing the metal layer, there is a demand of burr-free holes after the machining process. This leads to high challenges in optimizing the drilling process and the tool geometry in order to accomplish long and reliable tool service times when processing generally expensive aircraft components. In order to monitor the drilling process of CFRP-aluminum stacks, this work shows the application of acoustic emission (AE) measurement technology to investigate the cutting mechanisms and enable a prediction for bore hole quality. For this purpose, the spectrum of the acoustic emission of the cutting process is examined for the various stages when cutting the different layers of the stack. Here, varying cutting mechanisms in regard to chip, burr formation and jamming of chips are observed and discussed. In wear-out tests the tool service times, the development of tool wear and the generated workpiece quality are recorded and the applicability of AE for process control is shown. Afterwards, suitable approaches are introduced in order to relate the tool wear state to the burr height by tracking the acoustic emission.