Surface quality and burr characterization during drilling CFRP/Al stacks with acoustic emission monitoring

Abstract

Riveting and screwing are generally used as an important joining technique of composite/metallic stacks, which require one shot drilling of hybrid stacks to improve productivity and geometrical accuracy. However, due to the different optimal machining parameters between CFRP composites and Al alloys, surface damage is prone to occur during drilling process, which significantly affects the fatigue life of components. In this work, acoustic emission (AE) technology was firstly employed to investigate the characteristics of AE signal corresponding with material removal behavior and machining defect during drilling CFRP/Al stacks. Fast Fourier transform and short-time Fourier transform were used in processing AE signals. Results showed that time-frequency diagram and root mean square (RMS) curve of AE signals could be used to distinguish different stages of drilling process with sufficient accuracy. The specific frequency range of damage pattern when drilling CFRP composite was determined by analyzing the frequency domain characteristics of AE signals. In addition, AE signals had significant correlation with machined surface quality. Amplitude and fluctuation of AE signal generally increased with the increase of hole surface roughness. Moreover, by extracting the amplitude characteristics of AE signal in time domain, the initial fracture and formation mechanism of burrs at hole exit could be further understand.