Investigation of a pulsed laser ablation process for bonded repair purposes of CFRP composites via peel testing and a design-of-experiments approach

The present study aims to investigate an automatic material ablation process performed by a pulsed Ytterbium green fiber laser for bonded repair purposes of carbon fibre reinforced composites. The investigated process parameters are the scanning speed, the pulse frequency, the hatching distance and the hatching pattern whereas the targeted objectives are the material removal rate as well as the quality of adhesion as characterized by a novel composite peel test. The use of a green laser, if properly implemented and optimized, turns out to offer a series of advantages. Not only it allows the precise, controllable and automatic -without human intervention-material ablation but it can change the morphology of the CFRP surface through micro-machining effects such as the creation of novel highly regular microstructural formations that significantly enhance bonding. We provide with experimental evidence that this is not always the case but happens only for specific levels of the process parameters. A design of experiments approach is implemented to search as effectively as possible the process parameters space. We utilize a Box-Behnken four-level factorial design to define the minimum number of experimental trials and build an appropriate test matrix. Analysis of variance is finally implemented to assess the influential parameters. Finally, the set of parameters that optimize the objective of simultaneously high peel strength and material removal rate are identified following a response surface methodology.