Analytical and experimental investigation of the effects of the machining processes on the mechanical behaviour of carbon epoxy composite laminates
thesisposted on 23.05.2021, 16:44 by Muhammad Saleem
To join various components, drilling is the most frequently used machining process for carbon fiber-reinforced polymer (CFRP) composites. However, it induces various defects such as microcracks, resin degradation and fiber pull-out. In order to eliminate these problems, an appropriate machining process must be employed. Therefore, the main objective of this research is to conduct an analytical and experimental study to investigate the influence of the machining process on the mechanical behaviour of a CFRP structural component and assembly drilled with conventional (CM) and abrasive water-jet machining (AWJM) processes. All CFRP composite laminates did not demonstrate any prominent change in the mechanical properties during static tests. However, fatigue tests showed that the damage accumulation in conventional machining was higher than that in AWJM specimens. Thus, in the case of CM specimens the endurance limit was less than AWJM specimens. The difference in the mechanical behaviour of the composite laminates can be related to the initial surface integrity induced by the difference in the mechanism of material removal of each machining process. This difference in surface texture was responsible for the initiation of stress concentration sites as evident from infrared thermographic stress and thermal analysis. The heat dissipated from all laminates was also correlated to the damage accumulation. It was identified that the surface roughness criterion (Ra) used for the characterization of the surface roughness for metals was not suitable for composites as roughness values of all specimens were same, however different mechanical behaviour was observed. An IR thermographic damage criterion (TDC) was developed which related the temperature threshold area with damage evolution. TDC further confirms the superiority of AWJM compared to CM process. Further it was identified that AWJM process was less sensitive to the composite's stacking sequence compared to conventional machining. The main conclusion of this research is that the failure modes of composite structure parts and assembly are highly affected by the choice of the machining process. This study therefore confirms that the machining process has an important contribution for the design of composite parts and assembly in order to improve the service life of the machined composite structures.