Study of static and fatigue behaviour of RTM carbon epoxy laminates designed for innovative preforms

Composite materials have gained large interest during the last decades due to their high strength and high stiffness-to-weight ratio, corrosion resistance and outstanding fatigue resistance. Anyway even the most proven manufacturing techniques are labor intensive and thus costly and with poor repeatability, so that the future success of these materials probably depends on the development of the manufacturing processes. Lamiflex company (Ponte Nossa, BG) patented a new efficient method for the realization of Resin Transfer Moulding preforms which represents the core of a RTM process for the manufacturing of structural components with complex geometries, such as airframe. The analysis done in the present thesis work regards the study of the static and the fatigue behavior of the carbon-epoxy laminate designed for the Lamiflex new concept preforms, and the study of the static and the fatigue behavior of the holed material, considering a hole geometry taken from an helicopter fuselage frame. Moreover, the performance of a newly designed braided reinforcement for the holed zone, which was designed to delay delaminations initiation, has been assessed showing its inefficiency. Static tension and compression tests together with tension-tension fatigue tests have been performed and the static tension results, including the strain maps obtained with Digital Image Correlation (DIC), have been compared with linear elastic FEM simulations showing a good accordance. To obtain further information on the fatigue damage distribution and evolution non-destructive evaluation have been performed, using DIC during fatigue loading and Ultrasounds for post-fatigue damage observation. DIC on-line monitoring has highlighted the most strained zones on the external layer of the holed laminate and the change of the dominating damage mechanism with the change of the stress level. Through Transmission Ultrasounds have allowed the most delaminated zones to be highlighted, and their depth inside the specimen has been evaluated through Pulsed Echo Ultrasounds, identifying the weakest zones inside the holed laminate.