| Resumo : |
The use of composite materials has become current in aerospace industry due the development of their manufacturing processes and the increasing knowledge acquired regarding their mechanical properties and behavior. As a result, more complex ways to use composites have arisen, such as alternative bonding technologies to replace the traditional fasteners that are often a poor choice in terms of weight reduction and stress concentration. This dissertation investigates the fracture behavior of three types of carbon/epoxy composite joints, named co-cure, co-bonding and secondary bonding, and how hygrothermal aging affects their properties. Two campaigns were conducted: one to be the reference, performed at room conditions, and one to evaluate the influence of temperature and moisture. Fracture toughness was assessed through a set of mechanical tests carried out under pure mode I (Double Cantilever Beam), pure mode II (Four Point Bend, End Notched Flexure) and mixed-mode I/II at 35%, 50% and 75% of mode II (Mixed Mode Bending) in order to obtain the failure envelope corresponding to each bonding technology. The mechanical tests showed that co-cure presented the lowest values of fracture toughness at ambient conditions under any failure mode, while co-bonding and secondary bonding alternated as the highest fracture toughness during mixed-mode tests, with co-bonding ending up as the highest under pure mode II. For the aged set, however, co-cure presented by far the highest fracture toughness under pure mode II, while secondary bonding was the most tough under mixed-mode tests and all three technologies were almost equal under pure mode I. Fractography analysis was performed in tested specimens to determine the failure mechanisms involved during fracture process and how temperature and moisture affected it, showing that the different values of fracture toughness resulted from a change in failure mode derived from the hygrothermal influence. Quantitatively, Dynamic Mechanical Analysis and Differential Scanning Calorimetry tests determined that matrix and adhesive had their glass transition temperature lowered 20oC in average by the aging process, explaining the different results between dry and aged sets of tests, proving that the extreme ambient conditions cannot be neglected during project concept. |