Abstract
Thermoplastic composites have attracted increasing interest as alternative materials for primary and secondary structures of the next aircraft generation, owing to their fast processability and good reparability. The employment of these materials has triggered research in the fields of durability, fatigue, and damage tolerance, and prompted the development of alternative joining solutions that mitigate the dissimilarity between them and the remained metal parts in the aircraft. Among these technologies, Friction Riveting (FricRiveting) is an innovative, friction-based joining process suitable for polymers, composites and hybrid metal-composite structures. Prior to this work, the maturity of FricRiveting was limited to scientific knowledge at coupon level, including topics of heat generation, microstructure, physicochemical, and quasi-static mechanical properties. Moreover, no information on the behavior of joints under harsh environmental conditions, accidental damage scenarios or cyclic loading has been assessed, which are topics essential for the industrial transferability of this new joining technology. Therefore, this PhD work was devised to fill in the gaps in scientific and technological knowledge, with a focus on further develop and understand the fundamentals of the FricRiveting process, joint design, and mechanical integrity. Case study overlapped joints were produced using a titanium alloy Ti6Al4V rivet and woven carbon fiber reinforced polyether ether ketone (CF-PEEK) parts relevant to aviation.
By a stepwise analysis of the joining process along with X-ray micro-computed tomography and digital image correlation method, the joint formation and composite flow were assessed, showing the contribution of the squeezed material between the composite parts as an additional bonding mechanism to the mechanical interlocking of the plastically deformed rivet tip. The process temperature measured by thermography and thermometry exceeded the decomposition temperature of PEEK as well as the beta transus temperature of Ti6Al4V, leading to volumetric flaws in the rivet surrounding and morphological transformations in the plastically deformed rivet tip, which promoted local mechanical changes as confirmed by micro- and nanohardness measurements. Over the process temperature range analyzed in this work, three plastic deformation shapes of the rivet tip were detected and of these a bell-shaped rivet tip produced the strongest joints under shear loading. Through statistical analysis, a set of optimized joining parameters was obtained that produces sound joints with bell-shape rivet tip and above-average quasi-static strength. In addition, fundamental understanding of the effect of joint geometries on the joint strength was analyzed, in which by optimizing the joint design (washer size and tightening torque), 30 % increase in joint strength was achieved.
Although FricRiveting presented inferior quasi-static mechanical performance compared to reference lock bolting, the fatigue life of the joints showed an improvement up to 88 %, fulfilling aircraft industry requirements. The sensitivity of the friction riveted joints to impact damage and its propagation under quasi-static and cyclic loading was investigated through drop weight impact testing as well as microstructural characterization and post-impact single lap shear and fatigue testing. The joint strength and fatigue life were not compromised by barely-visible impact damage, which did not indicate a nucleation of critical delamination. However, visible impact damage introduced both delamination and premature failure at the metal-composite interface, leading to a 40 % decrease of quasi-static mechanical strength and the fatigue limit reached at load level of 58 % of the quasi-static joint strength. The residual quasi-static strength of those joints surviving 106 cycles of fatigue was evaluated revealing no critical damage accumulation at the examined load level for unimpacted and impacted joints.
The durability of the joints was assessed under hydrothermal and saline aging. With hydrothermal aging a 23 % increase of joint mechanical performance was observed after 28 days of exposure, as a result of PEEK post-crystallization. With saline aging a decrease up to 23 % of the quasi-static mechanical performance could be explained by corrosion induced in the external tightening elements, which no longer contributed to redistribution of the compression stress through the composite surface.
This PhD work succeeds in further developing the FricRiveting process by covering complex and relevant issues from scientific and engineering perspectives for the introduction of thermoplastic composites and providing a new joining solution for aircraft manufacturing.
DOI Link to Publication
