Design and mechanical integrity of friction riveted joints of thermoplastic composite

Thermoplastischer Verbundwerkstoffe sind aufgrund ihrer schnellen Verarbeitbarkeit und guten Reparaturfähigkeit alternative Materialien für Primär- und Sekundärstrukturen der nächsten Flugzeuggeneration. Die (industrielle) Verwendung dieser neuen Materialien hat die Erforschung der Haltbarkeit, Ermüdung und Schadenstoleranz sowie der Entwicklung alternativer Verbindungsverfahren eingeleitet. Vor allem die Auswirkungen der Artfremdheit zwischen ihnen und den verbleibenden metallischen Bauteilen im Flugzeug stehen dabei im Fokus. Reibnieten (Friction Riveting) ist ein innovatives, reibbasiertes Verbindungsverfahren, das für Kunststoff-, Verbundwerkstoff- und hybride Metall-Verbundstrukturen geeignet ist und daher für diese Art der Anwendung in Frage kommt. Bevor der vorliegenden Arbeit war die Reife des Reibnietens auf den Labormaßstab beschränkt, vor allem hinsichtlich wissenschaftlicher Erkenntnisse der Wärmeerzeugung, Mikrostruktur, physikalisch-chemische- und quasi-statische mechanische Eigenschaften. Darüber hinaus waren keine Informationen über das Verhalten der Verbindung unter kritischen Umgebungsbedingungen, Unfallschadenszenario und Ermüdungsverhalten verfügbar, die für die Übertragbarkeit auf den industriellen Maßstabunerlässlich sind. Diese Arbeit wurde durchgeführt, um die wissenschaftlichen und technologischen Wissenslücken des Reibnietens zu schließen. Dabei wurde sich vor allem auf das grundlegende Prozessverständnis, das Verbindungsdesign und die mechanische Integrität von Überlappverbindungen unter Verwendung der Titanlegierung Ti6Al4V und gewebtes kohlefasergewebes - Polyetheretherketon (CF-PEEK) fokussiert. Durch eine schrittweise Analyse des Fügeprozesses wurden die Verbindungsbildungsmechanismen und der Materialfluss bewertet. Es konnte gezeigt werden, dass das zusammengedrückte Material zwischen den Verbundteilen als zusätzlicher Haftmechanismus maßgeblich zum mechanischen Formschluss der plastisch verformten Nietspitze beiträgt. Die Prozesstemperatur übersteigt die Zersetzungstemperatur von PEEK sowie die β-Transus-Temperatur von Ti6Al4V, was zu Volumenfehlern im Verbundwerkstoff in der Nietumgebung und zu morphologischen Umwandlungen in die plastisch verformte Nietspitze führt. Über den in dieser Arbeit untersuchten Prozesstemperaturbereich wurden drei plastische Verformungsprofile der Nietspitze festgestellt, von denen die glockenförmige Nietspitze unter Scherbelastung die stärksten Verbindungen erzeugte. Ein optimierter Prozessparametersatz wurde ermittelt, um Verbindungen mit überdurchschnittlicher quasi-statischer Festigkeit herzustellen. Durch die Optimierung des Verbindungsentwurfes (Außendurchmesser der Scheibe und Anziehdrehmoment) wurde darüber hinaus eine Steigerung der Verbindungsfestigkeit um 30% erzielt. Obwohl die quasi-statischen mechanischen Eigenschaften reibgenieteter Verbindungen denen verschraubter Referenzverbindungen unterlegen sind, hielten die Verbindungen unter Ermüdungsbelastung mit 66 % ihrer quasi-statischen Festigkeit 105 Zyklen stand. Somit wurde eine Verbesserung von bis zu 88 % im Vergleich zum Ermüdungsverhalten herkömmlicher mechanischer Befestigungstechniken erreicht. Der Einfluss verschiedener Rissausbreitung auf die quasi-statischen und zyklischen mechanischen Eigenschaften sowie auf die Schadensausbreitung wurde untersucht. Die Festigkeit und die Ermüdungslebensdauer der Verbindung wurde bei kaum sichtbaren Aufprallschäden nicht beeinträchtigt, was auf keine Keimbildung kritischer Delamination hindeutet. Andererseits führten sichtbare Aufprallschäden zur Delamination und vorzeitigem Versagen der Metall-Verbund-Grenzfläche was zu einer Verringerung von etwa 40 % der quasi-statischen mechanischen Festigkeit sowie eine Ermüdungsgrenze entsprechend 58 % der quasi-statischen Festigkeit führt. Die Dauerhaftigkeit der Verbindungen wurde unter hydrothermaler und Salz Alterung bewertet. Durch Alterung in hydrothermaler Alterungsbedingung erhöhte sich nach 28-tägiger Aussetzung die mechanische Leistungsfähigkeit der Verbindung um 23 % als Ergebnis der Nachkristallisation des PEEK. Bei der Salzalterung konnte die Verringerung der Festigkeit um etwa 23 % durch die Korrosion der äußeren Spannelemente erklärt werden, die dadurch nicht mehr zur Umverteilung der Druckspannung durch die Verbundoberfläche beitrugen. Durch diese Arbeit gelingt es, das Reibnietverfahren weiterzuentwickeln, indem komplexe und relevante Themen aus wissenschaftlicher und technischer Sicht für die weitere Verwendung thermoplastischer Verbundwerkstoffe und dieser neuen Verbindungslösung in die Flugzeugfertigung behandelt werden.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.Brazilian National Council for Technological and Scientific Development- CNPq (Brazil) [grant number 200695/2015-0

Rebitagem por fricção de juntas híbridas de ti-6al-4v e Poliéster termofixo reforçado com fibra de vidro

Friction Riveting is an innovative spot joining technology for metal-polymer hybrid structures. This MSc thesis provided for the first time in the literature, a fundamental understanding on the Friction Riveting process for metal-thermoset composites joints. Joints of Ti-6Al-4V rivet and pultruded glass fiber reinforced thermoset polyester part were produced under three joining conditions with different heat input. Thorough analytical techniques were used to understand the physics of the process and the effect of the energy input on the final microstructure of the joined parts, the physico-chemical changes in the composite and the local and global mechanical properties of the joints. The process temperature reached values up to 761 ± 2°C indicating intrinsic degradation of the composite, formation of a softened/molten glass interlayer between the rivet and the composite and complex metallurgical transformations in the metallic rivet. Through monitoring of the process temperature and torque, an unstable friction regime was observed for FricRiveting of pultruded thermoset composites leading to distinguished extents of composite degradation. The microstructure of the Ti-6Al-4V alloy changed across the length of the rivet, from the equiaxed morphology to acicular structures in the rivet tip, where plastic deformation occurred. Three microstructural zones were proposed for each joint part including two thermo-mechanically affected zones and a heat affected zone. Microhardness mapping was performed in the metallic rivet evidencing an increase from the center to the tip of the rivet, with a hardness increment of over 20% compared to the base material (HVTi6Al4V= 300- 320 HV). The glass interphase consolidated in the metallic surface reached values of up to 974 HV followed by a drastic decrease to 24 HV in the polyester matrix located out of the joint area. The ultimate bearing strength ranged between 60 MPa and 166 MPa. Lesser composite degraded areas led to stronger joints. Two failure modes were observed combining initial composite bearing followed by final failure through shear of the rivet with partial rivet pullout or by full rivet pull-out. Complex failure micro-mechanisms were observed including the combination of cohesive and adhesive failures through the glass layer and the damaged composite interface. Friction-riveted joints achieved an ultimate lap shear strength of up to 80% to that of a similar bolted joint. A case study for a presumptive truss bridge application of friction-riveted joints showed a necessary of 92 rivets per truss node, 43% less than previous studies and with potential for further optimization.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Rebitagem por Fricção é uma tecnologia de união pontual inovadora para estruturas híbridas metal-polímero. Esta tese de mestrado apresenta pela primeira vez na literatura um entendimento aprofundado do processo de Rebitagem por Fricção para juntas de metal e compósito termofixo. Juntas de rebite de Ti-6Al-4V e componente pultrudado de poliéster termofixo reforçado com fibra de vidro foram produzidas seguindo três condições de processo com diferentes aportes térmicos. Diversas técnicas analíticas foram utilizadas para entender a física do processo e o efeito do aporte térmico na microestrutura final dos componentes unidos, mudanças físico-químicas no compósito, formação de camada vítrea intermediária entre o compósito e o rebite metálico e transformações metalúrgicas. Através do monitoramento da temperatura processual e do torque, um regime friccional não estável foi observado para a Rebitagem por Fricção de compósito pultrudado termofixo resultando em distintas extensões da degradação do compósito. A microestrutura da liga de Ti-6Al-4V transforma-se ao longo da secção transversal do rebite, de uma morfologia equiaxial no centro do rebite para uma estrutura acicular em sua extremidade, onde ocorre deformação plástica. Três zonas microestruturais foram propostas para cada componente da junta incluindo duas zonas termo mecanicamente afetadas e uma termicamente afetada. Mapa de microdureza foi realizado no rebite metálico evidenciando um aumento do centro para a extremidade do mesmo, próximo a 20% comparado com o material de base (HVTi6Al4V= 300-320 HV). A interfase vítrea consolidada na superfície do rebite metálico apresentou dureza em torno de 974 HV seguido de um drástico decaimento para 24 HV na matriz de poliéster localizada fora da região de união. A tensão máxima de apoio variou entre 60 e 166 MPa. Juntas com menor área degradada apresentaram os melhores desempenhos mecânicos em ensaio quase estático de cisalhamento. Dois modos de falha foram observados combinando um modo de falha inicial por deformação plástica severa no compósito seguida de falha final por cisalhamento no rebite metálico com parcial remoção do mesmo ou por remoção completa do rebite. Complexos micro mecanismos de falha foram observados incluindo a combinação de falha adesiva e coesiva através da interface entre interfase vítrea e compósito degradado. Juntas rebitadas por fricção atingiram resistência ao cisalhamento de 80% da obtida para juntas parafusadas. O estudo de caso para uma ponte hipotética treliçada revelou um número necessário de rebites de até 92 rebites por nó da ponte, 43% a menos que o encontrado em trabalhos anteriores, com potencial para futuras optimizações

The influence of clamping pressure on joint formation and mechanical performance of Ti6Al4V/CF-PEEK friction-riveted joints

This work aims at investigating the influence of pre-set clamping pressure on the joint formation and mechanical strength of overlapping direct-friction-riveted joints. A pneumatic fixture device was developed for this work, with clamping pressure varying from 0.2 MPa to 0.6 MPa. A case study on overlapping joints using Ti6Al4V rivets and woven carbon fiber-reinforced polyether-ether-ketone (CF-PEEK) parts were produced. Digital image correlation and microscopy revealed the expected compressive behavior of the clamping system and the continuous pressure release upon the joining process owing to the rivet plastic deformation and the polymer squeezing flow. Two preferential paths of material flow were identified through the alternate replacement of the upper and lower composite parts by a poly-methyl-methacrylate (PMMA) plate-the composite upward and squeezing flow between the parts which induced their separation. The ultimate lap shear forces up to 6580 ± 383 N were achieved for the direct-friction-riveted CF-PEEK overlap joints. The formation of a gap to accommodate squeezed polymer between the composite parts during the process had no influence on the joint mechanical performance. The increase in the clamping pressure for joints produced with a low friction force did not affect the joint-anchoring efficiency and consequently the joint strength. On the other hand, the combined effect of a high-friction force and clamping pressure induced the inverted bell shape of the plastically deformed rivet tip, a lower anchoring efficiency, and the delamination of the composite, all of which decrease the mechanical strength by 31%. Therefore, the higher the friction force and clamping pressure, the more defects would be generated in the composite parts and the more changes in the shape of the plastically deformed rivet tip, leading to a lower level of quasi-static mechanical performance. All the joints failed by initial bearing of the composite and final rivet pull-out. The findings of this work can contribute to further improvement of the clamping design for industrial application.CNPq, Brazil ; Helmholtz Association ; Austrian aviation program TAKE OFF ; BMVIT-Austrian Ministry for Transport, Innovation and Technolog

Influence of rotational speed on the microstructure and mechanical performance of friction-riveted thermosetting composite joints

Facing the actual demand for efficient joining technologies for multi-materials structures, Friction Riveting was shown to be an alternative joining technology for thermoset composite profiles in civil infrastructure. This process is based on plasticizing and deforming the tip of a rotating metallic rivet within a polymeric component through frictional heating. The feasibility of friction-riveted hybrid joints of Ti-6Al-4V/glass-fiber reinforced thermoset polyester was already demonstrated in a separate work. This paper complements this study by analyzing the rivet rotational speed effect on the process temperature, joint microstructure and the local and global mechanical properties of the joint. Joints were produced using two different levels of rotational speed: 9000 rpm and 10000 rpm (the other parameters were kept constant). The results showed process temperatures (655-765 °C) up to 96% higher than the onset decomposition temperature of the polyester matrix (370 °C); this led to severe degradation of the composite in the joint area. The increase in rotational speed, and therefore in heat generation, led to a statistically insignificant increase of the rivet penetration depth and the rivet diameter widening. However, the extension of the degraded composite area increased 47% which was responsible to deteriorate in 50% the joint tensile strength (from 4.0 ± 1.2 kN to 2.0 ± 0.7 kN). Moreover, the microhardness map of the joined rivet evidenced possible phase transformations in the alloy, favoring the material hardening by increasing in rotational speed. However, no correlations could be established between the changes in hardness and the joint tensile strength since the joints majority failure by full rivet pull-out. Thereby, for the improvement of friction-riveted Ti-6Al-4V/ glass-fiber reinforced thermoset polyester joints, the optimization of rotational speed is essential. This can guarantee the formation of efficient anchored joints and wider rivet tip deformation, concomitantly with the minimizing of the extension of the matrix degradation and finally leading to better tensile strength of the joints

On the process-related rivet microstructural evolution, material flow and mechanical properties of Ti-6Al-4V/GFRP friction-riveted joints

In the current work, process-related thermo-mechanical changes in the rivet microstructure, joint local and global mechanical properties, and their correlation with the rivet plastic deformation regime were investigated for Ti-6Al-4V (rivet) and glass-fiber-reinforced polyester (GF-P) friction-riveted joints of a single polymeric base plate. Joints displaying similar quasi-static mechanical performance to conventional bolted joints were selected for detailed characterization. The mechanical performance was assessed on lap shear specimens, whereby the friction-riveted joints were connected with AA2198 gussets. Two levels of energy input were used, resulting in process temperatures varying from 460 ± 130 °C to 758 ± 56 °C and fast cooling rates (178 ± 15 °C/s, 59 ± 15 °C/s). A complex final microstructure was identified in the rivet. Whereas equiaxial α-grains with β-phase precipitated in their grain boundaries were identified in the rivet heat-affected zone, refined α' martensite, Widmanstätten structures and β-fleck domains were present in the plastically deformed rivet volume. The transition from equiaxed to acicular structures resulted in an increase of up to 24% in microhardness in comparison to the base material. A study on the rivet material flow through microtexture of the α-Ti phase and β-fleck orientation revealed a strong effect of shear stress and forging which induced simple shear deformation. By combining advanced microstructural analysis techniques with local mechanical testing and temperature measurement, the nature of the complex rivet plastic deformational regime could be determined.Helmholtz Associatio