Adhesion modulation In bio-inspired micropatterned adhesives by electrical fields

With steps towards Industry 4.0, it becomes imperative to the development of next-generation industrial assembly lines, to be able to modulate adhesion dynamically for handling complex and diverse substrates. The inspiration for the design and functionality of such adhesive pads comes from gecko’s remarkable ability to traverse rough and smooth topographies with great ease and agility. The emphasis in this thesis was to equip artificial micropatterned adhesives with such functionalities of tunability and devise an on-demand release mechanism. The project evaluates the potential of electric fields in this direction. The first part of this work focusses on integrating electric fields with polymeric micropatterns and studying the synergistic effect of Van der Waals and electrostatic forces. An in-house electroadhesion set up was built to measure the pull-off forces with and without electric fields. As a function of the applied voltage, adhesion forces can be tuned. The second part of the work demonstrates a novel route that exploits the in-plane actuation of the dielectric elastomeric actuators integrated with microstructure to induce peeling in them. Voltage-dependent actuation has been harnessed to generate the requisite peel force to detach the micropatterns. Overall, the findings of this thesis combine disciplines of electroadhesion, electroactuation, and reversible dry adhesives to gain dynamic control over adhesion.Im Einklang mit dem Fortschreiten in Richtung Industrie 4.0, wird es auch für die Entwicklung von industriellen Montagelinien der nächsten Generation unerlässlich sein, die Handhabung komplexer und unterschiedlicher Objekte zu flexibilisieren. Bioinspirierte Haftpads nach dem Vorbild des Gecko könnten zukünftig hierzu wesentlich beitragen. Der Schwerpunkt dieser Arbeit bestand darin, künstliche mikrostrukturierte Haftpads mit einem elektrisch schaltbaren Adhäsions- und Ablösemechanismus zu funktionalisieren, um die Grundlage für einen schnell schaltbaren, intelligenten Greifer zu schaffen. Der erste Teil dieser Arbeit konzentriert sich auf die Kombination elektrischer Felder mit elastomeren Mikrostrukturen und die Untersuchung der synergistischen Wirkung von Van der Waals- und elektrostatischen Kräften. Zur Messung der Adhäsion wurde ein individueller Aufbau realisiert und mit diesem die Feldstärkeabhängigkeit der Haftkräfte nachgewiesen. Der zweite Teil der Arbeit demonstriert einen neuartigen Ablösemechanismus unter Ausnutzung der lateralen Bewegung dielektrischer elastomerer Aktuatoren, um so ein Abschälen der Haftpads vom Substrat zu induzieren. Durch Variation der elektrischen Spannung wurde untersucht, wie sich diese auf die Ablösegeschwindigkeit der Haftpads auswirkt. Insgesamt kombinieren die Ergebnisse dieser Arbeit die Disziplinen Elektroadhäsion, Elektroaktuation und reversible trockene Klebstoffe, um so eine dynamische Kontrolle über die Adhäsion zu erhalten

Testing mechanical performance of adhesively bonded composite joints in engineering applications: an overview

The development of new adhesives has allowed to expand the application of bonding into the most diverse industrial fields. This review article presents the commonly used experimental methods for the investigation of mechanical performance of adhesively bonded joints in the aerospace, wind energy, automotive and civil engineering sectors. In these sectors, due to their excellent intrinsic properties, composite materials are often used along with conventional materials such as steel, concrete and aluminium. In this context, and due to the limitations that the traditional joining techniques present, adhesive joints are an excellent alternative. However, standardized experimental procedures are not always applicable for testing representative adhesive joints in these industries. Lack of relevant regulations across the different fields is often overcome by the academia and companies’ own regulations and standards. Additional costs are thus mitigated to the industrial sectors in relation with the certification process which effectively can deprive even the biggest companies from promoting adhesive bonding. To ensure continuous growth of the adhesive bonding field the new international standards, focusing on actual adhesive joints’ performance rather than on specific application of adhesive joints are necessary.This work was supported by the European Cooperation in Science and Technology [CA 18120]

BEHAVIOR OF RC BEAMS STRENGTHENED IN FLEXURE WITH SPLICED CFRP ROD PANELS

FRP laminates and fabrics, used as an externally bonded reinforcement (EBR) to strengthen or repair concrete members, have proven to be an economical retrofitting method. However, when used to strengthen long-span members or members with limited access, the labor and equipment demands may negate the benefits of using continuous EBR FRP. Recently, CFRP rod panels (CRPs) have been developed and deployed to overcome the aforementioned limitations. Each CRP is made of several small diameter CFRP rods placed at discrete spacing. To fulfill the strengthening length, CRP’s are spliced together and made continuous by means of overlaps (or finger joints). In this doctoral dissertation, the effectiveness of spliced CRPs as flexural strengthening reinforcement for RC members was investigated by experimental, analytical and numerical methods. The experimental research includes laboratory tests on (1) RC beams under four-point bending and (2) double-lap shear concrete specimens. The first set of tests examines the behavior of concrete members strengthened with spliced CRPs. Several beams were fabricated and tested, including: (a) unstrengthened, (b) strengthened with spliced CRPs, (c) strengthened with full-length CRPs, and (d) strengthened with full-length and spliced CFRP laminates. The double-lap shear tests serve to characterize the development length and bond strength of two commonly used CRPs. Several small-scale CRPs, with variable bond lengths, were tested to arrive to an accurate estimation of development length and bond strength. Several other specimens were additionally tested to preliminarily examine the effects of bond width and rod spacing. A 3D nonlinear finite element simulation was utilized to further study the response of CRP strengthened RC beams, by extracting essential data, that couldn’t be measured in the experimental tests. Additionally, analytical tools were added to investigate the behavior of tested bond and beam specimens. The first tool complements the double-lap shear tests, and provides mathematical terms for important characteristics of the CRP/concrete bond interface. The second tool investigates concrete cover separation failure, which was observed in the beam testing, for RC beams strengthened with full-length and spliced CRPs

Shape memory polymers as direct contact dry adhesives for transfer printing and general use

For most diminutive life on Earth, control over external adhesive forces is crucial for survival. As humans, we pay little notice because at our scale inertial forces typically overwhelm adhesive forces by a wide margin. Nonetheless, the study and development of dry adhesives, which rely on ubiquitous intermolecular attractions to repeatedly form and break attachment to their adherends, have garnered substantial interest in recent decades. High performance artificial dry adhesives may unlock the door for many exciting new technologies from nanoscale manufacturing to wall climbing robots, but thus far the challenges have proven substantial and few successful commercial applications have come to fruition. This dissertation represents an initial investigation into the benefits and potential limitations of developing shape memory polymer (SMP)-based dry adhesives. Prior to the presentation of experimental results, a review of the current state of dry adhesive knowledge including both theory, observations of the natural world, and lessons learned by other researchers in their attempts to develop a wide variety of synthetic dry adhesives is provided. It is concluded that dry adhesives fundamentally function through careful control of elastic energy, an idea that is very well suited to explore using SMPs which offer a large change in compliance across their thermal transition temperature. Thermoset epoxy SMPs are identified as an ideal choice for the investigation due to their mechanical strength, chemical resistance, manufacturability and convenient glass transitions among other features. The dry adhesive performance of a selected SMP is first evaluated for the purpose of microscale transfer printing, wherein micro-objects are assembled through precise control of adhesive surface forces. Significant benefits over existing solutions in terms of maximum adhesive strength during loading (~7 MPa), minimum strength for release (~0 MPa), and process versatility are confirmed, culminating in demonstrations of several challenging assemblies. The increase in adhesive strength is explained by invoking arguments from linear fracture mechanics and considering the dramatic compliance change experienced by the SMP between bond and load events. Advanced methods of heating and meaningful steps towards commercial-scale parallel printing processes are demonstrated. The suitability of SMP for larger-scale applications is considered next. Strength rivaling or exceeding known alternatives is demonstrated, showing adhesion exceeding 2 MPa for 6 mm diameter adhesives while retaining excellent releasability through the use of microstructuring. A method of internally heating the SMP by adding conductive carbon nanoparticles is explored, including quantitative analyses of conductivity and the SMP composite's storage and loss moduli. The resulting flexible and conductive bi-layer SMP adhesive supports load while attached to surfaces of varied curvature. Variations on the SMP formula have their adhesive and mechanical properties tested, and are used to produce a self-contained SMP prototype wall-hanging adhesive

Characterization of Mechanical Properties at the Micro/Nano Scale: Stiction Failure of MEMS, High-Frequency Michelson Interferometry and Carbon NanoFibers

Different forces scale differently with decreasing length scales. Van der Waals and surface tension are generally ignored at the macro scale, but can become dominant at the micro and nano scales. This fact, combined with the considerable compliance and large surface areas of micro and nano devices, can leads to adhesion in MicroElectroMechanical Systems (MEMS) and NanoElectroMechanical Systems (NEMS) - a.k.a. stiction-failure. The adhesive forces between MEMS devices leading to stiction failure are characterized in this dissertation analytically and experimentally. Specifically, the adhesion energy of poly-Si μcantilevers are determined experimentally through Mode II and mixed Mode I&II crack propagation experiments. Furthermore, the description of a high-frequency Michelson Interferometer is discussed for imaging of crack propagation of the μcantilevers with their substrate at the nano-scale and harmonic imaging of MEMS/NEMS. Van der Waals forces are also responsible for the adhesion in nonwoven carbon nanofiber networks. Experimental and modeling results are presented for the mechanical and electrical properties of nonwoven (random entanglements) of carbon nanofibers under relatively low and high-loads, both in tensions and compression. It was also observed that the structural integrity of these networks is controlled by mechanical entanglement and flexural rigidity of individual fibers as well as Hertzian forces at the fiber/fiber interface

Grenzflächenverhalten zwischen Faserverbundwerkstoffen und quasi-spröden Oberflächen

The use of externally glued fiber-reinforced polymers (FRP) as reinforcement to overcome the tensile deficiency of quasi-brittle elements (e.g. concrete beams, shear walls, masonry arches) has gained great popularity during the last years. Experimental and theoretical studies demonstrated that, when the FRP-substrate joint is mostly stressed in shear, one of the princiapal failure mechanisms is the debonding. It occurs when the shear capacity of the system is reached and a crack develops underneath the bond plane a few millimeters inside the substrate, causing the detachment of the composite element. In the present work the interface behavior of FRP joints is studied by means of experimental and numerical studies. A new single-lap test setup is proposed allowing to stably follow, for the first time, the entire equilibrium path of this kind of reinforcement. The proposed setup is then adopted to study the bond performances of FRPs applied on both concrete and masonry. The test results highlight a dependence of the global behavior from the initial bonded length and suggest the presence of non-negligible stresses orthogonal to the bonding plane. Also, comparing the results on concrete and on masonry, it is shown how, for this latter kind of substrates, the behavior is strongly influenced by the material texture and composition. To reproduce the changes in behavior observed during the experimental campaign, a novel cohesive zone model that accounts for the presence and the coupling between normal and tangential stresses is proposed and validated. Furthermore, the problem of the fatigue failure for this joints is studied and a new thermodynamically consistent numerical model that couples damage and plasticity under pure shear conditions is formulated. The numerical simulations coming from the two proposed models are compared to experimental results coming from the performed tests as well as from the available literature. Moreover, the improvements with respect to the models to date available are highlighted. Finally, taking advantage of new experimental studies and starting from theoretical considerations, a modified practical design formula for the debonding capacity for FRP reinforcements applied on masonry substrates is proposed and calibrated over a large database of results collected form the literature.Der Einsatz außen geklebter Faserverbundwerkstoffe (fiber-reinforced polymers, FRP) als Verstärkung, um die Schwäche quasi-spröder Elemente bei Zugbelastung zu überwinden, hat in den letzten Jahren an großer Aufmerksamkeit gewonnen. Experimentelle und theoretische Studien haben gezeigt, dass wenn die FKV-Substrat-Verbindung überwiegend auf Schub belastet wird, die Ablösung eine der hauptsächlichen Versagenskriterien ist. Diese tritt auf, wenn die Schubtragfähigkeit des Systems erreicht ist und sich ein Riss unterhalb der Klebfläche, wenige Millimeter innerhalb des Substrats ausbreitet und die Ablösung des Verbundelements bewirkt. In der vorliegenden Arbeit wird das Grenzflächenverhalten von FRP-Verbindungen mittels experimenteller und numerischer Studien untersucht. Eine neue Zugscherversuchsanordnung wird vorgeschlagen, die es erstmalig für diese Art von Verstärkung ermöglicht, der gesamten Kraft-Weg-Kurve stabil zu folgen. Der vorgeschlagene Aufbau wird genutzt, um die Klebergebnisse von Faser-Kunststoff-Verbunden zu untersuchen, die sowohl auf Beton als auch Mauerwerken appliziert werden. Die Testergebnisse zeigen eine Abhängigkeit des globalen Verhaltens von der initialen Länge der Klebung auf und deuten auf das Vorhandensein nicht unerheblicher Spannungen senkrecht zur Klebfläche hin. Um die Veränderungen im Verhalten zu reproduzieren, die während der experimentellen Testreihen beobachtet wurde, wird ein neuartiges Kohäsivzonenmodell vorgeschlagen und validiert, welches das Vorhandensein und die Koppelung zwischen Normal- und Tangentialspannungen berücksichtigt. Außerdem wird das Problem der Materialermüdung für diese Verbindungen untersucht und ein neues, thermodynamisch konsistentes, numerisches Modell formuliert, das Schaden und Plastizität unter reinen Scherbedingungen koppelt. Die numerischen Simulationen auf Basis der beiden vorgeschlagenen Modelle werden mit experimentellen Ergebnissen aus den durchgeführten Tests sowie der verfügbaren Literatur verglichen. Außerdem werden die Verbesserungen im Vergleich zu aktuell verfügbaren Modellen dargestellt. Zum Schluss wird - den Vorteil neuer experimenteller Studien ausnutzend und mit theoretischen Überlegungen als Ausgangspunkt - eine modifizierte, praxistaugliche Berechnungsformel für die Schubtragfähigkeit von FRP-Verstärkungen, die auf Mauerwerk-Substraten appliziert werden, vorgeschlagen und mit Hilfe einer großen Datenbank aus Ergebnissen in der Literatur kalibriert