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Soft Contact Mechanics

Final Report Summary - SOFT-MECH (Soft Contact Mechanics)

The SOFT-MECH project was aimed at shedding light on the contact mechanics of soft materials, i.e. on the interactions between a “soft body” and other contacting objects. The main motivation for such a goal is given by the large variety of applications involving contact between soft rough bodies (e.g. automotive, aerospace, biomedical, earthquake engineering, microdevices) in all these cases, an optimized design strictly depends on the accurate knowledge of the interfacial phenomena, in terms of stresses, friction and wear.
Components of interest include not only “traditional” products such as tires, seals, wiper blades and dampers but also human and engineered soft tissues and a large number of soft polymers used in profusion to achieve cost and size reduction, increased efficiency and biodegradability. All this makes this project highly relevant to many new focus areas, such as bioengineering, nanotechnology and “green” technology, a key to the ERA quest for excellence and growth.
Indeed, the use and development of soft materials is partly hindered by the difficulty in accurately predicting their mechanical behavior. Phenomena, such as hyperelasticity, viscoelasticity and porosity, make engineering design complicated and problematic. To address this unresolved issue, advanced numerical methodologies to model soft contact mechanics have been developed and an extensive experimental validation programme, relying on a range of novel techniques, such as in-contact thermal microscopy and high speed imaging, has been carried out. More in detail, the following points have been pursued:
i) DRY CONTACT MECHANICS: The dry linear viscoelastic contact mechanics between rough surfaces have been analysed. An advanced mathematical formulation has been developed to estimate the viscoelastic friction entailed in the rough solid sliding contact mechanics (see Ref. [1] of the journal papers listed in the attached bibliography); the numerical investigation has shed light on the contact behavior resulting from the anisotropy of the solution, thus providing possible pathways to improve the design of mechanical rubber seals [1]. The following step has dealt with the effects related to the finite thickness of the layers into contact ([2] and [11]): this investigation has shown how the half space assumption, commonly accepted in literature, has to be carefully checked before being employed in numerical simulations. All these results rely on the steady state assumption, but in [3] an alternative formulation has been proposed to account for more complicated kinematic scenarios: in particular, the alternative harmonic periodic viscoelastic sliding case has been studied. Results, in terms of pressure distributions, displacements and dissipated power, can be considered important in a variety of applications, ranging from earthquake dampers to biological applications, such as in biological cells . All these achievements are summed up in [4], where the main advancements made with reference to the scientific literature are highlighted. Further developments are proposed in [5], where the role of the different roughness parameters in determining the viscoelastic friction is discussed. In parallel, experimental techniques have been developed to validate the numerical outcomes: in particular, thermal measures, carried out by means of the thermocamera FLIR X6540SC available at the host institution, have shown the influence that temperature can have on contact area [6] and friction [7]. Finally, an analytic solution for a simple punch shape, e.g. a sinusoidal profile, has been obtained in [8].

ii) WET CONTACT MECHANICS: A soft-ehl multigrid solver has been developed to deal with the lubricated contact of soft bodies. Some convergence issues, typically encountered when dealing with a strongly coupled problem, such as the lubrication of deformable bodies, have been resolved by employing a direct solver for the solution of the Reynolds equations [12]. Then, the formulation has been extended to account for a more complex rheology of the contact interface: in particular, the methodology include the linear viscoelasticity of the solid and the piezoviscosity of the fluid. Experimental comparison in terms of fluid thickness and friction is currently being finalized in collaboration with other researchers and industrial partners in the Tribology Group at IMPERIAL. Application of teh methodology to understand lubrication of elastomers in industrial environments is currently being pursued.

The project has also been extended beyond the original planned activities, looking, for example, at aspects of soft contacts linked to bio-adhesion: the peeling process and, in particular, its stability and its efficiency in presence of a rough substrate have been studied [9,10,11].

The outcomes of the project have wide-reaching impact for research and development of engineering solutions. The industrial relevance of the project is very significant . In particular, Bosch, Toyota Motor Europe and Shell have already invested in new research contracts aimed at the development of methodologies that, starting from the findings of this fellowship, will optimize sealing processes and elastomeric lubrication in automotive applications.

Furthermore, the Fellow has developed profitable collaborations with other Institutions, including Forschungszentrum Jülich (Germany) , Saarland University (Germany), Polytechnic of Bari (Italy), Michigan University (USA) and IMT (Italy) , and has carried out some teaching activities as senior tutor in Stress Analisys and Materials at Imperial College He has supervised a number of Master students and, since April 2015, he has co-supervised 2 PhD students. This has enabled him to successfully apply for a lectureship position in Applied Mechanics at the Polytechnic University of Bari, Italy.

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