Numerical study of shearing of a microfibre during friction testing of a microfibre array

Abstract

Friction testing of microfibre arrays is typically conducted under displacement boundary conditions. For example, after the application of a compressive preload, the translation stage is fixed in the vertical direction while a shear displacement is applied by translating the stage laterally. A nonlinear rod model is used to compute the normal and shear forces acting on a typical microfibre during such a test. The normal load acting on a typical fibre is found to switch from compression to tension as the shear displacement increases. The critical shear force to detach a fibre is found to depend linearly on the compressive preload. The fibre is also found to become more stable as it is sheared, hence it never buckles. On the other hand, instead of fixing the vertical displacement, when a fibre is subjected to constant normal load, it becomes unstable upon shearing. We show that the buckling load (the applied normal load to make a fibre unstable) of a microfibril is reduced by the application of a shear displacement.

Contact compliance effects in the frictional response of bioinspired fibrillar adhesives

“…0) to traction (N' 2 F N , 0). Such a behaviour has been predicted theoretically by Kumar et al [16] for a clamped fibre subjected to imposed normal and shear displacement.…”

“…These observations thus support the fact that a high-static friction force mostly requires good adhesion of the fibre tips. As argued by Kumar & Hui [16] and Jagota & Hui [2], the contrasting results of Varenberg and Gorb as well as Kim et al could result from the fact that the experiments are realized either at an imposed normal load [14] or at constant vertical displacement [15]. When a fibre is sheared, its buckling load can decrease significantly [10,17].…”

Surface contact and design of fibrillar ‘friction pads’ in stick insects (Carausius morosus): mechanisms for large friction coefficients and negligible adhesion

“…Alternatively, it is likely that the shear forces measured at the smallest normal load were already sufficient to induce large amounts of side contact. Side contact is more easily achieved by a combined normal and shear load [ 43 , 45 – 48 ], because shear increases both the compliance of fibrillar structures and their tendency to buckle ([ 46 , 48 – 50 ], see also the electronic supplementary material, video S1). Hence, shear forces may increase not only the amount of side contact, but also the density of acanthae in surface contact, as a result of the effective reduction in length of the deflected fibres [ 40 , 41 ].…”

Functionally Different Pads on the Same Foot Allow Control of Attachment: Stick Insects Have Load-Sensitive “Heel” Pads for Friction and Shear-Sensitive “Toe” Pads for Adhesion

“…Treating a single fibre as an elastica, [54] showed that fibres maintain side contact at zero normal load if the work of adhesion between the fibre and the surface exceeds the elastic energy stored in the deformed fibre. While vertical fibres must have a high aspect ratio to fulfil this criterion [55] , angled fibres require less energy to be sufficiently deformed to make side contact [54] , [56] , [57] . When strong shear forces act on the acanthae, the resulting moment may exceed the level required to bend the acanthae into side contact, so that they maintain surface contact even for zero or negative loads (adhesion).…”