Dynamic stiction without static friction: The role of friction vector rotation

Abstract

In the textbook formulation of dry friction laws, static and dynamic friction (stick and slip) are qualitatively different and sharply separated phenomena. However, accurate measurements of stick-slip motion generally show that static friction is not truly static but characterized by a slow creep that, upon increasing tangential load, smoothly accelerates into bulk sliding. Microscopic, contact-mechanical, and phenomenological models have been previously developed to account for this behavior. In the present work, we show that it may instead be a systemic property of the measurement apparatus. Using a mechanical model that exhibits the characteristics of typical setups of measuring friction forces-which usually have very high transverse stiffness-and assuming a small but nonzero misalignment angle in the contact plane, we observe some fairly counterintuitive behavior: Under increasing longitudinal loading, the system almost immediately starts sliding perpendicularly to the pulling direction. Then the friction force vector begins to rotate in the plane, gradually approaching the pulling direction. When the angle between the two becomes small, bulk sliding sets in quickly. Although the system is sliding the entire time, macroscopic stick-slip behavior is reproduced very well, as is the accelerated creep during the "stick" phase. The misalignment angle is identified as a key parameter governing the stick-to-slip transition. Numerical results and theoretical considerations also reveal the presence of high-frequency transverse oscillations during the "static" phase, which are also transmitted into the longitudinal direction by nonlinear processes. Stability analysis is carried out and suggests dynamic probing methods for the approaching moment of bulk slip and the possibility of suppressing stick-slip instabilities by changing the misalignment angle and other system parameters.

The contribution of plastic sink-in to the static friction of single asperity microscopic contacts

“…This region corresponds to the initial, low load region of the FS lateral load versus displacement curve studied in figure 1g, for which there is minimal tip sink-in, as was shown in figure 1h. The origins of this seemingly elastic region whose existence is contrary to interpretations of plasticity that feature a fixed yield stress Y 0 such as those we have employed here are not entirely clear at present; however, we speculate that it arises from a combination of instrumentation effects such as tip rotation upon the immediate application of Q x and interfacial friction, which may screen the bulk material from some of the added shear stress [16]. This region shall be the subject of a forthcoming publication.…”

“…To shear, the interface adhesion and chemical bonding have to be overcome. [6], [13], [20] The striking result of Figs. 7b & 7c is that for the plastic contacts, these contributors to frictional resistance occur at approximately the same 𝜇 = 𝑄 𝑥 𝑃 𝑧 ⁄ ~ 0.08.…”

“…For the elastic contact, resistance to sliding arises from physical phenomena located at the contact. To shear, the interface adhesion and chemical bonding have to be overcome [6,16,23]. The striking result of figure 7b,c is that for the plastic contacts, these contributors to frictional resistance occur at approximately the same μ = Q x /P z ∼ 0.08.…”

“…The origins of this seemingly elastic region whose existence is contrary to interpretations of plasticity that feature a fixed yield stress 𝑌 0 such as those we have employed here are not entirely clear at present; however we speculate that is arises from a combination of instrumentation effects such as tip rotation upon the immediate application of 𝑄 𝑥 and interfacial friction which may screen the bulk material from some of the added shear stress. [13] This region shall be the subject of a forthcoming publication. While the junction growth model outlined above initially works well for the sharp, deep indentation geometry imposed by the Berkovich tip in Fig.…”

“…Laboratory single asperity contacts are often taken as representative, if somewhat idealized, analogues for the points of contact on macroscopic mating surfaces, where due to roughness, the true contact area is limited to a miniscule fraction of the apparent contact area [15]. Experimental issues such as low support frame stiffness and inter-axial coupling in surface force apparatus and scanning probe microscopy studies [6,16] have made it extremely challenging to achieve in these experiments the high normal contact pressures and plastic deformation common to asperities on real engineering surfaces. Uncertainties in tip shape and contact area in the latter technique further cloud the picture [17].…”