Xi Shi

Measurement and Modeling of Normal Contact Stiffness and Contact Damping at the Meso Scale

Modeling of contact inteffaces that inherently include roughness such as joints, clamping devices, and robotic contacts, is very important in many engineering applications. Accurate modeling of such devices requires knowledge of contact parameters such as contact stiffness and contact damping, which are not readily available. In this paper, an experimental method based on contact resonance is developed to extract the contact parameters of realistic rough surfaces under lightly loaded conditions. Both Hertzian spherical contacts and flat rough surfaces in contact under normal loads of up to 1000 mN were studied. Due to roughness, measured contact stiffness values are significantly lower than theoretical values predicted from smooth surfaces in contact. Also, the measured values favorably compare with theoretical values based on both Hertzian and rough contact surfaces. Contact damping ratio values were found to decrease with increasing contact load for both Hertzian and flat surfaces. Furthermore, Hertzian contacts have larger damping compared to rough flat surfaces, which also agrees with the literature. The presence of minute amount of lubricant and wear debris at the interface was also investigated. It was found that both lubricant and wear debris decrease the contact stiffness significantly though only the lubricant significantly increases the damping.

An Elastic-Plastic Spherical Contact Model Under Combined Normal and Tangential Loading

Spherical contact under combined normal and tangential loading has been investigated by many researchers, and some physically based criteria were proposed to capture the sliding inception, e.g., the local yielding criterion of the Kogut-Etsion (KE) model and the tangential stiffness criterion of the Brizmer-Kligerman-Etsion (BKE) model. In this work, by utilizing the maximum frictional shear stress criterion for the sliding inception, a finite element model for obliquely loaded spherical contact has been developed, which realized a friction transition from the KE model to the BKE model, with an increasing normal approach. The stress, strain, tangential force, normal force, and contact area during tangential loading are investigated using different models. It was found that with an elastic normal displacement preload, material failure is initiated on the surface, while with an elastic-plastic normal displacement preload the failure is initiated under the surface and then extends to the surface with the increasing tangential load. With an elastic-plastic normal displacement preload, there is an obvious normal force release during tangential loading. Different from the full stick model, both the Coulomb friction model and the proposed model are partial slip models in nature. However, the Coulomb friction is more empirically determined with some arbitrary friction coefficient, whereas the proposed model is based on physics parameters. Furthermore, both the Coulomb friction model and the proposed model predict a lower tangential force at the same tangential displacement, a slower growth of the contact area under elastic normal displacement preload, and a faster growth of the contact area under an elastic-plastic normal displacement preload compared to the full stick model.

Investigation of Contact Stiffness and Contact Damping for Magnetic Storage Head-Disk Interfaces

As the areal density of magnetic disk storage continues to increase and head-disk spacing decrease, contact between the recording slider and the rotating media becomes imminent. In order to predict contact forces, fly-height modulations, and off-track motions, dynamic models are typically used. A critical element of these models is the contact stiffness and damping arising from the interfacial interaction between the slider and the disk. In this paper, we review different models for predicting contact stiffness based on roughness and layered media and then we report experimental data of both contact stiffness and contact damping of typical head-disk interfaces. It is found that the contact stiffness models (based on roughness alone), over-predict the contact stiffness of actual head-disk interfaces by as much as an order of magnitude. Also, it is found that the contact damping ratio is typically few percent and its behavior is substrate dependent. In addition, the effects of a molecularly thin lubricant and humidity on contact stiffness and damping were experimentally investigated and no significant effects were found.Copyright

A Dynamic Friction Model for Unlubricated Rough Planar Surfaces

Modeling dynamic or kinetic friction for realistic engineering surfaces continues to be a challenge, partly due to the coupling between system dynamics and interfacial forces. In this paper, a dynamic friction coefficient model for realistic rough surfaces under external normal vibrations is developed. From the system dynamic model, the instantaneous time varying normal separation at the interface is obtained under normal harmonic excitation. Subsequently, the instantaneous dynamic contact and tangential (friction) forces are calculated as a function of the instantaneous normal separation. The dynamic friction coefficient defined as the ratio of the time varying friction to the interfacial normal forces that explicitly includes interfacial damping, is also calculated. The results show that a mean increase in the instantaneous normal separation may or may not lead to a decrease of the mean friction force and the mean friction coefficient, which is supported by published data. For unlubricated elastic sliding contact conditions considered in this paper the effect of damping on the dynamic friction coefficient is found to be negligible, whereas loss of contact causes significant apparent dynamic friction force and dynamic friction coefficient reductions. Several different interpretations of the time varying dynamic friction coefficient are presented and the implications of using a simple constant value to represent the lime varying dynamic friction coefficient are discussed.

Adhesive Effects on Dynamic Friction for Unlubricated Rough Planar Surfaces

As the size of contacting and sliding tribosystems decrease, intermolecular or adhesive forces become significant partly due to nanometer size surface roughness. The presence of adhesion has a major influence on the interfacial contact and friction forces as well as the microtribosystem dynamics (microtribodynamics) and thus influences the overall dynamic friction behavior. In this paper, a dynamic friction model that explicitly includes adhesion, interfacial damping, and the system dynamics for realistic rough surfaces was developed. The results show that the amplitude and mean value of the time varying normal contact and friction forces increase in the presence of adhesion under continuous contact conditions. Also, due to the attractive nature of adhesion, its presence delays or eliminates the occurrence of loss of contact. Furthermore, in the presence of significant adhesion, dynamic friction behavior is significantly more complicated compared to the no adhesion case, and the dynamic friction coefficient predictions may be misleading. Thus, it is more appropriate to discuss dynamic friction force instead of dynamic friction coefficient under dynamic conditions.