Variation Of Contact Stiffness Research Articles

Modelling Friction-Induced Dynamic Instability Dedicated for Isogeometric Formulation

Flutter-type dynamic instability induced by friction is a highly nonlinear phenomenon and computationally expensive to model through transient analysis. An efficient way to make inference of such instabilities in a dynamical system is through analyzing the first-order effect of a perturbation at one of its equilibrium with eigenvalue analysis. The contact characteristics of such dynamical systems are typically modelled through the normal compliance approach with inference from experiments. In this case, the dynamical response of the system is implied to be sensitive to the contact stiffness modelled through the normal compliance approach. Typically, with the normal compliance approach, the continuum of the contact interface is approximated through a set of nonlinear springs which can be interpreted as a collocation method. Such approximations or the numerical implication of contact formulations in general for such problems is not largely studied. We focus on a variational formulation-based contact formulation without domain decomposition which is computationally efficient with small sacrifice in accuracy, where we imply that the dynamical response can be robustly modelled with the given accuracy. Further, we expose the inadequacy of the collocation method for such problems, where the dynamical system is observed to be sensitive to the extent of inaccuracy as a result of collocation for low values of contact stiffness. The inferences numerically imply the characteristics of the dynamical system for variation in contact stiffness.

Design and implementation of sequential excitation module for high fidelity piezoresponse force microscopy

The acquisition of accurate information through a contact resonance mode is critical for mapping weak electromechanical effect reliably by using piezoresponse force microscopy (PFM). However, it is very challenging to track resonance frequency shifting when the contact stiffness from the sample varies significantly. In this work, we have developed a sequential excitation (SE) module to enable high fidelity PFM. A customized discrete frequency sweep signal from an arbitrary waveform generator is used for drive excitation so that resonance frequency tracking is no longer necessary. Furthermore, the AC component of the piezoresponse is sampled by using an oscilloscope instead of using lock-in amplifiers. To accommodate high volume of data acquisition, a fast analysis method is also developed to fit the transfer function of the cantilever efficiently on the fly during scanning. Hardware implementation and data processing are described in detail. The capability of our SE module has been demonstrated on an ordinary PMN-PT film via first and second harmonic PFM, as well as a suspended freestanding MoS2 membrane that is very challenging to probe due to its substantial variation in contact stiffness.

Enhancement of hydrodynamic friction by periodic variation of contact stiffness

A common way to increase friction during lubricated sliding is to increase energy dissipation in the slider or the substrate, for example, by bulk viscoelasticity. It has recently been shown that lubricated friction can also be enhanced by surface architecture, specifically by periodic variation of near-surface stiffness. We study this phenomenon by considering a rigid cylinder undergoing lubricated sliding on a substrate with a periodic variation in mechanical stiffness. We model the process using the Reynolds theory for transient elasto-hydrodynamic lubrication (t-EHL). We developed a numerical scheme to solve this t-EHL problem and used it to study how surface variation in stiffness affects the motion of the cylinder, the distribution of hydrodynamic pressure, the liquid film thickness, and the friction force. Our results indicate that increasing the variation of the near-surface stiffness can significantly increase the friction force. The numerical method developed in this work can be applied to other transient EHL problems with slight modification. Our results provide insight into the mechanism of friction enhancement and can guide design of surfaces to control friction during lubricated sliding.

On the resolution of subsurface atomic force microscopy and its implications for subsurface feature sizing.

Ultrasound atomic force microscopy (AFM) has received considerable interest due to its subsurface imaging capabilities, particularly for nanostructure imaging. The local contact stiffness variation due to the presence of a subsurface feature is the origin of the imaging contrast. Several research studies have demonstrated subsurface imaging capabilities with promising resolution. However, there is limited literature available about the definition of spatial resolution in subsurface AFM. The changes in contact stiffness and their link to the subsurface resolution are not well understood. We propose a quantitative approach to assess the resolution in subsurface AFM imaging. We have investigated the influences of several parameters of interest on the lateral resolution. The quantification of the subsurface feature size can be based on threshold criteria (full width at half maximum and Rayleigh criteria). Simulations and experimental measurements were compared, revealing that the optimal choice of parameter settings for surface topography AFM is suboptimal for subsurface AFM imaging.

Decomposition failure of Li1.5Al0.5Ge1.5(PO4)3 solid electrolytes induced by electric field: A multi-scenario study using Scanning Probe Microscopy-based techniques

The failure phenomena of the full cell battery based on Li1.5Al0.5Ge1.5(PO4)3 ceramic electrolytes generally manifests as the macroscopic fading of the capacity, the governing mechanisms in the particular term of electrolytes remain elusive. In this study, Li1.5Al0.5Ge1.5(PO4)3 electrolytes after cycling are characterized by multiscale techniques, including Scanning Probe Microscopy, to systematically investigate the chemical and structural evolution processes. Secondary phases are proven to exist in the cycled Li1.5Al0.5Ge1.5(PO4)3. It is demonstrated that the Li1.5Al0.5Ge1.5(PO4)3 electrolytes suffer from both electrochemical and mechanical deterioration during the cycling processes. The changes in both chemical compositional and inhomogeneous mechanical stress result in the failure of the electrolyte and pose high resistance to lithium-ion transport. Furthermore, the ionic transport process is successfully manipulated by a local electric field with amplitude of 2 V and 4 V from the tiny tip in Scanning Probe Microscopy. Based on the observed electrochemical strain response and contact stiffness variation induced by dynamic Li-ion fluctuation under an applied AC electrical field, the degradation process of the Li1.5Al0.5Ge1.5(PO4)3 electrolyte is studied. It is determined that the chemo-mechanical degradation originates from the non-uniform Li ion distribution at the boundaries and/or voids in the interior of the electrolyte.

Time-dependent shear transformation zone in thin film metallic glasses revealed by nanoindentation creep

The holding time effect on the mechanical property, creep behavior and shear transformation zone has been systematically investigated by performing a series of nanoindentation creep measurements on magnetron-sputtered La-Co-Al, Zr-Cu-Ni-Al, Ni-Nb and W-Ru-B thin film metallic glasses. When the variation of contact stiffness with indentation depth is considered, the hardness, Young's modulus and strain rate have been found to decrease with increasing holding time. Meanwhile, strain rate sensitivity (m), and the volume and activation energy of shear transformation zone (STZ) have been analyzed, according to the cooperative shear model, at different stages of creep to explain the time-dependent mechanical behavior and properties. The higher strain rate results in few potential STZs with larger volume and activation energy, and the lower plastic stability of deformation manifested by smaller m, at the primary creep stage, as compared with those at apparent steady-state creep. Therefore, easier STZ activation, at longer holding time and lower strain rate, conduces to profuse operation of STZs and larger critical size for deformation mode transition from highly-localized to non-localized.

Dynamic analysis of torus involute gear including transient elastohydrodynamic effects

The torus involute gear can compensate large axial misalignments and may possess good meshing characteristics without lead correction. In order to study its dynamic characteristics and verify its feasibility of the practical application, a new efficient rigid-elastic coupling dynamic model of multi-tooth is established which includes effects of lubrication oil film and tooth deformations directly in contact simulation of gears. In this model, each tooth is connected with the gearwheel by a rotatable spring-damper element whose stiffness is calculated through analysis of tooth deformation. The normal tooth contact force is determined via Lankarani and Nikravesh model. Variation of contact stiffness and rotatable spring stiffness with contact points are both taken into account. Combined with tooth contact analysis, the computation of friction coefficient is implemented with high efficiency by introducing the average lubrication oil film height. A three-dimension multi-body model of a torus involute gear pair is employed and verified by an impact experiment. The simulated results provide useful information about tooth impacts, dynamic transmission error and lubrication conditions like oil film heights and friction coefficients, and show that this type of gear can work with good meshing characteristics. The contributions in this paper lay theoretical basis for the application of the torus involute gear.

Investigation of the contact stiffness variation of linear rolling guides due to the effects of friction and wear during operation

This study investigates the contact stiffness variation of linear rolling guides due to the effects of friction and wear during operation. The initial and final contact stiffness models were established. As a validation of the predicted varying stiffness, the experimental modal analysis was performed on a specialized linear guide system. The results show that the contact stiffness of linear guides decrease with the increase of running distance, and the entire stiffness decay could be divided into two distinct stages depending on how the thermal and wear effects influence the contact deformations of the balls at different running distances.

Torsional Vibration Effects on Induction Machine Current and Torque Signatures in Gearbox-Based Electromechanical System

The monitoring of heavy-duty electromechanical systems is crucial for their preventive maintenance planning. In these systems, the mechanical anomalies such as load troubles, great torque dynamic variations, and torsional oscillations lead to shaft fatigue and aging of other mechanical parts such as bearings and gearboxes. In this paper, a gearbox-based electromechanical system is investigated. Initially, a simple gearbox dynamic model is used to show the effects of rotating input, output, and mesh frequency components on the electromagnetic torque and consequently on the stator current signature. By this model, the influence of transmission error, eccentricities of pinion/wheel, and teeth contact stiffness variation is demonstrated for a healthy gearbox. Then, it is shown that the electrical machine can be considered as a torque sensor through electromagnetic torque estimation for torsional vibration monitoring without any extra mechanical sensor. A test-rig based on a 5.5-kW three-phase squirrel-cage induction motor connected to a wound-rotor 4-kW induction generator via a one-stage gearbox has been used to validate the proposed method.

Forced vibration analysis for dynamic coupling of finite-element spring-reduced suspension system

The current paper studies the dynamic behaviour of a coil spring in a vehicle suspension system. A portion of the coil spring is contacted with the top and bottom spring pads. The contact interface is modelled as a set of linear spring elements on the contact nodes of the coil spring. The eigensensitivity analysis is conducted to investigate the frequency variation of the stiffness-coupled system with respect to the contact stiffness variation. In the forced vibration analysis, the transfer function through the coil spring is obtained. The contribution of the coil spring modes to the load transfer is calculated by the modal contribution factor. It is shown that some of the longitudinal and bending vibration modes participate on the load transfer.

Coupled lateral bending–torsional vibration sensitivity of atomic force microscope cantilever

We study the influence of the contact stiffness and the ration between cantilever and tip lengths on the resonance frequencies and sensitivities of lateral cantilever modes. We derive expressions to determine both the effective resonance frequency and the mode sensitivity of an atomic force microscope (AFM) rectangular cantilever. Once the contact stiffness is given, the resonance frequency and the sensitivity of the vibration modes can be obtained from the expression. The results show that each mode has a different resonant frequency to variations in contact stiffness and each frequency increased until it eventually reached a constant value at very high contact stiffness. The low-order vibration modes are more sensitive to vibration than the high-order mode when the contact stiffness is low. However, the situation is reversed when the lateral contact stiffness became higher. Furthermore, increasing the ratio of tip length to cantilever length increases the vibration frequency and the sensitivity of AFM cantilever.

Parameters affecting contact stiffness of nominally flat surfaces

The problem of the elastic contact of nominally flat surfaces is reviewed towards the mathematical derivation of the resultant contact stiffness in terms of the surface topographies. The present study puts forth two approaches based on the elementary theories of elastic contact. When considering the theory of Greenwood and Williamson (GW model), it is found that the contact stiffness is dependent upon the standard deviation of height distribution of asperities, the effective radius of curvature of asperities and the density of asperities per unit area. When considering the theory of Onions and Archard (OA model), the contact stiffness is found to be dependent upon the correlation distance, the apparent area of contact and the standard deviation of height distribution of asperities. However, it is found that the standard deviation in both models strongly affects the value of contact stiffness. Moreover, the variation of contact stiffness with normal load is similar to that of a stiffening spring. It is also shown that the GW model underestimates the contact stiffness when it is compared with the OA model.