Estimation of the normal contact stiffness for frictional interface in sticking and sliding conditions

Modeling of frictional contact systems with high accuracy needs the knowledge of several contact parameters, which are mainly related to the local phenomena at the contact interfaces and affect the complex dynamics of mechanical systems in a prominent way. This work presents a newer approach for identifying reliable values of the normal contact stiffness between surfaces in contact, in both sliding and sticking conditions. The combination of experimental tests, on a dedicated set-up, with finite element modeling, allowed for an indirect determination of the normal contact stiffness. The stiffness was found to increase with increasing contact pressure and decreasing roughness, while the evolution of surface topography and third-body rheology affected the contact stiffness when sliding

Contact stiffness estimation for PMMA/STEEL contact pair

Modelling of frictional contact systems with high accuracy needs the knowledge of several contact parameters that are mainly related to the properties of the contact interfaces. While the interface parameters cannot be directly obtained by performing local measurements, the values estimated by means of analytical/numerical models are not reliable to describe the contact behavior, which affects in a prominent way the complex contact phenomena. This work presents a newer approach for identifying reliable values of the normal contact stiffness between rough surfaces in both sliding and sticking conditions as a function of contact pressure, surface roughness and materials. The combination of dynamic experimental tests, on a dedicated set-up, with finite element modelling allowed for an indirect determination of the normal stiffness at the contact

Numerical and experimental analysis of nonlinear vibrational response due to pressure-dependent interface stiffness

Modelling interface interaction with wave propagation in a medium is a fundamental requirement for several types of application, such as structural diagnostic and quality control. In order to study the influence of a pressure-dependent interface stiffness on the nonlinear response of contact interfaces, two nonlinear contact laws are investigated. The study consists of a complementary numerical and experimental analysis of nonlinear vibrational responses due to the contact interface. The laws investigated here are based on an interface stiffness model, where the stiffness property is described as a nonlinear function of the nominal contact pressure. The results obtained by the proposed laws are compared with experimental results. The nonlinearity introduced by the interface is highlighted by analysing the second harmonic contribution and the vibrational time response. The analysis emphasizes the dependence of the system response, i.e., fundamental and second harmonic amplitudes and frequencies, on the contact parameters and in particular on contact stiffness. The study shows that the stiffness-pressure trend at lower pressures has a major effect on the nonlinear response of systems with contact interfaces

Identification of dynamic contact instabilities generated by braking materials

The occurrence of unstable friction-induced vibrations is a major issue for braking manufacturers, as they lead to annoying noise, structure vibrations and brake surface degradation. Understanding the underlying causes of frictional instabilities, arising during the sliding between two bodies, is necessary for developing solutions and countermeasures. For this purpose, in this work, an experimental and numerical investigation of contact instabilities has been performed. Mode coupling and negative friction-velocity slope instabilities have been numerically investigated by both lumped-parameter and finite element models. As well, an experimental campaign has been carried out for recovering the frictional and vibrational response of braking materials under different boundary conditions. The comparison between numerical and experimental results allows validating a new methodology, based on the study of the phase shift between the tangential and normal vibrational responses, in order to distinguish the different types of contact instabilities

Numerical and experimental analysis of the bi-stable state for frictional continuous system

Unstable friction-induced vibrations are considered an annoying problem in several fields of engineering. Although several theoretical analyses have suggested that friction-excited dynamical systems may experience sub-critical bifurcations, and show multiple coexisting stable solutions, these phenomena need to be proved experimentally and on continuous systems. The present work aims to partially fill this gap. The dynamical response of a continuous system subjected to frictional excitation is investigated. The frictional system is constituted of a 3D printed oscillator, obtained by additive manufacturing that slides against a disc rotating at a prescribed velocity. Both a finite element model and an experimental setup has been developed. It is shown both numerically and experimentally that in a certain range of the imposed sliding velocity the oscillator has two stable states, i.e. steady sliding and stick–slip oscillations. Furthermore, it is possible to jump from one state to the other by introducing an external perturbation. A parametric analysis is also presented, with respect to the main parameters influencing the nonlinear dynamic response, to determine the interval of sliding velocity where the oscillator presents the two stable solutions, i.e. steady sliding and stick–slip limit cycle

Examination of stick-slip scenario on lubricated spring-brake systems

Several complex mechanisms can be responsible for undesirable friction-induced vibrations in many mechanical systems. This paper presents a tribological and dynamic analysis of the stick-slip problem, under greased lubrication, taking into account the practical application of a spring-brake system used in electric tubular motors. The main functioning of these brakes is based on the frictional greased contact between a stationary cylinder and a torsional spring, which rotates inside it. The identification of the parameters that most affect the stick-slip appearance in greased contacts requires a complete understanding and appropriate analysis of the entire system, to identify the effects of all physical parameters on the system. Here the global dynamics and the local contact behaviour is analysed, providing an in-depth examination of the stick-slip phenomenon on a greased contact

Synthesis and Biological Evaluation of Dantrolene-Like Hydrazide and Hydrazone Analogues as Multitarget Agents for Neurodegenerative Diseases

Dantrolene, a drug used for the management of malignant hyperthermia, had been recently evaluated for prospective repurposing as multitarget agent for neurodegenerative syndromes, including Alzheimer's disease (AD). Herein, twenty-one dantrolene-like hydrazide and hydrazone analogues were synthesized with the aim of exploring structure-activity relationships (SARs) for the inhibition of human monoamine oxidases (MAOs) and acetylcholinesterase (AChE), two well-established target enzymes for anti-AD drugs. With few exceptions, the newly synthesized compounds exhibited selectivity toward MAO B over either MAO A or AChE, with the secondary aldimine 9 and phenylhydrazone 20 attaining IC50 values of 0.68 and 0.81 μM, respectively. While no general SAR trend was observed with lipophilicity descriptors, a molecular simplification strategy allowed the main pharmacophore features to be identified, which are responsible for the inhibitory activity toward MAO B. Finally, further in vitro investigations revealed cell protection from oxidative insult and activation of carnitine/acylcarnitine carrier as concomitant biological activities responsible for neuroprotection by hits 9 and 20 and other promising compounds in the examined series

Heat generation and transfer in automotive dry clutch engagement

Dynamic behaviour of automotive dry clutches depends on the frictional characteristics of the contact between the friction lining material, the flywheel, and the pressure plate during the clutch engagement process. During engagement due to high interfacial slip and relatively high contact pressures, generated friction gives rise to contact heat, which affects the material behaviour and the associated frictional characteristics. In practice excess interfacial slipping and generated heat during torque transmission can result in wear of the lining, thermal distortion of the friction disc, and reduced useful life of the clutch. This paper provides measurement of friction lining characteristics for dry clutches for new and worn state under representative operating conditions pertaining to interfacial slipping during clutch engagement, applied contact pressures, and generated temperatures. An analytical thermal partitioning network model of the clutch assembly, incorporating the flywheel, friction lining, and the pressure plate is presented, based upon the principle of conservation of energy. The results of the analysis show a higher coefficient of friction for the new lining material which reduces the extent of interfacial slipping during clutch engagement, thus reducing the frictional power loss and generated interfacial heating. The generated heat is removed less efficiently from worn lining. This might be affected by different factors observed such as the reduced lining thickness and the reduction of density of the material but mainly because of poorer thermal conductivity due to the depletion of copper particles in its microstructure as the result of wear. The study integrates frictional characteristics, microstructural composition, mechanisms of heat generation, effect of lining wear, and heat transfer in a fundamental manner, an approach not hitherto reported in literature

Squeal propensity characterization of brake lining materials through friction noise measurements

Disc brake systems are a technology widely adopted within the automotive and rail industry, especially when high performance is needed. The interaction between the disc and the pads is responsible for friction-induced vibrations, leading often to squeal noise emission. Squeal vibrations are generated by the onset of an unstable mode, which is triggered by an external excitation. Local phenomena occurring at the contact interface, and resulting in friction noise, can be responsible of the dynamic excitation triggering the squeal instability. This work proposes a new approach for characterizing friction lining materials, by measuring the friction noise coming from the contact between different pad materials and a disc rotor, in order to quantify and compare the attitude of materials to trigger squeal. Then, a parametrical analysis has been carried out with the aim of highlighting the influence of the main parameters on the friction noise. When testing the same set of materials on a full brake disc system, the measured friction noise indexes resulted to be strongly correlated with the squeal occurrence, validating the proposed characterization method for the squeal propensity of lining materials