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Investigation of the Influence of Snow Track Density on Tire Tread Block Traction by Experiments and Discrete Element Method Simulation
Abstract
While in nature, snow properties change from day to day or even minute by minute, one of the great advantages of lab tests is the stability and reproducibility of testing conditions. In our labs at the Institute of Dynamics and Vibration Research, Leibniz Universität Hannover, we currently run three test rigs that are able to conduct tests with tire tread blocks on snow and ice tracks [1,2]: High-Speed Linear Tester (HiLiTe) [3], Portable Friction Tester (PFT), and Reproducible Tread Block Mechanics in Lab (RepTiL). In the past years, we have run a project on the influence of snow track properties on friction and traction test results with those test rigs. In this article, we will present a first excerpt of the results concentrating on the RepTiL test rig. Because this rig reproduces the movement of rolling tire tread blocks [2], we executed a test campaign with special samples for the analysis of snow friction mechanics. We evaluated penetration into the snow, maximum longitudinal force level, and longitudinal force gradient. On the other hand, we varied the snow density while preparing our tracks to assess the influence of the snow track density on the friction mechanics. In parallel, we have accompanied our experiments with discrete element method simulations to better visualize and understand the physics behind the interaction between snow and samples. The simulation shows the distribution of induced stress within the snow tracks and resulting movement of snow particles. Hypotheses for the explanation of the friction behavior in the experiments were confirmed. Both tests and simulations showed, with good agreement, a strong influence of snow density and sample geometry.
... In addition, the samples were driven on an artificially produced snow track under traction. The snow track was made using a process developed at the institute specifically for this purpose and is described in detail in [6]. Ice and snow were tested at a temperature of -7°C. ...
It is known that different weather conditions require a specific design to take into account the main mechanisms acting between the tire tread block and the road surface. When developing all-season tires, additional research is necessary to find the best solution considering various road conditions. This paper analyzes the influence of the inclination angle of tire tread blocks and the tire tread blocks siping design on different surfaces on the friction forces in lateral and longitudinal directions. The tests were conducted on the hybrid test rig Realistic Pattern Testing in Lab at the Institute for Dynamics and Vibration Research of the Leibniz University of Hanover with different single tread blocks. Tire tread blocks with different numbers of sipes were rotated with an angle between 0° to 90° in 15° increments. To simulate different road conditions, artificially produced ice and snow tracks and real road wet and dry asphalt were used. For a better understanding of the mechanisms, high-speed images of the same samples sliding over a wet glass track were taken from below.
On the one hand, the measurement results and videos help to understand the influence of the inclination angle of a tread block sample on the friction process and show the different friction mechanisms on different surfaces and resulting forces in the two directions. On the other hand, the results show clear favorites for optimizing performance on individual surfaces.
... In addition, the samples were driven on an artificially produced snow track under traction. The snow track was made using a process developed at the institute specifically for this purpose and is described in detail in [6]. Ice and snow were tested at a temperature of -7°C. ...
Inhalt
Reifenabrieb
Einfluss von Reifengröße und Fahrzeuggewicht auf Abriebrate und Laufleistung von PKW- Reifen . . . . . . . .1
Aufbau eines Prüfstands zur Messung von Reifen-Fahrbahn-Feinstaubemissionen auf realen Fahrbahnoberflächen . . . . . 15
Fahrwerksentwicklung
Investigation of the damage criticality of axle components of a four-link rear axle on the
driving simulator . . . . . . . . . . . .35
Comfort experience with automated driving – How good do chassis systems have to be? . . . .59
Fahrwerkskonzept für große Lastbereiche mit integrierter Hubfunktion. . . . . . . . . . . . . . . .73
Innovative Reifenentwicklung
Continuous Estimation of the Local Friction Value Potential based on Tyre Acoustic Data . . . . .83
Untersuchung der Druckverteilung zwischen Reifenwulst und Felgenhorn unter statischen Lastzuständen . . . . . . . . .97
Simulation im Versuch und Entwicklung
Hardware-in-the-Loop-Simulation mit erweiterten physikalischen Reifenmodellen zur virtuellen Erprobung von Steuergeräten . . . . . . 117
AUDEx – Automotive development in 1:x – Angewandte Lehre und Forschung m...
The failure of a weak snow layer buried below cohesive slab layers is a necessary, but insufficient, condition for the release of a dry-snow slab avalanche. The size of the crack in the weak layer must also exceed a critical length to propagate across a slope. In contrast to pioneering shear-based approaches, recent developments account for weak layer collapse and allow for better explaining typical observations of remote triggering from low-angle terrain. However, these new models predict a critical length for crack propagation that is almost independent of slope angle, a rather surprising and counterintuitive result. Based on discrete element simulations we propose a new analytical expression for the critical crack length. This new model reconciles past approaches by considering for the first time the complex interplay between slab elasticity and the mechanical behavior of the weak layer including its structural collapse. The crack begins to propagate when the stress induced by slab loading and deformation at the crack tip exceeds the limit given by the failure envelope of the weak layer. The model can reproduce crack propagation on low-angle terrain and the decrease in critical length with increasing slope angle as modeled in numerical experiments. The good agreement of our new model with extensive field data and the ease of implementation in the snow cover model SNOWPACK opens a promising prospect for improving avalanche forecasting.
We here introduce our cloud chamber which enabled us to grow snow crystals from water vapour under conditions similar to those in natural clouds. The snow crystals grew as a result of water vapour supersaturation as well as due to the interaction of ice crystals and water droplets within the cloud chamber. The cloud chamber, of cylindrical shape with a volume of about 2.7 m 3 , was positioned in a cold room with regulated temperature ranging from −1 to −20°C. A fine mist of water droplets was fed into the cloud chamber and a short pulse of pressurized air triggered the nucleation. Observations of the size and shape distribution of the ice crystals as a function of temperature and supersaturation were consistent with literature values. Different crystal shapes – e.g. plates, columns, hollow columns, dendrites – were successfully formed at different conditions within the chamber. The produced snow was collected at the bottom of the cloud chamber and was used for further experiments and measurements.
The failure of a weak snow layer buried below cohesive slab layers is a necessary, but insufficient condition for the release of a dry-snow slab avalanche. The size of the crack in the weak layer must also exceed a critical length to propagate over a wide surface. In contrast to pioneering shear-based approaches, recent developments account for weak layer collapse and allow for better explaining typical observations of remote triggering from flat areas. However, these new models predict a critical length for crack propagation that is almost independent of slope angle, a rather surprising and counterintuitive result. Our new mechanical model reconciles past approaches by considering for the first time the complex interplay between slab elasticity and the mechanical behaviour of the weak layer including its structural collapse. The crack begins to propagate when the stress induced by slab loading and deformation at the crack tip exceeds the limit given by the failure envelope of the weak layer. We are able to reproduce crack propagation on flat terrain and the decrease in critical length with slope angle modeled in numerical experiments. The good agreement of our new model with extensive field data and its successful implementation in the snow cover model SNOWPACK opens promising prospect towards improving avalanche forecasting.
We present an improved machine to produce nature-identical snow in a cold laboratory for reproducible experiments. The machine is based on the common supersaturation principle of blowing cold air over a heated water basin. The moist airstream is directed into a chamber, where it cools and the nucleation of ice crystals is promoted on stretched nylon wires. Snow crystals grow on the wires and are harvested regularly by a new automatic brush rack. Depending on the settings, different snow crystals can be produced, which are shown to be consistent with the Nakaya diagram. The main snow types are dendrites and needles. We prepared specimens from the snow produced by the snowmaker and analyzed them using microcomputer tomography. For dendrites we show that there are natural snow samples that have the same crystal shape and similar microstructural parameters, namely density and specific surface area. The machine can produce suitable amounts of snow for laboratory experiments in an efficient way. As an advantage over previous designs, uniform and reproducible snow samples can be generated under well-defined conditions.
Highly dendritic snowflakes are an aesthetic source of wonder, but the greater challenge for physicists lies in understanding the “simpler” prismatic and planar forms.
A detailed understanding of effects occurring in the contact patch between tire tread and snow surface is needed to maximize tire grip in winter conditions. The main focus of this study is quantifying the snow milling effects of individual tire tread block elements during sliding. Tests are carried out using the high-speed linear friction tester (HiLiTe), located at the Institute of Dynamics and Vibration Research at Leibniz University of Hannover, Germany. Test tracks are prepared using artificially produced snow.
To solely investigate snow milling effects and exclude material properties of rubber, in a first instance the tread block samples are made of polytetrafluoroethylene (PTFE). Because PTFE is at the same time rigid and hydrophobic, known friction mechanisms such as adhesion and hysteresis can be neglected, leaving only the tread pattern milling mechanics to transmit frictional forces to the snow track. The PTFE samples are shaped in such a way that they mimic the geometry of different siped rubber tread blocks under load, varying the sipes' number, shape, and tilt angle.
Results show the benefit of multiple sipes and give information on the evolution of transmittable forces with respect to sliding distance. It is found that the block element shape and tilt angle are directly linked to the frictional force, showing a distinct optimum for specific angle and shape combinations. In addition, forces are not depending on sliding speed, but on sliding distance.
The snow milling results of PTFE block elements are then compared to siped rubber block samples. Corresponding high-speed videos show that PTFE sample snow milling mechanics can be directly applied to rubber samples.
A review of tire testing in the winter environment is presented from the viewpoint of an independent testing laboratory. The independent tester, by necessity, must satisfy the particular requirements of individual customers. A description of the drive traction truck which was designed to meet these individual client requirements is presented. Also, a comparison of results obtained by the various techniques of analysis is included.
Due to general safety reasons and an increasing individual demand on more traffic safety, winter tires have become more and more important. This evolution results in a rising requirement of the customers concerning the tire performance on the one hand, and the effort of the tire industry to improve the tire traction performance on snow and ice on the other hand. To engineer winter tires in an effective way, the friction influencing factors as well as the contact mechanics should be well known. Normally the design and development of tires is strongly based on vehicle tests, but in modern tire development processes the simulation as well as the experimental investigation of tires and tire components in the lab have become more popular. This strategy plays an especially important role for reducing the time of development cycles but also the development costs. With simulation and experiments in lab, new challenges come up which have to be solved. Lab experiments compete with totally different problems: First of all, the environmental influences like temperature and humidity have to be controlled. Furthermore, the test tracks used inside must be comparable to the proving ground outside, hence the properties of snow and ice have to be investigated in detail. Therefore not only is it very important to understand the formation of snow and ice, but also to find characteristics of both materials which can be identified and measured with mobile measurement devices outside on the test track and inside in the lab. Within this publication a general overview of the tire tread block test method as well as the test rig, which are used for identifying relevant tread block friction mechanisms on snow and ice, will be given. The results of the measurement will be shown and the acting friction phenomena will be explained.
The distinct element method is a numerical model capable of describing the mechanical behavior of assemblies of discs and spheres. The method is based on the use of an explicit numerical scheme in which the interaction of the particles monitored contact by contact and the motion of the particles modelled particle by particle. The main features of the distinct element method are described. The method is validated by comparing force vector plots obtained from the computer program BALL with the corresponding plots obtained from a photoelastic analysis.
A numerical model for rock is proposed in which the rock is represented by a dense packing of non-uniform-sized circular or spherical particles that are bonded together at their contact points and whose mechanical behavior is simulated by the distinct-element method using the two- and three-dimensional discontinuum programs PFC2D and PFC3D. The microproperties consist of stiffness and strength parameters for the particles and the bonds. Damage is represented explicitly as broken bonds, which form and coalesce into macroscopic fractures when load is applied. The model reproduces many features of rock behavior, including elasticity, fracturing, acoustic emission, damage accumulation producing material anisotropy, hysteresis, dilation, post-peak softening and strength increase with confinement. These behaviors are emergent properties of the model that arise from a relatively simple set of microproperties. A material-genesis procedure and microproperties to represent Lac du Bonnet granite are presented. The behavior of this model is described for two- and three-dimensional biaxial, triaxial and Brazilian tests and for two-dimensional tunnel simulations in which breakout notches form in the region of maximum compressive stress. The sensitivity of the results to microproperties, including particle size, is investigated. Particle size is not a free parameter that only controls resolution; instead, it affects the fracture toughness and thereby influences damage processes (such as notch formation) in which damage localizes at macrofracture tips experiencing extensile loading.
The equation of motion of a system of 864 particles interacting through
a Lennard-Jones potential has been integrated for various values
of the temperature and density, relative, generally, to a fluid state.
The equilibrium properties have been calculated and are shown to
agree very well with the corresponding properties of argon. It is
concluded that, to a good approximation, the equilibrium state of
argon can be described through a two-body potential.
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