Short- and Long-Term Behaviour of Solid-Like Granular Materials

Granular materials conglomerations of discrete macroscopic solid particles are often found in na- ture. Processes like landslides, avalanches, erosion or plate tectonics are highly influenced by the static and dynamic behaviour of such materials. Granular materials also play an important role in many technical applications, like the production pro- cess of pills consisting of compressed granular materials including several additives. Sand brought into the wheel-rail contact for improving the traction and braking behaviour of railway vehicles is a further example for a technical application of granular materials as well as ballast of railway tracks. These kind of granular materials behave mainly like solids because the average energy of the individual par- ticles is low and the particles are fairly stationary relative to each other. The proposed project is focussing on granular materials under such conditions. The goal is to develop advanced particle based prediction models (Discrete Element Method (DEM) models) for the static and dynamic short- and long-term behaviour of solid-like granular materials. With respect to the short-term behaviour the modelling of friction between the particles and their con- tacts with their environment plays an important role. Currently Coulomb law of friction is mainly ap- plied where a constant coefficient of friction is assumed for the whole system. This approach is often insufficient to bring the models in accordance with experimental results. It is known from tribology that friction is influenced for example by loading conditions, surface-roughness and liquids within the contact and cannot be described by a constant coefficient of friction. For this reason one goal of the proposed project is to account for such effects by consideration of tribological effects within friction laws of DEM models. Together with an appropriate modelling of particle plasticity and particle break- ing due to exceeding the strength limit of the particles a significant improvement of the prediction quality for the short-term behaviour is expected. With respect to the long-term behaviour of granular materials under cyclic loading particle wear and breaking due to fatigue is of high relevance. Sufficient models for description of these effects are still not available. Thus, the development of physical fatigue models on a particle base taking into account the interaction with wear is a further goal of the proposed project. In this way a significant improve- ment of the prediction quality with respect to the long-term behaviour can be expected. For parameterisation of the developed advanced DEM models a method based on principal experi- ments will be developed. Thereby the kind of granular material and loading conditions will be taken into account. Finally the validity of the developed models and methods will be approved by applica- tion to examples of granular materials. Therefore several principal experiments will be performed within the proposed project.

Granular materials conglomerations of discrete macroscopic solid particles are often found in nature. Processes like landslides, avalanches, erosion or plate tectonics are highly influenced by the behaviour of such materials. Granular materials also play an important role in many technical applications. Sand brought into the wheel-rail contact for improving the traction and braking behaviour of railway vehicles represents a typical example as well as ballast used in railway tracks. These types of granular materials behave mainly like solids (but not always) because the average energy of the individual particles is low and the particles are fairly stationary relative to each other. This project focussed on granular materials under such conditions. Therefore, a so-called particle-based approach has been used to predict the behaviour of such granular materials. There, each particle is considered individually as well as its interactions with other particles and its environment. This approach causes high computational effort on the one hand, because of the generally high number of considered particles, but gives insight into phenomena occurring on particle scale on the other hand. For example, the particle movement due to loading or the development of force chains can be investigated in detail. Furthermore, physical phenomena occurring within the particle-particle contacts can be investigated in detail leading to a better understanding for granular materials. The main outcome of the project is the introduction of a so-called balanced approach to particle-based modelling. The basis therefore are the following three main steps of particle- based modelling: representation of particle geometry, contact modelling and model parametrisation. The introduced balanced approach breaks new scholarly ground, as it emphasizes that each of the three aspects should be addressed with a comparable intensity. Following this definition, an unbalanced approach is one, where one aspect is investigated with much more effort than the others. In the literature, many examples for unbalanced approaches can be found. To give an example, many papers exist where a very detailed representation of the particle geometry is combined with very simple contact laws, neglecting relevant physical phenomena occurring in the particle-particle interfaces. In this project the introduced balanced approach has been followed consequently. While the particle shape has been modelled relatively simple (but not too simple), additional physical phenomena observed in particle-particle contacts have been implemented in the models, like particle edge breaking or load dependent friction, which have not been considered in state of the art models yet. Furthermore, the parameterisation of these advanced models is based on different principal experiments with the aim to find one set of model parameter which is able to describe all experiments. This increases the reliability of the models especially when applying them to other (more complex) conditions.