Permanent deformation in ballast layers is a major contributing factor to the railway track geometry deterioration. In spite of a considerable amount of research on understanding and predicting performance of ballast layers, accurately capturing their settlements remains a challenge. In order to contribute to solving this important issue, a new numerical method for predicting ballast settlements is presented in this paper. This method is based on the finite element (FE) method combined with a constitutive model that captures permanent deformation accumulation in unbound materials under cyclic loading. This allows predicting permanent deformations of large structures and at large number of load cycles in a computationally efficient manner. The developed constitutive model is validated based on triaxial test measurements over wide range of loading conditions. Stress state in ballast layers has been examined with a 3D FE model, for several embankment structures and traffic load magnitudes. The determined stress distributions and loading frequencies were used as an input of the constitutive model to evaluate permanent strains and settlements of ballast layer. The influence of embankment structural designs and traffic loading magnitudes on the ballast layers settlements is examined and the results obtained are compared with the existing empirical performance models.

Interlayers in asphalt pavements are potential structural damage initiators. In order to better understand the quantitative role of interlayer parameters, such as surface roughness, binder type, binder content and loading type on interlayer shear strength, this paper focuses on the effects of particle interlock and contact conditions on interlayer strength through experimental and numerical modelling. Experimentally, interlayer shear box strength tests on a model material consisting of stiff binder blended with steel balls are performed with and without normal force confinement. A Discrete Element method model of the test is developed using measurements of the model material for calibrating the contact law and for validating the model. It is shown that this model captures adequately the measured force-displacement response of the specimens. It is thus a feasible starting point for numerically and experimentally studying the role of binder and tack coat regarding interlayer shear strength of real asphalt layers.

Presently used experimental techniques for the characterization of tensile and compressive behavior of active layers in lithiumionbatteries have limitations of different kinds. This is particularly true for measurements of compressive properties.Furthermore, the characterizations of time-dependent stress-strain behavior are largely missing. In order to characterize thestress-strain relationship for a dry cathode active layer in lithium-ion batteries, a mechanical testing method is presented thatpreviously has been applied to the testing of optical fibers. The method is based on U-shaped bending of single-side coatedaluminum foils, which enables separate measurements of tensile and compressive properties. In particular, the method has clearadvantages for measurements of compressive properties in comparison to previously reported techniques. Relaxation experimentsare also conducted in order to characterize the time-dependent properties of the dry active layer and to check if these effectscould explain the measured hysteresis. It is found that the elastic modulus in compression is significantly larger than the elasticmodulus in tension and that the compressive modulus increases with strain level. Contrary, the tensile modulus is approximatelyindependent of strain. Furthermore, hysteresis effects are present at loading-unloading measurements, both for tension andcompression. The low values of the measured elastic moduli show that the electrode properties are largely controlled by thebinder and carbon additives. It is concluded that the development of particle-particle contacts most likely is the reason for thehigher modulus in compression in comparison to tension. The time-dependent effects are significant, primarily for shorter timescales, which explains the relaxation behavior, but they cannot fully explain the hysteresis effects. Most likely non-linear micromechanismsdo contribute as well.

Material parameter curves in an advanced material model describing compaction of spraydried cemented carbide powder are determined successfully based on a general approach formaterial characterization of powder materials. Pressing forces from a production machineand equivalent finite element (FE) calculations are used in inverse modelling. A pressingmethod that includes multiple unloading steps is used. The material model is of DruckerPrager CAP kind and friction between powder and pressing tool is modelled as a function ofnormal pressure. The results are verified with density gradient measurements using aneutron source. The method is proven to be robust and the results show good agreementbetween experiment and simulation. Effects that have not been captured numericallypreviously are captured due to the high accuracy of material characterization. The presentapproach is detailed for tungsten carbide powders but is valid for other powder materialswhen properly calibrated for constitutive and frictional effects in the same manner asoutlined here.

The service life of a road is to great extent controlled by the performance of the unbound layers. Assessing the constitutive behavior of these layers is thus imperative for a sustainable pavement design. Adequate description and measurement of unbound materials resistance to aggregate crushing is an issue, in terms of the measured response coupled to intrinsic properties of the aggregates and unbound materials gradation. In this study, a new Discrete Element Method (DEM)—based model is developed to investigate aggregate damage in unbound road materials. In order to get better insight into micro-mechanics of aggregate crushing, the developed model incorporates granular mechanics-based particle contact and damage laws. By numerical analysis with the DEM, several unbound granular materials have been examined investigating the effect of the materials gradation and aggregates toughness properties on their performance in crushing tests. The capability of the model to capture the effect of unbound material properties on its crushing performance, is evaluated based on comparison with experimental findings. 

DEM modeling of granular materials composed of viscoelastic particles can provide valuable insights into the mechanical behavior of a wide range of engineering materials. In this paper, a new model for calculating the normal contact force between visoelastic spheres is presented based on contact mechanics that takes the mechanical behavior of the DEM particles into account. The model relies on an application of the viscoelastic correspondence principle to elastic Hertz contact. A viscoelastic relaxation function for the contact is defined and a generalized Maxwell material is used for describing this function. An analytical expression for the increment in contact force given an increment in overlap is derived leading to a computationally efficient model. The proposed model provides the analytical small deformation solution upon loading but provides an approximate solution at unloading. Comparisons are made with FEM simulations of contact between spheres of different sizes of equal and dissimilar materials. An excellent agreement is found between the model and the FEM simulations for almost all cases except at cyclic loading where the characteristic times of the viscoelastic behavior and the loading are similar.

This paper presents a numerical approach for predicting damage in rock materials caused by contact loading. The rock material is modelled using a constitutive description that combines pressure dependent plasticity, for capturing shear deformation under high confining pressure, with an anisotropic damage model for capturing mode I cracking in tension. Material parameters for the model are taken from a recently performed investigation on a granite material. The model has been used to simulate two types of contact loading experiments from the literature, cyclic loading and monotonic loading up to fracture. In order to achieve accurate predictions, the model has been extended to account for small loaded volumes which occur at contact loading. The results show that the main damage mechanism at cyclic loading is crack propagation due to Hertzian stresses whereas in the monotonic experiments sub-surface cracks could initiate. All features measured in the contact loading experiments are captured by the model and hence, the modelling framework is judged to be able to capture contact damage if real stone geometries are studied in FEM.

Spray-dried refractory carbide and metal powder mixtures, containing tungsten carbide, is compacted and sintered during the production of conventional cutting tool inserts. Since friction between the pressing tool and the powder gives rise to density gradients in the powder compact, shrinkage during sintering is uneven. The shape of the sintered blank is important and can be predicted with finite element (FE) simulations. To validate the simulation of the pressing procedure, the density gradients in the powder compacts must be measured with a high spatial resolution. Since tungsten has a high atomic number, it is hard to penetrate with X-rays and even cold neutrons. We show here that by using a polychromatic beam of thermal neutrons, along with beam-hardening correction, such measurements can be successfully realized. The obtained results show good agreement with corresponding FE-simulations. Also, deliberate differences in the compaction process could be verified with the neutron measurements.

During asphalt mixture compaction, loads in the material are mainly transferred through contact between the stones and the interaction between the stones and the binder. The behaviour of such materials is suitable to model using the Discrete Element Method (DEM). In this study, a new DEM modelling approach has been developed for studying the asphalt compaction process, incorporating contact and damage laws based on granular mechanics. In the simulations, aggregate fracture is handled by a recently developed method of incorporating particle fracture in DEM, based on previously performed fracture experiments on granite specimens. The binder phase is modelled by adding a viscoelastic film around each DEM particle. This surface layer has a thickness that obtains the correct volume of the binder phase and has mechanical properties representative for the binder at different temperatures. The ability of the model to capture the influence of mixture parameters on the compactability and the eventual stone damage during compaction is examined for the cases of compaction flow test and gyratory compaction. Explicitly, the influence of different aggregate gradations, mixture temperatures and binder properties are studied. The results show that the proposed DEM approach is able to capture qualitatively and quantitatively responses in both cases and also provide predictions of aggregate damage. One large benefit with the developed modelling approach is that the influence of different asphalt mixture parameters could be studied without re-calibration of model parameters. Furthermore, based on comparative DEM simulations, it is shown that the proposed approach provides more realistic force distribution networks in the material.

It is known that the shape of a cutting insert blank, after pressing and sintering, can be predicted with FE-simulations. It is also known that such simulations have the potential to save costs and time when used for press tool compensation. In such simulations, the frictional behaviour has a great impact. Here, two frictional models are discussed and implemented into FEM. The results from the different frictional descriptions, when for instance analysing density after compaction, shows a clear difference. It can be concluded that friction also at low forces must be modelled in detail.