The Effect of Particle Size Distribution And Shape On The Microscopic Behaviors of Loess Via DEM

Loess has loose metastable structure, which is not difficult to be destroyed under load. As the core and soul of loess structure, it is part of the main research directions of loess engineering properties at present to study its microscopic behaviors and then realize the interpretation and prediction of macroscopic mechanical properties. In this study, based on the analysis of the basic physical properties of loess samples from seven different places, each sample was scanned by X-ray with continuous slice CT, and the three-dimensional microstructure of loess samples was established. According to the computer graphics method, each particle is equivalent to an ellipsoid, and the flattening rate and elongation rate of particles in each sample are quantitatively counted. Taking the particle size distribution (PSD) and shape parameters (flatness and elongation) of each sample as the control factors for generating discrete element method (DEM) samples, a series of triaxial compression simulation tests were carried out, and the microscopic behaviors of each sample were studied within the whole test framework. Comparing the results of seven different samples, it is shown that both PSD and particle shape have effects on stress-strain relationship, dry density and the normal contact force of loess samples. Most of the sand particles (> 0.075mm) are flat particles, while the clay particles (< 0.005mm) are mainly near spheres. When the volume fraction of sand particles is large, the dry density of the sample is the lowest. However, when the content of near spherical clay particles is large and the particle size distribution is good, and the average coordination value is large, which shows that it has strong normal contact force, and thus higher shear strength.

Measurement of moment coupling in fretting fatigue experiments

Moment coupling in fretting fatigue experiments refers to the generation of moments when a shear force is applied, caused by the difficulty in designing fretting fatigue experiments where the shear is reacted out on the same plane as the contact. Digital Image Correlation is used to measure the effect of moment coupling, and a model is created to calculate the effect of the applied moment at any point during a loading cycle on near-edge contact properties. The effects of the changing contact pressure on the slip zone sizes are considered. Finally, the model derived is used to find a load cycle including the effect of normal contact force that creates a truly constant near-edge contact pressure distribution at one edge of the flat and rounded pad. Although the calibrations found in this paper are valid only for the specific rig and specimen geometry used in this paper, the method could be readily applied to other experiments.

Scanning control based on real-time contact force feedback for ultrasonic thickness measurement

An effective contact force control strategy is of great significance for accurate and stable ultrasonic thickness on-machine measurement. However, it is difficult to adjust the contact force dynamically due to the uncertainty of the geometric characteristics of the measured workpiece. In this paper, a contact force control method based on the combination of adaptive impedance controller and sliding mode variable structure position controller is proposed. First, the control process with the force tracking impedance control and a normal contact force calculation model is established. Then, a force-position conversion model and a sliding mode variable structure controller are proposed. Further, a simulation with a typical S-shaped measured surface is given to show that the algorithm for controlling contact force can achieve good real-time tracking performance and has stronger robustness than traditional methods. Finally, an arc-shaped aluminum alloy thin-wall part thickness is sampled along the scan trajectory to verify the effectiveness of the algorithm. The experimental results show that the proposed algorithm for controlling contact force can quickly adjust the measuring device to the target position and maintain the stability of the normal contact force to ensure the accuracy of ultrasonic thickness on-machine measurement.

Evolution of the coefficient of lateral earth pressure at rest with interparticle friction: a numerical study

The grain-scale nature of evolution of the coefficient of lateral earth pressure at rest (K0) with interparticle friction (µp) is poorly understood. This study aims to use discrete element method simulations of vertical one-dimensional compression on both face centred cubic (FCC) samples and random monodisperse (RM) samples to link K0 and µp, and the results show that K0 increases with reductions in interparticle friction. Although K0 is dependent upon the sample density, patterns of evolutions with strain levels are likely to be unchanged with initial confining pressures. The stress-induced fabric becomes more anisotropic for samples with high values of the interparticle friction. The percentage of particles with a high value of the normal contact force increases with increasing strain levels as the interparticle friction increases in the simulations.

Influence of Particle Orientation on the Performance of Geogrid Reinforced Ballast

To explore the initial orientation effect of ballast assembly on the reinforcement performance of the geogrid reinforced ballast, particles with random orientation and five prescribed rotational orientations were developed through particle flow code (PFC3D). The evolution laws of the pullout force and the principal directions of the normal contact force were systematically compared and analyzed. Furthermore, the mechanical responses such as pullout force, distribution of axial force, displacement vectors, force chain, and mesoscopic fabric were discussed. According to the displacement vectors of the ballast particles, the average thickness of the stable shear band is determined. The inherent relationships among the force chain, the rotational angle of the normal contact force, and the mesoscopic fabric parameters are revealed. The results show that the pullout force of specimens with the initial orientation of 45° increases monotonously during the pullout process, and the peak value of pullout force appears at the end of the test. The mesostructural analysis also confirms that the evolution of the principal direction of contact normal force is relatively steady during the pullout process, indicating that the specimen with 45° orientation possesses higher systematic stability and ductility. Moreover, the optimum interval from 56.68° to 57.30° is observed to remain in a self-adapting state for ballast assembly.

A Theoretical Model for the Normal Contact Force of Two Elastoplastic Ellipsoidal Bodies

To model the mechanical behavior of granular materials, a reliable description of the material properties is indispensable. Individual grains are usually not perfectly spherical. In batteries, for instance, lithium nickel manganese cobalt oxide (NMC) is a frequently used material, consisting out of particles with possibly ellipsoidal like shapes. As particles may plastically deform under increasing stresses, the paper presents a theoretical model for the normal contact force of elastoplastic ellipsoidal bodies for the use in the context of mechanical discrete element method (DEM). The model can be considered as extension of the elastic, elastic-plastic, fully plastic Thornton model by using a more general description to incorporate elliptical contact areas. The focus is on a normal contact force description as continuous function of time for all regimes, elastic, elastoplastic, and fully plastic loading, as well as unloading from elastoplastic loading, while the evolution of the plastic contact area is not considered here. All underlying formulae to describe the force-displacement relationship for the static contact problem are derived, partly based on finite element analysis (FEA). To verify the new model, FEAs are performed and their results compared with the model predictions.