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The mechanized tunnel construction is carried out by tunnel boring machines, in which the soil in front of the working face is removed, and the tunnel lining is carried out with shotcrete or the setting of segments and their back injection. Advancements in this field aim towards increase of the excavation efficiency and increase of the tool lifetime, especially in rock-dominated grounds. The latter is achieved by understanding the wear mechanisms abrasion and surface-fatigue, and by knowledge of the microstructure-property relation of the utilized materials. Improvements for tool concepts are derived, based on experiments and simulations. A key parameter towards efficient rock excavation is the shape of the cutting edge of the utilized disc cutters. Sharp cutting edges have proven to generate higher rock excavation rates compared to blunt ones. The compressive strength of the utilized steel has to be high, to inhibit plastic deformation and thereby to maintain sharp cutting edges. This requirement competes with the demand for toughness, which is necessary to avoid crack-growth in the case of cyclic loading. Solutions for this contradiction lie in specially designed multiphase microstructures, containing both hard particles and ductile microstructural constituents. Besides adapting the alloying concept, these required microstructures and the associated properties can be adjusted by specific heat-treatments.

The Discrete Element Method (DEM) is a particle-based approach, which is applied in a wide range of engineering fields. One problem of the DEM is the identification of the required input parameters that govern the material behavior.In this study, we develop an analytical prediction based on an energy minimization approach relating the macroscopic elastic parameters to the microscopic contact parameters. In order to validate these relations, a series of confined and unconfined compression tests are performed by PFC 3D . A more complex version of materials containing a mixture of different types of particle ensembles are used to study the effect of bonding and grain shape on the elastic properties of the materials. This study covered both solid (bonded) and granular (unbonded) materials.In the following, a variational approach is provided to relate the local contact parameters to the macroscopic material properties such as elasticity parameters and yield stresses. First, let us consider the energy stored within the contacts in a RVE in normal ψ n and tangential ψ t directions aswhere c n and c t are the stiffnesses and u n and u t,e are the displacements in normal and tangential directions, respectively. The macroscopic specific free energy ψ m of a corresponding continuum is then achieved by averaging the contact energies over all directions. In the energy minimization procedure, it is assumed that the material is arranged in such a way to minimize the total free energy for a specific macroscopic displacement gradient. Therefore, the Lagrangian function is formulated and minimized with respect to the relative displacement and rotation. Finally, the stationarity conditions lead to the following equations for the macroscopic Young's modulus and Poisson's ratio.This direct relation between the microscopic contact stiffnesses c n and c t on the one hand and the macroscopic elastic constants on the other, makes it possible to predict elastic constants directly from the chosen microscopic ones for particle ensembles in a DEM calculation. For bonded materials, the following additional relation is obtained to relate the macroscopic Rankine and Tresca yield limits to the bond strengths in normal and tangential directions at the contact level. σ Rankine y = ρ c rf n,c and σ T resca y = ρ c rf t,cwhere ρ c denotes the number of contacts between particles per unit volume and may be obtained directly from the DEM calculations. Numerical ResultsTo validate the predicted relations, triaxial test simulations are performed with a mixture of spherical and complex grains, which consist of 50% of 3-particle clumps, 25% of 2-particle clumps and 25% of spheres. Grain shapes are shown in Figure 1a. The macroscopic values can be extracted from the numerical calculations with three different procedures, either the force and displacements are measured from (a) the outer wall elements, (b) or from the outer particles, (c) or from the measurement spheres located in the specimen. Figure 1b illustrates the different measurement procedures. Th...

The prediction of failure mechanism in structures are always an important topic in the field of computational mechanics. Finite element computations of an inelastic material involving softening behavior (e.g. softening plasticity or damage) can suffer from strongly mesh-dependent results. Therefore, such continuum models should be equipped with a regularization (localization limiter) strategy to overcome the above-mentioned problem.In this study, we present a framework for gradient-enhancement for coupled damage-plasticity material model derived by means of Hamilton's principle for non-conservative continua. This model is applied for the numerical investigation of wear processes as they occur, e.g. in the case of mechanized tunneling. These investigations require a fine resolution of the involved constituents (cut sheet and abrasive particles in the soil). Consequently, a numerical strategy for the damage-plasticity model is demanded that allows for time-efficient simulations.In this paper, we present a first step to the mentioned ultimate goal. To this end, a numerical framework for gradientenhanced damage-plasticity coupling is proposed that is based on a combination of the finite element method with strategies from meshless methods. We demonstrate that this framework keeps the computational effort limited and for each load step close to the purely elastic problems. Several numerical examples prove the elimination of the pathological mesh dependency of the results. Furthermore, first results to the simulation of wear in tunneling machines are presented. Material ModelTo derive a model for a material undergoing isotropic brittle damage coupled with plasticity, we chose an energy-based, variational approach. To this end, we need to specify the Helmholtz free energy, the internal variables and the dissipation function. In the current material model, we investigate the linear isotropic hardening with the plastic potential w(α p ) = 1 2 K H α 2 p , and a twice differentiable damage function f (d) = exp(−d) with d ∈ [0, ∞). Utilization of the damage function f (d) leads to a softening behavior and causes the ill-posedness and numerically instability problems. Therefore, the regularization of the model is achieved by the gradient enhancement of the damage function f , specifically by adding a potential that is convex in the highest gradients to the mechanical contribution of the Helmholtz free energy. The total Helmholtz free energy thus readswhere β stands for the gradient parameter and used as a switch between local and enhanced model: setting β = 0 obtains a local coupled damage-plasticity model. In this study for brittle damage and classical rate-independent plasticity, we chose the dissipation function as D = D d + D p = r d |ḋ| + r p |ε p |, which is a homogeneous function of order one in both rates. Application of Hamilton's principle yields the evolution equations for the plastic strains, the hardening variable, and the damage function. Details are given in [1]. Thus, the evolution equation and yield function ...

Wear is defined as damage to the surface of a solid body, involving progressive material removal, due to relative motion and frictional contact with another surface. This process is usually slow but is considered as one of the major factors causing damage and consequently failure of component parts during the lifetime of tools or machines in different applications such as tunneling. The implementation of conventional local material models in finite element simulations involving softening behavior (e.g., softening plasticity or damage) tends toward an ill‐posed boundary value problem after the onset of softening due to non‐convex and non‐coercive energy functions and suffers from strongly mesh‐dependent results. Therefore, different regularization strategies are developed to overcome the mentioned problem, such as integral or gradient enhancement, introducing an internal length scale and subsequently increasing computation effort. In this study, we present a variational approach for regularization of damage material model coupled with local plasticity based on emulated representative volume element (ERVE). This model shall be applied for the numerical investigation of wear processes, where a fine resolution of the involved constituents (cut sheet and abrasive particles in the soil) are required. We start with the theoretical derivation of material model, briefly present the numerical treatment, and prove the efficiency of this new approach via some numerical examples. Furthermore, results to the simulation of different wear modes in single scratch tests are presented.