Soft Contact Model Research Articles

A layer discrete particle based soft contact model in corroded rebar monitoring using linear and nonlinear ultrasonic guided waves

In offshore reinforced concrete (RC) structures, the phenomenon of rebar corrosion is widespread, seriously threatening the durability of the structures. However, the issue of rebar corrosion detection especially for the early corrosion situation is also challengeable. It is of great significance to use ultrasonic guided waves (UGWs) for monitoring the situation of the rebar corrosion entire process. In this paper, a mechanical model was used to establish the relationship between different rebar corrosion expansion states and layer-surface contact pressures in the layered RC components with radial cracks. Based on this model, a soft pressure-dependent contact 2D model in Abaqus was used to simulate the local corrosion layer. A linear and nonlinear signal joint analysis (LNSJA) method using PZT-based UGWs was proposed to monitor rebar corrosions, and an LNSJA-based rebar corrosion damage index (RCDI) for corroded RC components was proposed. The proposed method which can effectively detect both the micro- and macro-thickness as well as local area of rebar corrosion layer was validated by the relevant experiment and finite element analysis (FEA).

Modeling and quantitative study of soft contact between abrasive medium and active abrasive particles in abrasive flow machining

In abrasive flow machining (AFM), material is abraded by active abrasive particles powered by solid-liquid contact between them and the abrasive medium. This contact characteristic is a key and unfortunately it is investigated scarcely.In this work, a soft-contact model was firstly proposed. Soft-contact parameters including protrusion ratio and covering inclination angle were then defined to characterize soft-contact characteristics. Finally, the effect of medium pressure and particle diameter on soft-contact parameters was studied by full factorial design. Results showed that average scratch depth of single active abrasive particle increases with increasing particle diameter and medium pressure. In addition, active abrasive particles are actually deeply wrapped to a large proportion by abrasive medium, and the descriptive protrusion ratio decreases with increasing medium pressure and particle diameter. Moreover, medium pressure offers tangential and normal force for active abrasive particle through a covering inclination angle, and the angle increases with increasing medium pressure and particle diameter. It provides guidance for further improvement of prediction accuracy of material removal in AFM process.

Physics engine based simulation of shear behavior of granular soils using hard and soft contact models

Physics engines have been widely used to simulate physical and mechanical processes in modern video games to create an immersive and realistic gaming experience. This study explores the feasibility of using an open-source physics engine, Chrono, as a discrete element method (DEM) platform to simulate shear behavior of granular soils. This study develops a series of pre-processing, servo-controlling, and post-processing functions to improve the Chrono platform, so that Chrono can generate soil specimens with designed packing densities, perform laboratory test simulations such as direct shear tests, and output stress-strain, fabric, and force chains. Traditional DEM codes use soft contact models (e.g., Hertzian contact model) to determine inter-particle contact forces, while most physics engines use a hard contact model (e.g., non-smooth contact dynamics) to determine inter-particle contact forces. The hard contact model enables physics engines to use large time steps in iterations without affecting the numerical stability and simulation accuracy, which remarkably accelerate simulation speeds compared with traditional DEM codes. Based on systematical comparisons between simulation results of two contact models and experimental results, this study demonstrates that the hard contact model can yield the shear behavior of granular soils observed in soft contact model simulations and laboratory tests.

Contact Area-Based Modeling of Robotic Grasps Using Deformable Solid Mechanics

In robotic grasp analysis, contact between an object and a hand is described using point contact models and soft contact models. Although soft contact models estimate both contact area and contact force for grasp analysis, most of the approaches use point contact models to analyze the stability of grasp because of the simplicity involved in computation and modeling. However, grasps suggested by these approaches are not intuitive to execute with and often fail in experiments. Since the hand–object interface is modeled using points, their physical characteristics are not taken into account in the analysis, thereby resulting in failed grasps. In this work, the contact models related to robotic grasping are briefly reviewed, and the discrepancies present in them are identified from a solid mechanics standpoint. Using the constitutive equations available in solid mechanics, these contact models are then reformulated based on contact areas for both deformable hard as well as soft fingertips. By performing Finite Element (FE) simulations of the contact situation considered, the contact models are validated. The proposed models are validated using robotic grasp experiments as well.

Accelerated simulations of direct shear tests by physics engine

Physics engines, originally developed to simulate physical and mechanical processes in modern video games, are increasingly used as a scientific computational platform in many disciplines due to their high computational efficiency. This study explores the feasibility of using an open-source physics engine, Project Chrono, to simulate direct shear tests. This study develops a series of pre-processing, servo-controlling, and post-processing functions in Project Chrono to generate soil specimens with designed packing densities, perform direct shear tests, and output simulation results including stress–strain relations, fabrics, and force chains. To determine inter-particle contact forces, typical DEM codes use soft contact models, while most physics engines use hard contact models. The hard contact model enables physics engines to use large time steps in iterations without affecting the numerical stability and simulation accuracy, which remarkably reduces simulation time compared with typical DEM codes. Based on systematical comparisons between simulation results of two contact models, this study demonstrates that the hard contact model can yield the same direct shear test results observed in soft contact model simulations, but is ten times faster than the soft contact model for simulating the same number of particles. This study may provide DEM modelers with the physics engine as one more option for soil behavior simulation.

A soft-contact model for computing safety margins in human prehension

The soft human digit tip forms contact with grasped objects over a finite area and applies a moment about an axis normal to the area. These moments are important for ensuring stability during precision grasping. However, the contribution of these moments to grasp stability is rarely investigated in prehension studies. The more popular hard-contact model assumes that the digits exert a force vector but no free moment on the grasped object. Many sensorimotor studies use this model and show that humans estimate friction coefficients to scale the normal force to grasp objects stably, i.e. the smoother the surface, the tighter the grasp. The difference between the applied normal force and the minimal normal force needed to prevent slipping is called safety margin and this index is widely used as a measure of grasp planning. Here, we define and quantify safety margin using a more realistic contact model that allows digits to apply both forces and moments. Specifically, we adapt a soft-contact model from robotics and demonstrate that the safety margin thus computed is a more accurate and robust index of grasp planning than its hard-contact variant. Previously, we have used the soft-contact model to propose two indices of grasp planning that show how humans account for the shape and inertial properties of an object. A soft-contact based safety margin offers complementary insights by quantifying how humans may account for surface properties of the object and skin tissue during grasp planning and execution.

Grasp modelling with a biomechanical model of the hand

The use of a biomechanical model for human grasp modelling is presented. A previously validated biomechanical model of the hand has been used. The equilibrium of the grasped object was added to the model through the consideration of a soft contact model. A grasping posture generation algorithm was also incorporated into the model. All the geometry was represented using a spherical extension of polytopes (s-topes) for efficient collision detection. The model was used to simulate an experiment in which a subject was asked to grasp two cylinders of different diameters and weights. Different objective functions were checked to solve the indeterminate problem. The normal finger forces estimated by the model were compared to those experimentally measured. The popular objective function sum of the squared muscle stresses was shown not suitable for the grasping simulation, requiring at least being complemented by task-dependent grasp quality measures.

Direct numerical simulation of particles in a fluid interacting with a wall

AbstractWe develop a code to be applied in the context of the cleaning of wafer surfaces by hydrodynamic forces. Our goal is to study the detachment of (submicron) particles, exposed to a shear flow, from a wall by means of direct numerical simulation. The particles are treated as rigid bodies fully interacting with the fluid.To simulate moving particles in the fluid we implement an immersed boundary method with direct forcing into OpenFOAM. The particle‐wall interaction is treated with a soft contact model. As first simple examples we study the elastic normal impact of a cylinder onto a wall as well as the onset of sliding of a cylinder pressed to a horizontal wall by gravity under a time‐depended drag force. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)