Mustafa I. Alsaleh

Soil – Machine Interaction: Simulation and Testing

Researchers at Caterpillar have been using Finite Element Analysis or Method (FEA or FEM), Mesh Free Models (MFM) and Discrete Element Models (DEM) extensively to model different earthmoving operations. Multi-body dynamics models using both flexible and rigid body have been used to model the machine dynamics. The proper soil and machine models along with the operator model can be coupled to numerically model an earthmoving operation. The soil – machine interaction phenomenon has been a challenging matter for many researchers. Different approaches, such as FEA, MFM and DEM are available nowadays to model the dynamic soil behavior; each of these approaches has its own limitations and applications. To apply FEA, MFM or DEM for analyzing earthmoving operations the model must reproduce the mechanical behavior of the granular material. In practice this macro level mechanical behavior is not achieved by modeling the exact physics of the microfabric structure but rather by approximating the macrophysics; that is using continuum mechanics or/and micromechanics, which uses length scales, that are larger than the physical grain size. Different numerical approaches developed by Caterpillar Inc. researchers will be presented and discussed.

A Mesh Free Method to Simulate Earthmoving Operations in Fine-Grained Cohesive Soils

Gross distortion and eventual fragmentation of soil, which generally occur during earthmoving operations such as dozing and excavation, pose significant computational challenges to simulation by conventional Finite Element Methods (FEM). This deformation behavior in cohesive soils poses even greater challenges for simulation by the Discrete Element Method (DEM), since without the firm mathematical basis offered by continuum mechanics, DEM is heavily reliant on a mixed semi-analytical and empirical formulation. This paper focuses on the development of a 3D Mesh Free Method (MFM), specifically to extend the predictive capability of existing soil-machine interaction simulation tools to a variety of earthen materials important to earthmoving machines. This discretization method is seen as ideally suited for the prediction of implement forces and overall soil motion resulting from earthmoving operations in a fragmenting medium such as fine-grained cohesive soil. It is here, for simulations involving gross deformation and eventual fragmentation, that the absence of fixed connectivity (or “mesh” as the name implies) gives MFM great flexibility, while still retaining the highly desirable characteristics of a continuum mechanics based formulation. This work documents the theoretical aspects of the formulation, beginning with the MFM discretization of the governing partial differential equations. In addition, it covers the description of the coupled damage mechanics and plasticity constitutive model used to represent the soil, as well as, the details of the treatment of discrete fracture. The work also contains example results from 3D simulations of a blade cutting and a bucket excavating clay-type soil. These results depict a first attempt at capturing soil plasticity coupled with damage evolution, soil fragmentation at the end-state of damage and sustained contact of soil fragments with the earthmoving implementation and amongst the fragments themselves.