Numerical Dispersion Error in Finite Methods, Exemplified by the Perfectly Straight Beam Undergoing Bending Oscillations

In chapter (1) we present numerical simulations of the eigenstates of a MBS-model (Multi-Body-System) for a continuous ring structure. Both eigenfrequencies and eigenforms show a systematic error for high mode numbers. In order to understand the source of this error, in chapter (2) we calculate analytical solutions for the eigenfrequencies of a MBS- and a FEM-model (Finite-Element-Method) for the straight beam undergoing bending oscillations. We summarize our results in chapter (3)

Crack nucleation in a peridynamic solid

A condition for the emergence of a discontinuity in an elastic peridynamic body is proposed, resulting in a material stability condition for crack nucleation. The condition is derived by determining whether a small discontinuity in displacement, superposed on a possibly large deformation, grows over time. Stability is shown to be determined by the sign of the eigenvalues of a tensor field that depends only on the linearized material properties. This condition for nucleation of a discontinuity in displacement can be interpreted in terms of the dynamic stability of plane waves with very short wavelength. A numerical example illustrates that cracks in a peridynamic body form spontaneously as the body is loaded

Peridynamic States and Constitutive Modeling

A generalization of the original peridynamic framework for solid mechanics is proposed. This generalization permits the response of a material at a point to depend collectively on the deformation of all bonds connected to the point. This extends the types of material response that can be reproduced by peridynamic theory to include an explicit dependence on such collectively determined quantities as volume change or shear angle. To accomplish this generalization, a mathematical object called a deformation state is defined, a function that maps any bond onto its image under the deformation. A similar object called a force state is defined, which contains the forces within bonds of all lengths and orientation. The relation between the deformation state and force state is the constitutive model for the material. In addition to providing a more general capability for reproducing material response, the new framework provides a means to incorporate a constitutive model from the conventional theory of solid mechanics directly into a peridynamic model. It also allows the condition of plastic incompressibility to be enforced in a peridynamic material model for permanent deformation analogous to conventional plasticity theory