H.H.M. Cleveringa

Simulation of X-ray diffraction-line broadening due to dislocations in a model composite material

X-ray diffraction-line profiles of two-dimensional, plastically deformed model composite materials are calculated and analysed in detail. The composite consists of elastic reinforcements in a crystalline solid and is subjected to macroscopic shear. Slip occurs in the matrix only due to the collective motion of discrete dislocations on a single set of parallel slip planes. The results of dislocation dynamics computations are used as input for the calculation of the line profiles. The line profiles are computed directly using the kinematics approach, without making a priori assumptions on the dislocation distributions. Two steps are required. First, the full intensity distribution of a single crystal of composite material is calculated. Then, assuming a perfectly random orientation distribution of such single crystals, powder diffraction-line profiles are determined. Results will be presented for several orders of reflection and in different crystallographic directions. The broadening of the line profiles is shown to be not only determined by the density of dislocations, but also by their spatial distribution.

Discrete dislocations interacting with a mode I crack

Small scale yielding around a plane strain mode I crack is analyzed using discrete dislocation dynamics. The dislocations are all of edge character, and are modeled as line singularities in an elastic material. At each stage of loading, superposition is used to represent the solution in terms of solutions for edge dislocations in a half-space and a complementary solution that enforces the boundary conditions. The latter is non-singular and obtained from a linear elastic, finite element solution. The lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation are incorporated into the formulation through a set of constitutive rules. A relation between the opening traction and the displacement jumps across a cohesive surface ahead of the initial crack tip is also specified, so that crack initiation and crack growth emerge naturally. Material parameters representative of aluminum are employed. Two cases are considered that differ in the strength and density of dislocation obstacles. Results are presented for the evolution of the dislocation structure and the near-tip stress field during the early stages of crack growth.

Temperature effects and fast-moving screw dislocations at high strain rate deformations

In this paper, shear deformation at high strain rates is modeled within the framework of discrete dislocation plasticity. The method of discrete dislocation plasticity is extended to incorporate the temperature rise induced by moving dislocations. Also, the stress and displacement fields of a screw dislocation on inclined planes in a periodic structure are developed. The influence on the temperature rise on various micro-mechanical processes is discussed.