Mechanics and Physics of Material Defects

Researcher: Prof. Nasr Ghoniem


Defects in materials are the most interesting features, because without them, materials would be very boring! It is remarkable that materials are strongly bonded in regular crystal structures that look the same if one looks under the microscope anywhere in the material. However, because natural processes do not occur slow enough, a variety of topological defects always exist.  These defects can be classified as point (atomic size), line, surface, and volume defects. Examples are vacancies, interstitials, precipitates, dislocations, disclinations, grain boundaries (tilt and twist), twin boundaries, and even cracks.  Illustrations of the topological features of material defects are shown below. In Professor Ghoniem’s lab, revolutionary computational methods to track complex defects in materials have been developed. In particular, the method of Discrete Dislocation Dynamics has been developed to study the evolution of large ensembles of topologically interaction dislocations, where the interaction field is long-range. When dislocation lines collide, junctions or annihilations take place. These events result in complex 3-dimensional structures that describe the plastic deformation of materials at the nano- and micro-scale.  Likewise, a new method has been developed in Ghoniem’s lab for 3D cracks in real engineering components.

Figure 1: Dislocation Types

Figure 2: Stress Field Around a Dislocation Loop

Figure 3: Disclinations

Figure 4: Tilt Boundary

Figure 5: Twin Boundary

Figure 6: Twist Boundary Between Grains

Figure 7: Cracks

Figure 8: Computer simulation of dislocation ensembles in tungsten

 UCLA Samueli Mechanical and Aerospace Engineering