Dislocation Pile-up in Ge ( MPG ) (Courtesy Francois Louchet. Used with permission.) — In situ TEM was used to capture this movie of dislocation pile-up in a semiconductor. Note the dislocation glide is much more restricted in semiconductors, due to both the crystal structure and bonding peculiarities of these materials, as compared to metals. As a result, bulk silicon and other semiconductors are brittle and do not exhibit any significant plastic yielding prior to fracture. In class, we noted that conclusive observations of this pileup mechanism have not been documented in ductile metals that exhibit Hall-Petch strengthening.
Beam Bending Moment ( PDF ) — One student asked why the cross-section used to define the moment of inertia in beam bending is the one that is parallel to the axis of loading. By definition, I = double integral of y, the distance from the neutral axis of bending. By definition, neutral axis defines the plane in the bent beam in which there are neither tensile nor compressive stresses. In a beam of rectangular cross-section, this is the “middle” of the beam height.
Class Forum — This page contains questions and answers on a variety of topics, mostly about the problem sets. These discussions originally took place in the course forum between students, the teaching assistant, and the professor.