Development of a tri-axial load cell based on strain gages

Experimental Techniques - The tri-axial load cell of cantilever type was designed for an axial force (F z ) and two bending forces (F x , F y ) measurement necessary to fracture toughness study....     

Lattice dynamics studies of grain boundary structures: I. Method and application to stability

The method of lattice dynamics has been applied to an atomistic model of a grain boundary with emphasis on its utility in examining structural instability. A 36.9° [001] coincidence symmetrical tilt boundary was used with a Morse pairwise potential. Several structures derived by a static relaxation procedure were studied. Stable structures are characterized by a vibrational spectrum consisting of real frequencies only, while one of the statically relaxed structures yielded a pair of imaginary frequencies and therefore was unstable. A comparison of the lattice dynamics method with other methods for testing stability of a defect configuration is given.     

Analysis of vertical cracking phenomena in tensile-strained epitaxial film on a substrate: Part II. Application to InxGa1−xAs/InP system

Cracking phenomena in tensile-strained In_xGa_1?xAs epitaxial film on an InP substrate are analyzed via the formulation given in Part I [Lee, S., Choi, S.T., Earmme, Y.Y., 2006. Analysis of vertical cracking phenomena in tensile-strained epitaxial film on a substrate: Part I. Mathematical formulation. International Journal of Solids and Structures 43, 3401–3413], where the solution for a dislocation in an anisotropic trimaterial is used as a fundamental solution and the crack is modeled by the continuous distribution of dislocations. Misfit strains and stresses are evaluated as a function of indium content x in an In_xGa_1?xAs/InP system. A single crack and periodic cracks, respectively, induced by the misfit stresses are considered. The crack opening profile, the crack mouth displacement, and the energy release rate as a function of the crack length are obtained. The critical conditions for a single crack and periodic cracks, respectively, are thus obtained, and are found to depend on the film thickness, the crack length, and the period of the cracks. The results of these analyses are also compared with published data obtained from experiments.     

Lattice dynamics studies of grain boundary structures: II. Thermodynamic functions

The change in the atomic vibrational frequency spectrum of an fcc crystal due to the presence of a grain boundary is obtained by lattice dynamics methods. The grain boundary is a 36.9° 〈100〉 symmetric tilt coincidence boundary. An orthorhombic computational cell of 58 atoms containing two such boundaries of opposite rotational sense is employed. The atomic interactions are simulated by a Morse potential developed by Cotterill and Doyama to fit aluminum. From the changes in frequency spectrum the excess thermodynamic functions of the grain boundary are calculated and compared with results from both experiment and the Einstein model. The general shift of frequencies toward lower values is probably due to the weaker binding of many atoms near the boundary, while there are some higher frequency states associated with a few more tightly bound atoms.