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Theoretical estimates or experimental determinations of stress fields associated with semiconductor devices are generally simplified with the aid of two elastic constants, Young's modulus E and Poisson's ratio ν. In this paper, a generalized expression for ν has been derived for arbitrary orientations of cubic semiconductor crystals, and the variation of E, ν, and E/(1-ν) for directions within the important {111}, {100}, and {110} planes is examined. The results show that isotropic elasticity theory is exact for all directions within {111} planes and that the composite elastic constant E/(1-ν) which frequently occurs in problems of practical interest is also invariant for all directions within {100} planes. Numerical values for the various elastic constants are tabulated for GaAs, GaP, Si, and Ge.

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Numerous discarded ProFile GT, ProFile, and ProTaper nickel-titanium rotary instruments obtained from two graduate endodontic clinics were examined by scanning electron microscopy. These instruments had an unknown history of clinical use and had fractured or experienced considerable permanent torsional deformation without complete separation. The failure processes generally exhibited substantial ductile character, evidenced by a dimpled rupture fracture surface. Crack propagation at grain boundaries and cleavage surfaces indicative of transgranular fracture were observed for some specimens. It appeared that oxide particles from the manufacturing process served as nucleating sites for the microvoids, leading to dimpled rupture. A previously unreported fracture mode also was observed, in which crack propagation, approximately parallel to the local flute orientation, connected pitted regions on the surface. Combining present and previous scanning electron microscopy observations of clinically failed instruments, suggestions are offered for improving their fracture resistance.

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The purpose of this article is to examine the soundness of conventional orthodontic bonding assessment methods. A classification of bond strength studies is proposed with the testing environment (in vivo, in vitro, and ex vivo), loading mode (shear, tensile, and torsion), and bonding substrate (enamel, restorative, and prosthetic materials) serving as discriminating variables. Inconsistencies throughout the various stages of research protocols are analysed. These include the following: tooth selection, storage, and preparation; bonding; testing; and data analysis with regard to the clinical applicability of the reported information, as well as the scientific integrity of the testing procedure. Contradictory models may partially account for the considerable variability noted for reported bond strength values of different orthodontic bonding systems. Such discrepancies may also explain the conflicting evidence reported on the failure characteristics of the components of the bonding system in different trials examining the efficacy of nominally identical materials. A novel approach to study the fatigue life of materials is proposed to understand the processes occurring prior to bond failure. Mock research data manipulation is also utilized to illustrate the correct statistical treatment of findings, and recommendations for future research are made to ensure scientific soundness and clinical applicability of data.