Shu Sawai

Nanostructural interpretation for elastic softening of amorphous carbon induced by the incorporation of silicon and hydrogen atoms

First-principles molecular dynamics simulation is used to investigate the elastic softening of amorphous carbon on the incorporation of silicon and hydrogen atoms, and the mechanisms responsible for this phenomenon are discussed from the viewpoint of atomic structure. With increasing silicon incorporation, it is found that the bulk moduli of silicon-incorporated amorphous carbon (a-C:Si) and silicon-incorporated hydrogenated amorphous carbon (a-C:Si:H) decrease, whereas the total number of sp3-bonded atoms increases. This is explained on the basis of interatomic bond structures such as: increasing silicon incorporation reduces the number of interatomic (both single and double) bonds between carbon atoms while increasing the number of interatomic bonds between silicon and carbon atoms. Furthermore, for a given density and silicon content, it is found that the bulk modulus of the a-C:Si structure is greater than that of the a-C:Si:H structure, though their interatomic bond structures are similar. The reduce...

Proposal of Rational Determination of Fracture Toughness Lower-Bound Curves by Master Curve Approach

The Master Curve gives the relation between the median of fracture toughness and temperature in ductile-brittle transition temperature region. The procedure to determine the Master Curve is provided in the current ASTM E1921 standard. Considering the substitution of the alternative lower-bound curves based on the Master Curve approach for the recursive KIc curves in the present codes, the statistical characteristic should be well incorporated into the determination of the lower-bound curves. The appendix in the ASTM standard provides the procedure to derive the lower-bound curves, however, it seems to be addressed without sufficient consideration on statistical reliability. In this study, we proposed a rational determination method of fracture toughness lower-bound curves based on the Master Curve approach. The method took account of the effect of sample size in the determination of the tolerance bound curve. The adequacy of the proposed method was then verified by comparing with a fracture toughness database for RPV steels. The method allows the application of the Master Curve using fewer specimens, which can coexist with the present surveillance program.Copyright