Papers by Author: Yoshimasa Takahashi

Abstract: The aim of this study is firstly to investigate the applicability of a sharp notch with Focused Ion Beam (FIB) as a crack for fatigue limit evaluation. Secondly we investigate a condition in which artificial defects (drilled hole, FIB notch) can be used as a crack for fatigue limit evaluation. To achieve the aim, the growth behaviors and the non-propagating crack sizes of small fatigue cracks initiated from a FIB notch and a drilled hole are carefully compared with those of an annealed fatigue crack which imitates an ideally sharp crack. The results show that a FIB notch can be used as a crack for fatigue limit evaluation under some conditions. The results also show that the condition which controls the applicability of an artificial defect as an ideal crack for fatigue limit evaluation is strongly dependent on the relation between (i) the length of a non-propagating fatigue crack and (ii) the crack length when the small fatigue crack growth behaviors from an artificial defect and an ideal pre-crack become almost the same. It is found that the length of (ii) can be obtained by the analyses using the number of cycles from a certain crack length to failure.

Abstract: The plasticity of a copper (Cu) nano-component is experimentally evaluated by a cantilever specimen with multi-layered structure. The cantilever is monotonically loaded by a diamond tip and the deflection at the free-end is precisely measured by a transmission electron microscope (TEM). The plastic deformation of the Cu nano-component is successfully monitored through the non-linear behavior of applied load, P, and cantilever deflection, δ. The plastic constitutive quation of the Cu component is inversely analyzed by finite element method (FEM) assuming that the component obeys the Ramberg-Osgood law. The parameters in the R-O law (σ0, n and α) are optimally fitted to reproduce the experimentally evaluated P-δ relation. The resultant parameter set is derived as (σ0, n, α) = (345 MPa, 3.2, 1.25). The Cu nano-component has a much higher yield stress and a hardening rate compared with the ones in a bulk Cu.

Abstract: The interface strength of low-dimensional nano-components such as films and islands formed on substrates has been investigated in this project, and the focus is put on the mechanics of crack initiation from the free interface edge and propagation along the interface. The series of experiments elucidates the applicability of fracture mechanics concept on the structures. We proposed experimental methods for evaluating the initiation strength of an interface crack in submicron films and islands deposited on substrates. The initiation is governed by the singular stress field, and the criterion is prescribed by the stress intensity parameter. Using special loading apparatus built in a TEM, we developed a crack initiation method for nano-components and the role of plasticity on the delamination is clarified. Subcritical crack growth along an interface between submicron films under fatigue was also investigated by modified four-point bend method.