Atsuhiro Koyama

Accurate SIF Analysis of a Partially Cylindrical Side Crack Opened by Far-Field Tension

In recent years, due to a remarkable progress of qualified mesh generation algorithms combinedwith computational image processing technologies, a complex shaped 3D crack analysis has been carried out more and more easily by finite element method. Generally speaking, in order to assess thereliability of numerical solution, existence of a closed-form solution or a practically exact numericalsolution is important and thank-worthy. In contrast with 2D problems, however, there are very fewnumber of closed form solutions regarding to 3D crack problems. In the present analysis, we examineda highly accurate body force method (BFM) analysis for a partially cylindrical 3D surface crackin which a suitable basic density function can be supposed reasonably by taking account the crackgeometry. The present SIF solution could be used effectively for the purpose of benchmark the finiteelement solution for general non-planar crack problems.

Analysis of Planar Crack Coalescence by Mesh-Free Body Force Method

In a standard body force method analysis, a mesh division is required to define the boundary of a problem and to solve a governing equation using discretization procedure. However, in the present study, a moving least square strategy is introduced to define a weight function for the density of body force doublet and therefore a crack analysis is carried out without providing a standard mesh-division. Hence, the standard crack face elements are not required at all. A variety of 3D crack problems can be analyzed simply by providing a data that only de nes a crack front. Besides the nodal points for crack front, several internal nodes are generated on the crack face to represent a distribution of unknown function. At the internal nodes, an unknown variable is assigned which uniquely de ne a distribution of the relative crack face displacement. In the present approach, a crack problem is formulated as a singular integral equation whose unknown is a value of the weight function at the internal nodal points. A crack growth can be simulated directly by changing the shape of crack front, by means of adding a new nodal point in the vicinity of the current crack front. In the present paper, the proposed method is used to simulate a coalescence of interacting planar cracks.

Basic Research on Simulation for 3D Crack Growth by Mesh Free BFM

A new methodology that enables us to compute the arbitrary shaped 3D crack problems is studied. In the present method, it is possible to analyze the 3D crack problems without preparing mesh data as in ordinary boundary elements but with defining a sequence of nodal points representing the crack front and the internal nodal points that define a crack surface as well as a shape function used for determining unknown variables. The present method has special potential for analyzing a complicated 3D crack geometry which is generally difficult to treat in usual element based methods. In the present research, we apply mesh-free body force method to analyze the growth of 3D planar cracks. In concrete, a crack growth analysis for initially rectangular or elliptical crack existing in an infinite solid under uniform tensile stress perpendicular to the crack surface at infinity is demonstrated

Nondestructive Thermal Wave Detection of Internal Micro-defects using Scanning Electron-induced Acoustic Microscope

The nondestructive instrument as the application of coupling phenomenon between the thermal wave by non-Fourier heat conduction and the thermal stress wave by elastic vibration has been proposed for more than twenty years. This technique is called scanning electron-induced acoustic microscope (SEAM). Our own-built SEAM has, so far, successfully provided some nondestructive observations of micro-defects such as the micro-voids of sintered materials, martensitic phase transformation, grain boundary without any chemical etching, and so on. Some defects of them were indeed confirmed by digging using focused-ion beam (FIB) fabrication to make certain of those beings. In order to fully understand this coupling phenomenon, this paper gives the simple one-dimensional computational model of the multiphysics to explore the detailed transportation mechanism. The cyclic chopping of electron beam with extremely high frequency yields the thermal wave according to non-Fourier heat conduction, which respects the thermal relaxation different from the ordinal diffusive heat transmission. The wavelength of the thermal wave, which may determine the essential SEAM resolution, decreases as increasing the thermal relaxation time of material and also the modulation frequency. The numerical summaries are qualitatively in agreement with some nondestructive observations with changing the blanking frequency.

Fatigue Crack Growth Behavior in Silicon Nitride under Constant and Variable Amplitude Load Sequences

In order to investigate cyclic fatigue crack growth behavior of a gas-pressure-sintered silicon nitride under constant and variable amplitude load sequences, fatigue crack growth tests were carried out using compact type (CT) specimens, The crack length and macroscopic crack closure were measured using an unloading elastic compliance method. Grain interlocking was observed around crack wake in all fatigue test conditions, which implied fatigue crack growth of this material was associated with progressive degradation of the grain interlocking by cyclic loading. The fatigue crack growth rate, da/dn, under constant amplitude loading was controlled not only by the maximum stress intensity factor, K max , but also the stress amplitude, while K max was the most important factor in cyclic fatigue crack growth. The overload and high-level load excursion produced the acceleration of fatigue crack growth and the downward shift of crack closure point under low-level loading. This decrease was thought to result from much severer crash or frictional wear of grain interlocking and the crack closure could explain the acceleration behavior qualitatively.

Fatigue Crack Growth Behavior of Silicon Nitride under Constant and Non-Stationary Variable Amplitude Loadings.

In order to investigate the effect of load variation on the cyclic fatigue crack growth behavior of gas-pressure-sintered silicon nitride, fatigue crack growth tests under constant amplitude loading, multiple peak overloading, Lo-Hi and Hi-Lo two-step loadings were carried out using compact type (CT) specimens. Crack length and macroscopic crack closure were measured using the unloading elastic compliance method. Grain interlocking was observed around crack wake in all fatigue test specimens by SEM. Fatigue crack growth rate, da/dn, under constant amplitude loading was controlled by not only maximum stress intensity factor, Kmax, but also load amplitude. Crack opening stress intensity factor, Kop, decreased as load amplitude increased, as was concerned with breaking of grain interlocking. Overload caused the acceleration of fatigue crack growth rate, which was in contrast to the retardation observed in metallic materials. The acceleration was due to the breaking of grain interlocking by overload. The crack growth rate recovered as crack grew in a relatively short distance after overload, because grain interlocking in crack wake was reconstructed during crack growth.