Yoshiaki Akiniwa

Modelling of small fatigue crack growth interacting with grain boundary

Abstract The slip band at the tip of a small fatigue crack interacting with grain boundaries is modelled for four cases: a slip band not reaching the grain boundary, a slip band blocked by the grain boundary, a slip band propagated into an adjacent grain, and a slip band propagated through one and then blocked by a second grain boundary. The theory for continuously distributed dislocations is used to calculate the crack-tip sliding or opening displacement and the microscopic stress intensity factor under tensile and shear loading. Assuming that the range of the tip displacement directly determines the propagation rate of both Stage I and II cracks, prediction of the propagation behavior of a small crack is made as a function of the distance between the crack tip and the grain boundary, and of the difficulty to propagate slip into adjacent grains, as well as a function of crack length and stress level. The directions for further development of modelling are discussed.

Resistance-curve method for predicting propagation threshold of short fatigue cracks at notches

Abstract A new resistance-curve method was proposed for predicting the growth threshold of short fatigue cracks near the notch root. The resistance curve was constructed in terms of the experimentally determined threshold value of the maximum stress intensity factor which was the sum of the threshold effective stress intensity range ΔK effth and the opening stress intensity factor K opth The ΔK effth value was constant, irrespective of crack length or notch geometry. The relation between K opth and crack length was independent of notch geometry. The predicted effects of the notch-root radius and the notch depth on the propagation threshold of short fatigue cracks were compared with the experimental data obtained using center-notched specimens with various notch-root radii and single-edge notched specimens with various notch depths. Excellent agreement was obtained between predictions and experiments.

Statistical characteristics of propagation of small fatigue cracks in smooth specimens of aluminium alloy 2024-T3

Abstract Smooth specimens of aluminium alloy 2024-T3 were fatigued under a nominally elastic condition. The irregular growth behaviour of many small fatigue cracks was discussed in relation to the material microstructure. The distribution of crack growth rate was fitted well by the three-parameter Weibull distribution. The changes in the Weibull three parameters, standard deviation and coefficient of variation with crack length were determined. The relation between crack growth rate and crack length was divided into three regions: A, B and C. In region C, where the crack growth rate was higher than 10 −8 m cycle −1 , the crack growth data followed the d a/ d N vs. ΔK relation for large crakcs. In region A, where the crack length was less than 0.1 mm, cracks grew along slip planes and the scatter of the crack growth rate was large. In region B, the crack growth rate was perpendicular to the stress axis; the crack growth rate was higher than that predicted from the d a/ d N vs. ΔK relation for large cracks. Comparison of the behaviour of main crack propagation with that of average crack growth rate showed that the life prediction based on the average growth behaviour of cracks gave a dangerous estimate.

Design of Engineering Diffractometer at J-PARC

An engineering diffractometer designed to solve many problems in materials science and engineering including investigations of stresses and crystallographic structures within engineering components is now being developed at J-PARC project. This instrument views a decoupled-poisoned liquid H2 moderator providing neutrons with good symmetrical diffraction profiles in the acceptable wavelength range. The primary flight path and the secondary flight path are 40 m and 2.0 m, respectively, for 90 degree scattering detector banks. A curved supermirror neutron guide will be installed to avoid intensity loss due to the long flight path and to reduce backgrounds from fast neutrons and gamma rays. Therefore, stress measurements with sufficient accuracies in many engineering studies are quite promising. The optimization of this instrument has been performed with a Monte Carlo simulation, and an appropriate resolution of less than 0.2 % in d/d has been confirmed. A prototyped radial collimator to define a gauge width of 1 mm has been designed and manufactured. From performance tests conducted at the neutron diffractometer for residual stress analysis RESA in JRR-3 of Japan Atomic Energy Agency, the normal distribution with a full width at half maximum of 1 mm was obtained in a good agreement with the simulation.

Propagation and non-propagation of small fatigue cracks

ABSTRACT The propagation and non-propagation of small fatigue cracks at the notch root were studied in the first part of the present paper. A resistance-curve method was proposed for predicting the growth threshold of mechanically short cracks on the basis of the experimental results on the development of crack closure. The effects of material, notch geometry, and mean stress on the growth threshold of short cracks and the notch fatigue strength were successfully predicted by the resistance-curve method. In the second part, the propagation behaviour of small fatigue cracks in smooth specimens was examined. A micromechanical model of small crack growth was combined with statistical simulation of crack growth for life prediction. The fatigue failure diagram was constructed to judge the threshold condition for small crack growth.

Changes of Internal Stress in Solid-Oxide Fuel Cell During Red-Ox Cycle Evaluated by In Situ Measurement With Synchrotron Radiation

The internal stress in anode-supported solid-oxide fuel cells (SOFCs) was evaluated by in situ measurement using high-energy x-ray synchrotron radiation. The oxidized cell had a compression of ∼400 MPa in the c-ScSZ electrolyte thin film and a tension of 50-100 MPa in the NiO-YSZ anode substrate at room temperature. The internal stress decreased with increasing temperature, becoming approximately zero at 1000 K. Although the internal stress returned to its initial value after the thermal cycle, the stress decreased to ∼200 MPa in the electrolyte after the reduction cycle because of the decrease of the coefficient of thermal expansion mismatch between the electrolyte and anode. The red-ox cycle would be detrimental for anode-supported SOFC.

Molecular dynamics analysis for fracture behaviour of single crystal silicon thin film with micro notch

Thin films of single crystal silicon are widely required in many applications of semiconductor devices and micro-electro-mechanical systems. In this study, molecular dynamics simulation is conducted to investigate the effect of notch depth on fracture strength. The specimen size is about 10 nm × 5 nm, and the periodic boundary condition is applied for thin films. The loading directions are [100], [110] and [111] direction. Brittle fracture occurs on the plane perpendicular to the loading direction. The stress intensity factor at the onset of the crack propagation increases with increasing notch depth. When the notch depth is longer than 2 nm, the stress intensity factor is nearly constant. At the onset of the crack propagation, the stress at the notch root becomes constant irrespective of the notch depth.

R-curve behavior in fracture of notched porous ceramics

Abstract Notched specimens of porous silicon carbide (SiC) with porosity 37% were fractured under four-point bending. A single edge notch with six depths ranging from 0.1 to 2.8 mm was introduced to the specimen with a height of 7 mm. The fracture of specimens with a notch depth of 0.1 mm did not start from the notch, but from the intrinsic defect. The size of the non-damaging notch is about 0.1–0.2 mm and roughly equal to the size of SiC particles. When the notch depth was larger than 0.4 mm, the fracture started from the notch for all specimens. The record of the strain gage glued on the compression surface of the specimen as a function of the load showed nonlinearity before reaching the maximum load. The critical stress intensity factor was nearly constant for crack initiation from the notch. The resistance curve was constructed by estimating the crack length from the compliance change of the specimen, and was used for determining the maximum load point in bending tests. Fractographic observations showed the fracture path along the binder phase between silicon particles.

In Situ Synchrotron Measurement of Internal Stresses in Solid-Oxide Fuel Cell during Red-Ox Cycle

The internal stress in solid-oxide fuel cells (SOFCs) was evaluated during the thermal, reduction and re-oxidation cycles by using high-energy X-ray synchrotron radiation of about 70 keV at Beam line BL02B1 of SPring-8. The oxidized cell has a compression of about 400 MPa in the c-ScSZ electrolyte and a tension of 50-100 MPa in the NiO-YSZ anode at room temperature. In-situ measurement during the thermal cycle in an air atmosphere, the internal stress decreased with increasing temperature, becoming approximately zero at 1000 K. After the thermal cycle, the internal stress returned to its initial value. In the measurement during the reduction cycle, the internal stress was smaller than that measured during the cooling cycle after the anode was reduced from NiO-YSZ to Ni-YSZ. In the re-oxidation cycle of a reduced cell, the internal stress in the electrolyte went into tension above 800 K when the anode was re-oxidized from Ni-YSZ to NiO-YSZ. This tensile stress is responsible for possible fracture of unit cells in SOFCs.

Re-evaluation of formulae for X-ray stress analysis in polycrystalline specimens with fibre texture: experimental confirmation

Line splitting or line broadening of an X-ray Bragg reflection owing to a three-dimensional or biaxial load on a cubic polycrystalline specimen with 〈111〉 fibre texture has been predicted by Yokoyama & Harada [J. Appl. Cryst. (2009), 42, 185–191] if the specimen is in the symmetry of the Laue class m{\overline 3}m. By using a TiN film specimen and a high-precision four-circle diffractometer with a laboratory X-ray source, it is shown that the profile of the 420 reflection is substantially different with and without biaxial load and also depends on the measurement direction, while the profile of the 222 reflection does not change. These results are quantitatively in agreement with the theoretical prediction, although the theory is based on the Reuss model of elasticity.