Shigetomo Nunomura

Effects of dendrite cell size and particle distribution on the near-threshold fatigue crack growth behaviour of cast AlSiCp composites

Abstract Fatigue crack growth tests and crack closure measurements were performed for A356 cast aluminium alloys reinforced by 10 or 20% SiC particles and their matrix A356 alloys with systematically controlled dendrite cell size and particle distribution. The cell size dependence of the fatigue crack growth behaviour in the composite was found to be quite similar to that of the matrix alloy. This suggests that the cell size rather than the particle-crack tip interaction is the most important factor to control the fatigue crack growth of the composites. Near threshold fatigue crack growth properties were improved in the composites with coarser cell size and inhomogeneous particle distribution due to the enhanced roughness induced crack closure effect. These results were compared to those of powder-metallurgically-processed materials.

Effects of the particle distribution on fatigue crack growth in particulate SiC/6061 aluminium alloy composites

Abstract Fatigue crack growth rates have been examined in powder-metallurgy-processed 6061 aluminium alloy composites containing 15 and 30 wt.% SiC particles with different particle distributions. Over the range 10 −9 to 10 −6 m/cycle the fatigue crack growth rates in the composite materials were lower than those reported for the monolithic alloy. SEM observations of the broken specimens showed very few SiC particles on the fatigue surfaces produced at low values of ΔK . An increased area fraction of the fractured coarse particles was evident on the fatigue surfaces of the bimodal distribution composites tested at high ΔK . Quantitative examination of the crack path profile was used to highlight the interaction between the particles and the crack path. The fatigue crack avoids SiC particles at low- ΔK ranges, but at high ΔK ranges the crack appears to proceed by linking fractured SiC particles ahead of the main crack front.

Functional evaluation device capable of evaluating an artificial device by the use of acoustic emission

In a functional evaluation device for use in evaluating functional degradation of an artificial device buried in a living body, frequency zones of acoustic emission are detected by the use of the fact that the functional degradation brings about variation of the frequency zones. The acoustic emission may be measured at two different positions adjacent to the artificial device so as to detect an arrival time difference of the acoustic emission and to thereby locate the functional degradation. The acoustic emission is therefore picked up by a pair of transducers placed at the two positions and is processed by a processing circuit for detecting whether or not the functional degradation takes place and by a determining circuit for locating the functional degradation.

The propagation velocity of the martensitic transformation in 304 stainless steel

Abstract The martensitic transformation propagation velocity in 304 austenitic stainless steel polycrystals has been studied by acoustic-emission-frequency spectrum analysis. The transformation propagation velocity was found to be 160–200 ms−1 in a 250 μm grain size specimen and 110ms−1 in a 110 μm specimen at −135°C. These results showed that the martensitic transformation propagation velocity is not constant, but depends on the propagation distance.

Determination of the duration of transient phenomena by frequency-domain analysis of acoustic emission

Abstract A method of estimating the duration of transient phenomena is proposed using frequency analysis of the acoustic emission (AE). The generation and propagation characteristics of the AE through an infinite elastic medium are examined. The time dependent distortion of the AE source is assumed to be a Gaussian error function. The relation between the rise time of the source (δτ) and the log-log gradient, m, of the observed AE spectrum is represented as m = ω2δτ2/8 for an infinite medium using a flat frequency response AE transducer, where ω is the angular frequency. This expression suggests that for a particular frequency, it is possible to obtain a value for the rise time, δτ, which corresponds to a higher frequency. This parameter has been examined experimentally in a quasi-infinite plate, and is compared with theoretical predictions.

Mechanism of the two stage plastic deformation following an overload in fatigue crack growth

The plastic deformation behaviour at a fatigue crack tip following a tensile overload has been dynamically analysed in pure titanium using an electrochemical method. Using this method, the occurrence of plastic deformation can be recognised by an increase in polarisation current. Two stage plastic deformation, which was associated with the load increase, is observed during the retardation period. The two stage plastic deformation is due to the difference in the load levels at which the plastic deformations start in the interior and at the surfaces of the specimen. The plastic deformation associated with the lower of these two load levels is caused at the plane stress portions and the plastic deformation associated with the higher load level is caused at the plane strain portion of the crack front. The changes in the plastic deformation behaviour is explained by the changes in the shape of the crack front.

Micromechanism of a deformation process before crazing in a polymer during tensile testing

Polymers are popularly used for housing and parts of machines and equipment. However, their mechanical properties, especially the deformation process, have not been clarified. During tensile testing, crazes are thought to be a source of microcracking and fracture, but the relation between the craze formation process and the deformation process before crazing is not understood. In the present work, scanning acoustic microscopy and X-ray diffraction were used to investigate the micromechanism before craze formation in polymethylmethacrylate (PMMA) and polycarbonate (PC). The velocity change of the surface acoustic wave and X-ray diffraction intensity indicated that molecular orientation occurred in a very small area from early stages of plastic deformation. From the results it was thought that texture was heterogeneous and anisotropic in a very small area, the shape of the area was spheroidal with a longer radius in the direction perpendicular to the applied stress, and the molecular chain in the area was oriented parallel to the stress axis. The area is thought to increase with increasing plastic strain.

Evaluation of the Fixation of Artificial Hip Joint by Acoustic Emission

The fixation between an artificial joint and the bone becomes loose with time. To avoid this loosening, and there by increase the life of the artificial joint, it is important to detect the loosening at a very early stage. In this paper, we tried to detect the loosening of an artificial hip joint by recognizing analyzing acoustic emission from it. More than 200 measurements were taken and it is clear that the AE method is very efficient and sensitive for evaluation of the fixation condition of an artificial joint.

Effect of load ratio on the deformation behaviour of fatigue crack tips

Abstract To investigate the change in the manner of plastic deformation at a fatigue crack tip under different loading conditions, the fresh surface area which is produced in one fatigue cycle at the crack tip was measured by an electrochemical method under various load ratios. The fresh surface area produced in each fatigue cycle is in proportion to the crack propagation rate in the Paris regime. This indicates that the crack tip geometry at K max maintains a similar profile through the Paris regime. Furthermore, the fresh surface area is independent of load ratio at a given d a /d N in the Paris regime. This suggests that the shape of the fatigue crack tip at K max should be independent of load ratio at a given d a /d N , and that the shape at K min is also independent of load ratio, whereas the size of the deformed region at the crack tip is in proportion to d a /d N .

Non-destructive evaluation of the micromechanism of the deformation process during tensile tests on polymers by the elastic-wave transfer function method

The elastic-wave transfer function method (ETFuM) was applied to make clear the micromechanism of the deformation process during dynamic tensile testing of polymethylmethacrylate (PMMA) and polycarbonate (PC). In PC, the transfer function began to change at a high frequency. After that, it decreased abruptly in the low-frequency region. The variation of the transfer function at high frequency was caused by the nucleation and growth of microdefects such as crazes and microcracks. The variation at low frequency was caused by plastic deformation such as inclined necking and microdefects due to shear stress. On the other hand, in PMMA the transfer function changed homogeneously with elongation at high frequencies and did not change at low frequencies. The variation of the transfer function during tensile testing related to the micromechanism of elastic and plastic deformation processes in both PC and PMMA. The results suggested that the ETFuM is a useful and powerful method for evaluating the micromechanism of deformation processes in polymers in a non-destructive and dynamic way.