Satoshi Morooka

Magnetic scattering in the simultaneous measurement of small-angle neutron scattering and Bragg edge transmission from steel

A technique for the analysis of magnetic scattering has been developed, where small-angle neutron scattering and Bragg edge transmission measurements are performed simultaneously. This technique is shown to provide crystallographic information for ferrite crystallites and nanostructural information for precipitates in steel.

Inverse pole figure mapping of bulk crystalline grains in a polycrystalline steel plate by pulsed neutron Bragg-dip transmission imaging

A new mapping procedure for polycrystals using neutron Bragg-dip transmission is presented. This is expected to be useful as a new materials characterization tool which can simultaneously map the crystallographic direction of grains parallel to the incident beam. The method potentially has a higher spatial resolution than neutron diffraction imaging. As a demonstration, a Bragg-dip neutron transmission experiment was conducted at J-PARC on beamline MLF BL10 NOBORU. A large-grained Si–steel plate was used. Since this specimen included multiple grains along the neutron beam transmission path, it was a challenging task for existing methods to analyse the direction of the crystal lattice of each grain. A new data-analysis method for Bragg-dip transmission measurements was developed based on database matching. As a result, the number of grains and their crystallographic direction along the neutron transmission path have been determined.

Hierarchical 3D/4D Characterization on Deformation Behavior of Austenitic and Pearlitic Steels

This study highlites deformation behavior of austenitic and pearlitic steels by in-situ neutron diffraction and 3D/4D EBSD measurement with a particular attention to their hierarchy.In particular stress partitioning in these microstructures is examined from macroscopic as well as microscopic scale length levels, and they are correlated to each other.

Topological Analysis of Martensite Morphology in Dual-Phase Steels

Martensite morphology such as connectivity or dispersivity in ferrite (F)/martensite (M) dual-phase (DP) steels was investigated from topological viewpoint to reveal the effect of the martensite morphology on the mechanical properties. Topological analysis permits evaluation of the microstructural connectivity and dispersivity by measuring the number of handles, independent bodies and genus, etc. The topological analysis was performed on three-dimensional (3D) reconstructed images of the microstructure with different connectivity, dispersivity, volume fraction and hardness of martensite in DP steels that were prepared by changing the intercritical annealing temperature. The connectivity and the volume fraction of martensite increased while the dispersivity and hardness of it decreased with increasing annealing temperature. The effect of connectivity and dispersivity as well as volume fraction and hardness, in particular, on work hardening behavior was individually evaluated at a given strain.

Numerical analysis of residual stress distribution on peening process

Various peening techniques are employed to prevent stress corrosion cracking or to extend the fatigue life of structures. In this study, to investigate the effect of shot peening on operation, an analysis method that predicts the stress distribution due to shot peening was proposed. Using the proposed method, the load distribution from the shot collisions was modeled, and it was integrated with a dynamic analysis method based on the idealized explicit FEM (IEFEM). The accuracy of the proposed method was confirmed by comparing the stress distribution from the collision of a single shot with the results analyzed using ABAQUS. A thermal elastic-plastic analysis method using IEFEM was applied to the analysis of the residual stress distribution of a multi-pass-welded pipe joint. The calculated residual stress distribution was compared with the measured residual stress distribution measured using X-ray diffraction (XRD). The results showed that the two welding residual distributions were in good agreement. Considering the calculated welding residual stress distribution, the modification of the stress distribution due to shot peening was predicted using the proposed method. A similar stress distribution was obtained using XRD for the case where a large number of collisions were considered.

Energy-resolved small-angle neutron scattering from steel

Recent progress of pulsed neutron sources has enabled energy-resolved analysis of neutron attenuation spectra, which include information on neutron scattering. In this study, a new analysis technique for small-angle neutron scattering (SANS) is demonstrated. A clear difference is observed in the neutron attenuation spectra between steels with different nanostructures; this difference can be understood as arising from attenuation due to SANS. The neutron attenuation spectra calculated from the corresponding SANS profiles agree well with the experimentally observed attenuation spectra. This result indicates that measurement of neutron attenuation spectra may enable the development of a novel experimental technique, i.e. energy-resolved SANS.

Finite element analysis of mode II delamination suppression in stitched composites using cohesive zone model

To provide an accurate prediction of mode II delamination properties of stitched composites, a single stitched laminate under pure shear load (interlaminar shear) is investigated. An interlaminar shear test (IST) is conducted, using a specially constructed fixture. A finite element model of the IST is developed, using a spring connector to represent the stitch thread. Furthermore, an end notch flexural test is simulated based on the IST model and with a cohesive zone element used to simulate crack propagation. The effects of the cohesive zone modelling parameters (interfacial strength, stiffness and viscosity coefficient) are also investigated. Close agreement is obtained between the simulated and experimental results. Finally, the verified model is extended to study the effect of the number of stitch rows in the crack region. The results confirm that the crack propagation starting point (peak load) depends on the number of stitch rows in the crack region.

Analyses of heterogeneous deformation and subsurface fatigue crack generation in alpha titanium alloy at low temperature

Subsurface crack initiation in high-cycle fatigue has been detected as {0001} transgranular facet in titanium alloys at low temperature. The discussion on the subsurface crack generation was reviewed. Analyses by neutron diffraction and full constraints model under tension mode as well as crystallographic identification of the facet were focused. The accumulated tensile stress along may be responsible to initial microcracking on {0001} and the crack opening.

Subjects on Fatigue Crack Generation in High Strength Alloys for Long-Life Design

The very localized deformation processes have been found to be decisive for subsurface fatigue crack generation at the lower stress level such as the elastic incompatibility at boundaries where only a very small fraction of plastically deformed grains was detected. The material design and its microstructure modification to achieve higher fatigue resistance in long-life range are needed for the high strength alloys, which is one of the ways developing an ecomaterial. Novel systems have employed to clarify the substance crack generation and growth mechanisms of high strength alloys. The initial crack size highly depends on the maximum cyclic stress range, which implies a threshold of stress intensity range controlling mechanism. Heterogeneous microplasticity due to planar slip and restricted system is considered to play an important role on making the subsurface crack. Then, it should be progressed in the understanding of damage stage in high-cycle fatigue fracture process.

Finite element analysis of 4ENF testing of stitched carbon/epoxy laminate

Stitching technique improves both mode I and mode II delamination toughness significantly by reducing the opening and sliding displacement, respectively. However, measuring mode II delamination behaviour of stitched composites using three point end notched flexure test (ENF) is facing a serious problem where the premature failure of specimen occurs prior to delamination propagation. This work is addressed to conduct numerical study of four points end notched flexure (4ENF) test of stitched composites. Four models were simulated including unstitched unidirectional, unstitched multidirectional laminate, stitched multidirectional laminate with and without tabs on top and bottom of specimen. Furthermore, stitch threads were modelled using spring connectors with user define force-displacement relationship. Load-displacement and crack propagation length vs. displacement curves were plotted. Stress distributions across the thickness were also presented to recognize the possibility of specimen damage during the test. The results showed that simple 4ENF test seems to be quite enough for evaluating mode II critical energy release rate (GIIC) at the initial crack length of 35mm. However to evaluate R-curves (GII vs. crack propagation length) of stitched multidirectional laminate, tabbed 4ENF test specimen is recommended.