Tadashi Hasebe

High Temperature Multiaxial Low Cycle Fatigue of CMSX-2 NI-Base Single Crystal Superalloy

This paper describes studies of the tension-torsion multiaxial low cycle fatigue of CMSX-2 Ni-base single crystal superalloy. Tension-torsion low cycle fatigue tests were carried out at 1,173 K using CMSX-2 hollow cylinder specimens aligned with the {l_brace}001{r_brace} axis. Several multiaxial strain and stress parameters were applied to the experimental data to examine the suitability of the parameters to life prediction. All the strain parameters proposed so far gave a large scatter of the data correlation. Discrepant data correlation with the strain parameters resulted from the anisotropic stress response due to the crystallographic texture. Larger Mises equivalent stress was applied in torsion tests than in tension tests at the same Mises strain. However, Mises stress and the equivalent stress based on crack opening displacement gave a satisfactory data correlation. This study developed a new equivalent strain, taking account of the anisotropy of the elastic constants, which correlates the multiaxial low cycle fatigue data with a small scatter.

New Physical Interpretation of Incompatibility and Application to Dislocation Substructure Evolution

Physical interpretations of the incompatibility tensor are extensively discussed and applied to model several practically-important dislocation substructures in metallic materials. This paper firstly performs a tangible decomposition of the incompatibility tensor into the two types of defects by introducing Nye’s contortion tensor, and also clarifies the interrelationship with expressions given based on differential geometry. The effects on the evolutions of intra-granular substructures like cells and geometrically-necessary type bands are examined based on finite element simulations on multi-grain models under tension and simple shear with several representative orientations.

High strain rate forming using an underwater shock wave focusing technique

Abstract This paper proposes an effective method for controlling the morphology of impulsive pressure waves in hydro-spark forming, making use of the technique of underwater shock wave focusing. Free forming experiments using ellipsoidal reflectors with two kinds of minor-to-major axis ratios b / a =0.6 and 0.8 are performed under several blank-setting conditions. The height of the formed product depends solely on the impulse of the shock wave, regardless of its focusing behavior, while the shape of the product is controlled by distribution of the applied pressure wave. It is demonstrated that the reflector having b / a =0.8 yields a wide range of pressure distribution, which can be estimated from the shock wave front distribution calculated considering the effect of non-linear reflection.

Continuum Description of Inhomogeneously Deforming Polycrystalline Aggregate Based on Field Theory

This paper presents a field theoretical approach toward a continuum description of polycrystalline media. Collective behavior of the composing grains is shown to produce large stress fluctuation which can be an essential feature of polycrystalline aggregates. Stress function tensor is proposed as a new mechanical parameter describing such fluctuated fields. Implementation to the constitutive model based on crystal plasticity is extensively discussed.

Impact compression behavior of FCC metals with pre-torsion strains

Abstract This paper investigates the effect of non-proportional pre-straining path on flow behavior of FCC metals at impact compression, where reversed torsion strain is introduced following 10% pre-torsion. Two kinds of FCC metals, Al and Cu, are employed to also examine the material dependency. Experimental results demonstrate that significant interaction exists between strain history and strain rate effects, which is strongly material dependent. The reversed torsion temporarily reduces strain history effect for Al. The trend is shown to be described qualitatively by analysis based on the crystalline plasticity theory where newly proposed back stress model considering piling-up behavior of dislocations is introduced.

A fundamental study of excimer laser ablation using experimental and MD simulation method

Abstract This paper focuses on the effect of pulse duration on ablation rate and behavior of plume in excimer laser ablation. Experimental results of laser ablation demonstrate that the 200 ns pulse laser exhibits a four to five times larger ablation rate than that with 20 ns pulse for the same laser fluence. CCD photography reveals that the plume velocity exceeds 4.5 km s −1 , corresponding to Mach 12. It is shown that these phenomena can be qualitatively explained by simulation results based on the molecular dynamics (MD) method. When the pulse duration becomes long, the number of ablated atoms increases accordingly under a constant laser fluence condition. The simulated results shows that the maximum velocity of expelled atoms ranges up to 20 km s −1 .

New Impact Testing Methods for Sheet Metals Based on SHPB Technique

This paper proposes new impact testing methods applicable to sheet metals both under tension and compression based on widely used split Hopkinson pressure bar (SHPB) technique. Explicit dynamic finite element simulations by using LS-DYNA 3D are systematically conducted for several specimen clamping conditions to seek the appropriate methodologies to realize the two tests. For the tensile test, a method which can reduce stress oscillations that usually appear in the measured stress-strain curves is proposed and is devised to be used with SHPB technique. For the compression test, a candidate which can restrict buckling of the specimen is proposed. The method uses a special die-set sandwiching the sheet metal specimen which is simultaneously compressed without disturbing the planar stress wave to be propagated.

On Micro Forming by Hydro Spark Forming Method

This paper proposes a seamless microforming technique from 100 micrometer order bulging all the way down to micrometer-order embossments by using hydro spark forming method. Sub-millimeter order bulging with 300 and 100 m diameter aluminum thin foils are demonstrated to be easily available without accurate positioning as in the conventional methods. The present technique is successfully applicable also to surface embossments of micrometer order, for a coin surface, an IC chip and hologram surfaces.