Microstructural and mechanical properties of the Ni-rich TiNi alloys using the elemental mixtures of pure Ti powder and pure Ni powder were investigated. Ti-Ni elemental mixture powders were consolidated by spark plasma sintering (SPS). SPSed TiNi alloy compacts were extruded and heat-treated subsequently. SPSed TiNi compacts consisted of TiNi matrix and Ti4Ni2O phases. The solute Ni content of the matrix could be controlled by Ti-Ni powder mixing ratio. The martensitic transformation temperature of TiNi matrix decreased and the amount of Ti3Ni4 precipitate increased with increasing the solute Ni content. Consequently, the heat-treated Ti-52.0 at.%Ni alloy showed a high plateau stress of 850 MPa and a good shape recovery of 87.3% by applying 8% strain in tensile. The strengthening mechanism of the Ni-rich TiNi alloys was mainly due to a decrease martensitic transformation temperature by an increase soluted Ni content of TiNi matrix.
The applicability of stress measurement method using electrodeposited copper foil under combined stress state of out-of-phase tension and torsion was examined. The density of grains grown by cyclic loading was measured and the crystallographic orientation of grains was also analyzed by EBSD. The pole figure suggests that grains grow in the direction of Tresca equivalent shear stress and the normal stress acting on the plane of Tresca equivalent shear stress also affects the grain growth. The equivalent shear stress that dominates the grain growth density is proposed to the condition of out-of-phase loading. Based on these results, stress measurement is possible by using the density and the crystallographic orientation of grown grains. However, this method can be applied only when the phase difference of tension and torsion is known and not equal to π/2.
Short crack propagation behavior in poled lead zirconate titanate was examined under cyclic electric loading. A crack located at edge of a partial electrode grew along the electrode edge during the loading. The crack growth rate decreased with increasing crack length until a non-propagating crack was reached. The growth rate and crack length of the non-propagating crack were affected by the amplitude, mean voltage of the electric loading and environment. In the case of high-amplitude loading or negative-biased loading, the crack growth rate varied considerably because of domain switching. At testing temperature of 20°C, moist atmosphere had no effect on the crack propagation behavior. However the crack growth rate fluctuated and non-propagating crack length was increased with increase in temperature to 40°C. Finite element analysis of a three-dimensional permeable crack showed that the mode III stress intensity factor range is independent of crack length, but it decreases as a result of the frictional force under positive electric field. Fracture surface observations showed that intergranular cracking is dominant near the tip of the non-propagating crack.
The susceptibility to hydrogen embrittlement of two types of high strength austenitic stainless steels containing a small amount nitrogen and niobium were investigated by conducting a series of slow strain rate tests (SSRT) and fatigue crack growth tests in hydrogen gas with a pressure of around 100 MPa. The JIS-SUS304 and JIS-SUS316L austenitic stainless steels were also tested for a comparative purpose. In JIS-SUS304, the tensile strength and reduction of area in hydrogen gas were much lower than those in air. In contrast, in JIS-SUS316L, the degradation of those tensile properties in hydrogen gas was not so significant. The high strength austenitic stainless steels also exhibited an excellent resistance both in tensile strength and ductility in hydrogen gas. In JIS-SUS304, the fatigue crack growth in hydrogen gas was 10 times as fast as that in air, while the factor of acceleration remained within 1.5 - 3 in JIS-SUS316L and the high strength austenitic stainless steels. It was presumed that, in those high strength austenitic stainless steels, a small amount of added elements, N and Nb, increased the strength level as well as the stability of austenitic phase, which thereby led to the excellent resistance against hydrogen embrittlement.
Rail vehicles have the particularity in that pluralities of vehicles are operated as a set of connected units. This particularity is to be considered in the crashworthiness design of the coupling end structure of the vehicles in a train unit. In the case of a collision between long trains that are composed by a rigid coupling end structure, a large space for energy absorption in the structure of the cab end of the leading car will be needed. However, there is the possibility that the appropriate energy absorption characteristics of the coupling end can reduce the load of energy absorption at the cab end. Calculations of a collision between train units using a mass-spring model were conducted to study the force-displacement characteristic of a coupling end structure that can absorb more crash energy. The collision scenario applied to the calculation was that of a stationary 8-car train unit colliding with another identical train unit traveling at a speed of 36km/h based on EN15227. The results showed that the deformation of the cab ends in the leading cars can be greatly reduced by adjusting the force displacement characteristics of the coupling end to be lower than the reaction force of the cab end and achieve a progressive profile.
Sandwich panels such as roll core sandwich panel (hereafter, RSP for brevity) and honeycomb core sandwich panel (hereafter, HSP for brevity) are superior in impact absorption under whole surface compression, because of the buffer effect of core. On the other hand, impact properties of sandwich panels under local compression such as drop weight impact are affected by mechanical properties of the plate as the face sheet in addition to the core layer. This paper describes drop weight impact properties of double layer RSP which consists of two core layers and three plates. In order to compare RSP with HSP and build up the experimental database for design of double layer sandwich panel, a series of drop weight tests was carried out by using double layer RSPs and HSPs. From test results, the follows were summarized. The RSP and the HSP have common characteristics: 1) Three peaks of the load appeared in the load-displacement curve. The peak loads were larger in sequence of their appearances. 2) In the load-displacement curve until the last peak of the load, there were load increase regions by deep drawing of the plates and load equilibrium regions similar to plateau region under whole surface compression. Then drop weight impact properties of the RSP are higher than the HSP in terms of 1) the deformation resistances of the load increase regions, 2) the peak loads, 3) the loads of the load equilibrium regions, and 4) the absorption energy until the 2nd peak of the load.
According to basic research on brain trauma caused by micro-level injury, brain deformation triggers nerve injury. In traumatic brain injury, brain deformation and nerve injury are both important factors for severe brain injury. The purpose of this study is to develop a human head dummy for nerve injury assessment, more specifically, an alternate brain developed for a human head dummy. According to basic research into nerve injury related to traumatic brain injury, mechanical strain to brain neurons causes increase in intercellular Ca2+ as a transmitter substance. Therefore, this alternate brain can rapidly increase the internal Ca2+ amount by mechanical strain. To realize this increase, the alternate brain has interior microcapsules. If the surface of a microcapsule is broken by mechanical strain on the alternate brain, the microcapsule releases Ca2+ into the alternate brain. According to tolerance research on nerve injury in swine cadavers, nerve injury occurs at a compression strain of 30%. Therefore, the microcapsules, which contain carrageenan, must rupture at a compression strain of 30% applied on the alternate brain. To evaluate various bruise situations, frontal, occipital, and side drop tests applied impact load on the evaluation dummy. In the frontal and occipital drop tests, the increase in Ca2+ occurs in the frontal region of the alternate brain. On the other hand, in the side impact test, the increase in Ca2+ occurs in the impact and non-impact regions. These results mimic real brain injury. Hence, this evaluation is a reproducible model for local brain nerve injury.
Low cycle fatigue tests at elevated temperature were conducted on test specimens with small holes made of a Ni-based directionally solidified superalloy, which are intended to be the cooling structures formed in components in fossil fuel power plants. The tests included cases with and without strain holding processes. The number of crack initiation cycles of the tests without hold processes for the one- and seven-hole specimens was about 1/30 that of the smooth one. The number of crack initiation cycles of the test with the compressive hold processes for the seven-hole specimen was smaller than that of the tests without hold processes while that of a tensile hold case was even smaller. The test results were evaluated based on the inelastic behavior around the center hole of the specimens, where the largest inelastic strain occurred, using finite element analysis that takes into account the inelastic anisotropy of material properties. The number of crack initiation cycles of the tests without hold processes and that with compressive hold processes correlated with the maximum tensile stress around the hole, while that of all the tests correlated with the frequency-modified strain energy by setting the appropriate material parameters based on the test results of smooth specimens.
In mathematical problems and mechanical engineering, there are a number of examples, in which a non-symmetric Jacobian matrix is involved in solution of simultaneous linear equations. The left and right eigenvectors of such non-symmetric matrices are in general complex and must be well discerned to each other. Their properties and practical meaning are, however, hardly discussed in engineering applications. Especially when the non-symmetric matrix is singular, the critical left eigenvector corresponding to null eigenvalues is of increased significance in the examination of the solvability of the problem. The present paper describes and interprets the substantial role of the critical left eigenvector of the non-symmetric singular matrix in mechanics. Model examples in applied mathematics, solids, structures and rigid bodies will illustrate the meaning of the critical left eigenvector, when the singularity is unavoidable in the problem to be solved. The discussion will be then extended to the critical left singular vector of a rectangular matrix.
Plastic deformation of amorphous metals is dependent on a mean stress (hydrostatic pressure), that is, compressible due to the random atomic structure. This property leads their intrinsic anisotropy on deformation. In addition, the localized shear bands occurring just after an elastic region do not allow the sufficient elongation. This is the crucial drawback of that material which has been strongly tried to overcome. In the present paper, a constitutive law based on the inhomogeneous defects theory and an evolutional law of defects density (equivalent to free volume) were formulated with the mean stress-dependent yield function. Several parameters used in the constitutive and the defects evolution laws were fitted to the experimental results. Finite element analyses were first performed using one element model to obtain the perfectly uniform deformation. Yield curves under some multiaxial stress states were obtained at room temperature. Employing the elastic limit as a yield stress and the parameter κ of 0.09 in Drucker-Prager yield criterion, the prediction agrees well to the FEM solutions. The uniaxial deformation behavior with an initial fluctuation of defects density using a block model, then, exhibits the localized shear bands after the maximum point, and the anisotropic angles of such bands to the stress axis were coincident with the experimental and the other computational results.
This paper describes elasto-plastic damage analysis based on divided back stress by full implicit algorithm. The proposed algorithm guarantees second order convergence for geometrical non-linear iterations using consistent tangent stiffness. We validate the present method in localized necking analysis for checking convergence of residual force and cyclic loading analysis to simulate elastic softening and work-softening due to damage. The residual force is well converged in elasto-plastic damage analysis and the point of crack initiation agreed with that of experiment in necking model. The work-softening and deterioration of macroscopic elastic stiffness are well simulated in cyclic loading analysis.
For the application of high-strength steel plate to an automobile chassis, prevention of fatigue failure is one of critical issues in terms of its reliability. In the manufacturing, the shear cutting process with a die is commonly used because of the economic efficiency. However, it has a drawback that the shearing edge provides cracking sites in fatigue. Although the shaving process, which removes the highly-deformed layer on the surface of shearing edge, is promising candidate to improve the fatigue strength, it has not been investigated systematically. Then, the fatigue experiments is conducted on a high-strength steel sheet with the shearing edge focusing on the effect of shaving. The results elucidate that the decrease in the tensile residual stress on the edge by the shaving improves the fatigue strength. In detail, the stress re-distribution during the fatigue contributes the reduction of residual stress as well. On the other hand, the surface roughness enhanced by an inappropriate shaving process degrades the fatigue strength.
A series of crystal plasticity finite element simulations are conducted to understand the grain-scale deformation process with crystallographic slip and to elucidate the development mechanism of the macroscopic yield strength for magnesium alloys. For the comparative study, different values of initial critical resolved shear stress are set for the basal slip system in a polycrystalline aggregate FE model. The main finding in this study is that the formation of shear bands with localized deformation, which are associated with the macroscopic strength and ductility, is strongly related to the evolution of deformation resistance of separate slip systems and depends on the latent hardening characteristics of the prismatic slip system.
Residual stress caused by welding process affects characteristics of strength and fracture of equipment and piping in nuclear power plants. Numerical analysis is powerful tool to evaluate weld residual stress in the actual plants. However, the three-dimensional precise analysis requires enormous computation time. In this study, the finite element analysis code based on iterative substructure method developed to speed up welding simulation was proposed to simulate welding process of the plant equipment and piping such as multi-pass welding, machining and post weld heat treatment. Furthermore, the proposed analysis code was validated by measurements and other analysis results.