Hiromasa Ishikawa

Influence of disc degeneration on mechanism of thoracolumbar burst fractures.

In order to clarify the pathomechanism of thoracolumbar burst fractures and to evaluate the influence of disc degeneration and bone mineral density, a biomechanical study was performed using cadaveric spines. Eleven motion segments of thoracolumbar spines from human cadavers were compressed vertically until a fracture occurred. In addition, bone mineral density and degree of disc degeneration were determined for each specimen. Compression of 7 of 11 specimens resulted in the typical burst fracture characterized by retropulsion of a bony fragment into the spinal canal and an increase of the interpedicular distance. All seven specimens showed disruptions of the middle end plate and disc materials in the vertebral body. The fracture line was located between the middle of the end plate and the middle of the posterior wall cortex. No burst fractures were seen in the specimens with severely degenerated discs and osteoporosis. In order to confirm the stress state in a vertebra that induces the burst fracture, finite element analysis of one motion segment was also carried out under the same mechanical conditions as the experiments in this study. As a result of calculation for the healthy disc, the highest stresses under axial compression were concentrated in the following areas: the middle of the end plate, the cancellous bone under the nucleus pulposus, and the middle of the posterior wall cortex. This implies that the above regions are more vulnerable to vertical compressive load. In the analysis of specimens with severely degenerated discs, stresses were very low at the end plate and cancellous bone under the nucleus. Therefore, axial compression was reconfirmed as inducing typical burst fractures in the thoracolumbar spine. Moreover, the mechanism of this fracture was influenced by disc degeneration and bone mineral density.

A THERMOELASTOPLASTIC SOLUTION FOR A CIRCULAR SOLID CYLINDER SUBJECTED TO HEATING AND COOLING

Abstract This paper presents a theoretical study of the stresses in an infinite circular solid cylinder subjected to rapid surface heating and cooling. A quasistatic, uncoupled, thermoelastoplastic analysis based on the incremental theory of plasticity is formulated, and a numerical procedure is developed for a method of successive elastic solutions. The material of the cylinder is assumed to have temperature-dependent properties and to be characterized by the Romberg-Osgood stress-strain relation. The transient and residual stress distributions are discussed in detail, along with variations of the equivalent stress and plastic strain with time.

Biaxial ratcheting deformation of type 304 stainless steel: Effect of memorization of back stress

Abstract This paper treats both experiments and simulations of biaxial ratcheting. The experiments are conducted using a tubular specimen of type 304 stainless steel at room temperature. The specimen was subjected to cyclic shear straining under the axial superposed stress. The experiments show that the biaxial ratcheting strain was affected by the cyclic shear strain amplitude, the shear strain rate and the superposed stress level. Larger biaxial ratcheting strain occurred in the case of tensile superposed stress compared with that in the case of the compressive superposed stress. Moreover, even under the zero superposed stress, biaxial ratcheting strain occurred in the axial direction due to the cyclic shearing straining. Finally, the biaxial ratcheting behaviours were simulated by the unified constitutive model proposed by the authors. The characteristic features of the biaxial ratcheting behaviour, especially the axial strain due to the cyclic shear straining superposed on the zero axial stress, are well simulated by the constitutive model.

On One Analysis of Axisymmetric Extrusion by the Use of Flow Lines

On the assumption that the flow lines of material might be approximated by the function of hyperbolic tangent an analytical method to examine the mechanics of material during forward extrusion is presented in this paper. For the application of this method to actual extrusion, the unknown quantity is readily determined with the use of material constant obtained through the compression test. Therefore, the mechanics of material during forward extrusion can be obtained approximately without a preliminary extrusion test.

An Analysis of Deformation of Steel Coated With Ceramics in Rolling-Sliding Contact

The stresses, strains, and deformations produced by repeated, two-dimensional rolling-sliding contacts are analyzed both in the coated layer of ceramics SiC and in the substrate of two types of steels using a finite element method. Rolling and sliding are simulated by translating an appropriate set of normal and tangential surface tractions across the surface of an elastic-plastic model. A peak-pressure to shear-strength ratio of p0 /k = 5 and normal to tangential force ratios of T/P = 0.2 are examined. The effect of thickness of the coated layer on the mechanical behavior of the substrate is discussed.

THERMOELASTOPLASTIC STRESS ANALYSIS DURING PHASE TRANSFORMATIONS

Abstract Numerical results obtained from the thermoelastoplastic analysis of low carbon steel under heating and cooling are presented using the procedure of Mendelsons method of successive elastic solutions. A1 and A3 transformations on heating, and martensitic or pearlitic transformation on cooling, are reasonably taken into account as the typical models of phase transformations. Mechanical and thermal properties of steel are considered to be temperature dependent. From stresses and strains calculated with consideration of phase transformations, it is found that phase transformations during rapid cooling have the greater effect on stresses and strains than those during slow cooling.

Viscoplastic Deformation of Lead Free Solder Alloy- Experiments and Simulations

Title Viscoplastic Deformation of Lead Free Solder AlloyExperiments and Simulations Author(s) Sasaki, Katsuhiko; Yanagimoto, Akiyuki; Ishikawa, Hiromasa Citation Engineering Plasticity from Macroscale to Nanoscale Pts 1 and 2, 779-784 Issue Date 2003 Doc URL http://hdl.handle.net/2115/15888 Rights the original is available online at www.scientific.net Type article (author version) Note Key Engineering Materials, Volume 233-236 File Information EPMN2003.pdf

Creep, Stress Relaxation and Biaxial Ratchetting of Type 304 Stainless Steel After Cyclic Preloading

A series of tests for creep, stress relaxation, and biaxial ratchetting of type 304 stainless steel after cyclic preloading were carried out to investigate their interaction. The interesting fact was pointed out that back stress in cyclic plasticity played an important role to describe creep, relaxation, and biaxial ratchetting following cyclic preloading. Then, the test results showed that the material behavior due to creep after cyclic preloading could be represented by the modified Bailey-Norton law with stress levels evaluated from the current center of the yield surface, i.e., back stress which was determined by the hybrid constitutive model for cyclic plasticity proposed by the authors. In addition, biaxial ratchetting of axial strain induced by cyclic shear straining after cyclic preloading was expressed by the shear stress amplitude, the number of cycle and the axial stress level from the current center.

Constitutive Modeling of Cyclic Plasticity Considering Induced Anisotropy

Pressure vessels, turbines, spacecrafts, and many equipments in nuclear technology are subject to cycles of load and to cycles of temperature, both or separately. Therefore, many researchers, such as Drucker and Palgen [1], Chaboche and Rousselier [2], Eisenberg and Yen [3], and Mroz [4] have investigated cyclic plasticity of materials. Among many models proposed to fit one or more aspects of the response under cyclic loading, only few models can be used effectively in computer programs for complicated structural stress analysis, and at the same time meet the essential features of plastic behaviour of materials under cyclic loading.

Equivalence of Back Stress during Plastic and Creep Deformation

Back stresses caused by both cyclic and creep deformations are discussed in this paper. The equivalence of the back stress due to cyclic and creep deformation are shown by a series of experiments. Different creep curves due to cycle number in preloading are simulated by a constitutive model based on dislocation density (Estrin et al. 1996), considering the equivalence of the back stresses.