Relaxation Of Stress Concentration Research Articles

Relaxation of Stress Concentration by Applying Clothoid Curve at the Intersection of Straight and Curved Sections in REBCO Racetrack Coil

Relaxation of Stress Concentration by Applying Clothoid Curve at the Intersection of Straight and Curved Sections in REBCO Racetrack Coil

Development of High-Strain-Rate Superplastic Oxide Ceramics Based on Flow Mechanism

In order to attain high-strain-rate superplasticity (HSRS) in ceramics, flow behavior was examined with ZrO2 reference sample. The results suggest that the enhancement of the accommodation processes of grain boundary sliding (GBS) is important in addition to the careful controlling the microstructural factors, such as stable fine grain structure, reducing residual pores and so on. The spinel particles dispersion can simultaneously provide the following positive factors to ZrO2: i) suppressed grain growth due to pinning effect of spinel particles, enhanced accommodation due to ii) accelerated relaxation of stress concentrations exerted by GBS through dislocation motion and iii) accelerated lattice diffusion caused by the dissolution of aluminum and magnesium into ZrO2 from the spinel particles. The positive factors due to spinel dispersion make it possible to attain HSRS in ZrO2 ceramics.

B219 動脈狭窄症治療用ステントの応力集中緩和による耐久性向上に関する研究(B2-4 医療機器2)

B219 動脈狭窄症治療用ステントの応力集中緩和による耐久性向上に関する研究(B2-4 医療機器2)

High-strain-rate superplasticity in oxide ceramics: a trial of microstructural design based on creep-cavitation mechanisms

From existing knowledge about high-temperature cavitation mechanisms, necessary conditions were discussed for the suppression of cavitation failure during superplastic deformation in ceramic materials. The discussion, where special attention was placed on the relaxation of stress concentrations during grain-boundary sliding and cavity nucleation and growth, leaded to a conclusion that cavitation failure could be retarded by the simultaneous controlling of the initial grain size, the number of residual defects, diffusivity, dynamic grain growth and the homogeneity of microstructure. On the basis of this conclusion, high-strain-rate superplasticity (defined as superplasticity at a strain rate higher than 0.01 s−1) could be intentionally attained in some oxide ceramic materials. This was shown in tetragonal zirconia and composites consisting of zirconia, α-alumina and a spinel phase.

Relaxation of Stress Concentration of a Crack by Stop Drilling Holes and Additional Holes and Its Practical Prediction

It is well known that a stress concentration of a crack can be relaxed by drilling a hole at the crack tip. The repair method is called a stop drilling procedure and often used to repair aircrafts, machines and so on. Then, when two additional holes are drilled near the stop drilling hole so as to face each other symmetrically, the stress concentration of the stop drilling hole can be relaxed further, and a fatigue life is extended. However, the size and the location of the additional hole suitable for repairing the fatigue damage are not clarified yet. In this paper, finite element simulations were performed on various plates containing the small center crack at which the stop drilling holes and the additional holes were drilled, and influences of the additional hole on the relaxation effect of the stress concentration of the stop drilling hole were investigated. Then, the size and the location of such additional hole that the stress concentration of the stop drilling hole is minimized were examined.

Effect of UIT on Fatigue Life in Web-Gusset Welded Joints

Ultrasonic impact treatment (UIT), which is a peening method, is usually used as a post-weld treatment in order to improve the fatigue strength of welded joints. In this study, fatigue tests were carried out on web-gusset welded joints treated by UIT and the results were compared with the fatigue lives of as-welded joints in order to examine the effects of UIT on the fatigue lives of welded joints. The fatigue lives of web-gusset welded joints treated by UIT increased to more than ten times those of as-welded joints. The introduction of compressive residual stress, relaxation of stress concentration at a weld toe, and refinement of grains under the weld toes were considered as possible reasons for the improvement in fatigue life caused by UIT. Residual stress near weld toes was measured using the X-ray diffraction method. The stress concentration factor at the weld toes was analyzed using the finite element method (FEM). The grain size under the weld toes was measured using electron backscatter diffraction pattern (EBSD) analysis.

The fracture mechanism of polylactic acid resin and the improving mechanism of its toughness by addition of acrylic modifier

AbstractThe fracture mechanism of polylactic acid (PLA) resin and the improving mechanism of its toughness by addition of an acrylic modifier were examined. Plane strain compression testing of PLA clearly showed strong softening after yielding. Because the stress for craze nucleation of PLA was close to the yield stress, brittle fractures resulted. The addition of an acrylic modifier to the PLA significantly lowered the yield stress and formed many voids. The release of the strain constraint because of the formation of many voids and the decrease of yield stress resulted in the relaxation of stress concentration, and the toughness was improved. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Toughening of Polyamide 66 Blended with SEBS and PTFE

The toughening of PA 66 blended with styrene-ethylene-butylene-styrene tri-block copolymer modified with maleic acid (M-SEBS) and the effect of addition of modified Polytetrafluoroetylene (PTFE) to PA 66/SEBS were examined. Toughness of PA 66 was improved remarkably by the blend of M-SEBS. The optimum amount of maleic acid was needed to improve the toughness of PA 66. Further amount did not contribute to the improvement of toughness. The addition of SEBS with low strength is effective to the nucleation of voids, and as a result, the toughness was effectively improved by the relaxation of stress concentration. The addition of modified PTFE to PA 66/SEBS blend was effective in further improving the toughness. It was presumed that the addition of modified PTFE results in the transformation of the spherulite of PA 66 and the formation of network structure of modified PTFE. As a result, the restraint of softening and the increase of orientation hardening were achieved.

LOW CYCLE FATIGUE BEHAVIOR OF Zn-22Al ALLOY IN SUPERPLASTIC REGION AND NON-SUPERPLASTIC REGION

The crack propagation properties for ultrafine-grained Zn -22 wt % Al alloy during low cycle fatigue (LCF) in the superplastic region and the non-superplastic region were investigated and compared with the corresponding results for several other materials. With the Zn - 22 wt % Al alloy, it was possible to conduct LCF tests even at high strain amplitudes of more than ±5%, and the alloy appeared to exhibit a longer LCF lifetime than the other materials examined. The fatigue life is higher in the superplastic region than in the non-superplastic region. The rate of fatigue crack propagation in the superplastic region is lower than that in the other materials in the high J-integral range. In addition, the formation of cavities and crack branching were observed around a crack tip in the supereplastic region. We therefore conclude that the formation of cavities and secondary cracks as a result of the relaxation of stress concentration around the crack tip results in a reduction in the rate of fatigue crack propagation and results in a longer fatigue lifetime.

In Situ Scanning Electron Microscopy Observation of Tensile Deformation in Sn-Ag-Cu Alloys Containing Rare-Earth Elements

The effects of rare-earth (RE) element additions on the tensile deformation mechanism of the Sn-3.8Ag-0.7Cu solder alloy have been investigated. The results show that adding RE elements can remarkably improve the tensile strength and elongation of the Sn-3.8Ag-0.7Cu alloy. The increase in the mechanical properties are attributed to the constraints of microcrack growth and grain boundary sliding in the eutectic phase as well as the relaxation of stress concentration in the β-Sn phase due to the addition of the RE elements. It is considered that the RE elements strengthen the eutectic phase and increase the deformation resistance of this alloy.

凍結状態にある岩石の強度と変形・破壊挙動

Understanding mechanical behavior of frozen rocks is useful for evaluating the stability of rock slopes in cold regions. In this study, uniaxial compression tests and Brazilian tests were carried out on frozen rock specimens of welded tuff and andesite under -20°C temperature. The fracture process was examined by observing deformation behaviors and AE activities. Effects of water saturation degree on strength and deformability were also investigated. The main results are as follows.1) The fracture process of the frozen rock was similar to that of non-frozen rock and was divided into the following regions. (I) Closure of cracks and pores. (II) Elastic deformation. (III) Fracture initiation and stable fracture propagation. (IV) Unstable fracture propagation. Also, stress levels at the beginning of the region (III) increased with the degree of water saturation. 2) Both strength and strain at peak stress increased as the degree of water saturation increased. The relations between them were able to be approximated by a straight line. Brittleness of rocks decreased with the degree of water saturation since indirect tensile strength was more sensitive to the degree of water saturation than uniaxial compressive strength. 3) The mechanical effect of frozen pore water was considered with an inclusion model. The model predicted the observed phenomenon. Namely, increment in fracture initiation stress, due to frozen pore water in uniaxial tension, was always higher than that in uniaxial compression. The increase in strength was thought to result from relaxation of stress concentration around pores.

Twinning behavior and deformation mechanisms of extruded AZ31 Mg alloy

Complicated deformation mechanisms such as twinning, non-basal slips and grain boundary sliding besides the basal slip occur at room temperature for Mg and its alloys. The roles of these mechanisms should be understood from the viewpoint of the homogeneous deformation of a polycrystalline and the relaxation of stress concentrations at the grain boundaries. In the present paper, twinning behavior of AZ31 Mg alloy extrusions with different texture intensity and grain sizes was investigated. Twinning played a vital role in a coarse-grained Mg, not in a fine-grained Mg. Thus, the deformation behavior was drastically changed by grain refinement. Based on the results, the deformation mechanisms are discussed from the viewpoint of the deformation and relaxation mechanisms.

Elastic Shear Lag Analysis of Short Fiber Composites with the End of a Fiber Coated by Soft Resin

This paper presents a shear lag analysis which enables us to calculate stress distribution in short fiber reinforced composites with soft resin coating at the end of fibers or fiber bundles. Stress distributions are obtained for the following two cases ; the fiber is bonded (Case A) and debonded (Case B) with matrix resin at the fiber ends. Finite element analysis is employed to verify the validity of the present analysis. It is confirmed that shear stress at the fiber/matrix interface is relaxed by the soft coating resin while the axial stress in the fiber decreases in both cases. Width and shear modulus of coating resin have larger influence on relaxation of stress concentration than its length. Shear stress distribution predicted using the shear lag analysis is in fairly good agreement with the finite element calculation.

Role of Deformable Fine Spinel Particles in High-Strain-Rate Superplastic Flow of Tetragonal ZrO2

AbstractThe role of MgAl2O4 spinel particle dispersion in high-strain-rate superplasticity (HSRS) of tetragonal ZrO2 was examined by characterizing microstructural changes during deformation. The dispersed spinel particles elongate with strain along tensile direction and the elongation tends to be pronounced with increasing strain rate. In the elongated spinel particles, intragranular dislocations lying along the elongated direction were observed, suggesting that the elongation relates to the dislocation motion. The flow behavior characterized by a stress exponent of ≈ 2.0 suggests that grain boundary sliding (GBS) is the predominant flow mechanism. The dislocation-induced plasticity in the spinel particles may assist the relaxation of stress concentrations exerted by GBS, leading to HSRS in tetragonal ZrO2.

Ｎｉ基耐熱合金溶接継手の表層軟化処理による余盛止端応力集中の緩和

Surface treatment by laser irradiation leads to softening in a surface region for Ni-base superalloys. This paper investigates the utility of surface softening to reduce a stress concentration at the weld toe of Ni-base alloy joints. Three-dimensional FE-analysis is conducted to obtain the stress-strain fields of the welded joints. It is revealed that surface softening is effective to reduce the stress concentration at the weld toe. The relaxation of stress concentration is almost proportional to the degree of softening. The demerit of surface softening is found in an elevation of strain level at the weld toe. However, the strain concentration can be marginal in the applied stress level below 0.5σYB(σYB: yield stress of base metal), at which initial yielding occurs at the weld toe. It is expected from the results that surface softening will improve a high-cycle fatigue property of welded joints.

ポリマーアロイにおける応力集中の緩和機構の塑性力学による解析

ポリマーアロイにおける応力集中の緩和機構の塑性力学による解析

Effects of addition of magnesium on interface structure and high-strain-rate superplasticity in Si 3N 4-reinforced Al-alloy composites

The mechanical properties and interface structure of fine-grained Al–Cu alloy and Al–Cu–Mg alloy composites, reinforced with Si 3N 4 whiskers (Si 3N 4w) and particles (Si 3N 4p), are investigated to reveal the role of magnesium addition in high-strain-rate superplasticity. The Al–Cu alloy composites, which exhibit lower elongations (<100%), have clear interfaces between the Al-matrix and Si 3N 4 crystals without any interfacial reaction. On the other hand, the interfaces in the Al–Cu–Mg alloy composites, which exhibit high elongations (>280%), show strong reaction between the Al-matrix and Si 3N 4 crystals. The result suggests that the addition of magnesium causes the reaction between the Al-matrix and Si 3N 4 crystals, and consequently that partially melting of the reaction phases at the tensile-testing temperature results in the relaxation of stress concentration and suppresses the development of microcracks and cavities at the interfaces during superplastic deformation.

On the transition to intergranular fracture during high temperature deformation

The nature of the transition from transgranular fracture to intergranular fracture by wedge cracking at grain boundary triple junctions or by cavitation at grain boundary precipitates during high temperature deformation of polycrystalline metals is analyzed. To do this, a model material is constructed that is composed of grain matrix and grain boundary “materials”, each having its own steady state deformation law. The physical and adjustable parameters used in these flow laws are fixed in reference to the high temperature deformation behavior of type 316 stainless steel. The flow incompatibility between the grain matrix and grain boundary reflected in these equations can produce stress concentrations sufficient to cause intergranular separation under certain loading conditions. Characteristic times associated with the build-up and relaxation of stress concentrations at precipitates and triple junctions are calculated using these flow laws and the applicable diffusion equation. These characteristic times are compared with the cavity nucleation incubation time, and the conditions for fracture mode transitions are thereby determined. Simple rate ratio criteria obtained from these results indicate that, at a given temperature, wedge cracking occurs below the applied stress level where the ratio of the boundary strain rate to the matrix strain rate equals the ratio of the grain size to the boundary width: dot ε b / dot ε m = L/δ . “r cavitation” occurs below the even lower applied stress level where dot ε b / dot ε m = 10 7 for type 316 stainless steel. Model predictions based on these criteria compare favorably with published data on material deformed monotonically and in fatigue, including the effects of hold periods.

Strength-porosity-microstructure relations in porous reaction-sintered SiC with free silicon removed

The effects of controlled porosity on the fracture strength of reaction-sintered SiC (RS-SiC) with free silicon removed and of RS-SiC further heat-treated at 1780 °C in a vacuum were investigated. Although further heat-treatment did not cause significant change in porosity of the porous RS-SiC, it was found that the heat-treatment caused strengthening of the porous RS-SiC with below 38% porosity and an intergranular-to-transgranular fracture transition. The strengthening effect may be attributed to increasing bonding area between grains and a relaxation of stress concentration by changing pore shape, resulting from evaporation-condensation and/or surface diffusion by the heat treatment.

Effect of Vacuum-Heating on the Fracture Strength of Reaction-Sintered SiC

A commercial reaction-sintered silicon carbide (RS-SiC) was heat-treated at 1600°C in vacuum to remove the free Si, and the resulting SiC bulk with about 19% porosity was further heat-treated at 1600°-2000°C for 0.2-20h in vacuum of 0.65Pa. Although the second heat-treatment did not significantly change the porosity, grain growth and oxygen content in the porous RS-SiC, the heat-treatment at 1780°C for 2h increased the bending strength of the porous RS-SiC from 205MPa to 322MPa and caused the transition from intergranular to transgraular fracture. The strengthening effect may be attributed to the increased bonding area between grains due to evaporation-condensation and surface diffusion, and to relaxation of stress concentration due to the change in pore shape. However, the strength decreased by heat-treatment at 1780°C for 10h or 2000°C because of the decrease in the bonding area and the volume of SiC matrix due to the decomposition of the SiC in vacuum.