Versatile analysis of limited plasticity by body force method

A problem of thermally induced residual stress under plane strain constraint is considered based on a body force method (BFM). As a numerical example, a simple problem of limited plasticity due to a uniform strength of transient line heat source of finite width, which is applied to a surface of a semi-infinite solid for a short duration of time, was considered. Although the out-of-plane component of normal stress (σzz) can be simply estimated from the in-plane normal stress components σxx, σyy and the Poisson\u27s ratio ν as σzz = ν(σxx + σyy) for elastic plane strain problems, this relation is violated when plasticity is considered. As a result, the residual stress in the direction of out-of-plane found to be a major component in the present problem

Transforming growth factor-β1 antisense oligodeoxynucleotides block interstitial fibrosis in unilateral ureteral obstruction

Transforming growth factor-β1 antisense oligodeoxynucleotides block interstitial fibrosis in unilateral ureteral obstruction.BackgroundInterstitial expression of transforming growth factor-β1 (TGF-β1) is important in tubulointerstitial fibrosis, a common process in most progressive renal diseases. However, no effective therapy for progressive interstitial fibrosis is known. Recently, we developed an artificial viral envelope (AVE)-type hemagglutinating virus of Japan (HVJ) liposome-mediated retrograde ureteral gene transfer method, which allowed us to introduce the genetic material selectively into renal interstitial fibroblasts.MethodWe introduced antisense or scrambled oligodeoxynucleotides (ODNs) for TGF-β1 into interstitial fibroblasts in rats with unilateral ureteral obstruction, a model of interstitial fibrosis, to block interstitial fibrosis by retrograde ureteral injection of AVE-type HVJ liposomes.ResultsTGF-β1 and type I collagen mRNA increased markedly in the interstitium of untreated obstructed kidneys, and those were not affected by scrambled ODN transfection. Northern analysis and in situ hybridization revealed that the levels of TGF-β1 and type I collagen mRNA were dramatically decreased in antisense ODN-transfected obstructed kidneys. Consequently, the interstitial fibrotic area of the obstructed kidneys treated with antisense ODN was significantly less than that of the obstructed kidneys untreated or treated with scrambled ODN.ConclusionThe introduction of TGF-β1 antisense ODN into interstitial fibroblasts may be a potential therapeutic maneuver for interstitial fibrosis

Compression induced shear damage in brittle solids by scattered microcracking

Failure observation and numerical analysis were conducted to understand how shear damage develops in brittle solids under biaxial compression. A biaxial compression often induces shear damaged in brittle solids, which is preceded by a formation of huge number of array cracks. Cracks in the array appeared gradually as applied compression increased. They were almost similar in shape; gently curved but were possible to approximate a troop of straight cracks almost parallel to each other without loss of essential characteristics. Under a uniaxial compression, a brittle material tends to fail exhibiting a crack growth almost parallel to the loading axis. In this situation, the crack propagates rather in a stable fashion since the stress intensity factor at crack tip generally decreases with crack extension. Under a biaxial compression, however, such a stable crack growth is strongly inhibited. Consequently, an array of microcracks often appears as a presage of the macroscopic shear failure. A mechanism of the appearance of damaged zone with increase of applied compression was discussed using a scattered cracking model. It was found that each crack composing the damaged zone has a possibility to open due to crack-to-crack interaction and a localized tensile stress appeared both in the interior and in the exterior of the damaged zone. The localized tension appeared in the interior of the damaged zone may increase a crack density, while that appeared in the exterior of the damaged zone would bring an enlargement of the damaged zone

Effect of Side Grooves for Three-point Bending Fracture Toughness Specimens : Three-dimensional Elastic-plastic Finite Element Analysis

Elastic-plastic stress states on three-point bending fracture toughness specimens are analyzed by the three-dimensional finite element method. Effect of side grooves on the stress state around the crack tip is discussed on specimens about 30 % side-grooved in U, V, and slit types comparing with a regular flat specimen. Side grooves restrict the plane stress type deformation, which is commonly observed near the side face of a flat specimen. Consequently, over 80-90 % of the thickness of the side-grooved specimens, practically even distributions of the stress intensity factors, plastic zone geometries, crack tip dilatant stresses, and three-dimensional J integrals are achieved

Fatigue Strength of Notched Specimen under Combined Stress

From the fatigue test of notched specimens of isotropic 70:30- brass, under bending, torsion, and combined stress state of them, a simple criterion for the slip-band crack initiation has been obtained. In spite of the complexity of the triaxial stress state below the notch root, only two stress components, the maximum shearing stress τ_ and the normal stress σ_n acting on a particular slip plane, are responsible for initiating slip-band cracks. Using the averaged stress values over the half-grain-size depth below the notch root, linear sum of them, τ_ + a・σ_n, acts just as the torsional stress in unnotched specimens. At the fatigue limit, therefore, the value for any combined stress state coincides with the torsional one. This relation is more general than the one which has been obtained before for unnotched specimens because of including the case for the unnotched as an extremely shallow notch

Temperature Variation associated with Stable Crack Growth and the Fracture Toughness Value

The temperature of specimens during the fracture toughness testing, varies due to the heat generation associated with a plastic flow near the crack tip. The temperature has been analyzed using a simple heat balance model and the generated heat has been calculated. Heat generation rate changes in a clear slope corresponding to the stable crack initiation. Hence, this phenomenon can be used for detecting the stable crack initiation. J-integrals were estimated based on the heat variation for three kinds of steels and were found to be close to the ones evaluated by the R curve method

Effect of Side Grooves for Three-point Bending Fracture Toughness Specimens : Three-dimensional Elastic-plastic Finite Element Analysis

Elastic-plastic stress states on three-point bending fracture toughness specimens are analyzed by the three-dimensional finite element method. Effect of side grooves on the stress state around the crack tip is discussed on specimens about 30 % side-grooved in U, V, and slit types comparing with a regular flat specimen. Side grooves restrict the plane stress type deformation, which is commonly observed near the side face of a flat specimen. Consequently, over 80-90 % of the thickness of the side-grooved specimens, practically even distributions of the stress intensity factors, plastic zone geometries, crack tip dilatant stresses, and three-dimensional J integrals are achieved