Circumferential Tensile Stress Research Articles

Increase of Burst Pressure and Optimum Design for CFRP Pressure Vessels Reinforced with SMA Wire

CFRP pressure vessels are now widely used in compressed natural gas vehicles (CNGV) and have a possibility to be employed in fuel cell vehicles (FCV) for highly compressed Hydrogen. In order to increase a driving distance in CNGV and FCV, lighter vessels proofed against higher pressure are required. In the case of internal pressure, the tensile circumferential stress at the inner radius should be decreased in order to increase the burst pressure. This paper presents a method how to increase the burst pressure of CFRP vessels. For increasing the burst pressure of CFRP vessels, the shape memory alloy (SMA) wire memorized a compressive stress is wound around the outer surface of cylindrical part of the CFRP vessel. When the internal pressure acts on the CFRP vessel, its tensile circumferential stress can be reduced by being received the compressive stress of the SMA wire and the experimental results of increasing the burst pressure are shown. The experimental results are compared with numerical ones and they agree well each other. Furthermore, optimum design of CFRP pressure vessels by the change of winding method of SMA wire is numerically obtained.

Maximum circumferential stress criterion applied to orthotropic materials

ABSTRACTThe topic of the paper is the extension of the Maximum Circumferential Tensile Stress Criterion to orthotropic materials, to obtain the crack initiation angle and the fracture loci. It includes the effects of orthotropy, load biaxiality and non‐singular terms. It requires the solution of the elastostatic problem of an orthotropic plate having a central inclined crack and subjected at infinity to a uniform biaxial load. It is assumed that the crack line does not coincide with an axis of elastic symmetry of the body. The original problem has been transformed by rotation into a system of coordinates with one axis along the crack. The effects of the rotation of the reference system on the stress–strain equations as well as on the equilibrium equations have been considered. The stress field is represented in terms of two potential functions and the crack problem is solved by means of the superposition principle.

Domain rearrangement during nanoindentation in single-crystalline barium titanate measured by atomic force microscopy and piezoresponse force microscopy

Nanoindentation tests in an aa-in-plane domain area of an {001} oriented barium titanate single crystal were performed using a conical indenter with a tip radius of 800 nm. The topography and the polarization vectors of the area after indentation were imaged afterwards by both atomic force and piezoresponse force microscopy (PFM), respectively. Two perpendicular oriented cracks in the {110} planes were identified in the topographic image. An unexpected considerable uplift occurs inside the residual impression, which was correlated with a sharp pop-out-like behavior observed in the force-displacement curve just prior to unloading. Furthermore, PFM revealed an almost a twofold symmetric arrangement of the domains around the indent, which can be explained by residual circumferential tensile stresses around a residual impression and was unambiguously correlated to the crystal orientation.

Effect of the Parameters of Helical Rolling on the Magnitude of the Circumferential Stresses that Develop in Solid Semifinished Products

It is shown that increasing the degree of deformation increases the tensile circumferential stresses in the semifinished product. Although these stresses are considerably greater than the ultimate and yield strengths of the metal, no lamination or internal cavities were seen in tests that were conducted.

Impact of Density Gradients on the Stress Level Within a Green Ceramic Compact During Drying

: Internal water content and mechanical stress distributions during convective drying were simulated for a homogeneous and a heterogeneous (with density gradients) annular compact of a green ceramic agglomerate. A diffusive model for water transfer and an elastic model for structural mechanics were applied. Based on experimental measurements, the material apparent density was numerically implemented as a function of spatial coordinates and the key material properties (moisture diffusivity and the Young's modulus) were implemented as functions of material density. The heterogeneous compact (with density peaks at the top edges) exhibited three times larger circumferential tensile stress at the top external radius corner than that exhibited by the homogeneous one.

Biaxial Load Effect on Crack Initiation for Orthotropic Materials

This paper is concerned with the study of the elastostatic fracture response of an orthotropic plate with an inclined crack and subjected at infinity to a biaxial uniform load. To this end an unconventional approach to the derivation of the complex variable expressions of the elastic fields is proposed. The above formulation has been used to solve the boundary value problem as superposition of Mode-I and Mode-II crack problems and it is shown that the near tip asymptotic expressions of stress and displacement fields are affected by non-singular terms originated by load biaxiality. The maximum circumferential tensile stress criterion is applied in order to investigate the effects of non-singular terms on the angle of crack extension.

Finite Element Analysis and Strain-gauge Studies of Vertical Root Fracture

Vertical root fracture seems to result from stresses generated within the root canal and typically occurs in a buccolingual direction through the thickest part of dentin. Because stresses in the canal wall are difficult to measure experimentally, we have attempted to correlate stress patterns derived from finite element models of maxillary and mandibular incisors with strain measurements on the root surfaces of extracted teeth. Finite element analysis indicated that circumferential tensile stresses were concentrated on the buccal and lingual surfaces of the canal wall, corresponding to areas of greatest canal wall curvature. Surface stresses were much lower and were consistently tensile on the proximal root surfaces but variable on the buccal and lingual surfaces. The measurement of root surface stresses does not provide a reliable picture of internal stresses in the root. Canal wall curvature is a major factor in stress concentration and hence in the pattern of fracture.

A Neutron Diffraction Study of Residual Stress and Plastic Strain in Welded Beryllium Rings

We present a study of residual stresses associated with the welding of beryllium rings. Using novel analysis techniques, information about residual stresses and plastic deformation of the base metal were obtained. In the post-welded state, the rings have a strong tensile circumferential residual stress and show evidence of significant plastic deformation.

Observations on fracture in axi-symmetric and three-dimensional cold upsetting of brass

This paper describes the appearance of fractures in cold forming of brass during axi-symmetric collar upsetting tests and also in the upsetting of hexagonal shaped bars. The latter has been introduced to examine fracture under non-axi-symmetric conditions. The collar tests produced typical ductile fractures but the hexagonal bars displayed fractures similar to brittle fractures even though they were preceded by large plastic strain. Each experimental test was also modelled using the finite-element method to examine local stress/strain conditions at fracture. The simulations of the collar tests showed that fractures appeared in the presence of a circumferential tensile stress together with either a tensile or compressive axial stress. The compressive axial stress was present when fracture occurred at a much lower level of deformation than when the axial stress became tensile. The simulations of the hexagon bar upsetting revealed a maximum plastic strain coincident with the most likely site of fracture initiation. This mixture of differing stress and strain states at fracture leads to the conclusion that a single fracture criterion is unlikely to satisfy all the conditions that lead to fracture.

Role of tensile stress and strain in the induction of cell death in experimental vein grafts

Tensile stress and strain are known to induce vascular cell proliferation, a process that is physiologically counterbalanced by cell death. Here we investigate whether tensile stress and strain regulate vascular-cell death by using an end-to-end anastomosed rat vein graft model. In such a model, the circumferential tensile stress in the graft wall was increased by ∼140 times immediately after surgery compared with that in the venous wall. This change was associated with an increase in the percentage of TUNEL-positive cells at 1, 6, 24, 120, 240, and 720 h with two distinct peaks at 1 and 24 h (10.1±3.5 and 14.4±3.2%, respectively) compared with that in control jugular veins (0.4±0.5 and 0.5±0.5% at 1 and 24 h, respectively). When tensile stress and strain in the vein graft wall were reduced by using a biomechanical engineering approach, the rate of cell death was reduced significantly (3.6±1.1 and 1.6±0.5% at 1 and 24 h, respectively). Furthermore, DEVD-CHO, a tetrapeptide aldehyde that inhibits the activity of caspase 3, significantly suppressed this event. These results suggest that a step increase in tensile stress and strain in experimental vein grafts induces rapid cell death, which is possibly mediated by cell death signaling mechanisms.

High-speed photography of low-velocity impact cracking of solid spheres

Results from a high-speed photographic study of low-velocity impact cracking of optically transparent acrylic resin and poly(methyl methacrylate) spheres of 12.7mm diameter are described. It is shown that, in a sphere undergoing an impact, cracking occurs after the sphere has been plastically deformed. The point of initiation of cracking is shown to be located at the circle of contact between the sphere and the impacting platen. The cracking then develops throughout the sphere, with cracks attaining velocities of 600-800ms-1. It has been suggested that crack initiation at the contact circle is caused by the tensile circumferential stress, which is generated owing to the plastic flow of the sphere.

Sintering of β.q. ss. and gahnite glass-ceramic/silicon carbide composites

The sintering of β.quartz solid solution, β.q. ss., and gahnite glass-ceramic/particulate SiC composites has been studied by two different sintering procedures. In one procedure, the composites were fired above the melting point of the crystalline phase, identified by DTA, at a very high heating rate (800 °C min −1) in air, nitrogen and argon atmospheres, for 1–6 min. It was found that the reduction of ZnO constituent of the glass by SiC particles gives rise to Zn, CO, and SiO gaseous products preventing complete densification of composites. In the other procedure, sintering was done at about crystallization peak temperature of the glass phase, employing a low heating rate (40 °C min −1) in air for 60 min. In this case, the circumferential tensile stress in the glass-ceramic matrix phase, caused by the presence of incompressible SiC particles, retards the densification of the composites. The maximum amount of SiC particles yielding a reasonably dense composite was found to be 9 vol.%.

Prediction of Ductile Fracture in Cold Forging of Aluminum Alloy

In this paper, the limitation and applicability of the ductile fracture criteria based on a work hypothesis and Cockcroft and Latham were investigated. For this purpose, experimental and numerical investigations for simple upsetting were conducted for aluminum alloys Al1100-O, Al2024-T3, Al6061-T4, and Al7075-T4. As a result, the fracture mode of each alloy was observed. The study was extended for pin-shape cold forging of Al1100-O and Al6061-T4 to compare the likeliness of fracturing according to two criteria. Based on experimental data of simple upsetting, the damage factors for the same two criteria were calculated by adopting rigid-viscoplastic finite element analysis. With this approach, the prediction of surface cracking was attempted by comparing the calculated limiting damage factors between simple upsetting and pin-shape forging. It was observed in simple upsetting that Cockcroft and Latham’s criterion gave a more reasonable prediction for crack initiation site than work hypothesis, but the limiting damage factors differ depending on the process. In spite of the differences, however, Cockcroft and Latham’s criterion might be useful in designing upsetting-like cold forging processes in which the influence of the induced circumferential tensile stress on failure is dominant.

Haemodynamics, wall mechanics and atheroma: a clinician's perspective.

Atherosclerosis, leading to myocardial infarction and stroke, is the major cause of death and morbidity in Western societies. Atheromatous lesions characteristically occur in regions of branching and marked curvature. Low shear stress and increased mural tensile stress may be major determinants underlying atheroma formation at these sites. Furthermore, the distribution of circumferential tensile stresses may play a critical role in where, why and when advanced atheromatous plaques rupture, leading to catastrophic ischaemic events. Recent advances in the application of computational modelling to in vivo vascular ultrasound and magnetic resonance imaging data should further elucidate the roles of haemodynamic factors and vessel wall mechanics in atherosclerosis. In future this is likely to lead to better use of currently available anti-atherosclerosis strategies. It may also facilitate the discovery, evaluation and development of novel treatments.

A mixed mode rock fracture model for the prediction of crack path

Crack propagation in rocks is simulated by using a displacement substitution method based on a mixed mode fracture criterion. The main advantage of this model is that it can distinguish between mode I and mode II stress intensity factors simultaneously. A typical finite element program is used to compute displacements adjacent to the crack tip. The maximum circumferential tensile stress is adopted as a ‘yield surface’ for the calculation of the load factor in each crack increment. Pure mode I and mixed mode examples have been analysed to validate the capability of the model. Copyright © 1999 John Wiley & Sons, Ltd.

Alleviation of quenching residual stresses by cold working the necks of seamless 6351 aluminium alloy thick-walled gas cylinders

The application of a compressive axial load to the apex of a gas cylinder neck results in the generation of Poisson-induced tensile circumferential stresses at the bore. As the compressive axial load is increased in magnitude, the stress distribution in the gas cylinder neck increases until finally yielding occurs at a specific location on the bore. Provided that the maximum compressive axial load is of sufficient magnitude, significant compressive residual circumferential stresses are generated at the bore. The purpose of this remedial action is to alleviate residual stresses that are introduced into the gas cylinder neck during the quenching stage of the manufacturing process. A destructive technique has been employed to determine quenching residual stresses in the neck of one production gas cylinder neck. This residual stress distribution is compared with one obtained by employing the destructive technique on a gas cylinder that has been cold worked by axial loading. Results indicate that tensile quenching residual circumferential stresses at the bore can be favourably modified resulting in compressive circumferential residual circumferential stresses at the bore.

A Fundamental Study of Stretch-Drawing Process of Sheet Metals : Single and Double Operations

Fundamental and informative data of axisymmetric stretch-drawing of several sheet metals with thichness of 0.7~1.0 mm are presented especially for single and double operations. Very small radius is applied to the die-profile (or -shoulder) in all operations to induce wall-thinning by the effect of bending-under-tension, from which the name ‘stretch-drawing’ comes. It is clearly demonstrated that deeper cups could be formed by the single and double stretch-drawings from smaller cirlcular blanks due to such wall-thinning action than in the usual deep-drawing of larger blanks. From this fact, it is emphasized that the deep-drawability of a sheet metal is not evaluated simply by the conventional LDR (=limiting drawing ratio), but the depth of the drawn cup should also be taken into account. Many experimental data about various metals and thicknesses given in this paper offer a valueable information on this process for more general use which recommends to replace the conventional deep-drawing process by the stretch-drawing process both for single and double operations. In the single stretch-drawing, it is also confirmed that a deeper cup can be produced by raising the blank-holding force at later stage of operation. Fracturing is found to occur at the middle section of the wall part or at the die-profile other than at the punch profile common in the usual deep-drawing process. Numerical simulation of the single stretch-drawing process is also performed by use of DYNA-3D code to confirm that a satisfactory prediction especially in the depth of the drawn-cup can be done at least in a practical sense, although this kind of numerical analysis is very difficult because of the severity or localization of deformation around the die profile. The drawn cup of SUS304 among others fractures in a couple of weeks after the operation due to the residual circumferential tensile stress, whereas that of SUS304L does not. In the double stretch-drawing, it is confirmed that very deeper cups can be produced when compared to the usual re-drawing process, which assures typically the usefulness of this operation as a practical process. Fracture often takes place circumferentially or, very peculiarly, even in a spiral mode for SUS304 at the middle of the wall part of the cup.

Postbuckling strength design of open thin-walled cylindrical tanks under wind load

Open cylindrical steel tanks for the storage of liquids tend to be rather thin-walled because they are primarily designed for the circumferential tensile stresses from hydrostatic internal pressure. It is desirable to take advantage of the postbuckling strength of the thin cylindrical wall when designing the empty tank against wind load. The theoretical external pressure carrying behaviour of edge-stiffened thin-walled cantilever shells is discussed with regard to both bifurcation and postbuckling phenomena. For experimental verification, a series of postbuckling tests on cylindrical PVC and steel specimens with large radius/thickness ratio ( r/ t=2500) under internal underpressure and under a “wind-like” load arrangement has been carried out. Based on the numerical and experimental results, recommendations are put forward for an economic postbuckling strength design strategy. They are compared with existing design rules in tank codes.

Changes in the organization of the smooth muscle cells in rat vein grafts.

Mechanical tensile stress in vein grafts increases suddenly under the influence of arterial blood pressure. In this study, we examined the influence of increased tensile stress on the organization of the smooth muscle cells (SMCs) in the neointima and media of the rat vein grafts. An autogenous jugular vein was grafted into the abdominal aorta of the rat, and changes in the organization of the vein graft SMCs were studied by observing the distribution of SMC actin filaments and nuclei at 3 min and 1, 5, 10, and 30 days after surgery. In a normal jugular vein, the average wall circumferential tensile stress was approximately 3 kPa at an internal pressure of 3 mm Hg. The SMCs, that contained long, slender actin filamentous bundles, were oriented mainly in the circumferential direction of the vessel, and constituted a 2- to 3-cell-thick medial layer underneath the endothelium. In a vein graft, the wall circumferential tensile stress suddenly increased by approximately 140 times compared with the control level. In response to this suddenly increased stress, the SMC layer was stretched into a structure with scattered pores and disrupted SMC actin filamentous bundles within 3 min. This initial change was followed by a rapid reduction in the density of the SMC nuclei and actin filaments within 1 day and progressive SMC proliferation, that was associated with medial thickening and a change in the SMC orientation from 5 to 30 days. Further studies showed that a local inflation of normal jugular veins to 120 mm Hg for 3 min induced a similar change as found in the vein grafts, whereas the organization of the SMCs was not significantly changed in vein-vein grafts, that did not experience a change in tensile stress. These results suggested that increased tensile stress contributed to the initial damage of the SMCs and played a role in the regulation of medial SMC remodeling in vein grafts.

A comparison of potential methods for the alleviation of residual stresses in the necks of aluminium alloy thick-walled gas cylinders

Tensile circumferential residual stresses are introduced into seamless 6351 aluminium alloy gas cylinder necks at the bore during the heat treatment stage of the manufacturing process. To ascertain the feasibility of a proposed corrective procedure (selective removal of material from the gas cylinder neck) finite element analyses that modelled material removal were undertaken. Results indicate that no significant benefit was gained from the partial removal of bore material. However, removing the inelastically deformed outer surface material is likely to provide the greatest benefit of the four options considered. The material removal option was then compared with an investigation into cold working the gas cylinder neck to favourably modify the quenching residual stresses. Experimental results have indicated that the application of an axially applied compressive load to the gas cylinder neck, inducing bore yielding, favourably modifies the quenching residual stresses. This alternative of cold working the gas cylinder neck appears to be a possible method of favourably modifying the quenching residual stress.