On the axial and interfacial shear stresses due to thermal mismatch in hybrid composite sheets

A simple analytical model has been developed which allows the determination of the axial and the interfacial shear stresses which can occur in hybrid fiber composites as a consequence of the mismatch in coefficients of thermal expansion and Young's modulus. The configuration considered is a finite-width hybrid composite monolayer with alternating high- and low-modulus fibers. To account properly for the interfacial shear between fiber and matrix, a modified shear-lag model is used which permits extensional deformation due to thermal expansion of the matrix in the fiber direction. Typical stresses due solely to temperature changes are calculated, and these show steep boundary-layer edge stresses at free corners.     

小锥角零件多道次剪旋成形试验研究

针对小锥角零件(半锥角α=7°)的成形,以LF2,LF6,LF21铝板料为原料,采用先预制坯后多道次剪旋的成形方案,试验研究了材料塑性、工艺参数和壁厚减薄率对旋压件成形质量的影响,确定了成形小锥角零件的合理方案.研究表明:材料的塑性对可旋性有很大影响,为获得较高的成形极限,材料应同时具有较好的塑性和适当的加工硬化性;在每道次前均退火的条件下,合理的道次分配方案是前一道次在材料的成形极限范围尽量减薄,而最终道次减薄率需控制在一定的范围内(50%以内);旋压过程中保持一定的负偏离(3%左右),同时采用较大的进给比有利于提高旋压件精度.     

Excitation of a quasi-plane shear wave by a surface acoustic source producing normal stresses

The generation of elastic volume waves by a source which is widely used in acoustic flaw detection is considered. This source produces normal stresses at the boundary of an elastic solid. The source is large compared to the wavelength of the waves that are launched. The possibility of exciting a shear wave with a plane front in the near zone is established, and an estimate of its intensity is made. Pis’ma Zh. Tekh. Fiz. 23, 91–94 (December 12, 1997)     

Effect of shear deformation on the elastica with axial strain

The problem of the elastica is generalized to include the effect if shear as well as axial strain. This leads to a non-linear two-point boundary-value problem composed of five first-order differential equations. At first the problem is linearized to study the way in which the bucking loads are modified by the combination of effects. For a simple column, the eigenvalues are found to be zeros of a third-order polynomial, and these are evaluated by a straightforward Newton–Raphson technique using a digital computer. The postbucking behaviour is then studied by using the computer and a ‘shooting’ method to solve the non-linear problem. For the special case of the prismatic column, graphs plotted by the computer are displayed to show the variation of load with both lateral and axial deflection for various combinations of axial compressibility and shear deformability. For one particular case, the deflected center line at various load levels is shown.     

On a circular shear crack in a semiinfinite composite with initial stresses

By the methods of the linearized three-dimensional mechanics of deformed bodies, we consider an axisymmetric problem of a circular subsurface crack of radial shear in a semiinfinite composite material with initial stresses. We deduce the resolving system of Fredholm integral equations of the second kind and the representations of the stress intensity factors in the vicinity of the crack tip depending on the initial stresses. For two types of composite materials (layered composites with isotropic layers and composites stochastically reinforced by short ellipsoidal fibers), we compute the stress intensity factors and study their dependence on the initial stresses, physicomechanical characteristics of the composites, and the geometric parameters of the problem. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 3, pp. 27–34, May–June, 2007.     

Evaluation of residual stresses formed in surface plastic deformation

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 25, No. 3, pp. 92–95, May–June, 1989.     

Effect of a high-density current pulse on the relaxation of shear stresses

The procedure and several results of experimental studies on the high-density current pulse-induced relaxation of shear stresses are presented. A significant nonthermal current effect on shear strain kinetics has been established. The strain rate of the metal in shear under the action of a current pulse is shown to be determined by the viscosity factor, like under high-rate inelastic deformation. Translated from Problemy Prochnosti, No. 6, pp. 44–53, November–December, 2008.     

Influence of transverse shear on an axial crack in a cylindrical shell

An axial crack in a cylindrical shell is investigated by use of a 10th order shell theory, which accounts for transverse shear deformations as well as a special kind of orthotropy. The symmetric problem is formulated in terms of two coupled singular integral equations, which are solved numerically. The asymptotic membrane and bending stress fields ahead of the crack are found to be self similar. Stress intensity factors are given as a function of the shell parameter for various values of the ratio shell radius to shell thickness. Considerable differences from 8th order shell theory results are found for the bending stresses, while the membrane stresses of the 8th order theory seems to be a lower limit reached for very thin shells.

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Résumé Une fissure axiale dans une enveloppe cylindrique est étudiée en utilisant la théorie du 10ème ordre des enveloppes, qui tient compte des déformations de cisaillement transversal ainsi que d'une sorte particulière d'orthotropie. Le problème de symétrie est formulé en terme de deux équations intégrales singulières couplées qui sont résolues numériquement. On trouve que la membrane asymptotique et les champs de contrainte de flexion en avant de la fissure sont respectivement similaires. Les facteurs d'intensité de contrainte sont donnés en fonction du paramètre d'enveloppe pour diverses valeurs du rapport du rayon de l'enveloppe à son épaisseur. Des différences considérables avec les résultats obtenus par la théorie des enveloppes du 8ème ordre ont été trouvées dans le cas des contraintes de flexion tandis que les contraintes de membrane obtenues par la théorie du 8ème ordre paraissent constituer une limite inférieure que l'on atteint dans le cas des enveloppes très minces.

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Part of the present work was carried out during a visit to Lehigh University supported by NSF under the Grant ENG 73-045053 A01 and NASA-Langley under the Grant NGR 309-007-011.     

On the shell theory on the nanoscale with surface stresses

Below we discuss the derivation of the governing nonlinear shell equations taking into account the surface stresses. The surface effects are significant for the modeling of some structures as nanofilms, nanoporous materials and other nano-size structures. In particular, the surface stresses are responsible for the size effect, i.e. dependence of the material properties on the specimen size. The theory of elasticity with surface stresses is applied to the modeling of shells with nano-scaled thickness. It will be shown that the resultant stress and couple stress tensors can be represented as a sum of two terms. The first term in the sum depends on the stress distribution in the bulk material while the second one relates to the surface stresses. Hence, the resultant stress and couple stress tensors are linear functions with respect to the surface stresses. As an example the effective stiffness properties of a linear elastic Cosserat shells taking into account the surface stresses are presented.     

Crack surface contact of surface and internal cracks in a plate with residual stresses

The problem of a mode I surface or internal crack in a plate with residual stresses is examined. The emphasis in this study is in accounting for crack surface contact. The importance of such problems is realized in both crack detection and in the stress intensity factor calculations. The line spring model is used to iteratively determine the border of the closed portion of the crack and the stress intensity factors along the open portion. It is demonstrated that the model is very versatile for solving contact problems. Examples for both surface and internal cracks are given. It is shown that if crack contact is ignored, the stress intensity factor may be significantly underestimated.     

Evaluation of stress-rupture strength with a programmed change in stresses

Published experimental data for stress-rupture strength with stepwise loading and cyclic overloads are analyzed. On the basis of generalizing these data a procedure is suggested for calculating the equivalent failure stress and calculation errors are determined. Statistical treatment shows that these errors are within the limits of the natural scatter of high-temperature material properties in stress-rupture tests. It is established that in order to carry out calculations normal stress-rupture strength curves are required.Translated from Problemy Prochnosti, No. 4, pp. 23–29, April, 1994.     

Nonstationary thermal stresses in a hollow cylinder with heat flux acting on the inside surface

Thermoelastic stresses in a long hollow cylinder are considered when a uniformly distributed heat flux is applied to the inside surface and the outside surface is heat-insulated.     

Designing and perfecting a tribometer for the study of friction of a concentrated clay-water mixture against a metallic surface

Extrusion shaping of tiles and bricks containing clay meets with lots of problems because of friction at the interface wall-paste in the extruder and in the die. These frictions lead to wear of the walls and create defects which alter the materials and its mechanical properties. In this paper a new device (tribometer) is presented allowing the study of friction of paste samples (kaolin+water) pressed against a plate put into translation. The analysis of the results obtained with the tribometer, together with observations made on a model, enables us to understand the mechanisms occurring at the interface paste-steel. It appears that two types of phenomena are present at the interface, depending on the paste composition and the plate surface characteristics:

- The flow of water coming from sample consolidation creating enough pressure to build the normal force.

- The movement of particles governed by the relationship between the particle size and the plate roughness amplitude.

     

Evaluation of the ultimate stresses in a cycle for welded structures with high residual stresses based on test results for small specimens without residual stresses

We propose an equation for the evaluation of the fatigue limits of welded joints of low-carbon and low-alloy steels with high residual stresses based on the results of testing of small specimens without residual stresses. The independence of the amplitude of stresses on the mean stress in a cycle is proved for welded joints with high residual stresses. __________ Translated from Problemy Prochnosti, No. 2, pp. 66–81, March–April, 2008.     

The stressed-strained state in the chip forming zone during the finish turning of workpieces of hardened steels

The finite element method has been used for modeling the stressed-strained state in the chip forming zone in cutting workpieces of hardened steels by tools equipped with cutting inserts of cubic boron nitride-based polycrystalline superhard material (PCBN). Special features of the stressed-strained state in the chip forming zone when using tools with a flat and a cylindrical rake faces have been discussed.