Crack initiation and growth in contact problems

The problem of crack initiation and growth in contact problems is studied by the strain energy density theory. This is accomplished by considering each point of the body as a possible source of crack initiation. The direction of crack initiation is determined by calculating the minimum value of the strain energy density function along a circular core area surrounding the point. The crack initiates from the point with the maximum of the local minimum values of the strain energy density function. Considered is a circular disc subjected to two equal and opposite forces and a circular cylindrical body pressed by a cylindrical punch. The stress field for the first case is obtained from the classical elasticity solution, while a finite element code is used for the second case. The locations of fracture initiation and the subsequent fracture trajectories for fast unstable fracture are determined.     

On the Galerkin vector and the Eshelby solution in linear elasticity

Displacement potentials in linear static elasticity consist of three functions which can be regarded as the three components of a vector, e.g., the Galerkin vector. This research note gives an explanation as to why the biharmonic equations govern these functions in isotropic elasticity as opposed to the sixth-order partial differential equations that govern them in anisotropic elasticity. This note also shows that the Eshelby solution in two-dimensional anisotropic elasticity can be derived from the method of displacement potentials.     

Boundary layers in highly anisotropic plane elasticity

The boundary layer method proposed by Everstine and Pipkin for the analysis of highly anisotropic materials, such as fibre-reinforced materials, in elastic plane strain is developed and extended also to include plane stress. It is applied to problems of point forces acting on half-planes, and to two crack problems. The boundary layer solutions are compared with known exact solutions in anisotropic elasticity, and it is found that the boundary layer theory gives good results for elastic constants typical of a carbon fibre reinforced resin.     

Love–Bishop rod solution based on strain gradient elasticity theory

In the present work, the propagation of longitudinal stress waves is investigated with a strain gradient elasticity theory given by Lam et al. In principle, the analysis of wave motion is based on the Love rod model including the lateral deformation effects, but in the same time is also taken into account the shear strain effects with Bishop?s correction. By applying Hamilton?s principle, a general explicit strain gradient elasticity solution is developed for the longitudinal stress waves, and it is compared with the special solutions based on the modified couple stress and classical theories. This work gives useful information with regard to the meaning of the three scale parameters in the strain gradient elasticity theory used here.     

On Sanders' energy-release rate integral and conservation laws in finite elastostatics

Sanders showed in 1960, within the framework of two-dimensional elasticity, that in any body a certain integral I around a closed curve containing a crack is path-independent. I is equal to the rate of release of potential energy of the body with respect to crack length. Here we first derive, in a simple way, Sanders' integral I for a loaded elastic body undergoing finite deformations and containing an arbitrary void. The strain energy density need not be homogeneous nor isotropic and there may be body forces. In the absence of body forces, for flat continua, and for special forms of the strain energy density, it is shown that I reduces to the well-known vector and scalar path-independent integrals often denoted by J, L, and M.     

On Saint-Venant's Principle in three-dimensional nonlinear elasticity

Consider a long elastic isotropic beam with a convex cross-section and a sufficiently smooth boundary. Suppose that a self-equilibrated load is applied at each end but the sides are stress-free and there are no internal body forces. It is proved in the context of three-dimensional, nonlinear elastostatics that if the first four derivatives of the displacement vector are a priori assumed to be everywhere sufficiently small with respect to the physical constants and the geometry of the cross-section, then the strains at any point decay exponentially with the distance of the point from the nearest end.This result is an extension of known results on Saint-Venant's Principle in linear and two-dimensional nonlinear elasticity.     

Integral bounds on the strain energy for the traction problem in finite elasticity

For the traction boundary value problem in finite elasticity, a bound is obtained for the total strain energy in terms of the L₂ integral norms of the surface tractions and body forces, under the assumptions that the unstressed body occupies a convex domain and the displacement gradients are sufficiently small.This is an extension of known results in linear (infinitesimal) elasticity into finite elasticity.     

Invariants for Rari- and Multi-Constant Theories with Generalization to Anisotropy in Biological Tissues

The rari-constant theory of linear elasticity is based on the assumption that elasticity in solids is caused by only pair potentials with coaxial forces acting between atoms. The strain energy of each pair potential depends on the square of the strain between the atoms in the pair. This strain can be determined by taking the inner product of the strain tensor with a structural tensor that is the tensor product of a unit vector with itself. It is shown that the 15 independent constants in the rari-constant theory can be generated by a complete set of 15 structural tensors. It is also shown that the 6 additional independent constants in the multi-constant theory can be generated by taking the inner product of 6 of these structural tensors with the square of the strain tensor. A generalization of these results for nonlinear elasticity is discussed with reference to recent work which compares the structural and generalized structural tensor approaches to modeling fibrous tissues.     

The continuum elastic and atomistic viewpoints on the formation volume and strain energy of a point defect

We discuss the roles of continuum linear elasticity and atomistic calculations in determining the formation volume and the strain energy of formation of a point defect in a crystal. Our considerations bear special relevance to defect formation under stress. The elasticity treatment is based on the Green's function solution for a center of contraction or expansion in an anisotropic solid. It makes possible the precise definition of a formation volume tensor and leads to an extension of Eshelby's [Proc. R. Soc. London Ser. A 241 (1226), 376] result for the work done by an external stress during the transformation of a continuum inclusion. Parameters necessary for a complete continuum calculation of elastic fields around a point defect are obtained by comparing with an atomistic solution in the far field. However, an elasticity result makes it possible to test the validity of the formation volume that is obtained via atomistic calculations under various boundary conditions. It also yields the correction term for formation volume calculated under these boundary conditions. Using two types of boundary conditions commonly employed in atomistic calculations, a comparison is also made of the strain energies of formation predicted by continuum elasticity and atomistic calculations. The limitations of the continuum linear elastic treatment are revealed by comparing with atomistic calculations of the formation volume and strain energies of small crystals enclosing point defects.     

Representations of ℂ, biharmonic vector fields,and the equilibrium equation of planar elasticity

The equilibrium equation for an elastic body subjected to surface forces asserts the linear dependence of the Laplacian and the gradient of the divergence of the vector field which gives the displacement at each point. James Clerk Maxwell (1831–1879) was the first to point out that the component functions of such a field are biharmonic, i.e., their Laplacians are harmonic functions. Using only algebraic tools familiar to advanced undergraduates we show that the usual complex variable representation of two-variable biharmonic functions falls naturally out of a power series construction based on matrix representations of . Under the assumption of linear stress and strain components, this construction is then used to describe the solutions to the planar equilibrium equation in terms of the geometry of the Moebius plane.     

Dispersion and polarization of surface Rayleigh waves for the Cosserat continuum

In the framework of the nonsymmetric theory of elasticity (the Cosserat contimum), we consider the problem of propagation of a surface acoustic Rayleigh wave in the half-space. The wave is represented as a wave packet of arbitrary form bounded both in the time space and the Fourier space. We assume that the material strain is described by not only the displacement vector but also an independent rotation vector. The general analytic solution of this problem is obtained in displacements. We perform comparative analysis of the obtained solution and the corresponding solution for the classical elastic medium. We introduce and analyze macroparameters characterizing the difference between the stress-strain state and the state predicted by the classical theory of elasticity: the elasticity coefficient, the wave number, and the phase and group velocities. It should be noted that these parameters can be measured experimentally.     

A method of three-dimensional strain measurement on non-ideal objects using holographic interferometry

Holographic interferometry has been applied to determine the components of tensile, compressive and shear strain in the surface layer of a non-ideal object subjected to high values of stress. The method consists of determining the three components of the vector displacement at a sufficient number of discrete points on the surface of the object. Functions in the form of truncated power series are fitted to the fringe order-distance data using the method of least squares. Interpolation of these functions is then carried out to obtain the fringe-order numbers at closely spaced equal intervals. The displacement in each interval is calculated and the displacement-distance relationship is then numerically differentiated to obtain strain. Components of stress at a particular point are then estimated by multiplying the strain value by the appropriate modulus. These stresses result from the application of considerable forces that are likely to cause large rigid-body displacements unless the object can be satisfactorily restrained. The difficulty of providing the required restraining forces has been avoided by mounting the object on a kinematic mount from which it can be removed before stressing and replaced afterwards. The accuracy of relocation of the object is sufficient to ensure the satisfactory formation of holographic interference fringes. As the use of the two-exposure holographic method results in a direction ambiguity of the displacement vector at any point of the surface, this method is complemented by electrical strain measurement. The strain is sampled in two convenient orthogonal directions at some part of the surface. The application of the uniqueness theorem of elasticity then permits the direction of displacement to be uniquely specified at any point of the surface. The electrical strain measurement also provides a check of the strain values determined by interferometry at various positions on the surface. This check is necessary since precise knowledge of the surface geometry of a non-ideal object may be lacking. Incorrect assumptions made about the geometry are shown to result in serious errors in the evaluation of strain. The method described has been used to estimate the stresses in the vicinity of a threaded hole in a sample of pipe for representative values of torque applied in screwing a connecting member into the pipe. Possible reasons for failure of the pipe in service are then deduced.     

弹性大变形问题中的应力状态描述、奇异性问题和余能原理

对弹性大变形理论中的3方面问题进行了综述.首先,对各种应变度量的共轭应力进行综述.大变形问题引起的应力状态描述的复杂性引起了许多学者的兴趣,对这个问题的研究也促进了大变形弹性理论的发展.在各种特定问题中,人们提出了不同的应力张量来描述应力状态,如Caucby应力张量、第一类和第一二类Piola-Kirchhoff应力张...     

Ellipticity conditions of the static equations of nonlinear elasticity

The relations of the nonlinear model of the theory of elasticity are considered. The Cauchy and the strain gradient tensors are taken to be the characteristics of the stress-strain state of a body. Sufficient conditions under which the static equations of elasticity are of elliptic type are established. These conditions are expressed in the form of constraints imposed on the derivatives of the elastic potential with respect to the strain-measure characteristics. The cases of anisotropic and isotropic bodies are treated, including the case where the Almansi tensor is taken to be the strain measure. The plane strain of a body is investigated using actual-state variables. Novosibirsk State University, Novosibirsk 630090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 2, pp. 196–203, March–April, 1999.     

Balance laws and energy release rates for cracks in dipolar gradient elasticity

It is the purpose of this work to derive the balance laws (in the Günther–Knowles–Sternberg sense) pertaining to dipolar gradient elasticity. The theory of dipolar gradient (or grade 2) elasticity derives from considerations of microstructure in elastic continua [Mindlin, R.D., 1964. Microstructure in linear elasticity. Arch. Rational Mech. Anal. 16, 51–78] and is appropriate to model materials with periodic structure. According to this theory, the strain–energy density assumes the form of a positive-definite function of the strain (as in classical elasticity) and the gradient of both strain and rotation (additional terms). The balance laws are derived here through a more straightforward procedure than the one usually employed in classical elasticity (i.e. Noether’s theorem). Indeed, the pertinent balance laws are obtained through the action of the standard operators of vector calculus (grad, curl and div) on appropriate forms of the Hamiltonian of the system under consideration. These laws are directly related to the energy release rates in the processes of crack translation, rotation and self-similar expansion. Under certain conditions, they are identified with conservation laws and path-independent integrals are obtained.     

A Variational Justification of Linear Elasticity with Residual Stress

We consider a residually-stressed, uniform hyperelastic body whose stored energy is quadratic with respect to the Green–St. Venant strain. We show that, in the limit of vanishing loads, suitable minimizing sequences converge to the unique minimizer of the energy functional of linear elasticity. We also deduce the standard stress-strain relations for linear elasticity with residual stress.     

An asymptotic strain gradient Reissner-Mindlin plate model

In this paper we derive a strain gradient plate model from the three-dimensional equations of strain gradient linearized elasticity. The deduction is based on the asymptotic analysis with respect of a small real parameter being the thickness of the elastic body we consider. The body is constituted by a second gradient isotropic linearly elastic material. The obtained model is recognized as a strain gradient Reissner-Mindlin plate model. We also provide a mathematical justification of the obtained plate model by means of a variational weak convergence result.     

On the existence of stress concentrations in loaded bodies made of polycrystalline materials

We study the character of stress distributions near the corner point of the interface between the two joined crystals. The interface forms a dihedral angle. The joined crystals have a cubic symmetry and consist of the same material. They have a single common principal direction of elasticity, which is parallel to the edge of the dihedral angle. The other principal directions do not coincide and are oriented arbitrarily.In the framework of elasticity, we consider problems of out-of-plane and plane strain of the twocrystal. We show that, in the case of longitudinal shear in the direction of the common principal axis of elasticity, there is no stress concentration near the corner point of the interface between the two joined crystals.For the case of plane strain in which all displacements and strains occur only in the planes perpendicular to the common principal direction, we use separation of variables to construct the characteristic equation that determines the stress concentration degree and find the roots of this equation, which determine the order of singularities of the stresses.     

A note on line forces in gradient elasticity

The theory of gradient elasticity is applied to line forces. Line forces acting on a point within the body and a concentrated normal force (Flamant problem) which acts on a half plane are studied. Closed analytical solutions which have a simple form are obtained for displacement fields of these forces. The gradient elasticity solutions are free from undesirable displacement singularities predicted by classical elasticity.     

On the nature of the relaxation time,the Maxwell–Cattaneo and Fourier law in the thermodynamics of a continuous medium,and the scale effects in thermal conductivity

We present the theory of space–time elasticity and demonstrate that it is the extended reversible thermodynamics and gives the coupled model of thermoelasticity and heat conductivity and involves traditional thermoelasticity. We formulate the generally covariant variational model’s dynamic thermoelasticity and heat conductivity in which the basic kinematic and static variables are unified tensor objects (subject, matter). Variation statement defines the whole set of the initial-boundary problems for the 4D vector governing equation (Euler equation), the spatial projections of which define motion equations and the time projection gives the heat conductivity equation. We show that space–time elasticity directly implies the Fourier and the Maxwell–Cattaneo laws of heat conduction. However, space–time elasticity is richer than classical thermoelasticity, and it advocates its own equations of motion for coupled thermoelasticity. Moreover, we establish that the Maxwell–Cattaneo law and Fourier law can be defined for the reversible processes as compatibility equations without introducing dissipation. We argue that the present framework of space–time elasticity should prove adequate to describe the thermoelastic phenomena at low temperatures for interpreting the results of molecular simulations of heat conduction in solids and for the optimal heat and stress management in the microelectronic components and the thermoelectric devices.