Anisotropy of creep of materials

It has been shown that when ordinary stress-strain (-) diagrams are constructed for 20-mm thick rolled duralumin plate at various temperatures, the alloy behaves like an isotropic material, while a considerable degree of anisotropy is observed in creep testing.Thanks are due to A. F. Nikitenko for his assistance in obtaining and processing the experimental data.     

Vibrational creep of polymer materials

In the mechanics of deformed solids it is usually assumed that superposing small amplitude vibrations on a static load has no effect on the over-all characteristics of a material under strain. This hypothesis is reflected in the fact that the existing equations of state for the case of static loads with superposed small vibrations give deformation characteristics which differ little from the corresponding parameters of deformation processes taking place in the absence of excitations. At the same time, substantial changes in the deformation characteristics of a number of materials are observed under certain conditions after the application of alternating stresses of small amplitude. Reports on studies of creep of metals [1, 2], elastomers [3], and concrete [4] have been published, in which the fatigue curves obtained with small vibrations superposed on static loads lie above curves obtained for static loads corresponding to the maximum pulsating load level. Attempts have been made to explain this effect from the standpoint of the molecular-kinetic [3] and phenomenological [5] theories. Certain theoretical considerations and experimental data, discussed in this article, show that superposing a small dynamic component on a static load leads to an increase in the rate of creep of several polymer materials. This effect, which is due mainly to an increase in the polymer temperature as a result of dissipation of vibrational energy, differs from the vibration effect observed on elastomers by Slonimskii and Alekseev [3], in which the temperature rise due to the heat generated by vibrations plays no substantial part.The authors thank V. A. Volodchenkova, N. I. Gal'china, Yu. S. Levshina, Yu. P. Maksimacheva, and V. V. Tikhomirova who participated in the experimental work.     

A method for the evaluation of design limits for structural materials in a cyclic state of creep

The purpose of the present paper is to demonstrate how the minimum theorems proposed in an accompanying paper (Ponter and Boulbibane, 2002) can be utilised in the prediction of the deformation and life assessment of structures subjected to cyclic mechanical and thermal loadings. The developed method, which is based upon bounding theorems and an associate programming method, the Linear Matching method, takes into account the changes in residual stress field occurring within a cycle. Although the solution provided a bound on the inelastic work, it also appears that generally the displacements predicted by this solution are smaller than those that would be predicted by the rapid cycle solution. By way of illustration a simple non-linear viscous model is adopted and a number of solutions are presented involving a Bree plate problem subjected to cyclic histories of load and temperature. An elastic follow-up factor is identified as a key design parameter for high temperature dwell periods.     

Mechanics of crack propagation in materials with initial (residual) stresses (review)

Major results on the mechanics of crack propagation in materials with initial (residual) stresses are analyzed. The case of straight cracks of constant width that propagate at a constant speed in a material with initial (residual) stresses acting along the cracks is examined. The results were obtained, based on linearized solid mechanics, in a universal form for isotropic and orthotropic, compressible and incompressible elastic materials with an arbitrary elastic potential in the cases of finite (large) and small initial strains. The stresses and displacements in the linearized theory are expressed in terms of analytical functions of complex variables when solving dynamic plane and antiplane problems. These complex variables depend on the crack propagation rate and the material properties. The exact solutions analyzed were obtained for growing (mode I, II, III) cracks and the case of wedging by using methods of complex variable theory, such as Riemann–Hilbert problem methods and the Keldysh–Sedov formula. As the initial (residual) stresses tend to zero, these exact solutions of linearized solid mechanics transform into the respective exact solutions of classical linear solid mechanics based on the Muskhelishvili, Lekhnitskii, and Galin complex representations. New mechanical effects in the dynamic problems under consideration are analyzed. The influence of initial (residual) stresses and crack propagation rate is established. In addition, the following two related problems are briefly analyzed within the framework of linearized solid mechanics: growing cracks at the interface of two materials with initial (residual) stresses and brittle fracture under compression along cracks     

One-dimensional strain-dependent creep damage in inhomogeneous materials

A local inhomogeneous strain-dependent creep damage theory is presented and then applied using creep damage data obtained at J.R.C. Ispra for specimens of various lengths. Solutions for one-dimensional constant stress and load are obtained, and are found to be in better agreement with observation than solutions based on previous homogeneous theory. Also, the effect of random material parameters on the rupture time is considered for the constant tensile stress test, and is found to be a significant factor.     

Theory of creep of initially strain-hardening isotropic materials

Conclusions We analyzed the status of the problem of developing governing equations of isotropic creep within the framework of a mechanical equation of state. The feasibility of using equations of the hereditary type to describe the third stage of creep was evaluated.We also constructed a creep theory based on a refinement of the principle of the similarity of isochronic curves. This can be regarded as an attempt to generalize the concepts of mechanical equation of state and nonlinear heredity. The theory makes it possible to consider the initial strain-hardening of the medium, evaluate the third stage of creep, and take into account the history and cyclicity of loading.It was shown that nonsteady creep develops in media which exhibit linear strain-hardening, while the development of all three stages of creep is possible in media characterized by exponential strain-hardening. It was discovered that there is a sudden increase in the rate of nonsteady creep under constant stress. The creep of certain structural materials under steady, stepped, and cyclic loading was calculated and satisfactory agreement was obtained with experimental results.Institute of Mechanics, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Prikladnaya Mekhanika, Vol. 25, No. 2, pp. 90–100, February, 1989.     

Transient and steady-state nanoindentation creep of polymeric materials

The transient and steady-state nanoindentation creep of polymeric materials was investigated. The creep model is used to explain the experimental data of transient and steady-state creep dominated by viscoelastic deformation and power-law creep deformation, respectively. The Burgers viscoelastic model was used to interpret the transient creep in polymers under nano-indentation. Explicit expression for the displacement of transient creep was derived using the correspondence principle of linear viscoelasticity theory. The power law of strain rate-stress relation was used to explain the creep displacement during the steady state. Three polymers of poly(methyl methacrylate), hydroxyethyl methacrylate copolymer, and the fast-cure acrylic resin were used to measure the nanoindentation creep. The transient creep data are in good agreement with the predictions from the Burgers viscoelastic model. The creep displacement is mainly attributed to the viscous flow of the Kelvin element, and the computed values of viscosities (η_1,c, η_2,c) increase with decreasing preloading rate. By comparing the steady-state creep data with the power law of strain rate-stress relation, the stress exponents for the above polymeric materials were quantitatively determined.     

Energy consumption for creep fracture of metallic materials

The energy conservation law is applied to formulate the ductile and brittle creep fracture criterion for metallic materials. The criterion contains a summary of heat and latent energies. Assuming that the heat energy is given out so it has no effect on the fracture process, the ductile creep fracture criterion is simplified. To take into account the evaluation of the damage state of materials the compressibility condition is introduced and the brittle creep fracture law is formulated.