Estimation of Isotropic Hyperelasticity Constitutive Models to Approximate the Atomistic Simulation Data for Aluminium and Tungsten Monocrystals

In this paper,the choice and parametrisation of finite deformation polyconvex isotropic hyperelastic models to describe the behaviour of a class of defect-free monocrystalline metal materials at the molecular level is examined.The article discusses some physical,mathematical and numerical demands which in our opinion should be fulfilled by elasticity models to be useful.A set of molecular numerical tests for aluminium and tungsten providing data for the fitting of a hyperelastic model was performed,and an algorithm for parametrisation is discussed.The proposed models with optimised parameters are superior to those used in non-linear mechanics of crystals.

Effects of densification on untreated and nano-aluminum-oxide impregnated poplar wood

Effects of densification of poplar wood（Populus nigra） impregnated with nano-aluminum oxide（NA） and pre-treated with water vapor for 4 and 6 h were investigated in the present study. Physical and mechanical properties of treated poplar wood were measured according to the ASTM D-143 standard specifications, and then compared with the untreated specimens. Results showed significant improvement in all properties as a result of densification. A 4-h vapor pre-treatment improved effects on both physical and mechanical properties. When the duration of vapor-treatment increased to 6 h, wood polymers degraded to the extent that the improvements due to the vapor pre-treatment decreased substantially, though the final results were still significant improvements compared with the control specimens. High thermal conductivity coefficient of NA slightly but not significantly improved properties. Due to the high spring-back after 15 days,densified poplar is not recommended for applications in which densified wood will be exposed for long periods to high humidity or to direct water.

An Internal Expansive Stress Model of Concrete under Sulfate Attack

Concrete experiences expansive deformation during sulfate attack due to internal expansive stress. Sulfate ions enter concrete pores, react with the pore solution, and produce ettringite. The production of ettringite explains the internal expansive stress that reduces the durability of concrete. In this study, the model of internal expansive stress was achieved through the Eshelby＇s theory, as well as the experimental results for concrete erosion. Numerical simulation indicated that internal expansive stress is not only determined by the water-to-cement ratio of concrete and the concentration of sulfate solution, but also affected by the relaxation time of concrete.

Rate-Dep endent Characteristic of Relaxation Time of Concrete

This study focused on the rate-dependent relaxation time of concrete using the split Hopkinson pressure bar test and the ultrasonic technique.Based on the experimental results,and considering the viscous effect in concrete,the viscoelastic stress-strain relationship was established in terms of the principles of irreversible thermodynamics and the generalized Onsager's principle,providing the convergence of an iterative scheme in terms of the Lipschitz's condition.It was found that the relaxation time of concre te depends on the strain rate.Furt her more,considering the state equations and damage evolution,a nonlinear viscoelastic constitutive model of concrete coupling damage was proposed.Compared with the experimental results,this model could better characterize the strengthening and softening characteristics of concrete prior to and beyond the strength limit.Further analysis indicated that the viscous effect is dominantly caused by the C-S-H in concrete.