The uniaxial strain compression of porous and fully dense beryllium

Compressive stress versus uniaxial compressive strain curves obtained using a simple closed die are presented for SP-100-C beryllium of initial fractional porosities varying from 0.05 to 0.26, and for fully dense ingot beryllium. The relationship between porosity and compressive yield stress is found to be linear over the entire porosity range with a discontinuity at 0.065 fractional porosity. It is suggested that the discontinuity is related to the transition from isolated to predominantly interconnected porosity. It is deduced that the pressure required for full compaction varies with density and is estimated to range from 3.65×10³ MN m^?2 at 0.05 to 17.2×10³ MN m^?2 at 0.26 fractional porosity.     

Volumetric strain measurement of polymeric materials subjected to uniaxial tension

A novel method for measuring and calculating volumetric strain in circular cylindrical uniaxial tension samples made from polymeric materials is proposed. It is shown that special considerations must be taken when calculating volumetric strain when a sample is in a postnecking state. Solely based on surface data, the key feature of the proposed correction is that it allows for an inhomogeneous distribution of longitudinal strain through the diameter of the sample, where a more traditional approach would be to assume a homogeneous distribution. These two approaches are evaluated by applying them to data from a close‐to‐incompressible steel sample. Whereas the proposed method indicates only a small positive increase in volume, the assumption of a homogeneous distribution results in substantial negative volumetric strains. Applying the two methods to tension samples made from HDPE and PVC, where plastic dilatation is nonlinear, again shows an initial negative volumetric strain for HDPE with the assumption of a homogeneous longitudinal strain. The proposed method predicts close‐to‐zero early‐stage volumetric strain for the same test. The differences are more subtle for samples of PVC. Micrographs obtained with scanning electron microscope show that the dilatation of PVC is related to voiding of the material around filler particles, while the underlying mechanism for HDPE is less clear. The results indicate that earlier reports of negative volumetric strain in polymers subjected to uniaxial tension might be artefacts of the implicit assumption made when calculating the volumetric strain.     

Thermal conductivity of porous materials in relation to the aggregate state of the filler

The effective thermal conductivity of porous materials is analyzed as a function of the temperature and in relation to the aggregate state of the filler.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 6, pp. 1028–1032, June, 1973.     

Ferromagnetic domains in uniaxial materials

The two basic remanent domain structures (RDS) observed in magnetically uniaxial platelets and their variations as a function of the thickness of the platelet and the value of the rotational susceptibility2piM_{s}^{2}/K_{1}of the material is discussed. The parallel-plate structure (PPS) first proposed by Landau and Lifshitz represents the lowest energy configuration or ground state for the given geometry, i.e., the easy axis normal to the platelets. The validity of their half-power law connecting the domain width with the thickness of the platelet is discussed in the light of subsequent theories and experiments and is shown to be correct over a limited thickness range only. The honeycomb domain structure (HCS) consisting of a closely packed array of circular cylindrical domains, found later experimentally, is shown by the latest calculations to be a RDS having a total free energy only about 0.4 percent higher than that of the PPS. The domain spacing is found to obey a similar thickness dependence as that in the PPS.     

An equation of state for porous materials under shock loading

Based on the physical interpretation of the linear equation of state (EOS) of dense solids under shock loading, which relates particle and shock speeds asU _s=C _b+gU _p, the EOS for porous solids has been developed and is expressed asU _s*=ΨC _b*+g*U _p whereC _b* andg* are effective bulk sound speed and effective inverse ultimate volume strain respectively. Ψ is a pore collapse function introduced specially to differentiate loading and unloading behaviour.C _b* andg* are derived theoretically whereas Ψ is established empirically as Ψ=f(U _p,C _b). This EOS does not call for any experimentally established material constant to describe the effect of porosity. Also its ability to describe the unloading behaviour distinguishes it from the presently available equations of state.     

Micromechanical enhancement of the macroscopic strain state for advanced composite materials

A computationally efficient method for determining the microscopic strain field within the separate phases of heterogeneous medium consisting of collimated, continuous fibers within an isotropic matrix is presented with the goal of providing one of the essential links in a multi-scale analysis of a composite structure which relates structural loading and deformations at the macro-scale to the state of strain within the fiber and matrix phases at the micro scale. The model utilizes a conventional influence function formulation and considers thermo-mechanical deformations.     

Polaritons in uniaxial materials propagating in hollow cylinders

The properties of polaritons propagating in hollow dielectric and magnetic cylinders embedded in an optically inert medium are studied. We pay special attention to those solutions of Maxwell's equations that give the behavior of the nonradiative modes (confined and localized) propagating in an optically active cylindrical medium. The dispersion relation of surface (localized) modes is obtained. Numerical results are presented for cylinders constituted by magnetic and dielectric materials, such as the uniaxial Heisenberg antiferromagnet MnF2 and the dielectric TiO2.     

About the dynamic uniaxial tensile strength of concrete-like materials

Experimental methods for determining the tensile strength of concrete-like materials over a wide range of strain-rates from 10⁻⁴ to 10² s⁻¹ are examined in this paper. Experimental data based on these techniques show that the tensile strength increases apparently with strain-rate when the strain-rate is above a critical value of around 10⁰-10¹ s⁻¹. However, it is still not clear that whether the tensile strength enhancement of concrete-like materials with strain-rate is genuine (i.e. it can be attributed to only the strain-rate effect) or it involves “structural” effects such as inertia and stress triaxility effects. To clarify this argumentation, numerical analyses of direct dynamic tensile tests, dynamic splitting tests and spalling tests are performed by employing a hydrostatic-stress-dependent macroscopic model (K&C concrete model) without considering strain-rate effect. It is found that the predicted results from these three types of dynamic tensile tests do not show any strain-rate dependency, which indicates that the strain-rate enhancement of the tensile strength observed in dynamic tensile tests is a genuine material effect. A micro-mechanism model is developed to demonstrate that microcrack inertia is one of the mechanisms responsible for the increase of dynamic tensile strength with strain-rate observed in the dynamic tensile tests on concrete-like materials.