The influence of the pore shape on the bulk modulus and the Biot coefficient of fluid-saturated porous rocks

Fluid-saturated rocks are multi-phasic materials and the mechanics of partitioning the externally applied stresses between the porous skeleton of the rock and the interstitial fluids has to take into consideration the mechanical behaviour of the phases. In these studies the porosity of the multi-phasic material is important for estimating the multi-phasic properties and most studies treat the porosity as a scalar measure without addressing the influence of pore shape and pore geometry. This paper shows that both the overall bulk modulus of a porous medium and the Biot coefficient depend on the shape of the pores. Pores with shapes resembling either thin oblate spheroids or spheres are considered. The Mori–Tanaka and the self-consistent methods are used to estimate the overall properties and the results are compared with experimental data. The pore density and the aspect ratio of the spheroidal pores influence the porosity of the geomaterials. For partially saturated rocks, the equivalent bulk modulus of the fluid–gas mixture occupying the pore space can also be obtained. The paper also examines the influence of the pore shape in estimating the Biot coefficient that controls the stress partitioning in fluid-saturated poroelastic materials.

A study on the micromechanical behavior of Ti-55531 titanium alloy with lamellar microstructure by in-situ neutron diffraction

High strength titanium alloys are promising materials for heavy component parts in the aviation industry. The limited combination of strength and ductility requires an understanding of deformation and stress partitioning between constituent phases. The micromechanical behaviors of Ti-55531 titanium alloy with lamellar microstructure are investigated by in-situ neutron diffraction. The phase strain and lattice strain evolution of α and β phase at loading direction and transverse direction are determined. The results show that the micromechanical behaviors of oriented grains of α and β phase are obviously different. Furthermore, the stress partitioning between α and β phase during the deformation is clearly illustrated. It is found that the β matrix is subjected to higher stress than α precipitates. In addition, the intergranular and interphase microstress is quantitatively characterized. It is found that the intergranular microstress in the β phase grains is predominant among these microstresses. Combining the in-situ neutron diffraction with microstructure characterization, the present work provides a guide for further microstructure optimization.

Stress partitioning in a near-β Titanium alloy induced by elastic and plastic phase anisotropies: experimental and modeling

The load transfer induced by the elastic and plastic phase anisotropies of a Ti–10V–2Fe–3Al titanium alloy is studied. The microstructure consists in α nodules embedded in elongated β grains. EBSD performed on the alloy shows no crystallographic texture neither for α nor β phase. Tensile tests along the elongation direction, at a strain rate of 2 x 10-3 s-1 give a yield stress of 830 MPa with 13% ductility. Simulations based on an advanced two-phase polycrystalline elasto-viscoplastic self-consistent (EVPSC) model predict that the β phase first plastifies with a sequential onset of plasticity starting from <110> oriented β grains, then <111> and finally <100> oriented β grains. This leads to a strong load transfer from the β grains to the α nodules whose average behavior remains elastic up to high stresses (~940 MPa). However, additional simulations considering exclusively β grains of specific orientation show that the behavior of α nodules is strongly dependent on the β texture in which they are embedded. Especially, in <001> β grains, which plastify the latest, the model predicts the onset of plasticity in favorably orientated α nodules. Moreover, the orientation spread within the β grains can modify the average plastic behavior of α phase. In future, these results will be compared to data obtained from in-situ High Energy XRD and SEM/EBSD experiments.