Niels Saabye Ottosen

Determination of Brain Interstitial Concentrations by Microdialysis

Abstract: Microdialysis is an extensively used technique for the study of solutes in brain interstitial space. The method is based on collection of substances by diffusion across a dialysis membrane positioned in the brain. The outflow concentration reflects the interstitial concentration of the substance of interest, but the relationship between these two entities is at present unclear. So far, most evaluations have been based solely on calibrations in saline. This procedure is misleading, because the ease by which molecules in saline diffuse into the probe is different from that of tissue. We describe here a mathematical analysis of mass transport into the dialysis probe in tissue based on diffusion equations in complex media. The main finding is that diffusion characteristics of a given substance have to be included in the formula. These include the tortuosity factor (λ) and the extracellular volume fraction (α). We have substantiated this by studies in a welldefined complex medium (red blood cell suspensions) as well as in brain. We conclude that the traditional calculation procedure results in interstitial concentrations that are too low by a factor of λ2/α for a given compound.

Properties of Discontinuous Bifurcations Solutions in Elasto-Plasticity

Explicit expressions for the spectral properties of the bifurcation problem involving discontinuities for general elastic-plastic materials are presented. It then follows that the classical value of the critical hardening modulus derived by Rice (1976. Proc. 14th IUTAM Congr., Delft, The Netherlands, pp. 207–220, North Holland, Amsterdam.) is, in fact, the only possible one. Furthermore, from the spectral analysis it follows in a very straightforward fashion that bifurcation displaying elastic unloading on one side of the singular surface can never precede bifurcation with plastic loading on both sides of this surface. Explicit analytical results for the critical bifurcation directions and the corresponding hardening modulus are derived for non-associated volumetric flow rules while the devialoric portion is associated. The considered yield and potential functions may depend on all three stress invariants and may involve mixed isotropic and kinematic hardening. The result obtained by Rudnicki and Rice (1975, J. Mech. Phys. Solids 23, 371–394) for a Drucker-Pruger material appear as a special case. Other criteria that are investigated arc those of Coulomb and Rankine. (Less)

Discontinuous bifurcations of elastic-plastic solutions at plane stress and plane strain

Abstract Conditions for discontinuous bifurcations of the incremental fields in elastic-platic materials subjected to the condition of either plane stress or plane strain are derived and explicit expressions for the critical hardening modulus and the corresponding bifurcation directions are obtained for a quite general class of plasticity models. The only restriction is that the gradients of the yield function and plastic potential, that defines the nonassociated flow rule, have the same principal directions and that two of these directions are located in the plane of interest. Drucker-Pragers and Mohr-Coulombs yield criteria are taken as typical for the behavior of pressure-dependent materials such as concrete and granular materials. For the latter criterion, results for plane strain have previously been obtained only for the very particular case when the intermediate principal stress is directed out-of-plane. These results are confirmed in this paper as a part of the investigation of the complete behavior.

Kinematic hardening in large strain plasticity

A finite strain hyper elasto-plastic constitutive model capable to describe non-linear kinematic hardening as well as nonlinear isotropic hardening is presented. In addition to the intermediate configuration and in order to model kinematic hardening, an additional configuration is introduced - the center configuration; both configurations are chosen to be isoclinic. The yield condition is formulated in terms of the Mandel stress and a back-stress with a structure similar to the Mandel stress. It is shown that the non-dissipative part of the plastic velocity gradient not governed by the thermodynamical framework and the corresponding quantity associated with the kinematic hardening influence the material behaviour to a large extent when kinematic hardening is present. However, for isotropic elasticity and isotropic hardening plasticity it is shown that the non-dissipative quantities have no influence upon the stress-strain relation. As an example, kinematic hardening von Mises plasticity is considered, which fulfils the plastic incompressibility condition and is independent of the hydrostatic pressure. To evaluate the response and to examine the influence of the non-dissipative quantities, simple shear is considered; no stress oscillations occur. (Less)

Consequences of Dynamic Yield Surface in Viscoplasticity

A theory of viscoplasticity is formulated within a thermodynamic concept. The key point is the postulate of a dynamic yield surface, which allows us to take advantage of the postulate of maximum dissipation to derive an associated formulation of the evolutions laws for the internal variables without using penalty techniques that only hold in the limit it when viscoplasticity degenerates to inviscid plasticity. Even a non-associated formulation is presented. Within this general formulation, a particular format of the dynamic yield function enables us to derive the static yield function in a consistent manner. Hardening, perfect and softening viscoplasticity is also defined in a consistent manner. The approach even includes associated and non-associated viscoplasticity where corners exist on the yield and potential surfaces.

Acceleration Waves in Elasto-Plasticity

A spectral analysis of the acceleration wave problem in general elasto-plastic materials is carried out, whereby explicit expressions for the eigenvalues and eigenvectors are obtained. In case of nonassociated plasticity, all eigenvectors become nonorthogonal and one eigenvalue always remains unchanged and equal to the shear modulus. For a very broad class of nonassociated plasticity models, it is shown that the eigenvalues are always real, implying that so-called “divergence” instability can occur, while “flutter” instability can never occur. It is found that a certain value of the hardening modulus exists for which specific propagation directions will always imply that all wave speeds are identical and equal to the elastic distortion wave speed. Moreover, in this situation the eigenvectors are arbitrary, corresponding to a state of diffuse wave modes. The criteria of von Mises and Rankine are used to illustrate some of the findings.

Rayleigh waves obtained by the indeterminate couple-stress theory

Rayleigh waves in a linear elastic couple-stress medium are investigated; the constitutive equations involve a length parameter l that characterizes the microstructure of the material. With cR=Rayleigh speed, cT=conventional transversal speed and q=wave number, an explicit expression is derived for the relation between cR/cT, lq and Poissons ratio ν. The Rayleigh speed turns out to be dispersive and always larger than the conventional Rayleigh speed. It is of interest that when lq=1 and ν≥0, it always holds that cR/cT=2. The displacement field is investigated and it is shown that no Rayleigh wave motions exist when lq→∞ and when lq=1, ν≥0. Moreover, a principal change of the displacement field occurs when lq passes unity. The peculiarity that no Rayleigh wave motions exist when lq=1, ν≥0 may support the criticism by Eringen (1968) against the couple-stress theory adopted here as well as in much recent literature.

Discontinuous bifurcations in a nonassociated Mohr material

Discontinuous bifurcations of an elastic-plastic material obeying Mohrs yield criterion and a nonassociated flow rule and subjected to general three-dimensional loading are investigated. The first possibility for bifurcation is identified and expressed in terms of the critical hardening modulus and the corresponding orientation of the slip planes. In the case of perfect plasticity the Mohr and Roscoe solutions for the orientation of the slip planes are derived. The corresponding strain rate fields are determined and it is shown that the Roscoe strain rate field differs significantly from the other fields. Moreover, the stress rate field is continuous across the singular surface for the Roscoe solution.

Corners in plasticity—Koiter's theory revisited

A general theory for plastic loading at corners is presented that includes Koiters theory as a special case. This theory is derived within a thermodynamic framework and includes non-associated as well as associated theory. The non-associated theory even allows the number of potential functions to differ from the number of yield functions. The properties of the matrix of plastic moduli as well as of another important matrix are discussed in detail and hardening, perfect and softening plasticity are concisely defined. The existence of limit points is also discussed. The strain driven format turns out to be the most general. Moreover, consistent loading and unloading criteria are established for general non-associated plasticity. An explicit criterion for uniqueness is derived, and finally, some of the general findings are illustrated by means of specific plasticity formulations often encountered in practice. (Less)

Viscoplasticity based on an additive split of the conjugated forces

A generalized form of viscoplasticity is proposed, within a thermodynamic concept. The theory developed is based on an expansion of the dissipation inequality, where additional quantities are introduced. These quantities are obtained by an additive split of the conjugated thermodynamic forces. Additional potential functions that depend on these quantities can be introduced, which enables one to achieve a generalized form of non-associative viscoplastic theory. Within this concept, the Duvaut-Lions formulation follows naturally as a special case and other important possibilities are also discussed. It is shown that the proposed concept can be generalized to the case where corners exist on the yield and potential functions. Finally, some specific models that all take the Tresca criterion as the yield surface are discussed and used to illustrate some of the findings.