Multiporoelasticity of Hierarchically Structured Materials: Micromechanical Foundations and Application to Bone

We here extend the theory of microporomechanics by Dormieux et al. to multiple pore spaces. As an application, we reveal, on the basis of a recently validated multiscale elastic model for bone tissues by Fritsch and Hellmich, the effects of multiple pore pressures in various, scale-separated pore spaces, on the overall behavior of the multiporous composite material. Thereby, our focus is on the lacunar pore space, and on its interplay with the pore spaces found further below: not only those between the mineral crystals (of some 10?nm characteristic pore size) but also those of the collagen molecules building up (micro-)fibrils (with a little more than 1?nm distance between these molecules). Our results clearly show that the interplay between pore pressure and skeleton deformation depends strongly on the loading direction and on the characteristic size of the pores—hence, we can conclude that the consideration of these strongly hierarchical and anisotropic effects in whole-organ simulations including fluid mass transport, would allow for valuable new insights into the ongoing discussion on poromechanobiology of bone.     

Consistent linearization in Finite Element analysis of coupled chemo-thermal problems with exo- or endothermal reactions

 In this work, after a short review of the respective thermodynamic formulation, the algorithmic treatment of coupled chemo-thermal problems with exo- or endothermal reactions is addressed. The Finite Element Method (FEM) is serving as the analysis tool. Consistent linearization of the discretized evolution equations results in quadratic convergence of the global Newton-Raphson equilibrium iteration. This renders solutions of practical engineering problems feasible. The range of these problems encompasses the early age behavior of concrete as well as agricultural applications. In order to demonstrate the applicability of the presented material law, a 3D material test for shotcrete is re-analyzed. Received 24 January 1998     

Chemoporoplasticity of Calcium Leaching in Concrete

This paper presents a macroscopic material model for calcium leaching in concrete, for the quantitative assessment, in time and space, of the aging kinetics and load bearing capacity of concrete structures subjected to severe chemical degradation (such as radioactive waste disposal applications). Set within the framework of chemically reactive porous continua, the model accounts explicitly for the leaching of calcium of portlandite crystals and C-S-H, and its cross-effects with the elastic deformation (chemical damage) and irreversible skeleton deformations (chemical softening) treated within the theory of chemoplasticity. In the first part of this paper the governing equations are derived focusing on the chemomechanical couplings between calcium dissolution, increase in porosity, and deformation and (micro-) cracking of concrete. Without any a priori assumption concerning local equilibrium between the solid calcium concentration s and the interstitial calcium concentration c the well-known calcium leaching state function s = s(c) is then derived using combined thermodynamic equilibrium and dimensional arguments relating to the structural dimension of containment structures. In the second part, this paper addresses the experimental determination of chemical damage and chemical softening of the calcium leaching. For chemical damage, a simple mixture rule involving different skeleton constituents suffices to capture the main chemoelastic features of leaching; in turn, microhardness measurements allow access to the chemical softening state function capturing chemoplastic cross-effects. The intrinsic nature of these functions, and of the proposed procedure, is validated by means of finite-element analysis of experimental compression tests of a degraded specimen with nonhomogeneous chemical degradation states.     

Effect of Inclusions on Friction Coefficient of Highly Filled Composite Materials

Highly filled composite material systems exhibit, in triaxial compression, a composite strength that is greater than either the weaker particulate or matrix strength. This is due to an amplification of the local confinement in the matrix activating frictional mechanism. The paper quantitatively addresses this increase of the friction coefficient of a matrix reinforced by rigid inclusions using assorted means of nonlinear micromechanics. The approach is based on a nonlinear elastic representation of a Drucker–Prager type frictional strength behavior of the matrix at failure. The key to success of the homogenization procedure relies on the appropriate definition of effective strains in the matrix, to capture local confinement effects and shear effects in the connected matrix phase. It is shown that an effective strain concept based on linear volume averaging (i.e., classical secant method) leads to overestimate the inclusion effects; while an effective strain concept based on quadratic volume averaging (i.e., modified secant method) provides a more realistic representation of shear strains and local confinement effects that develop in triaxial compression in the matrix. Finally, a combination of these two methods leads to a mixed secant method, which gives a relative friction increase of (volume fraction fI). This estimate accurately predicts the experimentally observed frictional behavior of unleached and leached cement-based mortars, composed of a cement paste matrix and rigid sand inclusions.     

Residual design strength of cement-based materials for nuclear waste storage systems

Calcium leaching of concrete may be critical for the mechanical integrity of hazardous waste storage systems, in which cement-based materials are employed as construction material or grouting material. Results from a recent triaxial strength test campaign on calcium depleted cementitious materials highlight that these materials exhibit decohesion and frictional softening and become increasingly pressure sensitive. But the overall emerging picture is one of a material with a residual, finite accountable strength, which is well above either the particulate or the matrix strength. This allows us to propose a ‘safe’ lower bound for calcium depleted cement-based composites, which may serve for the durability performance design of concrete structures subjected to calcium leaching-one critical design scenario of e.g. nuclear waste storage systems.     

Spatially resolved photoelectric performance of axial GaAs nanowire pn-diodes

The spatially resolved photoelectric response of a single axial GaAs nanowire pn-diode has been investigated with scanning photocurrent and Kelvin probe force microscopy. Optical generation of carriers at the pn-junction has been shown to dominate the photoresponse. A photocurrent of 88 pA, an open circuit voltage of 0.56 V and a fill factor of 69% were obtained under AM 1.5 G conditions. The photocurrent followed the increasing photoexcitation with 0.24 A/W up to an illumination density of at least 90 W/cm², which is important for potential applications in concentrator solar cells.     

Mechanical characterization of integral aluminum-FRP-structures produced by high pressure die-casting

Due to the growing demand for light-weight solutions in a wide range of industrial sectors, the selection and combination of different materials is becoming more and more important. As a result, there is an increasing need for suitable joining technologies. In a new joining process, flexible glass fiber textiles are integrated into aluminum by high pressure die casting in the first production step. These structures are used for the electrochemical insulation between aluminum and carbon fiber textiles, which are connected in the subsequent production step by textile technology. The finished compound is formed in a final resin impregnation process. Challenges faced by Fraunhofer IFAM lie in the positioning, pre-tensioning, and infiltration of the glass fiber textiles in the high pressure die-casting process. The advantage of this joining technology, in addition to the electrochemical insulation between aluminum and carbon fibers, is in a slim and light-weight connection. Therefore, no thickening of the individual joining partners is necessary, and the force flow lines are not deflected. Within mechanical investigations of those hybrid structures it was determined, that the infiltration content of aluminum has only a small influence on the achievable tensile strength. Rather, casting parameters such as the holding pressure have an influence. The subsequent resin infusion process enables an additional infiltration by the resin system of fiber bundles that have been only slightly infiltrated with aluminum. As a result, additional adhesion can be achieved and the infiltration gaps can be closed. Furthermore, an influence on the achievable tensile strength was observed regarding the use of the fiber material. Further increases in tensile strengths were also observed by adapting the textile parameters (e.g. reduction of the fiber undulations). A variety of failure behaviors could be observed in dependence on textile and process parameters. Tensile strength of the hybrid structures was compared to reference samples made of glass fiber reinforced epoxy resin, to determine the loss of strength caused by the joining technology. Further investigations were carried out, including a fracture surface analysis using a scanning electron microscope. Thus it was possible to determine mechanisms of adhesion between encapsulated glass fibers and the surrounding aluminum matrix.     

Assessment of Tractable Cysteines for Covalent Targeting by Screening Covalent Fragments

Targeted covalent inhibition and the use of irreversible chemical probes are important strategies in chemical biology and drug discovery. To date, the availability and reactivity of cysteine residues amenable for covalent targeting have been evaluated by proteomic and computational tools. Herein, we present a toolbox of fragments containing a 3,5-bis(trifluoromethyl)phenyl core that was equipped with chemically diverse electrophilic warheads showing a range of reactivities. We characterized the library members for their reactivity, aqueous stability and specificity for nucleophilic amino acids. By screening this library against a set of enzymes amenable for covalent inhibition, we showed that this approach experimentally characterized the accessibility and reactivity of targeted cysteines. Interesting covalent fragment hits were obtained for all investigated cysteine-containing enzymes.