Large Geometric Changes Research Articles

Thin shell and surface crack finite elements for simulation of combined failure modes

In this study we present a new approach to analyse cracked shell structures subjected to large geometric changes. It is based on a combination of a rectangular assumed natural deviatoric strain thin shell finite element and an improved linespring finite element. Plasticity is accounted for using stress resultants. A power law hardening model is used for shell and linespring material. A co-rotational formulation is employed to represent nonlinear geometry effects. With this, one can carry out nonlinear fracture mechanics assessments in structures that show instabilities due buckling (local/global), ovalisation and large rigid body motion. By numerical examples it is shown how geometric instabilities and fracture compete as governing failure mode.

Ab initio study of guanine tautomers in the S0 and D0 states

AbstractThe electronic and stabilization energies of the guanine six most stable tautomers were calculated in the neutral ground electronic, S0, and the cationic ground, D0, states. Calculations using ab initio MP2 method shows that guanine 9‐H keto is the most stable tautomer in the S0 state. It was found that upon ionization to the D0 state, guanine 9‐H‐enol‐trans is the most stable form. The ionization energy of the six tautomers of guanine was calculated and found to agree well with the values reported from experiments and with theoretically predicted ionization energies. The molecular structures in the S0 and D0 states were predicted and the results from geometry optimization show that all tautomers are nonplanar in the S0 state. It was predicted that the ionization of all guanine forms to the D0 state may originate from the removal of one of the electron pairs on the terminal NH2 group of the pyrimidine ring, and the other electron enhances the interaction between nitrogen atoms of the NH2 and the immediate carbon atom neighbors via hyperconjugation. These interactions force the molecule to become planar in the D0 state. The planar geometry is justified by the electron delocalization in and outside both rings. The calculations of atomic charges and dipole moments support the findings of electrons delocalization around the ring causing the geometrical changes in the S0 and D0 states of all tautomers. The calculations of the rotational constants show that the geometries are different for all tautomers, in both the S0 and D0 states, verifying the findings from atomic charge and dipole moment analysis that these tautomers suffer large geometrical changes upon ionization. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Aerodynamic and aeroelastic amplification in adaptive belt-rib airfoils

The belt-rib concept for variable-camber airfoils was developed at DLR (German Aerospace Centre) in the framework of the Adaptive Wing project (ADIF). Based on the idea of exploiting distributed structural flexibility – instead of using articulated mechanisms – in order to produce the required large geometry changes, the belt-rib concept implements a new design philosophy for light shape-adaptable structures in which the basic requirements of large allowable geometrical changes, high load-carrying capability and low weight are explicitly taken into account. Among several options for the activation of a belt-rib airfoil, the use of multifunctional materials like piezoceramics or shape memory alloys is a very attractive one, in particular as a consistent solution with the solid-state construction principle (virtual absence of moveable parts) of the structure. However, the high performance requirements (high actuator work, particularly in presence of high aerodynamic loading) are not likely to be met by multifunctional materials which are available to date. A very promising solution consists in exploiting aerodynamic and aeroelastic amplification effects in order to reduce the energy requirements of the actuator system. A study of such effects is presented in this paper, based on a modal theory of the airfoil mechanics. Due to its modal formulation, the theory can be applied to a conventional airfoil equipped with an additional control surface for camber adaptation as well as to a belt-rib airfoil, despite of the essential differences in the design philosophy. Aerodynamic amplification effects are described by means of an aerodynamic amplification factor, which is a function of the airfoil kinematics and of the aerodynamic load distribution. Quantitative results are presented for the case of the ADIF benchmark structure, the AIRBUS A340 landing flap. Further, aeroelastic amplification effects are analysed and an aeroelastic amplification factor is introduced as a function of properly chosen structural and aerodynamic operators. Some final remarks about the role of this contribution in relationship to the state of the research as well as on perspectives opened by the exploitation of the described amplification effects close the paper.

Fundamentals of a Vector Form Intrinsic Finite Element: Part I. Basic Procedure and A Plane Frame Element

AbstractIn a series of three articles, fundamentals of a vector form intrinsic finite element procedure (VFIFE) are summarized. The procedure is designed to calculate motions of a system of rigid and deformable bodies. The motion may include large rigid body motions and large geometrical changes. Newton's law, or a work principle, for particle is assumed to derive the governing equations of motion. They are obtained by using a set of deformation coordinates for the description of kinematics. A convected material frame approach is proposed to handle very large deformations. Numerical results are calculated by using an explicit algorithm. In the first article, using the plane frame element as an example, basic procedures are described. In the accompanied articles, plane solid elements, convected material frame procedures and numerical results of patch tests are given.

Large geometrical structure changes of photochromic diarylethenes upon photoirradiation

Photochromic diarylethene derivatives having phenylethynyl and pent-1-ynyl groups at the reactive carbons have been synthesized. These ethynyl groups enhance the cycloreversion quantum yields of diarylethenes without affecting the absorption maxima and the absorption coefficients of the closed-ring isomers. The derivatives exhibited large geometrical structural changes in the photoisomerization process.

On the validity of the counterpoise correction for the basis set superposition error including the fragment relaxation terms

It is demonstrated that relatively large geometrical changes make Emsley et al.'s assumption (J Am Chem Soc (1978) 100:3303) on the counterpoise correction for the basis set superposition error including the fragment relaxation terms unacceptable.

Structural Analysis of Host–Guest Systems. Methyl-substituted Phenols in beta;-Cyclodextrin

Complexation trajectories and the variation ofinduced circular dichroism are calculated for thedocking of phenol and 2,4,6-trimethyl-phenol withβ-cyclodextrin. The results are compared toexperimental chirality data to elucidate themechanism of nonspecific molecular recognitionprocesses in aqueous solution. Large geometricalchanges along nearly isoenergetic Dynamic Monte Carlotrajectories show the conformational flexibility ofsuch host–guest systems. This proves diffuseintermolecular interactions, van der Waals orelectrostatic in nature, as the main contributions to thebinding energy. The number and position of the methylsubstituents of the guest reduces the complexityof the conformational space as the guest‘s positionbecomes fixed by steric constraints. The solvation free energy is calculated from thesolvent accessible surface area weighted byrespective atomic solvation parameters. Consideringthe solvation term in the dynamic simulationsrestricts the conformational flexibility of themacromolecular system. The relative importance ofvarious contributions to the solvation energy isdiscussed and it is shown that those terms arisingfrom the interaction of hydrophobic groups with theaqueous environment are essential for thedetermination of the complex structure. Consideringthese terms in the dynamic simulation model, the signand strength of the calculated rotatory strength isin perfect agreement with induced circular dichroismobtained from experimentally determined averagedspectra. The results demonstrate the accuracy of thegeometrical properties of host–guest systems obtainedfrom these simulations.

Three dimensional forging simulation with finite element control

The three dimensional rigid-plastic code NSÆC (Nonlinear Solid Analysis by Element Control) with new ideas has been developed and utilized to forging simulation. New robust finite element scheme has been used to describe the materials incompressible behaviors and to trace large geometrical changes. The mixed type variational principle has been employed with penalty function method. The finite element control by direct tessellation algorithm has been used for generation of the initial model and has been proved promising for adaptive remeshing and Arbitrary Lagrangian-Eulerian geometry control.

Non-singular stress and velocity fields for crack-tip plastic zones and conditions for uniqueness

Non-singular plastic stress and velocity fields, for the tip of a crack of finite thickness and root radius, are developed as an elastic-plastic crack model that is likely to be more physically realistic than the classical infinitesimal crack with a plastic crack-tip singularity. With a non-singular plastic zone the velocity-field equations are not uniquely determined by the boundary conditions, under large geometrical changes, and they must therefore have the form of a wide set of kinematically-admissible velocity fields. These virtual velocity fields are used to establish the critical work-hardening rate to give a sufficient condition for uniqueness of the crack-tip velocity field in elastic-plastic fracture; it is shown that proof of uniqueness of the velocity field is likely to be an essential requirement for the valid application of elastic-plastic fracture mechanics. The elastic infinitesimal-crack model is shown to give an inadequate representation of the ‘circumferential’ T-stress distribution at the surface of a crack of finite root radius, and this requires the adoption of a finite-thickness elliptical crack model to give approximate consistency between the elastic stress field and the non-singular plastic stress field at the crack tip.

An explicit finite element formulation for very large deformations based on updated material reference frame

An explicit finite element formulation which has the capability of handling very large geometrical changes is developed. The formulation is based on an updated material reference frame and hence a true stress-strain test can be directly applied to properly characterize properties of materials which are subjected to very large deformations. For the large deformations, a consistent formulation based on the continuum mechanics approach is derived. The kinematics is referred to an updated material frame. Body equillibrium is also established based on the updated geometry. The second Piola-Kirchhoff stress tensor and the updated Lagrangian strain tensor are used in the formulation. Numerical examples of frame structures are solved to validate the presented formulation and to demonstrate the importance of imposing material properties properly.

Origin of dipole moment enhancement in the formation of SiF4–NH3 dimer

The dimerization of SiF4 and NH3 is accompanied by a substantial 1.63 a.u. increase in dipole moment. Theoretical analysis of the charge distribution of the SiF4–NH3 system using the theory of atoms in molecules shows that the origin of this increase is primarily the large geometrical change which occurs within the SiF4 molecule when the dimer is formed, rather than the changes in atomic polarization or interatomic or intermolecular electron transfer. The relative displacement of the highly charged fluorine atoms of SiF4 away from NH3 upon dimer formation accounts for nearly 90% of the dipole moment enhancement. This result is in agreement with predictions made recently based on experimental results.

Plane punch indentation of a ductile honeycomb

The stresses and deformations in an anisotropic half-space indented by a flat punch are calculated for material parameters that characterize a transversely crushed ductile honeycomb. After a small range of elastic deformations, the honeycomb crushes in localized bands that extend in characteristic directions within the half-space. Large geometric changes in the crushed cells modify their mechanical properties and constrain the deformation of adjacent cells. A lower estimate of the indentation force is obtained by comparing the peak stress beneath the indenter with the yield strength of the honeycomb. An upper estimate of the indentation force is calculated by equating the work-rate of indentation with the dissipation-rate of a flow-field that is compatible with the crushing bands.

An iterative franck-condon analysis procedure for the determination of ionic geometries for H 2O and H 2S

The pattern search optimization procedure is applied to a Franck-Condon analysis method with the use of the formulation proposed by Kupka and Cribb for evaluating vibrational overlap integrals [H. Kupka and P.H. Cribb, J. Chem. Phys., 85 (1986) 1303; H. Kupka and G. Olbrich, J. Chem. Phys., 80 (1985) 3163]. The method was applied to three different ionization processes accompanied by small to large changes in molecular geometry. It was found that ionic geometries thus deduced for transitions with small structural changes agree well with observation. However, the agreement is poor for transitions with large geometric changes although the method can still account for the observed vibrational intensity distributions to a certain extent. In addition, properties of the pattern search method as well as Kupka and Cribb's formulation for Franck-Condon analyses are discussed.

Plastic deformation of a polygon

A closed-form solution is derived for the large plastic deformation of a polygonal frame loaded by a uniform internal pressure. The material of the frame is assumed to be rigid, perfectly plastic, and the effects of large geometric changes are included. A comparison is made between theoretical results and available experimental data for a pressurized hexagonal tube.

ChemInform Abstract: SPECTROSCOPY OF THE BENZIL‐BIBENZYL MIXED CRYSTAL SYSTEM: HIGH RESOLUTION OPTICAL STUDIES

The S0→S1 excitation and T1→S0 emission spectra of a benzil doped bibenzyl mixed crystal at temperatures ranging from 300 to 2 K have been measured. Two kinds of spectra have been found and assigned to the benzil molecule trapped in two different molecular configurations. One of the observed sets of spectra is nearly identical to those of the benzil single crystal. The other, red shifted with respect to those of the benzil single crystal, is similar to the spectra of systems known to contain a trans‐planar 1,2‐dicarbonyl fragment. Such a geometry change is consistent with the manner in which the benzil molecule would most probably substitute into the bibenzyl lattice. The red shifted spectra are broad, beyond the extremely sharp 0–0, and this broadness is attributed to the strong excitation–phonon coupling expected for molecules that undergo large geometrical changes upon photoexcitation.

Suspended-sediment transport in the Seine estuary, France: Effect of man-made modifications on estuary—shelf sedimentology

Man-made modifications of the Seine estuary have produced large geometric changes with important hydrographic and sedimentological consequences affecting estuary—shelf interrelationships. The estuary once had shallow, meandering ebb and flood tidal channels and broad tidal flats. Today, after 130 years of landfill, marsh reclamation, jettying and dredging, there is only a narrow, deep, jettied channel extending to the sea. Most of the tidal flats and marshes have been filled. These modifications have concentrated river discharge and ebb flow into a narrow jettied channel which, in turn, has produced a seaward migration of the salt intrusion and the turbidity maximum. As the estuary volume was reduced, river flow dilution was also reduced and its mean residence time in the estuary was shortened. At the same time, sedimentation and landfill have reduced the area available for sedimentation of fluvial sediments. This change has amplified seaward transport of fluvial suspended sediment and increased the size of shelf mud zones off the estuary mouth. Man has thus changed the geologic role of the estuary from a sink for fluvial and marine sediment, to a source of fluvial sediment for the shelf.

Spectroscopy of the benzil–bibenzyl mixed crystal system: High resolution optical studies

The S0→S1 excitation and T1→S0 emission spectra of a benzil doped bibenzyl mixed crystal at temperatures ranging from 300 to 2 K have been measured. Two kinds of spectra have been found and assigned to the benzil molecule trapped in two different molecular configurations. One of the observed sets of spectra is nearly identical to those of the benzil single crystal. The other, red shifted with respect to those of the benzil single crystal, is similar to the spectra of systems known to contain a trans-planar 1,2-dicarbonyl fragment. Such a geometry change is consistent with the manner in which the benzil molecule would most probably substitute into the bibenzyl lattice. The red shifted spectra are broad, beyond the extremely sharp 0–0, and this broadness is attributed to the strong excitation–phonon coupling expected for molecules that undergo large geometrical changes upon photoexcitation.