A Constitutive Model of Shape Memory Alloys Based on Viscoplastic like Evolution Equations

The paper presents a new model that allows the simulation of the macroscopic thermomechanical behavior of shape memory alloys (SMA). In this paper we present a description of the austenite-martensite phase transition, which takes into account the two types of martensite by introducing suitable internal variables: the volume fractions of the self-accommodating product phase (pure thermal effect) and the oriented (stress induced) product phase. The evolution equations of the internal variables are similar to the evolution equation in viscoplasticity. Differing from previous models, the presented one is able to model the behaviour of the SMA under combined thermal and mechanical loads. A numerical example is given that illustrates the ability of the model to capture the thermomechanical behavior of shape memory alloys under temperature change and proportional loadings

Comparison of XFEM and Voxelbased FEM for the Approximation of Discontinuous Stress and Strain at Material Interfaces

When analyzing bimaterial problems, often stress concentrations appear close to material interfaces and cause nonlinear effects like damage or plastic yielding. Therefore, a precise approximation of stresses at interfaces is desirable. However, jumps in material parameters lead to non-smooth solutions (i.e. kinks in the displacements and jumps in the stresses and strains), which reduce the accuracy of standard discretization methods including the finite element method (FEM). The popular extended finite element method (XFEM) belongs to the class of methods, where additional unknowns are introduced to approximate jumps of the unknowns on meshes which are not aligned to the interface or boundaries. The field of application is the stress analysis of microstructures of two-phase materials with high contrast between the phases including nonlinear effects. The focus of this paper is to considers the XFEM approximation quality of strains and displacements at material interfaces. XFEM and standard FEM solutions are compared for varying mesh sizes. Therefore an analytical solution for a spherical inclusion in a finite domain under specific loading conditions is considered. This analytical solution is then compared in detail to different finite element discretizations with standard FEM and XFEM

Parallel software tool for decomposing and meshing of 3d structures

An algorithm for automatic parallel generation of three-dimensional unstructured computational meshes based on geometrical domain decomposition is proposed in this paper. Software package build upon proposed algorithm is described. Several practical examples of mesh generation on multiprocessor computational systems are given. It is shown that developed parallel algorithm enables us to reduce mesh generation time significantly (dozens of times). Moreover, it easily produces meshes with number of elements of order 5 · 107, construction of those on a single CPU is problematic. Questions of time consumption, efficiency of computations and quality of generated meshes are also considered

Multi-Scale Simulation of Viscoelastic Fiber-Reinforced Composites

This paper presents an effective algorithm to simulate the anisotropic viscoelastic behavior of a fiber-reinforced composite including the influence of the local geometric properties, like fiber-orientation and volume fraction. The considered composites consist of a viscoelastic matrix which is reinforced by elastic fibers. The viscoelastic composite behavior results anisotropic due to the local anisotropic fiber-orientations. The influence of the local time-dependent viscoelastic properties are captured within two elastic microscopic calculations for each fiberorientation in the composite part. These calculations can be performed within a preprocessing step, and thus no expensive, time-dependent viscoelastic multi-scale simulation has to be carried out to incorporate the local properties. The advantage of the presented approach is that the locally varying microscopic properties can be captured in a one-scale simulation within a commercial finite element tool like ABAQUS

A room temperature 19-channel magnetic field mapping device for cardiac signals

We present a multichannel cardiac magnetic field imaging system built in Fribourg from optical double-resonance Cs vapor magnetometers. It consists of 25 individual sensors designed to record magnetic field maps of the beating human heart by simultaneous measurements on a grid of 19 points over the chest. The system is operated as an array of second order gradiometers using sophisticated digitally controlled feedback loops.Comment: 3 pages, 3 figures, submitted to Applied Physics Letter

Interactions of Ar(9+) and metastable Ar(8+) with a Si(100) surface at velocities near the image acceleration limit

Auger LMM spectra and preliminary model simulations of Ar(9+) and metastable Ar(8+) ions interacting with a clean monocrystalline n-doped Si(100) surface are presented. By varying the experimental parameters, several yet undiscovered spectroscopic features have been observed providing valuable hints for the development of an adequate interaction model. On our apparatus the ion beam energy can be lowered to almost mere image charge attraction. High data acquisition rates could still be maintained yielding an unprecedented statistical quality of the Auger spectra.Comment: 34 pages, 11 figures, http://pikp28.uni-muenster.de/~ducree

Domain Dynamics of Magnetic Films with Perpendicular Anisotropy

We study the magnetic properties of nanoscale magnetic films with large perpendicular anisotropy comparing polarization microscopy measurements on Co_28Pt_72 alloy samples based on the magneto-optical Kerr effect with Monte Carlo simulations of a corresponding micromagnetic model. We focus on the understanding of the dynamics especially the temperature and field dependence of the magnetisation reversal process. The experimental and simulational results for hysteresis, the reversal mechanism, domain configurations during the reversal, and the time dependence of the magnetisation are in very good qualitative agreement. The results for the field and temperature dependence of the domain wall velocity suggest that for thin films the hysteresis can be described as a depinning transition of the domain walls rounded by thermal activation for finite temperatures.Comment: 7 pages Latex, Postscript figures included, accepted for publication in Phys.Rev.B, also availible at: http://www.thp.Uni-Duisburg.DE/Publikationen/Publist_Us_R.htm

Alternative shear reinforcement for reinforced concrete flat slabs

This paper presents the first series of validation tests for a patented shear reinforcement system for reinforced concrete flat slabs. The system, called “Shearband,” consists of elongated thin steel strips punched with holes, which undulate into the slab from the top surface. The main advantages of the new reinforcement system are structural effectiveness, flexibility, simplicity, and speed of construction. Four reinforced concrete slabs were tested in a specially designed test rig. The slabs reinforced in shear exhibited ductile behavior after achieving their full flexural potential, thus proving the effectiveness of the new reinforcement. This paper reviews briefly existing types of shear reinforcement and identifies the need for more efficient and economic solutions. Details of the experimental setup and results are given, including strain and deflection measurements as well as photographs of sections through the slabs. Finally, comparisons are made with the ACI 318 and BS8110 code predictions, which confirm that the system enabled the slabs to avoid punching shear failure and achieve their flexural potential. In addition, both codes are shown to lead to conservative estimates of flexural and punching shear capacities of reinforced concrete slabs

One- and two-photon resonant spectroscopy of hydrogen and anti-hydrogen atoms in external electric fields

The resonant spectra of hydrogen and anti-hydrogen atoms in the presence of an external electric field are compared theoretically. It is shown that nonresonant corrections to the transition frequency contain terms linear in the electric field. The existence of these terms does not violate space and time parity and leads to a difference in the resonant spectroscopic measurements for hydrogen and anti-hydrogen atoms in an external electric field. The one-photon 1s-2p and the two-photon 1s-2s resonances are investigated