Charakterisierung von Randentkohlungsvorgängen bei der Austenitisierung des Wälzlagerstahls 100Cr6. Teil 2: Modellierung des Kohlenstoff‐Tiefenverlaufs mit Hilfe der Methode der Finiten Elemente

Characterization of Decarburisation Processes During Austenitising of the Rolling Bearing Steel 100Cr6. Part 2: Modelling of the Carbon Concentration Profile by Means of the Finite Element Method The quantitative measurement of carbon concentration‐distance curves serves as fundamental prerequisite for the evaluation of rim zone properties connected with decarburisation processes in material science. This was shown in part 1 of the present work with two samples from through‐hardenable rolling bearing steel 100Cr6 (SAE 52100) austenitised in different oxidising atmospheres by position dependent determination of hardness, residual stresses, and X‐ray line broadening ({211} α’‐Fe diffraction line). In practice, it is important to predict carbon concentration‐distance curves under prevailing heat treatment conditions or to conclude conversely from profile measurements. Based on a refined kinetics model of a diffusion‐controlled process, part 2 therefore presents a simulation tool developed by means of the finite element method (FEM). Apart from the concentration dependence of the diffusion coefficient, it also considers the decarburisation induced austenite‐ferrite phase transformation, the time dependent influence of scaling, and variable atmosphere conditions. The interpretation of the carbon concentration‐distance curves, measured very accurately in the rim zone of both 100Cr6 samples by secondary ion mass spectrometry (SIMS), confirms the possibilities of application of the new numerical tool.     

Compression modeling of plain weave textile fabric using finite elements

Textile composite are used extensively in aerospace as they offer a 3D reinforcement in a single layer providing better mechanical properties in both in‐plane and transverse directions. This paper reports on the mechanical behavior of a plain weave textile fabric under the compressive loading. Unit cell geometry of the plain weave fabric structure is identified and its model is created using TexGen geometric modeling scheme developed by the University of Nottingham (U.K.). Later on its mechanical behavior is predicted using finite element modeling (FEM) based simulation software ABAQUS^® incorporating a transversely isotropic material law. Strain energy of the developed model has been compared with that of the published results and shows very good agreement. The analysis indicated that transverse‐longitudinal shear (TLS) modulus plays an important role in characterizing the behavior of the woven fabric under compression, while the friction between the yarns and longitudinal stiffness has less significant influence on compaction behavior. In order to ascertain the effectiveness of the developed model, exhaustive parametric studies have also been conducted to investigate the effect of transverse‐longitudinal shear modulus on some of the important parameters such as artificial strain energy, external work, frictional dissipation, internal energy, kinetic energy, strain energy and total energy of the model. The developed model has the capacity to predict and simulate the behavior of variety of fabric architectures based on their constituent yarn properties under various regimes of service loads.     

A new finite element technology for shape memory alloy composites

In this paper, we present a computationally efficient implementation of a continuum mechanical model for shape memory alloys into a finite element code. The model covers several thermomechanically coupled effects typical for the material behaviour of shape memory alloys, e.g. pseudo‐elasticity, the one‐way shape memory effect and the two‐way shape memory effect due to external loads. Via the use of a finite element formulation based on only one Gauss point, the computational effort is reduced enormously.     

Multiscale finite element modeling of superelasticity in Nitinol polycrystals

An efficient method is proposed for modeling superelastic polycrystalline NiTi by solving a two-scale problem. The RVE size of the fine scale is determined using a statistics-based approach. Both problems are discretized in space using the finite element method and their communication is effected using MPI. Representative simulations illustrate the modeling capabilities of the proposed approach.     

Neutron Stress Measurement of W‐Fiber Reinforced Cu Composite

Stress measurement methods using neutron and X‐ray diffraction were examined by comparing the surface stresses with internal stresses in the continuous tungsten‐fiber reinforced copper‐matrix composite. Surface stresses were measured by X‐ray stress measurement with the sin²ψ method. Furthermore, the sin²ψ method and the most common triaxal measurement method using Hooke's equation were employed for internal stress measurement by neutron diffraction. On the other hand, microstress distributions developed by the difference in the thermal expansion coefficients between these two phases were calculated by FEM. The weighted average strains and stresses were compared with the experimental results. The FEM results agreed with the experimental results qualitatively and confirmed the importance of the triaxial stress analysis in the neutron stress measurement.     

Simulation von Schädigungs‐ und Kriechvorgängen im Asphalt

Simulation of damage and creep processes in asphalt There is an increasing need for the use of numerical methods in road design. Here, the main difficulty lies in modeling the highly complex, nonlinear material behaviour of asphalt. The present paper deals with this problem and proposes a continuum mechanics model serving to simulate the main nonlinear mechanical processes taking place in asphalt. Plastic deformation, damage and crack development as well as viscous processes are treated. The model is implemented into the finite element program Abaqus and is used to simulate tests on asphalt specimens.     

Comparison between RVE and full mesh approaches for the simulation of compression tests on cellular metals

Metal foams are materials of recent development and application that show interesting combinations of physical and mechanical properties. Many applications are envisaged for such materials, particularly in equipments of passive safety, because of their high capacity of energy absorption under impact conditions. The damage analysis in metallic foams is a complex problem and must be performed in a finite strain context. Considering that compression is the dominant loading in impact situations, a finite deformation simulation including damage effects of a compression test on a cellular metal sample is shown in this work. The main objective of the paper is to compare simulations considering periodic boundary conditions, by means of a representative volume element (RVE) approach, with results obtained using full meshes. It is shown that, when the imposed deformation is high, the use of RVE does not describe in a proper manner the deformation that occurs at the walls of cells. This characteristic of RVE approach results in a too stiff behavior when considering load‐displacement relations. A comparison with experimental results is also presented.     

Adaptive Finite Element Simulations for Macroscopic and Mesoscopic Models of Steel

During heat treatment and other production processes, gradients of temperature and other observables may vary rapidly in narrow regions, while in other parts of the workpiece the behaviour of these quantities is quite smooth. Nevertheless, it is important to capture these fine structures during numerical simulations. Local mesh refinement in these regions is needed in order to resolve the behaviour in a sufficient way. On the other hand, these regions of special interest are changing during the process, making it necessary to move also the regions of refined meshes. Adaptive finite element methods present a tool to automatically give criteria for a local mesh refinement, based on the computed solution (and not only on a priori knowledge of an expected behaviour). We present examples from heat treatment of steel, including phase transitions with transformation induced plasticity and stress dependent phase transformations. On a mesoscopic scale of grains, similar methods can be used to efficiently and accurately compute phase field models for phase transformations.     

Simulation der geometrischen Oberflächen‐ wandlung beim Drücken optischer Bauteile

Simulation of the surface roughness alteration in spinning of optical components Forming technology is often used to manufacture work pieces with sophisticated surface topology as well as part shapes. Today, the production of the required surface qualities is still based on expert knowledge and “trial and error” techniques. Thus, in this paper an approach will be shown to compute surface quality by finite element analysis.     

Sythhesis of coral‐like and spherical nanoparticles of barium titanate using factorial and Taguchi experimental design

High purity barium titanate (BaTiO₃) nanoparticle and its coral like architectures were synthesized in a laboratory scale from BaO and TiCl₄ by microwave hydrothermal processing using a mixture of 0.1:2 volume ratio of Triethanalamine and distilled deionized water. The products were characterized by XRD, LLS, SEM and wet chemical analysis. The size of barium titanate particles was in the range of 5–25 nm with average particle size of 15.5 nm. The two dimensional coral – like product had the diameter of (3–18 nm and length of 140–420 nm). The experiments were designed to fulfill the 2³ factorial and L₄ Taguchi designs. Data analysis was performed using MINITAB software.     

Finite element analyses for design evaluation of biodegradable magnesium alloy stents in arterial vessels

Biodegradable magnesium alloy stents (MAS) can provide a great benefit for diseased vessels and avoid the long-term incompatible interactions between vessels and permanent stent platforms. However, the existing MAS showed insufficient scaffolding to the target vessels due to short degradation time. In this study, a three dimensional finite element model combined with a degradable material model of AZ31 (Al 0.03, Zn 0.01, Mn 0.002 and Mg balance, mass percentage) was applied to three different MAS designs including an already implanted stent (Stent A), an optimized design (Stent B) and a patented stent design (Stent C). One ring of each design was implanted through a simulation in a vessel model then degraded with the changing interaction between outer stent surface and the vessel. Results showed that a proper stent design (Stent B) can lead to an increase of nearly 120% in half normalized recoil time of the vessel compared to the Stent A; moreover, the expectation that the MAS design, with more mass and optimized mechanical properties, can increase scaffolding time was verified numerically. The Stent C has more materials than Stent B; however, it only increased the half normalized recoil time of the vessel by nearly 50% compared to the Stent A because of much higher stress concentration than that of Stent B. The 3D model can provide a convenient design and testing tool for novel magnesium alloy stents.     

The investigation of effect of adherend thickness on scarf lap joints

In this paper, the mechanical behavior of the Scarf Lap Joints (SLJs) bonded with adhesive under a tensile load was analyzed. The effects of adherend thickness at the interface stress‐strain distributions of SLJs were examined. The stress‐strain analyses were performed by Finite Element Method (3D‐FEM). The 3D‐FEM code was employed with Ansys (Ver.12.0.1). Experimental results were compared with the 3D‐FEM results and were found quite reasonable. It was concluded that both experimental and 3D‐FEM failure loads were increased with increased adherend thickness. The results indicated that the maximum failure loads were determined at t=8 mm in all joints. The analysis of the SLJs under tensile load showed that the stress and strain concentrations occurred around the edges of the joints.     

Development and characterization of active nanocomposites with embedded shape memory alloy wires

Active nanocomposites of epoxy resin containing bentonite clay and shape memory alloy (SMA) were made to evaluate the thermomechanical behavior in the range of phase transformation of shape memory alloy during heating. The epoxy resin system studied was prepared using bifunctional diglycidyl ether of bisphenol A (DGEBA), crosslinking agent diaminodiphenylsulfone (DDS), purified bentonite organoclay (APOC) and thin Ni‐Ti shape memory alloy wires. The evaluated ratio DGEBA/DDS was 100:40, for the epoxy resin/clay system was 100:1 and the shape memory alloy volumetric fraction of Ni‐Ti wires were 1.55%; 2.56%; 3.57% and 4.54%. The formation of nanocomposite was confirmed by X‐ray diffraction analysis. Phase transformation of the shape memory alloy wires were determined by differential scanning calorimetry (DSC). Specimens of the active nanocomposites were characterized mainly by dynamic mechanical analysis (DMA). According to the DMA results was evidenced a significant increase in glass transition temperature and storage modulus when 1 parts per hundred resin of clay is added to epoxy resin. A recover of storage modulus was observed in the active nanocomposite during heating in the range of the phase transformation of Ni‐Ti shape memory alloy wires when the volumetric fraction is above 3.5%.     

A new approach for the simulation of skeletal muscles using the tool of statistical mechanics

The structure of a skeletal muscle is dominated by its hierarchical architecture in which thousands of muscle fibres are arranged within a connective tissue network. The single muscle fibres consist of many force‐producing cells, known as sarcomeres. These micro biological engines are part of a motor unit and contribute to the contraction of the whole muscle. There are a lot of questions concerning the optimisation of muscle strength and agility. Standard experimental investigations are not sufficient to answer these questions because they do not supply enough information. Additionally, these methods are limited because not enough material for testing is accessible. To overcome these problems, numerical testing tools can be an adequate alternative. From the mechanical point of view the material behaviour of muscles is highly non‐linear. They undergo large deformations in space, thereby changing their shape significantly, so that geometrical nonlinearity has to be considered. Many authors use continuum‐based approaches in combination with the finite element method to describe such material behaviour. However, models of this kind require realistic constitutive relations between stress and strain which are difficult to determine in an inhomogeneous material. Furthermore, biomechanical information cannot be fully exploited in these models. The present approach is crucially based on the use of the finite element method. The material behaviour of the muscle is split into a so‐called active and a passive part. To describe the passive part special unit cells consisting of one tetrahedral element and six truss elements have been derived. Embedded into these unit cells are further truss elements which represent bundles of muscle fibres. Depending on the discretisation it is possible to simulate the material behaviour of e.g. artery walls characterised by oriented fibres or soft tissue including only non‐oriented fibres. In summary, the present concept has the advantage that a three‐dimensional model is developed which allows us take into account many physiological processes at the micro level.     

Experimentelle und numerische Untersuchungen zur Kennzeichnung von Sinterteilen mittels gezielt eingebrachter Fremdpartikel

Experimental und numerical investigation on the marking of sintered parts by the targeted application of particles Sintered parts from steel as well as light metal alloys are becoming popular in many industrial sectors due to their advantageous product properties. Beside the good utilization of raw material and the variability in alloying the sinter technology offers the opportunity to mark parts in a unique and invisible manner. Spheric particles are used as information carrier. A quadratic 4x4 matrix with a maximum of 16 particles is placed inside a sinter part for first experiments. The defined places and number of particles result in an identification number. After sintering process the two materials are inseparably connected. The readout of the data is realized with x‐ray or computer tomography due to different physical properties of the particles and the powder material.     

Mechanical property analysis of Nitinol defective stent under uniaxial loading/unloading

The vascular stents are important medical devices introduced into a vessel to protect the lumen from unfriendly stenosis. However, Nitinol stent is easy to fail in practice. The present paper focuses on the influence of defects on its mechanical behavior by finite element analysis. The essential stent cell is used with two different type of defects, which includes the face bulge defect resulted from laser burrs or TiC inclusion arises and C‐contained particle voids. Auricchio’s super‐elastic consititutive equations are used in the finite element simulations. It is found that the stress distribution is not only related to the defect type but also to the defect position. With the increasing distance from the TiC defect to the knot’s notch, the influence of defects on stress distribution of stents becomes small. For void defects, those near both the inner fillet and the outer fillet have grand influence on the stent’s global stress distribution. In particular, the higher stress concentrations are undergone near knot’s defects. For all models, the maximum Martensite Volume Fraction is near knots. The finite element analysis shows that cracks/fractures can easily appear near knots. A stent with a TiC inclusion or void defect is likely to fail. All obtained conclusions can be useful to design against stent premature mechanical failure.     

Finite element analysis of quasi‐static indentation of woven fabric textile composites using different nose shape indenters

The finite element FE analysis of quasi‐static indentation event of various nose shape rigid indenters into woven fabric composite with carbon fiber as reinforcement has been performed and discussed in detail. It was found that indenter nose shape has large influence in terms of absorbed energy, indentation at failure and damage area. The FE software, ABAQUS^® was employed to simulate quasi‐static response of woven composite unit cell. Exhaustive parametric studies have been conducted with an aim to analyze the effect of change in indenter geometry on the indentation response of the woven composite unit cell. The developed FE model for the purpose of validation was compared with available experimental results and was found to be in reasonably good agreement. The failure morphologies, damage shape and damage size were evaluated, compared and deeply discussed for different nose shape indenters. Largest damaged areas were observed for flat and truncated indenters while the smallest for the conical one.