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Influence of Heat-Treatment of Selective Laser Melting Products - e.g. Ti6Al4V
Usually additive manufactured metal parts are showing a different mechanical behavior compared to
conventionally produced parts used the same material. Apart from process-related macroscopic part
imperfections (pores, surface roughness, etc.) the microstructure has a decisive influence on the mechanical
properties of the materials. Thus, in order to optimize mechanical properties of metal parts a heat treatment for
changing microstructures is routinely applied in most production lines to meet the product requirements. By
means of the Titanium alloy Ti6Al4V the optimization of the static- and the fracture mechanical behavior by
changing the microstructure with a heat treatment after the SLM process is discussed on the present work.Mechanical Engineerin
Crack propagation in fracture mechanical graded structures
The focus of manufacturing is more and more on innovative and application-oriented productsconsidering lightweight construction. Hence, especially functional graded materials come to the fore. Due to theapplication-matched functional material gradation different local demands such as absorbability, abrasion andfatigue of structures are met. However, the material gradation can also have a remarkable influence on the crackpropagation behavior. Therefore, this paper examines how the crack propagation behavior changes when acrack grows through regions which are characterized by different fracture mechanical material properties (e.g.different threshold values ?KI,th, different fracture toughness ?KIC). In particular, the emphasis of this paper ison the beginning of stable crack propagation, the crack velocity, the crack propagation direction as well as onthe occurrence of unstable crack growth under static as well as cyclic loading. In this context, the developedTSSR-concept is presented which allows the prediction of crack propagation in fracture mechanical gradedstructures considering the loading situation (Mode I, Mode II and plane Mixed Mode) and the materialgradation. In addition, results of experimental investigations for a mode I loading situation and numericalsimulations of crack growth in such graded structures confirm the theoretical findings and clarify the influenceof the material gradation on the crack propagation behavior.KEYWORDS. Functional fracture mechanical gradation; Crack propagation direction; TSSR-concept;Experimental investigations; Numerical simulations
The effect of varying loading directions and loading levels on crack growth at 2D- and 3D-mixed-mode-loadings
While productâs operation the loading situation commonly changes. The local loading situation on an existing crack then can shift to a combined loading, composed of mode I, mode II and mode III, and consequently influence the productâs durability significantly. This influence on further fatigue crack growth and structuresâ failure can be positive or negative. Present article describes and discusses the effect of varying loading directions from mode I- to 2D-mixed-mode-loading as well as from mode I- to mixedmode I + III-loading. Moreover, experiments on varying loading levels are performed by interspersing mixed-mode block loads in cyclic mode I base load, cyclic mode II base load as well as in cyclic mode III base load
Part shrinkage anomilies from stereolithography injection mould tooling
The use of stereolithography (SL) tooling allows plastic parts to be produced by
injection moulding in a very short time due to the speed of mould production. One of the
supposed advantages of the process is that it provides a low volume of parts that are the
same as parts that would be produced by the conventional hard tooling in a fraction of the
time and cost.
However, this work demonstrates different rates of polymer shrinkage are
developed by parts produced by SL and conventional tooling methods. These revelations
may counter the greatest advantages of the SL injection moulding tooling process as the
parts do not replicate those that would be produced by conventional hard tooling.
This work identifies the different shrinkage that occurs in mouldings produced by
an SL mould as compared to those produced from an aluminium mould. The experiments
utilise two very different types of polymers and two mould geometries, which are
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processed in the same manner so that the heat transfer characteristics of the moulds are
isolated as the only experimental variable.
The work demonstrates how the two mould materials exhibit very different rates
of expansion due to the temperature profiles experienced during moulding. This
expansion must be compensated for to establish the total amount of shrinkage incurred by
moulded parts. The compensation is derived by a mathematical approach and by
modelling using finite element analysis. Both techniques depend upon knowledge of the
thermal conditions during moulding. Knowledge of these thermal conditions are obtained
by real-time data acquisition and simulated by FEA modeling. The application of the
findings provide knowledge of the complete shrinkage values relating to the mould
material and polymer used which would enable the production of geometrically accurate
parts
The Role of Friction in Compaction and Segregation of Granular Materials
We investigate the role of friction in compaction and segregation of granular
materials by combining Edwards' thermodynamic hypothesis with a simple
mechanical model and mean-field based geometrical calculations. Systems of
single species with large friction coefficients are found to compact less.
Binary mixtures of grains differing in frictional properties are found to
segregate at high compactivities, in contrary to granular mixtures differing in
size, which segregate at low compactivities. A phase diagram for segregation
vs. friction coefficients of the two species is generated. Finally, the
characteristics of segregation are related directly to the volume fraction
without the explicit use of the yet unclear notion of compactivity.Comment: 9 pages, 6 figures, submitted to Phys. Rev.
Electrodynamics with radiation reaction
The self force of electrodynamics is derived from a scalar field. The
resulting equation of motion is free of all of the problems that plague the
Lorentz Abraham Dirac equation. The age-old problem of a particle in a constant
field is solved and the solution has intuitive appeal.Comment: 5 page
Layer thickness and draft angle selection for stereolithography injection mould tooling
The introduction of rapid prototyping has allowed engineers and designers to generate physical models of required parts very early on in the design and development phase. Further to this, the use of stereolithography (SL) cavities as a rapid tooling method has allowed plastic prototype parts to be produced in their most common production manner -- by injection moulding. The process is best suited to small production runs where the high costs of conventionally machined tooling is prohibitive. One of the major drawbacks of the SL injectionmoulding process is the susceptibility of the tools to premature failure. SL tools may break under the force exerted by part ejection when the friction between a moulding and a core is greater than the tensile strength of the core, resulting in tensile failure. Very few justified recommendations exist about the choice of mould design variables that can lower the part ejection force experienced and reduce the risk of SL tool failure. This research investigates the ejection forces resulting from SL injection moulding tools which are identical in all respects except for their build layer thickness and incorporated draft angles in an attempt to identify appropriate evidence for recommendations with respect to these design variables and SL injection moulding. The results show that adjustment of draft angle results in a change of part ejection force as a reasonably linear relationship. An adjustment of the build layer thickness results in a change in part ejection force as a more non-linear relationship. The adjustment of build layer thickness had a greater effect on ejection force than the adjustment of draft angle. The results also show that the surface roughness of all tools remains unchanged after moulding a number of parts in polypropylene. A mathematical model was used in an attempt to predict ejection forces according to the moulding material used. This model reflected the experimental results in terms of relative values but not in absolute values, which may be due to inappropriate specific values used in their calculation. Finite element analysis (FEA) was used in an attempt to identify the factors involved in the ejection process. Results indicate that the effect of draft angle on ejection force is due to elastic deformation of the surface roughness. A fact borne out by the lack of damage to the surface after ejection
Using position dependent damping forces around reaching targets for transporting heavy objects:A Fitts' law approach
Passive assistive devices that compensate gravity can reduce human effort during transportation of heavy objects. The additional reduction of inertial forces, which are still present during deceleration when using gravity compensation, could further increase movement performance in terms of accuracy and duration. This study investigated whether position dependent damping forces (PDD) around targets could assist during planar reaching movements. The PDD damping coefficient value increased linearly from 0 Ns/m to 200 Ns/m over 18 cm (long PDD) or 9 cm (short PDD). Movement performance of reaching with both PDDs was compared against damping free baseline conditions and against constant damping (40 Ns/m). Using a Fitts' like experiment design 18 subjects performed a series of reaching movements with index of difficulty: 3.5, 4.5 and 5.5 bits, and distances 18, 23 and 28 cm for all conditions. Results show that PDD reduced (compared to baseline and constant damping) movement times by more than 30% and reduced the number of target reentries, i.e. increasing reaching accuracy, by a factor of 4. Results were inconclusive about whether the long or short PDD conditions achieved better task performance, although mean human acceleration forces were higher for the short PDD, hinting at marginally faster movements. Overall, PDD is a useful haptic force to get humans to decrease their reaching movement times while increasing their targeting accuracy
Weakly-Bound Three-Body Systems with No Bound Subsystems
We investigate the domain of coupling constants which achieve binding for a
3-body system, while none of the 2-body subsystems is bound. We derive some
general properties of the shape of the domain, and rigorous upper bounds on its
size, using a Hall--Post decomposition of the Hamiltonian. Numerical
illustrations are provided in the case of a Yukawa potential, using a simple
variational method.Comment: gzipped ps with 11 figures included. To appear in Phys. Rev.
Anomalous Pseudoscalar-Photon Vertex In and Out of Equilibrium
The anomalous pseudoscalar-photon vertex is studied in real time in and out
of equilibrium in a constituent quark model. The goal is to understand the
in-medium modifications of this vertex, exploring the possibility of enhanced
isospin breaking by electromagnetic effects as well as the formation of neutral
pion condensates in a rapid chiral phase transition in peripheral,
ultrarelativistic heavy-ion collisions. In equilibrium the effective vertex is
afflicted by infrared and collinear singularities that require hard thermal
loop (HTL) and width corrections of the quark propagator. The resummed
effective equilibrium vertex vanishes near the chiral transition in the chiral
limit. In a strongly out of equilibrium chiral phase transition we find that
the chiral condensate drastically modifies the quark propagators and the
effective vertex. The ensuing dynamics for the neutral pion results in a
potential enhancement of isospin breaking and the formation of
condensates. While the anomaly equation and the axial Ward identity are not
modified by the medium in or out of equilibrium, the effective real-time
pseudoscalar-photon vertex is sensitive to low energy physics.Comment: Revised version to appear in Phys. Rev. D. 42 pages, 4 figures, uses
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