Christoph Broeckmann

OXYCOAL-AC: Towards an integrated coal-fired power plant process with ion transport membrane-based oxygen supply

The cooperative project OXYCOAL-AC aims at the development of a zero-CO2-emission coal combustion process for power generation. The scope of the research comprises a multitude of aspects. This article focuses on membrane-based air separation modules and their design for oxycoal conditions, the specifics of coal combustion in a CO2/O2 atmosphere including related burner design as well as the cleaning of hot flue gas from oxycoal combustion.

Effect of subsequent Hot Isostatic Pressing on mechanical properties of ASTM F75 alloy produced by Selective Laser Melting

Abstract The Co-based alloy ASTM (Co – 28.5 wt.-% Cr – 6.3 wt.-% Mo) is widely used for medical implants, e.g. knee prostheses, and is commonly processed by investment casting. Selective laser (SLM) melting is supposed to be an efficient alternative for the production of individually designed knee implants regarding production time and production costs. The mechanical properties, in particular the fatigue strength, of the material have been studied in different states of the material. The mechanical properties of investment casted ASTM F75 and PM-SLM produced ASTM F75 were investigated. The focus in this study was on the PM-SLM material, the specimens were initially produced by selective laser melting and a part of the specimens were further processed by hot isostatic pressing (HIP). The PM-SLM material was mechanically tested in the as-SLM state as well as in the SLM+HIP state. It was found that the mechanical properties of the as-SLM material did not reach the level of the fatigue strength of as cast material. The post-densification treatment by HIP offers distinct improvements regarding the fatigue strength compared to the as-SLM material.

On the Statistical Determination of Yield Strength, Ultimate Strength, and Endurance Limit of a Particle Reinforced Metal Matrix Composite (PRMMC)

In this paper we present a numerical methodology to determine the load bearing capacity of a random heterogeneous material. It is applied to a particulate reinforced metal matrix composite (PRMMC), WC-30 Wt.% Co, to predict its strength against both monotonic and cyclic loads. In this approach, limit and shakedown analysis based on Melan’s static theorem [30] is performed on representative volume element (RVE) models generated from real material microstructure and the obtained results are converted to macroscopic load domains through homogenization. To evaluate microstructure’s impact on the overall material strength, the relationship between strength and composite structure is investigated by means of statistics. Meanwhile, several numerical issues, e.g. the impact of RVE’s size, mesh density, as well as the discrepancy between 2D and 3D models, are studied.

Development and characterization of powder metallurgically produced discontinuous tungsten fiber reinforced tungsten composites

In future fusion reactors, tungsten is the prime candidate material for the plasma facing components. Nevertheless, tungsten is prone to develop cracks due to its intrinsic brittleness—a major concern under the extreme conditions of fusion environment. To overcome this drawback, tungsten fiber reinforced tungsten (Wf/W) composites are being developed. These composite materials rely on an extrinsic toughing principle, similar to those in ceramic matrix composite, using internal energy dissipation mechanisms, such as crack bridging and fiber pull-out, during crack propagation. This can help Wf/W to facilitate a pseudo-ductile behavior and allows an elevated damage resilience compared to pure W. For pseudo-ductility mechanisms to occur, the interface between the fiber and matrix is crucial. Recent developments in the area of powder-metallurgical Wf/W are presented. Two consolidation methods are compared. Field assisted sintering technology and hot isostatic pressing are chosen to manufacture the Wf/W composites. Initial mechanical tests and microstructural analyses are performed on the Wf/W composites with a 30% fiber volume fraction. The samples produced by both processes can give pseudo-ductile behavior at room temperature.

A model for predicting crack initiation in forged M3:2 tool steel under high cycle fatigue

AISI type M3 class 2 tool steel (or in German designation DIN: HS6-5-3 tool steel) is most commonly used in tooling industry, and also in some engine parts. Those components are usually subjected to cyclic stresses and mostly fail by fatigue. Fatigue crack initiation in this material occupies large fraction of total lifetime and strongly depends on microstructural features of primary and eutectic carbides, such as shape, shape ratio, volume fraction, the distribution of carbides as well as load ratio. To model fatigue initiation mechanisms of forged M3:2 tool steel, McDowell’s model was modified and developed for different length-scales. For fatigue crack formation and short crack growth, a hierarchical approach was used and the life time of these stages were estimated based on the local cyclic plasticity. Through this relation the effect of microstructural features on both fatigue crack formation and short crack growth in the material were identified. The results of the proposed model have explicitly reflected the influence of microstructural features on both fatigue crack formation and propagation in forged M3:2 tool steel. Moreover, the model can be used for improving the fatigue resistance of a tool steel component.

Festigkeitskennwerte und mehrachsige Schwingfestigkeit von lamellarem Gusseisen

Kurzfassung Vorgestellt werden analytische und experimentelle Untersuchungen zum Ermüdungsverhalten von Gusseisen mit Lamellengraphit (GJL). Neben statistisch abgestützten Kennwertfunktionen für die zur Festigkeitsrechnung maßgeblichen Eingangsgrößen und zum Einfluss von Normal- und Torsions-Mittelspannungen, wurden Versagensgrenzen für GJL unter zweiachsig schwingenden Lastspannungszuständen durch Rechnung und Versuche verifiziert.

On the Size of the Representative Volume Element Used for the Strength Prediction: A Statistical Survey Applied to the Particulate Reinforce Metal Matrix Composites (PRMMCs)

Particulate reinforced metal matrix composites (PRMMCs) are typical random heterogeneous materials whose global behavior depends on the microstructural characterisics. Recently a numerical approach was developed (Hachemi et al., Int J Plast 63:124–137, 2014 [1], Chen et al. Direct methods for limit and shakedown analysis of structures, 2015 [2]), by applying it to a typical PRMMC material WC/Co, we presented how the ultimate strength and endurance limit can be predicted from the material microstructures. Due to the randomness in the microstructures of PRMMCs, size of the representative volume element (RVE) has a nontrivial influence over the predicted effective behaviors. In order to understand how size of RVEs contribute to the result and based on that to eliminate its influence, a numerical investigation is performed in the present study. In this study, a large number of representative volume element (RVE) samples representing a representative PRMMC material, WC-20 Wt% Co, were built from artificial microstructures. The samples are obviously different in size, and by deploying the established numerical approach to these samples, ultimate strength and endurance limit were calculated. Afterwards, the derived material strengths were analyzed by multiple inferential statistical models. The statistical study reveals how strength and other effective material properties react to the change of the RVE size. On that basis, the study proposed a feasible and computationally inexpensive solution to minimize the size effect.

Kriechen der teilchenverfestigten Typ-A-Legierung NiCr23Co12Mo

The presented work focuses on the creep mechanisms of the Ni base alloy NiCr23Co12Mo (Alloy 617). The aim is to develop a physical based, constitutive material model which can be used for the design of power plant components. Particle hardening and internal back stresses are quantitatively and qualitatively described based on the combination of the results from hot tensile tests and creep tests, as well as microstructural investigations. The particle hardening arises from the precipitation of secondary phases such as Cr rich M23C6 carbides and the ’ phase. The material model is implemented in the finite element software ABAQUS via user subroutine CREEP and can predict multiaxial creep deformation. As an example of application, the creep deformation of a thick walled pipe for a future 700°C power plant has been predicted by the FE model. Complex component tests allow the appropriate validation of the material model.

Prediction of fatigue limit of journal bearings considering a multi-axial stress state

Purpose The purpose of this paper is to investigate the applicability of the quadratic failure hypothesis (QFH) on journal bearings coated with a white metal sliding layer on the prediction of safe and unsafe operating conditions. The hypothesis covers operation conditions under static and dynamical loading. Design/methodology/approach Material tests and elastohydrodynamic, as well as structural, simulations were conducted to provide the required input data for the failure hypothesis. Component samples were tested to verify the results of the QFH. Findings The load bearing capacity of journal bearings was analysed for different operating conditions by the use of the QFH. Results allow for the identification of critical and non-critical loading conditions and are in accordance with component test results. Originality/value Today’s design guidelines for journal bearings do not consider a multi-axial stress state and actual stress distribution. The applied hypothesis enables consideration of multiaxiality inside the sliding surface layer, as well as determining the location of bearing fatigue due to material overload.

A NUMERICAL STUDY ON THE ENDURANCE LIMIT OF PARTICULATE REINFORCED METAL MATRIX COMPOSITES (PRMMCS) USING THE DIRECT METHOD AND THE STATISTICAL LEARNING

Abstract. Particulate reinforced metal matrix composites (PRMMCs) are characterized by their stochastic and irregular microstructure. The strength of the material has been noticed to have a large scatter and great dependence on the underlying composite structure. In order to understand how size and distribution of the reinforcement particles contribute to the effective material behaviors, we elaborated in present study a numerical approach which incorporates homogenization technique, shakedown analysis, and statistical learning. To demonstrate this approach, a typical PRMMC material, WC 20 wt.% Co, was taken as example and numerous representative volume element (RVE) samples of this material were built based on both real and artificial microscope images. Multiple effective behaviors, including the ultimate strength and endurance limit, were evaluated on each RVE sample using the direct method. In order to understand how multiple structural factors jointly affect the overall composite strength, the predicted strength and other selected features extracted from the samples are submitted to few statistical models such as logistic regression and artificial neural network (ANN) to establish predictive models. On the basis of these models we discussed how few structural factors, which have been identified to have nontrivial impact, contribute to the endurance limit of the selected PRMMC material.