Der Betonstahl trägt entscheidend zur Tragfähigkeit von Stahlbetonmodulen bei. Bevor Module wiederverwendet werden können, muss daher sichergestellt werden, dass der Betonstahl den Anforderungen an die Tragfähigkeit unter statischer und zyklisch‐mechanischer Belastung genügt. Die Berechnung der Tragfähigkeit von jahrzehntealten Betonstählen ist derzeit allerdings mit hohen Unschärfen verbunden, die sich aus unzureichenden Dokumentationen des verbauten Stahls, Veränderungen der Normenwerke, aber auch möglichen zeitabhängigen Veränderungen des Betonstahls selbst ergeben. Um die Unschärfe zu reduzieren, ist es daher nötig, den Betonstahl zu charakterisieren. Für die Charakterisierung stellt die Notwendigkeit der Erhaltung des Stahlbetonmoduls eine zentrale Herausforderung dar. Daher kommen nur zerstörungsfreie Prüfverfahren oder solche, die an kleinen Probenvolumina durchgeführt werden können, infrage. Weitere Herausforderungen ergeben sich aus der potenziell großen Menge der zu analysierenden Module. Es ist notwendig, den zeitlichen und personellen Aufwand zur Charakterisierung des Betonstahls so niedrig wie möglich zu halten, um eine wirtschaftliche Umsetzung der Wiederverwendung von Modulen erzielen zu können. Mit diesen Herausforderungen beschäftigt sich das Teilprojekt B01 im neu eingerichteten Sonderforschungsbereich 1683. Diese Veröffentlichung bietet einen Ausblick auf die Lösungsansätze, die zur Bewältigung der zuvor genannten Herausforderungen beitragen sollen.

Cold work tool steels (CWS) are designed to endure high mechanical loads and offer a high wear resistance, due to a martensitic matrix with hard carbides. These carbides are formed by alloying chromium (Cr), molybdenum, and vanadium in combination with high‐carbon contents. To reduce resource consumption and cost, there is a strong motivation to lower the content of alloying elements. High‐boron tool steels (HBTS) present a promising alternative, requiring fewer alloying elements while maintaining high mechanical properties. In this work, two HBTS Fe0.4C1B2.5Cr and Fe0.4C1B10Cr are processed via a powder metallurgical (PM) route, as well as a conventional casting and hot swaging route. Investigations of microstructure and fatigue behavior reveal the processing–microstructure–property relationships of the HBTS. The results show that HBTS achieve a similar hard phase content as the CWS X153CrMoV12. A comparison of the microstructures after swaging and heat treatment reveals a more homogeneous distribution and more spherical morphology of the hard phases in the HBTS. Under fatigue loading, low crack propagation rates and a high number of load cycles as well as stress intensities before fracture are endured by PM HBTS. The investigations show that alloys Fe0.4C1B2.5Cr and Fe0.4C1B10Cr are viable alternatives to conventional CWS.

The use of boron as a hard phase forming element in cold work tool steels can improve performance, price, and environmental considerations all at once. This is achieved by reducing costly and high footprint elements, avoiding retained austenite and thus simplifying heat treatment. However, in boron-alloyed tool steels the formation of boride networks during casting impairs the mechanical properties. This, in turn, makes the powder metallurgy (PM) route all the more interesting and important as well as effective in improving material properties. In this study, we present two innovative boron-alloyed PM tool steels that are atomized and consolidated by HIP. Microstructural analysis is performed using SEM, EBSD and XRD and dynamic mechanical properties are evaluated. The results show that a combination of cost reduction through reduced element content combined with the isotropic microstructure of the PM-HIP production route can be achieved while improving mechanical properties.

In this study, the fatigue crack growth rate in four different tool steel microstructures (hot rolled, powdermetallurgically processed, as-cast, and carbide-free) is experimentally measured and correlated with hard phase size and spacing, as well as with the roughness of the fracture surface that is created by crack kinking. Numerical simulations of crack growth in carbide-containing microstructures are conducted and investigated. The results indicate a favorable influence of carbides with a larger size and higher degree of roundness, as they create the largest mean free path between the individual carbides at the same hard phase volume content. This facilitates the formation of a plastic zone in the matrix, which dissipates crack energy and reduces the effective stress intensity. In addition, the effect of crack kinking is increased at larger carbide sizes. Concerning practical application, the results suggest that a high degree of deformation is favorable regarding the fatigue growth resistance of tool steels, and that the use of powder metallurgically (PM) grades with small carbides is discouraged, if the lifetime of a tool is mainly controlled by the crack growth rate and not crack initiation.

In the case of a heterogenous ground condition, tunneling tools are exposed to cyclic loading. The cyclic loads can exceed the materials' permanent load-bearing capacity, which is associated with fatigue crack growth. Subsequently, the coalescence of fatigue cracks can cause spalling of material from the tool surface, leading to the deterioration of their functionality. The problem of fatigue damage on tunneling tools has lately been recognized. However, the underlying micromechanism of subcritical crack growth and the counteraction of fatigue by advanced material concepts still require fundamental research. In this work, miniaturized cutting discs made of different tool steels are subjected to a laboratory scale fatigue test and subsequently analyzed concerning fatigue damages and the material response to cyclic loading. The hot work tool steel X40CrMoV5-1, widely used for cutting discs in industrial practice, showed fatigue crack growth and significant plastic deformation of the cutting edge under cyclic loading, leading to chip formation and spalling at the cutting edge. An improvement was found by using X153CrMoV12 ledeburitic cold work tool steel, which offers a higher strength due to the coarse chromium carbides distributed in the microstructure. Due to their high young modulus, the carbides can inhibit the plastic deformation of the softer metal matrix at the cutting edge, thus increasing the edge retention and suppressing chip formation.

AbstractThe mechanized tunnel construction is carried out by tunnel boring machines, in which the soil in front of the working face is removed, and the tunnel lining is carried out with shotcrete or the setting of segments and their back injection. Advancements in this field aim towards increase of the excavation efficiency and increase of the tool lifetime, especially in rock-dominated grounds. The latter is achieved by understanding the wear mechanisms abrasion and surface-fatigue, and by knowledge of the microstructure-property relation of the utilized materials. Improvements for tool concepts are derived, based on experiments and simulations. A key parameter towards efficient rock excavation is the shape of the cutting edge of the utilized disc cutters. Sharp cutting edges have proven to generate higher rock excavation rates compared to blunt ones. The compressive strength of the utilized steel has to be high, to inhibit plastic deformation and thereby to maintain sharp cutting edges. This requirement competes with the demand for toughness, which is necessary to avoid crack-growth in the case of cyclic loading. Solutions for this contradiction lie in specially designed multiphase microstructures, containing both hard particles and ductile microstructural constituents. Besides adapting the alloying concept, these required microstructures and the associated properties can be adjusted by specific heat-treatments.

The increasing demand for hot isostatic pressing (HIP) means that a reliable and efficient operation of modern HIP units with fast cool capability is indispensable. A key factor for efficient operation is the ceramic crucible used as the load basket. Its task is to keep as much of the HIPed parts as possible effectively within the hot zone and to prevent them touching the furnace wall. This work focuses on designing a gas-permeable ceramic structure with a high thermal shock resistance that can be scaled up to a load basket for future HIP applications. Stereolithography (SL) 3D printing of a ceramic resin is employed to build various scalable framework structures inspired by nature and by existing engineering applications. Thermal shock tests with water quenching reveal that framework structures with evenly distributed triangular bracings offer the highest flexural strength, whereas auxetic structures are best at retaining their flexural strength after thermal shock.

Indentation testing has been widely used in laboratory environments to investigate the processes leading to rock fragmentation in drilling, mechanized tunneling, and mining. Rock specimens for laboratory testing are limited to finite size, potentially causing size effects that have to be accounted for when transferring results to in situ applications. We present an integrated experimental and theoretical investigation of the specimen size effect in indentation testing (a) to address the limited understanding of its causes and the lack of tools to analyze tests on variable specimen sizes and (b) to identify to what extent an indenter mimicking the shape of a cutter on a tunneling machine can be approximated by a conventional indenter geometry. We performed indentation tests on cylindrical specimens of a porous sandstone with aspect ratios (diameter/height) ranging from 0.3 to 1.7, using a blunt-truncated indenter and monitoring the fracturing process by the acoustic emission technique. A damage zone, enclosing a zone of crushed grains immediately below the indenter tip, forms and grows due to tool penetration. Eventually, all specimens failed as a result of the propagation of a sub-vertical fracture, initiated close to peak indentation pressure. Peak force, its corresponding penetration depth, and peak indentation pressure increase with specimen size, more significantly with specimen diameter than with height. We developed a semi-analytical model based on cavity-expansion theory and linear elastic fracture mechanics for the formation of the damage zone and the nucleation and propagation of the macroscopic vertical fracture, respectively, whose predictions are in good agreement with our experimental data. The observed increases of peak indentation pressure with specimen size can be explained by the effect of the free surfaces on damage zone growth rather than on fracture propagation. The model permits evaluating the specimen size effect through the ratio between two geometrical parameters, specimen diameter and tip width of the truncated indenter, which has to be larger than around 10(2) for the size effect to be insignificant. The model permits upscaling of experimental results to in situ conditions based on geometrical indenter parameters and commonly used material parameters.

The deformation-induced phase transition from fcc to hcp causes local embrittlement of the metal matrix in Cobalt-base alloys, facilitating subcritical crack growth under cyclic loading and reducing fatigue resistance. Our approach to increasing the fatigue life of Co-based hard alloys is to suppress the phase transition from fcc to hcp by an alloy modification that increases the stacking fault energy (SFE) of the metal matrix. Therefore, we substitute various contents (15, 25, and 35 mass pct) of Co by Fe and analyze the effect on the fatigue life and resistance against subcritical crack growth. Subcritical crack growth in the specimens takes place in a cyclic load test. The proceeding crack growth and the occurrence of phase transformations are monitored by scanning electron microscope (SEM) investigations and electron backscatter diffraction (EBSD). We determined an SFE of 35 mJ/m 2 at an iron content of 35 mass pct, which leads to a change of the main deformation mechanism from deformation-induced martensitic transformation to deformation twinning. Analysis of cyclically loaded specimens revealed that the resistance against subcritical crack growth in the metal matrix is facilitated with increasing Fe content, leading to a significant increase in fatigue life.

This paper presents a novel concept for vibration-based feature extraction to identify damages in cutting discs of Tunnel Boring Machines (TBM). Defect frequencies resulting from repeated interaction of rock and disc defects are analysed. The data set is represented by the normal force acting on the edge of a cutting disc and the rock. Two different methods, the Hilbert transform and the complex demodulation, are used to generate the envelope of the time series, which was used to analyse whether the signal shows a feature representing an existing defect in the frequency domain. For the first proof of concept two numerical models were used - a multi-body system and a peridynamics 3D model simulating time series of normal forces. With both models, the linear motion of the disc on a rock sample with constant velocity was simulated. An experimental setup, mechanically similar to the simulations, was used in two experiments for further comparison. All implemented defects could be detected using vibration data of forces and one of the proposed data analysis techniques.

The efficient operation of mechanized tunnel drilling machines is strongly determined by the wear resistance of the applied mining tools. Especially in chisels, but also partly in cutting disks, metal matrix composites are used. Their wear mechanism is dominated by surface spalling, i.e. subcritical crack propagation through the material's microstructure, mainly consisting of a ductile metal matrix and carbide inclusions. Since this process is primarily governed by the morphology of the microstructure and the mechanical behavior of the individual phases, simulations at the microscale enable the design of improved materials. In this contribution, a method for the modeling of this process is presented. The approach is based on the eigenerosion framework introduced in [1] and an algorithmic scheme for large strains is given, which extends the small strain implementation in [2]. For the phases at the microscale, the finite strain plasticity formulation [3] is applied. Examples according to [4] are shown in order to demonstrate the mesh independence of the framework for ductile crack propagation. Furthermore, simulations are carried out on the microscale by applying the Finite Cell Method [5] on metal matrix composite microstructures. Here, a specific cell arrangement is constructed which minimizes the required number of cells for a given microstructure morphology. Additionally, an experimental setup for the validation of the eigenerosion framework on the microscale is presented. By evaluating the results of these simulations, failure of the material can be investigated on a microscopic level and improvements of the material morphology regarding wear can be realized.

In this work, the subcritical crack growth in Fe-, Ni- and Co-base hard alloys was investigated. Specimens were cyclically loaded in the pressure threshold range until a ring crack resulted as a failure criterion. Crack propagation along with the individual microstructural constituents and the associated resistance of the individual materials to crack propagation was investigated by scanning electron microscopy and by the methods adapted to it. For the Ni-base alloys, the formation of a closed ring fracture occurred after the lowest load cycle number, followed by the Co- and Fe-base alloys. Almost no crack deflection by the hard phases was detected in the Ni-base alloys. The higher number of loading cycles to produce a closed crack ring in the Fe-base alloys is attributed to the pronounced crack deflection by the hard phases and to the higher matrix strength. Besides, phase transformations were registered in front of the crack tip of the Co- and the partially austenitic Fe-base alloy. This phase transformation counteracts crack formation in the case of the Fe-base matrix but promotes crack propagation in the Co-base alloy.