Eberhard Kerscher

Mechanical Properties of Compound-Extruded Aluminium-Matrix Profiles under Quasi-Static Loading Conditions

Compound-extruded unidirectionally reinforced lightweight profiles are a novel class of materials for the realisation of load-bearing structures. They may be fabricated in a flexible and rapid near-net-shape process. The authors present investigations of the reinforcing effect of wires in compound-extruded aluminum profiles under quasi-static tension and compression. In particular, the compounds were characterized by metallographic examinations focusing on the fracture morphology. Furthermore, specimens subject to compression tests were examined using micro computer tomography (µ-CT) and light microscopy (LM). It is shown, that the mechanical properties of wire-reinforced profiles are improved under both positive and negative quasi-static loads in comparison to non-reinforced profiles.

Very high cycle fatigue of bearing steels with artificial defects in vacuum

At high-strength steels very high cycle fatigue (VHCF) failure always occurs at subsurface inclusions. As a result, the failure shows large scatter and is not observable or predictable prior to failure. In order to understand the mechanisms leading to crack initiation in VHCF, it is necessary to make the failure observable. Within our work, a new testing procedure is introduced that enables the simulation of subsurface conditions at the surface and thereby the localisation of the failure. Therefore, specimens with artificial surface defects were tested in vacuum atmosphere. In our work, we will show the testing procedure and the validation of the results. Further, the transferability of our results on the failure at subsurface inclusions is discussed. This paper is part of a Themed Issue on Recent developments in bearing steels.

A Constitutive Relationship between Fatigue Limit and Microstructure in Nanostructured Bainitic Steels

The recently developed nanobainitic steels show high strength as well as high ductility. Although this combination seems to be promising for fatigue design, fatigue properties of nanostructured bainitic steels are often surprisingly low. To improve the fatigue behavior, an understanding of the correlation between the nanobainitic microstructure and the fatigue limit is fundamental. Therefore, our hypothesis to predict the fatigue limit was that the main function of the microstructure is not necessarily totally avoiding the initiation of a fatigue crack, but the microstructure has to increase the ability to decelerate or to stop a growing fatigue crack. Thus, the key to understanding the fatigue behavior of nanostructured bainite is to understand the role of the microstructural features that could act as barriers for growing fatigue cracks. To prove this hypothesis, we carried out fatigue tests, crack growth experiments, and correlated these results to the size of microstructural features gained from microstructural analysis by light optical microscope and EBSD-measurements. Finally, we were able to identify microstructural features that influence the fatigue crack growth and the fatigue limit of nanostructured bainitic steels.

Influence of the Inclusion Type on the Threshold Value of Failure in the VHCF-Regime of High-Strength Steels

The very high cycle fatigue behaviour of high-strength steels is mostly affected by cracks which were initiated at non-metallic inclusions inside the fatigue specimens and hence under the surface. By separating the data points in the S-N-diagram by the chemical composition of the inclusion at which a crack is initiated the authors recognized that the lifetime depends strongly on the inclusion type. The authors further deduce that the chemical composition of an inclusion as well as its geometry influences the mechanism by which such an inclusion triggers crack initiation. Thus, titanium nitride and homogeneous calcium oxide inclusions have been observed to show fractures caused by the stress concentration in the inclusion. The broken inclusions then present sharp notches in the steel matrix and cause further damage from there. In contrast, aluminium calcium oxide inclusions decay or detach from the steel matrix during loading on account of a rather low interface stress. This detachment process results in holes in the steel matrix, which act as logical starting points of fatigue cracks. Both processes described above occur at different stress intensity factors and lead to failure before an ultimate number of cycles of 10^9. Furthermore, it was possible to determine threshold values of failure for each inclusion type by stressing run out specimens on a higher stress level. These threshold values are in accordance with those of specimens which failed during one-level stressing. The fatigue tests were performed with 100Cr6 in martensitic and bainitic condition. Tension/compression tests at a load ratio of R = -1 were conducted on an ultrasonic fatigue testing facility. Failures were only initiated at non-metallic inclusions. The fracture surfaces were analyzed by scanning electron microscopy and by energy dispersive X-ray spectroscopy.

Consequences of Micro-Milled and Laser Structured Surfaces of CP-Titanium on Tension-Compression Fatigue Behaviour

Component surfaces can be modified by micro-structuring processes like micro-milling or laser structuring for functionality reasons. This modification induces small notches, whose dimensions are in the same order as the grain size. This could have an influence on the mechanical properties. This paper presents the results of tension-compression fatigue tests with structured and – for comparison – with unstructured micro-tensile specimens of cp-titanium grade 2. Longitudinal metallographic microsections illustrate the grain size of the microstructure and the geometry of the notches. The results of the fatigue tests show the influence of the notches on lifetime and endurance limit. With a Scanning Electron Microscope (SEM) the fracture surfaces, the crack initiation sites, and the crack propagation areas of all samples were analyzed. With these analyses we want to determine which notch structure dimensions relative to the grain size are tolerable.

Increasing the fatigue limit of a high-strength bearing steel by a deep cryogenic treatment

High-strength steels typically fail from inclusions. Therefore, to increase the fatigue limit of high-strength steels it is necessary to modify the inclusions and/or the surrounding matrix. The goal must be a higher threshold for crack initiation and/or crack propagation. One possibility to reach this goal seems to be a deep cryogenic treatment which is reported to completely transform the retained austenite as well as to facilitate the formation of fine carbides. Therefore, specimens were annealed before or after deep cryogenic treatment, which was carried out with different cooling and heating rates as well as different soaking times at −196° C. Hardness and retained austenite measurements and fatigue experiments were used to evaluate the different sequences of treatments mentioned above. The fatigue limit increases only after some of the sequences. The results show that the soaking times are not relevant for the fatigue limit but it is very important to temper the specimens before the deep cryogenic treatment. Also, repeated deep cryogenic treatments had a positive influence on the fatigue limit.

Analysis of failure behaviour of carbon/carbon composite made by chemical vapour infiltration considering fibre, matrix and interface properties

Abstract The mechanical properties of a unidirectional carbon/carbon (C/C) composite with a mainly highly textured matrix are investigated in tensile tests for the as-received state and after heat treatment from 1600°C to 2500°C. In order to interpret the composite behaviour single fibre tests, analyses of fracture surfaces and tests on infiltrated fibre felts were conducted. The fibre–matrix shear strength was determined by push-out indentation tests. Strength and ductility only change slightly although the strength of the fibres decreases significantly with increasing heat treatment temperature (HTT). The reason is a reduced bonding strength between fibre and matrix with increasing HTT. The Youngs modulus of C/C composites increases with HTT. The best mechanical properties of the composite are reached by finding a compromise between sufficient fibre strength and weakened fibre-matrix strength at a HTT of 1600°C.

Surface Erosion of Carbon Steel 1045 During Waterjet Peening

The present study investigates the effect of waterjet treatment on the surface characteristics of the carbon steel 1045. The effect of waterjet treatment parameters namely number of jet passes and pressure was investigated. An increase in the number of jet passes as well as pressure leads to a higher roughness and more erosion of the surface. The damage features consist of various fracture mechanism modes occurred at the initial and evolved damage stage. The ferrite phase experienced more damage than the pearlite phase. However, the damage was more concentrated along the grain boundaries. The shearing force from the jet lateral flow raised the circumferential rim and created lateral cracks and sub-tunnels which might eventually be removed in the subsequent jet passes. The hardness of the treated specimens increased with an increase in the number of jet passes and pressure.

Analysis of the Crack Initiation at Non-Metallic Inclusions in High-Strength Steels

Abstract High-strength steels exhibit different fracture surface characteristics under cyclic load when failure is initiated by non-metallic inclusions depending on the lifespan. These can be attributed to different crack propagation mechanisms. These characteristics are correlated with the short and long crack propagation. As far as the short crack is concerned, the mechanism of the discontinous decohesion of carbides in the immediate vicinity of the inclusions and the merging of the resulting microcracks to form a macrocrack is discussed. This postulated mechanism is investigated by means of a combination of scanning electron microscope (SEM) analyses of the fracture surface and target preparation by means of focused ion beam (FIB) by a detailed resolution of the local microstructure around the crack-initiating inclusion and the crack path.

Influence of shot peening on the mechanical properties of bulk amorphous Vitreloy 105

ABSTRACT The quasi-static and cyclic properties of bulk glassy Zr52.5Cu17.9Al10Ni14.6Ti5 alloy (Vitreloy 105) were investigated under three-point bending conditions for two different shot-peened surface states. Residual stress analysis and nanoindentation measurements revealed the presence of compressive residual stresses and an enhanced hardness in the near surface layer after shot peening. Further investigations of the longitudinal cross-sections of the mechanically tested specimens by optical and scanning electron microscopy showed small cracks propagating along shear bands in the vicinity of the fracture surface. The results are in accordance with the improved plasticity of the shot-peened states under quasi-static loading conditions compared to the as-cast reference state. All mechanical testing was carried out with the aim to find a material’s state with improved mechanical properties with a special focus on the improvement of the fatigue lifetime and the endurance limit of Vitreloy 105 bulk metallic glass.