Aufbau und elektronische Funktion eines Detektorsystems für integrierende Messungen von Radon in der Bodenluft werden beschrieben. Es besteht aus einer Halbleiterdetektorsonde und einer separaten Auswerteeinheit.
The formation and clustering of non-metallic inclusions was investigated by applying a steel casting simulator. In a fully controlled atmosphere, the oxygen content of the steel melt was intentionally increased. At a specified level, the steel was deoxidized by pure aluminum. After the treatment, the melt was cooled down in the crucible. The effects of the inclusions and the cavities were determined by means of metallography, tensile tests, dynamic fracture toughness tests, and fractography. Metallographic results show that alumina particles have a strong tendency to aggregate at the walls of the crucible. Neglecting this aggregation, a relatively homogeneous distribution of alumina inclusions was observed. Furthermore, the solidified steel exhibited manganese sulphide inclusions and shrinkage cavities. The results of the tensile tests revealed a relatively low ductility. Fractographic examinations showed that both non-metallic inclusions and shrinkage cavities promoted ductile fracture. Results of dynamic fracture toughness tests revealed a relatively large scatter in the dynamic crack resistance. This was analogously attributed to the damaging effect of the non-metallic inclusions and the shrinkage cavities. Fractographic investigations showed that not only alumina inclusions but preferentially manganese sulphide inclusions affected the failure behavior of the investigated steel.
Dynamic crack initiation with crack-tip loading rates of K̇ ≈ 2.106MPa√ms-1 in a high strength G42CrMoS4 steel was investigated. To this end, a previously developed split Hopkinson pressure bar with four-point bending was utilised. V-notched and pre-cracked Charpy specimens were tested. The detection of dynamic crack initiation was performed by analysing the dynamic force equilibrium between the incident and the transmission bar. Additionally, the signal of a near-field strain gauge and high-speed photography were used to determine the instant of crack initiation. To account for vibrations of the sample, a dynamic analysis of the stress intensity factor was performed. The dynamic and static analyses of the tests produced nearly the same results when a force equilibrium was achieved. Fracture-surface analysis revealed that elongated MnS inclusions strongly affected both the dynamic crack initiation and growth. Blunting of the precrack did not take place when a group of MnS inclusions was located directly at the precrack tip. Due to the direction of the elongated MnS inclusions perpendicular to the direction of crack growth, the crack could be deflected. The comparison with a 42CrMo4 steel without elongated MnS inclusions revealed the detrimental effect in terms of resistance to crack initiation. Taking the loading-rate dependency into consideration, it was shown that there was no pronounced embrittlement due to the high loading rates.
Abstract This chapter presents results of investigations on the strength, deformation and toughness behavior of quenched and tempered 42CrMo4 steel. Intentional impurification and, afterwards, filtration by functionalized ceramic foam filters were applied in order to process cast steels with different amounts and distributions of non-metallic inclusions. As references, a hot-rolled steel batch and spark-plasma sintered materials were studied. The investigations focused on the loading rate and temperature effects. Both, tensile and fracture mechanics tests, were performed in order to investigate the damaging behavior due to non-metallic inclusions remaining after the melt processing of the steel. A further goal was to predict the fracture toughness of the material based on the combination of microstructural information on the inclusion distribution and the strain rate and temperature-dependent strength and deformation behavior. It was shown that the damaging effect of non-metallic inclusions, in particular agglomerated inclusions properties, is localized which leads to relatively low strain to fracture and fracture toughness, but also to crack path deflection. Furthermore, it could be observed that the small interparticle distances within agglomerated non-metallic inclusions determine the fracture toughness behavior of the materials. By analyzing the acoustic emissions, the onset of crack growth as well as the size of the plastic zone at the crack tip could be estimated.
Dynamic crack initiation with crack tip loading rates K ˙ of approximately 2 ‧ 10 6 MPa√ms − in high-strength 42CrMo4 steel was investigated. To this end, a recently developed split Hopkinson pressure bar with four-point bending was utilized. V-notched and precracked Charpy specimens were tested. The tests were performed at temperatures of –40 °C and 20 °C. The loading of the specimen was determined by analyzing the strain in the incident and transmission bars. Furthermore, strain gauges at the specimen’s surface were applied to measure the crack tip loading. High-speed photography complemented the analysis of the specimens loading and the detection of the crack initiation. Fracture surface analysis by means of scanning electron microscopy enabled the measurement of the fracture surface topography and, consequently, stretch zone height and width. Hence, the macroscopically measured dynamic crack initiation toughness was correlated with the toughness at microscopic scale. It was observed that the resistance against dynamic crack initiation decreased with decreasing temperature. Microscopically, a decrease in toughness was analogously observed. Non-metallic inclusions resulted in crack path deflection with localized shear zones. After a small stable crack extension, cleavage fracture was observed.
Abstract In this paper, the effect of microstructure of a thick-walled rotor shaft for wind turbines on fracture toughness properties has been investigated. The relevant nodular cast iron grade EN-GJS-600-3 was processed using chill casting technology. Due to different solidification conditions over the wall thickness, heterogeneous microstructures were formed. To illustrate the influence of the microstructure gradient caused by chill casting technology, specimens were taken from different sample positions in the cross section of the casting component. A detailed metallographic analysis revealed essential differences in microstructure. The crack growth resistance under quasi-static loading conditions and the fatigue crack propagation under cyclic loading conditions were measured. The results of the static fracture mechanics investigations revealed that fracture toughness is strongly influenced by the microstructure of this pearlitic ductile iron grade. On the other hand, cyclic fracture mechanics analysis showed that the complex formation of the microstructure has only a minor effect on the fatigue threshold value, but microstructure has a significant effect on the stable crack growth. For the assumed load cases, it was shown that microstructure can be a dominant factor on the mechanical and fracture toughness properties.
Carbon‐bonded ceramic foam filters with different functional coatings are immersed in a 42CrMo4 steel melt within a steel casting simulator. The solidified steel is analyzed with respect to the size distribution and the chemical composition of the remaining nonmetallic inclusions (NMI). Cyclic loading and quasi‐static tests are performed to determine the fatigue limit, the strength, deformability, and toughness of the steel after filter immersion. The immersion of filter with calcium hexaluminate (CA6) coating significantly reduces the population of small (4–20 μm) NMIs. This leads to an increased deformability and, thus, ability for energy dissipation during deformation. However, the maximum size of NMIs is increased from 100 to 150 μm, which results in fatigue limit reduction, despite the decrease in NMIs total density. The majority of inclusions are found to be pure alumina. Large (up to 150 μm) plate‐like alumina inclusions introduce most of the detrimental effects on cyclic strength, whereas significant effect on quasi‐static strength is not found.
Abstract The determination of velocity and displacement evolution of the specimen in a low-blow Charpy impact test was critically analyzed. To this end, a laser-based measurement of velocity and displacement was applied in addition to the standard integration procedure. The applicability of this non-contact method was investigated with respect to the measurement of velocity and detection of vibrations. It was shown that the velocity of the specimen could be determined in the case of unnotched specimens as well as of notched and precracked specimens. Utilizing the velocity evolution of the specimen determined by the laser, the dynamic force calibration was possible. The high sensitivity of the laser measurement to changes in velocity enabled the detection of pop-ins and, in some cases, the detection of stable crack initiation. The analysis of the specimen vibration frequency directly after the test could be utilized to estimate the final crack length.
