Kugelstrahlen bei erhöhter Temperatur mit einer Druckluftstrahlanlage

Shot peening using an air blast machine at elevated temperatures With a new system shot peening using an air blast machine at elevated temperatures is feasable. This is done by mixing the cold airflow which contains the peening media with a hot airflow heated up with a flow heating system. It will be shown for a quenched and tempered steel 42 CrMo 4 (equivalent to AISI 4140) that the fatigue strength increases with increasing peening temperature T_peen in the temperature range 20 °C ≤ T_peen ≤ 290 °C. On the one hand this is caused by slightly higher compressive residual stresses in the near surface area which is influenced by the shot peening process. On the other hand different dislocation structures are generated as a result of strain ageing which occurs while and after the shot peening treatment. This also causes a higher stability of the residual stresses compared to the conventional shot peening.     

Dualphasen-Stähle mit erhöhter Festigkeit und Verformbarkeit

Dual-Phase Steels with Improved Strength and Ductility. The term “dual-phase steel” refers to a new class of steels with a special ferritic-martensitic microstructure, which offers a superior combination of good formability, particularly concerning deep-drawing, and of high tensile strenght. This paper describes the alloys and the thermomechanical treatment to process a dual-phase microstructure. Comparing tensile tests of dual-phase structures and other ferritic-martensitic structures, factors are analysed which determine the typical mechanical properties of dual-phase structures. Their optimum mechanical properties of dual-phase structures. Their optimum mechanical properties, which are found to be better than those of conventional high-strength steels are due to the topology of the microstructure and to the addition of the unusual alloying element phosphorus. These new steels are of interest to all automobile manufactures because weight reduction of auto-body parts can be realized. The future perspectives for the application of dual-phase steels are discussed.     

Untersuchungen zur Schallemission von Rißbildung und -ausbreitung in Punktschweißverbindungen unter Last

Crack Initiation and Propagation in external loaded Spot Weld Joints controlled by Acoustic Emission The crack initiation and propagation in spot welded joints is controlled by acoustic emission. A high strength steel with different mechanical properties after heat treatment was welded and the different stages of the fracture and their influences are studied by analyzing methods like acoustic emission and metallographic and scanning electron microscopic techniques.     

Untersuchung der Rißbildung in Salzgestein mit Hilfe der Schallemissionsanalyse

Acoustic Emission Measurements During Hydraulic Fracturing Tests in Rock salt To investigate the state of stress in the rock, during a hydraulic fracturing experiment the hydraulic pressure in a sealed volume of a borehole is increased up to the initiation of a fracture. In this paper hydraulic fracturing experiments are described which were performed in the salt rock of the former Asse salt mine and in a laboratory specimen of 200mm diameter. Acoustic Emission measurements were applied to evaluate the three-dimensional orientation and the extension of the fracture area. A multi-channel signal analysis system was used. The results correlate fairly well with the fracture locations which were surveyed after the experiments.     

Das Kriechverhalten von stickstofflegierten,austenitischen Stählen mit erhöhter Festigkeit bei Raumtemperatur

The Creep Behaviour at Room Temperature of Higher Strength Austenitic Steels containing Nitrogen. The mechanical properties of special austenitic steels with increased Cr and Ni contents and N₂ content up to 0,35 % have been investigated. In the as-received condition i.e. annealed and water quenched, the 0,2 % offset yield strength of these steels ranges from 400 to 500 N/mm². However, regardless of these increased yield strengths, the stress-strain characteristics are the same as for conventional austenitic steels. Since first plastic deformations correspond to an applied stress equal to 50 % of the 0,2 % yield strength, in practical applications the creep behaviour of these steels may be significant. Therefore, the creep behaviour at room temperature of the base metals and weldments has been investigated to determine the influence of thermal and mechanical treatments after annealing. The elastic limit of airquenched steels is significantly higher than of water-quenched, even though the 0,2 % yield strengths are essentially the same. This different stress-strain dependence influences the creep behaviour of the steel. The cold working of these steels by plastic straining in both tension and compression results in a useful strain hardening effect only when the direction of applied stress is the same as the stress during cold working. When the direction of stressing was reversed, a considerable Bauschinger-effect was observed.     

Verhalten von Hochtemperatur-Lötverbindungen bei schwingender Belastung und erhöhten Temperaturen

Fatigue behaviour of High Temperature Brazed Joints tested at elevated temperatures Fatigue testing, on a systematic and statistical basis, was carried our on high temperature brazed joints produced in the base metal NiCr20TiAl joined with BNi-5 and BAu-4 filler metals. The single-stage tests were conducted with various temperatures of 500 and 700 °C. Microscopic examination of the fracture surfaces (SEM) yielded information about crack initiation and propagation and the fracture behaviour of the joints.     

Spannungsrißbildung und Spannungsrißkorrosion bei Kunststoffen

Environmental stress cracking and stress-corrosion cracking in plastics Plastics are being used to an increasing extent in fields which have hitherto been the reserve of other, usually metallic, materials. Owing to their good processability and chemical resistance, plastics engineering materials are for example employed for vessels, pipes, valves and fittings, blowers, pumps and gear wheels. In these applications they are not only exposed to liquid media but, are also subjected to occasional or continuous mechanical stress. The trouble is that there are media to which plastics in the stress-free state are absolutely immune, but that, when tensile stresses are present, cracking may occur; this phenomenon is called environmental stress cracking. It is therefore necessary for the design engineer both to know whether the medium to be handled causes environmental stress cracking, and to be familiar with the other properties of the material. The following article describes the most important methods of detecting environmental stress cracking and the most important media which cause this form of cracking in common plastics materials.     

Die Rißzähigkeit und Brucharbeit von SiC-faser-verstärkten Glasmatrix-Verbundwerkstoffen: Bestimmung durch die Chevron-Kerb-Technik

Fracture Toughness and Work of Fracture of SiC-Fibre Reinforced Glass Matrix Composites: Assessment by Means of the Chevron-Notch Technique The applicability of the chevron-notch flexural technique to determine the fracture toughness and work of fracture of fiber reinforced glass matrix composites was investigated. Borosilicate glass matrix composites containing SiC-Nicalon fiber reinforcement were considered in the as-received condition and after thermal aging. The thermal aging involved exposure of the samples to temperatures in the range 500–700 °C for up to 1000 hours in Argon. To detect the onset of unstable microcracking during the chevron-notch experiments, an acoustic emission technique was used. Except for the most severe conditions investigated (600 °C, 1000 hours and 700 °C, 100 hours), the values of the fracture toughness did not change significantly, and they were in the range 19-26 MPa m^1/2, in agreement with literature results. The possible degradation mechanisms which may be activated during high-temperature aging are discussed.     

Rißbeginn und Rißfortschritt in unlegierten Kohlenstoffstählen. Untersuchungen mit Hilfe der Schallemission

Investigations of Crack Initiation and Crack Propagation in Plain Carbon Steels by Means of Acoustic-Emission Crack initiation and propagation has been studied during fatigue tests with ferritic, ferritic-pearlitic and pearlitic specimens by means of acoustic emission (A.E.) instrumentation. The ferrite and the ferrite-pearlite showed acoustic emission only in connection with fast yielding of large volumes. Early steps of crack initiation showed no detectable acoustic emission because of very small yielded volumes and small amounts of crack surface friction. Gapping of the cracks inhibited A.E. due to friction during the early steps of crack propagation. Pearlitic specimens showed strong A.E. prior to microscopic detection of cracks, ascribed to fast fracture modes. Cracks in ferritic and ferritic-pearlitic CT-specimens are only detectable by crack surface friction. The increase of yielded volume with the propagating crack is too small to be distinguished from friction noise. Pearlitic specimens showed strong burst-signal activity ascribed to cleavage fracture and fast fracture modes. Great differences in A.E. of specimens with the same microstructure are in most cases the result of differing crack surface topography and therefore very differing friction areas. Friction noise depends clearly upon loading conditions and crack surface geometry.     

Gültigkeits- und Anwendungsbereich der Rißbruchmechanik

Fracture Mechanics Range of Validity and Application. The range of validity and application of the linear-elastic and elasto-plastic fracture mechanics is determined according to practicable quantitative criteria. The former is applicable to the low stress brittle fracture in brittle materials, the latter is applicable to the higher stress brittle and tear fracture in tougher materials. The real continuum mechanics fracture criterium is not yet determined. The linear-elastic fracture mechanics is widely applicable to fatigue fractures. Taking into account the interaction and sequence effect of low and high load amplitudes is necessary. The fracture mechanics approach to brittle and fatigue fracture is compared with traditional testing techniques.     

Messungen der Rißschließeffekte an Al 8090-T6 und 42CrMo4

Two different materials, a quenched and tempered steel with German designation 42CrMo4 and an Al-Li 8090-T6, were used to investigate the influence of crack closure on the lower part of the loading cycle. The usually measured closure parameter K_op was determined, too. But, the primary target of the present investigation was the lowest K_min-value experienced by the crack front in the tension-tension fatigue regime. This lowest K_min was denoted as K_min. In addition, the modification of ΔK to a lower value ΔK_red due to crack closure during the lower part of a fatigue cycle has been determined. It was found that for 42CrMo4 the influence of crack closure on K_min or ΔK is negligible, while for the Al-Li 8090-T6 closure can substantially alter K_min and ΔK. The two materials investigated can – in respect to their closure effects – be considered as the two boundary conditions enclosing the range of structural materials.     

Untersuchungen zur Haftreibung zwischen faserverstärktem Borosilicatglas und Kalk-Natron-Glas bei erhöhten Temperaturen

Investigation of static friction of fiber reinforced borosilicate glass mated to soda lime glass at elevated temperatures Tribological tests were carried out on a soda lime glass mated to SiC- and C-fiber reinforced borosilicate glass. The static coefficient of friction and the transition to sliding were measured using a ring-on-block geometry at temperatures between 25 and 500 °C. The fiber orientation (parallel or antiparallel) refering to the direction of sliding was varied. The static friction coefficient decreased with increasing temperature for both the SiC and the C-fiber reinforced glass owing to increasing tendency for adhesion and resulting material transfer. At temperatures up to 300 °C, there was no difference between the static and the kinetic coefficient independent of the material, whereas the static friction coefficient was lower for parallel than antiparallel fiber orientation. At test temperatures above 300 °C and antiparallel fiber orientation, stick-slip effects occurred under the experimental conditions used. The tribological mechanisms could be explained by SEM (scanning electron microscopy) studies of the worn surfaces.     

Neues und umweltfreundliches Entölungsverfahren durch Niederdruck-Verdampfung

Ecologically beneficial processes for deoiling metallic or plastic parts to be coated or brazzed gain more and more importance. Especially the low-pressure evaporation process shows recognizable advantages. The oily parts or devices are cleaned at medium temperature and at process-pressures of less than 0.1 mbar from the adhered machine oil. In opposite to liquid deoiling procedures also complete assembled devices can be treated in one process step only. Joined plasma processes activate or modify the surfaces for decoration or functional coatings. In the following contribution the process-technology and machine design is explained and as an example a vacuum-deoiling plant for automotive heat exchangers is presented.     

Das Verhalten rostfreier Stahlgußlegierungen unter gleichzeitiger mechanischer und korrosiver Belastung

Behaviour of stainless cast alloys under combined mechanical and corrosive attack In media with 3000 mg B/1 (as boric acid, H₃BO₃) and varying chloride contents down to 1 mg Cl^?/1 13%-Cr-steels show pitting. Under constant load cracks appear simultaneously. As can be expected, 17%-Cr-steels are more resistant and duplex ferritic austenitic steels exhibit no corrosion cracking in 3% NaCl-solution even after 10 000 hrs. Generally, corrosion resistance improves with increasing contents of ferrite forming elements. Corrosion cracks are interpreted in termes of anodic stress corrosion cracking. Notch impact bending tests with hydrogen charged specimens show that deformability is reduced.     

Aktivierungsenthalpien und Mechanismen der unterkritischen Rißausbreitung in Aluminiumoxidkeramiken

Activation Enthalpies and Mechanisms of Subcritical Crack Propagation in Aluminium Oxide Ceramics Progressive crack propagation has been investigated in the temperature range between 25 and 1000°C for two commercial varieties of Al₂O₃ ceramic, using double torsion specimens. 3 different temperature regions have been found, each of them with a different dependence of the effective activation enthalpie on the stress intensity factor. While at low temperatures the crack propagation rate is mainly controlled by the reaction of the grain boundary phase with the water vapor partial pressure of the environment, the softening of this phase is the predominent factor at high temperatures.