Modellierung des Einflusses von Temperatur und Verformungsgeschwindigkeit auf die Fließspannung von Ck 45 bei Temperaturen T ≲ 0,3 Ts

Modelling of the temperature and strain-rate dependance of the flow stress of Ck 45 at temperatures T ≲ 0.3 T_s After presenting the theoretical background of thermally activated dislocation slip an evaluation method is discussed, which allows to describe the influence of temperature and strain-rate on the flow stress. For this purpose it is neccessary to measure the flow stresses in tensile tests as well as the thermal flow stress jumps in strain-rate jump tests at different temperatures and strain-rates. The quality of this method is shown using experimental results obtained at specimens of normalized Ck 45 (SAE 1045). The constitutive law allows a reliable extrapolation of flow stress values to strain-rates up to 10⁺⁴ s⁻¹. The influence of plastic strain on the constitutive constants of the modelling law is discussed.     

Wechselverformung und Gefügeänderungen von Ck 15 und Ck 45

Microstructural Changes and Cyclic Deformation The crack initiation starts due to weakening and strengthening process during rotating bending. However a smaller plastic deformation amplitude is noticed at the same nominal stress compared to tension-compression stressed specimens. This results in a higher fatigue life. The different cyclic deformation behaviour was proofed by SEM (rotating bending specimens showed a lower slip line density compared to tension-compression specimens at the same nominal stress) and TEM investigations (the rotating bending specimens showed a smaller dislocation density at the same nominal stress). Furthermore it is showed, a correlation of cyclic stress strain data σ(ε_pls) between tension-compression and rotating bending specimens exists. This is also valid for the Manson-Coffin-relationship. the relation between lg ε_pls and lg N_B depends on the material (Ck 15, Ck 45) but not on the state of stress.     

Lebensdauerermittlung bei mehrachsigen wechselnden Beanspruchungen im niedrigen und hohen Temperaturbereich

Life time assessment on multiaxial cyclic loadings at low and high temperatures For the calculation of fatigue strength of components made out of ductile materials under complex cyclic load different assessments are present. As typical representatives of stress theories the shear stress intensity hypothesis (SIH) as well as the method of critical plane approach (MKS) are considered and compared for rigid and non rigid principle stress directions. Furthermore for synchronous loads the calculation methods are compared with Bach's method. The calculation method becomes more complex, if time dependent material properties at corresponding high temperatures have to be taken into account. In this case the application of viscoplastic material models is necessary, which allows the consideration of combination of creep and fatigue. As an example a modified material model by Chaboche / Nouailhas is used in order to present the calculation of multiaxial creep fatigue tests.     

Einfluß positiver und negativer Mittelspannungen auf das zyklische Spannungs-Dehnungsverhalten von normalisiertem Ck 45

Influence of Positive and Negative Mean Stresses on the Cyclic Stress-Strain Behaviour of Normalized Steel SAE 1045 In technical parts cyclic loads are often superimposed by static load components. In such cases, mean stresses influence the cyclic deformation behaviour and produce cyclic creep effects. Single step push-pull fatigue tests were carried out with normalized specimens of the steel SAE 1045. Using cyclic deformation curves and cyclic creep curves, the influence of different mean stresses will be discussed.     

Quasistatisches Verformungsverhalten reiner Sintereisen im Temperaturbereich von –184 bis 600°C

Quasistatic deformation behaviour of pure sintered iron in the temperature range between –184 and 600°C The deformation behaviour of pure sintered iron materials with densities between 6,88 und 7,57 g/cm³ was investigated in tension tests in the temperature range of –184 and 600°C. Supplementary compression tests were carried out at 20°C. Increasing density leads to increasing material resistances and ductility properties due to the increase of the bearing specimen cross sections as well as due to smaller numbers of pores, more spherical pores with smaller notch effects and smaller numbers of mircocracks, which are initiated at pores. After equal deformations, due to pore closing effects and the impediment of crack initation, the flow stresses of compressively deformed specimens are larger than those of tensily deformed. The deformation behaviour is dominated at low temperatures by thermal activated glide processes of dislocations and their interactions with short range obstacles, at middle temperatures by dynamic strain ageing due to elastic interactions of glide dislocations and diffusing carbon atoms and at high temperatures by recovery controlled dislocation creep processes.     

Verformungen und Spannungen im Bereich der plastischen Zone nahe der Rißspitze angerissener Proben aus Ck 45

Strains and Stresses in the Area of the Plastic Zone Near the Crack Tip of Precracked Steel Specimens. Models for shape and dimension of the plastic zone near the crack tip of precracked specimens are described and discussed. Using idealized assumptions an area of influence of the plastic zone is postulated in accordance with the shape of the plastic zone observed. In this area an estimation of the stress distribution has been made. X-ray stress measurements have been done in and near the plastic zone.     

Das Spannungs-Dehnungs-Verhalten von PTFE-Schälfolie im Temperaturbereich zwischen +25°C und −80°C und dessen Beeinflussung durch Kerben

Stress/strain properties of PTFE skived film over a temperature range of +25 °C to ?80 °C and the problem of notch effect Chemical plant under severe corrosive stress can be protected with “made-to-measure” liners welded up from PTFE skived film. For correct manufacture, handling and use of these chemical plant linings, a thorough knowledge of the semi-finished product and particularly of its load/deformation properties over a wide temperature range is required. The present paper deals with questions of strength and ductility and with the problem of notch effect at temperatures below + 25 °C. It is based on tensile tests carried out on PTFE skived film. Defined punched and cut notches were used to obtain information on notch sensitivity below room temperature. As the tests relate to two very different skived films, the reasons for these differences are analysed. The part played by the degree of crystallinity in determining many property differences is examined more closely with special reference to its effect on ultimate tensile strength, ductility and notch sensitivity. The aim of this study is not to determine material constants for PTFE but to identify tendencies in the behaviour of the semi-finished product, PTFE skived film, and so to gain useful information for its use in chemical plant lining.     

Einfluß von Spannungsfrei-Glühung und Abkühlungsgeschwindigkeit auf Mikrostruktur und Festigkeit von TiAl6V4

Influence of stress relieving of TiA16V4 The stress relieving of Ti6A14 V has been investigated by electron microscopy and by tensile tests. Partial recrystallization could be detected at 740 °C. The cooling rate from the stress relieving temperature exerts a marked influence on strength and elastic modulus. Slow cooling increases tensile strength, probably because of a higher volume fraction of alpha and because of an oxygen ordering reaction. It is concluded that after stress relieving, slow cooling down to a temperature of 300 °C is of advantage.     

Rissbildung und ‐fortschritt bei Schwingbelastung von Schraubenverbindungen

Crack initiation and crack propagation of fasteners under fatigue loading In this investigation the fatique strength of fasteners under variation of the property class, the sequence of manufacturing, the coating and the kind of lubrication, was determined. Thereby the crack initiation and the crack propagation of the individual variations were observed by suitable measurement instrumentation. As a result of the investigation the S‐N curves for crack initiation and for fracture of the different variations of fasteners are presented.     

Lokale Last- und Eigenspannungsverteilungen an rißbehafteten Oberflächen nach Korrosionsermüdung von vergütetem Ck 45

Distributions of Loading and Residual Stresses in Cracked Surfaces of Corrosion Fatigued Quenched and Tempered Steel Ck 45 Corrosion fatigue in the active state leads to a great number of short cracks and corrosion pits in the surface of metallic materials. This paper deals with the resulting distributions of loading and residual stresses in the case of bending fatigue. The material investigated was the quenched and tempered CBN-ground steel Ck 45. Results presented clearly indicate that stress distributions around cracks and corrosion pits are very inhomogeneous. Corrosion pits lead to a relaxation of manufacturing induced residual stresses. In the loaded state as a consequence of notch effects, maximum stresses are observed, which explains the importance of corrosion pits as crack initiation sites.     

Werkstoffanstrengung im Hertzschen Kontakt – Einfluß von Reibung und Eigenspannungen

Stress Fields in Concentrated Contacts – Influence of Normal Load, Tangential Load and Internal Stresses The stresses in the half space of a Hertzian Contact (coordinate stresses and equivalent stresses) are shown in dependence of geometry and Hertzian contact stress. The equivalent stresses were calculated with the Huber, von Mises, Henky criterion. The superposition of combined normal and tangential load with residual stresses in the material was calculated for point and line contact. The course of the maximum of the equivalent stress in dependence of the residual stresses with the parameter friction coefficient is shown.     

Eine Probeneinspann- und Heizvorrichtung für Dauerschwingversuche im Temperaturbereich 20 °C < T < 600 °C

Grip °and Specimen Heating-System for Fatigue Tests at the Temperature-Range 20 °C < T < 600 °C A grip and specimen heating system for fatigue tests is presented. The equipment is easily to handle, long-time stable and available up to a test-temperature of 600 °C also with inert environment. Specimens can be fixed without bending moments.     

Werkstoffanstrengung bei Wälzbeanspruchung – Einfluß von Reibung und Eigenspannungen

Material Stressing under Rolling Contact – Influence of Friction and Residual Stresses Material stressing of parts in rolling contact is in the main made up by the normal load to be transmitted, by surface friction due to slippage of the body in rolling contact and by residual stresses. The effects of varying slippage rates are described as well as the additional influence of residual stresses on magnitude, position and direction of load. By the aid of mere mechanical reflections the formation of flat and steep White Bands can be interpreted which are observed in long-lived ball bearings subjected to high stresses. The report shows that residual stresses and friction must not be neglected when describing the material stressing by rolling contact. This especially applies to residual stresses originating from rolling contact.