Ermüdungsverhalten von Schraubenverbindungen

Fatigue Behaviour of Bolted Connections In bolted connections the bolt must withstand under alternating forces the resulting fatigue loads at each local area. Therefore the endurance limit of the first stressed engaged thread in the nut or tapered hole is related to the thread diameter, the nut heigth and the strength ratio of the bolt and nut. It is shown which influence on the fatigue limit has the thread profile, the pitch-diameter-ratio, the prestress in relation to the strength grade of the bolt and the thread rolling process before or after heat treatment. Additional bending will cause a significant loss of fatigue strength, while high temperature application does not decrease the fatigue limit.     

Zum Ermüdungsverhalten von Hartschaumstoffen

Fatigue Behaviour of Rigid Cellular Plastics The fatigue behaviour of crosslinked and non-crosslinked rigid cellular PVC had been investigated for tensile, compressive, and shear stresses, and displayed by means of Wöhler diagrams. In dynamic extension/compression loading, precompression shortens the life of the specimen, while pre-extension lengthens it - within certain limits. This behaviour can be explained by the different forms of the stress-strain curves in extension and compression. Detailed analysis of the hysteresis loops, including evaluation of the strains, energy uptake, and damping value, allows conclusions to be drawn about the behaviour of the material when subjected to dynamic loads and the degree of approach to failure. Although cellular plastics are poor thermal conductors there is very little rise in temperature when they are subjected to dynamic loads.     

Ermüdungsverhalten von Betonstahl unter Korrosionsbedingungen

In the 2001 completely revised version of DIN 1045‐1 for the first time an explicit proof against fatigue, more a fatigue analysis, have been published. A limit on the fatigue strength at two million cycles, as it was formerly common in civil engineering, is no longer in use. The influence of corrosion on the fatigue behaviour has been investigated insufficiently. It is thus not satisfyingly clarified. To fill this gap of knowledge, a research program was launched, in which fatigue tests at steel samples with a diameter of 16 mm were performed under going corrosion. The fatigue behaviour of the reinforcing steel was determined for four different corrosive media. The aim of the investiagtions was to show that the SN lines, which are the result of the fatigue tests, have exposure caused very different patterns. On the other hand it is examined, whether crack initiation and crack growth of the steel specimen can be recorded separately from each other. Therefore, different testing methods for crack detection were used. They differ especially with respect to their methodology. The article briefly describes the use of the testing methods for crack detection, explains the fatigue tests with the different corrosive liquids and discusses the test results.     

Einfluß der Trennverfahren auf das Ermüdungsverhalten von Stahlblechen

Influence of Cutting Method on the Fatigue Behaviour of Steel Sheets The present paper gives the fatigue behaviour of test specimens which have been obtained by laser cutting, plasma cutting or punching from steel sheets. The paper demonstrates the different damage to the surface introduced by the three techniques. For thin sheets (1.5 mm) laser cut specimens show advantages, for thicker sheets (6 mm) plasma cut sheets exhibit better fatigue properties.     

Einfluß von Verfahrensparametern beim Fließpressen auf das Ermüdungsverhalten

Effect of Process Parameters in Extrusion on Fatigue Behaviour Fatigue test specimens were produced by forward extrusion and subsequent heading from conventional steels, P/M steels and AlMg4.5Mn. The strain, die-opening angle, temperature and in some cases the lubricant were varied. For purpose of comparation in some cases also specimens were prepared by machining. S/N curves or cyclic deformation curves were determined mainly by stress-controlled tests. For all materials the time strength was increased by cold forming whereby the effect of strain was stronger than that of the die-opening angle. The time strength of steel is less increased by warm (semihot) forming than by cold forming. The fatigue strength, however, is not necessarily increased by cold or warm forming. After cold forming the P/M steels contrary to conventional steels revealed mainly quasielastic deformation until fracture. After warm forming, however, P/M steels, due to the decrease of porosity, behaved almost like conventional steels. For AlMg4.5Mn specimens prepared by forming – probably due to worse surface quality – revealed a lower fatigue strength than those prepared by machining. Here also a feasible effect of the choice of the lubricant was stated.     

Ermüdungsverhalten von normalisiertem 42CrMo4 unter Torsionsbeanspruchung bei Raumtemperatur

Room Temperature Fatigue Behaviour of a Normalized Steel SAE 4140 in Torsion Cyclic deformation behaviour of a normalized steel SAE 4140 in shear strain-controlled torsion is characterized by cyclic softening and cyclic hardening. If mean shear stresses are superimposed to an alternating shear stress, cycle-dependent creep occurs, and the number of cycles to failure decreases. In shear strain-controlled torsional loading, mean stresses are observed to relax nearly to zero within a few cycles. Fatigue life is not influenced by mean shear strains.     

Ermüdungsverhalten und Dauerfestigkeit von Graphit und Aluminium – infiltriertem Graphit

Fatigue behaviour and endurance limit of graphite and of aluminium‐infiltrated graphite Fatigue properties of polycrystalline, isotropic graphite FU2590 and of FU2590 infiltrated with AlSi7Mg (FU2590/AlSi7Mg) were investigated in reversed bending tests at 25 Hz at numbers of cycles below 10⁷ and in tension‐compression tests at 20 kHz below 10⁹ cycles. The open porosity of Graphite (10‐11 Vol.‐%) was infiltrated with the aluminium alloy using the squeeze casting infiltration method, which led to an increase of the bending strength by 50 %, increase of tensile strength by 30 % and increase of stiffness by 15 %. Fully reversed tension‐compression loading of FU2590 delivers a mean endurance limit at 10⁹ cycles at the normalized maximum stresses (i.e. maximum tension stress of a cycle divided by the static strength) of 0,65±0,03. Mean numbers of cycles to failure of 10⁴ were found in fully reversed bending tests at the normalized maximum stress of 0,78. The infiltrated material shows approximately 30 % higher cyclic strength in reversed bending tests, and the mean endurance limit under tension compression loading increases by 15 %. The increased endurance limit of the infiltrated material is caused by the increased stiffness. The increased toughness of graphite due to the infiltration with aluminium is of additional beneficial influence at the higher cyclic stresses investigated in reversed bending tests and in static tests.     

Zum thèrmisch-mechanischen Ermüdungsverhalten von NiCr22Co12Mo9

On the Thermal-Mechanical Fatigue Behaviour of NiCr22Co12Mo9 The fatigue behaviour of the Ni-based alloy NiCr22Co12Mo9 (corresponding to Inconel 617) under combined cyclic thermal and mechanical “in-phase”- and “out-of-phase”-loading was investigated with a constant minimum cycle temperature of 473K and a constant total strain amplitude of 6,25% at maximum cycle temperatures T_o ranging from 873K to 1473K. It was found that the cyclic deformation behaviour and the corresponding development of the microstructure during the tests were mainly determined by the maximum cycle temperatures. With increasing T_o increasing recovery processes occurred accompanied by charakteristic changes in the microstructure which reduced cyclic hardening. In contrast, both maximum cycle temperature and cycle mode determined surface deteriorations, which were characterized by surface cracks, and fatigue life. At the highest temperatures during the in-phase-loading cycles, the occuring tensile stresses caused increasing amounts of intergranular damage with corresponding reductions of fatigue life.     

Ermüdungsverhalten von Schraubendruckfedern bei konstanten und variablen Beanspruchungsamplituden bei sehr hohen Schwingspielzahlen

A test rig for simultaneous testing of up to 88 compression springs under constant as well as variable amplitude loading is presented in this paper. The test rig utilizes a servo‐hydraulic testing machine. The results of long‐term fatigue tests of compression springs under constant and variable amplitude loading up to 5 ? 10⁸ and 1.4 ? 10⁷ cycles are presented. Experimental Woehler‐ and Gassner‐curves are obtained using the maximum likelihood method. Theoretical Gassner‐curves are generated using Miner's rule and experimental Woehler‐curves. The theoretical Gassner‐curves are compared to the experimental ones. The results of the constant amplitude loading tests are compared to literature data. The possibility to increase the testing frequency in variable amplitude loading tests is discussed. Thereto, the comparability of results from fatigue tests of material specimens using torsional ultrasonic fatigue testing equipment to results from fatigue tests on compression springs is addressed.     

Ermüdungsverhalten von Cu-35%Ni-3,5%Cr nach unterschiedlichen Wärmebehandlungen

Fatigue Behaviour of Cu-35%Ni-3.5%Cr after Different Heat Treatments Cu-35%Ni-3.5%Cr has been developed as a substitute for the conventional alloy Cu-30%Ni for marine applications. The Cu-Ni-Cr system exhibits a stable miscibility gap which contains the alloy Cu-35%Ni-3.5%Cr. Therefore, Cu-35%Ni-3.5%Cr can decompose in a wide temperature range. The fatigue behaviour of Cu-35%Ni-3.5%Cr was investigated over a wide decomposition range from solution treated to overaged condition both in air and argon. The most important results will he presented here in a comprehensive form.     

Ermüdungsverhalten einer lasergeschweißten pseudoelastischen NiTi-Legierung unter Biegebeanspruchung

Fatigue behaviour of a laser welded pseudoelastic NiTi alloy under bending stresses The mechanical fatigue of small specimens of a commercial pseudoelastic NiTi alloy produced by laser welding and laser cutting was investigated in strain-controlled bending tests. In comparison with the base material the butt welded specimens showed significantly reduced numbers of cycles to failure. This is mainly caused by higher local strains due to height reductions in the welded joints. Both the welded microstructure and the unwelded one was able to be loaded up to 10⁷ cycles without failure if the strain amplitude remained below 1%.     

Einfluß einer Mitteldehnung auf das Ermüdungsverhalten der Aluminiumlegierung AlMg5Mn

The effect of a superimposed mean strain on the fatigue behavior of the aluminum alloy AlMg5Mn The cyclic stress-strain response of the aluminum alloy AlMg5Mn in three different strength conditions is studied with emphasis on the influence of a superimposed mean strain and mean stress, respectively. The cyclic deformation behavior is characterized by a pronounced cyclic saturation independent of the prior heat treatment. Decreasing strength of the material is connected with a increasing extent of mean stress relaxation and cyclic creep, respectively. However, these transient effects are mainly restricted to the initial stage of cyclic life. In order to validate the lifetime data observed, a comparison with data from the literature is performed. For this purpose, it is necessary to apply a damage parameter which takes the damaging effect of a mean stress into account, since the cyclic lifetime values reported have mostly been obtained in tests without mean stress or mean strain. It is found that the damage parameter PSWT after Smith, Watson and Topper is suitable for the two conditions with higher strength. In the case of the soft condition, the damage caused by plastic mean strain is underestimated. As a consequence the cyclic lifes observed lie at the lower boundary of the scatter band of the literature data.     

Ermüdungsverhalten ausgewählter Werkstoffe und Bauteile bei sehr hohen Schwingspielzahlen

Very High‐Cycle Fatigue of Selected Materials and Components Results of several fatigue tests using a servohydraulic testing machine (VHF 50 D) and a spring testing machine are presented. Investigations were carried out at room temperature with. – smooth and notched specimens made of aluminium wrought alloy EN AW 6082,. – screws M 8 made of EN AW 6056,. – smooth specimens made of die cast magnesium AZ91 hp (here also tests at 125 °C) and. – shot‐peened helical compression springs of four different high strength steels. Tests were done with a frequency of 20 Hz (spring testing machine) and between 250 Hz and 400 Hz (VHF50D) up to a maximum number of cycles of N = 1,5 x 10⁹. Crack initiation sites were investigated and could be found at high number of cycles below the surface for smooth magnesium and aluminium specimens and also for most of the shot‐peened helical compression springs, but not for screws and other notched specimens.     

Einfluß der Temperatur und der Umgebungsbedingungen auf das Ermüdungsverhalten metallischer Werkstoffe

Influence of Temperature and Environmental Conditions on Fatigue Behaviour of Metallic Materials The corrosion fatigue behaviour of metallic materials is influenced by environmental conditions. With increasing temperature a similar diminishing of fatigue strength can be observed as in the usual tensile properties. Air and water vapour decrease the number of cycles to fracture because they accelerate the crack propagation by adsorptive and reactive processes at the crack tip. By the effect of pure hydrogen or hydrogen sulfide and mixtures from both appears a marked increasing of crack propagation rate in fracture-mechanical test pieces alternating loaded at low frequency (1 Hz). Corrosive mediums influence also the formation of cracks. A mechanical electrochemical failure mechanism leads to a total loss of fatigue strength. Depending on performance of materials in an aggressive solution active or passive corrosion fatigue occurs. With regard to the form of appearance the two kinds differ in characteristic manner.     

Einfluss der Schweißparameter auf das Ermüdungsverhalten hochfester Baustähle

High strength low alloy steels are used in many different engineering areas. A commonly used joining technique for those steels is fusion welding. Generally, these components have to withstand fatigue due to dynamic loading. Using thermal joining techniques affect the mechanical properties of the steel. This study focuses on the influence of the heat input on the microstructure of high strength low alloy steels (S690). Furthermore, the fatigue behaviour with special regard to crack initiation and crack propagation is characterized.     

Thermisch– mechanisches und isothermes Ermüdungsverhalten der Nickelbasis – Superlegierung IN 792 CC

Thermal-mechanical and isothermal fatigue behaviour of the nickel-base superalloy IN 792 CC Many components used in high temperature applications are exposed to complex thermal-mechanical loadings during operation. For this reason the effect of start-stop-cycles with thermalmechanical fatigue (TMF) as consequence was investigated by means of In-Phase(IP)and Out-of-Phase (OP) TMF tests. The fatigue life of the γ'hardend nickel-base cast superalloy IN 792 CC decreases with increasing maximum temperatures T_max of the TMF cycles, due to the increasing plastic deformations and the increasing mean stress (OP-TMF) or increasing intergranular; damage (IP-TMF), respectively. These relations can be satisfactorily described using the Manson-Coffin-relationship or the damage parameters of Smith-Watson-Topper and Ostergren. By contrast, the influence of different phase shifts between temperature and mechanical loading also cannot be approximately described with one consistant relation between damage parameters and fatigue life. The evaluation of TMF loadings based on results from isothermal LCF-tests with the same frequency and respective mechanical strain leads always to an overrating of the fatigue life, even if the temperature of the isothermal test is the maximum temperature of the TMF cycle. This applies when comparing mechanical loading values as well as when comparing damage parameters.     

Ermüdung von Keramik unter Schwingbeanspruchung

Fatigue of Ceramics under Cyclic Loading Fatigue of ceramics attracts more attention due to the expected availability of high-performance ceramic components e.g. for engines, However, the knowledge in fatigue of brittle materials is still inadequate, the actual standpoint as taken from literature is shortly reviewed. In this study two experimental-analytical ways are presented which manifest the chances and difficulties in this part of the fatigue research. First, the probabilistic approach to identify a true cyclic fatigue effect in brittle ceramics is envisaged. The second way applies direct observation of crack extension in static and cyclic loading mode in order to define the conditions and characteristic differences of both types of crack propagation. Results are presented from both test methods for several Si₃N₄, SiC and ZrO₂ materials and possible mechanisms are discussed. It is concluded, that toughened ceramics are more prone to cyclic fatigue effects than conventional and pure brittle ceramics (e.g. glass).     

Lebensdauerberechnung auf der Grundlage von Ermüdungsfunktionen

Fatigue Life Calculation on the Basis of Fatigue Functions Specific two-step tests constitute the basis of the fatigue functions determination. They consider the fatigue state reached during preloading in the action on the following stress step. The fatigue functions control the sequence of the fatigue life calculation at multi-step and random loadings.     

Ermüdung und Mikrostruktur von koronaren Stents

Fatigue and Microstructure of Coronary Artery Stents During implantation the structure of coronary artery stents is subjected to distinct plastic deformation. Subsequently the implant has to sustain up to 700 million cycles induced by the cyclic diameter change of coronary arteries. Regions of high deformation show characteristic deformation structures like slip traces or extrusions and intrusions of grains on the surface without failure of the material. In order to gain information about the microstructural and microtextural evolution during deformation microstructure characterisation by means of scanning electron microscopy as well as transmission electron microscopy and single grain orientation determination were carried out. The analysis of cyclic deformation with different frequencies is of special interest. Several correlations between microstructure and deformation state are observed. Few deformation characteristics occur in the solution annealed stents and increase after dilation. Inhomogeneous deformation behaviour depending on the deformation of individual grains is observed. After cyclic deformation further changes in the microstructure can be observed. These changes caused by cyclic deformation are sensitive to load frequency and generate the formation of a completely different microstructure. Knowledge of microstructure evolution caused by distinct plastic deformation and fatigue in so called oligocrystalline structures is basis for ongoing development in stent optimization.     

Rissfortschritts‐ und Ermüdungsverhalten der Aluminiumlegierung EN AW‐6060 nach ECAP und nachgelagerter Wärmebehandlung

Crack growth and high cycle fatigue behaviour of an AA6060 aluminium alloy after ECAP combined with a subsequent heat treatment Crack growth properties of the Al‐Mg‐Si alloy AA6060 as well as the high cycle fatigue behaviour have been investigated after equal‐channel angular pressing (ECAP). In our study, experiments have been conducted on different stages of microstructural breakdown and strain hardening of the material as they were present after different numbers of ECAP passes. A bimodal condition, obtained after two pressings, and a homogeneously ultrafine‐grained condition after eight repetitive pressings have been investigated. Furthermore, optimized conditions with an enhanced ductility, produced by ECAP processing combined with a following short‐time aging treatment were included into the study. Crack growth experiments have been conducted in the near‐threshold regime and the region of stable crack growth, covering a range of load ratios from R = 0.1 up to 0.7. It was found that the lowered fatigue threshold ΔK_th of the as‐extruded material can be enhanced by the combination of ECAP and short‐time aging, owing to the increased ductility and strain hardening capability of this material. By means of SEM investigations and tensile tests, the crack growth properties of the different conditions were related to microstructural and mechanical features. In fatigue tests, load reversals up to failure and the fatigue limit for an as‐extruded condition and an optimized condition after two ECAP‐passes have been compared to the coarse grained initial condition and a remarkable increase in fatigue strength was noted.