Einfluß einer Oberflächenkaltverfestigung auf die Schwingfestigkeit und das Wechselverformungsverhalten der normalisierten Kohlenstoffstähle Ck 15 und Ck 45 unter Zugdruck-und Umlaufbiegebeanspruchung

Effect of Surface Cold Working on Fatigue Strength and Cyclic Stress-Strain Behaviour of Normalized Plain Carbon Steels Ck 15 and Ck 45 under Compression-Tension and Rotating Bending Load Investigations on deep rolled specimens of normalized plain carbon steels showed that surface could working can improve the fatigue strength by about 15% in rotating bending tests and by about 5% in cyclic compression-tension tests. The better endurance properties are mainly caused by a reduced amplitude of plastic strain. From microscopical examinations of Ck 15 surfaces was deduced that crack initiation in deep rolled specimens resembles by far with that in specimens not surface treated. During fatigue tests the dislocation structure produced by deep rolling approached that developing in samples without cold worked surface.     

Stress Controlled Cyclic Deformation of Ck 15-steel at intermediate life

Experimental investigations have been carried out to study the cyclic deformation behaviour of Ck 15-steel in the intermediate life under constant stress amplitudes, 24 kp/mm² ? σ_a ≥ 30 kp/mm², in axial pushpull fatigue. In all the cases, an initial softening process followed by a hardening process has been observed during fatigue life. The plastic strain amplitudes near fatigue life of the specimens are linearly dependent on the stress amplitudes. From the experimental results, it has been observed that both the fatigue ductility exponent and the fatigue strength exponent have a value of around 0.32. Experimental observations on Ck 22 steel are in excellent agreement with these findings.     

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.     

The effect of steady torsion on fatigue crack growth in shafts

Most of catastrophic mechanical failures in power rotor shafts occur under cyclic bending combined with steady torsion: Mode I (ΔK_I) combined with Mode III (K_III). An analysis of the influence of steady torsion loading on fatigue crack growth rates in shafts is presented for short as well as long cracks. Long cracks growth tests have been carried out on cylindrical specimens in DIN Ck45k steel for two types of testing: rotary or alternating bending combined with steady torsion in order to simulate real conditions on power rotor shafts. The growth and shape evolution of semi-elliptical surface cracks, starting from the cylindrical specimen surface, has been measured for several loading conditions and both testing types. Short crack growth tests have been carried out on specimens of the same material DIN Ck45k, under alternating bending combined with steady torsion. The short crack growth rates obtained are compared with long crack growth rates. Results have shown a significant reduction of the crack growth rates when a steady torsion Mode III is superimposed to cyclic Mode I. A 3D Finite Element analysis has also shown that Stress Intensity Factor values at the corner crack surface depend on the steady torsion value and the direction of the applied torque.     

The influence of Al2O3 particulate reinforcement on cyclic stress response and fracture behavior of 6061 aluminum alloy

The cyclic stress response characteristics and cyclic fracture behavior of aluminum alloy 6061 discontinuously reinforced with particulates of Al₂O₃ are presented and discussed. The 6061/Al₂O₃ composite specimens and the unreinforced 6061 aluminum alloy were cyclically deformed using tension-compression loading under constant total strain amplitude control. Both the composite and the unreinforced alloy exhibited softening to failure from the onset of cyclic deformation. The degree of softening was observed to increase at the elevated test temperature for both the composite and the unreinforced counterpart. The intrisic micromechanisms controlling the stress response characteristics during fully-reversed cyclic straining are highlighted and rationale for the observed behavior is discussed. The cyclic fracture behavior of the composite is discussed in terms of the competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, cyclic stress response, and test temperature.     

Influence of changes of the bending plane position on the fatigue life

The paper concerns influence of changes of the bending plane position on the fatigue life. The obtained results were analyzed and compared with the fatigue results for oscillatory bending. The applied specimens were smooth, they had round sections, and they were made of the leaded brass CuZn40Pb2 (MO58). The results obtained under cyclic bending with the plane position change were compared with the results obtained for the specimens with the same parameters under pure oscillatory bending. A change of the bending plane position occurred every 10% fatigue life determined under pure oscillatory bending at the given amplitude of the bending moment, according to the defined fatigue characteristics. Calculated values of nominal stress in a cross section were recalculated according with cyclic material properties and values of elasto‐plastic stress were obtained.     

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.     

Die Schadenslinie im Wöhlerdiagramm

The Damage Line of the Wöhlerdiagram Sometimes, Wöhlerdiagrams are supplemented by so-called damage lines or French-lines, synonymously. Due to the statistical nature of fatigue events, however, Wöhlerdiagrams more and more are statistically ascertained and described by curves which commonly are referred as to probability-stress-cycle curves (P-S-N). Accordingly, it was to find out, whether the feature “damage” might be described in a similar way, aiming at P_d-S-N curves (P_d = Probability of damage) within the expected damage field. Using the steels Ck 35 and 34 CrMo 4 quenched and tempered, the statistically planned and evaluated investigations showed that within the Wöhlerdiagrams of these steels large damage fields can be defined which extend from far before the ranges of finite life till far into these ranges (of scatter). P_d-S-N curves are given. Satisfactory shortcut-methods for estimating such curves using features as specimens' temperature and deformation during cyclic pre-loading could not be developed. By progressively increasing and – another sample – decreasing load tests to the same account of 0,34 Palmgren-Miner-damage specimens showed remarkable differences when compared within French's concept of damage.     

FAILURE MECHANISMS IN IMPACT FATIGUE OF METALS

Abstract Impact fatigue tests were performed with smooth and notched specimens of low carbon steels under various impact loading conditions. The characteristic failure mechanisms in impact fatigue was discriminated by comparison with those in non-impact, ordinary fatigue. The fatigue life of smooth specimens was uniquely related to the range of plastic strain at the middle of the fatigue life in both impact and non-impact fatigue, although the characteristics of micro-structural deformation and cyclic stress-strain relationships were markedly different. The growth rate of a fatigue crack in impact fatigue of notched specimens was higher than that in non-impact fatigue when compared at the same stress intensity factor. Fractographic observations with scanning electron microscopy and the X-ray diffraction technique revealed more abundant cleavage facets and a smaller spread of the plastic zone beneath the fracture surface made by impact fatigue. Both nucleation and propagation lives in notched specimens were much shorter in impact fatigue than in non impact fatigue when compared at the same values of nominal stress and stress concentration factor.     

A comparison of uniaxial and rotating bending fatigue tests on an acetal co-polymer

Using the same type of injection moulded specimen, uniaxial and rotating bending fatigue tests have been carried out and the results compared. In each type of testing conventional fatigue or thermal softening failures occurred depending on the loading conditions. Much higher cyclic frequencies could be used in rotating bending without causing thermal softening failure. Injection moulding produces a skin at the surface of the specimens which is more resistant to fatigue crack initiation than the internal structure. Since the maximum stress in bending is at the surface, the skin effect contributes to the much larger fatigue endurances observed in rotating bending. A sharp V-notch, a diametral hole or a moulded weld line in the specimens reduced endurances in both types of fatigue loading to various extents.     

Statistischer Größeneinfluß unter mehrachsiger Wechselbelastung

Statistical Size Effect Under Alternating Load The results of 41 series of tests with smooth cylindrical and notched specimens, subjected to uni- and multiaxial alternating loads (tensioncompression, torsion, rotating bending and combined torsion and bending), are compared with theoretically predicted data. The prediction are based on the ?Weakest-Link-Concept”?. Multiaxial stresses are included either by means of suitable strength criteria, or by a fracturemechanical consideration of a crack in a multidimensional stress field. For the evaluation of the experimental data the 2-parametric Weibull's equation was used. The experimental and theoretical results may deviate by less than 20%, when the ratio between the strength at cyclic torsion and cyclic bending ist taken into account in the function of survival probability [eqns. (2), (7)].     

SMALL FATIGUE CRACK GROWTH IN A LOW CARBON STEEL UNDER TENSION–COMPRESSION AND PULSATING-TENSION LOADING

The growth behaviour of small fatigue cracks has been investigated in a low carbon steel under axial loading at the stress ratios R of –1 (tension-compression) and 0 (pulsating-tension). Crack closure was measured to evaluate the effects of stress ratio and stress level on small crack growth. Except for the accelerated growth at stress levels close to the yield stress of the material, at R=–1 small cracks grow faster than large cracks below a certain crack length, but at R= 0 the crack growth rates for small cracks are coincident with those for large cracks in the whole region of crack length investigated. The critical crack length, 2c_c, above which the growth behaviour of small cracks is similar to that of large cracks depends on stress ratio, being 1–2 mm at R=–1 and smaller than 0.7 mm at R=0. The 2c_c value at R=–1 agrees with that obtained under rotating bending (R=–1). The small crack data are closely correlated with large crack growth rates in terms of the effective stress intensity range, ΔK_eff; thus ΔK_eff is found to be a characterizing parameter for small crack growth including the growth at the higher stress levels.     

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.     

Die Scheiteldruckprüfung korrosionsbeanspruchter Hoch- und Höchstdruckrohre zur Untersuchung des Bauteil-Rißwachstumsverhaltens unter Modus II-Schwingungsrißkorrosion

External Crown Fatigue Loading of High and Ultrahigh Pressure Tubes Subjected to Corrosion – a Highly Informative Test Predicting the Crack Growth Behaviour of Tubular Components under Mode II Corrosion Fatigue Conditions Stressing high and ultrahigh pressure tubes by external static or fatigue loads has been qualified as a convenient method to simulate the load case “internal pulsating pressure” by analysing the stress state of thick walled tubes when loaded by internal pressure and external crown loads, respectively. The results of different analytical calculations were compared with that of a Finite-Element-Computation demonstrating, for tubes with nominal pressures in the range of 325–3600 bar, an excellent correspondence. Tests with 86 tube cuttings of steel X 6 CrNiMoTi 17 12 2 (W.-No. 1.4571; ASTM UNS S 31635; BS 320531) showed the following results: In air, pulsating pressures of 325 bar (corresponding to the maximum allowable operating pressures) are sustained indefinitely. Under mode II-corrosion fatigue in 0,1 N H₂SO₄ (30°C) failure occures after 3,8 · 10⁷ mode cycles. A twentyfold H₂SO₄ concentration will lower the number of cycles to fracture to one tenth of this value without leaving mode II. Under mode II corrosion fatigue crack growth will propagate faster in radial direction than in air, so that leak-before-break under internal pressure will be likely. Crack growth rates in radial direction increased with increasing acid concentration so that the probability for leak-before-break will further increase. Highest priority for the surveillance strategy of components loaded in mode II CF has, however, the prove that crack initiation in this environment is commencing much earlier than in air, and definitely earlier than found for compact specimens tested in a mode II pulsating fatigue or rotating bending test.     

Berechnung der Dauerschwingfestigkeit unter Berücksichtigung der spannungsmechanischen und statistischen Stützziffer

Fatigue Limit and Geometrical and Statistical Size Effect The ratio of the fatigue limit of an unnotched specimen to that of a notched one, the fatigue notch factor K_f, is usually smaller than the theoretical stress concentration factor K_t. With the assumption of a plastic cyclic deformation ?_apW at the level of the fatigue limit the fatigue limit for a notched specimen can be calculated. According to formula (4) this fatigue limit σ_naD is a function of K_t, ?_apW, the notch stress amplitude σ_a, the cyclic strengthening exponent n′ and the fatigue limit of a smooth specimen σ_W. Moreover, taking into account the statistical size effect with the known “weakest link concept”, see flow chart Fig. 9, the calculation is in a good agreement with 77 test results for steel.     

Deformation und Versagen von StE460 und AlMg4,5Mn bei mehrachsig-proportionalen Beanspruchungen mit konstanten und variablen Amplituden

Deformation and failure behaviour of FeE460 and AlMg4.5Mn under multiaxial proportional loading with constant and variable amplitudes To calculate the fatigue life-to-crack initiation of engineering components under combined cyclic loading, experimentally secured knowledge on the cyclic deformation and failure behaviour of the materials used under the certain multiaxial cyclic stress and strain conditions are required. To obtain this, strain-controlled fully reversed experimental tests at tensional, torsional and combined loading with constant and variable amplitudes have been conducted using thin-walled tube specimens of FeE460 and AlMg4.5Mn. Experimental tests on standard uniaxially loaded hourglass specimens have also been conducted to study specimen form effects. Cyclic deformation behaviour can be uniformly described by the stabilised cyclic σ-ε-curve, if stresses and strains are expressed as equivalent values according to the von Mises criterion. Failure behaviour at constant and variable amplitude loading is characterized by the initiation and growth of short cracks at right angle to the direction of the greatest principal stress (mode I) in the case of tensional or combined loading and by short crack growing in both shear stress directions (mode II+III) in the case of torsional loading. At fully reversed constant amplitude loading, all three types of load can be described by one constant amplitude strain life-to-crack initiation curve. At variable amplitude loading (notch strain simulation with gaussian spectrum, H₀=10⁵), the experimental fatigue life-to-crack initiation values are lower than estimated values based on Miner-calculations using an equivalent stress-strain supported P_SWT-N-curve. The question of mean stresses and their evaluation is discussed.     

Effect of cyclic hardening on fatigue performance of slide burnished components made of low‐alloy medium carbon steel

The slide burnishing process causes cyclic loading of the surface being treated, which provokes cyclic hardening. Using a forced‐controlled indentation test, the sixth “loading‐unloading” cycle was stabilised. The effect of the number of passes and the cyclic loading coefficient (CLC) on the fatigue performance of slide burnished specimens was investigated. Rotating bending fatigue tests were conducted using nine groups of hourglass shaped specimens, which were slide burnished through a different number of passes and CLC values. A stabilised cycle of the surface layer achieved with six passes, lead to largest fatigue limit, whereas the CLC exerted negligible influence on the fatigue performance. The observed phenomenon was explained through different residual stress relaxation rates, due to the rotating bending load, as well as with the obtained surface layer microstructure. The residual stress relaxation was investigated through rotating bending fatigue tests, using cylindrical fatigue specimens, followed by X‐ray stress analysis.     

Wechselverformungskurven und Mikrostruktur bei spannungskontrollierter Zug-Druck-Wechselbeanspruchung von Ck 45 im Temperaturbereich 295 K ≤ T ≤ 873 K

Cyclic deformation curves and microstructure of SAE 1045 after stress-controlled push-pull loading in the temperature range 295 K ≤ T ≤ 873 K Stress-controlled cyclic deformation tests were performed with normalized SAE 1045 in the temperature range 295 K ≤ T ≤ 873 K. From the measured mechanical hysteresis loops cyclic deformation curves were determined, which are characterized by temperature dependent cyclic softening and hardening processes. Due to the cyclic deformation processes at any temperature distinct dislocation structures are formed depending on the stress amplitudes and the number of cycles chosen. In the whole temperature range and for all stress amplitudes the plastic strain amplitude was proved to be a suitable parameter to describe the actual fatigue state.     

Lebensdauer- und Dauerfestigkeitsabschätzung von zugdruckwechsel- und zugschwellbeanspruchtem Stahl mittels Temperaturmessung

Estimation of the Endurance and the Fatigue Limit of Steel by Measuring Specimens′ Temperatures. Microplastic deformation processes are pre-requisites for fatigue crack formation within metallic materials. If the testing frequency of a specimen, cyclically stressed by a progressively-increasing load test, is not too low it is no great metrological problem to ascertain that special stress amplitude, σ_f,th (f ? fatigue limit, th ? thermometrical), at which specimen's temperature begins to rise due to the start of ‘remarkable’ microplastic deformations. Investigations of this kind, recently carried out by rotating bending showed a very good correspondence between σ_f,th and a statistically ascertained estimate of the fatigue limit, \documentclass{article}\pagestyle{empty}\begin{document}$ \hat \sigma _{({\rm P} \simeq {\rm 0}\%{\rm)}} $\end{document} (P ? Probability of fracture), derived from comparatively performed Wöhler-tests. The purpose of this paper is to investigate the relationship between σ_f,th and \documentclass{article}\pagestyle{empty}\begin{document}$ \hat \sigma _{({\rm P} \simeq {\rm 0}\%{\rm)}} $\end{document} for some carbon steels when cyclically stressed by push-pull and pulsating tensile loading, respectively. Both, unnotched and notched specimes were tested. Moreover, thermometrically monitored Wöhler-tests revealed that temperature measurements can provide a short-cut prediction of specimens′ lives. Above all it has to be mentioned that a reliable clue is gettable at a very early experimental stadium whether the cyclic stressed specimen will later become a ‘break’ or – normally much later – a ‘run-out’.     

The effect of notch plasticity on the behaviour of fatigue cracks emanating from edge-notches in high-strength β-titanium alloys

The present paper is aimed at investigating the behaviour of fatigue cracks emanating from edge-notches for two different microstructures of the Ti-6246 alloy, produced by two specific thermo-mechanical treatments and defined as β-annealed and β-processed, respectively. Pulsating four point bending tests were performed on double-edge-notched specimens. The initiation and early propagation of fatigue cracks were investigated at two relatively high nominal stress levels corresponding to 88 and 58% of the 0.2% material’s yield stress. Plastic deformation at the notch tip initially produced a local stress redistribution followed by elastic shake down due to the high cyclic strain hardening rates exhibited by both microstructures, as confirmed by finite element modelling. Crack closure effects, measured by an extensometric technique, and variations in crack aspect ratio were considered in the ΔK calculation. The obtained crack growth rate data were compared with those of long cracks measured on standard C(T) specimens as well as of microcracks measured on round, unnotched S-N type of specimens to evaluate the intrinsic fatigue crack propagation resistance of the two microstructures. The contribution of notch plasticization to crack closure was estimated by finite element modelling.