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.     

Inhomogene Verformungserscheinungen bei Schwingbeanspruchung des Vergütungsstahls 42 CrMo 4

Inhomogeneous Deformation Processes During Cyclic Loading of Quenched and Tempered Steel SAE 4140 The cyclic deformation behavior of a normalized steel SAE 4140 in the loading range σ_a < R_eS is characterized by softening and hardening phenomena. Cyclic softening processes are combined with inhomogeneous plastic deformations and the propagation of a fatigue Lüdersband. The following hardening process occurs homogeneously. At stress amplitudes σ_a < R_p0.2 the quenched and tempered SAE 4140 worksoftens continuously until crack initiation. Plastic deformation processes are extremely inhomogeneous and are concentrated on small fatigue zones. The inhomogeneity increases with decreasing tempering temperatures und stress amplitudes.     

Das totaldehnungskontrollierte Wechselverformungsverhalten unlegierter Stähle im normalisierten Zustand bei Raumtemperatur

In total strain-controlled tests without mean strain the cyclic deformation behaviour of unalloyed normalized steels with carbon contents 0,1 ≤ C ≤ 1,05 (wt.-%) is characterized by pronounced softening and slight hardening processes. With increasing carbon content the cyclic deformation curves are shifted to higher stress amplitudes. Cyclic stress strain curves of unalloyed steels determined in tests without mean load are independent of the testing procedure. With increasing carbon content the number of cycles for ε_a,e = ε_a,p (transition point) of strain-controlled SN-curves is shifted to smaller values.     

Influence of temperature on the low cycle fatigue behaviour of a modified 9Cr–1Mo ferritic steel

Low cycle fatigue (LCF) behaviour of a modified 9Cr–1Mo steel under normalized and tempered conditions is reported. The alloy was normalized at 1313K for 1 h followed by tempering at 1033K for 1 h, which resulted in a tempered martensitic structure. Total axial strain controlled LCF tests were conducted at a constant strain rate of 3×10⁻³ s⁻¹ at different strain amplitudes varying from ±0.25 to ±1.0% in the temperature range of 300–873K. The cyclic stress response behaviour, in general, showed an initial brief hardening for the first few cycles, followed by a continuous and gradual softening regime that ended in a stress plateau that continued up to the specimen failure. The fatigue life decreased as the temperature increased. The temperature effect on life was more pronounced at low strain amplitudes. The metallography of the failed samples revealed that the fatigue failure at high amplitudes of testing was marked by extensive crack branching and the formation of secondary cracks. Oxidation was found to exert major influence on LCF life reduction at 873K.     

HIGH TEMPERATURE CYCLIC DEFORMATION OF NICKEL

Abstract— High temperature cyclic deformation in nickel is investigated by comparing its cyclic hardening curves at constant strain amplitudes at four temperatures with the monotonic hardening curves and by detailed electron microscopic examination of the bulk. Just as at ambient temperature, there are broad similarities between the cyclic and monotonic hardening of nickel at high temperatures, but there are also significant differences in detail. It is shown that, as in monotonic deformation, increased dynamic recovery with increasing temperature is the main cause for the reduction in flow stress at high temperatures. However, the increased vacancy concentration produced by cyclic straining causes a linear reduction in flow stress (as distinct from the "athermal" behavior observed in monotonic deformation over a limited temperature range). Also the tendency for dislocations to agglomerate into sign-balanced, low energy configurations requires a temperature higher than that observed for monotonic deformation before high temperature softening is observed.     

Zur thermischen Ermüdung der Magnesium‐Basislegierung AZ91

Thermal fatigue of magnesium‐base alloy AZ91 Thermal fatigue tests of the magnesium‐base alloy AZ91 were carried out under total strain control and out‐of‐phase‐loading conditions in a temperature range between ‐50°C and +190°C. Specimens produced by a vacuum die casting process were loaded under constant total strain and uniaxial homogeneous stress. To simulate the influence of different mean stresses, experiments were started at different temperature levels, e.g. the lower, mean or upper temperature of the thermal cycle. The thermal fatigue behavior is described by the resulting stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. Depending on the maximum temperature and the number of loading cycles, cyclic softening as well as cyclic hardening behavior is observed. Due to the complex interaction of deformation, recovery and recrystallization processes and as a consequence of the individual temperature and deformation history, thermal fatigue processes of the material investigated cannot be assessed using results of isothermal experiments alone. The upper temperatures or the resp. temperature amplitudes determine the total fatigue lifetime.     

Influence of γ‐α′‐Phase Transformation in Metastable Austenitic Steels on the Mechanical Behavior During Tensile and Fatigue Loading at Ambient and Lower Temperatures

The present investigation is concerned with the three metastable austenitic steels AISI 304 (X5CrNi1810), 321 (X6CrNiTi1810), and 348 (X10CrNiNb189). In the temperature range ?60 °C ≤ T ≤ 25 °C tensile and fatigue tests were performed to characterize the mechanical and phase transformation behavior using stress‐elongation, stress–strain hysteresis, and magnetic measurements. The mechanical properties are significantly influenced by the temperature dependent deformation induced phase transformation from austenite to α′‐martensite which are combined with pronounced hardening processes. Furthermore microhardness measurements after fracture could be correlated with the results of the fatigue tests.     

Einfluß der Wärmeermüdung im Tieftemperaturbereich (77 K ⇔︁ 373 K) auf mechanische Eigenschaften der (α + β)-Ti-Legierung

Effect of the low-temperature thermal cycling (77 K ⇔︁ 373 K) on certain mechanical properties of a two-phase (α + β)-titanium alloy The paper reports an investigation on the effect of low-temperature thermal cycling (LTTC) within (77 K ⇔︁ 373 K) range on select mechanical properties and crystal lattice parameters of the constituent phase, α and β, of a double-phase titanium alloy as determined at temperatures of 295 K and 77 K. Fatigue tests were conducted under plane bending conditions at stresses exceeding the material endurance limit. It was found that the LTTC effects were closely related to the number of thermal cycles applied: after 100 cycles the fatigue strength at the two temperatures rose by 30% as compared with the no-LTTC condition: after 500 LTTC cycles a pronounced drop in the fatigue strength was observed ′ 87% at temperature of 295 K and 30% at 77 K. Microscopy and X-Ray studies showed that twinning had a significant effect on both the LTTC and mechanical fatigue processes. Residual stresses arising during the LTTC treatment were found to be a stimulating factor in the development of deformational processes occurring in the loaded material.     

Manifestations of dynamic strain aging under low and high cycle fatigue in a type 316LN stainless steel

Influence of Dynamic strain aging (DSA) under low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated by conducting LCF and HCF tests on specimens over a wide range of temperature from 573 to 973 K. DSA was found to be highly pronounced in the temperature range of 823–873 K. DSA was seen to have contrasting implications under LCF and HCF deformation. The cyclic hardening owing to DSA caused an increase in the cyclic stress response under LCF, leading to decrease in cyclic life. On the other hand, the DSA-induced strengthening suppressed the crack initiation phase under HCF where the applied stress remains fixed, leading to an increase in the cyclic life.     

Comparative evaluation of strain controlled low cycle fatigue behaviour of solution annealed and prior cold worked 316L(N) stainless steel

The effect of 20% prior cold work on low cycle fatigue (LCF) behaviour of type 316L(N) stainless steel (SS) was studied at 873 K by conducting total axial strain controlled tests in air with strain amplitudes in the range ±0.25% to ±1.0%. The cyclic deformation behaviour of 20% prior cold worked (PCW) material was compared with the LCF response of solution annealed (SA) alloy tested under similar conditions. The cyclic stress response (CSR) of 316L(N) SS in the PCW condition was characterized by a short period of hardening followed by prolonged softening prior to failure, whereas SA material exhibited a significant hardening regime followed by stress saturation. Interrupted tests on PCW material were carried out at different stages of CSR in order to determine the underlying mechanisms as reflected in substructural changes. The fatigue life in the solution annealed condition was similar to that of the PCW material at higher strain amplitudes of testing (≥±0.5%) while at lower strain amplitudes, the PCW material exhibited longer life.     

High-temperature deformation characteristics of Ti-15at.% Al alloy

The deformation characteristics of Ti-15 at.% Al alloy have been investigated by compression tests in the temperature range 873 to 1273 K (0.44 to 0.64T _m) and by extensive transmission electron microscopy. Two types of deformation patterns were identified depending on the temperature: at lower temperatures below about 1073 K, the yield stress of the sample showed inverse temperature dependence, and serrations were found on the flow curves, whereas the normal dependences of the yield stress on temperature and strain rate were found at higher temperatures above about 1073 K. Corresponding dislocation substructures were composed of coarse bands of localized slip at 1023 K, and of rather uniformly distributed dislocations at 1123 K, and sub-boundaries as well as free dislocations at 1273 K. The main operating mechanisms in these temperature regimes were assumed to be the co-operative movement of numerous dislocations under the condition of the dynamic strain ageing, viscous glide of dislocations and dynamic recovery, respectively.     

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.     

Effect of strain rate on low cycle fatigue of 316LN stainless steel with varying nitrogen content: Part-I cyclic deformation behavior

The effect of strain rate and nitrogen content on cyclic deformation and substructural changes in 316LN stainless steel is investigated at temperatures 773, 823 and 873 K. Dynamic strain aging (DSA) and/or thermal-recovery processes are observed to control cyclic deformation, and the regimes of their predominance are mapped. An increase in nitrogen content and DSA enhanced cyclic stress and are found to offset thermal-recovery induced cyclic strength reduction. In addition, strain localization in the form of slip-bands impinging on grain boundary is observed. The predominance of thermal-recovery over DSA manifested as dislocation-poor channels, dislocation cells within and in-between planar slip-bands.     

FATIGUE BEHAVIOUR OF THREE VARIANTS OF THE ROLLER BEARING STEEL SAE 52100

Stress-controlled, low-cycle, push-pull fatigue tests were performed on three variants of the bearing steel SAE 52100 with slightly different compositions and heat treatments. The experiments demonstrated differences in the cyclic plastic behaviour of differently hardened steels (bainitically-hardened and martensitically-hardened, respectively), whereas the two martensitic variants, which differ in composition, behaved very similarly. Bainitically-hardened SAE 52100 steel exhibited initial hardening followed by cyclic softening above a stress amplitude level of 1200 MPa. In contrast, the martensitically-hardened variants showed a pronounced cyclic hardening. The deformation behaviour of the martensitically-hardened bearing steel in a monotonic tensile test and during the first cycles can be well understood on the basis of the transformation of retained austenite. This process leads to an onset of plastic deformation at lower stresses compared to the bainitically-hardened bearing steel. As a result of the subsequent cyclic hardening of the martensitic variants, the CSS curves are almost identical for the differently hardened conditions under investigation. Additional tests under pulsating compression documented that a high negative mean stress enhances the cyclic plasticity.     

Correlation between cyclic deformation behaviour and microstructure in a duplex steel between 300 and 773 K

Cyclic tests performed in the temperature range 300–773 K on duplex stainless steel DIN 1.4460 show that the cyclic stress–strain behaviour of this steel is strongly temperature dependent. At 300 and 473 K an almost constant peak tensile stress stage, is followed by a slight softening that continues up to failure in the case of 300 K, but by a secondary hardening at 473 K. Pronounced initial cyclic hardening followed by secondary hardening was the main feature of the temperature range between 573 and 723 K. At 773 K, after a weak hardening stage, a strong softening continues up to failure. The mechanical behaviour and the evolution of the microstructure were analysed, and the internal and the effective stresses were studied. It was found that the internal stress is responsible for the strong hardening that occurs in the intermediate temperature range and for the softening at 773 K.     

Effects of temperature on the low cycle fatigue behaviour of nitrogen alloyed type 316L stainless steel

Strain-controlled low cycle fatigue tests have been conducted in air between 298–873 K to ascertain the influence of temperature on LCF behaviour of nitrogen-alloyed type 316L stainless steel. A strain amplitude of ± 0.60% and a symmetrical triangular waveform at a constant strain rate of 3 × 10⁻³ s⁻¹ were employed for all tests. Crack initiation and propagation modes were evaluated, and the deformation and damage mechanisms which influence the cyclic stress response and fatigue life identified. The cyclic stress response at all temperatures was characterized by an initial hardening to the maximum stress, followed by gradual softening prior to attaining saturation. Temperature dependence of fatigue life showed a maximum in the intermediate temperature range. The drastic reduction in fatigue life at elevated temperatures has been ascribed primarily to the combined influence of dynamic strain ageing effects and oxidation-enhanced crack initiation, while the lower life at room temperature is attributed to detrimental effects associated with deformation-induced martensite.     

Fatigue assessment and fatigue life calculation of quenched and tempered SAE 4140 steel based on stress–strain hysteresis, temperature and electrical resistance measurements

In this paper, mechanical stress–strain-hysteresis, temperature and electrical resistance measurements are performed for the detailed characterization of the fatigue behaviour of quenched and tempered SAE 4140 steel used for many applications in the automotive industry. Stress-controlled load increase and constant amplitude tests (CATs) were carried out at ambient temperature on servo-hydraulic testing systems. The applied measurement methods depend on deformation-induced changes of the microstructure in the bulk material and represent the actual fatigue state. The plastic strain amplitude, the change in temperature and the change in electrical resistance can be equally used for an assessment of baseline fatigue properties in generalized cyclic deformation curves as well as in generalized Morrow and Coffin–Manson curves. On the basis of comprehensive experimental fatigue data, the physically based fatigue life calculation method ‘PHYBAL’ based on the generalized Basquin equation was developed. S–N (Woehler) curves calculated with ‘PHYBAL’ using data from only three fatigue tests agree very well with the conventionally determined ones.     

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.     

Damage mechanisms of single and polycrystalline nickel base superalloys SC16 and IN738LC under high temperature LCF loading

The mechanical behaviour of polycrystalline and single crystal nickel base superalloys, IN738LC and SC16, respectively, has been investigated under strain controlled LCF loading at 1223 K. The alloys showed a very similar stress response: a long stable stress amplitude period preceded by either cyclic hardening at higher strain amplitudes or by cyclic softening at lower strain amplitudes. The fatigue life of the single crystal alloy was, however, measured to be about 10 times higher than that of the polycrystalline variant. This can be attributed to the differences in deformation and damage processes observed in the two alloys.