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.     

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.     

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.     

CYCLIC INDUCED CREEP OF A PLAIN CARBON STEEL AT ROOM TEMPERATURE

Abstract— Plain carbon steel specimens subjected to cyclic and superimposed mean stresses show significant plastic strain at intermediate life due to cyclic creep deformation at room temperature. Cyclic creep strains can also be observed in the high cycle region with N _f > 2. 10⁶. An extended Haigh-diagram provides information about the resulting mean strain at predetermined cycles.     

REVERSAL OF CYCLIC CREEP IN MILD STEEL AND COPPER

Abstract— The progressive change of mean strain during the cyclic plastic deformation of a material between fixed stress limits is commonly called cyclic creep. If, in tension-compression testing, the mean stress exceeds a certain critical small compressive value, shortening is to be expected; but, if it is less than this or if it is tensile, lengthening is to be expected. The value of the mean stress decides the eventual direction of cyclic creep but not necessarily the initial direction of cyclic creep. In a pre-strained metal, the form of the tension and compression curves differs because of the Bauschinger effect. This affects the behaviour of the material during the first cycle and also, to a decreasing extent, during subsequent cycles. Thus, if the mean stress tends to cause cyclic creep in a direction opposed to that in which pre-straining has induced initial creep, a reversal of creep can occur. Observations have been made of the phenomena in mild steel (for both directions of pre-strain) and also in high conductivity copper.
It was found that the phenomenon of creep reversal does not depend on the metal undergoing cyclic hardening or softening. However, changes in the strain range and changes in the mean strain caused by cyclic creep itself produced changes in the limits of true stress when cycling was carried out between fixed limits of nominal stress. It was also shown that a reversal of cyclic creep might occur as a result of cyclic hardening or softening of a metal which possesses similar tension and compression characteristics.     

INTERACTIONS BETWEEN TIME AND CYCLIC DEFORMATION PROCESSES IN A NIMONIC ALLOY

Interactive creep–fatigue behaviour of a nickel-base superalloy (IN 597) has been examined at 850 °C under various strain-limited, cyclic torsional loading conditions. In one test, forward creep deformation was reversed by creep under equal magnitude stress levels and strain limits. In other tests, forward creep strain was reversed by fast monotonic plasticity with and without a subsequent period of relaxation. These cycles were repeated within each test until fracture. This paper examines empirically the influence of a number of test variables upon cyclic creep curves, and demonstrates the usefulness of predictions based upon continuous low cycle fatigue and simple creep data when used in conjunction with a mechanical equation of state. A cyclic equilibrium condition was not achieved from these tests. Instead, a progressive softening occurred giving reductions to the amount of creep strain, creep time interval and reversed peak stress with each new cycle. Such reductions are expressed from derived formulae that embrace the range of inelastic strain, cycle number, creep dwell stress, reversed peak stress, and times expended in creep and relaxation.
Observations made on accumulated creep strain reveal the contribution to a creep–fatigue fracture from cyclic creep. This has led to a modified form of the linear damage rule which can provide conservative life predictions for components operating in service under similar cyclic conditions.     

Influence of prior cyclic hardening on high temperature deformation and crack growth in type 316L (N) stainless steel

For power generating equipment subjected to cyclic loading at high temperature, crack growth could arise from the combinations of fatigue and creep processes. There is potential for the material to undergo hardening (or more generally changes of material state) as a consequence of cyclic loading. Results of an experimental study to examine the influence of prior cyclic hardening on subsequent creep deformation are presented for type 316L(N) stainless steel at 600°C. Experiments were also carried out to explore creep crack growth at constant load, and crack growth for intermittent cyclic loading. For the as-received material there is substantial primary creep (hardening) at constant load, while for the cyclically hardened material at constant load the creep curves show recovery, and increasing creep rate with increasing time. Specimens subjected to prior cyclic hardening were also used for a series of creep and creep-fatigue crack growth tests. These tests demonstrated that there was accelerated crack growth compared to crack growth in as-received material.     

The relationship of creep,recovery and fatigue in polycarbonate

The quantitative relationship between creep and recovery which had been previously developed by Mindel and Brown [1] has been applied to interrupted creep tests. Single and multiple interruptions (fatigue) were investigated. In general it was observed that interruptions decreased the time to failure. The experiments were conducted in compression in the range of high stresses. Failure was caused by excessive deformation or accelerated creep which is produced by a softening mechanism that is common to all linear polymers. The temperature changes associated with the creep and recovery parts of the cyclic loading were measured. The temperature rise during loading exceeds the decrease during unloading so that there is a net rise in temperature if the creep and recovery intervals are equal. However, the temperature change is not the primary cause for the decrease in time-to-failure for cyclic loading as compared to the failure time during steady stress creep. It has been concluded that fatigue failure under compressive deformation is related fundamentally to the constant stress creep curve.     

Comparative study on stress–strain hysteresis response of SAC solder joints under thermal cycles

The present study attempts to evaluate the stress-strain hysteresis responses of SAC solder joints in Resistor and FleXBGA144 packages subjected to thermal cyclic loading using several constitutive models. The total deformation of the solder material consists of elastic, rate-independent plastic and rate-dependent creep components. The constitutive models discussed in this study each weighted elastic, plastic and creep deformations differently. At low stresses SAC solder alloys were found to be creep resistant, where at higher stresses, the influence of different microstructures disappears as matrix-creep dominates in this region. Thus, the proper constitutive model requires all the three ingredients of the elastic, the creep, and the time-independent plastic data for different stress levels to effectively predict the hysteresis behavior of the SAC solder alloys. The hysteresis loops predicted by constitutive models were also found in close agreement with the loops generated by FEM for the SAC solder joint subjected to thermal cycling.     

The enhancement of creep in Al–22?wt% Ag alloy by cyclic stressing

The plastic deformation behavior of Al–22 wt% Ag alloy during phase transformation was investigated by studying the creep behavior under cyclic stress reduction of low frequencies. The cyclic creep curves obtained describe clearly the cyclic stress acceleration behavior. Increasing frequency of cyclic stress reduction enhanced the creep deformation depending upon the combination of the experimental variables as testing temperature, aging temperature and static creep rate. The irregularity in the creep parameters, n, β and ε_st with increasing aging temperatures, has been explained on the basis of structure transformations occurring in Al–Ag system and their mode of interaction with mobile dislocations.     

The Influence of Superimposed Creep Loadings on the Thermal-Mechanical Fatigue Behaviour of the Ni-Base Superalloy IN 792 CC

The effect of superimposed creep loadings on the cyclic deformation andthe lifetime behaviour of the Ni-base superalloy IN 792 CC underout-of-phase TMF loadings is presented and discussed. The mean stressand the stress amplitude are not affected significantly by the creeploading. A slight cyclic hardening is observed even for cycles withsuperimposed creep loadings at a maximum temperature of 920°C whichmeans that the work hardening processes occurring under TMF loading arenot fully compensated by recovery during the creep phase. The plasticstrain amplitude increases with increasing creep stress. Using theManson–Coffin relationship, it is possible to calculate the influence ofthe superimposed creep loadings on the lifetime behaviour in goodaccordance with the experimental results.     

Fatigue behavior of ferritic–pearlitic–bainitic steel in loading with positive mean stress

The effect of positive mean stress on the fatigue behavior of ferritic–pearlitic–bainitic steel has been studied. Specimens, produced from a massive forging, were cycled with two constant stress amplitudes and various positive mean stresses. Plastic strain amplitude and cyclic creep rate were measured during cyclic loading and the effect of the mean stress on saturated plastic strain amplitude and mean strain at half-life was established. Plastic strain amplitude is weakly dependent but creep strain increases with the mean stress exponentially. Fatigue life decreases with the mean stress for both stress amplitudes. The contributions of cyclic plastic strain and cyclic creep to the fatigue damage were evaluated and discussed in relation with the Manson-Coffin curve.     

Influence of nitrogen and grain size on deformation behaviour of austenitic stainless steels

Abstract

The present study concerns the deformation behaviour of two austenitic stainless steels (base composition corresponding to 316L) with two different nitrogen levels, 0.14 and 0.29 wt-% respectively. The influence of grain size on the mechanical properties has been investigated. The grain size dependence of the monotonic as well as the cyclic stress-strain curves were shown to follow a Hall-Petch type relationship. The influence of the grain size was found to be lower for the cyclic thanfor the mOnotonic yield stresses. The lower grain size sensitivity of the cyclic yield stresses can be explained by the successive breakdown of the planar slip mode during cyclic straining. For both static and cyclic deformation modes the dependence of grain size increases with higher nitrogen content. Dislocation structures of the various conditions were studied with transmission electron microscopy and related to the mechanical behaviour of the two materials.     

Thermography in high cycle fatigue short‐term evaluation procedures applied to a medium carbon steel

This paper focuses on the fatigue life calculation for an unalloyed medium carbon steel SAE1045 (German DIN‐standard: C45E), by applying an energy dissipation‐based approach quantified through thermographic measurements. The purpose of this approach is to establish an intrinsic dissipation model and to predict characteristics derived from the cyclic deformation behavior of stress‐controlled fatigue tests, eg, the fatigue limit and the S‐N data by using simplified (zero‐dimensional, 0D) thermodynamic equations. In order to investigate the possibilities for a rapid evaluation while simultaneously reducing the experimental effort, one load increase test (LIT) and two constant amplitude tests (CATs) were carried out. The S‐N data evaluated on such a basis is competitive to conventionally determined S‐N data as will be shown.     

Shear Creep Deformation of the Super Alloy Single Crystal CMSX‐4 at High Temperatures and Low Stresses

In the present paper we investigate the shear creep behavior of the single crystal super alloy CMSX‐4 at temperatures between 950 and 1100 °C and shear stresses ranging from 80 to 155 MPa. A double shear creep test technique is used to study the shear creep behavior of four specific macroscopic crystallographic shear systems defined by a specific crystallographic shear plane and a specific crystallographic shear direction (systems investigated: {001}<110>, {100}<010>, {011}<01‐1>, and {111}<01‐1>). The shear creep behavior is analyzed in terms of the shape of individual creep curves and in terms of the stress and the temperature dependence of the secondary shear creep rate. Individual creep curves are generally characterized by a pronounced primary creep range where creep rates decrease by up to three orders of magnitude. A sharp creep rate minimum is not observed. The secondary creep range starts at shear stresses of the order of 0.02 and is followed by a secondary creep range which extends over shear strain ranges of the order of 0.1. No pronounced increase of shear creep rate in the later stages of creep is observed. Norton plots yield power law stress exponents ranging from 5.5 to 9.7. The temperature dependence of the secondary creep rate is of an Arrhenius type and apparent activation energies between 549 and 690 kJ/mol were found. There is a clear influence of crystallography on shear creep rates, which vary between different macroscopic crystallographic shear systems; this effect decreases with increasing temperature. The shear creep results obtained in the present study are discussed in the light of results from uniaxial testing and in the light of underlying microscopic deformation processes.     

Constitutive Equations of Strain-Hardening Theory for Nonisothermal Deformation Processes

A variant of the strain-hardening theory is proposed for describing nonisothermal deformation processes. The author postulates the dependence of parameters of the constitutive equations on stress and temperature. The influence of the loading history on the creep rate variation is allowed for by means of a scalar function of damage level. A procedure for more specific definition of the constitutive relationships is outlined. Efficiency of this approach is illustrated by describing creep curves for chromium-nickel steels over a fairly wide range of variation of stresses and temperature. __________ Translated from Problemy Prochnosti, No. 5, pp. 45 – 57, September – October, 2005.     

Martensitic Transformation and Residual Stress Generation During Thermomechanical Treatment

The influence of thermomechanical deformation on the residual stresses caused by quenching in bar shaped specimens of heat treatable steel SAE 4140 has been investigated using a mechanical method for determining the distribution of residual stresses of the first kind. The results obtained show that the residual stress distribution after quenching is affected by the strengthening and softening of the austenite and the modified transformation behavior in martensite stage. An attempt is made to discuss qualitatively the influence of these changes on the generation of residual stresses as compared to results obtained after conventional hardening.     

Dauerfestigkeit bei hohen Zugmittelspannungen

Fatigue strength under high tensile mean‐stress Several components are subjected to high static tensile loads while a dynamic load with small amplitude is superimposed. In the technical rules the maximum load is limited in a very conservative way by the yield strength to prevent local plastic deformations. In this paper investigations on the materials X22 CrMoV 12 1 and GGG 40 were made to estimate the influence of tensile mean stress on the fatigue strength and on the cyclic deformation behavior. A model to describe the cyclic creep as a function of mean stress, stress amplitude and the number of load cycles is developed and programmed for a calculation by means of finite‐element‐method. There are also two concepts to calculate the fatigue strength proposed.     

Microstructure evolution and its influence on deformation mechanisms during high temperature creep of a nickel base superalloy

The interaction of dislocation with strengthening particles, including primary and secondary γ′, during different stages of creep of Rene-80 was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). During creep of the alloy at 871 °C under stress of 290 MPa, the dislocation network was formed during the early stages of creep, and the dislocation glide and climb process were the predominant mechanism of deformation. The density of dislocation network became more populated during the later stages of the creep, and at the latest stage of the creep, primary particles shearing were observed alongside with the dislocation glide and climb. Shearing of γ′ particles in creep at 871 °C under stress of 475 MPa was commenced at the earlier creep times and governed the creep deformation mechanism. In two levels of examined stresses, as far as the creep deformation was controlled by glide and climb, creep curves were found to be at the second stage of creep and commence of the tertiary creep, with increasing creep rate, were found to be in coincidence with the particles shearing. Microstructure evolution, with regard to γ′ strengthening particles, led to particles growth and promoted activation of other deformation mechanisms such as dislocation bypassing by orowan loop formation. Dislocation-secondary γ′ particles interaction was detected to be the glide and climb at the early stages of creep, while at the later stages, the dislocation bypassed the secondary precipitation by means of orowan loops formation, as the secondary particle were grown and the mean inter-particle distance increased.     

Effect of cyclic loading on subsequent creep behaviour and its implications in creep–fatigue life assessment

Abstract

Evaluation of creep–fatigue failure is essential in design and fitness evaluation of high-temperature components in power generation plants. Cyclic deformation may alter the creep properties of the material and taking cyclic effects into account may improve the accuracy of creep–fatigue failure life prediction. To evaluate such a possibility, creep tests were conducted on 316FR and modified 9Cr–1Mo steel specimens subjected to prior cyclic loading; their creep deformation and rupture behaviours were compared with those of as-received materials. It was found that creep rupture life and elongation generally decreased following cyclic loading in both materials. In particular, the rupture elongation of 316FR in long-term creep conditions drastically decreases as a result of being cyclically deformed at a large strain range. Use of creep rupture properties after cyclic deformation, instead of those of as-received material, in strain-based and energy-based life estimation approaches brought about a clear improvement of creep–fatigue life prediction.