Thermo-mechanical behaviours of light alloys in comparison to high temperature isothermal behaviours

Abstract

In this article, out-of-phase thermo-mechanical fatigue (TMF) behaviours of light alloys were investigated in comparison to their high temperature low cycle fatigue (LCF) behaviours. For this objective, strain based fatigue tests were performed on the A356 aluminium alloy and on the AZ91 magnesium alloy. Besides, TMF tests were carried out, where both strain and temperature changed. The fatigue lifetime comparison demonstrated that the TMF lifetime was less than that one under LCF loadings at elevated temperatures for both light alloys. The reason was due to severe conditions in TMF tests in comparison to LCF tests. The temperature varied in TMF test but it was constant under LCF loadings. As the other reason, the tensile mean stress occurred under TMF loadings, in comparison to the compressive mean stress under LCF loadings. At high temperatures, the cyclic hardening behaviour occurred in the AZ91 alloy and the A356 alloy had the cyclic softening behaviour.     

Isothermal and thermomechanical fatigue behaviour of a high temperature titanium alloy

Abstract

Isothermal and thermomechanical fatigue (TMF) behaviour (including cyclic stress response and number of cycles to failure) of a Ti – 5.6Al – 4.8Sn – 2.0Zr – 1.0Mo – 0.32Si – 0.8Nd (wt-%) hightemperature titanium alloy was examined. The purpose of the present investigation was to understand the effect of temperature fluctuation on the cyclic behaviour and fatigue life of this alloy and to test the suitability of lifetime prediction based on isothermal laboratory data. The results indicated that both the level of peak stress and fatigue life were decreasing with increasing test temperature from 400°C to 650°C in isothermal fatigue (IF) tests. In TMF tests run between 400°C and 600°C, the peak stresses corresponding to 600°C coincide well with that found in IF tests run at 600°C, while a slight increase in cyclic hardening was found for peak stress corresponding to 400°C compared to that found in a 400°C/IF test. This increase in cyclic hardening became more pronounced when the maximum temperature increased to 650°C. Fatigue life in 'out of phase' (OP) condition was found to be shorter than under an equivalent 'in phase' (IP) condition, and this gap increased with decreasing mechanical strain amplitude. The results indicate that lifetime prediction based on isothermal laboratory data may lead to non-conservative results if thermal fluctuations are present in components made of the present alloy.     

Cyclic stress-strain response during isothermal and thermomechanical fatigue

Isothermal high-temperature low-cycle fatigue and in-phase and out-of-phase thermomechanical fatigue tests were carried out on 316L austenitic stainless steel specimens controlled by computer. A non-linear kinematic hardening model with internal variables was used to simulate the cyclic stress-strain behaviour of isothermal fatigue. This model was modified by considering thermal cyclic effects in order to describe the cyclic stress-strain behaviour of thermomechanical fatigue (TMF) using only isothermal fatigue data and the material performance data. A very good approximation of the hysteresis loops was obtained by comparing with experiments of both in-phase and out-of-phase cases. The thermomechanical fatigue behaviour described by isothermal fatigue data gives the possibility of developing the TMF lifetime prediction technique.     

The characteristics of fatigue under isothermal and thermo-mechanical load in Cr–Ni–Mo cast hot work die steel

ABSTRACT High temperature isothermal fatigue (IF) and in-phase thermo-mechanical fatigue (TMF) tests in load control were carried out in cast hot work die steel. At the same load amplitude, the fatigue lives obtained in the in-phase TMF tests are lower than those obtained in the isothermal tests. Observations of fracture surface and the response of stress–strain reveal that cyclic creep in the tensile direction occurs and the intergranular cracks dominate in TMF tests, whereas cyclic creep in the compressive direction occurs and the path of the crack growth is mainly transgranular in IF tests. A model of life prediction, based on the Chaboche law, was discussed. Damage coefficients that are functions of the maximum temperature and the variation of temperature are introduced in the model so as to evaluate TMF lives in load control. With this method, the lifetime prediction gives results corresponding well to experimental data.     

Stress–strain time-dependent behavior of A356.0 aluminum alloy subjected to cyclic thermal and mechanical loadings

This article presents the cyclic behavior of the A356.0 aluminum alloy under low-cycle fatigue (or isothermal) and thermo-mechanical fatigue loadings. Since the thermo-mechanical fatigue (TMF) test is time consuming and has high costs in comparison to low-cycle fatigue (LCF) tests, the purpose of this research is to use LCF test results to predict the TMF behavior of the material. A time-independent model, considering the combined nonlinear isotropic/kinematic hardening law, was used to predict the TMF behavior of the material. Material constants of this model were calibrated based on room-temperature and high-temperature low-cycle fatigue tests. The nonlinear isotropic/kinematic hardening law could accurately estimate the stress–strain hysteresis loop for the LCF condition; however, for the out-of-phase TMF, the condition could not predict properly the stress value due to the strain rate effect. Therefore, a two-layer visco-plastic model and also the Johnson–Cook law were applied to improve the estimation of the stress–strain hysteresis loop. Related finite element results based on the two-layer visco-plastic model demonstrated a good agreement with experimental TMF data of the A356.0 alloy.     

Cyclic stress–strain characteristics of two microalloyed steels

Abstract

The cyclic stress–strain behaviour of two microalloyed steels with different microstructures has been characterised at room temperature under strain controlled low cycle fatigue. The cyclic stress–strain curve in the double logarithmic plot shows a linear relation for both steels. A transition of the cyclic stress–strain curve from softening to hardening with increasing strain amplitude has been observed with respect to the corresponding tensile curve. The strain amplitude for the onset of cyclic softening to hardening transition has been found to be dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics of the two microalloyed steels have been compared with corresponding predeformed state carried out under stress controlled conditions. While, cyclic saturation was observed in case where the extent of predeformation was within the Lüders strain, cyclic softening occurred when it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding of the differences in the cyclic behaviour of the two steels owing to the microstructure and predeformation.     

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.     

Creep and Low Cycle Fatigue Investigations of Light Aluminium Alloys for Engine Cylinder Heads

Abstract: An influence of the chemical composition, porosity and ageing on mechanical behaviour of light, multifunctional aluminium alloys (AlSi8Cu3 and AlSi7MgCu0.5) subjected to creep and low cycle fatigue (LCF) was investigated. The materials were tested to verify their applicability as the cylinder heads in car engines. During creep tests, a strain response of the materials was observed under a range of the step‐increased stresses and different temperatures. The LCF tests were carried out under strain control in three blocks of 100 cycles each with a constant strain amplitude. The results of creep and LCF tests were analysed with regard to chemical composition, type of porosity and ageing of the materials tested. An influence of porosity on the creep resistance and lifetime was considered. The results of the LCF tests were compared for the materials in the as‐received state and after ageing. An experimental evaluation of cyclic behaviour because of the LCF was carried out to check whether the hardening or softening effects can be observed in the materials. Taking into account the various history of loading, a stress response of the materials was investigated.     

Effects of structure on the tensile,creep and fatigue properties of polyester fibres

The behaviour of two types of polyester fibres have been compared and the differences observed explained in terms of the differences in molecular structure. One fibre which had a higher crystallinity than the other showed improved creep behaviour however shorter fatigue lifetime. Crack propagation during fatigue suggests a macrofibrillar structure superimposed on the microfibrillar molecular arrangement. Fatigue lifetime is suggested as being greatly influenced by the time necessary for crack initiation which involves modification of the molecular structure. The influence of the minimum cyclic load on fatigue lifetime has been studied for both fibres.     

Nonlinear constitutive equations for transient creep of viscoelastic polymers including the effect of hydrostatic pressure

The behaviour of polymers is known to be significantly influenced by the hydrostatic pressure in creep deformation or elastic-plastic deformation. The effect of the third stress invariant on the nonlinear viscoelastic deformation is much smaller than that of the hydrostatic pressure. In this paper, a constitutive equation for transient creep is proposed, which includes the effect of the hydrostatic pressure on the yield function. The creep and plastic strains or the creep strain rate converge to zero with increasing hydrostatic pressure. The proposed constitutive equation is in good agreement with the actual creep data of cellulose nitrate and cellulose acetate, under various combinations of superimposed tensile and hydrostatic loadings.     

Cumulative damage model based on equivalent fatigue under multiaxial thermomechanical random loading

A new creep–fatigue damage cumulative model is proposed under multiaxial thermomechanical random loading, in which the damage at high temperature can be divided into the pure fatigue damage and the equivalent fatigue damage from creep. During the damage accumulation process, the elementary percentage of the equivalent fatigue damage increment is proportional to that of the creep damage increment, and the creep damage is converted to the equivalent fatigue damage. Moreover, combined with a multiaxial cyclic counting method, a life prediction method is developed based on the proposed creep–fatigue damage cumulative model. In the developed life prediction method, the effects of nonproportional hardening on the fatigue and creep damages are considered, and the influence of mean stress on damage is also taken into account. The thermomechanical fatigue experimental data for thin‐walled tubular specimen of superalloy GH4169 under multiaxial constant amplitude and variable amplitude loadings were used to verify the proposed model. The results showed that the proposed method can obtain satisfactory life prediction results.     

The interpretation of cyclic creep deformation mechanism of copper in terms of the anelastic recovery

The cyclic creep deformation behaviour of copper has been studied in the temperature range of 0.4 to 0.5T _m and under the constant stress range of (/E)=4×10^–4 to 10×10^–4. To see the effect of cyclic stress frequency, stress amplitude and the duration of the unloading time in a fixed frequency on cyclic creep behaviour, static and cyclic creep tests were conducted under the conditions mentioned above. The measured activation energies for static and cyclic creep were analysed in terms of the various experimental parameters. Anelastic behaviour during the unloading period was also studied to find out the possible assistance for the positive creep deformation in cyclic creep. Using the concept of anelastic recovery and the activation energy for the anelastic it is hypothesized that the accelerated cyclic creep deformation is controlled by the anelastic recovery during the unloading period.     

Towards improved ODS steels: A comparative high‐temperature low‐cycle fatigue study

Within the frame of this work, the mechanical behaviour of a bimodal ferritic 12Cr‐ODS steel as well as of a ferritic‐martensitic 9Cr‐ODS steel under alternating load conditions was investigated. In general, strain‐controlled low‐cycle fatigue tests at 550°C and 650°C revealed similar cyclic stress response. At elevated temperatures, the two steels manifest transitional stages, ie, cyclic softening and/or hardening corresponding to the small fraction of the cyclic life, which is followed by a linear cyclic softening stage that occupies the major fraction of the cyclic life until failure. However, it is clearly seen that the presence of the nano‐sized oxide particles is certainly beneficial, as the degree of cyclic softening is significantly reduced compared with non‐ODS steels. Besides, it is found that both applied strain amplitude and testing temperature show a strong influence on the cyclic stress response. It is observed that the degree of linear cyclic softening in both steels increases with increasing strain amplitude and decreasing test temperature. The effect of temperature on inelastic strain and hence lifetime becomes more pronounced with decreasing applied strain amplitude. When analysing the lifetime behaviour of both ODS steels in terms of inelastic strain energy calculations, it is found that comparable inelastic strain energies lead to similar lifetimes at 550°C. At 650°C, however, the higher inelastic strain energies of 12Cr‐ODS steel result in significant lower lifetimes compared with those of the 9Cr‐ODS steel. The strong degradation of the cyclic properties of the 12Cr‐ODS steel is obviously linked to the fact that the initial hardening response appears significantly more pronounced at 650°C than at 550°C. Finally, the obtained results depict that the 9Cr‐ODS steel offers higher number of cycles to failure at 650°C, compared with other novel ODS steels described in literature.     

ASPECTS OF HIGH-TEMPERATURE LOW-CYCLE THERMOMECHANICAL FATIGUE OF A SINGLE CRYSTAL NICKEL-BASE SUPERALLOY

High-temperature thermomechanical fatigue (TMF) tests were performed on monocrystalline specimens of the γ′-precipitate-strengthened nickel-base superalloy CMSX-6. The specimens had axial orientations near [001]. In-phase, out-of-phase, clockwise-diamond and counter-clockwise-diamond TMF tests were conducted in total strain control in symmetric push-pull. The temperature interval extended from 873 to 1373 K, the total strain amplitude was 0.5% with a cycle time of 300 s. The stress response, the plastic strain amplitudes and the fatigue lives were measured. In addition, fatigue-induced micro-structural changes were investigated. The shapes of the hysteresis loops and the mechanical TMF behaviour varied markedly with the shape of the strain-temperature cycles. The TMF life of the alloy was found to correlate with the stress amplitude in the tensile half-cycle. The evolution of the microstructure shows the formation of dislocation networks around the precipitates and in particular, severe changes in the size and the morphology of the γ'-precipitate structure. These changes vary with the strain-temperature cycle shape. Failure occurred exclusively by fatigue (and not creep) in the form of localized shear.     

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.     

In‐phase and out‐of‐phase thermomechanical fatigue behavior of 4Cr5MoSiV1 hot work die steel cycling from 400 °C to 700 °C

The hysteresis loops, stress and strain behavior, lifetime behavior and fracture characteristic of 4Cr5MoSiV1 hot work die steel at a wide range of mechanical strain amplitudes (from 0.5% to 1.3%) during the in‐phase (IP) and out‐of‐phase (OP) thermomechanical fatigue (TMF) tests cycling from 400 °C to 700 °C under full reverse strain‐controlled condition were investigated. Stress‐mechanical strain hysteresis loops of 4Cr5MoSiV1 steel are asymmetric, and stress reduction appears at high‐temperature half cycles owing to a decrease in strength with increasing temperature. 4Cr5MoSiV1 steel always exhibits continuous cyclic softening for both types of TMF tests, and the cyclic softening rate is larger in OP loading condition. OP TMF life of 4Cr5MoSiV1 steel is approximately 60% of IP TMF life at the same mechanical strain amplitude and maximum temperature. Lifetime determined and predicted in both types of TMF tests is adequately described by the Ostergren model. Fracture surfaces under IP TMF loading display the striation and tear ridge, showing quasi‐cleavage characteristics, and the cracks are less but longer. However, fracture surfaces under OP TMF loading mainly display the striation and dimple characteristics, and the cracks are more and shorter.     

Cyclic deformation behaviour and lifetime calculation of Al‐3Mg‐Mn

Plastic strain amplitude, temperature and electrical resistance measurements were performed on the aluminium‐magnesium alloy Al‐3Mg‐Mn (AA5454) in recrystallised condition to describe and evaluate the cyclic deformation behaviour in detail. The endurance limit was estimated in load increase tests (LIT). In stress‐controlled single step tests at ambient temperature the cyclic deformation behaviour is characterised by pronounced cyclic hardening, which leads to a saturation state with a plastic strain amplitude of nearly zero. Due to far‐reaching cross effects of the applied measuring techniques, the plastic strain amplitude, the change of the specimen temperature due to cyclic plastic loading and the change of the electrical resistance show a strong interrelation with the underlying fatigue processes. A new lifetime calculation method “PHYBAL” on the basis of the plastic strain amplitude, the change of the temperature and the change of the electrical resistance yields an excellent accordance with experimentally determined lifetimes. Microstructural details were investigated by light and scanning electron microscopy.     

THE INFLUENCE OF THE HARDENING STATE ON TIME DEPENDENT DAMAGE AND ITS CONSIDERATION IN A UNIFIED DAMAGE MODEL

Abstract— A model is presented for the prediction of the lifetime of metals in the high-temperature range under arbitrary variable multiaxial load. The definition of an internal variable for damage in continuum damage mechanics is adopted, which allows indirect measurement of damage via the deformation behaviour. To acquire some knowledge of damage evolution, damage is measured in two ways during uniaxial strain controlled cyclic tests: (a) a change of the modulus of elasticity and (b) a decrease of the peak stress. Surprisingly, both methods lead to results which are in good agreement. A new damage law is then developed (with reference to known models and lifetime rules) which is a modification of the creep damage law of Rabotnov that is extended by a dependence on the inelastic strain rate instead of the dependence on internal variables to take into account the hardening state. Uniaxial as well as multiaxial formulations of the new damage model (Inelastic Strain Rate Modified (ISRM) model) are presented.
The parameters of the ISRM model are determined with a view to applying them to AISI 316 L(N) austenitic steel. Some of the parameters are derived from standard creep experiments. To determine further parameters, the ISRM model is applied to uniaxial cyclic tests. Both failure behaviour and damage evolution are well described.