Experimental study on the whole‐life heterogeneous ratchetting and ratchetting‐fatigue interaction of SUS301L stainless steel butt‐welded joint

Uniaxial fatigue tests of butt‐welded joint, made from SUS301L stainless steel, were carried out under asymmetric stress‐controlled cyclic loading conditions in this work. The effects of stress amplitude and mean stress on the whole‐life heterogeneous ratchetting and fatigue life of the butt‐welded joint were investigated, respectively, for the specified subzones. The experimental observations show that the whole‐life inhomogeneous ratchetting strain concentrating in a specific fusion zone (denoted as the FZ‐1 subzone) of the welded joint becomes more significant as the stress level increases; the fatigue failure also occurs in the FZ‐1 subzone, and the fatigue life depends on both the applied mean stress and stress amplitude and is determined by the combination of ratchetting damage and fatigue one in the localized FZ‐1 subzone.     

Ratchetting behavior of advanced 9-12% chromium ferrite steel under creep-fatigue loadings

The ratchetting behavior of advanced 9-12% chromium ferrite steel was investigated by cyclic loading tests with various hold times and stress ratios at elevated temperature of 873 K. Particular attention was paid to the effect of hold time on the whole-life ratchetting deformation and failure mechanism. Results indicate that the total ratchetting strains under creep-fatigue loading can be decomposed into two parts, i.e., cyclic accumulated creep strain produced during the peak stress hold time (ε_R1), cyclic accumulated inelastic strain produced during the stress change process (ε_R2). A transition in ratchetting components and rupture behavior with the increase of hold time was observed. In the long hold time domain, a quick shakedown of ratchetting strain ε_R2 occurs after the very first few cycles and the rupture behavior is fully controlled by the time-dependent creep damage. In the short hold time domain, ratchetting strain ε_R2 increases till the specimens fails and a mixed damage mode is responsible for the failure. An attempt is made to explain the existence of these two domains in terms of the evolutions of three internal stress components (back stress, isotropic stress and viscous stress) measured at the end of the holding period.     

Ratcheting‐fatigue behaviour of bainite 2.25Cr1MoV steel with tensile and compressed hold loading at 455°C

The ratcheting behaviour of a bainite 2.25Cr1MoV steel was studied with various hold periods at 455°C. Particular attention was paid to the effect of stress hold on whole‐life ratcheting deformation, fatigue life, and failure mechanism. Results indicate that longer peak hold periods stimulate a faster accumulation of ratcheting strain by contribution of creep strain, while double hold at peak and valley stress has an even stronger influence. Creep strains produced in peak and valley hold periods are noticeable and result in higher cyclic strain amplitudes. Dimples and acquired defects are found in failed specimen by microstructure observation, and their number and size increase under creep‐fatigue loading. Enlarged cyclic strain amplitude and material deterioration caused by creep lead to fatigue life reduction under creep‐fatigue loading. A life prediction model suitable for asymmetric cycling is proposed based on the linear damage summation rule.     

Uniaxial ratchetting of polymer and polymer matrix composites: Time-dependent experimental observations

The uniaxial time-dependent ratchetting of polyester resin and glass fiber reinforced polyester resin matrix composites was observed by the stress-controlled cyclic tension–compression with non-zero tensile mean stress and tension–tension tests at room temperature. After the ratchetting of the polyester resin had been observed by the cyclic tests with different loading conditions including some time-related factors, such as stress rate and peak stress hold, the ratchetting evolutions of the continuous and short glass fiber reinforced resin matrix composites were also investigated by the stress-controlled cyclic tests, respectively. It is concluded that: both the polyester resin and its composites present apparent ratchetting deformation, i.e., the ratchetting strain accumulates progressively in the tensile direction during the cyclic tension–compression with non-zero tensile mean stress and tension–tension tests; the ratchetting depends on the applied stress amplitude, mean stress, stress rate and peak stress hold, and the time-dependent ratchetting is obvious even for the continuous glass fiber reinforced resin matrix composites with high fiber volume fraction (such as 40% and 50%); the time-dependent ratchetting of the polyester resin and its composites mainly stems from the viscosity of the polyester resin, while the addition of glass fiber into the resin matrix improves the resistance of the composites to the ratchetting deformation and lowers the time-dependence of the ratchetting simultaneously.     

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.     

Non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy: Experimental observations

Experimental observations are reported for the non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy performed under the stress-controlled cyclic tension–torsion loading conditions and at room temperature. The effect of axial mean stress on the evolutions of transformation ratchetting strain and dissipation energy per cycle during the cyclic tests is discussed firstly; and then the dependence of multiaxial transformation ratchetting on the different non-proportionally loading paths (e.g., linear, square, hourglass-typed, butterfly-typed, rhombic and octagonal paths) is investigated. The results show that the multiaxial transformation ratchetting occurs mainly in the axial direction because only the non-zero axial mean stress is used and the mean shear stress is set to be zero in the all prescribed multiaxial loading paths; and the axial peak and valley strains increase with the increasing axial mean stress and also depend significantly on the shapes of loading paths. Comparison with the corresponding uniaxial ones illustrates that the multiaxial stress states are more helpful to promote the development of transformation ratchetting, especially for the non-proportional ones.     

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.     

聚碳酸酯的单轴棘轮行为研究

在常温常湿下,对聚碳酸酯(PC)材料进行了一系列单轴应变循环和非对称应力循环实验。讨论了PC材料在不同加载水平、加载历史、应力率和峰值保持时间下的循环变形特征。结果表明:PC材料在应变循环过程中体现出了一定程度的循环软化特性,其响应应力幅值在应变循环中随着循环周次的增加而下降,但不是很明显;PC材料在非对称应力循环加载过程中产生明显的棘轮行为,棘轮应变随着平均应力和应力幅值的增加而增加,并且平均应力的影响大于应力幅值的影响;加载历史对于棘轮变形行为的影响较为明显,但对应变循环特性影响不大;PC材料的棘轮变形随着应力率的减小和峰值保持时间的增加而增加,体现明显的时相关性。     

Uniaxial ratchetting in steels with different cyclic softening/hardening behaviours

The cyclic deformation of three structural steels, SS316L stainless steel, 40Cr3MoV bainitic steel and 25CDV4.11 steel, were studied experimentally by uniaxial cyclic straining or stressing tests at room temperature. The cyclic softening/hardening behaviours of the steels were discussed first by cyclic straining tests; and then the effects of cyclic softening/hardening behaviours on the uniaxial ratchetting of the materials were investigated by asymmetrical cyclic stressing tests. It is concluded from the experimental results that the ratchetting greatly depends on the cyclic softening/hardening behaviours of the materials, as well as the loading history. Different ratchetting and failure behaviours are observed for the prescribed steels. It is also stated that the proposed unified visco‐plastic constitutive model can provide a fairly reasonable simulation of the uniaxial ratchetting of SS316L stainless steel and 25CDV4.11 steel; but cannot simulate the ratchetting of 40Cr3MoV bainitic steel since the dependence of cyclic softening behaviours on the applied inelastic strain amplitude cannot be reasonably described in the discussed constitutive model. Some significant conclusions are obtained, which are useful to construct constitutive model to describe the ratchetting of the materials with different cyclic softening/hardening behaviours.     

CDM prediction of creep failure modes in thin and thick section Cr‐Mo‐V Butt‐welded pipes under combined internal pressure and end loads at temperatures in the range 540–620 °C

The paper examines the dependence of creep failure modes of butt‐welded pipes upon: temperatures in the range 540–620 °C; the ratio of axial to pressure loadings (loading condition); and, load levels which result in failure times in the region 2000–250 000 h. Continuum damage mechanics (CDM) analyses have been used to analyse two extreme geometries: thin section pipes (4.52 mm) and thick section steam pipes (46 mm) of the type found in power generation plant. The thick pipes have been selected to allow through‐thickness constraint to be developed. Two predominant modes of failure have been predicted: Fusion Boundary failure and Type IV failure. For the thin section pipes the modes of failure are not strongly dependent on applied loading levels, and loading condition, but are dependent on temperature: at 540 °C failure is in the Fusion Boundary and at 620 °C it is in the Type IV region. Failure mode in the thick pipes is dependent on temperature, and applied loading levels, but is essentially independent of loading condition. At 540 and 565 °C failure is in the Fusion Boundary; whereas at 575 °C, failure is in the Type IV region at higher applied stress levels, and in the Fusion Boundary at lower applied stress levels.     

Stress Evolution With Temperature in Titanium‐Rich NiTi Shape Memory Alloy Films

Abstract: The effect of deposition temperature on residual stress evolution with temperature in Ti‐rich NiTi films deposited on silicon substrates was studied. Ti‐rich NiTi films were deposited on 3″ Si (100) substrates by DC magnetron sputtering at three deposition temperatures (300, 350 and 400 °C) with subsequent annealing in vacuum at their respective deposition temperatures for 4 h. The initial value of residual stress was found to be the highest for the film deposited and annealed at 400 °C and the lowest for the film deposited and annealed at 300 °C. All the three films were found to be amorphous in the as‐deposited and annealed conditions. The nature of the stress response with temperature on heating in the first cycle (room temperature to 450 °C) was similar for all three films although the spike in tensile stress, which occurs at ～330 °C, was significantly higher in the film deposited and annealed at 300 °C. All the films were also found to undergo partial crystallisation on heating up to 450 °C and this resulted in decrease in the stress values around 55–60 °C in the cooling cycle. The stress response with temperature in the second thermal cycle (room temperature to 450 °C and back), which is reflective of the intrinsic film behaviour, was found to be similar in all cases and the elastic modulus determined from the stress response was also more or less identical. The three deposition temperatures were also not found to have a significant effect on the transformation characteristics of these films such as transformation start and finish temperatures, recovery stress and hysteresis.     

An experimental study of creep‐ratchetting behavior of rolled AZ31B magnesium alloy at room temperature

To study the interaction effect of creep and ratchetting for rolled AZ31B magnesium alloy at room temperature, a series of stress‐controlled tests were designed. In the tests, four loading types with different mean stresses were considered, and dwell loading was applied to explore the creep effect on the ratchetting. The test results indicated that the sequence of ratchetting and creep loading is crucial for the strain evolution. The amount of twinning/detwinning increased as the mean stress decreased, leading to an exhaustion of nonbasal slip during ratchetting, and then suppressed the creep ductility. However, the creep sequence exerted little influence on the strain shift of ratchetting while large amount of twinning/detwinning was involved.     

Experimental characterisation of a CuAg alloy for thermo‐mechanical applications. Part 1: Identifying parameters of non‐linear plasticity models

Despite the wide use of copper alloys in thermo‐mechanical applications, there is little data on their cyclic plasticity behaviour, particularly for CuAg alloys. This prevents the behaviour of the materials from being correctly described in numerical simulations for design purposes. In this work CuAg0.1 alloy used for thermo‐mechanical applications was tested by strain‐controlled cyclic loading at 3 different temperatures (room temperature, 250°C, 300°C). In each test, stress‐strain cycles were recorded until the alloy had completely stabilised. These cycles were then used to identify material parameters of non‐linear kinematic and isotropic models. The focus was on plasticity models (Armstrong‐Frederick, Chaboche, Voce) that are usually implemented in commercial finite element codes. Simulated cyclic responses with the identified material models were compared with experiments and showed a good agreement. The identified material parameters for the CuAg alloy under investigation can be used directly in finite element models for cyclic plasticity simulations, thus enabling a durability analysis of components under thermo‐mechanical loads to be performed, particularly in the field of steel‐making plants.     

Impact of compressive hold on out‐of‐phase thermomechanical fatigue crack growth in IN 718

Accurate characterization and understanding of the fatigue crack growth behaviour of components in jet turbine engines is critical for successfully using a damage tolerant design method to maximise safety and efficiency. The hot section components experience changing loads and temperatures, and hence, fatigue crack growth rates are typically studied under thermomechanical loading. One question that remains unclear is the role of the compressive holds that are often part of an aircraft loading‐temperature spectrum. This experimental study was undertaken to investigate a turbine disk alloy, Inconel 718, subjected to different cycling and temperature profiles considering different lengths of hot compressive holds to determine its effect on the fatigue crack growth rate. It was found that the addition of a compressive hold at temperatures from 650 to 725 °C has no significant impact on the fatigue crack growth rate when compared with a cycle without a compressive hold. Fractographic analysis shows that crack growth is primarily transgranular in all cases studied suggesting that grain boundary oxidation, often observed during hot tensile holds, is insignificant.     

Einfluss von Randschichtzustand und Haltezeit auf die thermische Ermüdung der Magnesiumbasislegierung AZ31

In this paper thermal fatigue of magnesium base alloy AZ31 in the temperature range between +50°C and +290°C is investigated. Experiments were carried out under constant total deformation (out‐of‐phase loading) and the resulting stress amplitudes as well as the plastic strain amplitudes were recorded as a function of the number of thermal loading cycles. In particular the consequences of mechanical surface treatments (deep rolling) and of hold‐times were investigated. In both cases no particular influence compared with untreated specimens loaded without hold‐times was observed, which is due to the interaction of deformation and recrystallization processes during thermal fatigue.     

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.     

The stress–life fatigue behaviour of aluminium alloy foams

The tension–tension and compression–compression nominal stress versus fatigue life responses of Alulight closed cell aluminium alloy foams have been measured for the compositions Al–1Mg–0.6Si and Al–1Mg–10Si (wt %), and for relative densities in the range 0.1–0.4. The fatigue strength of each foam increases with the relative density and with the mean applied stress, and is greater for the transverse orientation than for the longitudinal orientation. Under both tension–tension and compression–compression loading the dominant cyclic deformation mode appears to be material ratchetting; consequently, the fatigue life is highly sensitive to the magnitude of the applied stress. A micromechanical model is given to predict the dependence of life upon stress level and relative density. Panels containing a central hole were found to be notch insensitive for both tension–tension and compression–compression fatigue loading: the net-section strength equals the unnotched strength.     

Modeling of creep and stress relaxation of the nickel‐base alloy NiCr20TiAl at isothermal and non‐isothermal loading conditions

The design and dimensioning of new as well as the assessment of operating high‐temperature components in service require a precise prediction of creep and stress relaxation. The increasing share of renewable energies forces fossil‐fired power plants for increasing numbers of start‐ups and shut‐downs. Consequently, transient loading conditions need to be taken into account. In order to meet this demand, non‐isothermal creep equations are necessary, which enables a consistent prediction of creep strain and stress relaxation in a wide range of temperatures and stresses. In this paper, an approach for the visco‐plastic modeling of creep and stress relaxation for non‐isothermal loading conditions is presented. The strain portions creep, “negative creep” and initial plasticity, occurring at elevated temperatures are described by temperature‐dependent phenomenological equations. Within this paper, the adjustment of the parameters is based on a wide database of hot tensile tests, creep and annealing experiments. The nickel‐base alloy NiCr20TiAl has been examined in a temperature range from 450 °C to 650 °C. The developed material models have been successfully validated with isothermal and non‐isothermal relaxation experiments. Further, the recalculation of a staged relaxation test demonstrates the capability of the defined material laws in a wide stress range under isothermal and non‐isothermal loading conditions.     

Fatigue thresholds of holed components under in‐phase and out‐of‐phase torsional and axial loadings

The resistance‐curve (R‐curve) method was applied to the prediction of the fatigue thresholds of notched components under in‐phase and out‐of‐phase combinations of cyclic torsion and axial loadings. The prediction was compared with the experimental data obtained from thin‐walled tubular specimen of medium‐carbon steel with a hole. The stress was completely reversed and the mean stress was zero. The crack was nucleated at the position of the maximum range of the circumferential stress on the periphery of a hole, and propagated almost straight for all cases examined. The experimental data of the thresholds for crack initiation and fracture agreed well with the predictions for in‐phase and for out‐of‐phase loadings with 45° phase difference. For out‐of‐phase loading with 90°, the threshold for fracture was close to the crack initiation limit, because of the reduction of crack closure due to crack face rubbing by mode II shear cycling.     

Werkstoffkennwerte für die Lebensdauerberechnung von Strukturen aus Stahlfeinblechen für den Automobilbau

Materials Data for Fatigue Life Calculation of Steel Sheet Structures for Automotive Engineering Within a joint project of the steel and automotive industry 17 steel sheet materials for automotive engineering in various delivery and forming conditions at temperatures of –40 °C, 22 °C and 100 °C were investigated. In the course of 37 test series strain controlled fatigue curves to crack initiation and stress‐strain‐curves under monotonic and cyclic loading were determined. All experimental data, hysteresis loops and determined cyclic properties are available in a database. A correlation between the mechanical properties from tensile tests and the properties from strain controlled cyclic experiments seems to be possible.