An investigation of the fatigue of CK45 steel in the as‐received state and after pre‐fatigue deformation

Fatigue failure, ratcheting behaviour and influence of pre‐fatigue on fatigue behaviour were investigated under uniaxial cyclic loading for CK45 steel at room temperature. The fatigue life was recorded for various stress ratios, and then, three mean stress models were considered. The Walker model showed an acceptable accuracy in comparison with Smith–Watson–Topper and Park et al. models. The ratcheting strains were measured for various loading conditions in order to evaluate the impact of mean stress, stress amplitude and stress ratio on ratcheting behaviour. The experimental results showed that the ratcheting strain increased with increasing mean stress, stress amplitude and stress ratio. In addition, the results of the post‐ratcheting‐fatigue tests showed that although the fatigue life decreased with increasing pre‐ratcheting strain (the ratcheting strain that is accumulated in pre‐fatigue), the loading condition that pre‐fatigue experiments were conducted has a significant effect on subsequent fatigue behaviour.     

Influence of reinforcement morphology and matrix strength of metal–matrix composites on the cyclic deformation and fatigue behaviour

The cyclic deformation behaviour of three metal–matrix composites, namely AA6061-T6 reinforced with 20 vol.% alumina particles and short-fibres, respectively, and pure aluminium reinforced with 20 vol.% short-fibres, has been investigated at temperatures between T=−100°C and T=300°C in total strain controlled symmetrical push–pull fatigue tests. The cyclic stress response exhibits initial cyclic hardening, subsequent saturation and cyclic softening, depending on the test parameters for temperatures lower than T=150°C. Initial cyclic hardening is less pronounced with increasing temperature and decreasing applied strain amplitude. Short-fibre reinforced composites — both with alloyed and unalloyed aluminium matrix — harden cyclically more than the particulate-reinforced composite. The comparison of the cyclic with monotonic stress–strain curves indicates that, depending on the testing conditions, both cyclic hardening and cyclic softening can occur.     

Biaxial fatigue for proportional and non‐proportional loading paths

In order to study the use of a local approach to predict crack‐initiation life on notches in mechanical components under multiaxial fatigue conditions, the study of the local cyclic elasto‐plastic behaviour and the selection of an appropriate multiaxial fatigue model are essential steps in fatigue‐life prediction. The evolution of stress–strain fields from the initial state to the stabilized state depends on the material type, loading amplitude and loading paths. A series of biaxial tension–compression tests with static or cyclic torsion were carried out on a biaxial servo‐hydraulic testing machine. Specimens were made of an alloy steel 42CrMo4 quenched and tempered. The shear stress relaxations of the cyclic tension–compression with a steady torsion angle were observed for various loading levels. Finite element analyses were used to simulate the cyclic behaviour and good agreement was found. Based on the local stabilized cyclic elastic–plastic stress–strain responses, the strain‐based multiaxial fatigue damage parameters were applied and correlated with the experimentally obtained lives. As a comparison, a stress‐invariant‐based approach with the minimum circumscribed ellipse (MCE) approach for evaluating the effective shear stress amplitude was also applied for fatigue life prediction. The comparison showed that both the equivalent strain range and the stress‐invariant parameter with non‐proportional factors correlated well with the experimental results obtained in this study.     

Microstructure‐oriented characterisation of the cyclic deformation behaviour of Ti‐6Al‐4V

In this paper, the cyclic deformation behaviour of the titanium alloy Ti‐6Al‐4V is characterised in uniaxial stress‐ and total‐strain‐controlled load increase and constant amplitude tests at ambient temperature by means of mechanical stress‐strain hysteresis and temperature measurements. The measured physical values obviously show a pronounced interrelation with the underlying fatigue processes and represent the actual fatigue state. In selected experiments the influence of elevated temperatures on the cyclic deformation behaviour was investigated. Using the plastic strain amplitude and the change of the specimen temperature with the physically based lifetime calculation “PHYBAL” an excellent accordance with experimentally determined lifetimes could be realised. Microstructural changes were evaluated by transmission electron microscopy in defined fatigue states, additionally, the fracture surface was analysed by scanning electron microscopy.     

Foreword – Materialwissenschaft und Werkstofftechnik 10/2011

Load controlled fatigue tests were performed up to 10⁷ cycles on flat notched specimens (K_t = 2.5) under constant amplitude and variable amplitude loadings with and without periodical overloads. Two materials are studied: a ferritic‐bainitic steel and a cast aluminium alloy. These materials have a very different cyclic behaviour: the steel exhibits cyclic strain softening whereas the Al alloy shows cyclic strain hardening. The fatigue tests show that, for the steel, periodical overload applications reduce significantly the fatigue life for fully reversed load ratio (R_σ = –1), while they have no influence under pulsating loading (R_σ = 0). For the Al alloy overloads have an effect (fatigue life decreasing) only for variable amplitude loadings. The detrimental effect of overloads on the steel is due to ratcheting at the notch root which evolution is overload's dependent.     

Experimental observations and modelling of cyclic and relaxation behaviour of the Ni-based superalloy Haynes 282

In this paper, the high temperature cyclic and relaxation behaviour of Ni-based superalloy Haynes 282 is investigated. Low-cycle fatigue (LCF) tests with and without hold time have been performed at two elevated temperatures, 650 and 730 °C. The test results are presented and analysed with respect to the cyclic behaviour and the stress relaxation behaviour. Based on this analysis, a Chaboche type of elasto-viscoplastic material model is formulated and calibrated with respect to the cyclic experimental data. Furthermore, the effect of the scatter, observed in the initial yield stresses of the LCF tests, is considered in the calibration of the material model. Finally, a Golos–Ellyin strain energy density fatigue criterion is used to predict the fatigue life and how the scatter in the LCF tests influences the predicted fatigue lives is studied. Moreover, to account for the reduction in fatigue life due to stress relaxation, a frequency modification of the Golos–Ellyin fatigue criterion is proposed and evaluated.     

Experimental and theoretical studies on thermo‐mechanical fatigue test for aluminium cast alloy

Evaluation of the thermo‐mechanical behaviour and prediction of the service life of cast aluminium alloys are important for the design of automobile engine cylinder heads. In this study, cast Al alloy specimens are extracted from cylinder heads and subjected to in‐phase thermo‐mechanical cyclic loading. The hysteresis curves related to stress and strain were recorded under the individual thermo‐mechanical loading conditions. The number cycles to failure corresponding to multiple mechanical strain and temperature ranges were obtained. It is found that the cyclic stress amplitude decreases and the cyclic softening rate increases with increasing maximum temperature rise. A modified fatigue‐creep model based on energy conservation has been developed for prediction of the fatigue life of cylinder heads. The proposed method shows good agreement with the well‐established Ostergren model and low standard deviations. In summary, the proposed method described in this study provides an option for prediction of the thermo‐mechanical behaviour of metals.     

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.     

Experimental and numerical evaluations of stress relaxation in A356 aluminium alloy subjected to out-of-phase thermomechanical cyclic loadings

Abstract

In this study, the stress relaxation has been measured experimentally and has been also calculated numerically by the finite element method in the A356·0 aluminium–silicon–magnesium alloy, under out-of-phase thermomechanical cyclic loadings. To get this objective, strain based thermomechanical fatigue tests were performed on cylindrical specimens, at an out-of-phase condition. In this loading condition, when the temperature was maximum, the mechanical strain was compressive and vice versa. These fatigue experiments were repeated at various dwell times, in which the temperature was held at the maximum temperature. This hold time was considered as 5, 30, 60 and 180 s and then the stress relaxation was measured during the mid-life cycle of each test. Besides, the finite element analysis was also conducted on the material to simulate the stress relaxation numerically. A two-layer visco-plastic model was applied to simulate the high temperature cyclic behavior of the material. Finite element results showed a good agreement with experimental results, which were obtained from thermomechanical fatigue tests on the A356·0 aluminium alloy. The two-layer visco-plastic model could properly predict the stress relaxation at elevated temperatures, during various dwell times.     

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.     

An energy‐based critical fatigue life prediction method for AL6061‐T6

An energy‐based critical fatigue life prediction method is developed and analysed. The original energy‐based fatigue life prediction theory states that the number of cycles to failure is estimated by dividing the total energy accumulated during a monotonic fracture by the strain energy per cycle. Because the accuracy of this concept is heavily dependent on the cyclic behaviour of the material, a precise understanding of the strain energy behaviour throughout each failure process is necessary. Examination of the stress and strain during fatigue tests shows that the cyclic strain energy behaviour is not perfectly stable as initially presumed. It was discovered that fatigue hysteresis energy always accumulates to the same amount of energy by the end of the stable energy region, which has led to a new ‘critical energy’ material property. Characterization of strain energy throughout the fatigue process has thus improved the understanding of an energy‐based fatigue life prediction method.     

Experimentelle Untersuchung und kristallplastische Simulation des Verhaltens kurzer Ermüdungsrisse in der ausgehärteten Aluminiumknetlegierung AlMgSi1 mit dem Ziel der Lebensdauervorhersage

Experimental investigation and crystal‐plastic simulation of short fatigue crack behaviour in the age‐hardened aluminium alloy 6082 with the purpose of fatigue life prediction Cyclic loading tests with various block sequences were carried out. Short blocks yielded shortest life times. The initiation of microcracks and crack growth was examined. Strain controlled tests were carried out to be evaluated in order to gain model parameters. Crystal plasticity and the resulting polycrystalline behaviour were modelled and programmed in form of Finite Element subroutines.     

Thermisch‐mechanisches Ermüdungsverhalten der partikelverstärkten Aluminiumknetlegierung EN AW‐6061‐T6 und die Entwicklung von Eigenspannungen im Matrixwerkstoff unter thermisch‐mechanischer Beanspruchung

Thermal mechanical fatigue behaviour of particle reinforced EN AW‐6061‐T6 and development of residual stresses in the matrix material by thermal mechanical loading The behaviour of non reinforced and 15 Vol.‐% α‐alumina particle reinforced wrought aluminium alloy EN AW‐6061‐T6 in thermal mechanical fatigue loading was investigated at different maximum temperatures. The tests were performed in strain controlled mode by means of an electro‐mechanical testing machine. Alternating load deformation and life cycle behaviour either materials were compared. It came out, that the reinforcement leads to an decreasing thermal mechanical fatigue life cycle while keeping constant the maximum temperature and mechanical loading. The two materials showed softening behaviour due to high maximum temperatures of 573 K to 673 K. However, there is an intense scatter of the number of cycles to failure of the non reinforced alloy aggravating the interpretation of the results. On the other hand the thermal mechanical life cycle increases in combination with increasing maximum temperatures. Simultaneously the part of plastic deformation in mechanical loading increases for both materials, while for a constant total strain range the effective maximum and minimum stresses are decreasing. Furthermore, the development of residual stresses in the matrix of the reinforced alloy by thermal mechanical fatigue loading was analysed. It was observed that only small absolute values of residual stresses will be obtained for these loads. Nevertheless, tendencies of mounting tensile residual stresses can be identified in the direction of thermal mechanical fatigue loading and subsequently reduction of the residual stresses.     

Zyklische Beanspruchung drahtverstärkter Verbundstrangpressprofile mit Aluminiummatrix

Cyclic loading of wire‐reinforced aluminium matrix composite extrusions Aluminium matrix composite extrusions reinforced with wires featuring high strength and stiffness represent an innovative materials concept for lightweight structures. The use of reinforcing elements should improve the mechanical properties and hence enhance the performance of the lightweight structures. Composite extrusions made from the aluminium alloy EN AW‐6060 reinforced with reinforcing elements made from the spring steel 1.4310 and the cobalt‐base alloy Haynes 25 were examined under cyclic loadings which are of vital importance for the desired applications. Initially, load controlled multiple step tests at a load ration of R = ‐1 allowed for the determination of the cyclic stress‐strain‐curve. Afterwards, lifetime predictions were determined from these results by using mechanical models proposed by Morrow and Basquin, which were reviewed Woehler tests without mean stress. Furthermore, light and electron microscopy served for the clarification of damage and failure mechanisms. The investigations have been carried out with varying materials, configurations and surface treatments of the reinforcing elements. The investigations strived for the identification of the parameters’ influence on the lifetime behaviour to optimize the materials systems regarding the fatigue behaviour.     

无铅焊料Sn-3.8Ag-0.7Cu的低周疲劳行为

测量了Sn-3.8Ag-0.7Cu无铅焊料试样的循环滞后回线、循环应力响应曲线、循环应力-应变和应变寿命关系,研究了焊料在总应变幅控制下的低周疲劳行为结果表明:该焊料合金在总应变幅较高(1%)时发生连续的循环软化,而在总应变幅较低(≤0.4%)时则表现为循环稳定.线性回归分析表明,该焊料的低周疲劳寿命满足Coffin-Manson经验关系式,由此给出了焊料在室温下的低周疲劳参数.采用扫描电镜观测和分析了焊料在疲劳前后的组织特征.     

A new energy‐based isothermal and thermo‐mechanical fatigue lifetime prediction model for aluminium–silicon–magnesium alloy

In this paper, a new fatigue lifetime prediction model is presented for the aluminium–silicon–magnesium alloy, A356.0. This model is based on the plastic strain energy density per cycle including two correction factors in order to consider the effect of the mean stress and the maximum temperature. The thermal term considers creep and oxidation damages in A356.0 alloy. To calibrate the model, isothermal fatigue and out‐of‐phase thermo‐mechanical fatigue (TMF) tests were conducted on the A356.0 alloy. Results showed an improvement in predicting fatigue lifetimes by the present model in comparison with classical theories and also the plastic strain energy density (without any correction factors). Therefore, this model is applicable for TMF, low cycle fatigue (LCF) and both TMF/LCF lifetimes of the A356.0 alloy. Furthermore, this model can be easily used for the estimation of thermo‐mechanical conditions in components such as cylinder heads.     

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.     

ELEVATED TEMPERATURE FATIGUE IN Ni3Al-BASED ALLOYS

Both high-cycle and low-cycle fatigue properties of hot-extruded powders of a Ni₃Al-based alloy, IC218, have been evaluated. High cycle fatigue measurements were performed under stress controlled conditions at temperatures ranging from 25°C to 850°C. Tests were made in both laboratory air and vacuum environments. Low cycle fatigue tests were conducted under total strain control in a laboratory air environment at 650°C. In high cycle fatigue, high ratios of the fatigue limit (Δσ at 10⁶ cycles) to monotonic yield strength (σ_ys), of approximately Δσ/σ_ys～1, were obtained in the powder extruded IC218 alloy for temperatures ranging from 25°C to 650°C. In low cycle fatigue, a substantial decrease in fatigue life occurred at 650°C, compared to results obtained previously at 25°C. High cycle fatigue performance at low stress/strain amplitudes is better than expected when compared to precipitation strengthened superalloys. The improved performance is explained in terms of the cyclic hardening behavior of the alloy.     

Fatigue behaviour and modelling of talc‐filled and short glass fibre reinforced thermoplastics,including temperature and mean stress effects

Effects of temperature and mean stress on fatigue behaviour of talc‐filled polypropylene (PP‐T) and short glass fibre reinforced polypropylene (PP‐G), polyamide‐66 (PA66), and a blend of polyphenylene ether and polystyrene (PPE/PS) were investigated. Load‐controlled fatigue tests were conducted under positive stress ratios (R = 0.1 and 0.3) and at several temperatures (T = 23, 85 and 120 °C). Larson–Miller parameter was used and a shift factor of Arrhenius type was developed to correlate fatigue data at various temperatures. Effect of mean stress on fatigue life was significant for some of the studied materials; however, for the PPE/PS blend no effect of mean stress was observed. Modified Goodman and Walker mean stress equations were evaluated for their ability to correlate mean stress data. A general fatigue life prediction model was also used to account for the effects of mean stress, temperature, anisotropy and frequency.