In this paper, the strain‐controlled low‐cycle‐fatigue (LCF) test and low‐cycle‐fatigue‐creep (LCFC) test of a cast Al‐Si‐Cu alloy at 350°C were investigated. A significant softening behavior can be ...observed in the tests, the strain amplitude has an effect on softening speed, and the dwell time has an effect on both softening ratio and softening speed. Microscopic observation indicated that the compressive load of creep could result in a significant amount of plastic deformation and lead to the eutectic silicon particles rupture and the propagation of dimples, causing the softening behavior and the decrease of failure life of Al‐Si‐Cu alloy. Based on the experiment data, a modified visco‐plasticity model combining Chaboche model and θ‐projection model was proposed to capture the cyclic stress response. The modified model could simulate the softening behavior and hysteresis loop for LCF test and LCFC test of Al‐Si‐Cu alloy accurately.
Highlights
The softening behavior of LCF and LCFC was experimented and studied.
The microstructure evolution of LCF and LCFC were studied.
The effect of creep on LCFC and the fracture mechanism of LCFC were investigated.
An improved modified model was established by combining Chaboche model and θ‐projection model
•A phase-field model for low-cycle fatigue of brittle materials is proposed.•The model introduces a fatigue degradation function, which modifies the fracture toughness.•The model can reproduce ...total-life and defect-tolerant approaches to fatigue.•It can handle complex geometries, loading conditions, and crack patterns without ad hocadditions to the theory.
A phenomenological phase-field model for the formation and growth of fatigue macro-cracks in brittle materials is proposed. Additionally to the classical elastic stiffness degradation in the phase-field approach to overload fracture, a fracture toughness degradation is further introduced. The model can reproduce both total life approaches, such as the Basquin empirical relation, and the Paris law, which lies at the core of most of the defect-tolerant approaches, with exponents characteristic of brittle fatigue crack growth. The numerical implementation of the model in an efficient scheme is described and its ability to handle complex geometries and loading conditions demonstrated.
The interaction between creep and fatigue damage in Ni‐base superalloy CMSX‐8 was explored using a sequential workflow that decouples creep and fatigue steps to quantify how accumulated creep damage ...interacts with subsequent isothermal low‐cycle fatigue (LCF) and vice versa. Two workflows were considered: creep followed by LCF and LCF followed by creep. For the first workflow, the subsequent LCF was evaluated at temperatures of 20°C, 750°C, and 1100°C. It was found that the extent of deleterious interaction of prior creep on LCF depended on increasing subsequent fatigue test temperature. For the LCF followed by creep, fatigue tests were first conducted at 20°C and 1100°C to 60% of life, and then creep was conducted at 800°C, where primary creep is promoted, or 900°C, where primary creep is absent. LCF damage accumulated at 1100°C accelerated creep and decreased ductility, while fatigue damage at room temperature had minimal effect on the creep behavior.
Highlights
Sequential experiments provide new sights into the interaction between creep and fatigue damage.
Pre‐creep reduces yield strength but does not reduce LCF life at room temperature or 750°C.
In contrast, pre‐aging and pre‐creep reduce LCF life at
Rε=0 and 1100°C.
Creep rupture lives are reduced when prior fatigue occurs at 1100°C but not at lower temperatures.
•A novel Fe-Mn-Si alloy U-shaped damper with excellent LCF resistance is proposed.•Both the monotonic and cyclic loading material tests are conducted.•Experimental study on 30 U-shaped dampers is ...conducted.•A combined kinematic/isotropic hardening model is used to simulate the Fe-Mn-Si alloy.•Finite element models are established and validated through comparisons against test results.
This study presents a new type of Fe-Mn-Si alloy U-shaped dampers (FMS-UDs) that exhibit extraordinary low cycle fatigue (LCF) performance. The basic material properties of Fe-Mn-Si alloy were first evaluated by a series of material tests considering both the monotonic and cyclic loading schemes, where normal carbon steel specimens were also tested for comparison. After the material tests, a total of 30 U-shaped dampers were examined, considering various parameters such as material type, geometry, loading direction and strain rate. Following the experimental program, finite element models were established to help interpret the test result. The material tests showed that Fe-Mn-Si alloy exhibited non-obvious yield plateau and moderate strain hardening under monotonic loading. Spring-back phenomenon, non Masing behavior, and excellent LCF performance were generally observed under cyclic loading. The damper tests showed that the FMS-UDs exhibited slightly “narrower” hysteresis responses compared with normal steel dampers. The LCF life and the total energy dissipation of the FMS-UDs are up to 7 times that of their steel counterparts, depending on the parameters considered. From a modeling point of view, the hysteretic behavior of the Fe-Mn-Si alloy could be adequately captured by a combined kinematic/isotropic hardening model. By using the calibrated parameters from the material tests, the hysteresis behavior and stress distribution of the FMS-UDs can be well represented. Critical locations which are prone to fatigue failure of the FMS-UDs were accurately captured by the numerical model, and the predictions agree very well with the test results.
•An all-steel bamboo-shaped energy dissipater (SBED) was investigated.•SBED has stable hysteretic behavior and controlled deformation patterns.•Geometrical parameters have significant influence on ...SBED.•Failure modes are related to bending deformation, stress concentration and torsion.•Recommended β values are given at different strain amplitudes.
Energy dissipaters constructed in precast structures play an important fuse-type role in concentrating damage and protecting the primary structure. The stable hysteretic behavior, easy fabrication and low cost are expected characteristics for high-performance energy dissipater. In this paper, an all-steel bamboo-shaped energy dissipater consisting of an inner bamboo-shaped core and an outer restraining tube was developed, which aimed at guaranteeing the performance of the energy dissipater under relatively high strain amplitude and avoiding the adverse effect of grouting and welding. Parametric studies on geometrical variables were performed to investigate the low-cycle fatigue behaviors and deformation patterns of the proposed bamboo-shaped dissipaters. Test results showed that all-steel bamboo-shaped dissipaters showed stable hysteretic curves and no local or overall buckling were observed. Failure modes of bamboo-shaped energy dissipater were affected by the lateral deformation resulted from bending, stress concentration around the fillet and the torsion in the segment. The torsion, contact conditions and the wavelength were discussed via finite element analyses and theoretical derivations.
Based on the physical phenomenon that the fatigue cracks initiate along specific slip plane, a slip plane damage‐based low‐cycle fatigue (LCF) lifetime model for the nickel‐based single‐crystal ...superalloy is established. The predicted results indicate that the lifetime model can reflect the orientation effect. In addition, in order to characterize the dwell‐time dependence of the LCF lifetime, creep damage and compression‐creep damage are introduced to the lifetime model. Finally, the lifetime predictions under LCF loading with tensile dwell time, compressive dwell time and tensile‐compressive dwell time are conducted by employing the lifetime model, respectively. The predicted lifetimes show a good agreement with the experimental data, which verifies the accuracy of the developed lifetime model in this paper.
•A simple and efficient CCF life prediction model is proposed to calculate the approximate threshold damage.•Fatigue tests of specimen are conducted for investigating the threshold damage.•Framework ...for estimating fatigue life involving threshold damage value under CCF loadings is presented by checking the threshold damage against current HCF damage.•The threshold damage-based model provides more accuracy predictions than others, especially near the level of threshold damage value.
Life prediction based on the damage mechanics of aero-engine turbine blades is crucial for performing their strength design and ensuring the operational reliability under combined high and low cycle fatigue (CCF) loadings. In view of this, a simple and efficient life prediction method is developed to consider the interaction of high cycle fatigue (HCF) and low cycle fatigue (LCF) without any additional material constants, and as a basis of that a new nonlinear damage accumulation model is proposed by introducing threshold damage of the component related to material type and current HCF damage. In order to validate the presented methodology, model predictions were compared with the experimental data sets of turbine blades and its alloy materials. The predicted results indicate that the simple life prediction model is capable of estimating the fatigue life quickly with an acceptable accuracy in contrast to other four existing ones. Moreover, the threshold damage-based method provides the higher prediction accuracy than others due to improved HCF damage determined by the damage threshold.
•Calibration of low cycle fatigue laws requires tests with variety of stress states.•Conducted novel testing with combinations of axial and torsional loading.•Got stress and strain histories at crack ...initiation location using finite element.•Unique data set for calibrating low cycle fatigue criteria with an example.
Structural steel components experience cyclic inelastic strains when subjected to extreme loading events such as strong earthquakes. In such cases, the steel may experience ultra-low cycle fatigue (ULCF) fracture, associated with the growth and coalescence of microscopic voids in the material. A number of ULCF criteria have been formulated and used in the literature for the analysis of fracture under cyclic loading. The calibration of such criteria requires experimental test data covering a variety of stress states. To address this need, a testing program, involving both monotonic and cyclic loading protocols, was conducted on 60 circumferentially notched coupons extracted from rectangular steel tube sections made of ASTM A1085 steel. A relatively novel approach was used to achieve a wide range of stress states by subjecting the coupons to combinations of axial and torsional loading. The experimental tests are accompanied by finite element analyses to elucidate the local stress and deformation states at the crack initiation location of each coupon. To demonstrate the importance of the experimental tests for the reliable characterization of ULCF in structural steel, an example calibration is conducted for an ULCF criterion from the literature using the experimental results.
The behavior of CrCoFeMnNi high-entropy alloy (Cantor) during cyclic deformation at room temperature was compared with those of a twinning-induced plasticity (TWIP) steel and 304 stainless steel ...(SS304). The three materials with similar grain sizes (~65 μm) were made by controlled heat treatment and tested to evaluate tensile and low-cycle fatigue (LCF) properties. The tensile strength and ductility of Cantor were much lower than those of TWIP steel and SS304. Cantor and TWIP steel revealed cyclic hardening and then softening; SS304 showed secondary hardening at the latest stage of cyclic deformation due to formation of α′-martensite, which led to a significant decrease in its LCF life. At high total strain amplitudes >0.4% LCF life was longer in Cantor than in SS304, but shorter than in TWIP steel. However, at total strain amplitudes ≤0.4%, Cantor showed the shortest LCF life. It was found that the Coffin-Manson plot of Cantor followed the trend of TWIP steel rather than that of SS304. Microstructural analysis of the Cantor with increasing cycles revealed the formation of twins and well-defined cell structures, the latter of which is the evidence of wavy slip.
•The LCF life of Cantor alloy was higher than that of SS304 at high strain amplitude, but lower than that of TWIP at all strain amplitudes.•In Cantor alloy, mechanical twinning formation was evidenced by TEM images.•The most active mechanism during cyclic loading of Cantor alloy was the rearrangement of dislocations in cell structures.
•Softening during interrupted low cycle fatigue at 650 °C was studied in 10%Cr steel.•Decrease in the friction stress is attributed to dislocation annihilation.•Gradual lath widening occurs due to ...disappearance of lath boundaries.•Additional precipitation on lath boundaries provides a more stable back stress.•Steady-state stage at 1–10 cycles is attributed to additional precipitation.
The correlation between cyclic softening and microstructural evolution using the interrupted low cycle fatigue (LCF) tests with a low strain amplitude of ±0.2% at 650 °C in a 10% Cr steel was studied. Cyclic softening was analyzed in terms of the friction and back stresses caused by short-range and long-range obstacles, respectively. Softening was mainly attributed to a decrease in the friction stress due to the dislocation annihilation. Whereas the back stress remained more stable due to additional precipitation on the lath boundaries, despite the intense lath widening caused by the disappearance of lath boundaries.
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