Creep-fatigue Interaction Loading Research Articles

A generalized physical-based failure indicator parameter used in crystal plasticity model to predict fatigue life under low cycle fatigue and creep-fatigue loadings

A dislocation movement-based physical failure indicator parameter (FIP) was proposed and incorporated into crystal plasticity finite element to assess the damage for components subjected to low cycle fatigue (LCF) and creep-fatigue interaction (CFI) loadings. The concept of net slip was introduced through the distance of dislocation movement whose evolution was analyzed and compared with other FIPs. The creep damage can only be captured by the proposed physical FIP. Life prediction based on net slip was developed and validated for different materials under LCF and CFI loadings at different temperatures. Most of the predicted life falls in the ±1.5 error bands.

Couple effects of temperature and fatigue, creep-fatigue interaction and thermo-mechanical loading conditions on crack growth rate of nickel-based alloy

The ambient and high-temperature fatigue crack growth behaviors in C(T) and SENT specimens of Ni-based superalloy for turbine disk application were studied in a wide interval of temperatures 25–750°C using a combination a electro- and servohydraulic test systems and fractographic investigations. The fatigue, creep-fatigue interaction and thermo-mechanical in-phase fatigue (TMF IP) crack growth tests are performed under isothermal and dynamic waveforms loading conditions. The interpretation of the experimental results is given in terms of the traditional stress intensity factors and C-integral as well as new normalized cyclic fracture diagrams. It is found that there are definite temperature-sensitive regions separate for harmonic fatigue and creep-fatigue interaction loading conditions in which the crack growth rate of Ni-based alloy increases sharply. Scanning electron microscopy in longitudinal sections containing cracks revealed the mechanisms responsible for fatigue crack initiation and growth. The couple effect of temperature ranging and isothermal and dynamic waveforms loading conditions on fatigue life was discussed.

Unraveling the Complexities of Deformation/Damage Incurred in P91 Steel Weld Joint During Creep–Fatigue Interaction Loading at 873 K

Unraveling the Complexities of Deformation/Damage Incurred in P91 Steel Weld Joint During Creep–Fatigue Interaction Loading at 873 K

An improved unified viscoplastic model for modelling low cycle fatigue and creep fatigue interaction loadings of 9–12%Cr steel

An accurate constitutive model is one of the basic aspects to ensure a precise simulation. This study presents an improved unified viscoplastic model to simulate the various behavior of P92 steel under low cycle fatigue (LCF) and creep fatigue interaction (CFI) loadings. In the proposed model, an accumulated inelastic strain dependent parameter is introduced into the nonlinear kinematic hardening rule to represent the evolutionary behavior of strain-stress hysteresis loops and the varied relaxation behavior during CFI loadings. The traditional isotropic hardening rule is modified as well to capture the accelerated cyclic softening phenomena observed in the prolonged hold time of CFI tests. To validate the accuracy and the predictive capability of the proposed model, LCF tests at various strain amplitudes and CFI tests at different hold time are conducted at elevated temperature of 650 °C. Good agreement between the experimental and simulated results verifies the robustness of the proposed model. In addition, the proposed model is distinguished from published models by few determined material parameters.

Low cycle fatigue and creep fatigue interaction behavior of 9Cr-0.5Mo-1.8W-V-Nb heat-resistant steel at high temperature

In this paper, Low Cycle Fatigue (LCF) and Creep-Fatigue Interaction (CFI) behavior of 9Cr-0.5Mo-1.8 W-V-Nb heat-resistant steel (ASME Grade P92 steel) at elevated temperature of 600 °C are investigated. Strain controlled LCF tests are conducted in fully reversed triangular waveform at different strain amplitudes ranging from 0.2% to 0.8%. CFI tests are conducted at 0.4% strain amplitude in trapezoid waveform with tensile hold time varying from 1 min to 60 min and compressive hold time varying from 1 min to 10 min. During LCF and CFI loadings, the evolution of cyclic stress, mean stress and stress relaxation behavior are investigated. It turns out that the softening behavior and lifetime degradation are dependent on strain amplitude, hold time and hold direction. In addition, the microstructure evolution and fracture behavior are characterized by optical, scanning and transmission electron microscope. The initial rapid softening behavior is attributed to the quick elimination of low angle boundaries, whereas no obvious microstructure alteration appears in the stable stage. Fracture behavior analysis reveals creep voids in long-term CFI tests facilitates the initiation and propagation of secondary cracks. The different responses of outer surface oxidation layer during cycling provides an explanation for severer damage of compressive hold and also accounts for the observed various fracture behavior of failed samples.

Comparative assessment of remnant tensile properties of modified 9Cr-1Mo steel under prior low cycle fatigue and creep-fatigue interaction loading

Progressive deterioration in the mechanical properties with increasing service period is commonly observed and hence needs to be addressed. Awareness about the rate of degradation of the properties can circumvent a lot of problems faced during service. Hence the present work aims to understand the degradation of tensile properties due to prior cyclic loading. Systematic comparative evaluation study for the evolution of tensile properties with prior fatigue loading and the deformation and damage evolution process in modified 9Cr-1Mo steel during low cycle fatigue (LCF) and creep-fatigue interaction (CFI) loadings has been carried out. For this, LCF and CFI experiments were carried out at constant strain amplitude of ±0.6%, strain rate of 3×10−3s−1 and temperature of 600°C. Experiments interrupted after 5, 10, 30, and 50% of the total fatigue life (Nf) under both LCF and CFI conditions were followed by monotonic loading at the same strain rate and temperature until fracture. Drastic reduction in the yield strength value due to both LCF and CFI prior fatigue loading was revealed and the extent of decrease in the YS values increased with the extent of prior fatigue damage imparted on the steel. Substantial difference in the YS evolution with prior fatigue damage under LCF and CFI conditions occurred only after 10% life fraction due to the predominance of creep recovery process. Factors such as microstructural changes due to creep alone or fatigue alone or both and surface oxidation changed the tensile properties under LCF and CFI.

Investigation on microstructure evolution and fracture morphology of single crystal nickel-base superalloys under creep-fatigue interaction loading

Creep-fatigue interaction behavior of a single crystal nickel-base superalloy was studied at 980°C and the microstructure was quantitatively analyzed. A three-stepped plate specimen had been designed to simultaneously observe microstructure changed under three different stresses. It was found that the distribution of γ′ size was basically consistent with lognormal probability scale under all three loading conditions. The average size of γ′ precipitates increased with the increasing of stress at the same interrupted time. Four different layers including oxide layer, γ′-free layer, γ′-reduced layer and γ/γ′ layer can be observed on the cross-section of the specimens. The thickness of γ′-free layer also increased with the increasing of stress. Numerous dimples and square-shaped facets were observed at the fracture surface and furthermore propagation of cracks from micropores to the boundary of the facets could be found, which showed the fracture behaviours of both creep and fatigue damage.