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•Rapid cyclic softening and slow stress relaxation were captured in the FCI tests.•Holding time leads to intensified oxidation and accelerates crack initiation.•Both creep voids and ...secondary cracks accelerate the failure of the specimen.•A new fatigue-creep life prediction model considering the effect of holding time.•The effectiveness of the proposed model has been confirmed.
The QCr0.8 alloy used in the thrust chamber liner wall of the reusable liquid rocket engine is subjected to high-temperature fatigue and high-temperature creep loading. This work investigates the effect of fatigue-creep interaction (FCI) on the fatigue life of QCr0.8 alloy. Experimental results reveal accelerated cyclic softening and decelerated stress relaxation in the alloy under FCI conditions, attributed to creep deformation and stress amplitude growth. The damage evolution mechanism of FCI is revealed by the results of the energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). The decrease in fatigue-creep life of the QCr0.8 alloy is attributed to the fracture of the outer surface oxide layer, as well as the coupling development of creep voids and secondary cracks. Finally, a new fatigue-creep life prediction model has been developed, taking into account the increase in plastic strain energy resulting from stress relaxation and creep deformation behaviors. The study results provide substantial theoretical justification for the utilization of QCr0.8 alloy in the thrust chamber wall of reusable liquid rocket engines.
To study the effect of the upper limit stress on the fatigue characteristics and crack propagation process of jointed rock masses under the interaction of fatigue‐creep, a series of fatigue‐creep ...tests with different cyclic stress levels were carried out on jointed sandstone samples. Acoustic emission, scanning electron microscopy, and digital image correlation technologies were used to analyze the fatigue characteristics and crack growth process of the specimen. The results show that as the stress level increases, the deformation of the sample increases, the strength decreases, the elasticity attenuates, and the damage increases. The splitting failure of grains is weakened, the area of fatigue folds increases, and the fracture roughness increases. Besides, the crack propagation process of the sample can be divided into three stages: crack initiation, stable crack growth, and unsteady crack growth. As the stress level increases, the failure mode of the sample gradually changes from brittle to ductile.
Highlights
Red sandstone containing pre‐existing crack under fatigue‐creep interaction is tested.
AE and DIC techniques are used to study the crack evolution process of jointed rock.
The fatigue characteristics of jointed rock masses are analyzed in macroscale and mesoscale.
Based on the deformation parameters, the damage model of the specimen is derived.
A series of strain-controlled Low-Cycle Fatigue tests (LCF) and Thermo-Mechanical Fatigue tests (TMF) were performed on silicon-molybdenum ductile cast iron SiMo 4.06. LCF tests were performed for ...various mechanical strain amplitudes at a high strain rate of 3×10−3/s for various temperatures between 20 °C and 750 °C. In addition, several LCF tests were performed for a low strain rate of 1×10−4/s and as fatigue–creep tests with a tensile strain dwell. TMF tests were performed as out-of-phase and fully constrained between 100 °C and 650 °C. The investigation of the damage mechanisms revealed that the predominant failure mode is a combination of graphite-matrix debonding and transgranular crack propagation through ferritic matrix. Finally, a novel damage model based on the dissipated hysteresis energy is proposed in order to effectively predict the lifetime of SiMo 4.06 under LCF and TMF loading. Creep and fatigue damage are each calculated separately and the oxidation effect is taken into account indirectly.
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•LCF and TMF tests were performed for SiMo 4.06 cast iron between 20 °C and 750 °C.•LCF tests were also conducted with a tensile strain dwell and for various strain rates.•Out-of-phase TMF leads to a significantly shorter fatigue life compared to LCF.•Transgranular crack propagation with graphite-matrix debonding is predominant.•The novel damage model incorporates the effect of frequency and mean stress.
Thermomechanical fatigue in distinct crystal orientations of nickel-base single-crystal superalloy was studied. Crack initiation modes and damage mechanisms were characterized on fracture surfaces ...and longitudinal sections by scanning electron microscope and optical microscope. Creep damage nucleated from casting pores in IP-TMF and propagated under mode I. The creep facets were controlled by the activated slip systems, whereas fatigue damage was orientation-dependent and developed by slipping along different crystallographic planes. Oxidation-assisted cracking in OP-TMF tests was non-crystallographic for all crystal orientations. Multi-layer structures near the crack tip revealed successive growth of oxide and the crack grew within the oxidized material.
•Revealed anisotropic TMF life distributions depending on three crystal orientations in a nickel-base single-crystal superalloy.•Confirmed the creep-dominant damage mechanisms of IP-TMF and oxidation-dominant fatigue damage in OP-TMF.•Displayed creep damage initiated from internal casting pores affected by different crystallographic slip systems.•Unveiled surface cracks initiated from oxide spikes under mode I condition in OP TMF, regardless of crystal orientations.
An improved unified viscoplastic model is developed within the framework of the Chaboche model in order to simulate the mechanical behaviour of a martensitic steel under Low-Cycle Fatigue (LCF) and ...Low-Cycle Fatigue-Creep (LCFC) loadings at 550 °C. The effects of strain rate sensitivity, static recovery, strain range-dependent cyclic softening, along with the effect of additional hardening under non-proportional loading are incorporated into the constitutive equations. The non-linear kinematic hardening rule is modified in such a way that it enables the simulation of the evolutionary behaviour of mean stress and the effect of decelerating stress relaxation. Finally, the prediction capability of the proposed model is illustrated for the experimental data obtained from the various strain-controlled axial-torsional tests.
•A modified viscoplastic constitutive model for martensitic steels is developed.•The effect of decelerated stress relaxation is incorporated into the static recovery.•A strain range dependency is used in the evolution of the dynamic recovery term.•The effect of additional hardening under non-proportional loading is incorporated.•Mean stress evolution is simulated by modifying the kinematic hardening rule.
Effects of different loading and uninstalling and safe loading time on the low-cycle fatigue-creep interaction behavior of GH4169 superalloy at 650°C/850MPa have been investigated. The study found ...that under the safe loading time, with the longer the loading and unistalling time, the fatigue-creep property and life of this alloy were gradually improved. However, under the same loading and uninstalling time, with the extension of the loading time, property of the fatigue-Creep of the alloy was gradually reduced, and its life was gradually shortened. In addition, The fracture failure behavior of the alloy under different conditions was studied in detail. Interestingly, With the extension of the holding time, the fracture failure of the alloy was changed from fatigue damage to creep damage, which makes its fracture mode transition the transgranular fracture into intergranular fracture extension.
Strain controlled fatigue of P92 steel with various strain hold dwells introduced at the peak loading point were conducted at 625 °C. Two features which depend on the cycle and strain range level ...were observed under the fatigue-creep condition for the viscous and cyclic softening material. The first one is the accelerated cyclic softening response which is ascribed to the accumulated inelastic strain transformation from the creep mechanism during the strain dwell period and becomes more significant with the decrease of strain ranges. The second one is the decelerated stress relaxation behavior which is caused by the reduced viscous stress related to the continuous cyclic softening and fades with the decrease of cyclic strain ranges. Accordingly, a new unified viscoplastic constitutive model within the framework of Chaboche model was developed by improving the nonlinear isotropic hardening rule and the kinematic hardening rule with a cyclic softening parameter. As a result, the accelerated cyclic softening and decelerated stress relaxation response of fatigue-creep interaction was finely reproduced by the proposed model.
•Strain controlled fatigue of P92 with various strain hold dwells introduced at the peak loading point were conducted at 625 °C.•The accelerated cyclic softening response increases with the decrease of strain ranges due to the strain dwell.•The decelerated stress relaxation behavior fades with the decrease of cyclic strain ranges due to the continuous cyclic softening.•A unified viscoplastic constitutive model was developed by improving the kinematic hardening rule with a cyclic softening parameter.
The tension-tension fatigue behavior of ductile adhesively-bonded double-lap FRP joints was experimentally investigated. An acrylic adhesive, which was in the rubbery state at ambient temperature, ...provided the joint ductility. The fatigue degradation of the joints was characterized by the cyclic energy dissipation, cyclic stiffness, cyclic creep displacement and self-generated temperature. The effects of elevated temperature on the joints’ static tensile and pure creep behaviors were also investigated. All joints failed in the adhesive layer at almost the same failure displacement, independent of the loading history (static, creep, fatigue, and temperature loading) due to the stretching of the adhesive’s molecular chains until the primary bonds failed. Fatigue failure was driven by cyclic creep; the cyclic creep displacements were accelerated mainly by the damage caused by fatigue at high load levels and by the damage caused by creep and self-generated temperature at low load levels.
Interaction of fatigue and creep behavior is the dominant failure mode for high cycle or very high cycle fatigue under high mean stress at high temperature. As for large rotating component working as ...high-speed at high temperature, such interaction drives the need to update strength assessment and design method. In this work, a constant life diagram incorporating dual-criterion for evaluating the interaction behavior between high cycle/very high cycle fatigue and creep at high temperature was proposed, and then verified using six types of materials data. This dual-criterion method was proved to be more precise for assessing the serious fatigue-creep interaction behavior in long-life regime, that was promising for design and safety assessment of high-temperature rotating components.
•Interaction of creep with high cycle or very high cycle fatigue is a dominant failure mode for high-temperature rotating components.•The dual-criterion assessment method considering HCF/VHCF and creep interaction was established and widely verified.•The dual-criterion assessment result is more precise with accuracy improvement for up to as high as 70 %.
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