Crack initiation and propagation induced by inclusions in a nickel-base P/M superalloy under fatigue load

In situ fatigue tests in special designed SEM were conducted to trace the whole process of crack initiation and propagation fill to fracture in mckel-base P/M superalloy seeded inclusions. The experimental results show that non-metallic inclusions can induce crack initiation. When the inclusion size is larger than the critical one, the crack can propagate as the main crack that induces the specimen to fracture. As a result, the LCF life of the specimen decreases.

Near-wellbore fracture initiation and propagation induced by drilling fluid invasion during solid fluidization mining of submarine nature gas hydrate sediments

Drilling in a natural gas hydrate formation is challenging due to the poor consolidation of the formation and the potential evaporation of the hydrate.The unreasonable down-hole pressure of the drilling fluid can not only lead to the wellbore instability,but also change the predrilling condition of the natural gas hydrate formation,thus leading to an instable wellbore.In this paper,the integrated discrete element method(DEM)-computational fluid dynamics(CFD)work flow is developed to study the wellbore instability due to the penetration of the drilling fluid into the hydrate formation during crack propagations.The results show that the difference between in-situ stresses and overpressure directly affect the drilling fluid invasion behavior.The lower hydrate saturation leads to an easier generation of drilling fluid flow channels and the lower formation breakdown pressure.The breakdown pressure increases with the increase of hydrate saturation,this also indicates that hydrates can enhance the mechanical properties of the formation.The induced cracks are initially accompanied with higher pressure of the drilling fluid.According to the rose diagram of the fracture orientation,a wider orientation of the fracture distribution is observed at higher pressure of the invasion fluid.

Microstructure evolution during heat treatment of Mg-Gd-Y-Zn-Zr alloy and its low-cycle fatigue behavior at 573K

In as-cast Mg?2.1Gd?1.1Y?0.82Zn?0.11Zr(mole fraction,%)alloy,lamellar microstructures that extend from grain boundaries to the interior ofα-Mg grains are identified as clusters ofγ′using a scanning transmission electron microscope equipped with a high-angle annular dark-field detector.Under a total strain-controlled low-cyclic loading at573K,the mechanical response and failure mechanism of Mg?2.1Gd?1.1Y?0.82Zn?0.11Zr alloy(T6peak-aging heat treatment)were investigated.Results show that the alloy exhibits cyclic softening response at diverse total strain amplitudes and573K.The experimental observations using scanning electron microscopy show that the micro-cracks initiate preferentially at the interface between long-period stacking order structures andα-Mg matrix and extend along the basal plane ofα-Mg.The massive long-period stacking order structures distributed at grain boundaries impede the transgranular propagation of cracks.

Effects of laser shock processing,solid solution and aging,and cryogenic treatments on microstructure and thermal fatigue performance of ZCuAl_(10)Fe_(3)Mn_(2)alloy

The materials used in variable temperature conditions are required to have excellent thermal fatigue performance.The effects of laser shock processing(LSP),solid solution and aging treatment(T6),and cryogenic treatment(CT)on both microstructure and thermal fatigue performance of ZCuAl_(10)Fe_(3)Mn_(2) alloys were studied.Microstructure and crack morphology were then examined by scanning electron microscopy(SEM)and energy-dispersive X-ray spectroscopy(EDS).The result showed that,after being subjected to the combination treatment of T6+CT+LSP,the optimal mechanical properties and thermal fatigue performance were obtained for the ZCuAl_(10)Fe_(3)Mn_(2) alloy with the tensile strength,hardness,and elongation of 720 MPa,300.16 HB,and 16%,respectively,and the thermal fatigue life could reach 7,100 cycles when the crack length was 0.1 mm.Moreover,the ZCuAl_(10)Fe_(3)Mn_(2) after combination treatment shows high resistance to oxidation,good adhesion between the matrix and grain boundaries,and dramatically reduced growth rate of crack.During thermal fatigue testing,under the combined action of thermal and alternating stresses,the microstructure around the sample notch oxidized and became loose and porous,which then converted to micro-cracks.Fatigue crack expanded along the grain boundary in the early stage.In the later stage,under the cyclic stress accumulation,the oxidized microstructure separated from the matrix,and the fatigue crack expanded in both intergranular and transgranular ways.The main crack was thick,and the path was meandering.

Cutting Behavior of Cortical Bone in Different Bone Osteon Cutting Angles and Depths of Cut

Cortical bone is semi-brittle and anisotropic,that brings a challenge to suppress vibration and avoid undesired fracture in precise cutting process in surgeries.In this paper,a novel analytical model is proposed to represent cortical bone cutting processes.The model is utilized to predict the chip formations,material removal behavior and cracks propagation under varying bone osteon cutting angles and depths.Series of orthogonal cutting experiments were conducted on cortical bone to investigate the impact of bone osteon cutting angle and depth of cut on cutting force,crack initialization and propagation.The observed chip morphology highly agreed with the prediction of chip formation based on the analytical model.The curly,serrated,grainy and powdery chips formed when the cutting angle was set as 0°,60°,90°,and 120°,respectively.Cortical bone were removed dominantly by shearing at a small depth of cut from 10 to 50μm,and by a mixture of pealing,shearing,fracture and crushing at a large depth of cut over 100μm at different bone osteon angles.Moreover,its fracture toughness was calculated based on measured cutting force.It is found that the fluctuation of cutting force is suppressed and the bone material becomes easy to remove,which attributes to lower fracture toughness at bone osteon cutting angle 0°.When the cutting direction develops a certain angle to bone osteon,the fracture toughness increases then the crack propagation is inhibited to some extent and the fluctuation of cutting force comparatively decreases.There is a theoretical and practical significance for tools design and operational parameters choice in surgeries.

Effects of loading waveforms on rock damage using particle simulation method

The particle simulation method is used to study the effects of loading waveforms （i.e. square, sinusoidal and triangle waveforms） on rock damage at mesoscopic scale. Then some influencing factors on rock damage at the mesoscopic scale, such as loading frequency, stress amplitude, mean stress, confining pressure and loading sequence, are also investigated with sinusoidal waveform in detail. The related numerical results have demonstrated that： 1） the loading waveform has a certain effect on rock failure processes. The square waveform has the most damage within these waveforms, while the triangle waveform has less damage than sinusoidal waveform. In each cycle, the number of microscopic cracks increases in the loading stage, while it keeps nearly constant in the unloading stage. 2） The loading frequency, stress amplitude, mean stress, confining pressure and loading sequence have considerable effects on rock damage subjected to cyclic loading. The higher the loading frequency, stress amplitude and mean stress, the greater the damage the rock accumulated; in contrast, the lower the confining pressure, the greater the damage the rock has accumulated. 3） There is a threshold value of mean stress and stress amplitude, below which no further damage accumulated after the first few cycle loadings. 4） The high-to-low loading sequence has more damage than the low-to-high loading sequence, suggesting that the rock damage is loading-path dependent.

The Relationship Between Oxidation and Thermal Fatigue of Martensitic Hot-Work Die Steels

Thermal fatigue behaviors of two forged hot-work die steels subjected to cyclic heating （650 ℃）-water quenching were investigated. A martensitic hot-work die steel containing 10% Cr （HHD）, showing superior oxidation resistance and thermal fatigue resistance to the commercial martensitic hot-work die steel （Uddeholm DIEVAR ）, was developed. The maximal crack length in HHD was 35% shorter than that in DIEVAR after 2000 thermal cycles, and the hot yield strength at 650℃ of HHD was 14% lower than that of DIEVAR prior to thermal fatigue testing, which is 30% higher after 1500 cycles. It is found that cracks initiated and propagated along the oxide layers in the grain boundaries, suggesting that the oxidation-induced thermal fatigue cracks can significantly reduce the mechanical performance and service life for the hot- work die steel. High-temperature oxidation behavior is crucial for thermal fatigue crack formation, while high-temperature yield strength and ductility play a less important role.