In-situ SEM (Scanning Electron Microscope) observation of fatigue crack propagation in aluminium alloys reveals that crack growth occurs in a continuous way over the time period during the load cycle. Based on this ob...In-situ SEM (Scanning Electron Microscope) observation of fatigue crack propagation in aluminium alloys reveals that crack growth occurs in a continuous way over the time period during the load cycle. Based on this observation, a new parameter da/dS is introduced to describe the fatigue crack propagation rate, which defines the fatigue crack propagation rate with the change of the applied stress at any moment of a stress cycle. The relationship is given between this new parameter and the conventional used parameter da/dN which describes the fatigue crack propagation rate per stress cycle. Using this new parameter, an analysis has been performed and a model has been set up to consider the effect of the applied stress ratio on the fatigue crack propagation rate. The obtained results have been used to correlate the published test data and a good correlation has been achieved. This method is very easy to use and no fatigue crack closure measurement is needed, therefore this model is significant in engineering application.展开更多
By comparison of the characteristics of existing models for long fatigue crack propagation rates, a new model, called the generalized passivation-lancet model for long fatigue crack propagation rates （GPLFCPR）, and ...By comparison of the characteristics of existing models for long fatigue crack propagation rates, a new model, called the generalized passivation-lancet model for long fatigue crack propagation rates （GPLFCPR）, and a general formula for characterizing the process of crack growth rates are proposed based on the passivation-lancet theory. The GPLFCPR model overcomes disadvantages of the existing models and can describe the rules of the entire fatigue crack growth process from the cracking threshold to the critical fracturing point effectively with explicit physical meaning. It also reflects the influence of material characteristics, such as strength parameters, fracture parameters and heat treatment. Experimental results obtained by testing LZ50 steel, AlZnMgCu0.5, 0.5Cr0.5Mo0.25V steel, etc., show good consistency with the new model. The GPLFCPR model is valuable in theoretical research and practical applications.展开更多
Hydraulic fracturing is a mainstream technology for unconventional oil and gas reservoirs development all over the world.How to use this technology to achieve high-level oil and gas resource extraction and how to form...Hydraulic fracturing is a mainstream technology for unconventional oil and gas reservoirs development all over the world.How to use this technology to achieve high-level oil and gas resource extraction and how to form complex fracture networks as hydrocarbon transportation channels in tight reservoirs,which depends to a large extent on the interaction between hydraulic and pre-existing cracks.For hydraulic fracturing of fractured reservoirs,the impact of natural fractures,perforation direction,stress disturbances,faults and other influencing factors will produce a mixed I&II mode hydraulic fracture.To forecast whether hydraulic fractures cross pre-existing fractures,according to elastic mechanics and fracture mechanics,a stress state of cracks under the combination of tensile(I)and shear(II)is presented.A simple mixed-mode I&II hydraulic fracture's crossing judgment criterion is established,and the propagation of hydraulic fractures after encountering natural fractures is analyzed.The results show that for a given approaching angle there exists a certain range of stress ratio when crossing occurs.Under high approaching angle and large stress ratio,it is likely that hydraulic cracks will go directly through pre-existing cracks.The reinitiated angle is always controlled within the range of approximately 30among the main direction of penetration.展开更多
基金the National Natural Science Foundation of China (Grant No. 10772063, 10572068, and 10772064)
文摘In-situ SEM (Scanning Electron Microscope) observation of fatigue crack propagation in aluminium alloys reveals that crack growth occurs in a continuous way over the time period during the load cycle. Based on this observation, a new parameter da/dS is introduced to describe the fatigue crack propagation rate, which defines the fatigue crack propagation rate with the change of the applied stress at any moment of a stress cycle. The relationship is given between this new parameter and the conventional used parameter da/dN which describes the fatigue crack propagation rate per stress cycle. Using this new parameter, an analysis has been performed and a model has been set up to consider the effect of the applied stress ratio on the fatigue crack propagation rate. The obtained results have been used to correlate the published test data and a good correlation has been achieved. This method is very easy to use and no fatigue crack closure measurement is needed, therefore this model is significant in engineering application.
基金supported by the Military Pre-study Project of General Armament Department of China(No.YG060101C)
文摘By comparison of the characteristics of existing models for long fatigue crack propagation rates, a new model, called the generalized passivation-lancet model for long fatigue crack propagation rates （GPLFCPR）, and a general formula for characterizing the process of crack growth rates are proposed based on the passivation-lancet theory. The GPLFCPR model overcomes disadvantages of the existing models and can describe the rules of the entire fatigue crack growth process from the cracking threshold to the critical fracturing point effectively with explicit physical meaning. It also reflects the influence of material characteristics, such as strength parameters, fracture parameters and heat treatment. Experimental results obtained by testing LZ50 steel, AlZnMgCu0.5, 0.5Cr0.5Mo0.25V steel, etc., show good consistency with the new model. The GPLFCPR model is valuable in theoretical research and practical applications.
基金supported by National Natural Science Foundation of China (52074248)Fundamental Research Funds for the Central Universities (2652019105,2652022207).
文摘Hydraulic fracturing is a mainstream technology for unconventional oil and gas reservoirs development all over the world.How to use this technology to achieve high-level oil and gas resource extraction and how to form complex fracture networks as hydrocarbon transportation channels in tight reservoirs,which depends to a large extent on the interaction between hydraulic and pre-existing cracks.For hydraulic fracturing of fractured reservoirs,the impact of natural fractures,perforation direction,stress disturbances,faults and other influencing factors will produce a mixed I&II mode hydraulic fracture.To forecast whether hydraulic fractures cross pre-existing fractures,according to elastic mechanics and fracture mechanics,a stress state of cracks under the combination of tensile(I)and shear(II)is presented.A simple mixed-mode I&II hydraulic fracture's crossing judgment criterion is established,and the propagation of hydraulic fractures after encountering natural fractures is analyzed.The results show that for a given approaching angle there exists a certain range of stress ratio when crossing occurs.Under high approaching angle and large stress ratio,it is likely that hydraulic cracks will go directly through pre-existing cracks.The reinitiated angle is always controlled within the range of approximately 30among the main direction of penetration.
