Search Results (4)

Aluminum alloys, serving as critical structural materials in the aviation and aerospace industry, frequently endure variable amplitude loading under complex service conditions. The resulting non-steady-state crack propagation behavior directly impacts structural safety. This study considers the engineering application requirements of the 7055-T7751 aluminum alloy and conducts fatigue crack growth experiments on compact tensile specimens subjected to constant amplitude loading and periodic variable amplitude overloading conditions. The findings indicate that the 7055 aluminum alloy exhibits an instantaneous acceleration period under tensile overload, which is important in the comprehensive analysis of crack growth life. The experimental findings show no significant correlation between post-overload minimum crack growth rate deviation and thickness or crack size at overload, where the values are 50.3% and 94.8% at 1.4 and 1.7 R_OL, respectively. An analytical model for the crack growth increment a_ii during this period was developed. Additionally, the delay distance influenced by overloads a_d and the number of delay cycles N_d are identified as effective parameters for evaluating the retardation effects induced by overloading. Our comparative analysis of crack growth experimental data under varying overload ratios R_OL and specimen thicknesses B revealed that existing plastic zone models inadequately assess a_d, prompting the establishment of a corresponding evaluation model. By incorporating the parameters a_ii and a_d into the Wheeler model, a method for calculating the delay cycles N_d was constructed, which effectively captured the variation trend. Finally, an analysis of fractography revealed numerous secondary cracks within the overload damage zone, and the ductile fracture characteristics in this region were significantly weaker compared to areas subjected to fatigue loading. Full article

This paper aims to investigate the response and fracture of EMT carbon steel round-hole tubes (EMT carbon steel RHTs) under cyclic bending loads. The study considers four different hole orientations (0°, 30°, 60°, and 90°) and five distinct hole diameters (2, 4, 6, 8, and 10 mm). The results reveal that hole orientation and diameter exert a minimal impact on the moment-curvature relationship, leading to the formation of stable loops. The ovalization-curvature graphs demonstrate a trend of asymmetry, serration, and growth with an increasing number of bending cycles. Additionally, larger hole orientations or smaller notch diameters result in reduced ovalization. Furthermore, the double logarithmic coordinates of the controlled curvature–number of cycles required to induce fracture reveal five parallel lines for different hole diameters when the hole orientation is fixed. Finally, in adopting the formulas for smooth tubes and for 6061-T6 aluminum alloy round-hole tubes (6061 aluminum alloy RHTs), this study adjusts the related material parameters. These modifications effectively describe the controlled curvature–number of cycles required to induce fracture for EMT carbon steel RHTs with different hole orientations and diameters under cyclic bending, demonstrating reasonable agreement with the experimental results. Full article

Unconventional oil and gas reservoirs, characterized by low porosity and permeability, often require multistage fracturing techniques for development. The high-pressure fracturing fluids with large volumes can easily cause alternating changes in both temperature and pressure within the casing. Using a theoretical model and field data from hydraulic fracturing operations, this paper calculated the alternating ranges of axial loads and temperatures in the reservoir section. Based on the calculation results, the temperature–load alternating coupling test of the P110 casing was carried out, and the tensile test was performed to analyze the yield strength variation law of the casing material. The results indicate that the yield strength, ultimate strength, and elastic modulus of casing materials are decreased under alternating thermal–mechanical coupling conditions. As the number of alternating cycles increases, there is an initial rapid decrease followed by a slower declining trend. Moreover, the tension–tension (T–T) cycles induce greater reductions in yield strength and ultimate strength than tension–compression (T–C) cycles. Meanwhile, under the same axial load condition, the higher the circulating temperature, the more significant the reduction in yield strength and ultimate strength. In essence, this is the result of the coupling effect of low-cycle fatigue and temperature aging. Finally, based on the experimental data, a yield strength prediction model of the P110 casing under the alternating thermal–mechanical coupling condition was established. The research results provide theoretical guidance for the safe design and material selection of a casing string under multistage volumetric fracturing conditions of shale gas exploration. Full article

The permeability of more than 70% of coal seams in China is less than 1 mD, creating difficulties in recovering underground coal methane. Therefore, a new technology of high-temperature nitrogen (HTN₂) injection into the coal seam was proposed to improve the coal permeability and gas extraction rate. In this paper, the effects of the N₂ temperature, injection pressure and cycle number on the permeability of naturally fractured coking coal has been investigated. When HTN₂ was injected into coal samples, the results indicated that the permeability decreased over time in the beginning, suddenly increased to a large value, and was subsequently maintained in a relatively stable range. The maximum permeability ratio increased with the rise of the N₂ temperature and injection pressure. An analysis indicated that the increase of coal permeability was the result of the increase of the global coal strain caused by thermal expansion and the adsorption-induced expansion. The maximum permeability ratios in various cycles of multicycle N₂ injection into the coal sample were all greater than 1.0 while progressively declining. Obviously, the alternating stress was conducive to the further expansion of the coal fractures to increase the coal permeability. However, on the basis of the first period of expansion, re-expansion was difficult and required more energy. The effects of multicycle N₂ injection on coal permeability have been considerably improved when compared with N₂ injection with only one cycle. The research results are helpful for rapidly extracting methane and guaranteeing mine safety. Full article