| A lot of engineering components and pressure vessels utilized in the machinery, petroleum, chemical and civil engineering fields are often subjected to cyclic loading, resulting in fatigue crack initiation and propagation, which will eventually bring about disastrous consequences. Therefore, the study of the fatigue crack growth behavior is very important practical significance. It is found that the changes of the crack profiles will directly have an impact on the fatigue crack growth rate, and then will affect the accurate evaluation for the residual life of pressure vessels and engineering components. At the same time, the stress intensity factor is an important fracture mechanics parameter. Therefore, in this paper, the compact tensile specimen(CT Specimen)of the Q345R steel is adopted to study the fatigue crack profiles and the stress intensity factor of the crack front. It mainly concludes these contents as follows:The crack profiles under mode I constant amplitude loading are fitted according to the ellipse equation by data processing software, and then the tunneling of the crack profile, which contains tunneling depth and surface angle, are analyzed. The results show that in the initial stage of crack propagation, the crack profile could be described by elliptic equations very well, the tunneling depth increases gradually and surface angle is falling sharply; In the stable crack propagation stage, when cracks propagate.the cracks keep similar crack profiles which are semi-elliptical; however, both of the long axis and the short axis of semi-elliptic equations vary continually while tunnel depth and surface angle gradually become stable. In addition, crack profiles under a single tensile overload are fitted by gauss equation and finally get a better result, it shows that crack profiles are very similar to the "bow" used in ancient Chinese archery.Under different crack lengths and different tensile load conditions, the stress intensity factor of the two dimensions CT specimen are analyzed by large finite element analysis software. The results show that when the crack length remains constant while the tensile load changes, it preserves a linear relationship between the stress intensity factor and the tensile load. Meanwhile, the stress intensity factor will increase faster and faster with the increasing of the crack length while the tensile load remains constant.Under three dimensional condition and plane strain condition, the stress intensity factors of the straight crack profile are calculated respectively. The results show that in the middle plane, the stress intensity factors in3D condition are greater than that in plane strain condition. Then finite element calculation is conducted on the stress intensity factor of both actual curving crack profile and average straight crack profile. The results show that the stress intensity factor range of the mid-plane is smaller than that of the surface, and the distributions of the intensity factor range along the thickness of the specimen are opposite under actual curving crack profile and average straight crack profile. The law of the crack initiation under the straight crack profile at the notch root is analyzed. The results show that the crack initiation of the middle plane is prior to that of the surface along the thickness of CT specimen. |
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