Notch effect analysis The axial stress on the cross section of the bolt rod is equal, no stress gradient, and there is a probability of crack initiation on the entire cross section. However, the grains on the free surface are less constrained by the adjacent grains than the inner grains, so the grains on the free surface are more susceptible to plastic deformation. According to modern fatigue theory, the initiation and propagation of microcracks are caused by cyclic plastic strain, and the magnitude of plastic deformation depends on the maximum shear stress component, so fatigue cracks always occur on the maximum shear stress plane of the free surface. When the applied stress is slightly higher than the fatigue limit, as long as the point on the free surface of the screw is cracked, the true stress here is greater than the other surface, the crack will continue to expand, and the other free surface with low stress will no longer crack, usually only one Crack source. Unlike a smooth screw, at the notch of the thread, the root of the notch can be a source of fatigue cracks. Because the applied axial (such as x direction) stress is R, due to the stress concentration effect of the thread notch, the maximum tensile stress in the axial direction is Rx>R, the Ry in front of the notch is the smallest, and the Rz is in the middle (plane strain condition), according to the slip. The line field theory derived Ry = 0 on the free surface, so the maximum shear stress here is Smax = 12 (Rx - Ry) = Rx2 (1) and the plastic deformation is limited to the xy plane. After leaving the root of the notch, Rx drops rapidly, and Ry rises. Therefore, after leaving the root of the notch, Smax drops rapidly, causing plastic deformation to occur in a small area near the root of the notch. Under the repeated action of fatigue stress, when the cyclic plastic deformation reaches a critical value. , the birth of fatigue cracks. Since the depth and radius of curvature of the thread notch are uniform, the stress concentration at each point is the same, and the probability of cracking is approximately equal. Therefore, multiple fatigue sources are formed along the root of the thread notch. The greater the stress concentration or the higher the applied stress, the number of fatigue sources. The more. Therefore, once the surface of the threading machine is improperly treated, defects or microcracks appear on the surface of the thread root, which will become the starting point of the fatigue crack and gradually expand with the action of the alternating load, eventually leading to fatigue fracture. Finite Element Analysis According to the literature <9>, the high-strength bolts used in the bolt-and-ball joint grid are mainly in the form of ordinary threads. The specification is M12M64. This paper mainly selects eight typical bolt specifications: M14, M20, M24. M30, M33, M52 and M60. The 40Cr high-strength bolts of these 8 specifications were studied and analyzed by finite element software. Computational model 1) Geometric model According to the research of thread by many scholars at home and abroad, the influence of the angle of the thread is ignored, and some scholars have pointed out that when the angle of the thread is less than 4b, the distribution of the load along the thread is almost unaffected by the angle of the helix. Under the axial load, the bolt can be simplified to the axisymmetric problem. The bolt angles of the bolt specifications studied in this paper are all less than 3b, which satisfies the axisymmetric requirements. Unit Type and Meshing Because the calculation model is a 3D solid, the unit type takes a 20-node SOLID95 tetrahedral unit. Meshing directly affects the accuracy of the calculation results. If surface element division is used, the unit is refined at the notch, and then the surface unit is rotated to form a body unit. Although the unit division is more regular, the vicinity of the gap is an abnormal unit, and the calculation result is not accurate enough. . Therefore, the body unit is directly divided into the three-dimensional solid model of the bolt, and the automatic unit is refined at the notch. The stress concentration caused by the gap effect is one of the main factors leading to the fatigue fracture of high-strength bolts. On how to improve the screw notch sensitivity and improve the fatigue performance, the following conclusions can be drawn: 1) From the analysis of the thread notch effect, the stress concentration on the root surface of the thread is the most serious, which is the source of fatigue crack. Therefore, the surface processing quality has a great influence on the fatigue strength. Under the conditions of the process permit, the surface quality of cold working should be improved as much as possible to reduce the surface roughness. 2) The use of a higher strength material for the purpose of improving the bolt strength does not improve the notch sensitivity of the bolt and improves the fatigue strength. It is recommended to properly reduce the grade of high-strength bolts under the condition of constant safety factor, so that the strength, plasticity and toughness have a good fit, and at the same time, the fatigue performance of the bolt can be improved. Pet Bottle,Honey Bottle,Pet Honey Bottle Feiyi Cosmetic Trading Co., Ltd. , http://www.stfycosmetic.com
Analysis of the post-loss effect of steel mesh bolt body frame type large hardness bolt
Notch effect analysis The axial stress on the cross section of the bolt rod is equal, no stress gradient, and there is a probability of crack initiation on the entire cross section. However, the grains on the free surface are less constrained by the adjacent grains than the inner grains, so the grains on the free surface are more susceptible to plastic deformation. Modern fatigue theory believes that microcracking