Fatigue condition of titanium alloy bolts and steel plates
Fatigue behavior of 30CrMnSiA steel plate The conventional fatigue SN curve of the perforated plate member shows the fatigue cycle maximum stress life curve of the 30CrMnSiA steel plate with ù8.1mm free center hole (no bolt connection in the hole). All the sample fracture occurs in the plate hole. The maximum stress concentration of the fatigued titanium alloy bolted steel plate specimen fracture morphology limit is about 200MPa. According to the SN curve shown, the fatigue behavior of 30CrMnSiA steel plate is compared under the maximum cyclic load of 300MPa. influences. The results show that the fatigue failure of the TC16 titanium alloy bolted high-strength steel plate structure with diameter ù8mm is the fracture of the contact hole between the 30CrMnSiA steel plate and the bolt contact. The macroscopic morphology of the sample fracture is shown. The crack initiation occurs in the direction of about 70b with the applied fatigue load, rather than the 90b position of the free orifice plate, the fatigue behavior of the TC16 titanium alloy bolt and its connection to the 30CrMnSiA steel plate hole (b).
The fracture morphology indicates that the fracture of the titanium alloy bolted 30CrMnSiA steel plate is caused by the fretting wear between the bolt surface and the inner wall of the steel plate hole and the combined cyclic fatigue load, that is, the fretting fatigue damage occurs. Comparing the fretting fatigue life of the 30CrMnSiA steel plate and the conventional fatigue life of the 30CrMnSiA steel plate with the maximum cyclic load, the former is 25,924 times and the latter is 65,413 times, that is, the fretting fatigue is lower than the conventional fatigue life. About 60%, that is, the micro-action on the contact surface significantly promotes the initiation and early expansion of fatigue cracks on the inner surface of the steel plate hole.
The damage characteristics of the fretting wear zone of the fretting fatigue plate indicate that the wear is characterized by rolling, abrasive wear and fatigue delamination. The surface of the micro-motion contact zone of the high-strength steel plate has high strain, large deformation and partial notch effect due to wear, which promotes the initiation of fatigue crack, and the external fatigue load promotes the expansion of fatigue crack. During the experiment, a large amount of black oxidized abrasive chips were observed due to the fretting wear of the titanium alloy bolts and the inner wall of the steel plate.
Fatigue behavior of TC16 titanium alloy bolts In order to study the fatigue fracture failure behavior of titanium alloy bolts in the joint structure, the hole margin of 30CrMnSiA steel plate is increased by appropriately reducing the fastening hole diameter to ensure the fracture of TC16 titanium alloy bolts without The 30CrMnSiA steel sheet was broken. The test conditions determined in this paper are as follows: the dimensions of the 30CrMnSiA steel plate are as shown, the bolt connection hole is ù6.1mm hole; the bolt diameter is ù6mm, and the fastening hole is clearance fit; the maximum cyclic load is 170MPa. The test results show that The bolts are all broken at the indicated a, ie the position of the maximum bending stress at the joint with the 30CrMnSiA steel plate. According to the given force diagram of the titanium alloy bolt in the joint structure, it can be seen that the bolt force can be simplified to the 3-point bending loading mode without considering the bolt head and the nut being restrained by the clamp, due to the TC16 titanium The alloy bolts are subjected to alternating one-way bending fatigue loads, and thus bending fatigue damage occurs. Although there is fretting wear on the contact surface between the bolt and the 30CrMnSiA steel plate hole or the jig, the maximum tensile stress position due to the fatigue load is on the back side of the fretting contact area (as in point a), or at the micro-motion area In the state of compressive stress, therefore, the fracture of the bolt is not a fretting fatigue fracture. This is because the bolt head and the nut are constrained by the side of the clamp, and the back side of the applied point is also subjected to bending stress, that is, it is also subjected to bending fatigue loading. In addition, if the test conditions in this paper are changed to tension and compression fatigue, due to the existence of alternating bending and tensile stress at the micro-motion zone, the micro-action will promote the initiation of fatigue cracks and synergistically accelerate the fatigue fracture of the bolt.
Analysis of stress distribution around the steel plate fastening holes For the 30CrMnSiA steel plate with free fastening holes, the stress distribution around the fastening holes is not uniform, as shown in (a). The stress at the edge of the hole perpendicular to the applied tensile load is the largest (indicated by Rmax), and the stress decreases sharply as the distance from the edge of the hole increases. For the 30CrMnSiA steel plate fastening hole bolted with titanium alloy, the stress distribution around the hole is analyzed by ANSYS finite element analysis software. The result is as shown in (b), that is, the stress field around the plate hole changes significantly. The stress concentration of the hole is more obvious. The stress concentration zone moves toward the direction in which the plate hole is pressed. At the same time, due to the most significant fretting wear between the bolt and the plate hole, the wear promotes the fatigue crack initiation, and the friction force and The additional stress superposition promotes crack propagation, so it becomes a crack source for fretting fatigue, which is in complete agreement with the experimental results.
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