Fatigue of titanium alloy bolts and steel plates during CNC machining
Fatigue behavior of 30CrMnSiA steel plate The conventional fatigue SN curve of a plate with holes shows the fatigue cycle maximum stress life curve of a 30CrMnSiA steel plate with a free center hole of 8.1 mm (no bolt connection in the hole). All sample fractures occur in the plate holes According to the SN curve shown, the fatigue stress behavior of the fatigue pole titanium alloy bolted steel plate sample is about 200MPa. According to the SN curve shown, the fatigue behavior of the TC16 titanium alloy bolted connection to 30CrMnSiA steel plate is compared under the maximum cyclic load of 300MPa. influences. The results show that the fatigue failure of the high-strength steel plate structure with TC16 titanium alloy bolts with a diameter of 8 mm is all shown as the fracture of the contact wear zone of the 30CrMnSiA steel plate connection hole, and the macroscopic morphology of the sample fracture is shown. Crack initiation occurs in the direction of about 70b with the applied fatigue load, rather than the 90b position of the free hole plate, the fatigue behavior of the TC16 titanium alloy bolt and its connection 30CrMnSiA steel plate hole (the position of the white dotted line marked in (b)).
Fracture patterns show that the fracture of the 30CrMnSiA steel plate with titanium alloy bolts is caused by the fretting wear effect and the combined action of cyclic fatigue load between the bolt surface and the inner wall of the steel plate hole, that is, fretting fatigue damage. Comparing the fretting fatigue life of the 30CrMnSiA steel-titanium alloy bolted connector with the maximum cyclic load of 300MPa and the conventional fatigue life of the free hole part shows that the former is 25724 times and the latter is 65413 times, that is, the fretting fatigue is reduced compared with the conventional fatigue life About 60%, that is, the micro-action on the contact surface obviously promotes the initiation and early propagation of fatigue cracks on the inner surface of the steel plate hole.
Fretting fatigue plate hole fretting wear zone damage characteristics show that the wear shows rolling, abrasive wear and fatigue delamination characteristics. The high strain, large deformation and local notch effect caused by the wear effect on the surface of the fretting contact area of ​​the hole wall of high-strength steel parts promote the initiation of fatigue cracks, and the application of fatigue loads promotes the growth of fatigue cracks. During the experiment, it was observed that a large amount of black oxidized abrasive debris was caused by fretting wear at the contact part of the titanium alloy bolt and the inner wall of the steel plate hole.
Fatigue behavior of TC16 titanium alloy bolts In order to study the fatigue fracture failure behavior of titanium alloy bolts in the connection structure, the method of increasing the hole margin of the 30CrMnSiA steel plate by appropriately reducing the tightening hole diameter to ensure that the TC16 titanium alloy bolts break without The 30CrMnSiA steel sheet is broken. The test conditions determined in this paper are as follows: the sample size of the 30CrMnSiA steel plate is as shown, the bolt connection hole is 6.1 mm hole; the bolt diameter is 6 mm, and the fastening hole is a gap fit; and the maximum cyclic load is 170 MPa. The bolts are broken at the position a, that is, the position of the maximum bending stress at the connection with the hole of the 30CrMnSiA steel plate. According to the given schematic diagram of the stress of the titanium alloy bolt in the case of the connection structure, it can be seen that without considering the bolt head and nut constrained by the clamp, the stress of the bolt can be simplified to the 3-point bending loading mode, because TC16 titanium Alloy bolts are subjected to alternating unidirectional bending fatigue loads, and thus bending fatigue failure occurs. Although there is fretting wear on the contact surface between the bolt and the hole or fixture of the 30CrMnSiA steel plate, the location of the maximum tensile stress due to fatigue load is on the back of the fretting contact area (such as point a), or the fretting area In the state of compressive stress, therefore, the fracture of the bolt is not fretting fatigue fracture. This is because the bolt head and nut are constrained by the side of the fixture, and the back side of the force point is also subjected to bending stress, that is, here is also bending fatigue loading. In addition, if the test conditions in this paper are changed to tensile and compressive fatigue, the micro-action will promote the initiation of fatigue cracks and coordinately accelerate the fatigue fracture of bolts due to the existence of alternating bending and tensile stresses in the fretting zone.
Analysis of stress distribution around steel plate fastening holes For 30CrMnSiA steel plate parts 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 (represented by Rmax). As the distance from the edge of the hole increases, the stress decreases sharply. For the fastening holes of 30CrMnSiA steel plate bolted with titanium alloy, the stress distribution around the hole is analyzed with the help of ANSYS finite element analysis software. The results are shown in (b), that is, the stress field around the plate hole has changed significantly. The stress concentration of the hole is more obvious. The stress concentration area moves in the direction of the plate hole being squeezed. At the same time, due to the most significant fretting wear between the bolt and the plate hole, wear promotes the initiation of fatigue cracks, and the friction force The superposition of applied stress promotes crack propagation. Therefore, this place becomes a source of fretting fatigue cracks, which is completely consistent with the test results.