| Fatigue failure is the main reason for bolt failure.In the service process,the fillet under the head is one of the main stress concentration parts of the bolt,and it is also the high incidence of fatigue fracture.TC4 bolts have the characteristics of high specific strength and yield ratio,excellent corrosion resistance and high temperature performance,and matching electrode potential and carbon fiber composite materials,and are widely used in the field of aerospace fasteners.Fillet rolling is the key process in the production of TC4 bolts,and the strengthening effect of its process parameters on the bolts is complex.In this paper,the rolling test and fatigue test of TC4 flat head and countersunk head bolts of M6 specification are carried out under different rolling pressures,and the corresponding fillet rolling and fatigue finite element models are established by ABAQUS combined with FE-safe,and the experimental verification is carried out.Based on this model,the influence of rolling process parameters on the residual stress distribution and fatigue life of bolts was studied,which provided a reference for the optimization of the bolt fillet rolling process.The main research contents and achievements of the paper are as follows:(1)The effect of rolling on the microstructure,morphology and fatigue life of the bolt fillet was studied.Rolling significantly reduces the roughness of the fillet,from about 1.6 μm to 0.8 μm,but it is not sensitive to the rolling depth;the surface of the fillet is rolled to form a work hardening layer,and the thickness of the deformed layer is about 2 μm.The inner β phase and secondary α are strongly deformed,and the grains are broken and elongated;the dimensional stability of the fillet of the flat head bolt after rolling is better,the fillet transition is still relatively smooth,and the deformation is not obvious,while the sinking after rolling.The dimensional stability of the fillet of the head bolt has not improved,and after 800 N rolling,the fillet will have obvious turning points,and the plastic deformation is more obvious;the countersunk head bolt is more likely to break at the head than the flat head bolt.The rolling process can significantly improve the fatigue life of the bolt.With the increase of the rolling force,the weak position of the fatigue fracture of the bolt is transferred from the fillet to the thread.The fatigue life of unrolled flat head bolts is about 30,000 times,and the fa tigue life of countersunk head bolts is about 9,000 times;after 200 N rolling,the fatigue life of countersunk head bolts is about 40,000 times,while the average life of flat head bolts is 200,000 times.Above;after 800 N rolling,the countersunk head and flat head bolts are basically broken at the thread,and the average life is more than 200,000 times.The fatigue fracture mechanism of bolts is fatigue ductile fracture,and the change of fracture position and the increase of fatigue life did not change the fracture mechanism.(2)The finite element model of the flat head bolt rolling and fatigue was established,and the effect of the rolling process(rolling depth,rolling angle and roller fillet size)on the residual stress distribution and fatigue lif e at the flat head bolt fillet was studied.When the rolling depth increases,the depth of the compressive stress layer and the maximum compressive stress of the bolt fillet increase rapidly and gradually tend to be stable.The maximum residual compressive stress does not exceed 1000 MPa,and the depth of the compressive stress layer does not exceed 0.1 mm.The fatigue life of the bolt increases first and then decreases with the increase of the rolling amount,and reaches the maximum value when the rolling d epth is about 0.02 mm.With the increase of the radius of the fillet of the roller,the area of compressive stress and the maximum compressive stress on the surface of the bolt after rolling also increase,and tend to be stable when the size of the roller is 90% of the size of the fillet.The size should be 90%-95% of the bolt fillet size.Too large or too small size will lead to a significant reduction in bolt fatigue life.The change of rolling angle within a small range has little effect on the residual stress and fatigue life of the bolt,and the rolling process can be selected in the direction of 45°.(3)The finite element model of the rolling and fatigue of the countersunk head bolt was established and verified by experiments,and the effect of the rolling process(rolling depth,rolling angle and roller fillet size)on the residual stress distribution and fatigue life at the countersunk head bolt fillet was studied.When the rolling depth increases,the depth of the compressive stress layer and the ma ximum compressive stress of the bolt fillet increase rapidly and tend to be stable,and the maximum residual compressive stress in the bolt does not exceed 800 MPa.The fatigue life of the bolt increases exponentially with the increase of the rolling amount and then tends to be stable.When the rolling depth is about 0.045 mm,the fatigue life reaches the maximum value.The change of the rolling angle makes the distribution of residual compressive stress correspondingly shifted.The fatigue life of the bolt first increases and then decreases with the increase of the rolling angle,and when the rolling angle is about25°,the fatigue life reaches the maximum value.With the increase of the fillet size of the roller,the maximum residual compressive stress decr eases and tends to be stable,but the formed compressive stress area is larger and more evenly distributed.In the range of 0.2 mm-0.45 mm,the fatigue life of the bolt increases with the roller The corner radius increases exponentially.(4)The effects of roughness,stress amplitude and residual stress at the fillet on the fatigue life of bolts are studied.The fatigue life of the bolt fillet is extremely sensitive to the surface roughness.As the roughness increases,the fatigue life of the bolt also drops sharply.The lower the roughness,the stronger the strengthening effect of residual stress;the effect of fatigue load on the fatigue life of the bolt fillet Large,the stress amplitude increases,and the fatigue life of the bolt decreases rapidly,but the residual stress influence factor increases instead. |
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