往复荷载下钢管与螺栓球组配试件超低周疲劳断裂试验研究

为探索螺栓球网格结构强震下的破坏特征,对五个螺栓球节点管球组配试件进行了轴向往复荷载作用下的超低周疲劳断裂试验研究。通过电镜扫描观察试件超低周疲劳破坏的裂纹萌生、扩展、断裂、断口形态,并分析了裂纹萌生原因、断裂机理。结果表明:当承受拉-压循环荷载时,试件均经历失稳弯曲、在杆件中点附近局部凹陷、在凹陷处表面萌生裂纹、进一步开裂、最后发生低周疲劳断裂的过程;其断口形状为椭圆,有明显塑性变形,为韧性断裂,疲劳寿命均很短。当试件承受较大压幅值的循环荷载时,试件首先失稳弯曲,然后在杆件中点附近局部凹陷,继而在凹陷处表面萌生裂纹,但同时因节点中高强螺栓承受弯剪作用,在螺栓牙底应力集中位置,也多处同时萌生裂纹并迅速发展,最后导致高强螺栓先于杆件局部凹陷处发生疲劳断裂;其断口表面光滑平整,没有明显塑性变形,疲劳寿命只有18次。

Experimental Study on Ultra-low Cycle Fatigue Fracture of Steel Tubes and Bolt Balls under Cyclic Loads

In order to explore the damage characteristics of the bolt-ball grid structure under strong earthquake, this paper studied the ultra-low-cycle fatigue fracture test of the five-ball-ball nodal assembly specimen under axial reciprocating load. Scanning electron microscopy was used to observe crack initiation, propagation, fracture and fracture morphology in ultra-low-cycle fatigue failure of the specimen, and the cause of crack initiation and fracture mechanism were analyzed. The results show that when subjected to tensile-compressive cyclic loading, the specimens first destabilize and bend, then partially dent in the vicinity of the midpoint of the rod, and then initiate cracks on the surface of the depression, further cracking, and finally low-cycle fatigue fracture. Its fracture shape is ellipse, there is obvious plastic deformation, it is ductile fracture, its fatigue life is very short; When the test piece is subjected to the pressure-pressure cyclic load that changes more greatly, the test piece is also first bent and then bent The local depression near the midpoint and then the surface initiates cracks at the depressions, but at the same time the high-strength bolts in the nodes are subjected to bending and shearing. At the location of stress concentration in the bottom of the bolts, multiple cracks develop at the same time and develop rapidly. Finally, the high-strength bolts lead to Fatigue fracture occurs in the local depression of the rod. Its fracture surface is smooth and smooth, without obvious plastic deformation, and its fatigue life is only 18 times.