First of all, it is necessary to find out the degree of loosening and what kind of load the bolt group bears, so as to judge whether the loosened bolt will break first. The discussion is divided into three situations.

    1. If the bolt is only subjected to static load, the bolt that is loosened first will not be broken first. If the bolt is loose, it means that the pre tightening force is smaller or the pre tightening force is zero. The smaller the pre tightening force is, the smaller the comprehensive working stress of the bolt under the external load. The bolt with the minimum working stress will not break first under the static load condition. Now take the bolt connection in the flange connection as an example to see how the stress condition of the bolt will be if one bolt is loose. The flange is connected by eight M16 grade 10.9 bolts, and the bolt number is shown in the following figure:

    Under normal conditions, the same pre tightening force is applied to the bolt. If one bolt is loose, it means that the pre tightening force is reduced or there is no pre tightening force. Here, the pre tightening force of bolt 1 is set to 30KN, which is 68kN less than that of other bolts. When the flange is subject to 600kN tension, see the difference of bolt stress;

    The stress of bolts is shown in the following table:

    According to the analysis results, due to the relaxation of bolt 1, the stress distribution of all bolts is changed. The tensile force of the relaxed bolt is smaller, while the tension of the two bolts (No. 2 and No. 8 bolts) adjacent to the relaxed bolt becomes larger, which means that the internal stress is relatively large. If the load continues to increase, the two bolts will break first.

    2. If the bolt group bears the fatigue load, the bolt is completely loose (i.e. there is no preload), even under the external load, the bolt will not be stressed, and the bolt next to it will bear the force it should bear. Therefore, the bolt loosened first will not break first, and its adjacent bolt may break first.

    Taking flange connection as an example, one bolt is completely relaxed, that is, the pre tightening force is zero, and the pre tightening force of other bolts is 98kn.

    It can be seen from the above results that when the flange connection is under external load, the fully relaxed bolt will not bear the tension, that is, the tension is zero.

     When the flange is subjected to alternating load from 0 to 600 kn, the tension range of each bolt is as follows:

    According to the fatigue characteristics of the bolt, the larger the stress range of the bolt, the earlier the fatigue fracture occurs. And the shorter the fatigue life. Therefore, the two adjacent bolts on both sides of the fully relaxed bolt will first fracture, while the fully relaxed bolt will not fracture.

    3. If only the pre tightening force of bolt is reduced, then under the action of fatigue load, its comprehensive stress range will be larger, and the fatigue life will be lower than the design life, and fatigue fracture may occur first.

    Assuming that the pre tightening force of bolt is 98kn, if one bolt is loose, it means that the preload is reduced or there is no preload. Here, the preload of bolt 1 is set as 30KN, which is 68kN less than that of other bolts. When the flange is subjected to alternating tension of 0 to 600kN, the stress and tension variation range of bolt are calculated as follows:

    From the calculation results in the table above, it can be seen that the stress variation range of incomplete relaxation bolt and its two adjacent bolts is relatively large, so the bolt with incomplete relaxation and the two bolts adjacent to both sides will first undergo fatigue fracture.

    From the analysis of the above three points, we can know why the bolt loosened first is more difficult to break. Because the bearing force of the loose bolt will be greatly reduced, so that the stress of the bolt next to it will be increased, so that other bolts will be fatigue fracture due to large stress, but the bolt loosened first will break later. The above is from the Internet.