The use of high-strength bolts is very widespread, such as in aerospace, petroleum machinery, large cars/trucks, etc. During the use of high-strength bolts, the common failure mode is fracture. The fracture situation of bolts varies depending on the usage. Some high-strength bolts are fatigue fracture, some are brittle fracture, and some are defect fracture. Based on our understanding of the use of fasteners and bolts, Zhonghua Standard Parts Network will share below common fracture morphology diagrams and corresponding reasons for high-strength bolts.
Example 1: Morphology diagram of fatigue fracture form of high-strength bolt
Figure 1 shows the fatigue fracture of high-strength bolts
Among them, fracture A is a ductile fracture, and fracture B is a fatigue fracture. When fatigue fracture and toughness fracture coexist, fatigue fracture is the first fracture, so it can be inferred that the b steel bolt is the first fracture. The loose fit of the threads in the A~B section of bolt B resulted in stress concentration at position B. Over time, microcracks gradually developed under the alternating stress of crankshaft rotation, ultimately leading to multi-source fatigue fracture. After the B steel bolt broke, the A steel bolt was unable to bear the force generated by the crankshaft rotation, resulting in overload fracture. In summary, the loose fit of the threads in the A~B section of the B steel bolt has caused wear on the threads of the bolt and screw hole in this area. The balance block and crank arm have also become loose, resulting in micro vibration spots on the connection surface between the two. At the same time, stress concentration occurs at position B, and under the alternating stress of the crankshaft rotation for a long time, micro cracks gradually form, ultimately leading to multi-source fatigue fracture. After the B steel bolt breaks, the A steel bolt is insufficient to bear the force generated by the crankshaft rotation, resulting in overload fracture, the balance block flying out, and damage to engine components. The fracture of steel bolts is related to insufficient tightening axial force of the balance block fixing bolts during installation.
Example 2: Morphology diagram of brittle fracture form of high-strength bolt
Figure 2 shows the brittle fracture of the bolt
Macroscopic analysis of the fracture surface of the high-strength bolt shows that the bolt in Figure 2 belongs to a brittle fracture surface. Further testing of the mechanical properties reveals that the hardness and strength indicators of the high-strength bolt are relatively high, with a high yield to strength ratio of 0.95; The elongation, cross-sectional shrinkage, and impact energy all decrease regularly with the increase of strength and hardness. Therefore, bolts are subjected to pre tightening force, repeated alternating stress, and high-pressure vibration loads during operation, and brittle fracture frequently occurs during on-site use. The tested mechanical performance data shows that improving material toughness is necessary. In the case of material fixation, reducing the strength index appropriately to improve toughness is a good rotation. The sacrificial strength can be compensated for by increasing the bolt diameter.
Example 3: Morphology diagram of defect fracture form of high-strength bolt
Figure 3 shows the defect fracture of high-strength bolts
Figure 3: When a high-strength bolt fractures, it begins to crack at the chamfer of the threaded groove with a high degree of stress concentration. The crack initiation site has many tearing edges, mainly in the form of cleavage, and exhibits intergranular fracture characteristics. The bolt is subjected to stress
Occurrence of intergranular fracture. After the fracture initiates from the crack source, the crack rapidly and unstably propagates until it fractures. The presence of coarse grains and grain boundary segregation defects inside the material leads to a decrease in the actual allowable stress, which is also a prerequisite for rapid unstable crack propagation. The formation of microcracks is related to incomplete degassing and slagging during smelting. The assembly torque of bolts fluctuates greatly, and there is a phenomenon of over tightening; The radius of the rounded corner at the joint of the bolt head and rod fluctuates greatly, and some of them do not meet the standard requirements. There is a problem of ineffective control of dimensional accuracy in the bolt production process.
During the bolt production process, defects such as wear on the R-angle forming surface and thermal fatigue cracks were found on the working surface of the high-strength bolt hot pier mold. The mold support surface was severely worn and corroded, and adjustments were made with adhesive tape. Additionally, the R value of the bolt head rod joint was not controlled on the production site. These defects prevent the mold from ensuring dimensional stability such as bolt coaxiality and perpendicularity, which can affect product quality and increase the risk of bolt fracture.
