Fasteners play a very important role in engineering structures such as aerospace, infrastructure, civil, automotive and other industrial sectors. Failure of fasteners can lead to catastrophic consequences and significant financial losses. One of the most common fastener failure modes is fatigue cracking. Insufficient design considerations, material problems, insufficient preload, looseness, and excessive load can cause fastener fatigue.
Similar to other metal parts, fastener failures include overload, corrosion-related cracking, embrittlement, creep, and fatigue. Due to external loads, fastener overload can occur during installation or use.Investigation of fastener overload faults should include evaluation of material properties, the primary purpose of which is to determine the cause of fastener damage, such as the strength of the fastener and the load applied to the fastener.
Stress corrosion cracking or hydrogen embrittlement, which is common in some metals, is also an important failure mode for fasteners when the fastener is subjected to static stretching during service.In view of the expected stress of the fastener, the material and environment must be carefully considered to properly mitigate stress corrosion cracking or hydrogen embrittlement.
During the electroplating process, hydrogen is likely to diffuse into the fastener, causing hydrogen embrittlement. In order to reduce the hydrogen embrittlement of the susceptible material, the fastener must be dehydrogenated.
Fatigue is the most common form of fracture in metal structures, accounting for 80% of fractures. Fasteners are no exception, and fatigue is also the most common cause of breakage in fasteners.When the cyclic load exceeds the fatigue strength of the material, the fatigue crack will sprout and expand on the fastener when sufficient cycles are applied. Fastener materials, geometry, stress amplitude, average stress, and assembly parameters all affect fatigue performance.
The fastener assembly process is the most important, but it is often overlooked when analyzing the cause of the fracture. It is also the main source of fatigue fracture of the fastener.
Broken bolt fastener
Bolt fracture macro image
Bolt fracture stereoscopic image
The fracture is divided into two areas, A and B, and zone A and zone B.
High-profile morphology in Zone A: Morphology of crack initiation zone
High-profile morphology in Zone B: Instantaneous fracture zone morphology
Bolt preload - torque diagram: as a percentage of yield strengthAccording to reports, the dynamic friction coefficient of the stainless steel bolt used in the paper is 0.53, the pre-tightening force of the bolt is 25% of the yield strength, far lower than the standard 50%, and far lower than the recommended pre-tightening force is 75% of the yield strength. . The reason for the fatigue fracture of the bolt in the paper is that the pre-tightening force is insufficient. Therefore, increasing the preload force is beneficial to reduce the fatigue sensitivity of the bolt and the probability of fatigue fracture.
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