The role of preload in fatigue fracture of bolt fasteners
Apr 22,2024
Fasteners play a very important role in engineering structures, such as aerospace, infrastructure, civil engineering, automotive, and other industrial fields. Failure of fasteners may lead to catastrophic consequences and significant financial losses. One of the common failure modes of fasteners is fatigue cracking. Insufficient design considerations, material issues, insufficient preload, looseness, and excessive loads can all lead to fastener fatigue.
Similar to other metal parts, fastener failure includes overload, corrosion related cracking, embrittlement, creep, and fatigue. Due to external loads, overloading of fasteners may occur during installation or use.
The investigation of fastener overload faults should include an evaluation of material properties, with the main purpose of determining the cause of fastener failure, such as the strength of the fastener and the load applied to the fastener.
Stress corrosion cracking or hydrogen embrittlement, commonly found in certain metals, are also important failure modes for fasteners when subjected to static tension during service.
Given the expected stress of fasteners, materials and environment must be carefully considered to appropriately mitigate stress corrosion cracking or hydrogen embrittlement.
During the electroplating process, hydrogen is likely to diffuse into the fasteners, causing hydrogen embrittlement. In order to reduce hydrogen embrittlement of susceptible materials, fasteners must undergo dehydrogenation treatment.
Fatigue is a common form of fracture in metal structures, accounting for 80% of fractures. Fasteners are no exception, and fatigue is also a common cause of fastener fracture.
When the cyclic load exceeds the fatigue strength of the material, when sufficient cycles are applied, fatigue cracks will initiate and propagate on the fastener. The material, geometry, stress amplitude, average stress, and assembly parameters of fasteners all affect fatigue performance.
The assembly process of fasteners is important, but it is often overlooked when analyzing the causes of their fracture, which is also the main source of fatigue fracture of fasteners.
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