A common cause of failures in various types of equipment is fatigue failure of bolts that secure parts together. Fatigue in any type of dynamically loaded mechanical component accounts for more than 80% of all failures. This post reviews some important factors in fatigue failure of metallic bolts.
Mechanical fatigue involves cyclic or fluctuating tensile stresses that lead to failure. The magnitude of the stresses often is below the yield strength of the material. These metals could easily withstand that same level of ambient temperature stress with no ill effects if the stress was applied statically. Fatigue starts by the initiation of one or more cracks on the surface, or just below the surface, of the stressed material. The crack (or cracks) then grows in size as it penetrates through metal cross sectional thickness. Crack growth continues until there is too little intact metal left to withstand the last cycle of stress and final fracture occurs. Corrosion fatigue combines mechanical fatigue and active corrosion. When all other conditions are equal, corrosion fatigue will cause bolt failure before mechanical fatigue acting alone.
Bolts always have threads and this makes them subject to early fatigue crack initiation. Threads create geometric discontinuities on the surface of the metal and thus they concentrate and significantly magnify any applied stresses. This is a major factor in fatigue failure of bolts.
The different ways that threads may be formed on smooth metal rods used for bolts are important to the bolts' final fatigue resistance. Threads may either be rolled on or machine cut on metallic rods. During rolling, rods are passed between serrated, high hardness die components that are forced together at high pressure. The serrations have the desired finished thread dimensions that are required on the bolts. During rolling helpful compressive stresses are imparted on the surface of the rods. These residual stresses counteract tensile service stresses that are necessary to start fatigue cracks. Machining of threads is done with a cutting tool applied to rods to generate the desired threads. Machining does not generate compressive stresses but it permits enhanced control of thread dimensional tolerances and, in general, it is more expensive than rolling. Typically any stress relief heat treatment of rolled bolts is done before a rolling process so as not to reduce the residual compressive stresses left after rolling.
Rolling has the harmful potential of generating surface discontinuities that slightly penetrate into the metal. These features are called laps and they appear as small metal folds on and slightly below the surface. They are generally caused due to misalignment between the forming dies used in rolling. Laps represent stress concentration points that lower resistance to initiation of fatigue cracks. ASTM standard F788 specifies the maximum permitted depth laps can penetrate into the metal. A lap of any size is undesirable but if a rolled bolt has one or more laps larger than the maximum permitted by F788 accelerated fatigue is probable even in the absence of other deleterious influences.
Bolted joints can lose their initial tightness - clamping force - especially during fatigue loading. Some potential causes of loosening are not applying the correct initial torque, not using the specified type of thread lubrication or not using the specified bolt material (correct initial torque is based on achieving a particular percentage of the intended bolt material's yield stress as tension in the bolt) or use of an incorrect gasket material. When one or more of these deficiencies exists a bolt can lose its initial tension and begin to slightly move with each cycle of stress. Accelerated failure due to fatigue follows.
Certain mechanical and chemical properties of the bolt material itself or its surface coating are significant factors in the incidence of fatigue failure. Both high strength and high ductility are important to providing good resistance. High strength bolt material is desirable because it resists the initiation of cracks and provides more resistance to crack growth. High ductility is desirable because this property allows local yielding of the stressed material and this often prevents the initiation of a crack. Unfortunately metal strength and ductility are inversely proportional. Therefore the best fatigue resistant material is one with a balance between strength and ductility. Desirable values of these properties may not be attained if the alloy used had the wrong composition or if the heat treatment applied was deficient. If a corrosive environment exists in the given application, along with cyclic tensile stresses, then a bare material with a corrosion resistant chemical composition might be used or a suitably resistant coating can be applied. Each approach can mitigate corrosion fatigue.
Gerald O. Davis, PE, President and co-owner of DM&ME, has over 40 years experience in Materials Engineering and Business. Mr. Davis is a Forensic Expert in Materials Usage, Corrosion, Metallurgy, Mechanical Failure, & Root-Cause Failure Analysis. His recent background includes work as a corrosion researcher, senior engineer, and program manager for Battelle Memorial Institute, DNV, Inc., Henkels & McCoy, Inc., respectively and, since 2004, as president of DM&ME.
©Copyright - All Rights Reserved
DO NOT REPRODUCE WITHOUT WRITTEN PERMISSION BY AUTHOR.