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fastener tension of only 40% of yield strength. Thus, in-
creasing the torque specification would be expected to de-
crease the screw’s susceptibility to fatigue failure.
Summary
Fasteners play a critical role in many engineering struc-
tures, and their failure can result in significant conse-
quences. The most common cause of fastener failure is
fracture by fatigue, which is often created by inadequate
tension and clamping force upon installation. Contrary to
what may be intuitive, susceptibility to fatigue can be re-
duced by increasing the amount of tensile preload in
screws or bolts. A case study of steel screw failures high-
lights some of the critical manufacturing and design fac-
tors that affect fastener fatigue. The investigation
determined that the steel screws fractured in fatigue for
two reasons. First, short intergranular cracks, likely caused
by tempered martensite embrittlement, contributed to fa-
tigue crack initiation. Second, the specified assembly
torque was insufficient to provide adequate preload and
clamping force between components. Lack of adequate
preload promoted fatigue, and led to fatigue crack growth
under service loads.
For more information:
Alex Hudgins is managing engineer,
Materials and Corrosion Engineering, Exponent Failure
Analysis Associates, 149 Commonwealth Dr., Menlo Park, CA
94025, 650.688.6724,
ahudgins@exponent.com,
exponent.com.
References
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Second
Edition, Prentice Hall, p 358, 1998.
3.
Metals Handbook, Volume 11: Failure Analysis and Preven-
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Introduction to the Design and Behavior of
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Ninth Edition, McGraw Hill, 2011.
5.
ASM Handbook Volume 19: Fatigue and Fracture,
ASM In-
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6. ASTM A 574-00, Standard Specification for Alloy Steel
Socket-Head Cap Screws, American Society for Testing and
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Steels: Heat Treatment and Processing Principles,
ASM International, 1997, p 229-239.
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Hemisphere
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10.
ASM Handbook Volume 1: Properties and Selection: Irons,
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ASM International,
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