A torque to yield fastener (TTY) or stretch bolt is a fastener which is torqued beyond the state of elasticity and therefore undergoes plastic deformation, causing it to become permanently elongated.[1][2]

Fastener

Torquing a fastener to its yield point results in a high preloading of the fastener which, depending on the load frequency and amplitude, can significantly increase the fatigue life of the fastener. When the applied load doesn't surpass the clamping force of the fastener, the strain of the fastener will be lower than when the preloading is smaller than the applied load. It is therefore beneficial in high-frequency high-load situations with a higher risk of fatigue related failure, like a bolted down cylinder head, to use torque to yield bolts.

  • Advantage: Compared to normally tightened hardware, a smaller TTY bolt/screw may be used while still maintaining the same clamping force.
  • Disadvantage: TTY hardware normally has to be replaced when loosened, for example when the cylinder head is removed.[3]

References

  1. ↑ "Archived copy" (PDF). www.acl.co.nz. Archived from the original (PDF) on 7 January 2012. Retrieved 13 January 2022.{{cite web}}: CS1 maint: archived copy as title (link)
  2. ↑ "Threaded Fasteners Torque-to-Yield and Torque-to-Angle - Engine Builder Magazine". 1 February 2001.
  3. ↑ "Torque-To-Yield (TTY) Cylinder Head Bolt Installation & Removal Tips". www.aa1car.com.


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