shot peening residual stress - Metal Improvement Company

C H A P T E R S I X
A X IAL F A T IGUE
26
A X I A L F A T I G U E
Axial fatigue is less common than other (fatigue) failure mechanisms. A smooth test specimen with axial
loading has uniform stress throughout its cross section. For this reason, fatigue results of smooth, axial
loaded shot peened specimens often do not show significant improvements in fatigue life. This is unlike
bending and torsion that have the highest applied stress at the surface.
In most situations, pure axial loading is rare as it is normally accompanied by bending. Shot peening of
axial loaded components is useful when there are geometry changes resulting in stress concentrations.
Undercut grooves, tool marks, cross holes and radii are typical examples of potential failure initiation sites.
A p p l i c a t i o n C a s e S t u d y
TRAIN EMERGENCY BRAKE PIN
Figure 6-1 is part of a hydraulic brake
assembly used in a mass transit system.
The undercut sections near the chamfered
end were designed to fail in the event of
axial overload. During the investigation of
premature failures it was found that a
bending load was also occurring. The
Figure 6-1 Brake Pin Schematic
combined axial and bending load when simulated in test caused fatigue failure between 150,000 –
2,600,000 cycles. Shot peening was added to the brake pin and all test specimens exceeded 6,000,000
cycles without failure [Ref 6.1].
A p p l i c a t i o n C a s e S t u d y
AUXILIARY POWER UNIT (APU) EXHAUST DUCTS
This type of APU is used to provide power to aircraft when they are on the ground with the main
engines turned off. The tubular exhaust ducts are a high temperature 8009 aluminum alloy welded in
an end-to-end design.
Tension-tension fatigue tests measured fatigue strength of 23 ksi (156 MPa) at 3000 cycles in the as
welded condition. Glass bead peening of the welds resulted in a 13% fatigue strength improvement to
26 ksi (180 MPa) [Ref 6.2].
REFERENCES:
6.1 RATP, Cetim; Saint Etienne, France, 1996
6.2 Internal Metal Improvement Co. Memo
www.metalimprovement.com