Sometimes stress risers with their related slip planes and micro cracks start to have oxidation form in the defects. This layer of oxidation acts like an expanding wedge in these small areas, which helps get the metal separated, greatly speeding crack formation. This is called stress corrosion fracture. It's common in areas where two dissimilar metals are in close proximity, like a piece of steel bolted to aluminum. Some hard alloys with certain types of heat treatment are also particularly sensitive to this sort of cracking as well. Stress corrosion fractures are particularly insidious because they are hard to detect and usually result in sudden catastrophic failure.
Polarized Cyclic Stress
Now that you know how cracks form, we can explain how shot peening works to prevent them. The force of hardened shot impacting the metals' surface at high velocities causes plastic deformation of the metal, which cold works the surface and refines the grain. Imagine a tiny forging hammer hitting the part all over millions of times. The shot hitting the part forges the metals surface on a micro scale. The peening action of the shot seals up stress risers as well as relieves welding stress. The compressive stress caused by cold working breaks up the metals' large crystal lattice structure and makes it more homogeneous with a finer lattice network to a depth of 0.003-0.004 of an inch. The uniform distribution of compressive stress around the outside of the part and the fine grain skin that is formed by the peening action of the shot strongly resists the formation of cracks. Slip planes have a hard time starting in a finely divided homogeneous structure and stress has a hard time finding an area to concentrate in an area under uniform compressive stress. Stress corrosion fracture has a hard time propagating in this fine-grained compressed layer as well.
It's important for the shot used to be uniform in size and round. Irregularly shaped media can produce sharp-edged holes instead of nice round dimples. Sharp-edged holes can produce stress risers in a part and hurt fatigue strength. That's why it's important not to confuse abrasive cleaning and sand blasting with shot peening
Shot peening is a near-miraculous process. It helps just about any metal last longer and work better. Shot peening can be used to help with breakage problems on many parts. Axles, rods, crank fillets, gears, and springs all benefit from shot peening. Shot peening is cheap-the cost to shot peen a bunch of parts is usually under $100. Many techniques can be used to get the most out of shot peening. One common example is two-stage shot peening. A part with complicated surface geometry like a splined axle or a gear is first hit with a large shot to develop a good amount of surface working, then hit with a finer shot to extend the surface working into the roots of the gear teeth or the splines. Parts that are heat treated, induction hardened, or nitrided require the use of special hardened shot and higher velocities. When looking for a shot peener, it is good to find one who has aerospace or motorsports experience and to stick with their recommendations. An experienced shot peener can help you with the right process for your situation.
Shot peening's only disadvantage is that it can leave a somewhat pebbly surface that can cause some distortion of finely machined parts. Surfaces that must remain smooth with tight tolerances, like crank journals, pistons, seal surfaces, bearing bores, and cylinder walls can't be shot peened. Delicate parts, like bearing and piston rings, can't either. Sometimes connecting rods and crankshafts must be straitened and resized after shot peening. Are you having a problem with parts breakage? You might want to try shot peening it.
These cross sections show...
These cross sections show the greatly improved grain refinement that WPC offers.