Parts do not fail from the inside. They fail from the surface.
Fatigue cracks start almost always from a point on the surface — a scratch, a machining mark, an inclusion brought to light.
That is why surface condition is not a question of appearance. It is a question of service life.
The rule of thumb
A observation from the classic machining literature is worth remembering: to double the service life of a part, surface integrity has to improve by an order of magnitude.
The formulation comes from foundational work on metal cutting principles and is frequently cited in discussions of finishing.
It is not a calculation formula. It is a proportion, and it says something important: durability gains are paid for dearly in surface quality. Small surface improvements yield small gains.
What “surface integrity” covers
Not just roughness. The term covers everything happening in the surface layer:
- Topography — roughness, profile, marks left by previous machining
- Stress state — tensile or compressive residual stress
- Microstructure — changes produced by machining heat
- Layer hardness — work hardening, or the opposite
A part can have excellent roughness and tensile residual stress. It looks good and fails early.
Why stress matters
A fatigue crack needs tensile stress to start and to propagate.
Classic machining — turning, milling, grinding — frequently leaves tensile stress in the surface layer. Exactly what you do not want.
Shot peening introduces compressive stress. The crack has to overcome the compression before it can start. That is where the increase in fatigue strength comes from.
What a designer takes from this
If the part works under variable loading — springs, gears, shafts, suspension components — finishing is not the last operation on the list. It is a design operation.
The choice between a merely clean surface and a controlled one is made at the design stage, not in the workshop with the part in hand.
Related reading
- What Does Shot Peening Do? Process, Standards and Applications
- Sandblasting vs shot peening: the difference and when to use each
- How clean is the surface: grades SA1, SA2, SA2.5 and SA3
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