Impact vs pressure. Crater vs smooth. Both improve fatigue.

Fatigue cracks nucleate at tensile stress. Surface under load: tension causes crack initiation. Shot peening/burnishing create compressive residual stress near surface — applied tensile stress must first overcome the compressive residual before creating crack-initiating tension. Result: crack initiation delayed or prevented. Effect most significant for fatigue-limited parts where surface stress dominates.

Thin flat parts experience distortion from shot peening — the surface compression causes the plate to dome (compressive side stretches slightly). For complex 3D parts, distortion usually manageable. For large flat panels, dimensional control required — masking, fixturing, or post-peen flattening. Controlled peening intensity (Almen strip measurement) keeps distortion predictable.

Ball burnishing uses precision ball rolled through bore under pressure. Compresses surface, improves finish, creates residual stress. Standard for: hydraulic cylinder bores (achieves Ra 0.1 µm + compression), small precision holes in instruments. Requires accessible straight bore. Not applicable to: blind holes, cross-drilled holes, complex internal geometry.

Almen strips are thin steel coupons with one side exposed to peening. Peening causes compressive residual stress on peened side, strip curves. Measured curvature (arc-height) quantifies peening intensity. Standard peening intensity specifications: "10A" (10 thousandths arc-height on A-type strip). Ensures repeatable peening between production runs. Aerospace specifies exact intensity.

Shot peening: $5-30 per part depending on size and complexity. Well-established industrial process. Automated line: high-volume very economical. Manual peening (aerospace): expensive due to inspection and documentation. Burnishing: $10-50 per part. Specialty equipment, trained operators. Not as widely available. For high-volume fatigue-critical parts: shot peening typically more economical.

Yes — for maximum effect. Process: CNC machine → shot peen (residual stress generation) → burnish (surface smoothing while preserving residual stress). Used for premium fatigue-critical parts. More expensive but combines advantages. Typical application: aerospace rotating shafts, premium hydraulic components. Specify only when standard single-process inadequate.