shot-peening-advanced

name	shot-peening-advanced
description	Advanced shot peening — Almen intensity (A, N, C strips), peening coverage (Avrami equation), compressive residual stress depth profile (X-ray diffraction), peening-induced work hardening, controlled shot peening (CSP) vs. laser shock peening (LSP), peen forming for airframe panels, fatigue improvement mechanics (Goodman diagram), stress peen, dual peening, SAE AMS 2430, and aerospace applications (wing skins, turbine blades).
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Advanced Shot Peening — Complete Skill

Shot Peening Mechanics

Stress State Induced

Process: hardened spherical media (shot) impact surface at high velocity → local plastic deformation → surrounding elastic material constrains plastic zone → biaxial compressive residual stress in surface layer

Compressive layer depth: typically 0.1–0.5 mm (conventional shot peening) Magnitude: σ_res = -(0.3 to 0.6) × σ_y (material yield strength) Peak at: ~25–50 μm depth (slightly subsurface); surface value lower due to elastic springback

Work hardening: ΔHV = 10–30% increase in surface microhardness; depth 0.05–0.3 mm HV_peened = HV_base × (1 + C_WH × (shot intensity / E_shot)^α) [C_WH, α = empirical; material-dependent]

Almen Intensity

Standard Almen Strips

Concept: Almen strip is thin SAE 1070 spring steel strip clamped flat; after peening, residual stress causes it to curve (arc); arc height = Almen intensity

Strip types:

• Type A: 1.295 mm thick × 19 mm × 76 mm; for standard intensity (coverage 0.15–0.25 mm arc)
• Type N (thin): 0.792 mm thick; for gentle peening (soft materials, blades)
• Type C (thick): 2.388 mm thick; for heavy peening (heavy components)

Reading: arc height measured with Almen gauge to ±0.001" (0.025 mm) Units: Almen intensity expressed in thousandths of inch (0.001") or mm (1 A = 0.001" arc height on type A strip) Type N conversion: N to A multiply by 3; A to C divide by 3 Intensity range: 4–8 A for thin sheets; 8–16 A for medium parts; 16–24 A for heavy forgings

Saturation curve: Plot arc height vs. exposure time (or passes); arc height increases then plateaus Saturation point = intensity at which doubling exposure time increases arc < 10% Specified Almen intensity at saturation = peening specification

Saturation curve fit (Baiker): h(t) = h_s × (1 - exp(-t/τ_s)) [h_s = saturation height; τ_s = time constant; t = exposure time] Saturation defined at h(2τ_s)/h(τ_s) < 1.10

Coverage

Avrami Coverage Equation

Coverage: percentage of surface area impacted by at least one shot

Avrami equation: Coverage(t) = 1 - exp(-N × π × r²) [N = impact density [impacts/m²]; r = indentation radius] Or equivalently for time-based: C = 1 - exp(-B × t) [B = rate constant for given conditions]

100% coverage: all surface impacted (C = 0.9999...) Practical "100%": C ≥ 98% (verified by fluorescent tracer media or dye penetrant on specific locations)

Minimum coverage per specification: MIL-S-13165 (general): 100% minimum AMS 2430 (aircraft): 100% minimum; critical areas (fillet, root): 200% (double pass) Note: 200% coverage means exposure for 2× the saturation time with 100% coverage

Coverage calculation from Hertz contact: Radius of plastic impression r_indent ≈ 0.4 × a_Hertz a_Hertz = ∛(3F×R_eff/(4E_eff)) for each impact F = m_shot × v_impact² / (2 × δ_max) [impact force from energy conservation; δ_max = penetration depth]

Residual Stress Profile Measurement

X-Ray Diffraction (XRD)

Method: Bragg's law diffraction from lattice planes; strain from d-spacing shift sin²ψ technique: σ_residual = -(E/(1+ν)) × ∂(d/d₀)/∂(sin²ψ) / d₀ [standard formula]

Depth profiling: Electropolish successive layers 10–20 μm deep; measure at each depth Full profile σ_res(z) from surface to tension crossover depth

Typical profile shape (steel, after shot peening): Surface (z=0): σ_res ≈ -300 to -500 MPa Peak compression at z = 50–150 μm: σ_res,max ≈ -600 to -900 MPa (depends on shot intensity) Crossover to tension at z_cross = 0.3–1.0 mm Self-equilibrium: ∫σ_res dz = 0 (zero net force)

Controlled Shot Peening (CSP)

AMS 2430 Requirements

CSP vs. conventional: controlled media, velocity, angle, coverage, intensity — all specified and verified AMS 2430 content:

• Shot type, size, hardness (hardness ≥ 55 HRC for cast steel; GH = glass beads)
• Almen intensity ± tolerance (typically ±1A on each A strip)
• Coverage (typically 100% minimum; 200% at critical zones)
• Saturation curve required for each setup change
• Inspection: Almen strip readings, microscopic examination of shot (no broken shot allowed)

Shot media types: Cast steel shot (SAE S110–S550: S110 = 0.30 mm dia; S550 = 1.40 mm): most common for structural Conditioned cut wire (CCW): more spherical; more consistent; preferred for CSP Glass beads (GH): gentler; for aluminum, thin sections, delicate parts Ceramic beads: for medical implants (no metallic contamination) Stainless steel shot: for stainless steel or titanium (no iron contamination)

Shot hardness specification: HRC ≥ 55 for steel shot on high-strength steel (otherwise shot deforms before workpiece) Shot hardness / workpiece hardness ≥ 1.0 (shots must be harder than workpiece)

Laser Shock Peening (LSP)

Principle

Process: short (ns) high-power laser pulse generates plasma → pressure pulse → shock wave → deep plastic deformation without high temperature

Advantages over shot peening:

• Deeper residual stress: 1–3 mm depth (vs. 0.1–0.5 mm for shot)
• No surface roughening (Ra often improves to 0.5–1.0 μm)
• Precision application (fillet roots, blade edges)
• Magnitude: σ_res ≈ -(0.5 to 0.8) σ_y

Laser parameters: Pulse energy: 1–50 J; pulse duration: 8–30 ns; spot size: 3–8 mm Pressure from plasma: P = 1–5 GPa (far exceeds dynamic yield strength → deep plastic zone) Confined ablation (water overlay): multiplies pressure 2–3× vs. direct ablation

Applications: GE90 fan blades (LSP at leading edge and fillet); turbine compressor blades; landing gear LSP at critical features on hard-to-peen areas (complex geometry)

Peen Forming

Wing Panel Forming

Concept: differential peening intensity across panel → controlled curvature for compound contours (wing skin) Heavy peening one side → that side in compression → panel curves toward peened side

Curvature from peening: 1/R_formed = 2 × ε_s × E_s × t_s / (E × t² × (1-ν)) [ε_s = surface strain from peening; simplified beam model] Controlled by: Almen intensity, coverage, shot size, peening pattern

Advantages over press forming: No spring-back uncertainty; works on already-assembled panels; residual stress benefit simultaneously induced Used for: Boeing 747/777 wing skins; Airbus wing covers; radar dishes

Forming accuracy: ±5 mm on large panels; requires numerical forming model (FEM)

Fatigue Improvement

Goodman Diagram Analysis

Mechanism: Mean stress σ_m decreases by |σ_res| → effective Goodman line shifts right σ_m_effective = σ_m_applied + σ_residual (negative → reduces mean stress)

Goodman improvement: σ_a_allow = σ_e × (1 - (σ_m + σ_res)/σ_u) [σ_e = endurance limit; σ_u = ultimate strength] With σ_res = -500 MPa (compressive): mean stress effectively reduced by 500 MPa → more amplitude allowed

Typical fatigue improvement: Ferrous parts in bending: fatigue life 2–10× (factor depends on applied mean stress level) Aircraft turbine disc bore: factor 3–5 improvement; tested per ASTM E466

Combined effects: residual stress + work hardening + surface finish improvement (smoother after CSP) Net improvement: often 3–8× in fatigue life; reduces Kt sensitivity

Standards and References

Standard	Scope
SAE AMS 2430	Shot peening (aerospace — controlled)
MIL-S-13165C	Shot peening of metal parts (military general)
SAE J443	Procedures for using Almen strips
SAE J2277	Shot peening coverage
ASTM E466	Fatigue testing for shot peening verification
AMS 2546	Laser shock peening

Output

Provide: part description (material; σ_y [MPa]; σ_u [MPa]; hardness HRC; geometry; critical region: fillet/bore/surface), peening objective (fatigue life increase ×; depth of compressive layer target [mm]; surface finish requirement Ra [μm]), process selection (conventional CSP / laser shock peening / peen forming; basis), shot specification (type: cast steel/CCW/glass; size SAE designation; hardness HRC; flow rate or velocity), Almen intensity (strip type A/N/C; target value ± tolerance; saturation curve verification required: yes/no), coverage (% minimum; critical zones: 200%; verification method: visual/fluorescent), residual stress profile (XRD measurement required: depth [mm]; expected σ_res_max [MPa] at peak depth; crossover depth z_cross [mm]), fatigue improvement (σ_res [MPa] applied to Goodman; estimated fatigue life multiplier ×; comparison with baseline S-N), work hardening (ΔHV; depth [mm]; any risk of over-peening/surface damage), peen forming (if applicable: target radius R [m]; differential intensity scheme; forming accuracy ±[mm]), and applicable standard (AMS 2430 / MIL-S-13165 / AMS 2546 for LSP; SAE J443 Almen).