Shot peening bombards a metal surface with a high-velocity stream of small hard spheres — the shot — to build a beneficial layer of compressive residual stress and extend fatigue life. Adjust the yield strength, Almen intensity and coverage to see the compressive residual stress, compressive-layer depth and fatigue-limit gain update in real time.
Parameters
Material yield strength σ_y
MPa
Sets the ceiling of the surface compressive residual stress
Almen intensity
mmA
Bombardment energy; sets the depth of the compressive layer
Coverage
%
Fraction of the surface covered by dimples; full benefit above 100%
Fatigue limit before peening
MPa
Fatigue limit of the untreated material (baseline)
Results
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Surface compressive stress (MPa)
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Compressive layer depth (µm)
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Coverage factor
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Improved fatigue limit (MPa)
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Fatigue-limit gain (MPa)
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Fatigue improvement (%)
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Shot peening visualization — impacts and residual stress
Top: the shot stream and overlapping dimples on the surface. Bottom: the residual-stress profile through the depth (strongly compressive near the surface, a small balancing tensile region deeper). The dashed line marks the compressive-layer depth.
Surface compressive residual stress σ_res is a representative 0.55 times the yield strength σ_y; the improved fatigue limit σ'_fatigue rises by up to 30% with the coverage factor C_cov (capped at 1.0).
The compressive layer depth d_c [µm] is proportional to the Almen intensity I_Almen [mmA]. The coverage factor is the coverage [%] divided by 100, capped at 1.0.
The compressive residual stress must be overcome before the surface feels net tension, so fatigue cracks are harder to start.
What is Shot Peening and Residual Stress?
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Shot peening means you deliberately blast small steel balls at a finished metal part, right? Why does damaging a perfectly good part make it stronger?
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It does sound backwards. But this is "controlled damage". When you bombard a surface with a high-velocity stream of small hard spheres — the shot — each impact plastically stretches a very thin layer of surface metal sideways, leaving the surface covered in tiny dimples. Because the deep, undisturbed elastic material below has not yielded, it squeezes that stretched layer, locking compressive residual stress into the surface. That compression is exactly what extends fatigue life.
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Why does having compression at the surface make a part better against fatigue?
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The key is that fatigue cracks are born and grow under tensile stress, and they almost always start at the surface. If you pre-load the surface with compressive residual stress, then any applied tensile load must first cancel that compression before the surface feels net tension. In other words, the effective stress driving crack initiation goes down. Take the "fatigue limit before peening" on the left as a baseline, and the tool reports the improved fatigue limit — typically a twenty to forty percent rise.
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There is an "Almen intensity" slider on the left. What does it represent?
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Almen intensity is a clever way to measure the "strength" of the bombardment. You peen a standard thin metal coupon — an Almen strip — under the same conditions; compressive residual stress goes into one face only, so the strip curls. The amount of curl, the arc height, gives you a number for the impact energy. The unit mmA means the arc height on an A-type strip. Raise the Almen intensity in the tool and you will see the compressive layer get deeper, because more energy reaches deeper into the material.
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The "coverage" defaults to 150%. Isn't more than 100% odd? If you hit the whole surface, that's 100%, surely?
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Good question. Coverage is the fraction of the surface struck by dimples, and by definition 100% means the whole surface has been hit at least once. But in practice any missed spot can become a fatigue-crack origin. So to peen the whole surface reliably and uniformly, engineers run 1.5 or 2 times the specified time — that is 150% or 200%. The tool caps the coverage factor at 1.0 above 100%, so 150% and 200% give the same maximum fatigue improvement.
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So in real factories, which kinds of parts get shot peened?
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Rotating and cyclically loaded parts with demanding fatigue duty are the classics: gears, coil and leaf springs, crankshafts, connecting rods, turbine blades, aircraft landing gear and airframe components. The more a part "must not fail" or "must be made light", the more shot peening pays off. Parts that only ever see static loads, on the other hand, gain little from it.
Frequently Asked Questions
Fatigue cracks are born and grow under tensile stress, and they almost always start at the surface of a part. Shot peening builds a layer of compressive residual stress into the surface, so any applied tensile load must first overcome that compression before the surface ever feels net tension. The effective stress driving crack initiation is therefore reduced, fatigue cracks are much harder to start, and the fatigue limit can rise by twenty to forty percent. This tool estimates that effect with standard engineering formulas.
Almen intensity is a measure of the bombardment energy, obtained by peening a standard test strip (an Almen strip) and reading how much it curls. It is quoted in mmA, the arc height of an A-type strip. A higher Almen intensity means more energy per impact, so the plastic deformation reaches deeper and the compressive residual stress layer becomes thicker. In this tool the depth of the compressive layer is taken to be proportional to the Almen intensity.
Yes. Coverage is the percentage of the surface that the dimples have struck, and 100% means the whole surface has been hit at least once. Any unpeened spot can become the origin of a fatigue crack, so a full and uniform benefit requires coverage of 100% or more. In practice engineers often aim for 150% or 200% (1.5 or 2 times the specified peening time) to be sure. This tool caps the coverage factor at 1.0 above 100%.
Each shot impact plastically stretches a thin surface layer sideways. The undisturbed elastic bulk beneath, which has not yielded, then squeezes that stretched layer, producing the compressive residual stress. The residual stress cannot exceed the value that would yield the material again, and it is set by the force balance between the plastic layer and the elastic bulk, so measured surface compressive residual stress typically falls at roughly 50-60% of the material's yield strength. This tool uses 0.55 as a representative factor.
Real-World Applications
Automotive powertrain parts: Gears, coil springs, leaf springs, crankshafts and connecting rods all carry cyclic loads and are classic shot-peening targets. Transmission gears in particular concentrate stress at the tooth root (the fillet), so introducing compressive residual stress there sharply raises the bending fatigue strength. The value on a mass-production line is that parts can be designed smaller and lighter while still meeting fatigue life.
Aerospace structural parts: Landing gear, turbine blades, discs and the regions around fastener holes — components where failure is not an option — are widely peened. In aviation the process is tightly controlled, with Almen intensity and coverage specified in standards. There is even peen forming, where flap peening deliberately curves a panel to shape the curved surface of a wing skin.
Welded structures and corrosion-exposed parts: Tensile residual stress is left around weld beads and acts as an origin for fatigue cracks and stress-corrosion cracking. Shot peening after welding can replace that harmful tensile residual stress with compression. It is also used as a corrosion-fatigue countermeasure for spring steels and high-strength steels, extending the life of bridges, construction machinery and ships.
Combination with CAE: In fatigue analysis (FEM), the compressive residual stress from shot peening is supplied either as an initial stress field or as a surface mean-stress correction. The practical workflow is to use a quick estimate like this tool to gauge how much compressive residual stress and depth to expect, and by what percentage the fatigue limit might rise, before moving on to detailed residual-stress measurement (X-ray diffraction) or non-linear FEM.
Common Misconceptions and Pitfalls
The most important point first: the calculations in this tool are standard engineering estimates (rough approximations). The formulas — compressive residual stress at 0.55 times the yield strength, compressive-layer depth proportional to Almen intensity, fatigue-limit gain capped at 30% — are representative values for understanding trends. The real residual-stress distribution, depth and fatigue gain depend on the material's work-hardening behaviour, the shot type, hardness and size, the projection speed and the part geometry. Formal design verification requires residual-stress measurement by X-ray diffraction and fatigue testing of actual parts.
A common misconception is that "the harder you peen, the better". Raising the Almen intensity too far does deepen the compressive layer, but it can also over-deform the surface and roughen it, and that increased surface roughness can itself create origins for micro-cracks. This is called over-peening, and it actually lowers fatigue life. An optimum Almen intensity exists for each part; stronger is not simply better.
Finally, beware the assumption that "the introduced compressive residual stress lasts forever". Residual stress relaxes and decays under high-temperature service (creep and annealing effects) or under excessive cyclic loads that exceed the yield strength. For turbine parts used at high temperature, or parts that see overloads beyond the design assumption, you should expect the peening benefit to fade over time.
How to Use
Select shot material (steel or ceramic) and enter shot diameter in mm (0.4–2.5 mm typical for aerospace)
Set impact velocity in m/s (40–90 m/s for pneumatic peening) and coverage percentage (100–300% for full surface saturation)
Input base material fatigue limit in MPa and click simulate to calculate residual stress depth, surface compression, and fatigue improvement factor
Worked Example
Aluminum 7075-T73 landing gear: base fatigue limit 285 MPa. Peening with 1.2 mm steel shot at 65 m/s, 150% coverage produces surface compressive stress of 580 MPa extending 180 µm deep. Improved fatigue limit reaches 385 MPa, a 100 MPa gain (35% improvement), significantly extending component life under cyclic landing loads.
Practical Notes
Coverage exceeding 100% overlaps impacts; 150–200% is industrial standard for consistent residual stress profiles in turbine blades