Improving Spring Fatigue Life: Design and Process Factors

Most springs do not fail because they were overloaded once — they fail because a small crack grew, cycle after cycle, until the wire could no longer carry the load. That is fatigue, and it is the number-one reason springs come back from the field. The good news is that fatigue life is governed by a short list of design and process decisions, and each one can be controlled. This guide walks through where cracks start and what you can do about it.

Where a fatigue crack starts

Fatigue almost always begins at the surface, because that is where bending and torsion stress is highest. A crack needs a starting point, and the usual suspects are a stress concentration, a surface defect, a corrosion pit, or a soft, decarburised skin. Remove or reduce those starting points and the spring lasts far longer. Spring surfaces are therefore usually checked for decarburisation, and cracks on safety-critical parts are checked with magnetic particle inspection.

Design factors

Keep stress inside the elastic range with margin — working stress should stay below the material’s limit with room left for surge and overload.
Control the spring index D/d — the ratio of mean coil diameter to wire diameter is normally kept between 4 and 12; a tight index raises the stress concentration on the inside of the coil.
Avoid sharp transitions — abrupt changes in section, poor end-coil geometry and sharp hook radii all act as stress raisers.
Watch solid height and travel — a spring compressed close to solid on every cycle is a fatigue risk; keep working travel within design limits.

Process factors

The way the spring is made matters as much as the drawing:

Heat treatment — quenched and tempered spring steels are typically used in the range 40–50 HRC. Too hard and the spring is notch-sensitive; too soft and it yields under load.
Decarburisation control — a soft surface skin cracks early, so surface condition is checked after heat treatment.
Shot peening — the single most effective process for raising fatigue strength. Shot peening is widely applied to suspension springs to raise fatigue strength: it hammers the surface into a state of compressive residual stress, which closes cracks before they can open. It is applied after heat treatment, not instead of it.
Surface finish and crack inspection — magnetic particle inspection catches cracks left by coiling, grinding or heat treatment before the part ships.

Corrosion and fatigue

Corrosion and fatigue work together: a corrosion pit is a ready-made crack starter, so a spring that would run indefinitely in a dry shop can fail quickly outdoors. This is why coatings are chosen as much for fatigue protection as for appearance. Zinc-nickel plating, zinc flake and epoxy powder coating are common corrosion protection systems for springs, and salt spray testing per ASTM B117 is commonly used to evaluate how well a coating resists corrosion.

A worked case: railway suspension springs

Railway suspension springs are a good example of everything above combined. Hot-coiled railway suspension springs are commonly made from bar diameters of about 20–45 mm and are shot peened to improve fatigue life. They also carry railway-specific testing for load and fatigue, and our own railway product line is certified to CRCC (certificate CRCC10219P11758R2M, valid to 2027-11-25). Because the service loads are high and the safety consequences are serious, the design margin, the heat treatment and the shot peening all have to be right at the same time.

Frequently asked questions

Q: What is the best way to improve spring fatigue life?
A: Control the design stress and spring index, ensure correct heat treatment, and shot peen the finished spring to create a compressive surface layer.

Q: Does shot peening replace heat treatment?
A: No. Shot peening is applied after tempering and adds a compressive surface layer; it does not change the hardness through the section.

Q: Why does corrosion shorten spring life?
A: A corrosion pit acts as a stress raiser and crack starter, so fatigue begins sooner. Coatings such as zinc-nickel, zinc flake or epoxy powder coating help.

Q: How is coating corrosion resistance evaluated?
A: Salt spray testing per ASTM B117 is commonly used.

Q: How are cracks found before shipping?
A: Magnetic particle inspection is used on safety-critical springs, along with checks for decarburisation.

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