Spring Wire Is a Fatigue Component: Design to Stress Range, Not Tensile Strength

A compression spring is not sized by whether it can carry the load once. It is sized by whether it can carry that load ten million times. The number that decides this is the alternating stress, not the tensile strength of the wire, and a design chosen purely on tensile strength is the most common reason a stainless spring breaks in service without ever being overloaded.

Two Stresses, Not One

Any spring working between a minimum and a maximum load sees a mean stress and an alternating stress at the same time:

• Mean stress is the average of the working and installed stress, and it sets how much of the material capacity is already used.
• Alternating stress is half the difference between the two, and it is the quantity that drives fatigue damage.

Both are calculated with the Wahl factor, which corrects the simple torsion formula for coil curvature and direct shear. On a 0.7 mm wire wound to a small index, that correction is not a refinement: stress on the inside of the coil can be materially higher than the average, and the inside of the coil is also where fatigue cracks usually start.

Why Tensile Strength Does Not Predict Life

A 302 spring temper wire at 0.7 mm typically sits at 1,850 - 2,150 MPa tensile, per ASTM A313 and the equivalent EN 10270-3 grades. Higher tensile generally means higher fatigue strength, but only if the surface and the residual stress state are equally good.

Three factors move life independently of the tensile certificate:

• Surface condition. A drawing line, a die mark or a transverse scratch is a crack starter sitting exactly where the stress is highest. A 10 micron deep scratch on a 0.7 mm wire changes the local stress field far more than a 50 MPa difference in tensile strength.
• Residual stress. Cold coiling leaves tensile stress on the outside of the coil and compressive stress on the inside. The inside condition helps; the outside condition does not, which is one reason a coil fails on the outside of the wire as often as the inside.
• Inclusions. A stringer near the surface acts as an internal notch. Cleanliness is a fatigue property as much as a corrosion property.

The Four Design Values That Matter

Spring design handbooks publish allowable stress range as a function not of one number but of four conditions: whether the spring is shot peened or unpeened, and whether the target life is of the order of one million or ten million cycles. Peened springs are allowed a higher alternating stress; longer life is allowed a lower one.

The absolute numbers belong to the design standard in use and to the wire diameter, so they should be taken from the handbook rather than from a supplier page. What matters here is that the four columns exist: a spring specified against only one of them will be either oversized or unreliable.

What Raises Fatigue Life in Practice

• Reduce the stress range before changing material. Increasing coil diameter or active coils lowers stress faster than upgrading the grade raises allowable stress.
• Shot peen after coiling, not before. Peening before forming destroys the compressive layer in the areas that are stretched during coiling.
• Stress relieve after coiling. For 302, a low temperature treatment in the region of 200 - 300 C for 30 to 60 minutes reduces coiling residual stress without softening the spring temper. It must stay well below the temperature at which the drawn strength starts to fall.
• Control the surface. Specify no transverse scratches and no die lines, and inspect at magnification rather than by eye at arm's length.
• Avoid sharp transitions. End coils, hooks and the transition into an active coil concentrate stress; a generous radius there is cheaper than a better wire.
• Keep the spring out of resonance. A spring that surges at its own natural frequency sees stress ranges far above the design value regardless of the wire.
• Do not let the soap film become a defect. The coating is a forming lubricant and should be removed or at least not allowed to hold grit on a peened surface.

How Fatigue Is Verified

• Pulsating shear test on finished springs at the service stress range, run to the target cycle count or to failure. This is the only test that evaluates the assembly rather than the wire.
• Load and rate check at installed and working height, since a rate error moves the whole stress range.
• Surface inspection of the finished spring at 10 to 20 times magnification, on the surfaces that see the highest stress.
• Wire tensile and wrap test per ASTM A313 as the incoming check, remembering that it predicts nothing about surface-initiated fatigue on its own.

Wire Specification

A spring fails at the point where the stress range and the weakest surface meet. Tensile strength sets the ceiling; the stress range, the surface and the residual stress decide where the part actually sits under it.