1. Springs Fail in Fatigue
Springs recover their shape, and that recovery is what makes them useful, but each deflection cycle also works the material. A spring in a valve, a clutch, a shock absorber or a connector cycles between two loads, and enough cycles eventually breaks it at a point far below its static strength. Fatigue, not static overload, is the failure mode that governs spring design. The spring that passes a static check will still fail in a week of service if the alternating stress is too high.
The fatigue process is cumulative and hidden. Every cycle leaves a tiny permanent change at the stress concentration; millions of cycles build these changes into a crack, and the crack grows until the remaining section cannot carry the load, releasing the spring suddenly. Because the damage is invisible until the end, spring life must be designed at the stress level, with a known margin, rather than discovered in service. The designer controls the stress through geometry, material, preload and surface quality.
This article follows the spring from material to finished part, showing how stress range, mean stress, surface condition and residual stress decide the number of cycles the spring survives, and how those variables are combined into a defensible life calculation.
2. Mean Stress and Alternating Stress
A cyclically loaded spring sees two stress components. The mean stress is the steady background level around which the loading swings, and the alternating stress is half the swing from minimum to maximum. A spring preloaded to a moderate force and cycled over a small range has a high mean stress and a small alternating stress; a spring released to zero each cycle has a zero mean and a large alternating swing. Fatigue damage scales with the alternating stress, but the mean stress changes how much alternating stress the material tolerates before yielding or cracking.
The relationship between the two components is captured in a Goodman diagram, a fatigue limit chart that plots the alternating stress the material can survive against the mean stress. A high mean stress reduces the allowable alternating amplitude, because the total stress at the high end of the cycle approaches the material strength. The Goodman line draws the safe envelope: points below the line meet the target life, points above it are expected to fail. Real design adds a margin below the line to cover scatter in material and manufacture.
For helical springs the stress is torsional, and the relevant stresses are the shear stress produced by the applied force and the correction factor that concentrates the stress at the inner surface of the coil. The alternating shear range, the mean shear and the endurance shear strength of the wire combine exactly as they do for axial loading. The same stress amplitude reasoning applies to leaf springs in bending and diaphragm springs in bending, each with its own geometry factors.
3. The Stress Curve in a Helical Spring
The stress in a compression spring is highest at the inner surface of the coil. When a coil spring is compressed, the wire twists, and the torsional stress is highest at the wire surface, higher still at the inner diameter where the curvature concentrates the stress. The Wahl correction factor accounts for this curvature and for the direct shear; springs with a small mean coil diameter relative to the wire diameter carry a larger correction and a lower allowable fatigue stress. Tightly wound springs are therefore weaker in fatigue than the same wire in a larger coil form.
The load cycle maps straight onto the stress cycle. The free length, the preload length and the working length define three corners of the operating window. Preload sets the mean stress; the working deflection, the distance between preload and maximum travel, sets the alternating stress. A spring specified only by its minimum and maximum force already carries its fatigue story. The spring rate turns force into deflection, and the design freedom is divided between the wire diameter, the coil diameter, the number of active coils and the preload.
| Load condition | Mean stress | Alternating stress | Typical life |
|---|---|---|---|
| Static, set at solid | high | near zero | infinite if below yield |
| Preloaded, low swing | high | low | very long |
| Zero to full travel | zero to mid | high | limited, needs margin |
Surge is a hidden fatigue enemy. A compression spring has a natural frequency along its length, and when the forcing frequency approaches that natural frequency the spring surges, the coils crashing together with stress several times the static level. Designing the spring so its natural frequency sits well above the forcing frequency removes surge as a fatigue multiplier, an easy design check that many failures miss.
4. Material Selection for Fatigue
Spring materials differ sharply in their fatigue behavior, and the difference is set by strength, cleanliness and surface condition rather than by chemistry alone. Music wire, the cold drawn high carbon wire used for small precision springs, has the highest tensile strength of the common spring wires and a fine surface, giving excellent fatigue life where it is not subjected to elevated temperature. Chrome silicon and chrome vanadium steels add alloying that retains strength at higher temperatures and gives a better response in heavy duty springs, and oil tempered wire adds a consistent surface state from manufacture.
Stainless steels trade strength for corrosion resistance. Austenitic stainless wire has roughly half the tensile strength of music wire, so its allowable stress and its fatigue life are lower, but the corrosion resistance that protects the spring in a humid or chemical environment is worth the penalty. Beryllium copper and phosphor bronze serve contact and high conductivity springs where no magnetic response and stable electrical conduction are required, with modest fatigue strength. Inconel and titanium alloys serve elevated temperature aerospace springs at a cost premium.
Internal cleanliness decides the fatigue floor. Non metallic inclusions in the wire act as a crack starter, and a spring made from a dirty heat will fail early no matter how well it is wound. High fatigue springs therefore specify vacuum melted or bearing grade steel, and the choice of material grade is written into the drawing along with the wire diameter and the tensile strength range. The tensile strength of the wire, not the bar stock, is the property that enters the fatigue calculation.
5. Surface Effects: Notches, Decarb and Shot Peening
Fatigue cracks start at the surface where the stress is highest and where flaws live. The wire surface carries the entire fatigue story of a helical spring, so surface condition is specified and inspected as carefully as the chemistry. Surface decarburization, a thin layer robbed of carbon during heat treatment, is soft and weak, and it lowers the fatigue strength of the outer fibers that carry the highest stress. Surface defects, laps, seams, die marks and scratches, act as notches that concentrate stress and start cracks early.
Shot peening converts the surface into a compressive layer. The spring is bombarded with small steel shot that plastically deforms the surface, leaving a residual compressive stress that opposes the tensile alternating stress at the surface. Because cracks need tensile stress to grow, the compressive skin postpones crack initiation and dramatically improves fatigue life, commonly doubling or more the allowable cycles for a given stress. Peened springs are specified for high cycle duty such as valve springs, suspension springs and truck clutch springs.
The peening quality is itself controlled. The intensity of the peening is measured on an Almen strip, a thin flat test strip whose curvature after peening reports the energy delivered. Coverage is checked by inspection under magnification. Peening after heat treatment and after any forming that would erase the layer, and the drawing names the peening specification and the covered zones. A spring shot peened in the wrong intensity, too heavy and it cracks the surface, too light and it gains nothing, is worse than no peening at all.
Order of operations that protects fatigue life: wind and form, heat treat, grind or set, shot peen, prestress set, then final inspect. Each step either improves the surface or removes a defect; none should be skipped for high cycle parts.
6. Prestressing and Set
Prestressing, also called setting or presetting, is a deliberate overload that improves the spring without breaking it. A compression spring is compressed solid, or nearly so, at manufacture, so that the wire stress exceeds the yield stress and the outer fiber yields in compression. When the load is removed, the spring returns slightly shorter than before, with a residual compressive stress at the surface just like shot peening, and with every subsequent load it operates within a much lower stress range.
Setting has two design consequences that must be negotiated deliberately. First, it changes the free length and the load at any set height, so springs made by a setting pass must be designed oversized and then verified at the working height. Second, the benefit only appears for springs that are stressed high enough to yield at setting; a lightly stressed spring gains nothing and the process is skipped. The process parameter, the set length and the final load tolerance are written on the drawing.
Presetting and shot peening are complementary tools. Peening compresses the whole surface skin; setting works the most stressed fibers through overload. A high cycle spring often uses both: peening to build the compressive layer, then setting to stabilize the length and the load. The residual stress they create is the reverse of the operating tensile stress, which is why the same nominal stress in a peened, set spring survives cycles that break an as wound spring many times sooner.
7. Other Failure Modes and Envelope Limits
Fatigue is the dominant but not the only spring failure. Relaxation, also called stress relaxation, is the slow reduction of load at constant deflection over time, and it is accelerated by temperature. A spring held compressed in a hot environment loses load month by month, which shows up as a lowered force at the working height in the field. Relaxation resistance requires the right material, chrome vanadium or inconel at temperature, a conservative stress, and often a presetting that stabilizes the load history.
Buckling is a geometry failure of long, slender compression springs. A spring whose free length is many times its mean diameter bows sideways under compression, losing its function and sometimes colliding with adjacent parts. The cure is not a material change but a geometry change: a shorter spring, a larger mean diameter, or a guide rod or housing that restrains lateral motion. The slenderness ratio is checked in design the same way a column is checked.
Corrosion, hydrogen ingress and fretting widen the failure menu. Corrosion pits act as notches and remove material, cutting fatigue life dramatically in humid service. Hydrogen absorbed during plating or pickling embrittles high strength wire; the standard defense is to avoid acid cleaning of hardened springs and to bake plated parts. Fretting at the bearing or on the end coils wears the surface and erodes the fatigue skin. For each application, the review lists the operating environment, the temperature, the humidity and the fretting contacts, because every one of them is a life multiplier.
8. The Fatigue Design Procedure
A complete fatigue design of a spring assembles the parts in a fixed order. The target life in cycles and the operating force range come from the application. The designer picks a material, grades it for cleanliness and surface, and chooses the wire diameter and coil geometry that fit the space. The mean and alternating shear stresses are calculated with the curvature correction, and the endurance strength of the spring wire is adjusted for size, surface finish, peening and reliability. The Goodman line is drawn, or equally a fatigue equation is solved, and the margin is checked against the required factor.
The design review then fixes the detail requirements: the natural frequency check for surge, the presetting and shot peening specification, the surface finish and decarb limits, the relaxation estimate at the service temperature, and the acceptance test, which for high cycle springs is often an actual fatigue test run to a proof of a set number of cycles. The drawing carries the material grade, the wire tensile strength, the surface, the peening class and the prestress recipe, so the fatigue story survives into production.
- Define the cycle count, force range and service temperature
- Select wire material and cleanliness grade
- Size geometry and calculate mean and alternating shear stress
- Apply the curvature and surface corrections to the endurance limit
- Verify the margin on the corrected Goodman diagram
- Check the natural frequency against the forcing frequency
- Specify peening, presetting and relaxation resistance
- Validate with a fatigue proof test on the production spring
Spring fatigue is won in the drawing office, not in the field. A spring specified with its stress range, its surface state and its residual stress plan will run to its design life, while the identical geometry without that specification fails early and without warning.
