Product Specification

Cyclic Bending Fatigue in Stainless Steel Wire: Where Failures Start

A wire that survives one bend and a wire that survives a million cycles are two different specifications, even when both are the same grade and the same diameter. Fatigue is a surface and stress-amplitude problem, and the grade only sets the starting point.

This wire is supplied from 0.15 mm to 12 mm, annealed at 550-750 N/mm2 or in spring temper, in 304 and 316L. For cycled duty the tensile band, surface inspection level and the bend radius in the design are agreed together, because each one moves the expected life.

Three variables decide whether a wire survives cycling: the stress amplitude it sees, the condition of its surface, and the mean stress left in it by the shape it was set to.

Product Specifications

The Three Variables

Stress amplitude, not tensile strength

A wire loaded to 60% of its tensile and released a million times will fail from fatigue long before it reaches that load once. The relevant figure is the alternating stress at the bend, which follows the radius: a tighter radius raises the surface strain on the outside of the bend.

Surface defects as crack starters

On a cycled wire the outside of the bend is in tension. A die line, a scratch or a sliver on that face concentrates stress and becomes the origin of the crack, while the same defect on a straight static part is harmless. This is why surface inspection is specified as a separate line item on cycled duty.

Mean stress from the installed shape

A wire pre-set into a shape carries residual stress before any load is applied. That mean stress shifts the whole cycle, and a part already sitting in tension has less room to absorb the alternating component.

Design Rules That Raise Life

Keep the radius above the forming limit

A bend radius of one wire diameter is the practical floor for annealed wire; below that the outside of the bend is overstrained before the part ever moves. On cycled parts the radius should be generous rather than at the limit.

Support the span

Long unsupported spans vibrate at their own frequency, and that resonance adds cycles the design never counted. Intermediate supports, or a shorter span, often buy more life than a stronger wire.

Keep welds out of the bend zone

A weld in a bend is both a stress riser and a coarser microstructure. Where a welded assembly will be cycled, the joint belongs on a straight run.

Applications

Vibrating screens and separation meshes

Woven and welded screen cloth in 0.5-3 mm, where the wire sees continuous small-amplitude flexing and surface defects decide the service interval.

Braided hose and cable reinforcement

Fine wire braided around a core, bent to a small radius and cycled as the hose flexes in service.

Clips, springs and retaining forms

Parts formed once and then flexed repeatedly, where a soft temper and a generous radius keep the bend zone below the point of work-hardening.

Product Advantages

• Surface treated as a fatigue variable - die lines and scratches are inspected on cycled duty, not averaged into a general surface check.
• Tensile band fixed per order - one band across coils keeps the forming force and the resulting residual stress consistent.
• Two tempers from one source - annealed for parts formed then cycled, spring temper for parts that must hold shape under load.
• Design guidance stated - radius floor, span support and weld position are described as part of the selection.
• Grade scope stated plainly - 304 for general service, 316L where chlorides or washdown add corrosion to the fatigue case.
• Full size range - 0.15-12 mm, so screen wire and structural wire are specified on the same basis.

Quality Control

Rod grade is verified before drawing. Diameter is measured inline and at coil ends, tensile and elongation are tested per heat per ISO 6892-1, and surface is inspected for die lines, scratches and slivers. Bend or torsion cycling can be run on sample lots where the application calls for it.

Selection and Ordering Notes

1. State the smallest bend radius and the approximate number of cycles expected in service.
2. Say which face of the wire is in tension at the bend, so surface inspection can be focused there.
3. Confirm whether the part is pre-set to a shape, since that adds mean stress.
4. Give the support spacing, so resonance is considered before the grade is chosen.
5. Provide sizes and annual volume for a tolerance and packing review.

Send the diameter, bend radius, cycle count and service environment for technical confirmation and quotation.