Crack detection for formed wire: visual, penetrant and eddy current

Where cracks start in formed wire

Forming bends the wire beyond its elastic limit. The outside of the bend carries the highest tensile strain, and that is where a crack will open if the material or the tool is wrong. The inside of the bend is in compression and rarely cracks, but it can wrinkle in a tight radius.

The leg root is next. It combines a bend with the full working moment, and a tight radius or a tool mark gives a crack a place to start. Cut ends are a third location: a poor shear leaves a burr that acts as a notch, and a crack can run from there.

Welds and heat affected zones deserve their own check. A weld can leave porosity, a lack of fusion or a hard zone that cracks under vibration. Plating adds risk, because hydrogen can enter during the process and the coating can hide a defect that was already there.

Visual inspection: the first and cheapest check

Visual inspection is the default for every formed part, and it catches more than a quick glance suggests. Good lighting, a clean surface and the right magnification will show an open crack, a lap, a deep tool mark, a torn cut end and a discoloured weld zone.

The limits matter just as much. Visual inspection finds defects that are open to the surface and wide enough to see. A tight crack, a shallow defect or one filled with oil, plating or buffing compound can pass. The method also depends on the inspector, the light and the time allowed, so it needs a written standard and a reference sample to be repeatable.

For a spring or a contact, inspect the outside of every bend, both leg roots and the cut ends. Use a magnifier of about 10x for the critical zones and a light at a low angle to cast a shadow into the defect. Record the acceptance criteria with photographs or a boundary sample.

Penetrant testing: finding the tight surface crack

Penetrant testing uses a liquid that enters surface-breaking defects, then draws it back out with a developer so the defect shows as a coloured or fluorescent indication. It works on non-porous materials such as stainless steel, copper alloys and aluminium, and is the usual choice where a tight crack is suspected.

The process has more steps than it looks. The part must be cleaned and dried, because oil or plating can block the defect. The penetrant needs a dwell time. The excess is removed without washing penetrant out of the defect. The developer is applied thinly, and the indication is read after a development time. Each step has a time, and skipping one produces a false accept.

Fluorescent penetrant under ultraviolet light is more sensitive than visible dye and easier to read on a complex shape. It needs a darkened booth and a suitable lamp. Small springs can go in a basket, but the coils must not trap penetrant or developer, or the reading becomes a pool of colour rather than an indication.

Eddy current: fast, sensitive, and setup dependent

Eddy current testing drives an alternating current through a coil near the part and measures the change in the induced field. A crack, a change in conductivity or permeability alters the signal. The method is fast, needs no consumable and can be automated for a high volume line.

Its strength is sensitivity to surface and near-surface defects on a conductive material. Its weakness is that the signal depends on the material, the geometry and the probe. A setup for one wire grade and diameter will not transfer to another without a new reference standard, and a magnetic stainless spring behaves differently from a copper alloy one.

Eddy current is a comparative method. The operator or the system compares the signal with a known good part and a known defect. Without that reference, a reading has no acceptance meaning. For small parts, a probe that follows the bend or a through-coil arrangement can be used, but coverage of a tight bend has to be demonstrated.

Comparing the three methods

The table sets out where each method fits. A visual check is always done, and a penetrant or eddy current check is added where the risk justifies it.

Matching the method to the material

Stainless steel and copper alloys are straightforward for penetrant testing. Carbon steel is also suitable, but the surface must be free of scale and rust, because both mask indications. Plated parts are a special case: the coating can bridge a crack, and penetrant may not enter. If hydrogen cracking is a risk, inspect before plating and again after.

Eddy current works on conductive materials: steel, stainless, copper and aluminium. It is less useful on a rough or heavily plated surface, and it needs a reference defect that represents the smallest rejectable flaw. For a magnetic material, permeability variation can dominate the signal, so the setup has to account for it.

The defect type matters as much as the material. A tight fatigue crack is a penetrant or eddy current job. A lap from the wire mill may be visible but shallow, and a destructive section is often the only way to see its depth. A weld defect may need radiography or a section.

Sampling and acceptance

Crack detection is usually a sample check, not 100% inspection, unless the part is safety critical. The sample size and frequency should reflect the risk, the process capability and the consequence of failure. A first article should check every critical bend.

When a crack is found, contain the batch and record the location, orientation and size. A crack on the outside of a bend points to a radius or material problem; a crack at a cut end points to the shear; a crack after plating points to hydrogen. Acceptance criteria belong on the drawing before production, with the method, standard and record stated.

What to put in the purchase order

Name the method, standard and acceptance level. Add the zones to inspect, the sample size and the record to be supplied. If the part is plated, say whether inspection happens before or after.

• Method and standard, for example penetrant to ISO 3452-1
• Zones to inspect: bend outsides, leg roots, cut ends, welds
• Acceptance criteria and a boundary sample
• Sample size and frequency
• Inspection stage: before plating, after plating or both
• Operator qualification and the record to be supplied