Crack Detection Systems

BTI engineers and manufactures a complete line of manual, semi-automatic, and fully automatic industrial eddy current crack detection equipment

Eddy current crack detection finds the flaws that scrap a part after value has been added to it — quench cracks in a forging, grinding burns on a journal, seams and laps carried in from the bar. It does it dry, in seconds, with no consumables and no operator judgment, which is why it is the method production lines use when every part has to be checked rather than a sample.

Balance Technology Inc. engineers and manufactures eddy current crack detection systems from single-station gages through multi-station and fully automatic in-line cells, for forgings, castings, machined components and assemblies.

How Eddy Current Works

A coil carrying alternating current induces circulating currents in a conductive part. A crack, a seam or a change in material condition disturbs those currents, and the disturbance reflects back into the coil as a change in impedance — a shift in amplitude and, more usefully, in phase.

Phase is what makes the method discriminating rather than merely sensitive. Lift-off, surface finish, geometry changes and genuine defects all produce a signal, but they produce it at different phase angles. Setting the instrument so that lift-off lies along one axis lets real indications be separated from the noise of a part moving slightly in its fixture.

What It Finds, and What It Does Not

Eddy current is a surface and near-surface method. It is excellent at tight, surface-breaking discontinuities — quench and grinding cracks, seams, laps, heat-treat damage — and it detects them without needing them to be open enough to see.

It does not see deep subsurface flaws. Penetration falls off exponentially with depth and with frequency, so raising sensitivity to small surface cracks costs you depth, and you cannot optimize the two at once. Inclusions, porosity and voids in the body of a part belong to ultrasonic inspection, not to eddy current.

It is also affected by things that are not defects. Material conductivity, magnetic permeability, residual stress, temperature and surface condition all move the signal. That is a limitation and an opportunity at once: the same sensitivity is what allows eddy current to be used for material and hardness sorting, catching a mixed lot or a missed heat treat that no dimensional check would find.

Probe Configuration Decides the Coverage

The coil arrangement is chosen from the geometry and the flaw orientation you need to catch.

Encircling coils surround the part and inspect the full circumference at once as it passes through. Fast, well suited to bar, tube and cylindrical components, less sensitive to short circumferential flaws.

Rotating probes spin a small coil around or along the surface, giving high sensitivity and good resolution on a defined area. Slower, but the choice when small cracks in a specific feature matter — a fillet, a journal, a bore.

Surface and pancake probes inspect a local area and suit flat or irregular features where an encircling arrangement is impossible.

Orientation matters as much as sensitivity: eddy currents are disturbed most by a flaw lying across their flow. A probe arrangement that finds longitudinal cracks reliably can miss transverse ones, so parts with both failure modes need either multiple probe arrangements or a scan pattern that covers both.

Setup Is Master-Based

An eddy current system is calibrated against a standard carrying artificial discontinuities of known size — normally EDM notches at specified depths. That standard defines what the system calls a reject, and it is the anchor for everything downstream.

Two consequences follow. The standard has to represent the production part in material, geometry and surface condition, because a calibration on the wrong material is a calibration on the wrong physics. And the standard needs periodic verification, since a worn or damaged master quietly moves the acceptance threshold on every part that follows it.

Where It Sits Against Other Methods

Magnetic particle inspection is sensitive and inexpensive, but it works only on ferromagnetic material, it needs cleaning before and after, and the call depends on an inspector looking at indications. Dye penetrant finds surface-breaking flaws in any material but is slow, wet and equally operator-dependent. Ultrasonic inspection reaches deep into the body but needs couplant and careful geometry.

Eddy current’s advantage is not that it finds more. It is that it is dry, fast, quantitative and automatable, which makes 100% inspection at production rate practical. Where a process needs subsurface coverage as well, the answer is usually both methods rather than a compromise on either.

Configurations

Layouts range from a single manual station for a cell or a repair operation, through semi-automatic stations with assisted load and part identification, to fully automatic multi-station systems with transfer, sorting of rejects, and data collection across the line. Configurations vary to accommodate any production rate.

Crack detection also combines readily with other measurements on one machine — see combination equipment — which frequently costs less in capital and floor space than separate stations, and removes a part handling step.

Specifying a System

The questions that determine the machine are:

• the material and its conductivity and permeability
• the minimum flaw size and depth that must be caught
• where on the part those flaws occur and in what orientation
• the surface condition at the point of inspection
• the part geometry and how it can be presented to a probe
• the production rate
• whether reject sorting and data collection have to be automatic

The last one is worth settling early, because in most audited processes the record of the inspection matters as much as the inspection.

Related Equipment and Applications

BTI also engineers dimensional gages, surface measurement systems and balancing machines, frequently on the same platform. Crack detection is commonly specified for crankshafts, turbine and jet engine components and brake rotors and drums.