Eddy-Current Inspection by the Numbers

About this book

Pass an alternating current through a coil and hold it near a metal, and the coil’s magnetic field induces circulating currents in the metal: eddy currents. They flow in a layer near the surface, and they push back on the coil, changing its impedance. Anything that changes how easily the currents flow, or where they can flow, changes that impedance: the metal’s conductivity and magnetic permeability, the distance between coil and metal, the thickness of the part and any crack across the currents’ path. Measuring the change lets an inspector check a heat treatment, measure a coating, find a crack or sort mixed parts, in seconds, without touching or marking the part.

This book shows how to use eddy-current testing with numbers. It starts with what the coil sees and how deep the currents reach, then works through conductivity and what it reveals about a metal, measuring it reliably, coating thickness, surface cracks, sorting steel parts and testing tubes. It finishes with the decisions: thresholds, false calls and missed defects, acceptance sampling and capability, and the procedure, people and records that make a test result trustworthy.

It is for quality engineers, inspection technicians, manufacturing engineers and supplier quality staff. It assumes engineering or trade training and comfort with basic electricity and statistics; the electromagnetism and the statistical methods are explained as they are used. Each chapter works the running case with real numbers, includes one plausible mistake and how it was found, links the KEVOS® tools that do the chapter’s calculations, and ends with an exercise and its worked answer.

The running case

Every chapter follows one product line. This is an illustration. A maker of hydraulic gear pumps in Dandenong South, Victoria, builds the PH-6 pump family and uses eddy-current methods at three points:

• Housings: machined from 6061-T6 aluminium bar and sulfuric acid anodised to 15 to 25 µm. Each incoming bar’s temper is checked by conductivity against the maker’s band of 42.0 to 45.0 %IACS, and the anodising thickness is measured on every batch.
• Drive shafts: 42CrMo4 steel, induction hardened at the two bearing journals and ground. The journals are scanned for grinding cracks with a four-element surface probe, and every shaft passes a comparator coil that sorts out any whose journals missed hardening.
• Oil cooler: built in-house from 90/10 copper-nickel tubes 12.7 mm in diameter with 1.0 mm walls, bought in lots of 2,000. The tube maker tests every tube with an encircling coil, and the pump maker audits the records and samples the tubes on its own test bench.

Conductivity bands, readings, signal amplitudes, noise levels, prices and test results are the pump maker’s working figures or are calculated from stated assumptions. They do not describe KEVOS® work or any real business, instrument maker, material supplier or customer. Material properties are typical values for illustration; real acceptance limits come from the material and product specifications a customer calls up. Prices are in Australian dollars excluding GST.

How to use the book

Read Chapters 1 and 2 first: they explain the coil’s response and the depth of penetration, which every later chapter uses. Chapters 3 and 4 go together on conductivity. Chapters 5 to 8 can then be read in any order. Chapter 9 brings the statistical decisions together, and Chapter 10 the procedure, records and cost.

Every calculation in the book was checked by running it, and the KEVOS® tools linked from each chapter were checked against the calculations they perform. Where a tool was built for a different purpose but does one step of a chapter’s calculation, such as the induction hardening plan’s reference depth, the chapter says which output is used and which are not.

Source limits and professional responsibility

This book is educational. Eddy-current testing of safety-critical parts, pressure equipment and aircraft is governed by product standards, customer specifications and written procedures, and is performed and interpreted by personnel certified for the method. The book names the standards that apply to its examples by number and scope only; it does not reproduce their requirements, and they must be read in full before use. Decisions about real parts remain with the competent people responsible for them.