Must Inspect with Eddy Current
A good understanding of eddy current applications, testing scenarios and best practices can go a long way toward a successful eddy current inspection.
The words were right there in the footnotes of the engineering drawing: “Must inspect with eddy current.” Yet, there were no indications of what areas of the component were to be inspected, and there were no notes specifying what the eddy current test was supposed to do. Find cracks? Verify heat treat? Check threads?
The good news is that the engineer wanted to use a solid, proven technology to validate the quality of the component. The not-so-good news is that eddy current testing cannot be used as a blanket test term. It is important that the application and best practices of the test method be fully understood to achieve the right outcomes.
Eddy Current Testing Applications
So what can eddy current testing do? In what areas can the technology help verify your production processes and reduce the risk of shipping bad components?
Here are some of the major eddy current testing scenarios:
1. Crack and Flaw Detection
Eddy current testing is very effective at finding surface cracks and flaws in metal components. Alternating magnetic fields are induced into the component which cause eddy currents to flow. If these eddy currents are interrupted by a crack or flaw, the eddy current probe and instrument detect the change in eddy current flow and register a test failure.
Using eddy current technology to test for cracks and flaws is clean, fast and repeatable. The inspection is easily integrated into production processes, and the technology can find flaws that visual inspections can miss. Probes can be custom made to fit into small, hard-to-reach, critical areas. Array probes and material handling systems help inspect large or complex surface areas.
Typical components tested for cracks and flaws with eddy current include:
- Wheel bearings (hubs or outer rings and spindles)
- Roller bearings (tapered rollers)
- Tubing (medical and automotive)
- Railroad rails
2. Heat Treat and Material Structure Verification
Eddy current testing is ideal for validating whether a component under test has the correct material structure as compared to a known good component. Material structure differences may occur due to variations in heat treat processes, or differences in alloys used. These processing differences can result in surface hardness and case depth variations. Eddy current technology detects these variations as changes in material conductivity and permeability. Eddy current testing technology helps validate the component’s structural integrity and durability as defined by the product development engineer.
Eddy current testing for heat treatment and material structure verification is fast and clean, requiring in most cases less than 1/10th of a second to perform. Testing results are displayed as a pass/fail indication.
Typical components tested for proper heat treatment processing and material structure with eddy current include:
- Wheel bearings (hubs or outer rings and spindles)
- Ball and roller bearings
- Pinion pins
- Ball studs
- Powdered metal (sintered) components
- Locking pins and clips
- Drive train components (engine and transmissions)
- Medical components
3. Thread, Feature and Assembly Verification
Similar to structure testing, eddy current inspections can be used to verify the quality or presence of a thread, verification of a component feature, or verification that an assembly is properly put together.
Eddy current thread inspection can help identify:
- Partial and damaged threads
- Oversized and undersized threads
- Broken taps
- Missing holes
- Incorrect threads
- Short tap
Component features that can be validated include:
- Keeper grooves
- Key ways
Assembly verification testing includes:
- Installation of all ball bearings in a bearing race
- Inclusion of shaft retaining keys or pins
- Verifying metal keepers in rubber molded components
- Verifying proper number of burst discs inside an air bag inflator
- Proper orientation of piston rings
- Valves and fittings component assembly (material presence, orientation and correct alloy)
Effective Eddy Current Testing Scenarios
Eddy current testing works very well in a variety of scenarios.
High volume testing. Eddy current systems offer the highest return on investment in high volume production.
Low volume testing. These inspections can be worthwhile with components that have a high monetary value, are “safety critical” or have the potential to be a high cost of failure component. For example: a small heat treated c-clip is a low cost part, but if it holds an automotive brake system together, the cost of failure could be high.
Integration into production lines. Speed and repeatability are key features of in-line eddy current inspections. When the test is incorporated directly into the production line, the eddy current instrument I/O efficiently communicates with a PLC to quickly sort out bad components.
Eddy Current Inspection Best Practices
Like any technology, if eddy current testing is not implemented correctly it will deliver inconsistent or even incorrect results. A good reliable test requires the proper pairing of an eddy current sensor (coil/probe assembly), with a modern low-noise eddy current instrument suitable for the test application. It also requires a good material handling system and properly trained operators who have some basic eddy current knowledge.
Best practices include:
Having a good understanding of the conditions you may encounter. For a material structure or heat treat test, create test standards that represent both good and bad component conditions. These should include “real world” conditions such as short heat, no quench, air cooled or misplacement of an induction heating coil (due to bent or damaged coil scenario).
Understand how component surface, geometry, and orientation affect the test. A very rough, inconsistent surface may introduce additional system noise, which could potentially hide very small flaws of interest. Component geometry and orientation must also be taken into consideration.
Use “good” components to set up a test, and samples of “bad” components to validate the test setup.
Ensure a consistent placement of the eddy current sensor (coil/probe) relative to the component under test. If the spacing or placement is inconsistent, the test results will also be inconsistent.
Provide a reliable material handling system. The best instrument and probe cannot make up for improper tooling. Regular system checks help ensure consistent component and sensor positioning.
If tests are to be conducted by hand, ensure mechanical tooling is used to reduce undesirable test variances due to hand placement of the component.
Use a stable, low noise eddy current instrument. The lower the noise, the smaller the flaws that you can find and the smaller the material difference you can identify. System stability ensures consistent measurements.
Properly train operators and perform an operational system review before problems occur. While eddy current systems are often designed to run 24/7, it is best to run a set of component test standards, also known as masters or rabbit samples, to run through the test system once a shift to ensure results. If conditions change, the trained operator can then evaluate the problem, make required test adjustments, and identify other corrective actions such as sensor replacement.
Get an eddy current instrument that is easy to use. Modern systems have touch screen displays and intuitive user interfaces. This makes the training of new personnel in today’s high turnover environment much easier.
Get an evaluation of your component inspection application before committing to a purchase of an eddy current test system. Some eddy current test system manufacturers offer a free evaluation and test report to validate test feasibility.
Identify a trusted resource for the technology and work closely with them in advance of a purchase and implementation.
A good understanding of eddy current applications, testing scenarios and best practices can go a long way toward a successful eddy current inspection. Design engineers can then correctly specify which type of eddy current test to apply and where to apply it, in order to meet the organization’s quality and manufacturing goals.