Using Eddy Current for Bolt Hole Inspections

The phrase “nuts and bolts” refers to the most basic details of a subject. It is an ironic turn of phrase because nuts, bolts, and bolt holes require significant engineering to determine specifications and nondestructive testing (NDT) to ensure that they meet those specifications. Multilayer bolt hole inspections in aerospace are one of the more challenging NDT inspections as it is difficult to identify the layer in which the flaw occurs. This is particularly true when the flaw is very close to the transition between layers. The current method requires the use of a marker or tape on the probe to indicate depth. 

Simple, reliable, and repeatable bolt hole inspections are best accomplished through eddy current array (ECA) testing. It uses electromagnetic fields to detect very small scale irregularities and discontinuities in metals. This is especially important in bolt hole inspections since flaws tend to be small and difficult to detect. Choosing easy-to-use tools that help with data interpretation can speed up NDT inspection, boost accuracy, and improve the reliability of the components used in aircraft. 

Choosing Eddy Current Equipment for Bolt Hole Inspections

Effective bolt hole inspection depends on a rotating scanner, rotary probe, and instrument that records and displays the resulting data. Each of these elements plays a role in fast and effective bolt hole inspection. Bolt holes are regularly machined into difficult-to-reach places on components. Equipment needs to be easy to handle so technicians can conduct NDT bolt hole inspections efficiently. The quality and speed of the analysis of the resulting probe data can depend on device quality.

For fast and effective bolt hole inspections, use instrumentation that offers:

• A lightweight and ergonomic form factor
• Portability combined with a long-lasting battery so technicians will not need to stop and change or charge batteries mid-shift
• Color 2D and 3D C-Scan displays to see the different layers
• High signal to noise ratio for a better probability of detection (POD)
• The ability to store and easily switch between testing configurations so it takes less time to set up for a bolt hole inspection and analyze the resulting data
• Clear and intuitive user interfaces that enable a wide range of functionalities like pan, zoom, rotate, and side-by-side channel comparisons

One of the keys to better eddy current array bolt hole inspections is to make every part of the inspection—from the initial insertion of the scanner and probe to the final viewing of the data—intuitive to the inspecting technician. This reduces the time it takes to conduct the inspection and lessens the time a technician needs to interpret and analyze the data. It also increases the likelihood of detecting fine-scale flaws that may be overlooked with displays that may be harder to interact with or read.

Improving Eddy Current NDT for the Inspection of Bolt Holes

Eddy current array testing is a fine-scale NDT technique that can detect very small flaws. It is especially valuable for bolt hole inspection because of fatigue cracking. Fatigue cracking, crack initiation, and the deformations that lead to them are very small scale structures that are below the detection threshold of some other NDT techniques. Metal fatigue detection is challenging but of vital importance. Fatigue cracks can initiate and grow much faster than expected, leading to catastrophic failure at stress levels that are well below expected stress levels. Metal fatigue has been a factor in over 300 fatal aircraft accidents since the 1930s, and fatigue cracking of bolt holes has been a cause in everything from aircraft engine failure to wings separating from the airframe. Detecting bolt hole metal fatigue early is a safety priority, as are reliable NDT methods for aerospace in general. 

ECA has the sensitivity needed to detect developing fatigue cracks, However, that same sensitivity poses the potential for false readings due to noise from within the bolt hole. Manufacturing processes like drilling can create rotational scraping that may remain if a bolt hole is not properly deburred and polished following machining. Similar phenomena can occur as tightened fasteners scrape the interior of a bolt hole. During bolt hole inspections, both can be read as cracking or other flaws, or as so much noise that they hide actual flaws from the technician.

Eddy current arrays can provide the comparative data needed to detect small crack initiation and other micro flaws like pitting due to corrosion. These arrays can also be used for bolt hole inspection for bolts that interface with multiple plates, layers, or structures. Software that supports eddy current arrays, rotating scanners, and viewing tools for Impedance, Sweep, waterfall, and C-scans can help equipment make sense of noise. Such software can also manage signal-to-noise ratio (SNR) so that technicians can better differentiate between noise generated by surface scratching and fine-scale flaws or developing defects. 

Given how critical bolt holes are for applications that range from containing steam in power generation to preventing critical airframes failures in flight, lives quite literally depend on using top-shelf eddy current software alongside intuitive equipment and ergonomic probes.

Zetec is a global leader in eddy current testing for bolt hole inspection and eddy current and ultrasonic testing for routine and highly specialized testing. To learn more about our NDT solutions, contact Zetec today.