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Fastener Hole Inspection with Eddy Current without Filter Adjustment

Ultrasonic velocity and attenuation measurements in geological samples


Aircraft are constructed using thousands of fasteners and bolts. As the metal around these bolts is susceptible to stress cracking, the fastener holes are a critical zone for structural integrity testing. Consequently, aircraft manufacturers routinely recommend crack detection using eddy current technology in combination with a MiniMite rotary scanner.

With the eddy current flaw detectors used on the market today, operators are required to perform a certain amount of filter adjustment to obtain a clear signal. Filter adjustment is a difficult and time-consuming technique to master. It can take a lot of trial and error to obtain the proper high-pass and low-pass filter settings. Furthermore, every time scan speed is changed, these filter settings must be readjusted. Operator skill level, as well as the differences between the instruments used during inspection, can impact the results.

The Key to Easy and Consistent Rotary Fastener Hole Inspection Solution

The NORTEC 600 eddy current flaw detector offers a solution to the time and consistency issues commonly experienced with fastener or bolt hole inspection. This portable instrument pre-sets the high- and low-pass filters, reducing the vibrational and electrical noise, respectively.

The new application menu of the NORTEC 600 includes a predefined factory setting for fastener hole inspection. The fastener hole application pre-set contains all the necessary parameters to obtain a clear signal with the required figure 6 and figure 8 responses. Using these pre-set filter settings provides consistent results from one operator to the next. In addition, the NORTEC 600 is equipped with a LINK function that automatically adjusts filter settings when scan speed changes.

Fastener hole inspection with the NORTEC 600 and MiniMite scanner
Fastener hole inspection with the NORTEC 600 and MiniMite scanner

The new application menu of the NORTEC 600 was established based on customer recommendations, including the fastener or bolt hole application with pre-set filter adjustments.

The new application menu of the NORTEC 600

The figure 6 filter response
The figure 6 filter response
The figure 8 filter response
The figure 8 filter response

The figure 6 and figure 8 signal types are used and recognized by the aircraft manufacturing industry. They are given this designation because the shape of the signal resembles the numbers six and eight on the instrument display (as shown above). With the NORTEC 600, the figure 6 and figure 8 pre-set filters provide consistent signals, without the need for filter adjustment by the operator.

LINK function
LINK function

As shown in the screen captures (above) the LINK function adjusts the high- and low-pass band filters whenever the RPM changes, with little effect on the signal. In this example, the speed doubles from 1500 to 3000 RPM without an impact on the signal. Consequently, a recalibration is not necessary.

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Products used for this application


The new NORTEC 600 incorporates the latest advancements in high-performance eddy current flaw detection into a compact, durable unit. With its vibrant 5.7 inch VGA color display and true full-screen mode, the NORTEC 600 produces user-selectable, highly contrast eddy current signals.
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