Application: Detection of fatigue cracks in marine propellers using Eddy Current testing.

Background: Propellers used on boats of all sizes and types undergo cyclic fatigue. Cracks occur as a result of cyclic fatigue causing potentially serious consequences including total loss of water craft, loss of lives or delay in time of arrival. Inspection of propellers is applicable across the maritime industry. Many different types of materials are used for propellers including bronze, aluminum, stainless steel and carbon steel, all of which are susceptible to cyclic fatigue. Cracks can originate at many locations on propellers. In most cases the crack begins at points of highest stress loads associated with stress risers such as sharp edges, thick to thin transitions and areas of where weld repair has occurred. The most common point of failure on propellers is where the blade joins to the hub. Cracks occur anywhere along the axis of the blade to hub radius.

The advantages of eddy current testing in this application are as follows:

• In many cases, especially in large ocean going vessels, inspection of the propeller in-situ is a big advantage. Eddy current testing can be performed under water while many of the other NDT is very difficult or impossible.
• Eddy current can be used on all the aforementioned materials of construction. In some cases where propellers are coated the inspection could possibly be conducted without removal of the coating.
• Eddy current can be used to detect anomalies slightly subsurface such as casting imperfections.
• Depth of anomalies can be estimated in some cases.
• Can be performed underwater.
• In the case of larger propellers, Eddy Current Array testing (ECA) can speed inspection and also provide a permanent record.

Applicable NDT techniques are subject to material of construction, type of defect to be detected and location of subject being inspected. Magnetic particle inspection, dye penetrant and visual inspection may also be used where applicable.

Equipment:
Nortec 500D eddy current flaw detector
Pencil Probe, 100-500 KHZ, part number 9222164
Right Angle Weld Probe, 100-600 KHz, part number WCD90I-5-50

As with most NDT techniques, a reference standard with known flaws should be constructed for calibration and performance verification.

Procedure: In order to achieve optimum results, two scans using two different eddy current probes were used to complete the inspection. The first scan used a conventional pencil probe, a common probe used in detecting surface cracking. The second scan was performed using a Nortec Weld probe, chosen due to its contoured face.

For each probe two scans were made and an image captured for each corresponding scan. One scan was made across the entire area of interest while the other scan was made directly across the reference notch to simulate the response from a surface breaking defect. A scan from a smaller reference notch on a separate standard was also done with each probe for comparison purposes.

Example of scanning a propeller with eddy current

Comparison of crack signal to defect-free area using weld probe

Finger tip probe can be used as well.