Crack Detection in Aircraft Components

The Problem:

Crack detection in aircraft engine and other high-energy rotating parts is extremely important, because uncontained failures can result in serious damage to equipment and loss of life. (For an example, see NTSB report NTSB/AAR-98/01). High length-to-diameter (L/D) bores, like tie-rod holes found in aircraft engine fan hubs, present a particularly difficult challenge for airline NDT staff. Visual inspection of these small diameter and deep holes can be compromised by poor access, operator subjectivity and fatigue.

Application Notes

The Solution:

LTC's patented laser-scanned penetrant inspection (LSPI™) technology is a perfect fit for this situation. In this automated fluorescent penetrant inspection (FPI) method, operators apply fluorescent penetrant to the surface in question, then they scan the surface with a LSPI™ sensor. Advantages of this method include:

Small sensor size - to less than 0.25 inch (6.35 mm) in diameter
Applicable to difficult-to-access bores
Accurate measurement of feature geometry
Data archival for future analysis and comparison - can be used to track sub-critical crack growth over time
Can be integrated with other NDT technologies, such as eddy current

The Details:

A major airline contacted LTC regarding the possibility of adapting LSPI™ for crack detection in aircraft engine parts with high L/D holes. LTC was provided a set of low-cycle fatigue (LCF) test samples for initial evaluation of the LSPI™ technology. The challenge was to locate and accurately measure the length of every one of the small and tight cracks on the internal surface of a 0.25 inch (6.35 mm) bore. The crack indications ranged from approximately 0.018 inch (0.46 mm) to 0.050 inch (1.27 mm) in length.

LTC built a prototype LSPI™ sensor for evaluation on these small diameter deep holes. Several scans were taken on three of the test plates. All 18 indications (six indications per plate) were successfully detected using the miniature LSPI™ sensor.

The image below shows of three of the indications from LCF sample 1291-98. Because the software that was used for this test compresses the image into a square pattern, the aspect ratio of the indications appears distorted. However, a "zoomed" image of one of the features reveals its true shape.

A typical example of an LSPI™ crack indication is shown in the "zoomed" image. The scan shows a crack length of approximately 0.051 inch (1.3 mm). The records for the plate indicate that the visual measurement (using a microscope) of the crack was 0.049 inch (1.24 mm) in length. The FPI estimate (using the unaided eye) was estimated to be 0.045 inch (1.14 mm).

In all cases, the LSPI™ crack detection measurements were closer to measurements acquired using a microscope than those obtained using the conventional FPI method (unaided eye). The LSPI™ method has not only proven to be very sensitive, but it also has demonstrated excellent resolution. Features as small as 0.010 inch (0.25 mm) in length have been repeatably detected and mapped on laboratory samples.

This advancement in automated fluorescent penetrant inspection technology provides NDT professionals the ability to acquire digital, high-resolution images of features such as cracks, laps and porosity in a rapid and cost-effective manner. Using the LSPI™ technology, NDT professionals can remotely and automatically locate, measure and display quantitative images of surface-breaking cracks in small diameter bores and tubes. Most importantly, the LSPI™ method will provide operators with a tool that reduces the laborious and subjective nature of visually inspecting safety-critical parts. The net result will be improved quality of inspections and therefore, increased safety of operation.