Defect Testing: A Key Stage Of Pre

Mar 09, 2024

Each fan blade must be inspected for internal defects before it is approved for installation.

Modern jet engines improve aircraft efficiency by 10-20% thanks to advanced materials and precise machining. The fan at the front of the engine sucks large amounts of air for compression, combustion, and expansion through the engine core. The majority of air from the fan is bypassed through the secondary passage (outside the core), adding to the engine thrust.

The fast-rotating fan comprises carbon composite blades that result in lighter and more efficient engines, allowing airlines to save fuel by shedding weight.

Carbon-fiber composite blades are manufactured using heat-treated carbon fibers and specialized epoxy resin. The precise layering of fibers allows for improved impact resistance. Moreover, a unique form of glass fiber composite is used on the trailing edge to deflect impact stress from the blade.

Fan blades must be rigorously tested before they are approved for installation. Fan blade manufacturers utilize state-of-the-art equipment and machinery to inspect each blade for defects.

Non-destructive testing (NDT) refers to a wide group of analysis techniques used to evaluate the properties of a material, component, or system without damaging or altering it. Ultrasonic, radiographic, and penetrant testing are some of the most common examples of NDT used on fan blades.

Ultrasonic testing (UT) is based on the propagation of ultrasonic waves within the fan blades. Short ultrasonic pulse waves (of 0.1 to 50 MHz frequency) are transmitted into the blade to detect internal defects of the materials. Various quantities, including depth and thickness, are measured to monitor material inconsistencies.

While some defects can be inherited from the parent materials, others may arise during the manufacturing process. It is noteworthy that the UT produces the highest resolutions when performed on metals (including alloys). However, the technique can be used on concrete, wood, and composites.

Manufacturers such as General Electric uses such techniques to identify any anomalies in the composite materials used on fan blades. Based on the results, future materials and the manufacturing process can be improved to obtain greater yield. For example, GE has been able to increase the fan blade yield on the GE90 engine from 30% in the initial days of manufacturing to 97%.

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The radiography technique uses ionizing radiation to inspect materials and components which have the potential to fail engineering structures. Industrial radiography uses X-rays, gamma rays, or neutrons that are captured by specialized detectors to view the internal form of the fan blades.

Aside from welding and casting, it is also used in composite piece inspection. Fan blade manufacturers use this common technique for surface characterization or to identify inherent surface defects. According to GE,

The workers inspected every single blade with X-rays, ultrasound, laser, and other tools for defects.

The Penetrant Inspection (PI) is typically used to check surface-breaking defects in non-porous materials, including composites. The method involves the pressurization of the interior cavity of the blade with nitrogen gas. Hairline or other cracks force the loss of pressure in the cavity, detected by the sensor built into the root to detect changes in pressure.

Pl is also used to detect surface cracks and leaks due to casting, forging, and welding. It is noteworthy that a similar inspection method may be used during the operational maintenance of the engine.

What are your thoughts on the different types of defect tests for fan blades? Tell us in the comments section.

Writer - Omar is an aviation enthusiast who holds a Ph.D. in Aerospace Engineering. With numerous years of technical and research experience under his belt, Omar aims to focus on research-based aviation practices. Apart from work, Omar has a passion for traveling, visiting aviation sites, and plane spotting. Based in Vancouver, Canada