Early Detection Prevents Destruction

Real-time crack detection system detects component and tooling damage during low-cycle fatigue testing.

The development of jet engines and other high-speed machinery requires extensive testing to establish the integrity and useful life of high-speed rotating components.

A fundamental evaluation technique for turbine engine rotors is to empirically test and validate the number of cycles the rotors can withstand, either to a predetermined number or all the way to failure. This type of test is typically referred to as a "Low Cycle Fatigue" (LCF) test.

During a standard LCF test, an engine rotor component is cycled up and down in speed in a spin rig in order to validate a prescribed number of cycles. Traditionally, these LCF tests end in either successful completion of the full cycle count, or in a complete rotor burst caused by a fatigue-induced crack in the part. This often results in loss of the component and can also cause damage to tooling and other test facility equipment.

In order to avoid premature component burst, either from crack formation in the component itself or the tooling (the arbor, or spindle that holds the component in place), Test Devices Inc. (TDI) has developed an application known as the Real-Time Crack Detection System (RT-CDS), for detecting the initiation of cracks during LCF tests.

TDI's exclusive Real-Time Crack Detection System automatically detects fatigueinduced cracks during an LCF test with 80% to 90% accuracy. When the RT-CDS detects a crack beginning to develop, it automatically stops the LCF test before component failure. This exclusive crack detection interlock capability allows an attended or unattended LCF test to be halted when components or tooling develop cracks - before a damaging rotor burst event.

No other LCF test method used in the industry employs Real-Time Crack Detection.

Conventional LCF procedures require that the testing is stopped periodically in order to inspect for cracks in the component and tooling. This involves removing the test article from the spin rig frequently, thereby greatly extending the cost and schedule of the testing program. In contrast, the Real- Time Crack Detection System can reduce the number of required inspections, thereby enabling more cycles between inspections.

Real-Time Crack Detection in Jet Engine Aplication

A major aerospace jet engine OEM recently ran a series of LCF tests on a critical component, which involved the testing of multiple sets of rotors. The company contracted TDI and another vendor to perform testing on two rotor sets each. TDI used its Real-Time Crack Detection System for its testing and monitoring, while the other vendor used a conventional LCF testing procedure.

TDI and the alternate vendor began their testing at the same time. Early in the testing procedure, the OEM notified TDI that one of the rotors at the alternate vendor was destroyed at approximately 80% of its expected life because of a crack rupture in the spin tooling hardware that was designed and manufactured by the OEM. The test component was destroyed and the spin test equipment incurred substantial damage.

During TDI's testing, however, the Real- Time Crack Detection System detected a signature indicative of a crack and automatically stopped the test. The customer then performed a component ultrasonic inspection and found two cracks. Early detection by the RT-CDS prevented the destruction of the part, tooling, and equipment. If the cracks had not been discovered by the RTCDS, the test article would have been lost, without preserving the initiation site and crack face (which are critical for follow-up metallurgical analysis).

Fatigue may cause cracks in the part or a tool, which can lead to the component bursting.

TDI's Real-Time Crack Detection System again proved its effectiveness in subsequent testing. A second rotor failed at the alternate vendor after approximately 70% of its expected life as a result of an undetected crack in the OEM tooling. In contrast, TDI's Real-Time Crack Detection System detected and stopped LCF testing of a second rotor when a crack occurred in the tooling, again before any damage was done.

An additional set of spin tooling was provided by the OEM and LCF testing resumed on the same rotor in a short amount of time. The two rotors that were tested using conventional methods at the alternate vendor were destroyed as a result of fatigueinduced cracks in the tooling. As a result, the engine manufacturer lost two valuable components that could have been saved and testing completed had they used TDI's Real-Time Crack Detection System. The approximate loss to the OEM as a result of the lost components and associated test time was $2 million.

Because of the RT-CDS system's capabilities, the customer decided to redirect testing of additional rotors from the alternate vendor to TDI.

Real-Time Crack Detection Provides Real Benefits

By detecting cracks in the rotors before they failed, the RT-CDS preserved the substantial test costs and prevented damage to the valuable rotors under test and the test equipment. The RT-CDS can save time by reducing the number of periodic inspections, while at the same time eliminating the risk of component and tooling failure.

The result is a significant cost savings by preserving pre-failure test cycles, reducing the frequency of test interruptions, saving valuable test components, and avoiding time lost in replacing damaged components and equipment.

In addition, the RT-CDS can provide valuable initiation and crack growth analysis data that gives customers the ability to determine the location, mode, and cause of a crack. This information is nearly impossible to obtain if the part is destroyed in a premature burst during conventional LCF testing.

The Real-Time Crack Detection System contributes significantly to the knowledge base of the fatigue life of spinning rotors and other application-critical components, and ultimately to the safety of the products in which these components are installed.

The Real-Time Crack Detection System can be used on a variety of equipment including jet engine rotors, turbochargers, rocket pumps, electric motors, gas turbine rotors, and compressor rotors. TDI's RTCDS can be employed during outsourced testing at its facility in Hudson, MA, or it can be installed at a customer's site and monitored remotely by TDI engineers.

Test Devices Inc.
Hudson, MA
testdevices.com