Tech Briefs

Test method simulates, in a laboratory environment, the service conditions ceramic-matrix composite material would experience in turbine engine exhaust applications.

There is growing interest in developing, testing, and deploying ceramic-matrix composites (CMCs) into commercial and military aerospace gas turbine engines due to their durability under extreme conditions and weight-savings potential. For military applications, the focus is on the afterburning section of the turbine engine, including the flameholder, augmentor liner, and both the convergent and divergent segments of the exhaust nozzle. These are demanding applications because of the high temperatures and rapid thermal cycles.

Each year, a large number of exhaust nozzle divergent flaps and seals are replaced before reaching their scheduled life, causing a large maintenance burden. Engineering evaluation and lab testing have shown that premature distress of divergent seals is due to severe environmental exposure, including: high-temperature-induced creep deformation, rapid thermal cycles, severe thermal gradients, and high acoustic loading.

Illustration of a ply-to-ply angled interlock weave. A ply-to-ply angle interlock is used to reduce delamination sensitivity that is common for 2-D CMC materials and to minimize the loss of in-plane properties associated with many through-thickness orthogonal 3-D weaves.

CMCs are being evaluated to replace traditional nickel-based superalloys because they offer high-temperature capability, limited or no property debit with temperature, they have a density that is a third of that of the nickel-based superalloy, and they do not require cooling air. CMCs being considered for gas turbine applications cover a wide range of fibers and matrices, and are fabricated by chemical-vapor infiltration (CVI), sol-gel techniques, polymer infiltration and pyrolysis (PIP), and melt infiltration (MI). For exhaust nozzle divergent flaps and seals, there is a large body of work that was performed on CVI- and PIP-processed CMC materials. It was in 1997 that the United States Navy (USN) approved full production qualification of SiC/C CMC divergent flaps and seals for the F414 engine that powers the F-18-E/F Super Hornet.

A CMC system developed by SPS was evaluated for a divergent seal application. This material is SEPCARBINOX® A500, a C/SiC composite with a 3-D weave configuration for improved delamination resistance and a sequenced CVI matrix that is self-sealing for protection of the carbon fibers.

Material acceptance testing at the coupon level had already been completed and reported. A database of 65 coupons from nine production runs of A500 showed that the material consistently exceeded the acceptance criteria for ultimate tensile strength (UTS) for the application. The UTS of the material in the database is 252 ± 25 MPa, compared to the acceptance level UTS of 170 MPa. The final stage of testing involved what the industry calls “killer tests” – a series of tests involving aggressive environmental conditions. The aim was to identify and expose any material weaknesses prior to proceeding to ground testing and insertion into actual fielded applications. Such tests were performed on material coupons; this earlier testing involved a combination of two-hour dwell fatigue testing in both air and steam environments. Results showed that the A500 CMC consistently performed better than other C/SiC CMC systems.

As part of the effort to expand the aggressive evaluation of the A500 material, burner rig tests were undertaken at the USN test facility at Naval Air Systems Command (NAVAIR). Burner rig testing simulates the in-flight condition of applying an afterburner, as used on military aircraft engines for takeoff and reaching high-speed supersonic flight. The lighting of an afterburner results in a very rapid thermal spike of the exhaust nozzle hardware. The longer the afterburner is operated, the higher the temperature rises. In addition, aircraft are often operated near a marine saltwater environment. Therefore, in addition to the burner rig exposures, the test specimens were exposed to salt fog before and between intervals of burner rig testing.

Furthermore, some specimens were creep tested and others fatigue tested prior to the salt fog and burner rig exposure with the intention of “pre-conditioning” them by exposing them beyond the predicted in-flight mechanical loads in order to introduce additional matrix cracks.

This work was done by Larry P. Zawada, Universal Technology Corporation; Jennifer Pierce, UDRI; and Craig Przybyla, AFRL/RXCCP for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) here under the Test & Measurement category. AFRL-0262