AFRL Completes CMC Aft Fairing Heat Shield Subcomponent Testing

  • Published
  • By Materials and Manufacturing Directorate
  • AFRL/ML
WRIGHT-PATTERSON AIR FORCE BASE, Ohio --  As part of a Dual-Use Science and Technology (DUS&T) program conducted with Boeing, AFRL researchers successfully completed the design and preliminary test of an oxide-oxide ceramic matrix composite (CMC)-based structure intended for use as an aft fairing heat shield (AFHS) on military and commercial aircraft. Incorporating the CMC material into the product design would facilitate construction of lighter and less costly AFHS structures.

High-temperature turbine engine exhaust washes over the aluminum struts that hold engines in place on both commercial aircraft (such as the Boeing 777) and military aircraft (such as the C-17). Because this thermal exposure can eventually cause the aluminum struts to fail, engineers use the AFHS structure to protect the struts from the exhaust. The AFHS is most commonly made of titanium, which is an expensive material. AFRL and Boeing entered into the DUS&T effort to study the suitability of a CMC-based AFHS. Boeing also teamed with Teledyne Scientific and ATK-COI Ceramics, Inc. (COIC)--each an organization with considerable experience in the area of advanced CMC design and development.

A COIC-manufactured oxide-oxide CMC became the prime candidate for meeting the AFHS service requirements. The team conducted extensive thermal stress and acoustic load modeling of various AFHS designs that incorporated the identified CMC material. This modeling activity eventually converged on a design that exhibited adequate mechanical properties and also met weight- and cost-related objectives. Using the COIC-produced material and the chosen design, engineers fabricated an AFHS subcomponent and associated T-joints. (A T-joint is a right-angle-shaped joint having no additional layers of composite reinforcement.) The researchers then conducted mechanical testing of the material panels.

The team devoted significant effort both to devising optimal T-joint design and to developing mechanical test methods capable of accurately evaluating those joints. The researchers ultimately found a pi-joint arrangement to be the best configuration. (A pi joint is a right-angle joint having an extra layer of reinforcement on each of its sides. This extra layer is commonly termed a "doubler.") Mechanical testing of the pi joints included tensile loading, which yielded the most revealing results. During tensile load testing, a test machine grips the joint at both ends, pulling until the joint delaminates from the doubler. The test provides information on proof stress, as well as tensile and interlaminar strength.

The team's successful modeling, mechanical testing, and subcomponent fabrication demonstrated the viable use of an oxide-oxide CMC structure as an AFHS. This work may lead to larger-scale testing and, eventually, the manufacture and aircraft testing of a full-scale CMC AFHS.