AFRL completes testing of CMC aft fairing heat shield subcomponents Published Nov. 8, 2007 By Mindy Cooper AFRL Materials and Manufacturing Directorate WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- The Air Force Research Laboratory's Materials and Manufacturing Directorate here, under a Dual Use Science and Technology program with Boeing Company, has successfully completed design and the first steps of testing an oxide-oxide ceramic matrix composite material for use as aft fairing heat shields on military and commercial aircraft. "High temperature turbine engine exhaust washes over the aluminum struts which hold the engines in place on commercial aircraft such as the Boeing 777 or military aircraft such as the C-17," said Dr. Randal Hay, senior materials research engineer who managed the program. "Eventually, this thermal exposure could cause the aluminum struts to fail. For this reason, the struts are protected from the exhaust with a structure called an aft fairing heat shield." The AFHS is most commonly made of expensive titanium. Boeing is interested in using a ceramic matrix composite (CMC) that would reduce both the cost and weight of the AFHS. "RX (Materials and Manufacturing Directorate) entered into a Dual Use Science and Technology program with Boeing to study the suitability of a CMC-based AFHS," said Dr. Hay. "Boeing also teamed with ATK-COI Ceramics Inc. (COIC) and Teledyne Scientific, organizations with considerable experience with advanced CMC development and design." An oxide-oxide CMC manufactured by COIC was identified as a prime candidate to meet the service requirements of the AFHS. Extensive modeling of thermal stress and acoustic loading of various AFHS designs using the CMC was conducted. The modeling eventually converged on a design with adequate mechanical properties and which also met the weight and cost objectives. An AFHS sub-element was fabricated of the chosen CMC, and T-joints and panels of the material were mechanically tested. "T-joints, joints that are right-angle shaped but have no additional layers of composite reinforcement, are mechanically critical to the AFHS because they hold the structure together," Dr. Hay explained. "The T-joints in some cases have the highest stress in the AFHS. For this reason, it was imperative to optimize the design." A significant effort was devoted to the optimal T-joint design and development of mechanical test methods to accurately evaluate the joints. A Pi-joint configuration was found to be the best configuration. A Pi joint is a right-angle joint that has an extra layer of reinforcement on each side of the joint. This extra layer is commonly termed a "doubler." According to Dr. Hay, the Pi joints underwent mechanical testing that included tensile loading, which proved to be the most revealing. During this test, the joint is gripped at both ends in a test machine and pulled until the joint delaminates from the doubler. The test provides information on proof stress, tensile strength, and interlaminar strength. The modeling, mechanical testing, and successful sub-element fabrication demonstrated that an oxide-oxide CMC structure is viable as an aft fairing heat shield. This work may lead to testing on a larger scale, and eventually a full scale CMC AFHS may be manufactured and tested on an aircraft.