Airframe structures and components on many existing and future Air Force aerospace systems require operation in elevated temperature. Examples include hypersonic vehicle airframes, engine related components (such as engine ducts, engine vanes, and exhaust flaps), and hot trailing edges of B-2 and C-17 wings. Material systems that show improved fatigue performance, excellent thermal resistance, and damage tolerance are prime candidate materials for potential air vehicle structural components. Polymer matrix composites (PMCs) and ceramic matrix composites (CMCs) are two types of composites used in aircraft structures subjected to high temperatures. The polymer matrix in most PMCs cannot withstand the temperatures required for many aerospace structural applications. Therefore, either improvements in temperature capability of polymer matrix materials or developing novel thermal protection systems are desired for elevated temperature applications. Any new material system intended for aerospace applications must be studied and tested to verify that the mechanical properties are sufficient for use in the operating environments. This study investigated the mechanical properties and tension-tension fatigue behavior of two newly developed material systems for use in structures subjected to elevated temperatures, namely a 2D weave PMC and a 2D weave unitized composite (or PMC/CMC, consisting of a PMC co-cured with a CMC layer to act as a thermal barrier). These two material systems are two of three new composites developed under contract through the Air Force Research Laboratory (AFRL) and investigated during a master’s thesis research program at the Air Force Institute of Technology (AFIT) [1].
The 2D PMC investigated in this effort consisted of an NRPE (a high-temperature polyimide) matrix reinforced with carbon fibers. The fiber architecture of the PMC was an 8 harness satin weave fiber fabric. The PMC portion of the unitized composite had the same constituent properties and weave as the aforementioned 2D PMC. The CMC layer consisted of a zirconia-based matrix reinforced with an 8 harness satin weave quartz fiber fabric. For both material systems (PMC and PMC/CMC), material properties were investigated for both on-axis [0°/90°] and off-axis [±45°] fiber orientations. Tensile properties were evaluated at (1) room temperature and (2) with one side of the specimen at 329 °C and the other side exposed to ambient air. Tension-tension fatigue tests were conducted at elevated temperature at a frequency of 1.0 Hz with a ratio of minimum stress to maximum stress of R = 0.05. Fatigue run-out for this effort was defined as 2×105 cycles. Elevated temperature had little effect on the tensile properties of both material systems with the 0°/90° fiber orientation; however, specimens with the ±45° fiber orientation exhibited a significant increase in failure strain at elevated temperature. The ultimate tensile strength (UTS) of the 2D PMC with the ±45° fiber orientation decreased slightly at elevated temperature, but the UTS of the unitized composite with ±45° fiber orientation showed no significant change. The unitized composite did not exhibit an increase in tensile strength and stiffness compared to the 2D PMC. However, the 2D PMC with ±45° fiber orientation produced significantly greater failure strain. The 2D PMC showed slightly better fatigue resistance than the unitized composite with the 0°/90° fiber orientation. For the ±45° fiber orientation, the fatigue limit for the 2D PMC was approximately two times greater than that for the unitized composite. Microstructural investigation of tested specimens revealed delamination in the 2D PMC and very severe delamination in the unitized composite.
