Fracture Mechanics Analyses of Reinforced Carbon-Carbon Wing-Leading-Edge Panels

Fracture mechanics analyses of subsurface defects within the joggle regions of the Space Shuttle wing-leading-edge RCC panels are performed. A 2D plane strain idealized joggle finite element model is developed to study the fracture behavior of the panels for three distinct loading conditions – lift-off and ascent, on-orbit, and entry. For lift-off and ascent, an estimated bounding aerodynamic pressure load is used for the analyses, while for onorbit and entry, thermo-mechanical analyses are performed using the extreme cold and hot temperatures experienced by the panels. In addition, a best estimate for the material stressfree temperature is used in the thermo-mechanical analyses. In the finite element models, the substrate and coating are modeled separately as two distinct materials. Subsurface defects are introduced at the coating-substrate interface and within the substrate. The objective of the fracture mechanics analyses is to evaluate the defect driving forces, which are characterized by the strain energy release rates, and determine if defects can become unstable for each of the loading conditions. EINFORCED carbon-carbon (RCC) is used in the wing-leading-edge (WLE) panels of the Space Shuttle orbiter wings. The RCC in the panels provides thermal protection from the intense heat during entry of the Orbiter Vehicle. RCC is a composite laminated structure and consists of substrate material between two layers of coating. During manufacturing, craze cracks form in the coating. Each of the RCC panels has two joggle regions, which aid in assembly of the WLE. The regions between the joggles of the panels are called acreage regions. During the lift-off and ascent phase of flight, the panels experience mechanical loads and negligible thermal loading. During the on-orbit phase of flight, the panels experience a severe cold soak, with negligible mechanical loads, and during entry, the panels experience extreme heating, with negligible mechanical loads. As such, the liftoff and ascent phases are predominantly mechanical loading cases, while the on-orbit and entry phases are predominantly thermal loading cases. Thus, mechanical loading and thermal loading cases can be analyzed separately.