Fireproof Testing of Pressurized Acoustic Organic Composite Sandwich Structures

Organic matrix composites provide significant weight benefits over traditional metal based structures in turbofan and engine nacelle applications. The larger axisymmetric engine structures such as inlets, casing and ducts have... [ view full abstract ]

Organic matrix composites provide significant weight benefits over traditional metal based structures in turbofan and engine nacelle applications. The larger axisymmetric engine structures such as inlets, casing and ducts have noise and fire certification requirements which are progressively becoming more stringent. As a result, these structures must be acoustically treated and yet maintain their fireproof capability.

This acoustic treatment utilizes a sandwich structure consisting of a perforated inner skin, a honeycomb core and an outer skin. During engine operation the acoustically treated area serves to meet noise requirements while the outer skin contains the internal engine flow static pressure. During a fire event, the outer skin absorbs heat from the external flame and rapidly exceeds the glass transition temperature of the organic matrix composite material. The internal static pressure imposes a hoop load which can transition into a tensile membrane dominant load if the outer skin disbonds from the honeycomb core in the heat affected area. In this environment the outer skin fiber material temperature exceeds the material allowable and tensile rupture of the fibers occurs with subsequent loss of static pressure containment contrary to the certification requirements of a firewall.

Several fireproof strategies were explored including parasitic ceramic fabric, parasitic thermal blankets, increased outer skin thickness and a structurally integrated fire blanket. Each was fireproof tested and provided a solution which imposed added weight and cost to the baseline design.

Future research is required to find an optimized fireproof solution for pressurized acoustic organic matrix composite sandwich structures.