Testing of a cocured compsite frame panel by the applications of vacuum as an alternate to frame bending test

icaf2023 Tracking Number 143

icaf2023 poster

The Frame Bending Test (FBT) of fuselage panels is plagued by complex design at load introduction regions, high workload for assembly of specimen to test rig and the need for disassembly for access to stiffened structure. An alternative to the FBT was explored by the application of vacuum on skin side of panel using a metallic fixture while frame side of panel is subjected to atmospheric pressure. The vacuum level can be controlled to obtain the desired differential pressure. A curved composite panel was designed with three cocured corrugated frames and eight stringers under the InFuSe (Integral Fuselage Shell Concepts) Project between CSIR-NAL and Airbus. The shear clips, to stabilise the frame web laterally, were eliminated by the corrugation of web. A metallic fixture was developed to mount the panel to enable application of vacuum. The finite element (FE) analysis of panel mounted on fixture was carried out to understand the structural response. The desired circumferential strains in panel were achieved by the proper sizing of vacuum fixture. The location of strain gauges, Digital Image Correlation (DIC) regions and dial gauges were guided by FE analysis. Acoustic Emission (AE) was also monitored during the test. Two vacuum tests were carried out with vacuum levels of 100mbar and 20mbar. The structural responses were measured both during loading and unloading. The first test was stopped at 100mbar vacuum pressure because of increased AE activity. Post-test ultrasonic scan of cocured joints showed disbonds in the Frame and Stringer crossover regions in proximity to metallic fixture. Rivets were installed on disbonds to prevent the further growth. Subsequently, the panel was loaded up to 20mbar vacuum pressure and the panel withstood the vacuum pressure successfully. Ultrasonic scan was carried out on cocured joints showed no disbonds. The structural response in terms of deflections and strains were correlated well with FE simulations. The proposed Vacuum Test has advantages like smooth and uniform load introduction, quick assembly and economical. It also allows quick access to specimen for DIC, NDE and other sensors on frame side during the test and presents itself as an alternate to FBT.