Determination of the out-of-plane tensile strength using four-point bending tests on laminated L-angle specimens with different stacking sequences and total thicknesses

Laminated composite materials are increasingly used for the design of aircraft primary structures subjected to complex 3D loadings. The delamination observed in curved parts ensuring the junction between the different perpendicular panels is one of the most critical failure mechanisms. The present article proposes a complete protocol to identify the out-of-plane tensile strength of specimens composed of unidirectional plies. Firstly, a method to design a four-point bending (4 PB) test on L-angle specimens has been proposed. Secondly, a test campaign on T700GC/M21 laminated L-angle specimens has been performed at ONERA. Thirdly, the analysis of these tests with different methods has been performed to demonstrate that such a test is relevant to determine the material out-of-plane tensile strength, which seems to be independent of the stacking sequence and of the total thickness of the specimen, thus allowing the use of this strength in a 3D failure criterion. Finally, the different advantages and drawbacks of 4 PB tests performed on curved beams are discussed.