Damage tolerance of a hybrid lower wing bonded stiffened panel with a Fiber Metal Laminate skin

Abstract

Fiber Metal Laminates (FML) represent a highly damage tolerant option for lower wing skins that can be included in an advanced high performance metallic wing box. The purpose of the present study is to assess the damage tolerance behavior of a panel made of FML with 0.8 mm 2024 T3 sheet and bonded stringers.A 5-stringer test panel, 800 mm wide, is tested at R=0.1 at a maximum stress of 135 MPa. This stress conservatively represents a spectrum with a sustained flight stress (1 g stress) of 90 MPa corresponding to a 20% increase versus a conventional 2024 T351 lower wing allowable stress.Crack growth from an initial defect is monitored using Thermal Surface Analysis and visual inspection. The propagation life between the chosen detection event and the residual strength test is 137580 cycles. This detection event is a broken stringer; it occurs after the crack in the FML skin has reached a 39 mm length. The residual strength exceeds the required limit load. The total life from crack initiation at the notch to the crack length reached before the residual strength test is 220000 cycles.Combining the stress allowable increment and density reduction, the test justifies a 25% weight saving potential versus a lower wing in 2024 T351, with 45000 flight cycles inspection intervals and 90000 flight cycles Design Service Goal.