The concept of crenellations was proposed by ALCAN five years ago with the presentation of simulation results. Tests on mid-size demonstrators (typically 1-m wide stiffened panels) have shown that over-thicknesses in the skin retard the crack largely so that a 15 to 25% weight reduction potential is achieved. The crenellations were shown to work under variable amplitude loading, for bottom wing and upper wing alloys, as well as on welded fuselage sections. Tests were conducted internally on a 1000 KN, a 2500 KN machine, and in external labs including the GKSS.

 The experience gained from several series of tests allowed derivation of analytical rules to help optimize the design of crenellations.

 The concept of local over-thicknesses can also be interesting in compression as is demonstrated with sub-stiffened panels (stiffened panels having small intermediate pad-ups of different geometries stabilizing the skin). Tests on sub-stiffened panels were made at ALCAN and in collaboration with the Queen’s University Belfast.

 Bonding, in particular of top-hat stringers was known as an efficient concept and merits reconsideration. Tests on panels with bonded stringers show a 15% benefit in compression and large benefit in fatigue crack growth over riveted or integral designs. In addition, bonding of aluminum-lithium alloys does not bring any specific issue.

 A five years program of demonstrators testing shows that combining new alloys and new concepts can improve the weight of aluminum aircraft structures by about 25% versus the conventional 2000 technology.