Differences Between the Fatigue Behaviour of Longitudinal Lap Joints in a Pressurized Fuselage and Laboratory Lap Joint Specimens

As already said earlier, a primary loading for the fuselage skin is hoop tension. However, the total effect of pressurization is a combination of hoop and longitudinal tension and local out-of-plane bending of the skin, the so-called pillowing. Besides the circumferential pillowing between the stiffeners shown in Fig. 2.1a, pressurization also causes a longitudinal pillowing of the skin and stringers between the frames, Fig. 2.1b. Because the stringers hardly restrain the skin pillowing, this effect will be strongly dependent on the type of the stringer-frame connection, for example the shear-tied frame or “floating” frame, cf. Figs. 1.2 and 1.3. Due to the pillowing, the hoop stress is not uniformly distributed between the frames. The loading complexity is additionally increased by the tear straps. A comparison between the measured and predicted (from FE analysis) membrane hoop stresses in a panel typical for the Boeing narrow-body fuselage airplane is shown in Fig. 2.2. The structure consists of “floating” frames and riveted tear straps. The data are for the location along an axial line adjacent to the lap joint. The stresses are normalized by the nominal hoop stress in an equivalent unstiffened cylinder of the same radius R and lap joint skin thickness t. The maximum stress equal to 80% of the nominal stress occurs midway between the tear straps, while the stresses in the vicinity of the tear straps are much lower.