Andrzej Skorupa

Fatigue crack growth in the aluminium alloy D16 under constant and variable amplitude loading

Fatigue crack growth experiments were carried out on sheet specimens of the Russian alloy D16Cz (Al-Cu-Mg alloy). Constant-amplitude tests were performed at different stress ratios. Variable-amplitude tests included experiments with a single overload, periodic overloads and underloads, and the flight-simulation load history miniFALSTAFF. Fractographic observations were made to obtain information from striations and the development of shear lips. Results are presented and compared to literature data for 2024-T3. Attention is paid to incompatible crack front orientations and consequences for crack growth prediction models.

An Experimental Investigation on the Fatigue Performance of Riveted Lap Joints

Results of an experimental research on the influence of several factors on the fatigue behaviour of simple riveted lap joint specimens under constant amplitude loading conditions are presented. The variables considered are the rivet type and material, sheet material and the squeeze force. Also, the effect of sheet thickness staggering in the overlap region on the joint fatigue life is studied in the context of secondary bending. The measurements of the driven head dimensions for a range of squeeze force levels and fatigue test results for rivets installed with various squeeze forces indicate that the squeeze stress rather than the rivet driven head dimensions is a safe standard for the quality of the rivet installation. The superior fatigue performance of a rivet with the compensator compared to the round head and universal rivet is noted. The underlying reason is the better hole filling achieved due to the presence of the compensator. Fatigue lives observed for the staggered thickness specimens are consistently longer than for the standard specimens. The analysis of these preliminary results suggests that a primary reason for the improved fatigue performance is the reduction of secondary bending.

An Experimental Investigation on Crack Initiation and Growth in Aircraft Fuselage Riveted Lap Joints

Effects of variables related to design and production of riveted lap joints representative of longitudinal sheet connections for a pressurized transport aircraft fuselage were experimentally investigated. The specimens from an aircraft Al alloy D16 Alclad sheets of three different thicknesses (1.9, 1.2 and 0.8 mm) were assembled under load control using round head rivets and rivets with the compensator from a P24 Al alloy. For the joints from 1.9 mm thick sheets fatigue tests indicated a dependency of the crack initiation site and crack path on the squeeze force level and on the rivet type. At the same time, increasing the squeeze force led to improved fatigue properties of the joints, specimens assembled using the rivets with the compensator showing fatigue lives consistently longer than joints with the round head rivets. All observed trends have been explained based on hole expansion and load transfer measurements. For thin sheets connected using the round head rivets, local deformations and indentations under the driven rivet head promoted crack initiation and failure in the adjacent sheet. Fatigue test results indicated that the detrimental effect of this type imperfections could outweigh the benefits associated with a decrease in secondary bending due to thinning the sheets. The rivets with the compensator were observed to cause significant local imperfections beneath the manufactured head, which adversely affected the joint fatigue performance.

Influence of pressure on tribological properties of multilayer surfacing welds of bronze CuSn6 padded onto steel substrate by means of the MIG method

This article presents the results of testing the influence of pressure on the tribological properties of single- and double-layer surfacing welds of bronze CuSn6 applied onto steel substrate by means of the MIG welding method. The welds have been created with fixed technological parameters of the surfacing process, which guaranteed an adequate quality of the welds and low content of substrate material in the weld, while ensuring sufficient adherence between the weld and the substrate. The tribological tests were conducted using the T-05 tribotester, and the results were compared with similar results obtained for cast bronze of the same chemical composition. Friction and linear wear coefficients of joints intended for sliding applications were determined in the lubricating environment of mineral and synthetic oils with pressures of 3 and 10 MPa. The tests with higher loads were designed to indicate how much pressure increase influences the tribological properties of the tested joints. The test results have shown a dependence of the friction coefficient on the number of layers in the surfacing weld and on the type of lubricating environment. Changes in the linear wear were observed for different loads and for single-layer samples compared with double-layer samples and comparative samples.

Applicability of Empirical Formulae for the Fatigue Notch Factor to Estimate Riveted Lap Joint Fatigue Strength

A semi-empirical fatigue life prediction model under development by the present authors for riveted lap joints used in aircraft structures is outlined. In contrast to existing models, it will account for the influence of the rivet squeeze force on the fatigue life of riveted joints. To determine the effect of rivet-hole interference on the fatigue behaviour of a riveted joint, a series of fatigue tests on filled hole coupons with different amounts of interference will be carried out under loading conditions representing the bypass load, transfer load and secondary bending. These experiments will allow evaluating of the dependency of the fatigue notch factors on rivet hole expansion. Preliminary results obtained so far are presented in this paper.

Load Transfer in Lap Joints with Mechanical Fasteners

The role of a fastener is to transfer the load from one sheet to another sheet in the overlap region. For a configuration with more than one row of fasteners, the applied force P is split at the first row into the bypass load (T BP), which remains in the sheet, and the transfer load (T TR) transmitted to the other sheet, as schematically shown in Fig. 5.1. The T TR-load is comprised of a bearing force (T BR) resulting from the bearing pressure exerted by the rivet shank on the hole surface and a friction force (T FR) induced by friction between the mating sheets. Friction is localized mainly beneath the rivet heads where maximum clamping occurs. Works reviewed in the present chapter reveal that, for well-tightened fasteners, frictional forces can transmit a significant portion of the transfer load.

Multiple-Site Damage in Riveted Lap Joints – Experimental Observations

The present chapter is divided into two sections. In Sect. 8.1, two passenger aircraft accidents due to multiple-site damage (MSD) of riveted lap joints are described in detail. Section 8.2 presents some results of experimental investigations into MSD performed on full-scale fuselage panels and lap joint specimens.

Riveted Lap Joints in a Pressurized Aircraft Fuselage

The contemporary transport aircraft fuselage is a skin structure supported by frames and stringers, Fig. 1.1. The skin and the stiffening elements carry the flight loads including those due to cabin pressurization. The stringers are joined by riveting, bonding or spot welding directly to the skin, as exemplified in Fig. 1.2. In order to connect the skin with the frames, two different solutions can be applied. One of these, addressed as the shear-tied frame, involves attaching the frame directly to the skin and tear straps, Fig. 1.2. Another possibility, illustrated in Fig. 1.3, is to connect the frames only to the stringers by means of stringer clips. This solution is referred to as the floating frames. The internal tear straps, Fig. 1.4, located at and sometimes also between the frame stations, force longitudinal skin cracks (parallel to the fuselage axis) to turn circumferentially. If this situation takes place between the tear straps, a segment of the skin bends back creating an opening. This phenomenon, described as “flapping”, is a safe failure mode that limits the failure to the affected bay only (Kosai et al. 1992). A thin skin will flap more easily than a thick one (Maclin 1991). More information on flapping one can find in Swift (Swift 1987). Some aspects are also considered further on in this book. The straps can carry the load of the cracked skin. These crack stopper bands, although undesirable from the production point of view, are applied in several types of aircraft. Sometimes, instead of the fail-safe straps, waffle pattern doublers bonded to the skin are used, Fig. 1.5. Different types of crack stopper bands (integral, riveted and bonded made of the 2024-T3 and 7075-T6 Al alloys, a Ti alloy and ARALL) and their capability to cause crack growth retardation are reported by Schijve (1990).

Crack Initiation Location and Crack Shape Development in Riveted Lap Joints – Experimental Trends

In this chapter the nucleation and shape development of cracks in longitudinal lap joints under static and fatigue loading will be considered. The issues of special attention are the influence of a squeeze force on the mode of fatigue failure and the significance of fretting on a sheet interface for fatigue crack initiation.

Secondary Bending for Mechanically Fastened Joints with Eccentricities

In the case of joints eccentric with respect to the load path, like lap joints or single strap butt joints, out-of-plane deformations of the sheets referred to as secondary bending (SB) occur under nominally tensile loading. Maximum moments due to SB are induced at load path eccentricities, namely at the rivet rows. As said earlier, for a lap joint with more than two rivet rows, the maximum bending moment is always induced at the outer rows. The out-of-plane deformation of a lap joint with three rivet rows associated with SB is shown in Fig. 6.1a. The largest tensile stresses due to SB are produced along the faying surface at location A of sheet 1 and location B of sheet 2. Because outside the outer rivet rows either sheet carries the full applied tensile load, A and B are also locations of the maximum combined tensile stresses and, therefore, the most commonly observed fatigue crack initiation sites.