— Fatigue tests under spectrum loading were conducted to evaluate hole cold-expansion in thick-section open-hole aluminium alloy specimens, some of which contained residual fatigue cracks before cold expansion. Cold expansion resulted in an increase in life by a factor of about 7. Small residual fatigue cracks did not inhibit the effectiveness of the cold-expansion process, indicating that it may not be essential to remove such cracks prior to hole cold-expansion. The increase in life is primarily associated with a reduced crack propagation rate compared with that for cracks from non-cold-expanded holes. Cold-expanded hole fractures displayed a marked disparity in crack depths adjacent to the two faces of the specimens. Considerable differences were evident in crack depths and fatigue crack areas at failure between cold-expanded and non-cold-expanded hole specimens. These findings have ramifications in the damage tolerance assessment of aircraft structures.

Fatigue of thick‐section cold‐expanded holes with and without cracks

The influences of both hole cold expansion and interference-fit fasteners for extending the fatigue life of multi-layer aluminium alloy joint specimens under variable-amplitude loading have been examined experimentally. Improvements in fatigue life were markedly dependent on the degree of load transfer in the specimen joint. Secondary bending in the joint also had a major influence, reducing the effectiveness of these life improvement techniques. Depending on the joint configuration, improvements in fatigue life ranged from nothing at all to a factor of about 40. Fretting appeared to be involved in the initiation of all cracks and where this occurred on faying surfaces only small, if any, improvements in fatigue life were obtained.

Extending the fatigue life of multi-layer metal joints

The split-sleeve cold expansion process has been used successfully for over 20 years to extend the fatigue life of holes in aircraft structures. Cold expansion technology can also be applied to enhance engine low-cycle fatigue (LCF) performance in both production and repair applications. Specific test data are presented showing that fatigue life extension can be attained by cold expansion of holes in a wide range of situations (including non-round hole geometries and low edge margins), and in components subjected to high operating temperatures. A cold expanded bushing system is compared to standard shrink- fit bushing installations. Finally, two case studies are used to illustrate the application of cold expansion to full-scale engine components.

Extending the Fatigue Life of Aircraft Engine Components by Hole Cold Expansion Technology

An advanced procedure has been developed to analyse the fatigue crack growth in the residual stress field. The method is based on an extension of the strip yield crack closure model for part-through crack problems. In this method, the residual stress relaxation due to both the fatigue loading and the crack growth is considered. It has been shown that the effect of the residual stress can be reasonably analysed according to fracture mechanics methods so long as the residual stress field and its redistribution can be correctly evaluated and the elastic-plastic crack closure analyses are performed. Several examples are provided for the constant and variable amplitude loading conditions.

A strip yield analysis of fatigue crack growth in the residual stress field

: The detection of fatigue cracks at bolt holes in the main spar of the Mirage III wing during full-scale fatigue tests led to a requirement for refurbishment procedures to extend the fatigue lives at a number of critical locations. One of these, which is covered by this investigation, was the spar lower front flange. Flight-by-flight fatigue tests have been carried out to determine the relative fatigue performance of aluminum alloy bolted joint specimens of 28 mm thickness incorporating close-fit bolts, interference-fit bolts (0.4%), hold cold-expansion (3%), interference-fit steel bushes (0.3%) and a combination of cold-expansion and interference-fit bushes. Fractographic measurements of crack development from the bores of holes in specimens incorporating close-fit bolts in non cold-expanded (reamed) and cold-expanded holes clearly indicated much slower fatigue crack propagation rates from the cold-expanded holes until the crack length was close to the nominal region of transition from the residual compressive to tensile stress zone around the cold- expanded holes. Fatigue tests on cold-expanded hole specimens at different spectrum scaling factors indicated that, under the loading sequence used, each 10% increase in stress reduced the life to about half that at the lower stress level.

The Use of Interference-Fit Bolts or Bushes and Hole Cold Expansion for Increasing the Fatigue Life of Thick-Section Aluminium Alloy Bolted Joints

Flight simulation tests on 2024-T3 and 7075-T6 lug specimens indicated significant fatigue life improvements obtained by expanding the holes in accordance with the split-sleeve cold work method. Geometrical distortions induced by the expansion were measured. The significance of residual stresses is analysed with reference to depth of penetration of cold work, to relaxation of residual stress by cyclic loads and to some other practical aspects.

J. Schijve

Papers

Flight simulation fatigue tests on lugs with holes expanded according to the split-sleeve cold work method