Split Sleeve Cold Expansion of High Strength Aluminum Alloys; Lessons Learned

High strength aluminum alloys, 7050, 7010 and 7085, are less susceptible to stress corrosion cracking, have higher strength and fracture toughness but have low elongation properties associated with the L-S grain direction.  Machining large structural components from solid billets means that structural attachment holes may be in this short transverse (ST) plane.  Holes are a high stress concentration subjected to the same fatigue issues identified in more typical aluminum alloys.  Low elongation properties in the ST plane is an issue when installing high interference fit fasteners, bushings, or when enhancing the fatigue and damage life by split sleeve cold expansion.  Cold expansion induces beneficial residual compressive stress around the hole shielding it from cyclic tensile loads.

 

The nature of the cold expansion process, radial expansion (typically 3.5% to 5%) coupled with the axial pull of the mandrel through the sleeve in the hole, induces a bi-axial strain in the material, which is magnified in the region of the sleeve split. Cracks have been observed at the bore of the hole following cold expansion in holes drilled in the ST plane.  The cause is attributed to the high shear stress/strain perpendicular to the short-transverse grain direction in the longitudinal grain direction.  Most frequently observed cracks developed at 45º to the L and S direction and was most prevalent when the sleeve split was placed in this direction.  The split being placed parallel to the L direction, or perpendicular to the S direction, followed this.  Conversely, the split was placed towards the L direction, reduced the propensity to crack; especially when coupled with a reduced applied expansion.

 

Considerable experimentation and analysis of this cracking phenomenon, including the impact on fatigue life even if these small cracks or “shear tears” are evident, has been conducted.  The need to fatigue life enhance these holes still exists and this paper will discuss the nature of the problem, what has been tried to reduce or mitigate the problem such as reduced applied expansion and changing the strain rate of the process, the results of these initiatives, the current position and lessons learned.