Fatigue of Thick‐section Cold‐expanded Holes With and Without Cracks

Abstract

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.

Relaxation of Residual Stresses at Cold‐worked Fastener Holes Due to Fatigue Loading

“…Distance from Hole (mm)5. Residual hoop stress distributions at reamed 4 % FTI expanded holes cycled at the fatigue limit.…”

Numerical and experimental study of the cold expansion process in 7085 plate using a modified split sleeve

“…Various methods are used to measure this stress field, such as mechanical [2,3], optical [4] and diffraction techniques [5][6][7], confirming the presence of a compressive stress field around the hole. Moreover, many studies (e.g., [8][9][10][11]), show that cold expansion improves the fatigue life of hole containing specimens subjected to cyclic loading. Cold expansion can be applied during manufacturing or repair of airframes components [12].…”

Residual stress field and reduction of stress intensity factors in cold-worked holes

“…Subtopics considered include the consideration of thickness effects [5][6][7][8][9][10][11][12][13], the consideration of eventual pre-existence of cracks of various sizes before hole expansion is carried out [8], the possible re-cold-working of already cold-worked holes [14], and the stress analysis of neighbouring cold-worked holes [15].…”