Aircraft designers typically use hole centerline spacing of 4 times the diameter (4D) of the hole between adjacent holes. In some circumstances however the spacing may be less to accommodate a repair or to ensure adequate load transfer. Sometimes in repairs, holes may be inadvertently drilled close to another hole; or even mis-drilled beside another hole. If these holes are in fatigue sensitive locations, the question then arises, can you split sleeve cold expand these closely spaced holes as part of a structural requirement to increase the fatigue life and damage tolerance of the structural assembly or to incorporate a terminating repair solution. Concerns have been raised about the possible interaction of the residual compressive and tensile stresses induced by the cold expansion process when the holes are in close proximity to each other.
As shown in Figure 1, the residual compressive stress zone around a cold expanded hole typically extends up to one diameter around the hole with a balancing tension field extending beyond that. The tension field is equal to about 10 to 15% of the tensile yield stress of the material at the compressive stress boundary which rapidly diminishes further from the hole. Holes of 4D spacing typically will not leave residual tensile stresses at the edge of the adjacent hole, however as holes get closer together the residual tensile stresses will most likely come in contact with the nearby hole. Under cyclic tensile loading these tensile stresses will add to the applied stress and could lead to an increased probability of crack initiation at the edge of the adjacent hole.
Figure 1. Typical Residual Stress Distribution around a
Split Sleeve Cold Expanded Hole.
Placing a hole in a structure creates a point of high stress concentration when subjected to an applied load. The effect it has on the local applied stress distribution is dependent on the shape of the hole which will change the stress concentration factor, or stress multiplier, when analyzing the effect on fatigue life. Figure 2 shows the localized stress distribution around a circular hole. When there are multiple holes in close proximity to each other, the respective stress concentration factor for these configurations will be different when the holes are oriented differently to the applied stress (as shown in the Figure 3). When two adjacent holes are oriented perpendicular (horizontal in our example) to the applied load, the stress concentration factor is elevated above that of a single hole for all hole spacing's and therefore are more critical in a fatigue design case. For holes in-line with the applied load (vertical), the stress concentration factor is less than for a single hole.
Figure 2. Typical Stress Concentration around Single Hole under a Uniform Load
(note how it is highest on the sides normal to the applied load)
Figure 3. Stress Concentration Factors for Two Holes extracted from R.E. Peterson;
"Stress Concentration Factors" (Wiley, 1974)
To evaluate the effect on fatigue life of a hole in close proximity to a cold expanded hole, FTI conducted an extensive coupon test program where two holes were positioned at different distances apart and either horizontal to, or vertical with, the applied load. The coupons were made from 2024-T351 Aluminum with 0.250 inch diameter holes. The effect on fatigue life was evaluated for different hole configurations with either only one or both holes cold expanded. As a means to understand the overall effect on the fatigue life of the double hole coupons, the fatigue lives were compared to baseline "single hole" coupons, with the hole either cold expanded (Cx) or non-cold expanded (NCx).
In all the cases where only one of the adjacent holes was cold expanded, the fatigue lives were significantly reduced, particularly when the holes were closer than about 2D spacing. The reduction was most noticeable in the horizontal hole orientation (see Figure 4) where the fatigue lives were even less than the non-cold expanded single hole coupons when only one of the two holes was cold expanded. The stress concentration factor effect discussed previously explains this. The balancing tensile stresses that contacted the edge of the adjacent hole added to the applied stress which was then amplified by the stress concentration factor of the second hole.
Figure 4. Horizontal Hole Configuration Life Comparison FTI Technical Report #35979
In the vertical hole configuration, the reduction in life improvement was not as great as the horizontal hole orientation when both holes were cold expanded (see Figure 5). The life improvement factor was influenced by a combination of the reduced stress concentration effect of the "vertical" hole placement and the relative location of the tensile stress zone from the cold expansion residual stress around the hole. The tensile stresses were at the bottom of the hole and not in the zone of highest stress concentration induced by the applied load (see Figure 2). The overall life improvement factor for this configuration was greater than the horizontal case.
Figure 5. Vertical Hole Configuration Life Comparison
The optimum spacing where cold expansion of one hole has no significant effect on the fatigue life of the adjacent hole varies with material type, hole size, applied expansion and applied stress level. In general, it was found that for holes of greater than 3D spacing, the distribution of residual tensile stresses will not impact the adjacent hole. Therefore it is advisable to cold expand both holes when they are less than 3D spacing to avoid having a possible detrimental effect on the fatigue life of the adjacent non-cold expanded hole. Cold expansion of two adjacent holes in any orientation, even when very close, will provide significant fatigue life improvement over a non-cold expanded configuration for the same or similar material, conditions and stress levels tested.
