Stress-concentration factors at intersecting and closely approaching orthogonal coplanar holes

Using two models, each containing three groups of intersecting or closely approaching holes, 18 different cases were investigated by three-dinensional frozen-stress photoelasticity. In all cases, the coplanar holes were orthogonal to the applied uniaxial tension. Included were three cases of intersecting hoes with square corners and varying hole-diameter ratios. Five cases of intersecting holes with rounded corners were studied for two hole-diameter ratios and varying corner radius. Stresses were determined for two cases where the end of one drilled hole partially penetrated another hole leaving acute corners or feathered edges at the intersection. Three types of closely approaching holes were studied: two cases in which the end of one drilled hole nearly intersected the side of an-other hole, two cases in which the ends of two drilled holes approached each other along a common axis, and four cases (called corner-approach cases) in which the ends of two drilled holes approached each other along orthogonal axes. A stress-concentration factor of 13 was found for one of the partially penetrating drilled-hole cases. The upper limit on stress-concentration factor for a very small hole intersecting a large hole in an infinite body subjected to uniaxial stress is 8.4 for metals. This factor is reduced to 5.2 as the diameters of the intersecting holes become equal. Rounding the corners on intersecting holes reduced the stress concentration by only four to seven percent. Closely approaching drilled holes results in higher stresses than fully intersecting holes when the minimum ligament width is less than ten percent of the hole diameter. Corner approach cases do not result in high stress-concentration factors as long as the ligament width is greater than the percent of the hole diameter.