Assessment of Residual Stresses and Hole Quality on the Fatigue Behavior of Aircraft Structural Joints. Volume 2: Fastener Hole Drilling Quality and Fatigue Lives

摘要

A study was conducted on the effects of drilling fastener holes in 2024-T3 aircraft-quality aluminum. The effects were researched using three techniques: a comparison of four hole surface-quality metrics, x-ray diffraction, and fatigue at three stress intensities. Six production-line factor variables were considered: the experience of the operator, the use of a pilot hole, the length of the bit, the condition of the bit, the axial pressure or feed rate, and the rotational speed of the bit upon withdrawal from the hole. The four hole-quality metrics employed were surface roughness, conicality, the number of large gouge marks, and the angles of those marks. The pilot hole was found to produce the greatest overall effect on hole quality, improving hole quality with its use. X-ray diffraction was used to attempt to measure the relative residual hoop stresses induced by the six drilling factors. Tests found that specimens could not be compared using this method due to problems resulting from the combination of the specimens geometry, hole surface quality, and the properties of the aluminum alloy. Fatigue tests were conducted at 25, 21, and 17.5 ksi on as-drilled and chemically polished specimens. These tests suggested that, although residual stresses do appear to have an effect, hole quality is the predominant factor affecting fatigue life.