A Fastener Analysis Addressing Various Types of Misfit and Its Damage Life Calculations 2013-01-2312
In a fastening system when there is a small misalignment of the holes, the holes are enlarged to align the axes and a next size fastener is used to fit the joint. But when the misalignment is large then the enlargement need to be proportionally large. In this case a bushing is press fit onto the hole to handle the fastening. If we press fit a bushing, it generates residual stresses in the panel. These residual stresses reduce the damage life of the components on which the bushings were press fit. In the aircraft engine nacelle components the damage life is very critical in various failure conditions such as fan blade out condition, wind milling and bird strike. It increases the flight time in these events. Here four different case studies were considered to study the damage life of the aircraft components made of Aluminum or composite material.
Case-study 1 (Aluminum panel and full ring bushing): A knock down factor to the damage life for the component material (Aluminum - in this case) is tabulated in this paper to account for the pre stresses added to the nacelle components due to the press fit of the bushing, which is added due to the misalignment of the holes.
Case-study II (Aluminum panel and split ring bushing): An attempt is made to reduce the residual stresses on the Aluminum panel containing the fastening system for the non critical components. Here a split ring is used to press fit into the enlarged hole  in place of the bushing used in the Case-study I. The split ring reduces the tolerance requirements and decreases the manufacturing cost & time. This also reduces the stress on the panel and contains the fastening system.
Case-study III (Composite panel and full ring): In this case- study the bushing is press fit into composite sandwich panel instead of the Aluminum panel that is used in previous Case-studies for a treatment on the misalignment. The residual stresses developed are used to recalculate the damage life of the composite panel. Also a knock down factor chart is developed for the fiber with +45. −45, 0 and 90 degree ply orientations.
Case-study IV (Composite panel and split ring): In this case-study the split ring is used instead of full ring bushing on to the composite panel.
These studies can be used to modify the damage number for the given material of any part subjected to a peak pre stresses between 4 to 12 Ksi. By computing the modified damage number, the RUL (Rest of Useful life) number can be computed. This helps in accessing the health of the part as well as can decide in reusing a part in non critical system as a carry over part.
The split ring bushing is studied only to be used in the non-critical parts or in other systems such as automotive or consumer industrial components. For the critical aircraft components the added residual stresses will necessitate to compute the modified margin of safety as well as the modified damage life for the part.