Scatter in tensile and fatigue properties of aluminium alloys was investigated for the various geometric stress concentration effects and variables arising in a bolted lap joint. The extent of the scatter is important in determining the likeli hood of failure during the service life of structures designed with such joints. The risk of structural failure is determined from the possible variation of both loads and strengths during the service life of a structure. For bolted joints in aluminium alloy sheet under constant amplitude loading, a log-normal probability distribution of fatigue lives to failure and a standard deviation of 0.2 was found to be reasonable. Using this data the implications of fail-safe and safe-life design for a given level of risk were investigated. In the safe-life approach the probability of a fatigue failure occurring during the service life is set equal to the acceptable level of risk chosen for the structure. By the end of one service life very little of the fatigue endurance of the structure will have been consumed. However, the structure must then be retired since the risk of a failure from fatigue will be increased above the acceptable level. Safety factors on life of the order of 10 are necessary using this approach, where the level of risk is set at 10-6. The fail-safe approach adopts the principle of active redundancy employing multiple load paths to transfer loads. Using this approach the designer must accept the failure of one or two of the load paths, whilst demonstrating that the strength of the remaining structure is capable of carrying the limit load. It will then be necessary to introduce inspections in order to detect the damage and restore the structure to its original strength. Even so the safety factor on life for the fatigue sensitive structure will still be of the order of 6 for a risk of 10-6.
|Date of Award||1974|