This SAE Aerospace Information Report (AIR) presents flexure fatigue strength of tube fitting joints in the form of graphs showing applied deflection D and total calculated axial stress S versus number of cycles to failure N. The graphs have been prepared from data obtained by actual testing conducted in accordance with ARP1185.

Purpose:

The accompanying D/N (deflection versus cycles) and S/N (stress versus cycles) curves for tube fitting joints are presented over the widest practical range to enable a designer to choose the optimum design for a particular application. Requirements may vary from missile applications with a very short cycle life to a commercial airplane application with a very long cycle life. S/N curves can predict a life for projected designs where given stresses are forecast. D/N curves can compare similar fittings used on different tubing with widely varying levels of elasticity. The curves may also be used as the base line for qualification testing of alternate fitting joint designs.