H-46 Tie Bar Fatigue Failure Predictions by Multiaxial Fatigue Theory

The initial H-46 Tie Bar assembly design consisted of 35 stainless steel 301 and ¾ hard stamped straps. The straps are stacked together and separated by thin metal spacers and held together by Bronze dowel pins. The service inspections of the tie bar from 1990 to 1994 revealed early individual strap cracking at a slot radius. Boeing Helicopter thoroughly investigated the failures by conducting coupon tests, usage survey, strain survey, bending analysis, finite element analysis (FEA), and manufacturing improvements and recommended to design and fabricate an upgraded tie bar. For fatigue qualification and service life assignment, 32 full-scale tie bars were fatigue tested under 11 different loading spectrums from 1995 to 1999. However, the fatigue failure mode in an upgraded tie bar shifted from a slot radius to the pinbores. It was thought that the fatigue test cycles to failure are inconsistent with respect to the severity of the spectrum loading. In this study, to investigate the problem, load to strain/stress transfer functions of centrifugal force (CF), bending deflection, and torsional twist loads acting on the tie bar were developed from the previous configurations strain survey data. A finite element model (FEM) was developed to verify the load to stress transfer for the present configuration and to determine effective notch factors for axial loads and torsion and bending displacements. The principal strains were computed in time sequence as the tie bar was subject to preflight twist control checks, CF, and twist and bending loading cycles. The notch strain history was cycle counted with the peak and valley method to identify fatigue cycles. Using this information, the 11-fatigue stress spectrums corresponding to the 11-fatigue tests were created. The multiaxial fatigue failure theories, such as maximum principal strain, von Mises, and maximum shear strain, Kandil, and Lohr, were used to predict fatigue failure life. The predicted fatigue life using the maximum principal strain theory correlated well with test life. The Von Mises strain theory generally overpredicted fatigue test life; whereas, the Kandil theory underpredicted fatigue test life. The maximum shear strain theory and Lohr's theory also correlated well with fatigue test data. However, their predicted values are slightly higher than the maximum principal stress theory. The investigation helped to explain the perceived inconsistency between the spectrum severity and fatigue life. The service fatigue life predictions using the maximum principal strain, Lohr, and maximum shear strain theories do not differ significantly. The results of the investigation indicate that the multiaxial fatigue theory that predicts the test life accurately should be used to predict the service fatigue life. Also, the fatigue life should be determined by various theories to obtain the lower and upper bounds of the prediction.