Experimental and Modeling Damage Limits Study for Straight Ti-3A1-2.5V Tubes

Abstract : To insure the safety of the V-22 aircraft over long period of operation, damage limits should be established for Ti alloy hydraulic tubes In a previous report, the damage limit results for straight Ti-3Al-2.5V tubes with outer diameter (OD) of 3/8 inch and tube wall thickness of 0.032 inch were summarized. In this report, we present the fatigue life cycle (internal impulse pressure) test results and give the fatigue life trends for the straight Ti alloy tubes with ODs of 1/4 inch, 3/8 inch, 1/2 inch, 5/8 inch and 3/4 inch and the corresponding tube wall thicknesses of 0.022 inch, 0.032 inch, 0.043 inch, 0.054 inch and 0.065 inch, respectively. Presented results are worst case life cycles for short and long cracks or scratches based on sharp notch studies. Post-fracture crack profiling results for both short and long cracks are also presented. Finite element modeling was conducted for the prediction of the damage limits of various tubes and the results are compared with experiments with good agreement. Periodic inspection of Ti alloy tubes in combination with damage limit curves will provide a powerful tool to fleet maintainer for change of unsafe tubes. This was a successful collaborative effort between the researchers at the University of Virginia, V-22 program office, NAVAIR materials and hydraulics testing laboratories. Main conclusions of this study are: (1) the 1/4 inch OD tube is the worst case: (2) cracks longer than 0.020 inch in length with nominal depth of 0.0O2 inch will have reduced life cycle than expected value of 200,000; (3) none of the tubes survived expected 200,000 fatigue cycles for long cracks.