Simulation Fatigue Testing of an Autofrettaged Cylinder with an Outside Diameter Notch: The Effects of Root Radius and Surface Condition. Part 1. Very Low Cycle Applications,
ARMY ARMAMENT RESEARCH DEVELOPMENT AND ENGINEERING CENTER WATERVLIET NY CLOSE COMBAT ARMAMENTS CENTER
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The effects of notch root radius, electropolishing after machining, and glass ball peening after machining have been studied in simulation tests of an outside diameter OD notched, autofrettaged thick-walled cylinder subjected to cyclic internal pressure. Fatigue life measurements were made using specimens designed such that the stress at the notch root due to both autofrettage and pressure were simulated exactly. Fatigue life was defined as the number of cycles required to initiate a crack at the notch. Crack initiation was determined by ultrasonic measurement and verified by visual examination of the fracture surface. The notch root radius was varied from about 0.010 in. 0.254 mm to 0.020 in. 9.508 mm. This range of root radius corresponds to stress concentration factors from between 3.9 and 5.4. The total fatigue life of full-size cylinders in this configuration is between 3000 and 5000 cycles. Attempts were made to predict fatigue crack initiation using local strain analysis. Experimental results show that fatigue initiation life may be increased by about 300 with increased root radius. Neither electro-polishing nor glass peening showed any improvement over the as-machined notch. Further testing is needed to determine if surface treatments affect fatigue life at longer lives. Also, the local strain analysis yielded a lower limit prediction of the actual behavior.
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