Issue 30

G. Zucca et alii, Frattura ed Integrità Strutturale, 30 (2014) 409-416; DOI: 10.3221/IGF-ESIS.30.49 416 Figure 17 : Transition between two halves of the actuator case. D ISCUSSION he examination of fracture surface showed a multisite fatigue phenomenon started from the external edge of actuator case. Chemical analysis and hardness measurement indicated that no unusual features were found in the material and his heat treatment. Metrological checks showed interference between the cap and the actuator case. The gap between the actuator and the cap has to be between 25.4 and 101.6 µm as design requirements. Tolerance between cap and actuator case is established taking in account thermal expansion. FEM analysis showed that less than 50 µm of gap between the two parts would induce abnormal stresses on the outer edge of the case assy due to the normal thermal conditions. These, in addition to the other normal operative loads, such as the actuation pressure, started the multiple initiation site fatigue The evaluation of striations spacing features an high-cycle fatigue, compatible with vibration during flight [8] [3]. C ONCLUSIONS he Dual Booster Actuator failed because of a high-cycle multiple initiation sites fatigue mechanism. This was due to stress intensification caused by dimensional non conformances. Recommendations were issued in order to improve dimensional controls. R EFERENCES [1] Oddone, G., Guida alle interpretazioni delle caratteristiche morfologiche delle superfici di frattura al microscopio elettronico a scansione, Ragno Editore, (1981). [2] Amura, M., Allegrucci, L., De Paolis, F., Bernabei, M., Fatigue fracture of an AMX aircraft main weel, Engineering Failure Analysis, 27 (2012)194-202. [3] Cameron, D., Fatigue properties in engineering. Fatigue and fracture., ASM Handbook, (1996). [4] Handbook, Metallography and Microstructure, George F. Vander Voort, 9 (2004). [5] Handbook, Properties and Selection - Nonferrous Alloys and Special-Purpose Materials, (1990). [6] MatWeb, www.MatWeb.com (2014). [7] Taylor, R., Zienkiewicz, O.C., The Finite Element Method, 5th Ed., 2 (2000). [8] Gugliotta, A., Introduzione alla meccanica della frattura lineare elastica., Levrotto & Bella, (2002). T