Analysis of cracked tube (slithering handle) of a helicopter

A cracked slithering handle was sent to the laboratory for investigation. Examination revealed that there were three cracks at the bent regions of the handle; two in the longitudinal direction and the other in the circumferential direction of the tube. Fractographic examination confirmed that these cracks were fatigue cracks. Evidences suggest that there were preexisting cracks which got generated in the tube either during metal forming operation or at a later time due to presence of residual stresses. It is believed that some of these cracks have propagated by fatigue under the vibratory load. No metallurgical or mechanical abnormalities were responsible for the fatigue crack initiation. Also, there was no corrosion at the fatigue crack origins. The tube material was found to conform to the specification 3.1354 Al-alloy. A detailed analysis of the failure is presented in this report