Analysing the reason behind HAL Dhruv boom break

The HAL Dhruv is a utility helicopter developed and manufactured by India’s Hindustan Aeronautics Limited (HAL). The Dhruv entered service in 2002. It is designed to meet the requirement of both military and civil operators, with military variants of the helicopter being developed for the Indian Armed Forces, while a variant for civilian/commercial use has also been developed.

A while back during a rappelling exercise, the boom of the Dhruv gave up. Boom is the part where the rope is attached for soldiers to slide down from the helicopter.

A quick look at the boom attachment to the frame and the mating part tells us something obvious. Take a look at  the pictures below.

A look at the picture on the right (top side) shows the characteristics of a ‘tear’. Tear usually happens on a cantilever when the stress of a portion of the beam exceeds ultimate tensile strength. At this point, the portion fails and crack develops. Once a minor crack develops in the particular area, the section tears of completely as stress concentration subsequently shifts to the ‘first joint portion’. This continues until full separation.

The boom can be considered a cantilever for all purposes.

Another thing we can notice here is the holes (half-holes) in the torn section. This means they were either bolted or riveted in this area. Creating holes in such areas is not recommended as holes lead to decrease of the part’s section modulus (Z) leading to failure much faster than it originally would.

Take a look at the below representation.

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