Analysis of cracked MGB housing of a helicopter

The MGB housing of a helicopter had cracked by fatigue. The fatigue crack had initiated at the radius region of the cutout adjacent to the booster mounting area and propagated progressively in the vertical web over a distance of about 30 mm. The nature, location and length of the crack in this MGB housing were found to be similar to those observed in the MGB housings of helicopters PPA-01 and PPA-02. No metallurgical or mechanical abnormalities were responsible for fatigue crack initiation and fatigue appeared to be stress related. The most probable reason for fatigue crack initiation appears to be the high stress concentrations at the cutout surface. It also appears that with the existing design, the fatigue cracking of MGB housing is unavoidable. Certain design modifications are suggested to tackle this problem