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The torsion bar of the emergency braking system of a military aircraft, a forged block of a maraging low-carbon steel, was found fractured during an in-service inspection. Visual examination of the bar showed several defects associated to the missing and blistering of finishing on both external and internal walls. Further examinations using a Field Emission Scanning Electron Microscopy (FESEM) as well as the X-ray Energy Dispersive Spectroscopy (X-EDS) and metallographic microscope showed pits in the protective finish from which started inter-granular corrosion. FESEM observation also revealed grains disbonding, in according to a hydrogen embrittlement phenomenon. Finite Element Analysis (FEA) also performed to evaluate stress distribution pointed out the orthogonality between the main stresses direction and the helical fracture.
Considering all the evidences, it was possible to ascribe the torsion bar failure to the hydrogen embrittlement.
The pits, acting as raisers, just accelerated the failure mechanism.
In addition, it was suggested to intensify visual inspections, by checking of the finishing and eventually to replace the bars when defects are found.