Following, activities and results obtained by fractography, metallographic analysis, chemical analysis and numerical
simulation of the investigation carried out at Flight Test Center of the Italian Air Force, showing the real root cause
of the fatigue fractures occurred to the crankshaft of the object internal combustion engine.
2. Instruments
Optical examination was carried out using a Leica M 205 C microscope. Microfractographical evidences were
acquired by a Gemini Ultra Plus Field Emission Scanning Electron Microscope (FESEM) equipped with an INCAx-
Sight Oxford Instruments Xray Energy Dispersive Spectroscopy (EDS) to perform a semi-quantitative microanalysis.
Microstructural examination was performed using a Leica CTR 6000 metallographic microscope, whereas a Perkin
Elmer Optima 2100DV Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and a Leco CS-224
were used to determine the chemical composition of the forging. Hydrogen content was determined by Leco RH-402
Analyzer. Micro Hardness Vikers measurements (MHV) were carried out by Leica VMHT Auto. FEA was carried
out using ANSYS 14.0 software program.
3. Results
3.1. Visual and Optical examination
The part of the crankshaft interested by cracks was carefully divided into sections to open correctly the cracks in
order to observe them. Crack n. 1 propagated from oil hole; visual examination shows a darkened area inside the
oil chamfer near the journal surface. The path of growth was oriented at 45° to the crankshaft rotation axis, along
the oil chamfer and becoming normal to the journal surface in the deepest part. Beach marks and ratchet marks are
well visible even without magnification and these features, are typical evidences of a crack propagation in a fatigue
mechanism (Fig. 3).
Optical microscopy on crack n. 1 showed two different morphologies:
• The first, was located in the area near the intersection between oil chamfer and journal surface, and its extension
was about 1.5 mm in the direction of fracture propagation. It showed a smooth and bright grain, without relevant