PSI - Issue 17

Felix Kresinsky et al. / Procedia Structural Integrity 17 (2019) 162–169 Felix Kresinsky / Structural Integrity Procedia 00 (2019) 000 – 000

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3.2. Transition from keyway expansion to fatigue strength

In addition to the keyway expansion, numerous cracks were observed at the keyed shaft-hub connection above the permissible torsion moments. A specimen which exemplary shows all possible cracks after a load of M t = 2100 Nm with 5 mill. load cycles is shown in Fig. 4. In experiments with a maximum of 5 million load cycles, crack I occurs slightly above the permissible moment M t = 1400 Nm (swelling) for l tr /d = 1.3 starting from the joint. Crack II occurs from approx. M t = 2000 Nm, starting from the keyway rounding. Furthermore, fine cracks in the keyway bottom radius and the loaded keyway flank were also observed. However, fractures of the shaft were only detected at higher loads (approx. M t > 2200 Nm) due to the critical growth of crack II. In order to determine the fatigue strength level, it must therefore be clarified whether the observed cracks can become critical at lower loads and higher load cycles. This was to determine whether crack I or the cracks in the keyway can also became critical and thus led to the failure of the entire joint. For this reason, selected specimens were carried out with up to 10 million load cycles. Contrary to expectations, at M t = 1800 Nm the shaft failed via crack in the rounding (crack II) after 9.3 million load cycles. At M t = 1600 Nm only crack I occurred, without becoming critical up to 10 million loads. This also additionally secures the terminated maximum allowable keyway expansion with the resulting maximum torsional moment of 1400 Nm. It is therefore below the fatigue strength of the shaft.

Fig. 4. Confocal microscopic image of the different cracks occurring above the permissible keyway expansion (specimen after 5 mill. load cycles with M t =2100 Nm)

Experience (Vidner (2007)) has shown that a failure at the (high) number of load cycles (9.3 million) is primarily a consequence of the fretting fatigue in the contact between key and shaft keyway and less due to mechanical stress. The crack/fracture was therefore opened and examined microscopically. The keyway bottom radius was identified as the location for crack initiation (Fig. 5 b). Therefore the keyed shaft-hub connection is fractured along crack II in the keyway rounding initiated by the fine cracks in the keyway bottom radius. However, there is no contact in the keyway bottom radius, which indicates a mechanically induced crack.

Fig. 5. Selected opened fractured keyed shaft-hub connections with confocal microscopic details: a) crack initiation at free surface near the keyway rounding; b) crack initiation in keyway bottom radius