PSI - Issue 5

Lucjan Witek et al. / Procedia Structural Integrity 5 (2017) 369–376 Lucjan Witek et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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6. Results of numerical modal analysis In piston engines the unbalanced crankshaft is a source of vibrations (Feng at al., 2011). Intensity of vibrations depends on many factors (ie. cylinders layout, number of cylinders, kind of fuel, power of engine). The characteristic of diesel engines (low rotational speed and high torque) causes that these engines are susceptible to resonant vibration. During the mechanical resonance, the large stress amplitude causes high-cycle fatigue (HCF) conditions (Zhao, Wang, 2014). As a result, the crankshaft can be damaged in relatively short time. The numerical modal analysis of the crankshaft was made using Abaqus solver. As results both the natural frequencies and the modes of blade free vibrations were obtained. As seen from Table 5 the natural frequency (for mode I of vibration) equals 699 Hz. The next frequencies of free vibrations (II-VII) are in the range of 2047-6676 Hz. First 3 modes can be classified as longitudinal. Next are: transverse, complex and torsional.

Table 5. Values of natural frequencies and classification of modes. Mode no. Natural frequency [Hz]

Type of vibrations

699

Longitudinal Longitudinal Longitudinal Transverse

I

2047.3

II

3445

III IV

3788.6

3879

Complex Complex Torsional

V

4610.5 4676.6

VI

VII

In Figure 8 the maximum principal ( σ 1 ) stress distributions for mode II of crankshaft free vibration is presented. This result is related to the maximum amplitude of blade vibration of 0.1 mm. The maximum stress area (306 MPa) is located in the critical zone of the crankshaft where the crack was initiated. For modes: I and III-VII location of maximum stress zones not covers the crack initiation area. Results of performed in this study modal analysis showed that the mode II of resonant vibration could be excited during work of the engine. At this mode only the high stress occurs in the critical region of crankshaft where the fracture was observed.

Fig. 8. Maximum principal (  1 ) stress distribution for the crankshaft during mode II of free vibration (2047,3 Hz, vibration amplitude of 0,1 mm). Model scale of deformation 50:1.