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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Fig. 2. (a) Magnified view of the fracture area. (b) View of crankshaft fracture.

Figures 2a and 2b show that the crack origin was located on the corner (fillet) of crank pin no 4. The crack front just after initiation has a circular shape. At depth of about 10 mm, the crack was divided (on the oil hole) on 2 separate cracks. On the surface of hole the cracks propagate in different directions. As a result the crack has an offset (after joining). The fracture in preliminary stage of fatigue (to about 50 % of cross section area) is smooth and propagates in one plane. In the central part of the fracture a large technological hole is located. In next stage of fatigue process (when the crack touches the stopper) the sudden change of crack direction is observed (Fig. 2). In last stage of damage the fracture is jagged and has additional radial lines (Figs 2b and 3a). There is visible that the rupture zone is very small (about 10 % of cross-section area). It means that only small amplitude of loading can create this kind of failure. The fracture is characteristic for a high-cycle fatigue (HCF). After large magnification of the fracture, the small micro-crack (about 150  m long) was detected near the crack initiation zone (Fig. 3b). It means that the crack initiation process could be accelerated by the preliminary cracks created in production process of the crankshaft.

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3. (a) Fracture area with fatigue striations. (b) Magnified crack initiation zone. Small crack is visible in central part of the picture.

3. Determination the mechanical properties of material of damaged crankshaft

In next step of analysis the crankshaft was subjected to the material investigations. In order to check the real material properties the shaft segment (in area of crank pin no 4) was cut on 3 mm thick specimens. In this operation the wire cut machine was used. The tension test of specimens was performed using Zwick-Roell tension machine. Obtained results showed that the crankshaft material has the following mechanical properties: yield strength (YS) of 480 MPa, ultimate tensile strength (UTS) of 758 MPa, modulus of elasticity of 207 GPa, total elongation of 20.6 %. The maximum value of true stress (after reduction of cross-section area of specimen) was equal to 870 MPa (Fig. 6). The tension plot of specimen cut from damaged crankshaft is presented in Fig. 4.