PSI - Issue 76

Vladimír Chmelko et al. / Procedia Structural Integrity 76 (2026) 131–137

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the cut side, which increases the fatigue strength of the material. However, the cold cutting process creates two different parts on the cut surface, as can be seen in Fig. 3b. The cutting process (shear stress) changes in the final part to tearing with bending (normal stress), which leaves not only a high degree of plastic deformation (high hardness in Table 2) but also high roughness of the corresponding part of the cut surface. A detailed examination of the fracture surfaces under a microscope revealed that all fatigue fractures initiated in this part (Fig.6). This means that the negative effect of high surface roughness outweighed the positive effect of plastic deformation on the cut surface.

Fig.5 S-N curves of the three materials examined in the R=0 mode.

Fig.6 Fatigue crack initiation on the punching (tool cutting) surface

In the continuation of the cyclic tests, the edge after tool cold cutting was gently ground to remove the surface hardened and rough area. The result of this treatment was an increase in fatigue strength (11 to 15% in stress amplitude, as documented in Fig. 7. All fatigue cracks were initiated from the edge of the sample, which is also a stress concentrator, thus affecting the fatigue strength. Modifying the sharp edge can also have a positive effect on the fatigue life of the material.