PSI - Issue 76

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

134

Table 2 Vicker´s hardness and surface roughness. S355

S500

S700

laser cutted side

rolled surface

rolled surface

cutted side shear

Cutted side tear off

rolled surface

laser cutted side

HV30

158

204

209

270

305

263

298

Ra (µm)

3.7

3.7

12.3

52.8

3.7

Each cutting operation increased the surface hardness: laser cutting by 14 to 30%, punching by up to 50%. The increase in hardness after laser cutting is a result of thermal influence and rapid cooling. Punching causes plastic deformation on the cut edge – shear plasticization during initial cutting, followed by plasticization by bending or tearing in the final stage. After fine grinding of the cut edge, the hardness decreased to the value of the rolled surface. This difference in surface hardness corresponds to the measured roughness values of the individual surfaces. As can be seen in Table 2, the highest roughness is found on the surface after cutting in the area where the material is torn off - this value is up to an order of magnitude higher than that of the rolled surface and the laser-cut surface. Significant differences in surface hardness and roughness after individual material cutting technologies gave reason to expect differences in the fatigue properties of individual materials. Cyclic tests were performed in a load frame of an MTS electrohydraulic pulsator in force control mode. The shape of the samples for cyclic testing is shown in Fig. 4. Considering their thickness, a load cycle of R=0 was selected.

a/ b/ Fig. 4. Test specimens for cyclic tests of materials: a/ S500; b/ S355, S700. Cyclic tests controlled by force amplitude were performed for all three sets of material samples without any additional modifications.: S355 – laser cutting S500 – punching (tool cold cutting) S700 – laser cutting The results of cyclic tests in the form of S-N curves for all three materials tested are shown in Fig. 5. The position of the S-N curve for material S500 in contrast to the strength properties (Fig. 2) is lower than the curve for material S355, which is a result of the different cutting technology. Tool cold cutting causes significant plastic deformation on