*3.2. Experimental Methods of Investigation of the Materials Properties and the Bending Sti*ff*ness*

The mechanical properties of the base, i.e., laser-unprocessed metal, i.e., stress-strain diagram, modulus of elasticity *E<sup>b</sup>* , offset yield stress (proof stress) σ0.2,*B*, ultimate and fracture stresses as well as the fracture strains *ul*,*<sup>b</sup>* , were determined by the standard tensile test according to LST EN ISO 6892-1 [17]. The corresponding mechanical properties of the laser-processed metal, i.e., *E<sup>l</sup>* , σ0.2,*<sup>l</sup>* and σ0.2,*<sup>B</sup>* were determined indirectly by using the empirical relationships between the corresponding mechanical properties and hardness which was determined according to EN ISO 6507-1 [18]. The series of three samples were used for the mechanical tests—the estimated means *m<sup>x</sup>* = P*N i*=1 *x<sup>i</sup>* are presented hereafter and were used for the calculations presented below. Unmachined samples of type B, with initial gauge length *L*<sup>0</sup> = 80 mm, length *L<sup>c</sup>* = 120 mm and initial cross-section *S*<sup>0</sup> = 40 mm<sup>2</sup> were used for the tensile test.

A universal tensile-testing machine TIRAtest 2300 (TIRA, Schalkau, Germany) with a bending test tool and Catman-Express software (version 5.1, HBM, Germany) was used for the tensile and bend tests. A tension dynamometer up to 50 kN was used for the tensile test, while a compression dynamometer up to 1 kN was used for the bending test, described in the last paragraph of this subchapter. Steel hardness was determined using a Zwick/Roell ZHU (Ulm, Germany) universal hardness tester using the Vickers method. Steel hardness was determined on the surface with a load of 10 N. The hardness of the laser-processed layer was determined using a Zwick/Roell ZHµ (Ulm, Germany) hardness tester with a diamond, square-based tetrahedral pyramid tip with a load of 2.942 N. Hardness was measured on the surface of the laser-processed layers and on its cross-section.

The chemical composition of the steel was determined using the PMI Master PRO Oxford Instruments (High Wycombe, UK) optical emission spectrometer.

The metallographic examination of the steel and the laser-processed layer and analysis of the geometry and dimensions of the laser-processed layer was carried out using a Nikon Eclipse MA200 optical microscope (Tokyo, Japan) with a Lumenera Infinity 2-2 video camera and a JEOL JSM-7600 (Tokyo, Japan) scanning microscope with an energy dispersive spectrometer (EDS) Oxford INCA Energy X-Max20 (Oxford, UK) at different magnifications (up to ×1500).

The qualitative X-ray diffraction (XRD) analysis of the phase composition of materials was carried out by applying diffractometer DRON-7 (Burevestnik, Saint-Petersburg, Russia). Graphite-monochromated Cu Kα radiation (λ = 0.154178 nm) was used. The parameters of the tests were as follows: voltage—30 kV; current—12 mA; the range of the diffraction angle—from 4◦ to 120◦ , with detector movement steps of 0.04◦ ; the duration of the intensity measuring in each step—2.0 s.

The elastoplastic bending of the thin plate was conducted by using a 3-point bending device (see Figure 2), analogous to the device used in the metal bending tests, according to LST EN ISO 7438 [19]. The calculation scheme of the bending test is shown in Figure 2b. The distance between supports was 76 mm, the rounding radius and the Poisson's width were 10 mm. The test was conducted by using the strain-control mode with a velocity of displacement of 1.5 mm/s. The load *Fexp* was imposed at the middle of the span, at point B; see Figure 2b. The load *Fexp* ranged from 0 N up to the maximum load, corresponding to the greatest deflections of the experiment, which equal 2 mm. In general, *Fexp* < 500 N. The deflections were measured at the middle of the span of the plate, i.e., at point B; see Figure 2b. It should be noted that the distance between the supports, 76 mm, was greater than required according to LST EN ISO 7438 [19] to decrease the influence of shear strains on the deflections.

**Figure 2.** The principal view of the bending test device (**a**) and corresponding calculation scheme (**b**).

In the case of bending the one-side laser-processed plate, the laser-processed layer may be under tension or under compression. To evaluate the influence of the stress-strain state of the laser-processed layer on the deflections or the bending stiffness of the plate, 3 different loading variants were considered. In Variant A, the bending load *Fexp* was imposed on the laser-processed side of the one-side laser-processed plate, so that the laser-processed layer was under compression. In Variant B, *Fexp* was imposed on the laser-unprocessed side of the one-side laser-processed plate, so that the laser-processed layer was under tension. In Variant C, *Fexp* was imposed on the two-side laser-processed plate. In this case, the top laser-processed layer was under compression, while the bottom laser-processed layer was under tension. Also, 4 already mentioned laser-processing cases were considered with each loading variant: case I, laser-unprocessed plate; case II, 38 laser tracks; case III, 17 laser tracks; and finally, case IV, 14 laser tracks. In total, 10 different force–deflection experiment cases were conducted. These cases are summarized in Table 5. In general, three measurements were performed to determine each quantity. Three plates were tested for case I, and 9 plates for the rest of the laser-processed plate cases: case II–case IV. Thus, in general, 39 plates were tested for the bending experiment. The estimated mean values *m<sup>x</sup>* = P*N i*=1 *x<sup>i</sup>* are presented and analyses hereafter are in the present article.

