# Stick-Slip Phenomena and Acoustic Emission in the Hertzian Linear Contact
## 2 Experimental Model--Geometry and Material of Specimens  
The flat specimen was made of unhardened and unalloyed steel, type R260 (EN 13674-1:2018) with a Brinell hardness of 285 HBW, and the cylinder specimen was made of surface-treated steel, type ER7 (EN 13262:2021) with a Brinell hardness of 265 HBW. These wear-resistant materials are used for the rails and wheels in railway systems. The surface quality of the specimens was initially evaluated by measuring the roughness with a Gaussian filter of 0.8 mm in the longitudinal (\(L\)) and transverse (\(T\)) directions with Surtronic 25 equipment. Thus, the average roughness values of the cylinder specimen were 0.734 \(\upmu\)m (\(L\)) and 2.15 \(\upmu\)m (\(T\)), and for the flat specimen, 1.13 \(\upmu\)m (\(L\)) and 1.17 \(\upmu\)m (\(T\)). The sizes of the cylinder and flat samples are shown in Figure 1. The fixing drawbar of the cylindrical specimen was incorporated into the tribometer loading device. Applying a known tangential force on its free end caused bending of the drawbar-cylinder system, which could be measured, and thus, the sample stiffness was obtained (straight slope--30.427 N/mm). This stiffness influences the amplitude of the stick-slip phenomenon; hence, the greater the stiffness, the more the stick-slip phenomenon will decrease until it disappears. Although the effect of the frictional force on deformation is essential, frictional force does not depend on stiffness. The cylindrical sample moved vertically until it arrived at the flat specimen under predetermined conditions. In this position, it was loaded at a normal force (\(F_{n}\) = 20, 40, 60 N). The flat test piece moved linearly on a horizontal plane with a driving speed, thus achieving the frictional force. This device simulated linear contact with tangential and normal forces at very low sliding speeds, specific to contact with forced rolling. The device allowed for the experimental detection of one of the effects of rolling motion micro slips, namely the dynamics of sliding friction at low speeds and contact pressures with elliptical Hertz type distribution. This distribution led to a flat surface of the elastically deformed bodies, whilst the friction force between the specimens depended on the evolution of the friction coefficients with the driving speed (\(v\)) and sliding speed (\(v_{slip}\)). The experimental setup presented in this paper was carried out in CERT UMT-2 Tribometer, which is used to test the stick-slip phenomenon of different materials. Figure 2 shows the test system with the upper and lower specimens connected to the tribometer. The tribometer was adapted to the specific slip conditions for Hertzian linear contact to perform these experimental tests. Thus, the clamping systems of the lower flat sample and  
Figure 1: Dimensions for cylindrical and plane specimens used for tests. the cylindrical sample with the bending stiffness calibrated to detect the SS phenomenon were designed and made for these experimental tests. The tribometer was equipped with a two-dimensional force sensor DFH-20 that was used to measure the control of a normal loading force and friction force between the upper and lower test pieces. The damping system, located between the force sensor and the upper sample support, was used to maintain a constant load force during the tests. In order to maintain the normal force as constantly as possible at the contact of the samples, the tribometer was equipped with a device with a spring and shock absorber that allowed for the continuous adjustment of the normal force depending on the \"response\" of the material of the flat sample. The upper specimen was plugged into the monitoring system, while the lower specimen was connected to the L20 HE linear motion unit. The adhesion at the molecular level and the elastic-plastic deformations at the roughness level are both responsible for friction, which in turn is responsible for tangential force [39; 40; 41].