*2.2. Experimental Set-up*

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The set-up used for all the experimental measurements is presented in Figure 1. The system was designed to estimate the contact stiffness between two rough interfaces of different sample materials, within a range of average contact pressure up to 1 MPa, in both sticking and sliding conditions [18]. The system consists of a sample in contact with a massive steel disc and loaded by the dead weights on a guide bar, as shown in Figure 1.

The guide is maintained by an air bearing, to enable it to oscillate and not introduce further stiffness and friction in the vertical direction.

The tested sample consists of aluminium (Al). The material properties and surface roughness are presented in Table 1.

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An impulsive-type force is applied by an instrumented impact hammer (Brüel and Kjær type 8202), while the dynamic response of the system is recorded by an accelerometer placed on top of the guide bar.

All tests presented in the paper are performed on the system, with the overall weight of the guide bar on top of the sample, generating a static equilibrium pressure σ<sup>0</sup> = 0.14 MPa.

Measurements of the force and acceleration of the system are recorded using the acquisition system (SIRIUS–DEWESOFT), based on DualCoreADC® technology with dual 24-bit delta-sigma analogue to digital converter (ADC). An anti-aliasing filter on each analogue channel achieves a 160 dB dynamic range in time and frequency domains with 200 kHz sampling rate per channel. Data are post-processed by Matlab.

We assume that there is no interface separation between the two contact surfaces during the tests. This assumption is supported by the numerical simulations for the levels of impulsive excitation used.
