PSI - Issue 44

Ivan Marenda et al. / Procedia Structural Integrity 44 (2023) 2152–2157 Ivan Marenda et al./ Structural Integrity Procedia 00 (2022) 000–000

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In this case, the friction force is proportional to the applied load regardless of the apparent contact area. In point of fact, the friction coefficient depends on several factors including: • Axial load, • Sliding velocity. When the test temperature of the polymer sample approaches the glass-transition temperature a strong dependence of the friction coefficient on the velocity is observed and it is not easy to separate the effects of the velocity and temperature, • Temperature. Polymers is a viscous-elastic materials, hence they are very sensitive to frictional heating, • Wear. When the adhesion force of the adhesive exceeds the adhesion force of the polymer, the polymer is transferred to the sliding surface. Many authors have tried to describe, at a macroscopic level, the variations in the coefficient of friction with the aforementioned parameters. For instance, Gambelli et al. define the friction coefficient as the product of two functions: µ(N,V,c)=f NV (N,V) f c (c) (7) where f NV is a function that accounts for the effects of the values of axial load N and velocity V, while fc is a function that accounts for the cumulated heat generated at the sliding surface. The function f NV can be developed starting from the following standard formulation (Constantinou et al.): = #6 − ( #6 − 76 ) (8⌈6⌉ (8) 3. Hirun Sliding Material: HI-M The HP device can dissipate energy through the friction between the concave sliding surface and the special thermoplastic material (HI-M). To predict its dynamic behaviour, it is necessary to know both the static and the dynamic coefficient of friction. As regards the static friction coefficient, it is mainly a function of the applied pressure and the roughness of the surfaces in contact. For these reasons, the HI-M material manufacturing process is controlled and standardized. This process allows obtaining a well-defined statistical description of the joint. As for the dynamic coefficient, it depends on variation of temperature in the HI-M, contact area during motion, and velocity. Fig. 1 shows the temperature variation of the HI-M during a repeated cyclic test. The average pressure on the disc is 40 MPa, the imposed displacement is ± 170 mm, and the frequency of the sine wave is 0.270 Hz the two probes are placed just below the stainless-steel sheet. The plummet of the blue line, just after the test, is due to the presence of a central hole in the sliding material. a) b)

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Fig. 1. (a) HI-M disc and HP device (b) Temperature profiles of the two probes (blue and red lines) located just below the stainless-steel sheet under 5 cycles of displacement (green line)