RESUMEN
Soft polymers such as the investigated polyurethane, characterized by low Young's moduli and prone to high shear deflection, are frequently applied in pneumatic cylinders. Their performance and lifetime without external lubrication are highly determined by the friction between seal and shaft and the wear rate. FEM simulation has established itself as a tool in seal design processes but requires input values for friction and wear depending on material, load, and velocity. This paper presents a tribological test configuration for long stroke, reciprocating movement, allowing the generation of data which meet the requirements of input parameters for FEM simulations without the geometrical influences of specific seal profiles. A numerical parameter study, performed with an FEM model, revealed the most eligible sample geometry as a flat, disc-shaped sample of the polymer glued on a stiff sample holder. At the same time, the study illustrates that the sensitivity of the contact pressure distribution to Poisson's ratio and CoF can be minimized by the developed and verified setup. It ensures robust, reliable, and repeatable experimental results with uniform contact pressures and constant contact areas to be used in databases and FEM simulations of seals, enabling upscaling from generically shaped samples to complex seal profiles.
RESUMEN
The origin of friction and wear in polycrystalline materials is intimately connected with their microstructural response to interfacial stresses. Although many mechanisms that govern microstructure evolution in sliding contacts are generally understood, it is still a challenge to ascertain which mechanisms matter under what conditions, which limits the development of tailor-made microstructures for reducing friction and wear. Here, we shed light on the circumstances that promote plastic deformation and surface damage by studying several face-centered cubic CuNi alloys subjected to sliding with molecular dynamics simulations featuring tens of millions of atoms. By analyzing the depth- and time-dependent evolution of the grain size, twinning, shear, and stresses in the aggregate, we derive a deformation mechanism map for CuNi alloys. We verify the predictions of this map against focused ion beam images of the near-surface regions of CuNi alloys that were experimentally subjected to similar loading conditions. Our results may serve as a tool for finding optimum material compositions within a specified operating range.
RESUMEN
Although acoustic-emission (AE) piezoelectric transducers have distinctive variations in sensitivity, depending on frequency, propagation medium, and coupling, the vast majority of AE research is conducted by utilizing uncalibrated AE transducers. As a consequence, most results obtained by different groups are not comparable among each other. In this work, primary calibration by the method of reciprocity is shown. Rayleigh and longitudinal wave calibration curves are presented for piezoelectric high-frequency broadband transducer, mounted on steel and aluminium, in the frequency range 300 kHz-4 MHz. Influences on primary calibration of AE transducers, namely, by coupling medium, contact pressure, and propagation medium, are investigated.
RESUMEN
We performed large-scale molecular dynamics (MD) simulations to study the transient softening stage that has been observed experimentally in sliding interfaces subject to strain path changes. The occurrence of this effect can be of crucial importance for the energy efficiency and wear resistance of systems that experience changes in the sliding direction, such as bearings or gears in wind parks, piston rings in combustion engines, or wheel-rail contacts for portal cranes. We therefore modeled the sliding of a rough counterbody against two polycrystalline substrates of face-centered cubic (fcc) copper and body-centered cubic (bcc) iron with initial near-surface grain sizes of 40 nm. The microstructural development of these substrates was monitored and quantified as a function of time, depth, and applied pressure during unidirectional sliding for 7 ns. The results were then compared to the case of sliding in one direction for 5 ns and reversing the sliding direction for an additional 2 ns. We observed the generation of partial dislocations, grain refinement, and rotation as well as twinning (for fcc) in the near-surface region. All microstructures were increasingly affected by these processes when maintaining the sliding direction but recovered to a great extent upon sliding reversal up to applied pressures of 0.4 GPa in the case of fcc Cu and 1.5 GPa for bcc Fe. We discuss the applicability and limits of our polycrystalline MD model for reproducing well-known bulk phenomena such as the Bauschinger effect in interfacial processes.
RESUMEN
We consider a nanomachining process of hard, abrasive particles grinding on the rough surface of a polycrystalline ferritic work piece. Using extensive large-scale molecular dynamics (MD) simulations, we show that the mode of thermostating, i.e., the way that the heat generated through deformation and friction is removed from the system, has crucial impact on tribological and materials related phenomena. By adopting an electron-phonon coupling approach to parametrize the thermostat of the system, thus including the electronic contribution to the thermal conductivity of iron, we can reproduce the experimentally measured values that yield realistic temperature gradients in the work piece. We compare these results to those obtained by assuming the two extreme cases of only phononic heat conduction and instantaneous removal of the heat generated in the machining interface. Our discussion of the differences between these three cases reveals that although the average shear stress is virtually temperature independent up to a normal pressure of approximately 1 GPa, the grain and chip morphology as well as most relevant quantities depend heavily on the mode of thermostating beyond a normal pressure of 0.4 GPa. These pronounced differences can be explained by the thermally activated processes that guide the reaction of the Fe lattice to the external mechanical and thermal loads caused by nanomachining.
