Wear | EPFL Graph Search

Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology. Wear in machine elements, together with other processes such as fatigue and creep, causes functional surfaces to degrade, eventually leading to material failure or loss of functionality. Thus, wear has large economic relevance as first outlined in the Jost Report. Abrasive wear alone has been estimated to cost 1-4% of the gross national product of industrialized nations. Wear of metals occurs by plastic displacement of surface and near-surface material and by detachment of particles that form wear debris. The particle size may vary from millimeters to nanometers. This process may occur by contact with other metals, nonmetallic solids, flowing liquids, solid particles or liquid droplets entrained in flowing gasses. The wear rate is affec

Variational phase-ﬁeld study of adhesive wear: elastic and elastic-plastic models

Sylvain Collet

Wear is well known for causing material loss in a sliding interface. Available macroscopic approaches are bound to empirical ﬁtting parameters. We propose a study of the physical processes that lead to wear at the scale of the surface roughness, where adhesive junctions are formed between the asperities on the surface of the materials. Using a brittle formulation of the variational phase-ﬁeld approach to fracture, we demonstrate that the failure mechanism of an adhesive junction can be linked to its geometry. By imposing speciﬁc couplings between the damage and the elastic energy, we further investigate the triggering processes underlying each failure mechanism. We show that a large debris formation is mostly triggered by tensile stresses while shear stresses are dominating the mechanisms leading to small or no particle formation. The interaction of two neighboring junctions is analyzed and shows that macro-particles which embody the two junctions can be formed under certain conditions. A ductile phase-ﬁeld formulation permits us to assess the effect of the material’s ductility on the failure mechanism of adhesive junctions. We observe that the formation of large particle becomes less likely to happen as the ductility of the material increases.

2020

Transmissivity of rock fractures: Normal loading and shear reactivation

Monia Woehner

Enhanced Geothermal Systems represent a major field of study in the context of renewable energy resources. To create extractable energy from those reservoirs, a high enough fluid flow rate for production needs to be achieved. This fluid flow rate is directly related to the permeability of the fracture system in the reservoir. Hence, by increasing the reservoir’s permeability the fluid flow increases as well. For this purpose, shear stimulation has proven itself effective but remains a challenging field of study. Many aspects of this mechanism are still unknown and have to be explored further. This study focuses on the fracture transmissivity evolution of low porosity rocks (i.e. granite and marble) with smooth fracture surfaces subjected to progressing shear displacement until several millimetres of offset. The aim of this study was to record fracture transmissivities continuously throughout the experiment without stopping the mechanical loading processes and thereby using the oscillatory pore pressure method. In this context, a new experimental procedure was created making a direct shear configuration using a standard triaxial apparatus possible. Both samples were exposed to a constant confining pressure of 25 MPa which can be directly reflected as normal stress on the fracture. The granite fracture transmissivities were in the range of 6.8317·10−20 to 3.4429·10−19 m3. They first followed a decreasing trend until reaching the minimum value. Then a strong peak could be observed after which transmissivities were increasing again until the end of the experiment. Numerous peaks and deviations from the linear trend could also be observed. The marble fracture transmissivities were in the range of 1.4604 · 10−20 to 2.1110 · 10−20 m3 and were generally lower than the granite’s. Also, the transmissivity evolution followed a different path, continiously and slowly decreasing until the final position. The curve was generally more flattened and did not show any strong peaks. These results showed that rock fractures behave in different manners according to their lithology and more precisely, to the material’s hardness. Rock fractures often form wear products which can either obstruct the flow paths or enhance them as a function of the wear mechanism and wear volume. The granite sample encountered the debris formation mechanism that would transition from mild to severe wear. This transition could be observed in its fracture transmissivity passing from a decreasing to an increasing trend. On the other hand, the marble sample was subjected to a continual asperity smoothing mechanism, thereby slowly closing up the fracture and decreasing its fracture transmissivity. Within the frame of EGS, the results of this work can have interesting implications for production rates. Results showed that not only loading conditions can affect the fluid flow through rock fractures. Rock lithologies and ranges of shear displacement also play an important role and should be explored further.

2020

Variational phase-field continuum model uncovers adhesive wear mechanisms in asperity junctions

Sylvain Collet, Jean-François Molinari

Wear is well known for causing material loss in a sliding interface. Available macroscopic approaches are bound to empirical fitting parameters, which range several orders of magnitude. Major advances in tribology have recently been achieved via Molecular Dynamics, although its use is strongly limited by computational cost. Here, we propose a study of the physical processes that lead to wear at the scale of the surface roughness, where adhesive junctions are formed between the asperities on the surface of the materials. Using a brittle formulation of the variational phase-field approach to fracture, we demonstrate that the failure mechanisms of an adhesive junction can be linked to its geometry. By imposing specific couplings between the damage and the elastic energy, we further investigate the triggering processes underlying each failure mechanism. We show that a large debris formation is mostly triggered by tensile stresses while shear stresses lead to small or no particle formation. We also study groups of junctions and discuss how microcontact interactions can be favored in some geometries to form macro-particles. This leads us to propose a classification in terms of macroscopic wear rate. Although based on a continuum approach, our phase-field calculations are able to effectively capture the failure of adhesive junctions, as observed through discrete Molecular Dynamics simulations

2020

Transmissivity of rock fractures: Normal loading and shear reactivation

Monia Woehner

2020