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Michele Scaraggi - Research Interests
His actual research activities concentrate on the mechanics of dry/lubricated contacts of real (rough) surfaces. Elasto-hydrodynamic lubrication, mixed lubrication, lubricant rheology, adhesion are some reaserch topics, with applications, e.g., in the field of high-pressure contacts mechanics (as bearings, gears, continuously variable transmissions) and soft contacts mechanics (rubber sealings, cosmetics). Some recent investigations:
 
 
 
 

Role of lubricant rheology in hard-EHL squeeze

We analyze the influence of di fferent fluid rheologies on the high loaded normal approach of elastic balls, which is of utmost importance in gears, bearings and continuously variable transmissions. The analyzed lubricant rheologies are 1)Newtonian (linearly viscous), 2)Maxwell (linear viscous - linear elastic) and 3)Rabinowitsch (non-linear viscous, shear thinning) constitutive laws. For the Newtonian fluid, we show that the spatial pressure distribution is characterized by an annular (sharp) pressure peak, which first appears in the external region of the contact domain and after moves toward the center of the pin with rapidly decreasing speed. This high pressure fi eld determines the formation of a high viscosity oil dimple in the center of contact. The lifetime of this pressurized oil dimple, which corresponds to the time required by the lubricant to be expelled from the conjunction, actively determines the friction and wear characteristics at the interface. In the case of Maxwell rheology we show that the pressure field is exactly the same as in the Newtonian case but with a deep reduction in the annular pressure peak value, which explains the non-failure behavior of such contacts; thus we find that the Maxwell rheology enables a more realistic prediction of high loaded lubricated contacts (for lubricants not exhibiting limiting shear stress or shear thinning). The latter case is investigated with a Rabinowitsch constitutive law. We show that if the shear stress threshold, which characterizes the transition from the linear viscous to the non-linear viscous lubricant behavior, is sufficiently small the annular pressure peak may even disappear. In this case the squeeze process occurs faster (shorter lifetime), the fi lm thickness distribution is reduced and the lubricant may not be able to avoid direct asperity-asperity contact between the two approaching surfaces. The lubrication models is applied to the investigation of the pure squeeze motion at the pin-pulley interface in continuously variable transmissions (CVTs).
 

Newtonian and Maxwell lubricant pressure field. Comparison.

The oil pressure field for Newtonian and Maxwell fluid film. The oil viscoelasticity intervenes locally to smooth the annular pressure spike.
 

The film thickness as a function of the radial coordinate for different time instants. The radial displacement of the position of the minimum film thickness during time is due to the corresponding variation of normal load.

The fi lm thickness as a function of the radial coordinate for diff erent time instants in the case on non-constant load condition. The radial displacement of the position of the minimum film thickness during time is due to the corresponding variation of normal load.
 
 
 
 

The transition from boundary to hydrodynamic lubrication in soft contact

We consider the contact between elastically soft solids with randomly rough surfaces in sliding contact in a fluid, which is assumed to be Newtonian with constant (pressure-independent) viscosity. We discuss the nature of the transition from boundary lubrication at low sliding velocity, where direct solid-solid contact occurs, to hydrodynamic lubrication at high sliding velocity, where the solids are separated by a thin fluid film. We consider both hydrophilic and hydrophobic systems, and cylinder-on-flat and sphere-on-flat sliding configurations. We show that for elastically soft solids such as rubber, including cavitation or not result in nearly the same friction.
 

An asperity contact region observed at a given magnification

An asperity contact region observed at a given magnifi cation.
 

Fluid and asperity contact pressure surfaces.

Fluid and asperity contact pressure surfaces.
 

Friction coefficient for PDMS-PDMS interaction. The green line corresponds to the predicted Couette friction.

Friction coefficient for PDMS-PDMS interaction. The green line corresponds to the predicted Couette friction, while the other curves have been obtained by Bongaerts et al., 2007.
 
 
 
 

Mixed lubrication theory in soft contacts: The case of lip sealings.

We consider the contact between a soft rough sealing lip and a smooth rigid rotating shaft. We model the asperity-asperity and asperity-fluid interactions with a deterministic or a statistical approach depending on length scale at which the contact region is observed. Indeed, the roughness at large length scales, which mainly determines the fluid flow at the interface, is deterministically included in the model while the roughness at short-wavelengths, which strongly contributes only to the friction, is included by means of a homogenization process. This contact scheme allows to correctly capture the shear-induced deformation of the roughness asperities occurring in soft mixed lubrication contacts.
 

A schematic of a typical lip seal construction.

A schematic of a typical lip seal construction.
 

Flux lines at the contact interface (red curves are). The velocity field is shown in the vector form (black arrows) and in module (white-blue color gradient, where blue color is used for the higher values).

Flux lines at the contact interface (red curves are). The velocity fi eld is shown in the vector form (black arrows) and in module (white-blue color gradient, where blue color is used for the higher values).
 

Average interfacial separation, average fluid and solid pressure.

Average interfacial separation, average fluid and solid pressure.
 
 
 
 

The transition from hydrodynamic to mixed lubrication in high loaded squeeze contacts.

We analyze the high loaded strongly non-stationary squeeze process of an oil fi lm sandwiched between an elastic spherical ball and a rigid rough substrate. We show that the coupling between the elastic properties of the contacting solids, the oil rheology, the surface roughness and the applied load determines a wide range of lubrication conditions from fully elasto-hydrodynamic to mixed and even boundary lubrication. In particular we fi nd that increasing (decreasing) the surface roughness (the applied normal load) speeds up the squeeze process, anticipates and shrinks the time interval during which the transition to mixed lubricated conditions occurs. On the contrary, the initial separation between the approaching bodies only marginally aff ects the transition time. We also observe that, in mixed-lubricated conditions, the highest asperity-asperity contact pressure occurs in the annular region where the separation between solids takes its minimum value. One then conclude that surface damage and wear should nucleate in the outer region of the contact
 

A film of lubricant squeezed between a smooth elastic sphere and a rough rigid substrate.

A film of lubricant squeezed between a smooth elastic sphere and a rough rigid substrate.
 

The typical spatial distribution of fluid pressure, solid-solid contact pressure and interfacial separation for mixed lubrication squeeze contacts.

The typical spatial distribution of fluid pressure, solid-solid contact pressure and interfacial separation for mixed lubrication squeeze contacts. Observe that in the gray area across the minimum value of separation, where the solid-solid pressure takes its maximum value, the solid-solid contact spots may coalesce and obstruct the fluid passage. The oil then may not be squeezed out and remain entrapped between the two solids.
 
 
 
 

Adhesive contact of rough surfaces.

We have employed a numerical procedure to analyze the adhesive contact between a soft elastic layer and a rough rigid substrate. The solution of the problem, which belongs to the class of the free boundary problems, is obtained by calculating the Green's function which links the pressure distribution to the normal displacements at the interface. The problem is then formulated in the form of a Fredholm integral equation of the first kind with a logarithmic kernel, and the boundaries of the contact area are calculated by requiring that the energy of the system is stationary. The methodology has been employed to study the adhesive contact between an elastic semi-infi nite solid and a randomly rough rigid profi le with a self-affine fractal geometry. We show that, even in presence of adhesion, the true contact area still linearly depends on the applied load. The numerical results are then critically compared with the prediction of an extended version of the Persson's contact mechanics theory, able to handle anisotropic surfaces, as 1D interfaces.
 

The logarithm of the probability as function of pressure and contact magnification.

The logarithm of the probability as function of pressure and contact magnification. Points are numerical predictions whereas dashed lines are Persson's results. We observe that the tail of the probability distribution at large values of pressure follows exactly a Gaussian distribution.
 
 
 
 
 

Michele Scaraggi

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Mobile: +39 320 149 7255

Fax: +39 080 596 2777

Email: m.scaraggi@poliba.it

 

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