Elastic contact to a coated half-space - Effective elastic modulus and real penetration

A new approach to the contact to coated elastic materials is presented. A relatively simple numerical algorithm based on an exact integral formulation of the elastic contact of an axisymmetric indenter to a coated substrate is detailed. It provides contact force and penetration as a function of the contact radius. Computations were carried out for substrate to layer moduli ratios ranging from 0.01 to 100 and various indenter shapes. Computed equivalent moduli showed good agreement with the Gao model for mismatch ratios ranging from 0.5 to 2. Beyond this range, substantial effects of inhomogeneous strain istribution are evidenced. An empirical function is proposed to fit the equivalent modulus. More importantly, if the indenter is not flat-ended, the simple relation between contact radius and penetration valid for homogeneous substrates breaks down. If neglected, this phenomenon leads to significant errors in the evaluation of the contact radius in depth-sensing indentation on coated substrates with large elastic modulus mismatch

An approximate model for the adhesive contact of rough viscoelastic surfaces

Surface roughness is known to easily suppress the adhesion of elastic surfaces. Here a simple model for the contact of \emph{viscoelastic} rough surfaces with significant levels of adhesion is presented. This approach is derived from our previous model [E. Barthel and G. Haiat {\em Langmuir}, 18 9362 2002] for the adhesive contact of viscoelastic spheres. For simplicity a simple loading/unloading history (infinitely fast loading and constant pull-out velocity) is assumed. The model provides approximate analytical expressions for the asperity response and exhibits the full viscoelastic adhesive contact phenomenology such as stress relaxation inside the contact zone and creep at the contact edges. Combining this model with a Greenwood-Williamson statistical modeling of rough surfaces, we propose a quantitative assessment of the adhesion to rough viscoelastic surfaces. We show that moderate viscoelasticity efficiently restores adhesion on rough surfaces over a wide dynamic range

Elastic contact to nearly incompressible coatings -- Stiffness enhancement and elastic pile-up

We have recently proposed an efficient computation method for the frictionless linear elastic axisymmetric contact of coated bodies [A. Perriot and E. Barthel, J. Mat. Res. 19 (2004) 600]. Here we give a brief description of the approach. We also discuss implications of the results for the instrumented indentation data analysis of coated materials. Emphasis is laid on incompressible or nearly incompressible materials (Poisson ratio $\nu>0.4$): we show that the contact stiffness rises much more steeply with contact radius than for more compressible materials and significant elastic pile-up is evidenced. In addition the dependence of the penetration upon contact radius increasingly deviates from the homogeneous reference case when the Poisson ratio increases. As a result, this algorithm may be helpful in instrumented indentation data analysis on soft and nearly incompressible layers

Observation directe et analyse de la morphologie d'un front de fracture piégé dans une interface hétérogène

Nous avons étudié le piégeage d’une fissure par une interface hétérogène lors d’un test de clivage. Pour différentes structurations macroscopiques simples, nous montrons que la morphologie du front de fissure peut être décrite par une approche perturbative du premier ordre. Cette description nous a permis de déterminer les ténacités locales dans les zones de piégeage ainsi que le taux de renforcement de l’interface. Ainsi, on obtient une mesure locale d'adhésion entre deux couches structurées

On the plastic deformation of soda-lime glass - a Cr3+ luminescence study of densification

International audienceSilicate glasses are known to experience an anomalous plastic behavior at micron-scale: 1) they exhibit densification when flowing plastically and 2) hydrostatic pressure affects the yield point. We have previously shown (A. Perriot et al. J. Am. Ceram. Soc. 89 (2006) 596) that densification maps are useful to infer a reliable constitutive law for the plastic response of silicate glasses. It is shown here that for soda-lime glass Cr3+ luminescence microspectroscopy may be used for that purpose. We also show that the constitutive law we have previously developed for amorphous silica provides a qualitative description of normal glasses although it is unable to account for the finer details. More work is needed to quantitatively model normal-glass plasticity at the continuum length-scale

Plastic flow and structural heterogeneities in silicate glasses - A high throughput investigation

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