Attenuation of a plane compressional wave by a random distribution of thin circular cracks

The problem of the diffraction of obliquely incident plane compressional waves by a thin circular crack located in an infinite isotropic elastic medium is considered. An analytic estimation of the scattering cross-section is obtained for the Rayleigh limit. The attenuation coefficient associated with a random distribution of such cracks is derived for not too closely-spaced cracks, even when weak scattering density conditions are not satisfied. It is found that, for long wave-lengths, the attenuation varies directly with the average of the sixth power of the radius and inversely with the fourth power of the wave-length, with a coefficient which depends on Poisson's ratio. Comparisons are made with known results for spherical cavities and with the attenuation we could calculate for scalar waves. Conclusions are drawn about attenuation measurements used as a non-destructive test to characterize crack distribution in a sample.     

A non linear integral constitutive equation for viscoelastic fluids

An intégral constitutive equation is written using a particular reference frame, built with unit vectors along the principal axes of the rate-of-deformation tensor, and using the associated intrinsic rate-of-rotation. This equation is easier to handle in calculations than corotational or codeformational models. The material functions for a rheological model including the first six terms of the constitutive equation have been studied in steady and unsteady shear flows, as well as in elongational flows. Material functions are readily written from six memory functions and no inconsistency comes out.     

1st instance of the absence of an erythrocyte membrane protein (band 4(1)) in a case of familial elliptocytic anemia

In four members of a family presenting hereditary elliptocytosis erythrocytes were studied, membranes were extracted and proteins were analysed by gel electrophoresis in a polyacrylamide gradient. In one of the patients suffering from severe haemolytic anaemia successfully treated by splenectomy, an almost complete deficiency in band 4(1) was discovered. Endogenous protein kinase activities revealed the absence of radioactivity of band 4(1) in the proband, a result not modified by cAMP. The kinase activity was normal in the parents, which confirms the almost complete absence of protein 4(1) seen in the stained gel of the proband. A moderate increase in the phosphorylation of band 3 was observed in all the members of this family. Deformability was measured by a visco-diffractometric method (Ektacytometry) in a medium of low viscosity (11 cp at 22 degrees C), allowing the recording of curves characteristic of elliptocytosis and revealing a markedly reduced deformability index (DI). At low shear stress, elliptocytes were oriented perpendicular to the flow (a result common to all elliptocytosis). In the proband several years after splenectomy, the DI was extremely reduced at high shear stress, which can be explained by the simultaneous presence of elliptocytes, schizocytes and spherocytes. A more detailed comparative study of the proteins of the various members of the family could lead to more precise information on the possible role of band 4(1) as a linkage protein maintaining the erythrocyte membrane stability.     

Peeling of Acrylic Pressure Sensitive Adhesives: Cross-Linked versus Uncross-Linked Adhesives

In this paper we analyze the adhesive properties of two kinds of adhesives, determined by a 90[ddot] peeling test on a Pyrex^TM substrate. Simultaneously, we observe the mechanisms of flow at the peeling front. An uncross-linked acrylic pressure-sensitive adhesive is used, whereas the second one, of the same class, is slightly cross-linked. The mechanisms of peeling are compared with the ones of our previous study (Benyahia et al [8]) and are found to be identical in the case of uncross-linked adhesives. On the other hand, we find new regimes of flow when the adhesive is cross-linked.

To investigate these differences further, we determine the rheometrical properties of the adhesives in dynamic shear tests and in uniaxial elongational experiments. Furthermore, surfaces are characterized.

A discussion of the peeling curves is finally presented, showing the combined effects of the rheological properties and the surface ones. Conditions for predicting the type of regimes and transitions are also investigated.     

Hardening parameters of elastic-viscoplastic materials

A study by the author [1–3] of the delayed behavior of perturbations in isotropic elastic-viscoplastic media is extended to include usual macroscopic hardening parameters and particular evolution laws proposed in plastic theories. The results obtained are made explicit for some well-known cases where stability conditions and an evaluation of the intrinsic relaxation time are given.     

Crack scattering in polycristalline media

The attenuation of plane elastic compressional waves due to scattering by a distribution of thin circular cracks (in Rayleigh approximation) is compared to the attenuation due to grain-scattering in polycristalline media. An estimation of the range of crack distributions we are able to determine in such media by means of attenuation measurements is then proposed. The influence of porosity is also considered.     

Study of the influence of experimental conditions applied to the indirect tensile test versus type of bituminous mixture

The reference modulus of bituminous mixes, used for road design in France, is determined by the complex modulus measurement, carried out in the frequency domain at 15°C, 10 Hz or by the secant modulus in direct tension at 15°C, 0.02 s. However, there is strong interest to access this modulus using the indirect tensile (IT) test carried out in the time domain. Actually, this test appears to be easier to implement due to the sample manufacturing and numerous laboratories are already equipped. The present study proposes to determine the experimental condition to be applied to the IT test in order to get the reference modulus. A theoretical approach, based on the Maxwell and Huet viscoelastic linear models, is used to find the testing conditions at which the IT test has to be performed to give as precisely as possible the reference modulus. This procedure is applied on a large database of materials for which the viscoelastic spectrum has been provided. The influence of the loading waveform, which cannot be systematically controlled, is studied. For a force increasing linearly with time, the temperature at which the IT test has to be performed is close to 10°C, whatever the bituminous mix type. Moreover, the error that would have been committed by applying the equivalent temperature T_eq = 10°C during the indirect tensile test is evaluated. This error depends on the loading waveform but always remains, theoretically, under 1000 MPa. This study has been carried out in the framework of collaboration between LCPC and USIRF (Union des Syndicats de l’Industrie Routière française).     

Delayed behavior of double shear perturbations of isotropic incompressible elastic-viscoplastic materials

A study of the instantaneous and delayed behavior of a double shear perturbation superimposed on an equilibrium state of an isotropic incompressible medium with internal variables is presented. Elastic media with general internal variables and true viscoplastic media where an intermediate configuration and particular internal parameters are chosen, are successively considered. In both cases, conditions on evolution laws and free energy are proposed, and proved to be sufficient to obtain a stable system of differential equations for the perturbations. As a consequence, the two delayed wave speeds are then real and less than or equal to the instantaneous elastic wave speeds. When the equilibrium state lies on a viscoplastic yield surface, delayed wave speeds and loading conditions may be identified with those we obtain in a plastically deforming (rate-independent) medium, with the same surface as a plastic yield surface. The signification of the relaxation time introduced is also discussed.     

The Mechanisms of Peeling of Uncross-Linked Pressure Sensitive Adhesives

We analyze the peeling properties of an uncross-linked pressure sensitive adhesive. 90° peeling master curves on Pyrex^TM and PMMA (polymethylmetacrylate) are constructed The shift coefficients a_T are compared with the ones obtained from rheometrical shear tests.

With our machine, the peeling front is kept fixed, enabling us to observe the mechanisms of deformation of the adhesive. We count four different mechanisms of peeling in cohesive failure, and three in interfacial peeling (the last being unstable); they correspond to various slopes that we identify. The flow patterns at slow reduced velocities are two-dimensional. Then they undergo transitions to three-dimensional periodic complex flows, due to instabilities in the flow of thin adhesives. Interpretation of these peeling master curves are discussed in terms of rheology and physico-chemistry. It appears necessary to take into account the elongational properties of the adhesive, as well as the surface energy properties, to predict adhesion.