Interaction between gravitational sedimentation and shear diffusion in a suspension moving in a rotational viscometer gap

The start up behavior of a concentrated suspension of nonaggregating settling particles in a rotational viscometer is examined. The results of one of the experiments are presented.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 44–51, September–October, 1990.     

Investigation of stress intensity factors near interacting cracks in linear viscoelastic plates

Stress intensity factors as a function of time near interacting cracks during creep in plates made of linear viscoelastic materials are investigated. The interaction of collinear as well as parallel cracks disposed on different levels perpendicular to the tensile stress was studied. It was found that during interaction of collinear cracks, the stress intensity factor K₁(t) grew in creep and varied mainly due to the change in the distance between the tips of interacting cracks. There was practically no increase in the length of the crack. During interaction of parallel cracks, the value of K₁(t) decreased slightly in time, which was also due to an increase in the distance between the tips of the cracks in creep.Institute of Mechanics, Lomonosov Moscow State University, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 5, pp. 643–650, September–October, 1998.     

Study of stress intensity factors near cracks in plates made of linearly viscoelastic fiber composites

For the first time a method is proposed for determining the stress intensity factors (SIF) near cracks in linear viscoelastic fiber composites using the data from polarization-optical measurements. The character of the change in the SIF in time near cracks in plates made of optically sensitive viscoelastic fiber composites exposed to constant tensile stresses (creep) was investigated. In describing the mechanical and optical properties of the materials, the composite was considered a homogeneous linearly viscoelastic orthotropic body. Equations were derived which correlate the fringe ordersm(t) with SIFK _j (j=1,2) and the longrange field stress _Ox in the vicinity of the apex of a crack whose direction coincides with the main direction of the axis of orthotropy. The SIF near linear cracks whose direction forms angles =90° and =45° with the direction of the tensile stresses were determined. It was found that the character of the change in SIF in time near cracks in creep, is a function of the angle between the direction of the crack and the direction of the tensile stress.Scientific-Research Institute of Mechanics, M. V. Lomonosov Moscow State University, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 33, No. 3, pp. 321–328, May–June, 1997.     

Cracks in fiber composites: Determination of the stress intensity factor by the polarization-optical method

Composite plates with cracks coinciding with one of the principal axes of anisotropy of the material were examined. The mechanical behavior of the composite is described by the model of a homogeneous orthotropic body. A method was proposed for determining the stress intensity factor K₁ with the known values of the fringe orders m in the vicinity of the tip of the crack. It was found that the value of K₁ is a function of the ratio of the length of the plates to their width d/b. The stress intensity factor K₁ increases with a decrease in ratio d/b. Tlie effect of the orthotropy of the material on the value of K₁ is also a function of the ratio of the length of the plate to its width d/b.Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 6, pp. 769–775, November–December, 1996.     

Modeling the mean stresses in structural elements made of fiber composites

Conclusions The results of solution of the above-examined problems show that the method proposed here makes it possible to satisfactorily (with an error smaller than 5% for the maximum stresses) recalculate the mean stresses obtained in models made of fiber composites (such as by the photoelastic method) for the corresponding full-scale parts. The method does not require special election of the model materials, which is its main advantage and makes it easy to use.Translated from Mekhanika Kompozitnykh Materialov, No. 3, pp. 495–500, May–June, 1990.     

Experimental investigation of the dependence of the birefringence effect on the elastic and rubbery strains for certain epoxy polymers at room temperature

The strain dependence of the optical path difference δ=δ(e, ?*) has been investigated for certain network polymers. It is shown that this dependence can be represented in the form: δ=αe+β?*, where α and β are the elastic (e) and rubbery (?*) strain optic coefficients. Values of the coefficients α and β are determined and their properties are investigated. The possibility of determining the strains in parts made of the investigated polymers from the known effects is examined.     

Model studies of stress-intensity factors near cracks interacting with fibers

The problem of determining the stress-intensity factors near cracks interacting with fibers is solved for the first time using data from optical-polarization measurements. Simplified models of composites are investigated — plates reinforced with single short fibers with cracks assigned on one of the ends of the fibers and tensioned in the direction of the fibers. The plates (dies) were fabricated from a photosensitive material, and the fibers were modeled by bars of steel, glass, and polymer. The stress-intensity factors were determined from Eq. (5) using data derived from optical-polarization measurements at several points by the nonlinear method of least squares. We investigated the influence exerted by several physical and geometric parameters on the stress-intensity factors. It was established that the stress-intensity factor K₁ near the end of a crack interacting with a fiber is higher than the factor K⁰ ₁ near a crack of corresponding length in the unreinforced plate. The ratio K₁/K⁰ ₁ depends on the mechanical properties of the fiber and die materials and the geometric dimensions of the crack and fiber. Despite the fact that these results were obtained using simplified models of composites, they are of interest for evaluation of the failure and serviceability of real composites.Scientific-Research Institute of Mechanics, M. V. Lomonosov Moscow State University, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 32, No. 4, pp. 493–501.     

Modeling of blood rheological behavior in unsteady-state shearing flows

Issues of blood flow modeling under unsteady-state conditions at moderate shear rates are considered using a blood rheological model accounting both for the viscoelastic properties and the thixotropy caused by erythrocyte aggregation. The resulting shear stress versus time relations for single shear rate steps and the dependence of the complex viscosity components on the shear rate amplitude in oscillating shear flow show good qualitative correspondence with the experimental data reported in the literature.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 6, pp. 26–30, November–December, 1995.