For the first time a method is proposed for determining the stress intensity factors (SIF) near cracks in linear
EQUATIONS FOR DETERMINATION OF SIF WITH KNOWN FRINGE ORDERS re(t) IN THE VICINITY OF THE APEX OF A CRACKConsider a plate made of a linearly viscoelastic fiber composite with a central crack whose direction coincides with the direction of predominant reinforcement. The plate is strained by constant, uniformly distributed stress a o (MPa) at angle a to the direction of the crack. We will assign the crack to a rectangular system of coordinates x, y with the origin at the apex of the crack and axis Ox along the crack.We obtain the expressions for the stress components axx, ayy, rxy near the apex of a crack in a plate made of a linearly viscoelastic composite based on the quasi-elastic method of solving viscoelasticity problems [1] from the correspondhag equations of the mechanics of fracture of elastic orthotropic bodies [2], substituting the elastic constants in them by creep functions Lij(t):-s.,(t) p2(t)] + crox;
%~(t)-K~(t) K~t) &r, t) [sat) n,(t) -s~(t) n~(t)] + ~ LO.~t) -pl(t)];
~(t) = K~t) st(t) s.z(t)Kz~t) A(r, t) Lot(t) -p2(t)] + ~ [s2(t) n2(t) -st(t) nl(t)].
In polymeric composites reinforced with short fibers, the extremities of the latter are, as a result of stress concentration, sources of crack initiation. Investigation of the formation and behavior of these cracks as a function of the mechanical properties of the components in the composite and the linear dimensions of the fibers and cracks is of interest for estimation of composite serviceability. Numerical solutions of model problems of crack-fiber interaction are known at present. Thus, Selvadurai [ I] determined the stress-intensity factors (SLY) near circular cracks at the extremities of a single cylindrical fiber in an tmrestrained elastic die under uniaxial tension by the boundary-element method.In our study, we examine the problem of a plate of finite width, which is reinforced by a fiber with a given crack on one of its ends and tensioned along the fiber. Plates were fabricated from a photosensitive material, and the fibers were modeled by circular rods formed from various materials. The stress-inteusity factors at the ends of the cracks was determined from data derived by the optical-polarization method.We investigated the dependence of the SIF on the following physical and geometric parameters: the ratios of the elastic moduli of the fiber and matrix Er/Em; the ratios of crack length to fiber diameter 2c/d, and, the ratios of fiber length to fiber diameter lid.
PROCEDURE FOR MODEL FABRICATIONModels of plates reinforced with short discrete fibers with cracks at the ends were prepared in the following manner. Blanks for the cracks, which were formed from metallic plates of a given width and a thickness of 0.1-0.12 mm and coated with an amiadhesive, were first placed in a special mold. These blanks were fixed in clamps perpendicular to the plane of the plate. The mold was filled with a potting compound (epoxy resin ED-20 with a modified hardener -dicyanethyldiethyientriamine and polyethylene polyamine with an additive consisting of a dibutylphthaiate plasticizer -with a formulation such that we were able to produce a plate with a thickness t = (l/2)(h -d), where h is the proposed thickness of the plate and d is the
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