Effect of certain structural parameters of reinforced plastics on strength and stiffness in interlaminar shear

This study is concerned with the manner in which strength and stiffness in interlaminar shear of orthogonally reinforced laminates depend on the mechanical characteristics of their components, on the volume fraction of reinforcing material, on the coefficients of reinforcement distribution in the stacking direction, and on the microporosity of the polymer matrix, as well as other microdiscontinuities in the various structural components of the plastic. For the purpose of assessing the strength and the stiffness in interlaminar shear, with the volume fraction of reinforcing material and the interlaminar microporosity of the composite material varied, approximate formulas have been derived and their practical applicability subsequently confirmed in tests on glass-resin textolite specimens. It is shown that the indicated mechanical properties of a plastic are appreciably affected by these two structural parameters, especially so by its interlaminar microporosity.     

Approximation of strength surfaces in plane stress

Approximation of strength surfaces in plane stress is considered with the use of the method of expansion of the function on a sphere by means of tensors. Examples of the approximation are given for certain composite materials.     

Strength of an organic/glass fiber-reinforced textolite in plane stress

Conclusions An analysis of the results of testing hybrid glass organic textolites, containing layers of glass and organic fabric reinforcement in various proportions, along characteristic simple short-time quasi-static loading paths served as a basis for determining a family of strength surfaces for plane stress in the plane of reinforcement. The strength of the five materials investigated is described by a second-order surface equation with allowance for the difference in compressive and tensile strengths. The dependence of the strength surface tensor components entering into the strength equation on the structure parameter representing the relative content of organic and glass fabric in the hybrid textolite is investigated and described. The results obtained can be used in practical calculations for determining the optimum ratio of organic to glass fabric in hybrid material with allowance for the specific requirements to be met by the strength properties of the material when used for structural purposes.Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 1021–1026, November–December, 1979.     

Analysis of methods of determining the interlaminar shear strength of composite materials

Conclusions The range of geometrical dimensions of a sample in which the values of the interlaminar shear strength determined by four of the five methods discussed are practically constant has been established. Good agreement is shown in the values of the characteristic being determined which are obtained by each of the four methods. It has been established that the experimental data obtained by the method of three-point deflection of short beams does not characterize their interlaminar shear strength. It is shown that the interlaminar shear strength of glass-plastic and the shear strength of the adhesive with which it is prepared are very similar in value.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 640–648, July–August, 1976.     

Long-time strength of isotropic polymers in plane stress state

A criterion of strength of isotropic polymers in the form of the critical work done by stress was postulated and shown, by statistical analysis, to be independent of the time and form of loading. The introduction of the time factor in the existing theories of strength of isotropic materials was postulated and experimentally verified with the aid of this criterion.Mekhanika Polimerov, Vol. 2, No. 6, pp. 837–844, 1966     

Fracture of fiber-reinforced materials

The fracture behaviour of fiber-reinforced materials is studied in this paper. Using a simple shear lag model, which includes friction at the debonded interface and the Poisson contraction of the fiber, the fiber-matrix debonding problem is solved. This gives the relationship between debonding load and debonded length. Interfacial friction is shown to have a significant effect on the debonding load. The fracture toughness of fiber-reinforced materials due to fiber debonding, frictional dissipation at fibre-matrix interface following debonding and other micro-fracture mechanisms is discussed with reference to strong and weak fibres. Finally, the strength and toughness of short fibre-reinforced materials are given.On leave from Harbin Shipbuilding Engineering Institute, Harbin, China     

Experimental investigation of the impact strength of Caprolon in linear and plane stress

An apparatus for investigating the impact strength of materials in plane stress is described. The results of testing Caprolon in plane stress and linear tension over a broad range of strain rates are presented.Leningrad Kalinin Polytechnic Institute. Translated from Mekhanika Polimerov, No. 2, pp. 332–334, March–April, 1971.     

Evaluation of the stress singularities of plane V-notches in bonded dissimilar materials

According to the linear theory of elasticity, there exists a combination of different orders of stress singularity at a V-notch tip of bonded dissimilar materials. The singularity reflects a strong stress concentration near the sharp V-notches. In this paper, a new way is proposed in order to determine the orders of singularity for two-dimensional V-notch problems. Firstly, on the basis of an asymptotic stress field in terms of radial coordinates at the V-notch tip, the governing equations of the elastic theory are transformed into an eigenvalue problem of ordinary differential equations (ODEs) with respect to the circumferential coordinate θ around the notch tip. Then the interpolating matrix method established by the first author is further developed to solve the general eigenvalue problem. Hence, the singularity orders of the V-notch problem are determined through solving the corresponding ODEs by means of the interpolating matrix method. Meanwhile, the associated eigenvectors of the displacement and stress fields near the V-notches are also obtained. These functions are essential in calculating the amplitude of the stress field described as generalized stress intensity factors of the V-notches. The present method is also available to deal with the plane V-notch problems in bonded orthotropic multi-material. Finally, numerical examples are presented to illustrate the accuracy and the effectiveness of the method.     

Creep and long-time strength of oriented glass-reinforced plastics in interlaminar shear

The creep and long-time strength in interlaminar shear and the creep in compression in the direction of the reinforcement have been experimentally investigated for certain types of oriented glass-reinforced plastics. The specimens in the interlaminar creep tests took the form of short beams loaded in bending. The experimental creep data for shear and compression are well described by the hereditary theory with a kernel of the Abel type (shear) or in the form of a Rabotnov function (compression). If the stresses are constant in time, good agreement with experiment is also given by Findley's form of the aging theory. A deformation criterion of interlaminar shear strength is also obtained. The experimental curves and values of the creep and long-time strength constants are presented.Translated from Mekhanika Polimerov, No. 6, pp. 1003–1012, November–December, 1971.     

Some negative characteristics of fiber-reinforced materials

The effect of low resistance to shear and tension-compression at right angles to the layers of reinforcement on the state of stress and strain of elements composed of fiber-reinforced materials is analyzed. It is shown that to take these characteristics into account the calculations must be refined. In certain cases this leads to the appearance of qualitatively new effects. Typical problems in which the negative characteristics of the material must be allowed for are examined.Review of research conducted in the Polymer Structures Laboratory of the Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 5, No. 1, pp. 140–149, January–February, 1969.     

Testing the interlaminar shear strength of laminated reinforced plastics

An improved method of determining the interlaminar shear strength of reinforced plastics from the results of mechanical bending tests on beams of different lengths is described and the results of an experimental verification of the method on glass-reinforced textolite and longitudinal-transverse-wound (LTW) glass-reinforced plastic are presented.     

Stability of members composed of fiber-reinforced materials

Equations for the stability of a centrally loaded compression member are derived from the general relations of the theory of elasticity of an anisotropic body. Other methods of deriving these equations are discussed and their accuracy evaluated. It is shown experimentally and theoretically that shears sharply reduce the critical load for members composed of fiber-reinforced materials. Limits beyond which increasing the stiffness in the axial direction ceases to be effective are established.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 4, No. 5, pp. 911–917, September–October, 1968.     

Study of microbuckling of fiber-reinforced composite materials

On the basis of the theory of microbuckling of lamina-reinforced composites and formula predicting the critical composite stress for microbuckling in the shear mode _cs published in the literature, a FORTRAN program for study of the behavior of microbuckling of fiber reinforced composites has been developed. Some types of composite materials (reinforcement of different fibers and epoxy matrix) have been studied. Graphics and curves, accounting for the dependences of the compressive stress at failure _cs from the reinforcement volume k, specimen length L, and shear modulus of resin G_r have been obtained. The comparison of the theoretical diagrams presented here and experimental and theoretical results, published in the literature shows good agreement. The basic conclusion of the work presented here is that the study could be used for other fiber reinforced composites (with different mechanical properties of matrices and fibers).Institute of Mechanics, Bulgarian Academy of Sciences, Sofia, Bulgaria. Published in Mekhanika Kompozitnykh Materialov, Vol. 32, No. 4, pp. 531–538, July–August, 1996.