Ply Strength Research Articles

Analysis of first ply failure in composite laminates

The mechanics of transverse cracking in an elastic fibrous composite ply is explored for the case of low crack density. Cracks are assumed to initiate from a nucleus created by localized fiber debonding and matrix cracking. It is found that cracks may propagate in two directions on planes which are parallel to the fiber axis and perpendicular to the midplane of the ply. In general, crack propagation in the direction of the fiber axis controls the strength of thin plies, while cracking in the direction perpendicular to the fiber axis determines the strength of thick plies. The theory relates ply thickness, crack geometry and ply toughness to ply strength. It predicts a significant increase in strength with decreasing ply thickness in constrained thin plies. The strength of thick plies is found to be constant, but it may be reduced by preexisting damage. Results are illustrated by comparison with experimental data.

A Stochastic Model for the Growth of Matrix Cracks in Composite Laminates

This paper presents a stochastic simulation model for the growth of multiple matrix cracks in composite laminates subjected to both static and fatigue loads. Working within the premise of ply-elasticity, a new concept of effective flaws is introduced which replaces the conventional constant ply strength criterion. Thus, the model con sists of an application of fracture mechanics and a rational representation of material flaw distributions. Simulation examples are presented on [02/902]s and [0 2/903]s graphite-epoxy laminates which undergo characteristic transverse cracking under uniaxial tension.

Basic Problems in the Theory of Bond Strength between the Plies of a Rubber Article

Abstract The problem of ensuring adequate bonding strength between uniform and nonuniform layers in composite vulcanized rubber articles is particularly vital in the production of high grade multi-ply articles for heavy duty (high speeds, high dynamic loading and considerable heat build-up) and also in the production of multi-ply articles from dissimilar compounds based on synthetic rubbers of different structural type. In works practice various methods are known for the production of two-ply and multi-ply composite vulcanized rubber articles, based on plying-up in the hot state (sometimes combined with buffing of the unvulcanized parts and the use of special materials to increase the tackiness of the stocks) and also on the use of solvents and the cementing of the stocks by a cement, sometimes of several types applied in sequence. Difficult technological tasks arise in the production of composite articles with dissimilar plies based on rubbers of different chemical composition, for instance of natural and sodium butadiene (or chloroprene) or of natural (or butadiene) and butyl rubber. In spite of considerable practical successes, many of the tasks enumerated above remain unsolved, and there is hardly any great hope of success without a previous examination of a number of basic theoretical questions. A theory of strength has not been worked out even for the most simple case—the stretching of a strip of unloaded vulcanized rubber. There are many questions unstudied, the examination of which must precede the development of a theory of the strength of dissimilar plies in a multi-ply composite rubber article. Before considering the separate questions we regard it as worthwhile to formulate the general tasks of a theory, keeping within the bounds of the problem under discussion, i.e., bond strength. According to the minimum requirements, theoretical investigations ought to enable the selection of correct basic (determinant) scientific lines and to facilitate the development of rational methods of investigation, reflecting the essential aspects of the phenomenon being studied. Of course, major failures and unplanned and wasted labor are often caused by the choice of mistaken lines of investigation or are the outcome of the use of mistaken methods of testing.

Effect of Single and Ply Twists on the Properties of a 31 /2 Carded Cotton Yarn

The effect of single and ply twists on the properties of a 31/2 cotton carded yarn was studied. A widely grown cotton variety (Deltapine) of average fiber properties was spun into a 31/1 yarn (Z twist) using a range of twist multipliers (2.9-6.9). Each single yarn twist multiplier construction was then 2-ply twisted (S twist) with a range of twist multipliers (1.9-7.9). Maximum ply strength was obtained when low-twist single yarns were 2-ply twisted with a high twist multiplier. Maximum elongation at break was obtained with a similar single-ply twist construction. For certain twist constructions single and ply twist may be varied within reasonable limits without materially decreasing strength. When the same twist multiplier was used in the single and ply yarn, maximum ply yarn strength was realized with that twist multiplier which resulted in maximum single yarn strength. Elongation is related linearly and directly to equivalent twist multiplier constructions. The ratio of ply to single yarn strength, based on maximum ply yarn strength, was highest when single yarn strength was lowest (low twist), decreased rapidly as the single yarn strength approached maximum, became approximately constant in twist combinations resulting in single yarns of maximum strength, and increased slightly as single yarn strength decreased (very high twist).