Analysis of the adhesive strength variance in expoxy matrix-boron fiber joints

A new approach to the problem of adhesive joint failure based on the study of the scale effect of the adhesive strength variance is applied to an epoxy matrix-boron fiber system. The shear adhesive strengh is measured using the pull-out technique. It is found that the variance D practically does not depend on the joint area S, whereas the adhesive strength markedly decreases with increase in S. It is noted that the scatter of the values of by far exceeds the experimental errors. The –S and D–S curves obtained behave similarly to the corresponding curves for EDT-10-steel wire joints. The independence of D from S confirms the concept that the failure of fiber-polymer joints in pull-out tests starts near the end of the joint at which the external force is applied. An analysis of the adhesive strength distribution curves within sufficiently narrow (S=0.1 mm²) intervals of S values also confirms this concept.Presented at the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000).Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 4, pp. 435–444, 2000.     

Adhesion of Thermoplastic Matrices with Different Molecular Weights to Fibers

The dependence of the interfacial strength in polymer-fiber systems on the molecular weight (MW) of the matrix is investigated. Adhesive joints of poly(phenylene-sulfide sulfones) and polysulfones with a steel wire 150 m in diameter and glass fibers 200-300 m in diameter are examined. The MW of both polymers was controlled during the synthesis process. The shear adhesive strength was determined by the pull-out technique. For all the systems investigated, it is shown that the adhesive strength in relation to the duration t _f of their isothermal formation is described by curves with a maximum, whose values depend on the formation temperature T _f. For each polymer with a given MW, the greatest possible of these values, called optimum and denoted by _opt, are determined. It is found that _opt increases with molecular weight. It is speculated that the effect observed stems from the different packing of polymer chains with distinct lengths in the near-surface layers.     

Correlation between the strength of fiber-reinforced plastics and the adhesive strength of fiber—Matrix joints

The relationship between the strength (σc) of unidirectional fiber-reinforced plastics in different stressed states and the interfacial strength of their components is investigated. The shear adhesive strength (τ0) of fiber—matrix joints determined by the pull-out technique is used as a measure of the interfacial strength. To obtain the correlation curves betweenσc andτ0, the experimental results are used, where both the plastic and adhesive strength change under the influence of a single factor. In this case, such factors are the fiber surface treatment, nature and composition of polymer matrices, and test temperature. It is shown that the strength of the glass, carbon, and boron plastics increases practically linearly with increased interfacial strength. Such a behavior is observed in any loading conditions (tension, shear, bending, and compression). Sometimes, a small (10–20%) increase in the adhesive strength induces a significant (50–70%) growth in the material strength. Therefore, the interface is the “weak link” in these composites. The shape of theσc—τ0 curves for composites based on the high-strength and high-modulus aramid fibers and different thermoreactive matrices depends on the nature of the fiber and the type of stress state. In many cases, the composite strength does not depend on the interfacial strength. Then, the fiber itself is the “weak link” in these composites. Submitted to the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000). Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 3, pp. 291–304, May–June, 2000.     

Interfacial strength of joints between fibers and dispersedly filled epoxy matrices

The concentration dependences of adhesive strength are investigated for fiber/dispersedly filled epoxy matrix systems. The measurements were carried out using an improved model of adhesiometer under normal conditions at a constant rate of loading. It is shown that the adhesive strength as a function of filler concentration has a maximum, which is more or less pronounced. The location of the maximum depends on the nature of filler and particle geometry. The increase in the adhesive strength at the maximum reaches 20–30% in comparison with that for the unfilled epoxy matrix. Since the interfacial strength between steel wire and all the mineral powders investigated is zero, the growth in the adhesive strength upon introduction of a finely divided filler in polymer binders is rather un expected. The possible reasons for the phenomenon observed are discussed. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 43, No. 1, pp. 3–14, January–February, 2007.