The resonant frequencies of unidirectional graphite epoxy composite beams were found to deviate markedly from classical beam theory predictions at higher modes of vibration. Timoshenko beam theory was used to account quantitatively for the dependence of experimental resonant frequencies on mode of vibration, length/thickness ratio, and the ratio of Young's modulus/shear modulus of the composite beam. By using reasonable values for the longitudinal-transverse shear moduli, the longitudinal Young's modulus of anisotropic composite beams under vibration agreed well with values determined by static tests and became independent of mode of vibration and L/t. The vibrating beam test method was employed for the determination of E 11 , E 22 , and G 12 .

Recent developments in nonlinear finite element methods (FEM) and mechanics of composite materials have made it possible to handle complex tire mechanics problems involving large deformations and moderate strains. The development of an accurate material model for cord/rubber composites is a necessary requirement for the application of these powerful finite element programs to practical problems but involves numerous complexities. Difficulties associated with the application of classical lamination theory to cord/rubber composites were reviewed. The complexity of the material characterization of cord/rubber composites by experimental means was also discussed. This complexity arises from the highly anisotropic properties of twisted cords and the nonlinear stress—strain behavior of the laminates. Micromechanics theories, which have been successfully applied to hard composites (i.e., graphite—epoxy) have been shown to be inadequate in predicting some of the properties of the calendered fabric ply material from the properties of the cord and rubber. Finite element models which include an interply rubber layer to account for the interlaminar shear have been shown to give a better representation of cord/rubber laminate behavior in tension and bending. The application of finite element analysis to more refined models of complex structures like tires, however, requires the development of a more realistic material model which would account for the nonlinear stress—strain properties of cord/rubber composites.

The polymer—solvent interaction parameter for an ethylene—propylene ter-polymer containing 53 mole-% ethylene was determined as a function of degree of crosslinking in n-heptane and benzene. A well characterized EPR gum vulcanizate was used to obtain the polymer-solvent interaction parameter in fourteen solvents, and the cohesive energy density of the terpolymer was evaluated. The composite tensile creep curves for gum and filled EPR vulcanizates were obtained and compared with SBR creep curves.

The tensile strength and elongation at break of EPR and SBR gum vulcanizates were determined as a function of network chain density. A large viscous contribution to the tensile force was found at low crosslink density, and the maximum in the tensile strength versus chain density curve decreased as more nearly equilibrium conditions were approached. The rate dependence of tensile strength of black‐filled EPR was studied by creep rupture experiments. The temperature dependence of the tensile strength of filled EPR and SBR was studied and compared. The same limiting degree of cure was obtained for both filled and gum vulcanizates of EPR.

SynopsisThe effect of low plasticizer concentrations on the glass transition temperature of poly(methy1 methacrylate) (PMMA) has been studied experimentally for diethyl phthalate (DEP) and methyl methacrylate (MMA) as plasticizers. Volume dilatometry, differential thermal analysis, and a torsional pendulum were used to determine glass transition temperatures. The monomer-plasticized PMMA samples were prepared by polymerizing MMA to a limiting conversion which was found to depend primarily on temperature. At temperatures slightly above the glass transition temperature of a MMA-PMMA system, a polymerization reaction occurred at a rate rapid enough to complicate the interpretation of the dilatometric and differential thermal analysis methods for determining the glass transition temperature. However, the torsional pendulum method could be used since it did not require measurements to be made at temperatures where polymerization could occur. The differential thermal analysis resultq showed that the temperature at which the polymerization reaction was first detectable was related to the glass transition temperature of the MMA-PMMA solution. The measured glass transition temperatures were compared with the predictions of the theories of Fox, Kelley, and Bueche, and Dimarioz and Gibbs on the effect of plasticizer concentration on the depression of the glass transition temperature. * Present address: Lord Manufacturing Company, Erie, Pennsylvania. 3795 3796 T. J. DUDEK AND J. J. LOIIHaccepted dilatometric method were complicated by polymerization in the region of the glass transition temperature. This made the determination of T , from the dilatometric data potentially unreliable. For this reason, differential thermal analysis (hereafter referred to as DTA) and torsional pendulum methods of measuring glass transition temperatures were also evaluated. Glass transition temperatures measured by these two methods were related to the dilatometrically determined T , for a PMMA-DEP system which is free of the complications of polymerization.This relation is then used to determine the "true" dilatometric To of the PMMA-MMA system from the glass transition temperatures measured by the torsional pendulum method and by DTA. Finally, the FOX,^,^ Kelley-Bueche,' and Dimarzio-Gibbs5 relations of glass transition temperature depression due to plasticizer content are compared with the measured T,. EXPERIMENTAL MaterialsMethyl methacrylate (MMA) obtained from the Rohm and Haas Company was washed with 5% NaOH and with water to remove the inhibitor and was then dried and distilled. A middle fraction was collected for use in the preparation of polymer samples. -Gas chromatography showed that the distilled monomer was free of impurities.Diethyl phthalate (DEP) (Eastman, White Label) was used as received, as was cY,a'-azobisisobutyronitrile (AIBN) (duPont's Vazo).