Journal of Composite Materials

The increasing interest in utilizing carbonaceous-fiber-reinforced carbon matrix materials in high temperature structural applications has resulted in the generation of an increasing amount of experimental data, primarily for unidirectionally reinforced systems. While the longitudinal tensile strength and stiffness properties presently being achieved are attractive relative to those of unreinforced carbonaceous materials such as bulk graphite, they remain well below their apparent potential. Utilizing micromechanical analyses and taking into consideration the highly anistropic crystallo graphic structure of graphite, possible explanations for the observed properties degradation have been formulated. Suggestions for modifying the fabrication and thermal processing as a means of improving axial properties, and tensile strength in particular, are presented.

The notch sensitivity of boron film (B/PI) reinforced graphite fiber composites was compared to the behavior of fiber-only material by means of uniaxial tests on tensile coupons with two sizes of holes (.052 and .104 in.) drilled at midgage length, and on undrilled control coupons. The re sults indicated that B/PI additions to graphite fiber laminates reduced the notch sensitivity, while at the same time substantially raising both the tensile strength and the work-to-fracture of the material.

Strain and load controlled fatigue tests were run on boron-aluminum and boron-epoxy angle-plied specimens. Acoustic emission data was recorded using a gating technique that eliminated most extraneous noise. Various material parameters were monitored during the test. A good corre lation between acoustic emission and damage extent and propagation was obtained. This was evidenced by an apparent first order linear relationship between dynamic compliance and totalized acoustic emission. A basis for an energy based failure model is formulated and discussed.

An ultrasonic resonance technique for compressional and shear waves velocity measurements in thin composite laminates is described. The method utilizes a computer digitizing scheme and Fourier transform from time to frequency domain. Results showed an eight percent difference in compressional wave velocity across plies for specimens of two different fiber/resin systems. No difference was observed for specimens having different ply-orientations within a system. Shear wave velocity in one specimen was found to be 46% of the compressional wave velocity. The new technique could also be used for thickness measurements in thin-skin honeycomb structures.

Interface slip in a composite material is modeled by considering a finite length, rigid line inclusion (fiber) embedded in a two-dimensional elastic matrix with sliding fiber-matrix interface cracks extending from the fiber tips. The crack faces are assumed to remain in contact; the resulting fric tion forces between the crack faces are taken to be constant. A closed form solution for this problem is presented which is used to study the stable size of interface cracks based on the concept of a critical stress intensity factor. The relaxation of axial fiber force as a result of constant load debonding is examined. In addition, the change in fiber force due to stable unbonding under monotonic increasing loads is considered. Finally, the loading conditions leading to closed sliding cracks are identified.

Continuous filaments and eutectic composite materials have been treated as homogeneous and orthotropic and transversely isotropic elastic materials, respectively, in stress analysis problems. Based on these assump tions the elastic moduli of boron-epoxy and Al-Al₃Ni eutectic composite materials are measured using pulse-echo techniques. The shear wave measurements show that the elastic anisotropy assumed for each material is only an approximation. The measurements are also influenced by defects in the fiber arrangement and, m the eutectic composite, by the presence of a solidification substructure.

A theoretical study has been made to predict overall thermal expansion characteristics of a two-phase material consisting of a matrix and aligned ellipsoidal inclusions. The calculation is based on Eshelby's theory on the transformation problem; an approximate method is Introduced to account for the effect of finite concentrations of the inclusions. The following two cases are treated: elastic matrix-elastic inclusions and elastoplastic matrix- elastic inclusions. The results are explicitly obtained when shapes of the inclusions are spherical, disc-shaped, and fiber-shaped. The calculated over all thermal expansion coefficient for the case of elastic matrix-elastic spheres agrees with Kerner's prediction.