The influence of interfaces and interphases on the mechanical behavior of composite materials has been widely discussed in the literature, particularly in the last few years. Several books have been devoted to the subject. Despite this fact, the systematic representation of the mechanics of this subject remains grossly incomplete, and because of this, no consistent approach has been developed towards the determination of the influence of the interphase on the properties and performance of composite materials and laminates.The present paper advances a fundamental approach to this subject. The approach is based on the premise that the "composite effect"—that which defines the difference between the mechanical response of constituents acting without interaction and the response when they are bonded rigidly together — is the proper definition of the mechanical effect of the interphase. Within these two bounds, the "real" situation is determined by the extent of this interaction. The present paper attempts to specify these two bounds for typical loading and material situations. The nature of the boundary value problems associated with the two extremes as well as the intermediate cases is discussed.The incorporation of an interphasial region into micromechanical analyses of composite stiffness and strength will necessitate a reevaluation of the boundary value problem. The anisotropic, inhomogeneous, time-dependent quality of the interphase demands a more thorough treatment in existing micromechanical models. Yet, the level of complexity modeled is limited by our ability to measure interphasial properties. The interphase offers a wealth of opportunity on both analytical and experimental aspects of the problem.

No abstract

The effect of interleaving on the fatigue damage mechanisms and residual tensile strength of two graphite/epoxy material systems subjected to cyclic axial loads is investigated. The two materials, AS4/985 and AS4/1808, have stacking sequences of [0/45/90/-45]s2 and [0/-45/90/45]s4, respectively. All specimens have centrally located through-holes. The 16-ply specimens are subjected to tensile fatigue loads, while the 32-ply specimens are subjected to fully reversed fatigue loads. Interleaving these laminates at every interface with a thin (on the order of 10% of the ply thickness), tough, thermoplastic film results in an expected increase in mass and decrease in laminate stiffness, but also in higher virgin tensile strengths, lower normalized residual tensile strengths, and slightly altered damage patterns and rates of damage growth in both laminate types. The presence of the interleaves alters the baseline virgin tensile load-to-failure and damage patterns to a greater extent in the 1808 material than in the 985 material.

In the continuing effort to understand the processes involved during the compressive failure of statically and dynamically loaded composite structures, this work is concerned with the role the fiber, matrix, and interphase play in determining a composite's compressive performance. Fourteen material systems representing permutations of four carbon fiber systems, three matrix systems, percentages of fiber surface treatment, and three sizing conditions have been laminated and cut into coupons. The bond strength arising from the particular combination of fiber, surface treatment, sizing, and matrix is quantified by measuring the transverse flexural strength of each material system. Center-holed, cross-plied specimens of each of the 14 materials were tested in quasi-static compression to determine their structural strength. An attempt was then made to correlate differences in compressive strength to differences in the properties of the three phases. A fixed percentage of each strength value was then utilized to establish the tension-compression fatigue stress level employed for each of the 14 material systems. Damage analysis revealed the mechanisms responsible for the wide variation in fatigue life witnessed. The interphase is seen to play a second-order role in determining the compressive strength of these particular notched cross-plied laminates. On the other hand, the interphase is shown to be a key variable in dictating these laminates' compression-dominated fatigue behavior.

The investigation presented examines the compressive strengths, Xc, of thermoplastic-toughened epoxy composites with fiber surface chemistry altered by various levels of surface treatments and two distinctly different sizings. The thermoplastic, polyvinylpyrrolidone (PVP) size improved the IITRI Xc by 51% over an unreacted bisphenol-A epoxy size for a 100% surface-treated fiber. It is important to point out that effects were produced with the sizings making up less than 1% (by weight) of the composite. Thus, appropriate selection of sizings may present an effective and economical means for tailoring composite material performance. It is suggested that properties other than interfacial shear strength may be important in the description of composite compression strength, e.g., interphase modulus and fracture resistance.