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doi: 10.1002/pc.750080503pmid: N/A
The interfacial region between fibers and matrix in fiber composites governs the transfer of forces between the relatively weak and compliant matrix and the reinforcing fibers. An effective interphase can ensure that the mechanical properties of the composite reflect the high strength and modulus of the fibers. Although composites can be made with the expected strengths and moduli, it is not entirely clear why this is achieved: Tests with critical composites, i.e., those containing very short aligned fibers, do not show the expected stress‐strain behavior. This paper examines the effect of an interphase having a shear modulus that is less than that of the matrix. It is found that to explain the Young's moduli of the short fiber composites, the interphase must have a very low modulus indeed; i.e., a few kPa at most. In addition, the strength results can be accounted for only if we assume that the short lengths of fiber used in the experiments had higher strengths than anticipated. Although agreement between experiments and theory is thus not very good, the small amount of experimental evidence available indicates a need for further systematic experiments on critical (i.e. short aligned fiber) composites before firm conclusions are drawn.
doi: 10.1002/pc.750080504pmid: N/A
Fatigue crack propagation (FCP) of injection‐molded glass‐fiber‐reinforced poly(vinyl chloride) was examined as a function of fiber‐matrix adhesion (coupling) and fiber content at different load levels. Considering the entire FCP history, from crack initiation to critical propagation, it is shown that fatigue lifetime and fracture toughness of coupled composites increase with fiber weight fraction. Uncoupled material exhibits the highest fracture toughness at 10 wt% fiber, yet its fatigue life is considerably shorter. Damage analysis indicates that fiber debonding, pullout, and particularly fiber fracture seem to contribute to the higher fatigue lifetime noted in coupled composites. The Crack Layer Theory is employed to describe the observed FCP behavior. The effective enthalpy of damage parameterizes the resistance of the composite to FCP in terms of the observed mechanisms.
doi: 10.1002/pc.750080505pmid: N/A
The dynamic mechanical properties of unidirectional glass‐fiber‐reinforced polyester measured along the fiber direction were recently investigated. In the same polyester, the type of organosilane coated on the glass fiber, the amount of organosilane, the fiber volume fraction, and the fiber diameter affect the value of the loss tangent, tan δ, at the glass‐transition temperature of the glass‐fiber‐reinforced polyester. The interfacial shear strength and the tan δ at the glass‐transition temperature of the glass‐fiber‐reinforced polyester show good correlation suggesting that the latter can be used to characterize the quality of the interphase. Factors affecting the glass‐transition temperature and the application of Zorowski and Murayama's equation in the characterization of the interfacial adhesion are also discussed.
Roulin‐Moloney, A. C.; Cantwell, W. J.; Kausch, H. H.
doi: 10.1002/pc.750080506pmid: N/A
The effects of such parameters as the filler volume fraction, particle size, aspect ratio, modulus and strength of filler, resin‐filler adhesion, and toughness of the matrix on the stiffness, strength, and toughness of particulate‐filled epoxy resins were evaluated. The mechanisms of crack initiation and subsequent crack propagation in these multiphase materials are discussed and illustrated by scanning electron microscopy of fracture surfaces.
Roulin‐Moloney, A. C.; Cudré‐Mauroux, N.; Kausch, H. H.
doi: 10.1002/pc.750080507pmid: N/A
The experimental arrangement for conducting fracture tests in the scanning electron microscope (SEM) are described together with the particular problems in applying this technique to polymers and composite materials. Several examples are given of mechanisms of fracture in epoxy resins both unfilled and reinforced with particulate and fibrous fillers. The paper is illustrated by still photos taken from the video recording of the fracture experiments and by photos taken from the SEM raster.
doi: 10.1002/pc.750080508pmid: N/A
Procedures for measuring the crack initiation and arrest toughnesses in Mode II interlaminar fracture in composite materials were analyzed. Different techniques using flexural specimens were studied. The strain energy release rate, G, which is the energy available for crack propagation was calculated using simple beam theory. The calculation takes into account the transverse shear effect. Stable and unstable fractures are analyzed, and conditions required to measure the arrest toughness of interlaminar fracture are discussed. The methodology was applied to the measurement of fracture energy at the onset and arrest of delamination in glass/epoxy laminate.
doi: 10.1002/pc.750080509pmid: N/A
The interlaminar fracture and fatigue properties of AS/3501‐;6 graphite/epoxy are discussed from a mechanistic point of view. Particular emphasis is placed on the interaction between the loading mode and the local geometry of the interlaminar zone and on how this affects the stresses close to the crack tip and the resulting failure path. Delamination growth under Mode I loading is shown to depend on the likelihood of fiber bridging occurring and on how effective these bridged fibers are at diverting strain energy away from the crack tip. The Mode II behavior is controlled by both the work required to shear the fibers from the matrix and the ease with which tensile failure of the matrix between the fibers can occur.
doi: 10.1002/pc.750080510pmid: N/A
A failure model is proposed to calculate the minimum expected lifetime of a graphite‐epoxy structure under cyclic loading conditions. The theoretical approach is based on the statistical theory of strength and the mechanical redistribution of stresses between piles of a damaged laminate. The use of the acoustic emission technique has permitted the development of internal damage to be monitored during cyclic loading of unidirectional laminates. The experimental results indicate that the composite can withstand greater damage at low level cyclic loading than observed during monotonic loading to failure. The theoretical modeling under development aims at predicting this behavior.
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