Fibers orientation and concentration

One of the main aims of a big research project in our group is the generation of a comprehensive database which is necessary for the applicability of fatigue life calculation with local S/N curves for fiber reinforced composites. In this work the effect of fiber orientation and concentration as well as stress ratio R on fatigue behavior of two materials was investigated. The stress Ratio R is defined by equation 1, where a is the lower stress amplitude and CTq is the higher stress amplitude of a sinusoidal loading. 

Selected RPs as a particular structural component, micromechanics analyses, together with classic mechanics theory, should provide a means for predicting optimum fiber orientations and material thicknesses for specific load conditions. In addition to the analytical, predictive type of micromechanics research, there is also a significant amount of experimental micromechanics research that has been done, i.e., determination of stress concentration at fiber ends and crossovers, investigations of deformation and firacture modes, and crack propagation studies. Such work helps the analyst in establishing realistic assumptions of material behavior and in comparing observed mechanical behavior with predicted behavior. 

The increase in oxygen concentration accompanies the deterioration of fiber physical properties, but the deterioration does not necessarily relate to the increase directly. Factors such as fiber orientation and the conditions, primarily temperature at which the fiber is drawn, can affect the rate of deterioration [113], The reduction of elongation that accompanies exposure to UV light is less for a highly oriented fiber because the photooxidation reaction is initially limited to the fiber surface. The photooxidation reaction is accompanied by the formation of cracks on the thin surface [114]. Cold-drawn fiber reacts throughout, degrades faster, but does not form surface cracks. Fiber wettability increases with increased UV exposure [112], which is additional evidence of the formation of surface oxygen bonds. 

The matrix material used in polymer-based composites can either be thermoset (epoxies, phenolics) or thermoplastic resins (low density polyethylene, high density polyethylene, polypropylene, nylon, acrylics). The filler or reinforcing agent can be chosen according to the desired properties. The properties of polymer matrix composites are determined by properties, orientation and concentration of fibers and properties of matrix. 

For the random in-plane orientation of the fibers, it can be shown that TTlo = 0.375 [2, 3]. Equation (5) has been verified experimentally for glass-fiber polypropylene GMT over a wide range of fiber length and concentration and over the temperature range —50 to 150°C when the appropriate temperature-dependent parameter values are used [4]. 

At higher fiber contents, we must also consider the relationship between fiber aspect ratio and the theoretical maximum achievable volume fraction. As fibers are laid down randomly in a plane, the probability of fibers crossing each other and thus being oriented out of plane increases as the fiber length and concentration are increased. This will result in a lower sti ess in the X-Y plane and a subsequent increase in stiffness in the Z direction. For instance, it has been shown [2] that, in the fiber aspect ratio range of 300-lKX), the maximum achievable volume fi action in a two-dimensional random in-plane laminate is 18-20% v/v. If the laminate volxune fraction is greater than this, then there are a number... 

The factors affecting the properties of the reinforced thermoplastics are type of glass reinforcement, sizing, interface between glass fiber and plastic matrix, fiber dispersion, fiber concentration, fiber orientation, and fiber length and diameter. These factors are discussed in the paragraphs that follow. 

The presence of texture in the samples is important in polymer processing, as (1) laminar mixing produces texture (2) the mechanical properties of polymer blends depend on the texture (e.g., skin-core formation, fiber orientation, and distribution of voids along the thickness in structural foams) and (3) the lack or the presence of texture is required in certain products. Samples can exhibit a certain texture, and at the same time they do or do not exhibit concentration gross uniformity. 

The microductile/compliant layer concept stems from the early work on composite models containing spherical particles and oriented fibers (Broutman and Agarwal, 1974) in that the stress around the inclusions are functions of the shear modulus and Poisson ratio of the interlayer. A photoelastic study (Marom and Arridge, 1976) has proven that the stress concentration in the radial and transverse directions when subjected to transverse loading was substantially reduced when there was a soft interlayer introduced at the fiber-matrix interface. The soft/ductile interlayer allowed the fiber to distribute the local stresses acting on the fibers more evenly, which, in turn, enhanced the energy absorption capability of the composite (Shelton and Marks, 1988). 

Flow imparts both extension and rotation to fluid elements. Thus, polymer molecules will be oriented and stretched under these circumstances and this may result in flow-induced phenomena observed in polymer systems which include phase-changes, crystallization, gelation or fiber formation. More generally, the Gibbs free energy of polymer blends or solutions depends under non-equilibrium conditions not only on temperature, pressure and concentration but also on the conformation of the macromolecules (as an internal variable) and hence, it is sensitive to external forces. 

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