Composite unidirectional properties

Furthermore, the absorption of water by the interphasal polymer can reduce its yield strength below the interfacial bond strength. Thus, the apparent interfacial shear strength will be reduced, and a yield front, rather than a debond, will propagate along the fibre interface modifying the stress transfer micromechanics at a fibre break. A consequence is that the stress concentrations in adj acent fibres to the fibre break will be reduced, and the probability of the formation of a flaw of critical dimensions is also reduced. The number of interacting fibre breaks associated with a flaw of critical dimensions will increase. 

In compression, the mechanism of failure changes from fibre fracture to fibre buckling. Here, the modulus of the matrix is a critical factor in supporting the fibre and by prevention of kinking (of the fibres) in a shear band. Soutis [53] has discussed the modelling of these types of failure using Eqns (12.17) and (12.18)  

Test temperature rc) Compressive strength MPa Expt Compressive strength MPa prediction Young s Modulus (GPa) Shear strength (MPa) Shear yield stress (MPa) Shear modulus (GPa) 

Moisture content, = 1.42%. Assumed initial fibre misalignnoent, = 1-75 kink band inclination angle, A = 15 the value of pQ is not affected by the environmental test conditions. () estimates from Eqns (12.17) and (12.18) and the measured unidirectional compressive or shear strength properties. 

The compressive properties of a composite under aU loading conditions are strongly affected by moisture absorption because of the reduction in shear properties of the matrix polymer. The design of artefacts with polymer matrix composites needs to reflect the limitations of these materials in compression, especially in service where environmental conditioning is likely. 

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Other semi-2-IPNs [52] processed by the freeze-drying method included IPN from 4,4 -bismaleimido diphenylmethane (BMI) and linear BTDA/3,4 -ODA polyamic acid that were dissolved in 1,3,5-trioxane (Fig. 16). The resulting semi-2-IPNs exhibited higher Tgs and reduced phase separation, and contained no plasticizing solvent. A comparison of unidirectional properties of composites prepared by the freeze-dry process to those by traditional solvent evaporation process is presented in Table 13. The freeze-drying method for the preparation of IPNs appears to be superior to previous technology. 

Two types of composite physical property tests were conducted to measure properties which are sensitive to the degree of adhesion and failure mode of the fiber-matrix interphase. Short beam shear tests (ASTM D2344-84) were conducted on 18 ply unidirectional laminates. The support span-to-thickness ratio... 

Range of mechanical properties of graphite fiber/BMI composites (unidirectional) ... 

The morphological structure of the multicellular fibers makes them analogous to the modern-day fiber-reinforced, rigid-matrix composites materials in which (a) the fibers are all aligned in the same direction (unidirectional), (b) the volume fraction of fibers is very high, and (c) the fibers have discrete (discontinuous filaments) lengths. As in composites, the properties of multicellular fibers are determined by the physical, mechanical, and chemical properties of the morphological constituents. 

For the transverse shear modulus, the approach designated self-consistent was based on the formula obtained by the self-consistent method for the plane-strain bulk modulus (11.61), on the transverse modulus calculated using the Chamis approach (11.49b) and the in-plane Poisson s ratio given by the rule of mixtures. Except when used to predict the axial modulus and the major Poisson s ratio, the rule of mixtures underestimates the remaining composite elastic properties. The Bridging Model proved to be a very effective theory to account for all five elastic properties for unidirectional composites that are transversely isotropic. 

The model material used is a unidirectional carbon fiber-reinforced, thermoplastic polyether-etherketone (PEEK) composite with fiber diameter of = 8 fim. The fiber orientations studied are shown in Figure 1. The mechanical properties of this material are listed in Table 1. The first principal material direction, indicated by 1 in Table 1, is parallel to the fiber direction (Figure 1), the second and the third one are perpendicular to the fiber orientation. The anisotropic composite material properties were specified using the rule of mixture type equations. ... 

There are three basic types of engineered composites (1) laminates, (2) particle-reinforced composites, and (3) fiber-reinforced composites. In particle-reinforced composites, one can make the distinction between small (submicron) particle composites, where the particles are incorporated in the microstructure, vs. large particle composites, where the particles themselves actually do the work or carry the load. The reinforcing fibers can be discontinuous or continuous. The fibers in discontinuous fiber-reinforced composites can be randomly oriented to provide isotropic properties or aligned to enhance a specific property in a specific direction. Continuous fiber composites are generally designed for their unidirectional properties but can be crisscrossed to obtain multidirectional property enhancement such as in a filament-woimd pressure container. All possible permutations of metal, ceramic, and pol)uner are foimd in the laminated as well as in the reinforced composites. 

The difference between the bounds defined by the simple models can be large, so that more advanced theories are needed to predict the transverse modulus of unidirectional composites from the constituent properties and fiber volume fractions (1). The Halpia-Tsai equations (50) provide one example of these advanced theories ia which the rule of mixtures expressions for the extensional modulus and Poisson s ratio are complemented by the equation... 

The tensile strength of a unidirectional lamina loaded ia the fiber direction can be estimated from the properties of the fiber and matrix for a special set of circumstances. If all of the fibers have the same tensile strength and the composite is linear elastic until failure of the fibers, then the strength of the composite is given by... 

This is an important relationship. It states that the modulus of a unidirectional fibre composite is proportional to the volume fractions of the materials in the composite. This is known as the Rule of Mixtures. It may also be used to determine the density of a composite as well as other properties such as the Poisson s Ratio, strength, thermal conductivity and electrical conductivity in the fibre direction. 

Example 3.2 PEEK is to be reinforced with 30% by volume of unidirectional carbon fibres and the properties of the individual materials are given below. Calculate the density, modulus and strength of the composite in the fibre direction. 

Some typical elastic properties for unidirectional fibre composites are given in Table 3.4. 

Example 3.6 A unidirectional composite consisting of carbon fibres in a PEEK matrix has the fibres aligned at 25° to the loading axis. If the fibres and matrix have the properties indicated below, calculate Ex, Ey, Gxy, Vxy, and Vyx. 

Example 3.7 A thin unidirectional Kevlar fibre/epoxy composite has the properties listed below. If the fibres are aligned at 6 to the x-axis, show how... 

Fig. 3.11 Variation of properties in unidirectional composite as a function of fibre angle...

The previous analysis has shown that the properties of unidirectional fibre composites are highly anisotropic. To alleviate this problem, it is common to build up laminates consisting of stacks of unidirectional lamina arranged at different orientations. Clearly many permutations are possible in terms of the numbers of layers (or plies) and the relative orientation of the fibres in each... 

As an illustration of the results of the measurements just described, the mechanical properties for four unidirectionally reinforced composite materials, glass-epoxy, boron-epoxy, graphite-epoxy, and Kevlar 49 -... 

The results of the micromechanics studies of composite materials with unidirectional fibers will be presented as plots of an individual mechanical property versus the fiber-volume fraction. A schematic representation of several possible functional relationships between a property and the fiber-volume fraction is shown in Figure 3-4. In addition, both upper and lower bounds on those functional relationships will be obtained. 

R. L. Foye, An Evaluation of Various Engineering Estimates of the Transverse Properties of Unidirectional Composites, Proceedings of the 10th National Sympo-... 

Fibers are often regarded as the dominant constituents in a fiber-reinforced composite material. However, simple micromechanics analysis described in Section 7.3.5, Importance of Constituents, leads to the conclusion that fibers dominate only the fiber-direction modulus of a unidirectionally reinforced lamina. Of course, lamina properties in that direction have the potential to contribute the most to the strength and stiffness of a laminate. Thus, the fibers do play the dominant role in a properly designed laminate. Such a laminate must have fibers oriented in the various directions necessary to resist all possible loads. 

Tests by Roe et al. [63] with unidirectional jute fiber-reinforced UP resins show a linear relationship (analogous to the linear mixing rule) between the volume content of fiber and Young s modulus and tensile strength of the composite over a range of fiber content of 0-60%. Similar results are attained for the work of fracture and for the interlaminate shear strength (Fig. 20). Chawla et al. [64] found similar results for the flexural properties of jute fiber-UP composites. 

Tabie 6 Comparative Fiber and Unidirectional Composite Properties... 

Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

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