Aligned short fiber

It is assumed that the maximum stress in the next fiber located at a distance dj away is approximately equivalent to the composite tensile strength, cr. It follows then that the damage zone size, cq, for aligned short fiber composites is given by (Lauke et al., 1985)... 

Figure 5.97 Plot of composite tensile strength versus fiber volume fraction for an aligned, short fiber-reinforced polymer matrix composite. The dotted lines show the corresponding values for continuous fibers for comparison purposes. Reprinted, by permission, from N. G. McCrum, C. P. Buckley, and C. B. Bucknall, Principles of Polymer Engineering, 2nd ed., p. 281. Copyright 1997 by Oxford University Press.

Piggott MR, Ko M et al (1993) Aligned short-fiber reinforced thermosets experiments and analysis lend little support for established theory. Compos Sci Technol 48 291-299... 

For aligned short fibers subject to a stress along the fiber axis, the stress borne by the fibers is no longer Of as defined in Eq. (2.6), but it assumes a lower mean value that takes into account the effect of fiber ends and the rate of stress... 

An interpolation procedure applied by Halpin and Tsai [17,18] has led to general expressions for the moduli of composites, as given by Eqs. (2.18) and (2.19). Note that for = 0, Eq. (2.18) reduces to that for the lower hmit, Eq. (2.8), and for = infinity, it becomes equal to the upper limit for continuous composites, Eq. (2.7). By empirical curve fitting, the value of = 2(l/d) has been shown to predict the tensile modulus of aligned short-fiber composites in the direction of the fibers, and the value of = 0.5 can be used for the transverse modulus. Other mathematical relationships for modulus calculations of composites with discontinuous fillers include the Takaya-nagi and the Mori-Tanaka equations [20]. 

Consider a subunit that comprised of fully aligned short fibers. The subunit is said to be transversely isotropic, if it is isotropic in all planes perpendicular to the fiber direction, and the fiber axis is the axis of asymmetry. For a transversely isotropic unidirectional stiffness tensor Cijki, orientation averaging can be done to give the composite stiffness, Cijki), as follows (Advani and Tucker 1987) ... 

Hsu P-L, Chou T-W (1985) Corrosion effects on e-glass filaments, bundles, and their aligned short-fiber composites. In Harrigan Jr WC, Strife J, Dhingra AK (eds) Proceedings of the fifth international conference on composite materials (ICCM/5), San Diego, 29 July-1 August... 

Whisker and Short Fiber Composites. Whiskers and short fibers tend to align during forming, lea ding to anisotropic properties and large... 

Depending on x, Gf, and g, two failure modes are possible for aligned short brittle fiber composites fiber pullout and fiber fracture. Let us consider a composite in which > e. If / < 4, the average stress in the fiber will be given by Eq. (15.47), so the strength in the composite can be written... 

The properties are anisotropic to varying degrees (i.c.. mechanical, thermal, and electrical properties vary with direction in the material). The highest anisotropy is illustrated by the properties of a fully aligned 60% (by volume) carbon fiber epoxy laminate, where the properties parallel with the fiber direction can be thirty times greater than in the perpendicular direction, whereas in a molded short-fiber system the ratio of properties in perpendicular directions may only be a factor of two. The fibers themselves may have even higher anisotropy (e.g.. carbon and aramid fibers). 

In flexure or shear, as in the previous case of compression, plastics reinforced with short fibers are probably better than those with continuous fibers, because in the former with random orientation of fibers at least some of the fibers will be correctly aligned to resist the shear deformation. However, with continuous-fiber reinforcement if the shear stresses are on planes perpendicular to the continuous fibers, then the fibers will offer resistance to shear deformation. Since high volume fraction (f>() can be achieved with continuous fibers, this resistance can be substantial. 

Short fiber-reinforced systems are particularly vulnerable to formation of weld lines because the fibers tend to align with the direction of the flow. When flow fronts join, there is no fiber crossover or homogeneity, and the problem is intensified with faster reaction and gelling times. To minimize this, consider locating the gate closest to the largest obstruction to the flow. Computerized moldflow analysis can help predict where problems may occur. 

Aveston, Mercer and Sillwood [71,72] derived crack theory for cement reinforced with continuous and short fibers and found that the tensile strength of the composite for the aligned fibers was ... 

Flemming el al [120] have described the production of aligned short carbon fiber reinforced thermoplastic prepreg. 

Hoffman WP, Scanning probe microscopy of carbon fiber surfaces, Carbon, 30, 315, 1992. Flemming T, Kress G, Flemming M, A new aligned short carbon fiber reinforced thermoplastic prepreg, Advanced Composite Materials, 5(2), 151-159, 1996. 

Fiber alignment in short fiber composites (SFCs) is very difficult to achieve, however the results obtained with anisotropic aU-cellulosic based composites point out for a simple and new way of producing fiber aligned SFCs with enhanced mechanical properties, as discussed later on. 

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