Halpin-Tsai

The characteristic features of a cord—mbber composite have produced the netting theory (67—70), the cord—iaextensible theory (71—80), the classical lamination theory, and the three-dimensional theory (67,81—83). From stmctural considerations, the fundamental element of cord—mbber composite is unidirectionaHy reinforced cord—mbber lamina as shown in Figure 5. From the principles of micromechanics and orthotropic elasticity laws, engineering constants of tire T cord composites in terms of constitutive material properties have been expressed (72—79,84). The most commonly used Halpin-Tsai equations (75,76) for cord—mbber single-ply lamina L, are expressed in equation 5 ... 

In practical terms the above analysis is tcx) simplistic, particularly in regard to the assumption that the stresses in the fibre and matrix are equal. Generally the fibres are dispersed at random on any cross-section of the composite (see Fig. 3.8) and so the applied force will be shared by the fibres and matrix but not necessarily equally. Other inaccuracies also arise due to the mis-match of the Poisson s ratios for the fibres and matrix. Several other empirical equations have been suggested to take these factors into account. One of these is the Halpin-Tsai equation which has the following form... 

As in the case of the transverse tensile modulus, 2. the above analysis tends to underestimate the in-plane shear modulus. Therefore, once again it is common to resort to empirical relationships and the most popular is the Halpin-Tsai equation... 

Not much work is available regarding micromechanical theories of strength. However, considerable work as been done on micromechanical theories of stiffness. We will concentrate on those aspects of stiffness theory that are most prominent in usage (e.g., the Halpin-Tsai equations) in addition to those aspects that clearly illustrate the thrust of micromechanics. Available strength information will be summarized with the same intent as for stiffness theories. 

The mechanics of materials approach to the micromechanics of material stiffnesses is discussed in Section 3.2. There, simple approximations to the engineering constants E., E2, arid orthotropic material are introduced. In Section 3.3, the elasticity approach to the micromechanics of material stiffnesses is addressed. Bounding techniques, exact solutions, the concept of contiguity, and the Halpin-Tsai approximate equations are all examined. Next, the various approaches to prediction of stiffness are compared in Section 3.4 with experimental data for both particulate composite materials and fiber-reinforced composite materials. Parallel to the study of the micromechanics of material stiffnesses is the micromechanics of material strengths which is introduced in Section 3.5. There, mechanics of materials predictions of tensile and compressive strengths are described. 

The variational energy principles of classical elasticity theory are used in Section 3.3.2 to determine upper and lower bounds on lamina moduli. However, that approach generally leads to bounds that might not be sufficiently close for practical use. In Section 3.3.3, all the principles of elasticity theory are invoked to determine the lamina moduli. Because of the resulting complexity of the problem, many advanced analytical techniques and numerical solution procedures are necessary to obtain solutions. However, the assumptions made in such analyses regarding the interaction between the fibers and the matrix are not entirely realistic. An interesting approach to more realistic fiber-matrix interaction, the contiguity approach, is examined in Section 3.3.4. The widely used Halpin-Tsai equations are displayed and discussed in Section 3.3.5. 

Note that the expressions for E., and v.,2 are the generally accepted rule-of-mixtures results. The Halpin-Tsai equations are equally applicable to fiber, ribbon, or particulate composites. For example, Halpin and... 

Figure 3-32 Halpin-Tsai Calculations (Circles) versus Adams and Doner s Calculations for E2 of Circular Fibers in a Square Array (After Halplrt ar d Tsai [3-17])...

Figure 3-35 Halpin-Tsai Calculations (Circles) versus Foye s Calculations...

Figure 3-38 Modified Halpin-Tsai Calculations versus Adams and Conner s Calculations for G 2 Circular Fibers In a Square Array (After Hewitt and de Malherbe [3-23])...

The mere existence of different predicted stiffnesses for different arrays leads to an important physical observation Variations in composite material manufacturing will always yield variations in array geometry and hence in composite moduli. Thus, we cannot hope to predict composite moduli precisely, nor is there any need to Approximations such as the Halpin-Tsai equations should satisfy all practical requirements. 

Some physical insight into the Halpin-Tsai equations can be gained by examining their behavior for the ranges of values of and t. First, although it is not obvious, can range from 0 to . When = 0,... 

The term r Vf in Equation (3.71) can be interpreted as a reduced fiber-volume fraction. The word reduced is used because q 1. Moreover, it is apparent from Equation (3.72) that r is affected by the constituent material properties as well as by the reinforcement geometry factor To further assist in gaining appreciation of the Halpin-Tsai equations, the basic equation. Equation (3.71), is plotted in Figure 3-39 as a function of qV,. Curves with intermediate values of can be quickly generated. Note that all curves approach infinity as qVf approaches one. Obviously, practical values of qV, are less than about. 6, but most curves are shown in Figure 3-39 for values up to about. 9. Such master curves for various vaiues of can be used in design of composite materiais. 

There is much controversy associated with micromechanical analyses and predictions. Much of the controversy has to do with which approximations should be used. The Halpin-Tsai equations seem to be a commonly accepted approach. 

The modulus of the composites can be theoretically calculated using the well-known Halpin-Tsai equation [181], given by ... 

A.2.3 Composite Moduli Halpin-Tsai Equations. Derivations of estimates for the effective moduli (tensile E, bulk K, and shear G) of discontinuous-fiber-reinforced composite materials are extremely complex. The basic difficulty lies in the complex, and often undefined, internal geometry of the composite. The problem has been approached in a number of ways, but there are three widely recognized... 

The Halpin-Tsai equations represent a semiempirical approach to the problem of the significant separation between the upper and lower bounds of elastic properties observed when the fiber and matrix elastic constants differ significantly. The equations employ the rule-of-mixture approximations for axial elastic modulus and Poisson s ratio [Equations. (5.119) and (5.120), respectively]. The expressions for the transverse elastic modulus, Et, and the axial and transverse shear moduli, Ga and Gf, are assumed to be of the general form... 

Observations By virtue of the Halpin-Tsai equation, there is an inherent strength... 

The effect of polymer-filler interaction on solvent swelling and dynamic mechanical properties of the sol-gel-derived acrylic rubber (ACM)/silica, epoxi-dized natural rubber (ENR)/silica, and polyvinyl alcohol (PVA)/silica hybrid nanocomposites was described by Bandyopadhyay et al. [27]. Theoretical delineation of the reinforcing mechanism of polymer-layered silicate nanocomposites has been attempted by some authors while studying the micromechanics of the intercalated or exfoliated PNCs [28-31]. Wu et al. [32] verified the modulus reinforcement of rubber/clay nanocomposites using composite theories based on Guth, Halpin-Tsai, and the modified Halpin-Tsai equations. On introduction of a modulus reduction factor (MRF) for the platelet-like fillers, the predicted moduli were found to be closer to the experimental measurements. 

The elastic modulus of composite materials reinforced by discontinuous cylindrical fibers or lamellar shapes is expressed by the Halpin-Tsai equations [106, 230], as shown in the following equation ... 

YM Actual Guth Gold A mod Gulh Gold xHalpinTsai mod Halpin Tsai I... 

The Halpin-Tsai equation can thus be tailored into a much simpler form for PNCs comprising matrix-filler combinations having inordinately disparate sets of Young s moduli by addressing the shape-, size-, and aggregate-related factors a priori in order to adequately supplant those in (22). 

As seen, the Halpin-Tsai equation has a term a, raised to the power of one, to accommodate the filler aspect ratio. Since IAF intends to supplant the same, the new equation is expected to have a reduced dependence on the aspect ratio. Thus, the presence of aspect ratio in the equation needs to be diluted. Two constitutive equations are suggested the first one contains a correction term in the form of a shape reduction factor (a0 5) (24), while the second (25), is devoid of any extrashape related corrections Modified Halpin-Tsai I ... 

Since the polymer-filler interaction has direct consequence on the modulus, the derived function is subjected to validation by introducing the function in established models for determination of composite modulus. The IAF is introduced in the Guth-Gold, modified Guth-Gold, Halpin-Tsai and some variants of modified Halpin-Tsai equations to account for the contribution of the platelet-like filler to Young s modulus in PNCs. These equations have been plotted after the introduction of IAF into them. 

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