ANGLE-PLY

In situ allowable matrix strain for compression, shear and tension. Ply angle measured from x-axis. 

Figure 5.2 Stress-strain curves for laminates with different ply angles under uniaxial loading (CFRP T300/914c) [8]...

Reinforcements can be uni directional, cross ply, angle ply or random in their arrangement. In any one direction, the mechanical properties of the composites will be proportional to the volume fraction of the fibres oriented in that direction and the components due to other fibre directions. 

Figure 22.3 shows the large in-plane anisotropy for the elastic constants E and G of a Type I carbon fiber in an epoxy matrix tested at various angles to the fiber direction. The interlaminar shear stress is shown as a function of ply angle in Figure 22.4. 

Figure 22.4 Interlaminar shear stress as a function of ply angle for Type I high modulus carbon fiber epoxy resin laminate tested in uniaxial tension in x-direction. The maximum interlaminar shear strength occurs at 0 = 35° and the stresses are zero at 0, 60 and 90°. Source Reprinted with permission from Pipes RB, Pagano NJ, Interlaminar shear stress in composite laminates under axial tension, J Composite Mater Sci, 13, 2131-2136, 1978. Copyright 1978, Sage Publications.

For combined normal and shear in-plane loading, provide multiple or intermediate ply angles for a combined load capability. 

In-plane laminate properties depend on layer-to-layer high-strength-direction sequencing—in this regard, there are fonr laminate types nnidirectional, cross-ply, angle-ply, and multidirectional. Mnltidirectional laminates are the most isotropic, whereas unidirectional laminates have the highest degree of anisotropy. 

Maximum stress theory. Includes unloading of cracked lamina as well as geometric nonlinearity due to changes in the ply angle. 

Plywood is a panel made from wood veneers (thin shces or sheets) bonded to one another. Generahy each ply is oriented at right angles to the adjacent ply, and the two face pHes should have the grain direction parahel to each other. Thus most plywood wih have an uneven number of pHes, such as 3, 5, 7, or more. An exception to this is a four-ply constmction in which the two core pHes are oriented parahel to one another and perpendicular to the two face phes. 

Lumber core is a five-ply panel, usually about 19 mm (3/4 in.) thick, in which the bulk of the thickness, about 16 mm (5/8 in.) is edge-glued lumber. Yellow poplar and red gum are desired species for lumber core. Cross-pHes of lower value wood veneers are laid at right angles to the core grain direction, followed by two thin surface pHes of the decorative face veneer in a parallel direction to the core. This assembly is pressed and bonded to form a panel of exceptional quaHty, provided all steps are accompHshed in a desirable manner. 

Fig. 5. Calendered unidirectionaHy reinforced single-ply cord—mbber lamina where 9 is the helix angle, E is lamina, and Tis tire.

Fig. 6. Variation of Young s modulus (-) and shear modulus (—), with cord angle 9 for one-ply nylon—mbber system (88).

Ran Y. Kim, personal communication, 8 April 1981. For additional information, see Ran Y. Kim, On the Off-Axis and Angle-Ply Strength of Composites, in Test Methods and Design Allowables for Fibrous Composites, C. C. Chamis (Editor), Dearborn, Michigan, 2-3 October 1979, ASTM STP 734, American Society for Testing and Materials, 1981, pp. 91-108 (reprinted with permission). 

The aforementioned coupling that involves Aig, Ags, Dig, and D2g takes on a special form for symmetric angle-ply laminates. Those stiffnesses can be shown to be largest when N = 3 (the lowest N for which this class of laminates exists) and decrease in proportion to 1/N as N increases (see Section 4.4.4). Actually, in the expressions for the extensional and bending stiffnesses Aig and Dig,... 

Thus, for many-layered symmetric angle-ply laminates, the values of Afg, A2g, Dig, and D2g can be quite small when compared to the other Ajj and D j, respectively. 

The purpose of the remainder of this section is to discuss two important classes of antisymmetric laminates, the antisymmetric cross-ply laminate and the antisymmetric angle-ply laminate. Neither laminate is used much in practice, but both add to our understanding of laminates. 

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