Stacking sequences

In the case of the fibrous laminate not much work has been done, but it has been observed that a significant loss of stiffness in boron—aluminum laminate occurs when cycled in tension—tension (43,44). Also, in a manner similar to that in the laminated PMCs, the ply stacking sequence affects the fatigue behavior. For example, 90° surface pHes in a 90°/0° sequence develop damage more rapidly than 0° pHes. In the case of laminates made out of metallic sheets, eg, stainless steel and aluminum, further enhanced resistance against fatigue crack propagation than either one of the components in isolation has been observed (45). 

There are three possible arrangements of atoms in a layer of SiC crystal, and each type has the same layers but a different stacking sequence (29). Designation (30) is by the number of layers in the sequence, followed by H, R, or C to indicate whether the type belongs to the hexagonal, rhombohedral, or cubic class. 

Fig. 1. Crystal structures of (a) cubic diamond and (b) lonsdaleite. A, B, and C indicate the stacking sequence of sheets of atoms.

On silicon carbide, it is easier to see and measure step heights than in crystals like beryl, because SiC has polytypes, first discovered by the German crystallog-rapher Baumhauer (1912). The crystal structure is built up of a succession of close-packed layers of identical structure, but stacked on top of each other in alternative ways (Figure 3.24). The simplest kind of SiC simply repeats steps ABCABC, etc., and the step height corresponds to three layers only. Many other stacking sequences... 

Example 3.16 A unidirectional carbon hbre/PEEK laminate has the stacking sequence [O/SSa/—352]t- If it has an in-plane stress of = 100 MN/m applied, calculate the strains and curvatures in the global directions. The properties of the individual plies are... 

Solution This stacking sequence is similar to that in Example 3.11 except that in this case the laminate is not symmetrical. As shown in Fig. 3.27, the centre line of the laminate is in the middle of one of the plies and... 

First, we need a common, unambiguous manner of writing how a laminate is specified to be laid-up, i.e., stacking-sequence notation. Then, quasi-isotropic, balanced, and hybrid laminates are defined. 

The laminate stress-analysis elements are affected by the state of the material and, in turn, determine the state of stress. For example, the laminate stiffnesses are usually a function of temperature and can be a function of moisture, too. The laminae hygrothermomechanical properties, thicknesses, and orientations are important in determining the directional characteristics of laminate strength. The stacking sequence... 

The example considered to illustrate the strength-analysis procedure is a three-layered laminate with a [4-15°/-15°/+15°] stacking sequence [4-10]. The laminae are the same E-glass-epoxy as in the cross-ply laminate example with thickness. 005 in (.1270 mm), so that the total laminate thickness is. 015 in (.381 mm). In laminate coordinates, the transformed reduced stiffnesses are... 

N. J. Pagano and R. Byron Pipes, The Influence of Stacking Sequence on Laminate Strength, Journal of Composite Materials, January 1971, pp. 50-57. 

What kinds of configurations are possible for composite structures The most obvious is that of a fiber-reinforced laminate. With a laminate, we can change laminae orientations, stacking sequence, and laminae materials to arrive at a suitable structure. We can stiffen the laminate, or we can put a sandwich core in the middle of those laminae. We can do all of those possibilities, but recognize that we will also have, in vir-tuaiiy any structure, some kind of hoie or a cutout for some reason. Thus, we must have a procedure to place an appropriate amount of reinforcement around those cutouts so that ioad can be transferred around them. Without that reinforcement, the structure cannot do the job it is required to do. These various possibie configurations are shown in Figure 7-38. 

Figure 7-57 Effect of Changing Laminate Stacking Sequence...

CHANGE THE STACKING SEQUENCE AND/OR THE THICKNESS TO CHANGE THE BEHAVIOR UNTIL THE DESIRED RESPONSE IS OBTAINED... 

That is, the fundamental laminate design problem can be expressed as given the loading N, Ny, and N, find the laminate stacking sequence in Figure 7-51. That is, what are the laminae orientations 01, 02 03- nnany of each orientation are needed, i.e., what... 

TOO LOW OR TOO HIGH, CHANGE STACKING SEQUENCE AND/OR NUMBER OF LAMINAE... 

Suppose we replace the 90° layers with a laminae in an attempt to increase the axial stiffness and to increase the first-ply failure load as in Figure 7-61. The load-deflection curve slope after first-ply failure also increases when a laminae replace the 90° layers. However, the energy absorption decreases with such a stacking sequence change. The associated fatigue lives are not known unless both laminates are made and subjected to fatigue loading. 

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