Laminating strength

LAMINATE STRENGTH EVALUATION Figure 4-35 Laminate Strength-Analysis Elements... 

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... 

Finally, both the state of the material and the state of stress affect the laminate strength evaluation. That is, the actual temperature and moisture conditions influence the laminae strengths. Taken together with the laminae stresses, the laminae strengths and the laminate loads lead to an evaluation of the laminate capabilities. 

The overall procedure of laminate-strength analysis, which simultaneously results in the laminate load-deformation behavior, is shown schematically in Figure 4-36. There, load is taken to mean both forces and moments similarly, deformations are meant to include both strains and curvatures. The analysis is composed of two different approaches that depend on whether any laminae have failed. 

Figure 4-36 Analysis of Laminate Strength and Load-Deformation Behavior...

The procedure of laminate strength analysis outlined in Section 4.5.2, with the Tsai-Hill lamina failure criterion will be illustrated for cross-ply laminates that have been cured at a temperature above their service or operating temperature in the manner of Tsai [4-10]. Thus, the thermal effects discussed in Section 4.5.3 must be considered as well. For cross-ply laminates, the transformations of lamina properties are trivial, so the laminate strength-analysis procedure is readily interpreted. 

The significance of interlaminar stresses relative to laminate stiffness, strength, and life is determined by Classical Lamination Theory, i.e., CLT stresses are accurate over most of the laminate except in a very narrow boundary layer near the free edges. Thus, laminate stiffnesses are affected by global, not local, stresses, so laminate stiffnesses are essentially unaffected by interlaminar stresses. On the other hand, the details of locally high stresses dominate the failure process whereas lower global stresses are unimportant. Thus, laminate strength and life are dominated by interlaminar stresses. 

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

A collection of the basic building block, a lamina, was bonded together to form a laminate in Chapter 4. The behavior restrictions were covered in the section on classical lamination theory. Special cases of laminates were discussed to learn about laminate characteristics and behavior. Predicted and measured laminate stiffnesses were favorably compared to give credence to classical lamination theory. Then, the strength of laminates was discussed and found to be reasonably predictable. Finally, interlaminar stresses were analyzed because of their apparent strong influence on laminate strength (and life). 

The manner in which the laminate design is approached can be expressed in flow-chart form as in Figure 7-59. There, some initial laminate is arbitrarily selected to start the procedure. Then, the laminate load-deflection behavior is evaluated by use of the laminate strength analysis procedure described in Section 4.5. That evaluation is theoretical in nature. The next step is to evaluate the laminate fatigue life, and that evaluation can only be done experimentally, although progress is... 

Pagano, N.J. and Pipes, R.B. (1971). The influence of stacking sequence on laminate strength. J. Composite Mater. S, 50-57. 

Polyester laminate strength data obtained by the Bjorksten group using their BJY treatment are shown in Fig. 1. Fiberglass treatments with DuPont s hydro-phobic chrome complex (Volan 114) and no treatment (112) are also shown for comparison. 

Traditionally, laminate strength theories are based on ply strengths and models that use them to predict when a laminate would fail. The complexities associated with failure of a single ply are compounded in this case by the fact that damage created in one ply can interact with damage in other plies and evolve very differently depending on the laminate. For example, matrix cracks may evolve to delaminations or may branch out to adjacent plies as shown in Figure 6.6. 

P(l) Laminate strength should be determined by experimental testing or taken from manufacturers data. The Hart-Smith failure criterion shall be used for design. 

The above steps for calculating laminate strength are summarised in the following flow chart, Figure 4.12. 

Figure 4.12 Flow chart for calculation of laminate strength.

Lamina and laminate strengths are dealt with simultaneously in this document. The strength of a laminate is a function of the strengths of its laminae. 

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