Stresses interlaminar

Moreover, classical lamination theory often implies values of Oy and where they cannot possibly exist, namely at the edge of a laminate. Physical grounds will be used to establish that  

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However, it is possible that the constant rate of AE activity is interrupted by local peaks of high rate of AE. This is due to the formation of local (internal) delaminations because of interlaminar stresses arising due to the presence of transverse cracks. This is more accentuated in less severe loading conditions. Under severe loading conditions = 80% CTu, R = 0.1) the rate of damage development (delamination growth) is so fast that leads to an overall high rate of AE emission. 

The existence of interlaminar stresses means that laminated composite materials can delaminate near free edges whether they be at the edge of a plate, around a hole, or at the ends of a tubular configuration used to obtain material properties. In all cases, delamination could cause premature failure so must be considered in specimen design because othen/vise the specimen does not represent the true physical situation. 

In summary, three classes of interlaminar stress problems exist ... 

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. 

Note that no assumptions involve fiber-reinforced composite materials explicitly. Instead, only the restriction to orthotropic materials at various orientations is significant because we treat the macroscopic behavior of an individual orthotropic (easily extended to anisotropic) lamina. Therefore, what follows is essentially a classical plate theory for laminated materials. Actually, interlaminar stresses cannot be entirely disregarded in laminated plates, but this refinement will not be treated in this book other than what was studied in Section 4.6. Transverse shear effects away from the edges will be addressed briefly in Section 6.6. 

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). 

R. Byron Pipes and N. J. Pagano, Interlaminar Stresses In Composite Laminates Under Uniform Axial Extension, Journal of Composite Materials, October 1970, pp. 538-548. 

Pipes, R.B. and Pagano, N.J. (1970). Interlaminar stresses in composite laminates under uniform axial extension. J. Composite Mater. 4, 538-548. 

Hu FZ, Soutis C, Edge EC. Interlaminar stresses in composite laminates with a circular hole. Compos Struct 1997 37(2) 223—32. 

It should be noted that the above equation, which is based on the assumption of a homogeneous plate, is independent of the stacking sequence. Rearranging the plies in a laminate in any order will give the same answer. This means that interlaminar stresses, which for some stacking sequences can be very important, are neglected. 

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