Laminate interlaminar stresses

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. 

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. 

Disbond between plies caused by high interlaminar stresses Typically, a phenomenology-based equation that attempts to predict failure of a ply given a certain loading situation Any departure of the resulting laminate from the nominal expected... 

Force resultant Fracture plane Interlaminar stresses Kink-band Laminate... 

Friberg M. Fastener load distribution and interlaminar stresses in composite laminates [Licentiate thesis]. Stockholm Royal Institute of Technology 2000. 

The transverse contraction of these laminates is completely different under uniaxial applied strain. The 90° laminate contracts very Uttle (Vjj 0.02), whereas the contraction of the 45° laminate is very large (Vjj 0.773). This difference in the Poisson s ratio causes additional stresses in orthotropic laminates, where the constituent plies have different ply orientations. The contraction of an angle-ply laminate, for example with 90° plies, is hindered by the small Poisson s ratio of the 90° plies. This leads to transverse stresses in other plies. Near free edges and around unloaded holes, the transverse stresses are equilibrated by interlaminar stresses. For... 

In addition to influencing the anisotropic response of the laminate, the stacking sequence can cause interlaminar through-thickness stress to occur at the free edge of the laminate. Interlaminar tension stresses can cause delamination under static, cyclic and impact loading. 

Modify the stacking seqnence of the laminate phes to reduce interlaminar stresses or... 

Critical ply termination When a laminate with a 90° ply at mid-plane is subjected to tension, peak interlaminar stresses are generated at the interface between the ply or within it. This is because the Poisson s ratio is so different from the surrounding plies. One solution has been to terminate the 90° ply away from the free edge. 

The element sizes within the composite adherends were chosen such that two of the integration points of the second layer of elements adjacent to the adhesive were coincident with the interface between the first two plies of the laminate. Therefore, the stresses would be accurately predicted at the point where interlaminar failure was predicted by Adams et al. [17]. These Gauss point values were chosen as they are the only points within the finite element model at which the solution is fully integrated, and are therefore the most accurate predictions of stress and strain. The stresses and strains were monitored at these points in order to predict the interlaminar stresses so that the influence of temperature could be quantified. 

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