Interlaminar stresses appears due to load transfer between adjacent layers in composite materials laminate. The interlaminar shear strength is a measure of the strength of the bond that exists between the various layers within the composite.
The function of the matrix to transfer the loads from the ends of the fibers to the adjacent fibers. These loads make the apparition of what are known as interlaminar stresses which may lead to microcracking and edge delamination. These stresses are normal (peel stress σz) and shear components (τxy, τxz) are only present in a small region near the free edge. The reason behind the origin of these stresses are the mismatch of Poisson’s ratio and in-plane shear stiffness, the gradients of these stresses would be higher. τxy appears although no normal stress is applied to the laminate, it is equilibrated with the interlaminar shear stress at the near free edges τxz.
As it is explained in the Classical laminate theory (CLT) post, it assumes a state of plane stress, unable to predict the edge stresses. The free edges may be at the boundaries of a laminated plate, around a cutout or hole, or at the ends of a laminated tube. As a result of high interlaminar stresses delamination may initiate at the free edges. These stresses can be known using finite element analysis for example.
How to reduce interlaminar stresses?
- Changing the laminate stacking order.
- Minimizing the mismatch of the Poisson’s ratio
- Inserting an inner layer which has a lower shear modulus and a finite thickness between laminae.
Recommendations about interlaminar stresses in composite materials
- In an angle-ply laminate, txz is the most significant interlaminar stress at the interfaces laminas. Its magnitude and direction depend strongly on the fiber orientation angle (Figure 1). Furthermore, txz has a higher value at the interfaces in a clustered [(θ/- θ]s laminate than in an alternating [((θ/ -θ)n]s laminate.
- [0/90] laminate has σzz and 𝛕yz interlaminar stresses. In the [0/90/90/0] laminate, the σzz is tensile at the midplane, while [90/0/0/90] is compressive. So, in the first case the laminate will likely delaminate. In the Figure 2 it is shown the effect of the laminate sequence in interlaminar stresses.
- In a general laminate, different magnitudes of txz, tyz, and σzz may be present. In the case of the [45/4570/0/–45/45] laminate, shear stress txz between adjacent 45/45 laminas are higher than between adjacent 0/45. In the same way, interlaminar shear stress tyz between 0/45 laminas is higher that between 45/45 laminas. Finally, the maximum σzz occurs at the laminate midplane.
Stacking sequence have a great influence on interlaminar stresses. A lay-up with interspersed ±45º layers (separated by 0º or 90º layers) have lower interlaminar stress than those with adjacent ±45º layers, and, therefore, are less likely to delaminate.
Conclusions about interlaminar stresses
To conclude, free edge effect has been extensively studied under tension or compression. However, for other non in-plane loads such as out-of-plane shear/bending, in-plane bending, twisting moments, and combined loading have a higher magnitude of interlaminar stress.
