Laminate uncoupled laminates

Table 4.2 Number of symmetric, anti-symmetric and non-symmetric fully uncoupled laminates with seven through 21 plies...

After a layer fails, the behavior of the laminate depends on how the mechanical and thermal interactions between layers uncouple. Actually, failure of a layer might not mean that it can no longer carry load. In the present example of a cross-ply laminate, the inner layer with fibers at 90° to the x-axis has failed, but, because of the orientation of the fibers (perpendicular to the main failure-causing stress), the failure should be only a series of cracks parallel to the fibers. Thus, stress can still be carried by the inner layer in the fiber direction (y-direction). 

For cross-ply laminates, a knee in the load-deformation cun/e occurs after the mechanical and thermal interactions between layers uncouple because of failure (which might be only degradation, not necessarily fracture) of a lamina. The mechanical interactions are caused by Poisson effects and/or shear-extension coupling. The thermal interactions are caused by different coefficients of thermal expansion in different layers because of different angular orientations of the layers (even though the orthotropic materials in each lamina are the same). The interactions are disrupted if the layers in a laminate separate. 

Obvious and sometimes drastic simplifications occur when the laminate is symmetric about the middle surface (By = 0), specially orthotropic (all the terms with 16 and 26 subscripts vanisn in addition to the By), homogeneous ( = 0 and Djj = Ayr/12), or isotropic. In all those cases. Equations (5.6) and (5.7) are coupled to each other, but uncoupled from Equation (5.8). That is. Equation (5.8) contains derivatives of the transverse displacement w only, and Equations (5.6) and (5.7) contain both u and V but not w. Accordingly, only Equation (5.8) must be solved to determine the transverse deflections of a plate with the aforementioned... 

The use of a simple, analytical approach to damage tolerance enables the selection of optimum (fuUy uncoupled) stacking sequences for which effective stress levels may exceed 400 MPa, see Figure 4.13. The approach assumes that laminates are locally... 

Sloan and Talbot [113] cathodically coupled 26-ply quasi-isotropic T300/ 934 carbon/epoxy laminate coupons to magnesium in natural Pacific sea water at 40°C for 140 days. In the uncoupled state in tapwater or natural sea water, the moisture uptake was -0.85% and calcerous deposit was 0.14% and 0.36%, respectively. For the coupled materials, the moisture uptake was -1.3% and the calcerous deposit was 16%. There was some leaching loss from the coupled material. The shear strengths in four point bend tests showed negligible degradation for the soaked samples, but a 30% reduction was measured for electrically coupled materials. This reduction was correlated to delaminations, reducing the effective specimen width by 20-40%. 

P(4) Specially orthotropic laminates are those with uncoupled in plane and flexural behaviour and that do not exhibit bending stretching torsion coupling. Normally this is reached with a laminate structure that consists of 0° and/or 90° plies only. 

In the case of symmetric laminate, the in-plane deformations become uncoupled from bending, i.e. [S] = 0. The fundamental equation of classical lamination theory reduces to... 

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