Delamination-suppression

Figure 4-61 Free-Edge Delamination-Suppression Concepts...

Bianchi,F. and Zhang, X. (2012) Predicting mode-n delamination suppression in z-pinned laminates. Composires Science and Technology, 72 (8), 924—932. 

In particular, the techniques based on the termination of certain plies within the laminate has also shown promise. Static tensile tests of [30°/-30°/30°/90°]s carbon-epoxy laminates containing terminals of [90°] layers at the mid-plane show that premature delamination is completely suppressed with a remarkable 20% improvement in tensile strength, compared to those without a ply terminal. Cyclic fatigue on the same laminates confirms similar results in that the laminate without a ply terminal has delamination equivalent to about 40% of the laminate width after 2x10 cycles, whereas the laminates with a ply terminal exhibit no evidence of delamination even after 9x10 cycles. All these observations are in agreement with the substantially lower interlaminar normal and shear stresses for the latter laminates, as calculated from finite element analysis. A combination of the adhesive interleaf and the tapered layer end has also been explored by Llanos and Vizzini, (1992). 

Kim, D.M. and Hong, C.S. (1992). A simple sublaminate approach to the design of thick composite laminates for suppression of free-edge delamination. Composites Sci. Technol. 43, 147-158. 

Lagacc, P.A. and Bhat, N.V. (1992). Efficient use of film adhesive interlayers to suppress delamination. In Composite Materials Testing and Design (lOlh Volume), ASTM STP 1120 (G.C. Grimes ed.), ASTM, Philadelphia, PA, pp. 384-396. 

These somewhat surprising results imply that aligning initially curved plates may not be remarkably effective in suppressing delamination. It should be noted that if the initial curvatures are aligned, the final curvature of the plate will be twice the initial curvature the internal moments generated by the internal stresses introduced during pressing are complementary. Conversely, as one would expect, the final curvature of the bilayer is zero if the initial curvatures are opposed and identical. 

Some modes may dominate for example, for large bending strains in a flexible structure, fibre fracture in tension and fibre kinking in compression wdl dominate near both surfaces. Matrix cracks can cause delamination when they reach a ply interface. If the structure is stiff enough to resist with a significant force, then local indentation damage, and shear-driven delamination in the interior, wdl occur. Figure 9.2 shows schematically the different modes of fadure in three zones of a laminate. The peanut shape deformations (3) have this shape because the compression under the impact force suppresses the delaminations. 

For the time being, suppose that > (7 > 0 which corresponds to a film with residual tensile stress. For this case, it was noted in Section 4.4 that the phase angle of the stress concentration field at the edge of a delamination is approximately -if = 52.1° and that Kj > 0. Because the phase of a delamination crack with this t3cpe of loading is constant, the interfacial fracture energy will be written simply as F, as before, thus suppressing any dependence of this quantity on -if under these special conditions. 

The failure mechanisms which occur due to thermal cycling differ for underfilled parts compared to those with no underfill. In the absence of an underfill, area array solder joints fail due to solder fracture during thermal cycling. With an underfill, solder fracture is suppressed and other parts of the structure fail. These failure modes include chip fracture and delamination of the underfill from a chip, chip carrier, and solder joints. When the underfill adhesion fails, the solder joints locally are subjected to very high strain levels and quickly fracture. 

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