Interlaminar normal stress

At the free edges of a laminate (sides of a laminate or holes), the interlaminar shearing stresses and/or interlaminar normal stress are very high (perhaps even singular) and would therefore cause the debonding that has been observed in such regions. 

Figure 4-58 Interlaminar Normal Stress (After Pagano and Pipes [4-15])...

Historically, most attention has been devoted to mode I delamination growth to assess defects critically, as the first-generation composite systems exhibited low mode I fracture toughness when subjected to interlaminar normal stresses. The... 

Fig. 8.13. (a) Distribution of interlaminar normal stress, <7 , and interlaminar shear stress, r-,. in (0°/90°]s laminate under axial tension, (b) Effect of stacking sequence on through-the-thickness distribution of... 

Fig. 5 Results for the axial stress along the crack plane and the interlaminar normal stress in axial direction starting from the crack tips...

Many reports have shown that for a tc/4 quasi-isotropic laminate, when the interlaminar normal stress is tensile and is the dominant component among the free edge stresses, then open mode delamination may occur. Examples of such laminates include [ 45/90/0]s, [ 45/0/90]s, and [0/ 45/90]s laminates. In these laminates, the open-mode delamination crack may propagate into the laminate before final failure of the laminate. For these laminates, classical failure criteria are not suitable for laminate strength prediction. 

Fig. 8.14. Normalized interlaminar (a) normal stress,

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). 

Test results showed that open-mode delamination occurred along the interfaces adjacent to the 90 layer. From the analytical results of interlaminar stress distributions along interface 3 of [45/-45/0/90]s laminate, at interface 3, high normal stress along the free edge is evident. The interlaminar failure criterion given by Equation (2) was employed to predict open mode delamination load. The delaminate on-set stress for each interface adjacent to the 90 layer was calculated and the results presented in Table 6. The lowest interface failure... 

The principle pattern of the interlaminar stresses are comparable with those of Wang-Crossman [20], Raju-Crews[21] and Wu[16j. With the CFRC data lower stress values are obtained. For the normalized maximum stresses near the 0/90 interface different results are reported ... 

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