Delamination between planes

The large thermal expansion anisotropy often results in large Internal stresses and structural problems such as delamination between planes as will be seen in Ch. 5. Sec. 3. 

Thermal Expansion. The thermal expansion of pyrolytic graphite, like that of the ideal graphite crystal, has a marked anisotropy. It is low in the ab directions (lower than most materials) but an order of magnitude higher in the c direction. The effect of temperature is shown in Fig. 7.10. Such a large anisotropy may lead to structural problems such as delamination between planes, especially in thick sections or when the material is deposited around a sharp bend. 

The interlaminar shear strength (ILSS) is the value of the shear strength producing a delamination between two composite layers along the plane of their interface. The measurement is made by a three-point deflection test with the supports very close together. 

Composite structures in service are often subjected to complex 3-D load paths. In general, a delamination will be subjected to a crack driving force with a mode I opening, a mode II forward shear and a mode III anti-plane shear, as illustrated in Fig 3.29. Because delamination is constrained to grow between individual plies, both interlaminar tension and shear stresses are commonly present at the... 

Based on the failure mechanisms and stress distributions at the interface between steel and composite adherends of the co-cured single and double lap joints, tensile load bearing capacities of the two joints were evaluated. Since failure started at the edge of the interface between steel and composite adherends, it is important to consider the failure criterion using interfacial out-of-plane stress distributions at the interface. Three-dimensional Tsai-Wu and Ye-delamination failure criteria were used to predict partial cohesive failure or interlaminar delamination failure in the co-cured single and double lap joints. 

Where S, and are out-of-plane peel and shear strengths between steel and composite adherends, respectively. is out-of-plane shear strength in the plane (i.e. zy-plane) perpendicular to the fiber direction. All the stress components are defined as the averaged stresses over a distance from the free edge, c. Table 2 shows various values used to calculate Ye-delamination failure indices. The distance c was defined as one-ply thickness and S =5 was assumed [23]. 

There is a considerable difference between initiation values from the insert starter film determined from the maximum load/5% offset (MAX/5%) and the non-linear or visual point (NLWIS) for the two cross-ply laminates (NLA IS-values not reported in the Tables since they were not determined for all specimens). The NLATS-values, when determined, are comparable to initiation from the insert for the unidirectional material (determined as MAX/5%- or NL/VIS-values). The first few mm of delamination propagation in the cross-ply laminates usually occur in the mid-plane of the beam, but deviating from it before the maximum load value is reached. The difference between the NL/VIS- and MAX/5%-values quite likely reflects this transition from mid-plane to saw-tooth pattern. 

It could be argued that even in the unidirectional lay-up the delamination is not strictly miming along the mid-plane, at least on a microscopic scale (Fig. 6). Deviations are on the order of 0.05 mm. In the case of the cross-ply material, the maximum deviation is limited by the distance between the 0°-plies, i.e. about 0.17 and 0.38 mm, respectively, as long as the delamination does not further deviate into the unidirectional plies. This is about 3 and 7 times, respectively, more than in the unidirectional laminates. Sufficiently small , limited deviations will not affect the data analysis in a way that renders the values meaningless. In the following, it is proposed that the validity of cross-ply data can be defined by requiring the delamination not to deviate into the adjacent unidirectional plies. This criterion could easily be verified by inspection of the fracture surface. 

Matrix cracking in angle-ply laminates also introduces the coupling between extension and shear. The axial/transverse shear-extension coupling coefficients [5] that characterise shearing in the xy plane caused by respectively axial/transverse stress are plotted in Fig. 5 as a function of the relative delamination area D . There is no experimental data to compare our analytical predictions and this is the topic of current work. Results will be presented at a future event. 

F (23°C) and 50% relative humidity for 17-24 h. Test specimens, 10 X 15 cm, are cut from the sealed paper and delaminated at the rate of 5 in./min. The open ends of the seal are in the same plane, with a 180° angle between the ends. The unseparated portion is perpendicular to this plane. Seahng strength is the force measured in grams per centimeter required to separate the sealed strips. 

A mechanism which begins with delaminations that are initiated at opposite ends of a single grain, but on different basal planes. With further deformation, the torque separates the lamellae between the two delamination cracks to ultimately form crack bridges not unlike those shown in Figure 7.18b. 

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