Ply interfaces

Let us first consider the synergistic elfect that water has on void stabilization. It is likely that a distribution of air voids occurs at ply interfaces because of pockets, wrinkles, ply ends, and particulate bridging. The pressure inside these voids is not sufficient to prevent their collapse upon subsequent pressurization and compaction. As water vapor diffuses into the voids or when water vapor voids are nucleated, however, there will be an equilibrium water vapor pressure (and therefore partial pressure in the air-water void) at any one temperature that, under constant total volume conditions, will cause the total pressure in the void to rise above that of a pure air void. When the void pressure equals or exceeds the surrounding resin hydrostatic pressure plus the surface tension forces, the void becomes stable and can even grow. Equation 6.5 expresses this relationship... 

Models of the intimate contact process that have appeared in the literature are commonly composed of three parts or submodels. The first submodel is used to describe the variation in the tow heights (surface waviness or roughness) across the width of the prepreg or towpreg. The second submodel, which is used to predict the elimination of spatial gaps and the establishment of intimate contact at the ply interfaces, relates the consolidation pressure to the rate of deformation of the resin impregnated fiber tow and resin flow at ply surface. Finally, the third submodel is the constitutive relationship for the resin or resin-saturated tow, which gives the shear viscosity as a function of temperature and shear rate. 

Probably the two most commonly used techniques for measuring the overall quality of the composite consolidation are optical photomicrographs and through transmission C-scan. Both of these techniques can be readily adapted to measuring the degree of intimate contact at the ply interfaces. 

In the following example, scanning acoustic microscopy is combined with optical microscopy to measure intimate contact at the ply interfaces of a [0o/90o/0°]r graphite-PEEK laminate. First, eight holes, 1.5 mm (0.059 in.) in diameter, were drilled into the composite specimen. These holes, shown in Figure 7.12, were used to locate where on the composite specimen the scanning acoustic microscope images were taken. 

The intimate contact data shown in Figure 7.16 were obtained from three-ply, APC-2, [0°/90o/0o]7- cross-ply laminates that were compression molded in a 76.2 mm (3 in.) square steel mold. The degree of intimate contact of the ply interfaces was measured using scanning acoustic microscopy and image analysis software (Section 7.4). The surface characterization parameters for APC-2 Batch II prepreg in Table 7.2 and the zero-shear-rate viscosity for PEEK resin were input into the intimate contact model for the cross-ply interface. Additional details of the experimental procedures and the viscosity data for PEEK resin are given in Reference 22. 

Considering the complexities involved in the formation of the cross-ply interface during consolidation, the cross-ply interply interface model predicted degrees of intimate contact... 

The interply bond strength for thermoplastic matrix composites has been shown to be dependent upon the processing parameters, pressure, temperature, and contact time. If the temperature distribution in the composite is nonuniform during processing, the ply interfaces will bond (or heal) at different rates. Thus, for a specified processing cycle, it is important to know precisely the temperature and degree of autohesive bonding at every point in the composite laminate in order to estimate the required process time. 

The process by which a thermoplastic matrix composite consolidates to form a laminated structure has been attributed to autohesive bond formation at the ply interfaces. Autohesive bond formation is controlled by two mechanisms (1) intimate contact at the ply interfaces, and (2) diffusion of the polymer chains across the interface (healing). The rate of autohesive bond formation and hence the speed of the composite consolidation process is directly related to the temperature-pressure-time processing cycle. 

Once intimate contact is achieved, bonding of the ply interfaces can occur. The mathematical relationships between interply bond formation and processing temperature and time were discussed in Section 7.3. The analyses are based on the theories explaining strength development of a polymer-polymer interface and crack healing in polymers. 

Results from the initial resin studied are also being employed in the development of additional experimental procedures. Plans are currently being drafted to prepare three-ply test specimens that are similar to the specimens used in the initial study, with the middle ply consisting of solid polystyrene. Comparing specimens with and without the graft polymers introduced to the ply interfaces should provide additional information on the ability of the cellulosic graft polymers to facilitate bonding between wood and plastic materials. If this approach proves successful, additional procedures will then be developed for the production of simple composite specimens. 

In the general approach, the loads are applied incrementally until first-ply failure occurs. The type of failure, matrix or fiber, determines which properties of the failed plies must change to reflect the damage created. This is subjective and can cover a range of possibilities. The most conservative approach would completely discard affected properties for the failed plies. So for fiber failure, E would be set to zero. For matrix failure, E22 and G12 would be set to zero. Then, the loads would be incremented until another ply fails, and the procedure would be repeated to complete failure of the laminate. Less conservative approaches attempt to only partially discount stiffness values of the failed ply and even differentiate between tension and compression moduli. These methods can be reasonably accurate if they are accompanied by selected tests that help better define adjustment factors for the stiffness properties of failed plies. However, they are limited in applicability and accuracy because they are affected by the first-ply failure criterion used to trigger the failure sequence and because they do not correctly capture damage modes such as delamination and the interaction between them such as matrix cracks causing delaminations in adjacent ply interfaces. 

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. 

The failure strain of a unidirectional fibre composite in the transverse direction is normally low because the matrix resins have relatively low failure strains while the large difference between the moduli of the components magnifies the strain in the matrix under stress. Thus, in an angle ply laminate, the first failure event occurs in the transverse ply or phes. Reloading of the transverse ply via shear stress transfer at the ply interfaces leads to multiple cracking before the fibres reach their failure... 

WUks, C. E. (1999), Characterization of the Tool/Ply interface during forming , PhD Dissertation, School of Mechanical, Materials, Manufacturing Engineering and ManagemenL University of Nottingham, UK. 

Dzenis Y A and Reneker D H (2001) Delamination resistant composites prepared by small diameter fiber reinforcement at ply interfaces, U.S. patent 6265333. 

Polymer composites reinforced with electrospun polymer nanofibers have so far been developed mainly for providing some outstanding physical e.g., optical and electrical) and chemical properties [20,21], Most of the applications at present focus on very small quantity usage for instance, reinforcement of dental resins, thin films, or in the case of large scale composites parts, only as additional ply interface reinforcement between composite laminates [21], As mentioned before, the biggest issues remain fiber alignment and collection at reasonable production rates. Therefore, large scale production (both quantity and size) of SPCs may benefit more from the methods described in Sections 19,2.3 and 19.4.1. 

Braiding. The elimination of a weak ply interface can be accomplished by braiding. It also has some drawbacks, namely a complicated manufacturing process and a loss of the ability to tailor other properties of the laminate. 

As can be seen from Fig. 11, the magnitude of the peel stress at the first ply interface is not significantly affected by the inclusion, or not. of the effects of the thermal shrinkage and, indeed, it is seen to drop slightly when the thermal effects were accounted for. 

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