Surface plies

Figure 4.15 shows the results from such a sensor controlled run in an autoclave. FDEMS sensor-measured viscosities from two sensors at the center ply and one sensor at the surface ply are shown in Figure 4.15. Air autoclave temperatures and the temperatures at the surface and center ply are also shown. The starting time for the FDEMS sensor-controlled autoclave run and the manufacturer s cure cycle run are defined as file time at which the tool surface temperature starts to increase.

The FDEMS sensor-controlled run significantly reduced the time lag and viscosity difference between the center ply and the tool surface ply. The amount of flow as measured by the magnitude of the viscosity minimum was greater in the FDEMS sensor-controlled run. The approach of ds"/dt to zero was used to determine cure completion. The total cure time of 200 min in this FDEMS controlled run is 40 min less than the conventional cure cycle. 

Lumber core is a five-ply panel, usually about 19 mm (3/4 in.) thick, in which the bulk of the thickness, about 16 mm (5/8 in.) is edge-glued lumber. Yellow poplar and red gum are desired species for lumber core. Cross-plies of lower value wood veneers are laid at right angles to the core grain direction, followed by two thin surface plies of the decorative face veneer in a parallel direction to the core. This assembly is pressed and bonded to form a panel of exceptional quality, provided all steps are accomplished in a desirable manner. 

Plywood furniture core panels, also about 19 mm (3/4 in.) thick, were normally made of a number of layers of relatively thick, 1.5—3.0 mm (1 /16—1 /8 in.) lower value wood veneers combined with thin surface plies of the decorative veneer. These assemblies were laid-up from glued veneers and then pressed while the bonding occurred. Both lumber core and plywood core have been almost totally displaced in recent years by particleboard or medium-density fiberboard, both discussed herein. This change resulted from the increasing availability and improved finishing characteristics of composites and from decreasing supplies of core lumber or veneer of suitable quality. 

The outer two surface plies are called faces (often one surface ply will be called the face and the other surface the back). The innermost ply is called core in a three-ply configuration. Other layers are called centers and in some cases crossbands. 

Pemsler et al. earlier introduced the concept of carbon fiber reinforced internal barrier, where diffusion barrier metals are infiltrated electrochemically into matrix-free carbon fibers in a specially oxidized surface ply of C-C. Infiltration was carried out successfully with Re, Rh and Ir. The rhodium coatings were found non-cracked, adherent, and survived thermal cycles to as high as l,600 C. 

The use of Eqn (4.12) in laminate design is illustrated for two different stacking sequences in Figure 4.12 where it can be seen that the four 45° surface plies in the [(45/—45/45/—45)2/(90/0/90/0)2ls laminate provide enhanced damage tolerance. This is due to the combined effect of delayed buckling (Figure 4.12(a)) and reduced All for the surface plies of this laminated. In the case of the [45/0/ 45/90]4s laminate, the near-surface presence of the 0° ply has the opposite effect. 

Fiber breakout Fiber separation or break on surface plies at drilled or machine edges. 

Figure 11.4 Surface ply buckling growth characteristics for the 3 series composites fatigued at 70% of UCS...

In research as well as there in industry, delamination during drilling FRP is recognized as one of the most critical problems. As defined by various researchers, it is an interlaminar or inter-ply failure phenomenon or behavior. When occurred at the topmost surface ply around the drilled hole periphery, it is called peel-up delamination or simply hole entry delamination of the composite workpiece. In addition, this interlaminar shearing of the last ply at the bottommost surface of the FRP composite material is usually more severe and is generally called push-out delamination or in simple words hole exit delamination. Delamination can be introduced by three mechanisms peeling up of the topmost layer, pushing out of the bottommost layer, and an additional cause, called thermal stress mode. 

When the laminate surface plies are of coarse fabrics or woven rovings thiah areal weioht hiah tPY niimheri a neel niv should he used on the... 

Figure fO Effect of surface ply on the joint efficiency, single-lap joints, L/t=5. 

In addition to any cost savings for long production runs, there are two key quality assurance factors that favor the use of automated lay. These are the greatly reduced chance that release paper or film could be retained, which would destroy shear and compressive strength if undetected, and the reduced probability of the addition or loss of a single ply that could cause warping because of the laminate s lack of symmetry and balance. New automated layup machines have a laser ply mapping accessory that can verily the position of the surface ply. 

Advanced FRP composites for structural applications Surface plies... 

Use fabric, particularly fiberglass or Kevlar, as a surface ply to restrict surface (handling) damage. 

A final use of adhesives for composite structure is as a surfacing ply. In this application a low flow adhesive is applied to the composite surface prior to cure in order to produce a cosmetically smooth surface that is ready for painting without further finishing operations. In addition, surfacing ply adhesives as well as interleaf films have been effective in reducing the porosity of composite skins. This has led to their applications in wet wings to prevent fuel leakage and in composite skin sandwich structure to prevent moisture penetration. 

Failure in the laminate is predicted by.three failure criteria. Three modes of failure are included, i.e., in-plane, interlaminar, and surface ply free edge failures. 

Since the in-plane stresses (Cn, CT22 12) ibe surface ply near the free edge are not constant, the average stresses over a distance of 2t from the free edge were used in the Tsai-Hill criterion for failure prediction. Further, since the surface plies are partially free from constraints (the lamination effect), the in-plane shear strength should be lower than that measured with [ 45]2s specimen. Thus, we took the value S = 14.4 ksi (for AS4/3501-6) reported in most literature. For T300/5208 graphite/epoxy composite we found S = 8.2 ksi was suitable. 

Here, we modify the free edge failure criterion by including the surface ply failure mode. The modified criterion states... 

Angle-ply [ 0]2s laminates of T300/5208 graphite/epoxy composite were analyzed using the proposed failure criterion. In ensence, Tsai-Hill, interfacial failure, and surface-ply failure loads were obtained first and the laminate strength was taken to be the lowest failure load among the three. 

Table 3 lists the predicted failure stresses for the three modes of failure, i.e., in-plane (Tsai-Hill), interlaminar, and free-edge surface-ply failures. The test data were reported by Rotem and Hashin [3] and by Oplinger et al [4]. Note that Rotem and Hashin used T300/5208 graphite/epoxy while Oplinger et al used MOD 11/5206 graphite/epoxy. However their data appeared similar, and thus were treated equally. 

For small 0 values (0 = 5° - 10°), failure is dominated by the surface-ply failure at the free edge. Both Tsai-Hill and interlaminar failure criteria predict much higher strengths in this region. 

In the region 0=15°- 30°, free edge (interlaminar or surface-ply) failure dominates. 

Criterion 0 Tsai-Hill Interlaminar Failure Surface-Ply with S =8.2 Experimental Failure... 

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