This process is illustrated in figure 1457 in a

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Unformatted text preview: ferable because substantial energy is absorbed both during plastic deformation of the matrix and during delamination. These processes contribute to the overall toughness of the composite materials. In the previous example widespread damage develops in the composite prior to fracture, leading to a tough material. On the contrary, if the interfaces are so strong that no σ σ Fracture Fracture Localized damage Distributed damage ε ε (a) (b) | v v FIGURE 14.5–6 (a) Schematic of a tensile stress-strain diagram and the progress of damage in ductile composites, and (b) the same as part a but in a brittle system. | e-Text Main Menu | Textbook Table of Contents 13.01.98 plm QC1 rps MP 601 pg602 [V] G2 7-27060 / IRWIN / Schaffer 602 Part III iq 13.01.98 plm QC2 rps MP Properties delamination is possible and the matrix is not ductile enough to effectively blunt the fiber cracks, then the progression of damage remains localized and a low-energy fracture results. This process is shown schematically in Figure 14.5–6b. The same principles apply to ceramic-matrix composites. Typically the cracking begins in the matrix and the toughness is enhanced by fiber bridging and fiber pullout, as described in Section 14.4. 14.5.5 Fatigue Behavior of Composites As discussed in Chapter 9, fatigue failures in metals generally initiate at the specimen surface because of microplasticity, which leads to crack formation. These cracks propagate and become larger, causing the final fracture. Fatigue mechanisms in composites are considerably different. The different stages of fatigue of laminates consist of ply cracking, delamination, and ultimately fiber fatigue. This process is illustrated in Figure 14.5–7 in a laminate with fibers in the 0 and 90 orientations. The number of applied fatigue cycles is divided by the number of cycles of failure to derive a cycle ratio. The microstructural changes that result from fatigue damage are then correlated with the cycle ratio. The extent of ply cracking is best represented by the crack density in the laminae oriented at 90 to the loading axis. As shown in Figure 14.5–7a, the crack density increases rapidly at first and then reaches a constant value. Delamination does not occur initially, but occurs rapidly after saturation of ply cracking (Figure 14.5–7b). Delamination saturates when the last stage of composite fatigue, fiber fatigue, begins. As shown in Figure 14.5–7c, when enough fibers have fractured because of fatigue, the composite 90 0 0 Delamination Crack density 0 Ply cracking 0 90 Delamination 1 0 1 Cycle ratio Cycle ratio (a) (b) Modulus 0 90 0 Fiber fatigue 0 1 Cycle ratio (c) | v v FIGURE 14.5–7 Schematic representation of stages of fatigue damage accumulation: (a) ply cracking, (b) delamination, and (c) fiber cracking. (Source: H. T. Hahn and L. Lorenzo, Advances in Fracture Research, Pergamon Press, 1984.) | e-Text Main Menu | Textbook Table of Contents 0 5 0.8 = 0 E0 0.9 E/ = (σ max/ σ uts) E0 E/ during a fa...
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