Fiber composites, crack propagation

In fiber-reinforced composites the deformation of the matrix is then used to transfer stresses by means of shear tractions at the fiber-matrix interface, to the embedded high-strength fibers. On the other hand, fibers retard the propagation of cracks and thus produce a material of high strength. 

Ceramic composites, which use ceramic fiber or whisker reinforcement in a ceramic matrix, are less susceptible to brittle failure since the reinforcement intercepts, deflects and slows crack propagation. At the same time, the load is transferred from the matrix to the fibers to be distributed more uniformly. These ceramic composites are characterized by low density, generally good thermal stability, and corrosion resistance. 

Similarly, by directly measuring changes in electrical resistance during mechanical deformation it is possible to monitor crack propagation in hierarchical composites with non-conductive fibers and CNTs dispersed in the matrix [48]. Figure 8.7(b) shows... 

Fig. 8.7 Examples of hierarchical composites where the presence of CNTs is used for SHM. (a) Damage detection through thermal imaging of resistively-heated CNTs in an alumina composite [47] and (b) detection of crack propagation by monitoring electrical resistance (normalized by specimen length) in a CNT/glass fiber/epoxy composite [48], With kind permission from IOP (2011) and Wiley (2006).

Mandell J.F., Huang, D.D. and McGarry F.3. (1982). Crack propagation modes in injection molded fiber reinforced thermoplastics. In Short Fiber Reinforced Composite Materials, ASTM STP 772 (B.A. Sanders ed.), ASTM, Philadelphia, PA, pp. 3-32. 

Aveston, J., Cooper, G.A., Kelly, A. (1971), Single and multiple fracture , in The Properties of Fiber Composites, IPC Science Technology Press, Guildford, UK, 15-22 Bahr, H.A., Weiss, H.J., Maschke, H.G., Meissner, F. (1988), Multiple crack propagation in strip caused by thermal shock , Theor. Appl. Fract. Mech., 10, 219-226. Bannister, M.K., Swain, M.V. (1990), Thermal shock of a titanium diboride based composite , Ceram. Int., 16, 77-83... 

Figure 8. Montage of photomicrographs showing crack propagation through the matrix and between plies and fibers in a flexure test of a 2D composite.

C. P. Ostertag, H. Xu, L. Braun, and J. Rodel, Pressureless Sintered Fiber Reinforced Composites Part II. In Situ Crack Propagation, J. Am. Ceram. Soc., in press. 

Crack Propagation Resistance of Random Fiber Composites... 

In practical cases, composite materials contain voids formed by air or vapors trapped in the resin and in the resin/fiber interface. These voids are a source of weakness, points of stress concentration that favor crack propagation. In advanced composite materials the proportion of voids is limited to about 0.5% (2,5,7). 

Standard Mode I Double Cantilever Beam specimens for delamination testing of a unidirectional (UD) IM7/977-2 composite were Z-pinned with two separate blocks of Z-Fiber reinforcement. The reinforced beam configuration was such as to provoke an unstable delamination, propagating between the two Z-pin blocks. Crack resistance curves for these specific geometry specimens of IM7/977-2 indicate that the unstable delamination cracks are arrested by the second Z-pin block, with the crack propagation resistance being dictated primarily by the Z-pinning density within a block. Acoustic emission analysis is used to interpret visual observations and other test data. 

The materials with through-the-thickness fibers had a much higher fracture toughness than ordinary layered composites, therefore the specimen arms were broken before the onset of crack propagation. In order to prevent this, metal tabs were glued to the bottom and upper sides of the specimens. Steel tabs 30 XTCA (so called "spring steel") of thickness 2,0, 2,2, and 2,5 mm and width 17 mm were used. 

New types of ceramic composites with high thermal shock resistance have recently been developed that show some promise for gas turbine applications. These composites consist of a ceramic matrix reinforced by ceramic fibers or platelets inside the matrix. The fibers pull out of the matrix during fracture to resist crack propagation. Such composites can be readily fabricated using a new process developed by Lanxide Corporation [18]. The process uses directed oxidation reactions of molten metals to grow a ceramic matrix around a reinforcing material. 

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