Composites crack propagation resistance

Crack Propagation Resistance of Random Fiber Composites... 

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. 

Figure 3.9 Crack propagation resistances of alumina, 3Y-TZP, 12Ce-TZP, Mg-PSZ (Mg-doped ZrOj), AlOZOY (ZTA with lOvol.% yttria-free zirconia), micro-nano-AljOj-ZrOj composite, and nano-nano-Ce-TZP/AljOj composite.

Other researchers have substantially advanced the state of the art of fracture mechanics applied to composite materials. Tetelman [6-15] and Corten [6-16] discuss fracture mechanics from the point of view of micromechanics. Sih and Chen [6-17] treat the mixed-mode fracture problem for noncollinear crack propagation. Waddoups, Eisenmann, and Kaminski [6-18] and Konish, Swedlow, and Cruse [6-19] extend the concepts of fracture mechanics to laminates. Impact resistance of unidirectional composites is discussed by Chamis, Hanson, and Serafini [6-20]. They use strain energy and fracture strength concepts along with micromechanics to assess impact resistance in longitudinal, transverse, and shear modes. 

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. 

Crack toughness behavior of MWCNT/polycarbonate nanocomposites was also reported (Xiao et al., 2004 Foster et al., 2005). When 2 wt.% MWCNTs was added into the composites, the resistance to crack propagation was markedly increased compared to pure Polycarbonate. At 4 wt.% MWCNTs, a tough-to-brittle transition has been observed. The attack of crack initiation needs shorter time for nanocomposites with 4 wt.% MWCNT compared to the composites with 2 wt.% MWCNTs, which supports that a tough-to-brittle transition exists in these nanocomposites. 

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).

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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