Microcracks, initiation

Step 3 transverse microcracks initiate in the longitudinal tows (deformations larger than 0.2%). These microcracks are confined within the longitudinal tows. They do not propagate in the rest of the composite. The matrix in the longitudinal tows experiences a fragmentation process and the crack spacing decreases as the load increases. 

Microcrack initiation Microcrack propagation Transverse to load... 

Experimental work on nanocomposites generally demonstrated new or improved properties compared to themicro-andmacrocompositecounterparts.Nanotechnology has proven to be an effective tool in increasing the fracture properties of materials (Kinloch and Taylor, 2006). Dispersion of particles in a polymer matrix usually results in the presence of stress concentrations, in particular when the moduli and Poisson s ratio are different from the matrix. The particles act as microcrack initiation points or may debond, resulting in increased toughness. As the size of the particles moves toward the nanometer scale, one may wonder whether finer particles eventually will lead to even better toughening effects. 

It seems that in concrete composites with brittle matrix, in the crack tip area, the most probable directions of microcracks formation are those which are perpendicular to the isostatic lines. Principal stress trajectories in the vicinity of a notch are connected with the polar coordinate system -microcracks initiate radially and circumferentially, so the body may be considered as cy1indrically-anisotropic. 

Cooper, C.V. Fine, M.E. Fatigue microcrack initiation in polycrystalline alpha-iron with polished and oxidized surfaces. Metall. Trans. A 1985, 16, 641-649. 

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