Brittle crack macroscopic

ESC is mostly a surface-initiated failure of multiaxially stressed polymers in contact with surface-active substances. These surface-active substances do not cause chemical degradation of the polymer, but rather accelerate the process of macroscopic brittle-crack failure. Crazing and cracking may occur when a polymer under multiaxial stresses is in contact with a medium. A combination of external and/or internal stresses in a component may be involved. 

Brittleness is the opposite of toughness. A brittle crack propagates with little or no macroscopic plastic deformation. Crack propagation requires very little energy and is rapid, usually resulting in rupture. Engineering codes contain rules to avoid brittle fractures. 

When cyclic loads are applied to a material, cracking and fracture may occur by the process of fatigue. On a microscopic scale, fatigue occurs by localized plastic deformation, resulting in the initiation and growth of macroscopically brittle cracks. 

Last is that the mechanism of compact failure also depends on strain rate. Figure 21-118 illustrates schematically the crack behavior observed in compacts as a function of capillary number. At low Ca, compacts fail by brittle fracture with macroscopic crack propagation, whereas at high Ca, compacts fail by plastic flow, which is more desirable to promote growth. 

In comparison the refractory intermetallic Ti5 Si3 compound is brittle at deformation temperatures below 950 °C and no macroscopic ductility has been observed under compressive load. At the test temperature of 1000 °C a remarkable high flow stress of about 1050 MPa was achieved. Crack initiation revealed after plastic deformation of Spi = 1.5%. 

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