Non-Metallic Fracture

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Non Metallic Fractures • Ceramic• Plastics or Polymers

Ceramic Fracture• Brittle and with very less toughness• This makes detection of flaws and the precracking

fracture toughness difficult.• critical flaw sizes in ceramics range from ~1 to ~500

μm.

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The fracture surface of a ceramic material normally consists of four distinct regions : 1. The Source of Failure ( Crack) 2. the smooth mirror region that separates the original crack (the flaw origin) 3. the mist region, which is a transitional area preceding the rough/hackle region. 4. Hackle region

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• Consider a very small flaw originating on a ceramic surface as shown in Fig. 1, which schematically depicts a fracture surface typically found in fractographs of ceramics and glasses.

• The mirror region is unique i.e. flat, smooth, and brittle in nature. For a wide variety of ceramics, the ratio of the mirror size to the flaw size is a constant with a value of 13.

• The mist region is somewhat less smooth. • The rough hackle region is easily recognized by the

outward divergent lines running along the crack propagation direction. This region is associated with a large amount of strain energy absorption and is thus somewhat ductile in nature.

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Stress Intensity Factor for Ceramic Fracture• Since the mirror radius (rM) is measurable, the flaw dimensions a

and b can be estimated. • The initial and critical flaw sizes are often the same in ceramics; • therefore, the critical stress-intensity factor can be calculated by

modeling the crack as an elliptical surface flaw. The stress-intensity expression in this case would be:

and Where, • ф is crack shape parameter,• S is the Stress• K becomes KIc when a = acr and b = bcr.• Now this Equation can be used for calculating KIc or to estimate

the critical flaw size when a separately determined KIc value is available.

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Due to the brittleness of ceramics, ASTM E 399 test methods are not applicable to fracture and crack growth testing of them. Instead, a number of specially prepared standards have been developed. Typical KIc values for some common ceramics are available. They are listed along with other mechanical properties in Table.

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