Non-Metallic Fracture

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.

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 3/15/2016www.ishuchita.com3

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. 3/15/2016www.ishuchita.com4

Stress Intensity Factor for Ceramic Fracture Since the mirror radius (r M ) 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 = a cr and b = b cr. Now this Equation can be used for calculating KIc or to estimate the critical flaw size when a separately determined KIc value is available. 3/15/2016www.ishuchita.com5

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. 3/15/2016www.ishuchita.com6