ASTM C1421 Fracture Toughness of Advanced Ceramics

Scope:

The ASTM C1421 test methods are used to determine the fracture toughness of advanced ceramics at ambient temperature. The methods determine K_Ipb(pre-cracked beam test specimen), K_Isc(surface crack in flexure), and K_Ivb(chevron-notched beam test specimen). These test methods are applicable to materials with either flat or with rising R-curves.

Beam test specimens having a sharp crack are used to assess fracture toughness values. The pb (pre-cracked beam) and vb (chevron-notched beam) fracture toughness values indicate the fracture resistance of advanced ceramics with big sharp cracks, whereas the sc (surface crack) fracture toughness value indicates the fracture resistance of small cracks similar in size to natural fracture sources.

Test Procedure:

The pre-crack, surface crack, and chevron notch methods are described in ASTM C1421 for determining fracture toughness values and testing ceramic beam specimens:

➢ Precracked Beam Method :  

To create a straight-through pre-crack in a beam test specimen, the bridge-flexure technique is used. The fracture force of the pre-cracked test specimen is measured as a function of displacement or alternatively as a function of time in three or four-point flexure (for example,  time, back-face strain). Using the fracture force, test specimen size, and pre-crack size measurements, the fracture toughness, k_Ipb, is calculated. 

➢ Surface Crack in Flexure Method:  

The fracture force of the test specimen is obtained, and the fracture toughness, k_Isc, is computed using the four-point flexure method. For this ASTM C1421 test method, a Knoop indenter is used to indent a Beam test specimen, which is subsequently polished until the indent and associated residual stress field are removed. 

➢ Chevron-Notched Beam Method : 

A chevron-notched beam is loaded using three- or four-point flexure. The fracture toughness,  K_Ivb, is determined by applying the greatest force to a test specimen after the crack has been stretched in a steady manner. The test isn’t valid in unstable parameters like time, back-face strain, or actuator displacement, but it can detect unstable fractures. 

Specimen Size : 

The crack of the ASTM C1421 test specimen is either a straight-through crack (pb), or a semi-elliptical surface crack (sc), or it is propagated in a chevron notch (vb) with specific dimensions,  tolerances, and finishes.

Data:

For the pb and sc test methods, the relevant maximum force is the greatest force occurring during the test, according to ASTM C1421.

Conclusion:

The ASTM C1421 test methods are used to determine the fracture toughness of advanced ceramics at ambient temperature.