ASTM E561 K-R Curve Determination 

Scope:

ASTM E561 test technique is used to estimate the K-R curve. A K-R curve is a continuous record of toughness growth (resistance to fracture extension) in terms of K_R plotted against crack extension in the specimen as the stress intensity component, K, increases. The test is suited to conditions in which a material exhibits slow, steady crack extension as crack driving power increases, which may exist in reasonably resistant materials under plane stress crack tip settings. The K-R curve can be matched with the crack driving (applied K) curves for an untested shape to predict the degree of stable crack extension and the conditions required to trigger unstable crack propagation.

Test Procedure:

For the ASTM E561 test procedure, data were collected on a regular basis to establish effective fracture size and quantify K values, which are individual data points that construct the K-R curve for the material under specific test conditions. This test method basically consists of two methods for establishing effective crack size. Direct measurement of the physical crack size, which is then adjusted for the developing plastic zone size and compliance measuring techniques, which produce the effective crack size immediately. It must be specified how to measure crack extension and make plastic-zone repairs to the physical crack size. Expressions for the calculation of crack-extension force K are also given.

Specimen Size:

The ASTM E561 test process employs either the middle-cracked tension M(T) specimen or the compact tension C(T) specimen.

Read More :- ASTM E561-10 Standard test for K-R curve determination

Data:

The K-R curve is a plot of crack extension resistance K as a function of effective crack extension Δa_e , determined using the ASTM E561 test method. 

Conclusion:

ASTM E561 test technique assesses the fracture resistance of metallic materials. This method simply covers two of the many alternative specimen types that could be used to build K-R curves.