ASTM E647 Test for Measurement of Fatigue Crack Growth Rates

Scope:

ASTM E647 test method is used to determine fatigue crack growth rates from near-threshold to K_max controlled instability. Crack growth equations are used to predict the crack size starting from a given initial flaw. The growth of fatigue cracks can result in catastrophic failure. A crack growth equation can be used to ensure safety, both in the design phase and during operation, by predicting the size of cracks. In critical structure, loads can be recorded and used to predict the size of cracks to ensure maintenance or retirement occurs before any of the cracks failing. 

This test method is used for the following purposes: 

-To determine the effect of the fatigue crack growth on the life of components subjected to cyclic loading. 

-To select material for durable products. 

-To determine the effects of metallurgical, fabrication, environmental, and loading variables on fatigue crack growth.

Test Procedure:

This test method involves cyclic loading of notched specimens that have been acceptably pre-cracked in fatigue. Crack size is measured, either visually or by an equivalent method, as a function of elapsed fatigue cycles, and these data are subjected to numerical analysis to establish the rate of crack growth. Crack growth rates are expressed as a function of the stress-intensity factor range, ∆K, which is calculated from expressions based on linear elastic stress analysis. 

Specimen size:

The specimen should be notched, and it should be predominantly elastic at all values of applied force. 

Data:

Threshold equation: To predict crack growth rate at the near-threshold region, the following equation is used: