ASTM E1457-19e1 Test for Measurement of Creep Crack Growth Times in Metals

Scope:

ASTM E1457-19e1 test method is used to determine CCG and CCI. The creep crack growth (CCG) rate, da/dt, is the rate of crack extension caused by creep damage and expressed in terms of average crack extension per unit time. The creep crack initiation (CCI) time is the time it takes for the initial crack to reach an extension of δa_i = 0.2 mm from the onset of the first applied force. This test method is applicable for base materials with homogeneous properties and mixed base/weld materials with inhomogeneous microstructures and creep properties. Extreme environmental conditions can alter creep crack growth behavior, therefore, environmental conditions are carefully selected and maintained during the test. 

Data gathered from this ASTM E1457-19e1 test can be used as follows:

1. To generate models to predict crack incubation periods and growth using analytical and numerical techniques. 
2. To determine the influence of creep crack development and growth on other component life under sustained loading and elevated temperatures where creep deformation might occur. 
3. To determine material selection criteria and inspection requirements for damage tolerant products. 
4. To calculate the individual and combined effects of metallurgical, fabrication, operating temperature, and loading variables on creep crack growth life.

Test Procedure:

In ASTM E1457-19e1 test method, the test specimen is a sharply notched, fatigue pre-cracked, compact test specimen (CT). It is heated to the test temperature in a furnace. A load is applied to the test specimen. It could be in a three-point bending test or some other test method. The applied force is held constant throughout the test. The temperature is constantly monitored to ensure that it remains at the specified level within allowable limits during the test.

Three different loading methods are available for creep crack growth testing. Dead weight loading is the best and the most commonly used method. Constant displacement and constant displacement rate loading are only used when working with extremely brittle materials. The ASTM E1457-19e1 test is carried out for a long time, usually between 5000 to 10,000 hours to reduce crack tip plasticity and allow for steady-state creep cracking to take place. The crack size and force-line displacements are continuously recorded. 

Video 01: Flexural test (3-point bending)

Specimen size:

For ASTM E1457-19e1, the size of the specimen depends upon material availability, the capacity of the loading system, whether the specimen can fit in the heating furnace with enough room for extensometers, and adequate ligament size for growing the crack stably to permit the collection of crack growth data. 

The notch can be machined in the specimen by electrical-discharge machining, milling, broaching, or saw cutting. Compact test specimen (CT) is the primary choice for the test specimen. 

Data:

In ASTM E1457-19e1, the force, force-line displacement and crack size data are numerically processed to obtain the crack growth rate versus C*(t), Ct, or K relationship.

Conclusion:

ASTM E1457-19e1 test method is used to determine CCG and CCI. The creep crack growth (CCG) rate, da/dt, is the rate of crack extension caused by creep damage and expressed in terms of average crack extension per unit time. The creep crack initiation (CCI) time is the time it takes for the initial crack to reach an extension of δai = 0.2 mm from the onset of the first applied force.