The standard “ASTM C1368-18 Standard Test Method for Determining Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress Rate Strength Testing at Ambient Temperature” developed by the American Society for Testing and Materials (ASTM) describes test methods for determining slow crack growth parameters of advanced ceramics using constant stress rate rectangular beam bending test, ring-ring biaxial disk bending test or direct tensile strength. Here, the strength is determined as a function of the applied stress rate in a specified environment at ambient temperature. The strength degradation exhibited with decreasing applied stress rate in a specified environment forms the basis of this test method, which allows the evaluation of the slow crack growth parameters of a material.
This test method is often called dynamic fatigue testing. Here, the term fatigue is used interchangeably with the term slow crack growth. To avoid possible confusion with the phenomenon of fatigue of a material occurring only under cyclic loading, as defined in the standard “ASTM E1823-13 Standard Terminology for Fatigue and Fracture Testing”, this test method is called constant stress rate testing rather than dynamic fatigue testing.
In glass and ceramic technology, long-term static tests are called static fatigue tests and this type of test is called stress rupture.
For many structural ceramic components in service, their use is often limited by the lifespan controlled by the slow crack growth process. This test method provides experimental parameters to evaluate the relative susceptibility of ceramic materials to slow crack growth in specified environments. Furthermore, this test method can determine the effects of processing variables and composition on slow crack growth and the strength behavior of newly developed or existing materials, thus allowing the material processing to be adapted and optimized for further modifications.
In short, this test method can be used for materials development, quality control, characterization, and limited design data generation purposes. Traditional analysis of constant stress rate testing is based on a number of critical assumptions.
The calculation of bending stress for rectangular beam test specimens or coaxial disk bending test specimens is based on simple beam theory with the assumptions that the material is isotropic and homogeneous, that the moduli of elasticity in tension and compression are the same, and that the material is linearly elastic. The average grain size shall not be greater than one-fiftieth of the beam thickness.
For rectangular beam test specimens, the test specimen dimensions and fixtures shall conform to the test method defined in the standard “ASTM C1161-18 Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature”, which provides a balance between practical configurations and introduced errors. In this test method, only four-point test configurations are allowed for rectangular beam specimens, three-point test configurations are not allowed.
The mathematical relationship between strength and stress ratio is derived based on the assumption that the slow crack growth parameter is at least 5 and above. Therefore, if a material exhibits a very high susceptibility to slow crack growth, i.e. below 5, special care should be taken when interpreting the results.
The slow crack growth behavior of ceramic materials exposed to stress-corrosive gases or liquid environments can vary as a function of mechanical, material, and electrochemical variables. It is therefore essential that test results accurately reflect the effects of the particular variables under investigation. Only then can data be compared on a valid basis from one study to another or used as a valid basis for characterizing materials and evaluating structural behavior.
Among the numerous testing, measurement, analysis and evaluation studies that our organization has provided for businesses in various sectors, our organization also provides testing services defined in the standard “ASTM C1368-18 Standard test method for determination of slow crack growth parameters of advanced ceramics by constant stress ratio strength test at ambient temperature” with its trained and expert staff and advanced technological equipment.
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