ASTM E561 test method uses different specimens such as the middle-cracked tension specimen or the compact tension specimen to determine the fracture enduring and resisting capabilities of the metallic materials by plotting the KR Curve against increasing stress intensity factor, K. The final results are expressed in SI units.
ASTM E561 test method determines the crack extension values of the materials by careful studies of the KR Curve. The yield strength and toughness of the materials are adjusted. Slow crack extension resulting from the growth of plastic zones in the presence of a progressing crack driving force contributes to the toughness and reliability of the materials. The specimen thickness, temperature, and strain rates are determined. This helps detect the instability and crack behavior of the material.
A testing machine with grip fixtures is used to distribute the stress uniformly. Displacement gages are installed to measure the crack-mouth opening displacement. The thickness and width of the specimen are measured. The specimens are pre-cracked and aligned in the testing machine for getting rid of misalignment and eccentricity. The rate of crack extension is increased through the application of force and displacement. Crack size and K values are determined. Direct measurement and compliance measurement techniques are applied to determine the elasticity of the specimen. Data recording equipment is used for an accurate record of force and displacement.
Specimen size should be as required, within the desired range of radius 0.08 mm. The fatigue pre-crack shall extend from the starter notch by 1.3 mm length.
The initial crack size ao In the specimens are recorded between 0.35 and 0.55W. The KR Value is calculated based on adequate crack size where P stands for applied force, B stands for specimen thickness, W is the width of the specimen, and α is the crack size.