In the ASTM E2760-19e1 method, cyclic loads are applied with sufficiently long loading/unloading rates or hold-times, or both, to cause fatigue and creep deformation at the crack tip. Fatigue and creep deformation enhance crack growth per loading cycle. Cyclic loading is defined as the loads that are applied, removed, and reapplied, for example, on a pavement, in a relatively rapid and repetitive form. This ASTM E2760-19e1 test is intended as a guide for creep-fatigue testing performed on materials for research and development, mechanical design, process and quality control, product performance, and failure analysis.
The fatigue crack will begin on the point of the notch and extend through the sample. The result from this test method can be used to establish material selection and inspection requirements for resilient materials. And it can be used to calculate the effects of metallurgical, fabrication, operating temperature, and loading variables on creep-fatigue crack growth life.
In the ASTM E2760-19e1 method, the specimen is installed on the machine by inserting both pins. After that, a small force is applied (about 10 % of the intended test force) to remove slack from the loading train. A current source is required for supplying current. A displacement gauge and an extensometer are attached to the specimen and a thermocouple to the appropriate potentiometer. The furnace is positioned, and the specimen is heated until it reaches the predetermined temperature. The heat is maintained at that temperature for at least 30 minutes before loading.
Before initiating the ASTM E2760-19e1 test, the extensometer output is brought to a null value and the appropriate cyclic force level is determined. The current is turned on at the same time as the furnace.
The test is initiated by applying minimum force on the specimen and then subjecting it to the determined cyclic force. The force and temperature are maintained within their prescribed limits throughout the test. The electric potential voltage, force, force-line displacement, and test temperature are recorded continuously during the test. The test is terminated when both the potential drop and the displacement measurement indicate that rapid crack growth has begun and that the final failure of the specimen is close. After the test is completed, the cyclic force is removed, and the furnace is turned off. Once the specimen has cooled down, the specimen is removed from the machine without damaging the fracture surface. The crack size is measured.
Video 01: Fatigue test
The specimens are compact tension specimens (CT). This ASTM E2760-19e1 test requires testing of at least two specimens that yield overlapping crack growth rate data. Notches should be machined, and notched specimens should have a fatigue crack.
The following data is calculated in ASTM E2760-19e1:
- Force, P [F] is the force applied to a test specimen or to a component
- Maximum force, Pmax [F] in fatigue, is the highest algebraic value of applied force in a cycle. By convention, tensile forces are positive and compressive forces are negative.
- Minimum force, Pmin [F] in fatigue, is the lowest algebraic value of applied force in a cycle. By convention, tensile forces are positive and compressive forces are negative.
- Force ratio (also stress ratio), R in fatigue, is the algebraic ratio of the two loading parameters of a cycle. The most widely used ratio is as follows:
R= Minimum load/ Maximum load = Pmin/Pmax
- Force range, ∆P [F]—in fatigue loading, is the algebraic difference between the successive valley and peak forces (positive range or increasing force range) or between successive peak and valley forces (negative or decreasing force range). In constant amplitude loading, the range is given as follows:
∆P = Pmax × Pmin
In the ASTM E2760-19e1 method, cyclic loads are applied with sufficiently long loading/unloading rates or hold-times, or both, to cause fatigue and creep deformation at the crack tip.