Is the fatigue test of smooth metal specimens a crack or a failure condition until it breaks?

Low-cycle fatigue is to achieve a certain percentage decrease in peak force, or the compression section of the hysteresis curve is deformed, or there are obvious cracks. High-cycle fatigue needs to be determined according to the type of testing machine, and the electro-hydraulic servo-fatigue testing machine generally achieves the fracture position; The high-frequency fatigue testing machine with electromagnetic excitation generally has cracks that are enough to make the test frequency drop enough to stop, or find an expert institution (such as 725 Research Institute) for specific consultation!

If there is a crack, it will definitely fail, and if there is no crack stress in the strain control test, it will be considered a failure if the crack stress drops to a certain extent.

6.Fatigue test methods and fatigue curves:

Principle: The stress of the actual machine parts is simulated with a small sample, and the fatigue curve of the material is systematically measured on the fatigue test machine, so as to establish the fatigue limit and fatigue stress criterion.

Test equipment: the most commonly used rotational bending fatigue testing machine will be the same size of fatigue specimens, from the range of several different maximum cyclic stresses to each specimen to carry out a cyclic loading test, determine their stress cycles from the beginning of loading to the specimen fracture N cycles, and then the test data is plotted as a Max N curve or Max-LGN curve, that is, fatigue curve.

3. Test procedure Load the sample into the testing machine, clamp it firmly and keep it well coaxial with the main shaft of the testing machine. When rotating, the radial runout measured on the free end of the specimen should not be greater than. No-load operation, the radial runout measured at the afterburner part of the spindle barrel should not be greater than.

The above values must be verified before afterburner. Do not touch the surface of the test part when loading. The test speed range is 900 10000r min.

The test speed should be the same for the same batch of tests. The test speed that causes the specimen to resonate must not be used.

3. Test procedure The test shall be carried out until the sample fails or terminates when the specified number of cycles is reached, and the test shall not be interrupted in principle. The criterion for specimen failure is fatigue crack or complete fracture visible to the naked eye. If the failure of the specimen occurs outside the maximum stress position, or if there is an obvious defect in the fracture or abnormal data occurs in the middle of the test, the test result is invalid.

Fourth, the determination of the conditional fatigue limit The stress increment is generally 3 5 of the predicted conditional fatigue limit -1. The test should be carried out at a stress level of 3 to 5, and the stress level of the first specimen should be slightly higher than the expected conditional fatigue limit. According to the test result of the previous specimen is broken or passed, that is, the specimen is destroyed or passed before the specified life is not reached for 10 weeks, the stress of the next specimen is reduced or increased until the whole test is completed.

When the part is aware of the possibility of fatigue fracture, it should be calculated according to the () criterion.

a.Strength shirt.

b.Stiffness. c.Life span.

d.Vibration stability.

The correct answer is pure: a

Due to the presence of rust at the crack, the main component of the rust is iron oxide, which can react with hydrochloric acid to form chlorinated iron and water

Due to the presence of rust at the crack, the main component of rust is iron oxide, which can react with hydrochloric acid to form ferric chloride and water, and the crack will become larger after the reaction and easy to observe