Creep fatigue testing machine is a precision equipment used to simulate the failure behavior of materials under the combined action of high temperature and cyclic loading. Its working principle combines dynamic cyclic loading of fatigue testing and static constant load maintenance of creep testing.
1、 Working principle of testing machine
The core system of the testing machine includes:
Loading system: Apply precise and controllable axial tension tension or tension compression cyclic loads to the specimen through servo motors or hydraulic actuators.
Heating system: High frequency induction furnace or resistance furnace is usually used to heat and stabilize the sample at the target temperature (usually above 0.3 times the melting point of the material).
Measurement and Control System: High precision sensors monitor load, strain (usually using extension rods to connect the gauge length section of the specimen), and temperature in real-time, and ensure that the test parameters strictly follow the preset waveform (including load and holding time) through a closed-loop control system.
The typical creep fatigue test waveform introduces a holding time at the peak or valley load during a fatigue cycle. During this period, the load remains constant, but the material will continue to undergo creep deformation due to high temperatures, leading to creep damage.
2、 Analysis of creep fatigue interaction mechanism
The creep fatigue interaction refers to the fact that two damage mechanisms are not simply superimposed, but accelerate each other, resulting in a material life much lower than the predicted results of pure fatigue or pure creep. The micro mechanism mainly originates from:
Grain boundary slip and void nucleation: During the tensile holding phase of fatigue cycles, the combined effect of high temperature and constant stress promotes grain boundary slip, and stress concentration occurs at grain boundary obstacles (such as second phase particles and triple junction points), leading to creep void nucleation. The subsequent cyclic loading will accelerate the growth and connection of these voids.
Environmental oxidation and crack propagation: High temperature environments cause severe oxidation on the surface of materials. During the holding time, oxygen diffuses along the grain boundaries, forming brittle oxides and weakening the strength of the grain boundaries. The repeated plastic deformation generated by fatigue cycles can damage the surface oxide film, expose fresh metal, and promote the initiation and propagation of intergranular cracks caused by oxidation.
Stress relaxation and redistribution: During the retention period, the internal stress of the material will relax due to creep deformation. When the load changes again, the stress needs to be redistributed, and this repeated relaxation redistribution process will exacerbate the accumulation of damage to the microstructure.
In summary, the creep fatigue testing machine physically simulates the working conditions through "cyclic loading+constant load holding". The essence of its interaction mechanism is that fatigue load provides nucleation sites and driving forces for creep damage (voids, oxidation); The creep process (voids, oxidation embrittlement) creates shortcuts for the initiation and propagation of fatigue cracks, and the synergistic effect of the two ultimately leads to early failure of the material.