JPS60151542A - Method for testing low-cycle fatigue of material - Google Patents

Abstract

PURPOSE:To prevent damage on a ruptured surface by judging the time point when a ratio of the tension-side max. load to the compression-side max. load at each cycle of repeated tensile and compressive loads becomes a prescribed value as the rupture life of a test piece and stopping the load supply. CONSTITUTION:The repeated tensile and compressive loads are loaded on the test piece 2 by an actuater 3, the load generated thereat is detected by a load cell 6, the strain quantity is detected by a strain meter 4, measured and recorded by a measuring unit 7. As the test proceeds, cracks occur and grow in the piece 2. An arithmetic 8 judges that the rupture life is attained when a ratio of the tension-side max. load to the compression-side max. load becomes the prescribed value, outputs the signal to a waveform generator 9, and it generates the signal for stopping operation of the actuater 3 to a control unit 10. In such a way, the repeated load supply is stopped exactly at the time point when the piece 2 attains the rupture life.

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The present invention relates to a test in which a test piece undergoes low-cycle fatigue by applying repeated tensile and compression loads to a test piece while controlling the amount of strain to be at a constant amplitude. Regarding the method.

[Prior art and its problems]

If the number of stress repetitions under the plate is approximately 105 times during the service life, the results of a fatigue test conducted at a higher stress than a general fatigue test can be helpful, and fatigue phenomena in such cases can be reduced to a lower cycle. This is called fatigue or plastic fatigue. This low-cycle fatigue can be measured by subjecting a test piece to repeated strain with a constant amplitude or a tensile/compressive load with a repeated load of a constant amplitude.

When a test piece breaks during a test, a crack first occurs in the test piece, and this crack grows until it finally breaks. However, in the case of a constant strain low cycle fatigue test in which a constant amplitude strain is repeated, due to the control function of the testing machine, even if the specimen completely separates and breaks, the strain amplitude may be repeated. The fracture life cannot be accurately determined from the number of times the material is broken, and the fracture surfaces collide with each other even after fracture, resulting in damage, making it impossible to observe the fracture surface, which is essential for understanding the fracture mechanism of the material.

Conventionally, the following methods have been adopted to prevent these problems. That is, when a test piece is repeatedly subjected to strain with a constant amplitude, a tensile and compressive load is generated with each cycle. As cracks occur and grow in the test piece, the cross-sectional area of the unbroken portion decreases, and therefore the tensile load decreases. Therefore, a method has been adopted in which the point at which this tensile survey large load falls below a predetermined value is determined to be the rupture life of the test piece, and at that point the supply of the tensile and compressive load to the test piece is stopped.

Generally, when a metal material is subjected to repeated loads, it softens or hardens as the number of repetitions increases. When a strain of constant amplitude is repeated, this phenomenon appears in the magnitude of the load, which becomes lower when the material softens and higher when it hardens.

This softening and hardening phenomenon becomes almost saturated by about 100 repetitions, and the load level also stabilizes at a constant value.

In the method described above, a predetermined percentage (for example, 5%) is determined from the tensile survey large load when it is stable.

The point in time when the tensile survey large load decreased by 10%, 25%) was defined as the fracture life of the test piece, and at that point, the supply of the tensile compression load to the test piece was stopped.

However, in the case of a constant strain low cycle fatigue test, the load level that occurs varies depending on the material, test temperature, strain amplitude, and repetition rate, and repeated hardening and softening phenomena occur, so it is difficult to predict the load level in a stable state. is difficult. Therefore, depending on the level of the load value preset before the test, there is a drawback that the cyclic loading may be stopped before the actual rupture life is reached, or the test may be continued even after the rupture life.

[Purpose of the invention]

An object of the present invention is to provide a low-cycle fatigue testing method for materials that can accurately determine the fracture life of a material due to low-cycle fatigue and can also prevent damage to the fracture surface.

[Key points of the invention]

According to the invention, this object is achieved in a low-cycle fatigue test method for materials of the type mentioned at the outset, by comparing the tensile survey maximum load and the maximum compressive load for each cycle of repeated tensile-compressive loading during the test; This can be achieved by determining the time at which the ratio of both reaches a predetermined value as the rupture life of the test piece, and stopping the supply of tensile and compressive loads to the test piece at that time.

[Embodiments of the invention]

Next, the present invention will be described in detail based on embodiments shown in the drawings.

In Fig. 1, the electro-hydraulic servo type fatigue testing machine 1 includes:
In this example, a test piece 2 of 5TJS304 stainless steel is set. This test piece 2 was heated to 550° C. and was subjected to repeated tensile and compressive loads in the first and second stages.

In order to conduct a constant strain low cycle test, this tensile and compressive repeated load is applied using a servo so that the strain detected by the extensometer 4 installed on the test piece 2 has a constant amplitude as shown in Figure 2a. It is controlled by a controller 5. The load generated at this time is detected by a load cell 6 provided in the testing machine, and this load changes as shown in FIG. 2 in accordance with the amount of distortion in the waveform shown in FIG. 2a. Strains and loads are continuously measured and recorded by a measuring unit 7.

Test piece 2 of this material shows a repeated work hardening phenomenon, but in the stable state after work hardening, the load (converted to stress in the figure) - strain hysteresis curve becomes as shown in Figure 3a (4 , 500 cycles), the tensile survey large load and the compression side maximum load are almost equal, in this case about 20. 9/m1n2. The number of repetitions of constant strain increases,
When a crack occurs in specimen 2, this hysteresis curve changes as shown in Figure 3 (at 8,700 cycles), and the maximum load on the compression side is the same as in the stable state, but
The maximum load on the tensile side decreases as the cross-sectional area decreases.

The present invention focuses on this point, and compares the tensile measurement large load and the compression side maximum load of each cycle of repeated tension and compression loads during the test, and the ratio of the two is set to a predetermined value, for example, 3:
4 is determined to be the rupture life, and the supply of tensile and compressive load to the test piece 2 is stopped at this point.

For this purpose, a computing unit 8 is provided which compares the tensile measurement large load detected in each repeated cycle with the compression side maximum load. Control to a constant amplitude (7) The waveform generator 9 that outputs the distorted waveform is controlled by the output of this calculator 8. The waveform sharpener 9 is controlled by the output.
The servo controller 5 is controlled via the servo controller 10. The waveform generator 9 also outputs electrical signals to the measurement unit 7 at the peak points on the tension and compression sides.

During the test, according to the output of the waveform generator 9, the actuator 3 continuously applies a controlled repeated tensile and compressive load (Fig. 2b) to the specimen 2 so as to cause a constant amplitude strain (Fig. 2a). give to the target. As the test progresses, cracks occur and grow in the test piece 2. 1 When the calculator 8 outputs a signal indicating that the tensile survey large load and the compression side maximum load have reached a predetermined ratio, that is, the rupture life has been reached, this output signal causes the waveform generator to 9 issues a signal to the control 22) 10 to stop the operation of the actuator 3. In this way, when the test piece 2 reaches its rupture life, the supply of repeated loads is accurately stopped.

Note that the present invention can also be applied to materials that soften due to repeated loads in the same manner as described above. In some cases, it is also possible to define the rupture life not by the load level but by the ratio of the crack propagation area in the cross-sectional area of the test piece.

〔Effect of the invention〕

According to the present invention, the following advantages can be obtained.

(1) Since the degree of decrease in the tensile survey large load is detected based on the ratio of the tensile survey large load to the compression side maximum load, the rupture life can be accurately determined even if the material hardens or softens due to work.

(2) The fracture life can be detected simply by detecting the tensile survey large load and the compression side maximum load with a load cell and comparing the two with a calculator, so the supply of repeated loads to the test piece can be stopped at the optimal point. Improves testing efficiency.

(3) Damage caused by mutual contact between the separated fracture surfaces of the test piece can be minimized, and fracture surface observation will not be hindered.

[Brief explanation of drawings]

Fig. 1 is a control system diagram of a low cycle fatigue testing machine based on the present invention, Figs. Figure 3 shows load-strain hysteresis curves of the test piece before and after crack initiation, respectively. 1: Fatigue tester, 2: Test piece, 3: Actuator, 4: Extensometer, 5: Servo controller, 6: Load cell, 7° measurement unit, 8: Arithmetic unit, 9: Waveform generator, 1o: Control unit. Figure 1 Figure 2 α W・Force (K9/Tnm2) Stress (Kg/mTT12)

Claims (1)

[Claims] ■) Test method C in which the material is subject to low-cycle fatigue by applying controlled repeated loads of tension and compression so that the amount of strain is at a constant amplitude on the test piece. Compare the maximum tensile limit load and the maximum compressive load for each cycle of repeated compression loading, and determine the rupture life of the test piece when the ratio of the two reaches a predetermined value of 75; A method for low-cycle fatigue testing of materials, characterized by stopping the application of tensile and compressive loads to the specimen. 2. Claim 1 (: In the method described in C2,
The maximum tensile limit load and the maximum compressive 9AII load for each cycle are detected by a load cell installed in the testing machine, and both are compared by a computing unit, and the output signal of this computing unit is used to control the actuator that repeatedly loads the test piece. A low cycle fatigue testing method for materials characterized in that the material is stopped.