JPH07113597B2 - Fatigue crack growth test equipment - Google Patents

Description

[Detailed Description of the Invention] A. Field of Industrial Application The present invention provides:
The present invention relates to a fatigue crack growth test apparatus for controlling a crack growth rate of a test piece by controlling a stress intensity factor width ΔK of a crack tip to a predetermined value.

B. Prior art Since the life of a structural member in which a fatigue crack occurs depends on the speed at which the crack propagates, correctly measuring the crack growth rate of each material ensures the safety of the structure. Important above. It is known that the crack growth rate becomes faster as the change width (amplitude value) of the cyclic load increases, and that the initial crack length becomes longer for the cyclic load of the same change width. .

By the way, the stress intensity factor K and the applied load P have the following relationship.

K = P · f (a) ... where a is a crack length and f (a) is a function with the crack length a as a variable, and f increases as the crack length a increases.
(A) also tends to increase.

In addition, the stress intensity factor width Δ, which is the fluctuation range of the stress intensity factor K,
K is expressed by the following equation.

ΔK = (P _MAX −P _MIN ) · f (a) Here, P _MAX is the maximum value of the cyclic load, and P _MIN is the minimum value of the cyclic load.

As is clear from the equation, the stress intensity factor width ΔK is the crack length a and the change width of the cyclic load (P _MAX −P _MIN ).
Both of them are included as parameters, and thus the stress intensity factor width ΔK is used to represent the crack growth rate, which fluctuates according to the crack length a and the change width of the cyclic load, by one parameter. Is convenient.

Therefore, in this type of fatigue crack growth test, the stress intensity factor width ΔK is changed stepwise as a single parameter, and the crack growth rate at each ΔK, that is, per cycle of repeated load (sine wave). The amount of advance of crack length a (da / dN) is measured.

FIG. 2 is a block diagram showing an outline of the configuration of such a conventional fatigue crack growth testing device.

In FIG. 1, reference numeral 1 is a tester body, 2 is a test piece having a notch formed in a side surface, 3 is an actuator which is a double-acting hydraulic cylinder, and 4 is an amount of oil supplied to the actuator 3. The servo valve 5 is a load cell for detecting the load acting on the test piece 2, and 6 is a load amplifier for amplifying the detection signal of the load cell 5. A deviation between the load detection signal L which is the output of the load amplifier 6 and the load signal P as shown in FIG. 3 is given to the servo valve 4. Test piece described above
2, the actuator 3, the servo valve 4, the load cell 5, and the load amplifier 6 constitute a negative feedback control system so that the load detection signal L and the load signal P are equal.

By the way, as described above, in the fatigue crack growth test, the stress intensity factor width ΔK as shown in FIG. 4 is measured by gradually changing the stress intensity factor width ΔK and measuring the crack growth rate at each ΔK. And the crack growth rate da / dN are obtained, the stress intensity factor width ΔK is used as the set value. Therefore, in this type of test apparatus, after converting the stress intensity factor width ΔK into the load signal P, the load signal P
Is applied to the above-mentioned negative feedback control system, and the load is controlled so that ΔK becomes constant. Below, the stress intensity factor width ΔK
A means for converting the load signal P into the load signal P will be described.

First, the load intensity calculation unit 7 is provided with the stress intensity factor width ΔK and the load ratio R as set values. The load ratio R is a ratio between the maximum value P _MAX and the minimum value P _MIN of the load signal P and is represented by the following equation.

R = P _MAX / P _MIN ... On the other hand, a clip gauge 8 is attached to the cutout portion of the test piece 2, and the detection signal of the clip gauge 8 is given to the crack length calculation unit 10 via the displacement amplifier 9. To be The crack length calculation unit 10 calculates the length a of the fatigue crack generated in the test piece 2 based on the detection signal of the clip gauge 8. The crack length a is given to the load calculation unit 7.

The load calculation unit 7 calculates the maximum value P _MAX and the minimum value P of the load signal P from the set values ΔK and R and the crack length a given from the crack length calculation unit 10 based on the formulas and formulas.
Calculate P _MIN and further, based on this P _MAX , P _MIN ,
The amplitude value ΔP of the load signal P is calculated from the following equation, and the average load value P _MEAN is calculated from the following equation.

ΔP = (P _MAX −P _MIN ) / 2 ...... P _MEAN = (P _MAX + P _MIN ) / 2 …… The amplitude value ΔP calculated in this way is given to one input of the multiplier 11. The multiplier 11 multiplies the sine wave given from the sine wave generator 12 by the amplitude value ΔP to calculate the amplitude of the load signal P by the set stress intensity factor width ΔK and the crack length at that time. Convert to a value corresponding to a. Then, the final weight signal P is obtained by adding the average weight value _PMEAN output from the weight calculator 7 to the output signal of the multiplier 11.

By the load calculating unit 7 and the multiplier 11 as described above, the amplitude value of the load signal P and the average load value P are determined according to the crack length a.
_A load is applied to the test piece 2 while changing the _MEAN and keeping the stress intensity factor width ΔK constant.

C. Problems to be Solved by the Invention By the way, this type of test apparatus acts on the test piece 2 as the crack length a of the test piece 2 progresses even if the set value ΔK is constant. Since the applied load becomes smaller and the ΔK value is generally decreased as the crack propagates, the applied load becomes smaller as the crack length a increases. That is, from the equation, the load signal (load) P is expressed as P = K / f (a) ... ′, and therefore the crack length a increases and f
(A) also increases and P decreases.

Therefore, according to the conventional device, the value of ΔP which is one input of the multiplier 11 becomes extremely small as the crack length a becomes longer. Generally, in this type of device, the load calculation unit 7
Since the multiplier 11 and the multiplier 11 are digitally processed by the computer, the calculation accuracy is deteriorated when the value of ΔP is small, the output of the multiplier 11 is easily varied, and is easily influenced by noise. There is a problem that is worse.

The present invention has been made in view of the above circumstances, and provides a fatigue crack growth test apparatus capable of accurately applying a load to a test piece even if the applied load becomes smaller with time. Has an aim.

D. Means for Solving Problems The present invention has the following configuration in order to solve the above problems.

That is, the present invention provides a negative feedback control system for controlling an actuator such that a load signal with a constant stress intensity factor width is calculated and the load signal is equal to the detected load signal detected by the load cell. In a fatigue crack growth test apparatus provided with, arithmetic means for calculating the amplitude value of the load signal from the stress intensity factor width, the crack length of the test piece and the load ratio which is the ratio of the maximum value and the minimum value of the load signal. According to the reciprocal of the amplitude value of the load load signal, a multiplier for changing the level of the load detection signal detected by the load cell, and the reciprocal of the amplitude value of the load load signal, the load load signal And a calculation unit for changing the level.

E. Operation According to this invention, the level of the load detection signal detected by the load cell is changed according to the reciprocal of the amplitude value of the load load signal, and the level of the load load signal is changed according to the reciprocal of the amplitude value of the load load signal. Since the level is changed, the level of the load detection signal fed back by the negative feedback control system becomes almost constant regardless of the magnitude of the load.

F. Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing the outline of the configuration of an embodiment of the present invention.

In the figure, the parts designated by the same reference numerals as those in FIG. 2 are the same parts, and therefore the description thereof is omitted here.

In this embodiment, the load calculation unit 7 ′ given ΔK and R as set values and the crack length a from the crack length calculation unit 10 calculates the reciprocal 1 / ΔP of the amplitude value of the load signal P. The 1 / ΔP and the load detection signal L obtained via the load amplifier 6 are multiplied by the multiplier 11. On the other hand, the load calculation unit 7 ′ outputs a value obtained by multiplying the average load value P _MEAN by 1 / ΔP, and this value is added to the output of the sine wave generator 12. The deviation between the sine wave added with P _MEAN / ΔP and the output of the multiplier 11 is given to the servo valve 4 of the test apparatus body 1.

As described above, according to this embodiment, even when the crack length a becomes long, or when a small ΔK value is set and the applied load becomes small, the amplitude value output from the load calculating unit 7 ′ is The reciprocal 1 / ΔP becomes large. By multiplying the output signal of the load amplifier 6 by this 1 / ΔP, the level of the load detection signal taken out as the output of the multiplier 11 becomes almost constant regardless of the magnitude of the load load. Can be raised.

The present invention is not limited to the above-described embodiment, and for example, when the applied load is relatively large, the crack growth rate was measured by the conventional device shown in FIG. 2 and the applied load was reduced. In this case, the crack growth rate is measured by the device shown in FIG. 1, and the device shown in FIG.
It is also possible to configure the apparatus shown in the figure so that it can be used by switching according to the magnitude of the applied load.

G. Effect of the invention As is apparent from the above description, the fatigue crack growth test apparatus according to the present invention changes the level of the load detection signal according to the reciprocal of the applied load. Regardless, the level of the load detection signal that is returned is substantially constant, so that the load can be controlled more accurately than in the conventional device.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of a fatigue crack growth test apparatus according to an embodiment of the present invention, and FIG. 2 is a block showing a schematic configuration of a fatigue crack growth test apparatus according to a conventional example. FIG. 3 is a waveform diagram of a load (load signal), and FIG. 4 is a characteristic diagram showing the relationship between the stress intensity factor width ΔK and the crack growth rate da / dN. 1 …… Test device body 2 …… Test piece 3 …… Actuator 4 …… Servo valve 5 …… Load cell 6 …… Load amplifier 7,7 ′ …… Load calculation unit 8 …… Clip gauge 9 …… Displacement amplifier 10… … Crack length calculator 11 …… Multiplier 12 …… Sine wave generator

Claims (1)

[Claims] 1. A negative feedback control system for calculating an applied load signal such that the stress intensity factor width is constant and controlling the actuator so that the applied load signal and the detected load signal detected by the load cell become equal. In a fatigue crack growth test apparatus, a calculation means for calculating the amplitude value of the load signal from the stress intensity factor width, the crack length of the test piece, and the load ratio which is the ratio of the maximum value and the minimum value of the load signal, , A multiplier for changing the level of the load detection signal detected by the load cell according to the reciprocal of the amplitude value of the load load signal, and the level of the load load signal according to the reciprocal of the amplitude value of the load load signal And a fatigue crack growth test apparatus.