JPS6161618B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a material testing method for determining the growth rate of cracks due to fatigue.

It is generally known that when fatigue, ie, repeated loading, is applied to a material, cracks in the material gradually develop, eventually leading to fracture. Therefore, if test data regarding the development of fatigue cracks for a specific material is known, it becomes possible to quantitatively treat the growth behavior of cracks and the process leading to failure in actual structures. From the dimensions, it is possible to determine the number of load repetitions until the crack grows to the critical crack dimension, that is, the life. This is extremely useful for safety design of structures.

Generally, when conducting a test to determine the crack growth rate, a test piece 1 called a compact tensile standard test piece having a shape as shown in Fig. 1 is used.
The dimensions of each part are when W = 1, H = 0.6W, H ₁
= 0.275W, D = 0.25W, L = 1.25W, and plate thickness B = W/4 to W/20, respectively. Note that the hole 1' drilled in the test piece 1 is for applying a tensile load to the test piece 1.

When performing this test, the stress coefficient is called a coefficient that indicates the intensity of the stress distributed near the crack tip of the material.
A coefficient K having the dimension “length”=“force”ד(length)〓〓 is introduced.

By the way, the load F applied to crack 1
When the stress intensity factor K changes over time as shown in FIG. 2a, the stress intensity factor K also changes over time as shown in FIG. 2b. Now, the fluctuation range of load F and the fluctuation range of stress intensity factor are ΔF and ΔK, respectively.
Then, even if ΔF is constant as shown in FIG. 3a, if the crack length a increases as shown in FIG. 3b, ΔK changes as shown in FIG. 3c.

Pauli et al.'s calculation formula for calculating the above ΔK is as follows: ΔK=Fmax−Fmin/BW〓 [4.55−40.32(a/w)+41.47(a/w) ² −1698(a/w) ³ +3781( a/w) ⁴ -4287(a/w) ⁵ +2017(a/w)
⁶ ] ...(1) is proposed. In the formula, B and W are the respective dimensions of test piece 1 in Fig. 1, Fmax and
Fmin indicates the maximum and minimum loads, respectively. In equation (1), if the constants determined by the shape of test piece 1 are collectively expressed as k, equation (1) becomes ΔK=(Fmax−Fmin)・k・f(a) ……(2) Be able to write.

As is clear from the above explanation, ΔK is the difference between the maximum stress intensity factor Fmax at the maximum load Fmax and the minimum stress intensity factor Kmin at the minimum load Fmin, and Kmax and Kmin are respectively, Kmax=Fmax・K・f(a) It is expressed as Kmin=Fmin・k・f(a). Then, if these ratios are R and R=Kmax/Kmin=Fmax/Fmin, then from the above formula (2), ΔK=Fmax(1-R)・k・f(a) Fmax=ΔK/(1-R )・{1/k・f(a)}……(3) Fmin=R・Fmax=R/(1−R)・{ΔK/k・f(a)}……(
4) It will look like this.

As can be seen from the above formula, Fmax and Fmin can be determined by specifying R, ΔK, and a. Therefore, by applying repeated loads while keeping R and ΔK constant, the crack length a By sequentially measuring , it is possible to determine the crack growth rate da/dN at a specific ΔK. Also,
The crack length a has a specific relationship with the reciprocal of the spring constant of the material, and is determined by the displacement of the crack opening of the test piece and the magnitude of the load at that time. Therefore, by determining the crack growth rate da/dN at a large number of ΔKs, a fatigue crack growth curve as shown in FIG. 4 can be obtained for a particular material at a constant R.

Figure 5 shows the principle of the material testing device that conducts the test to determine the crack growth rate.
is held between the movable member 2a of the actuator 2 and the load cell 3, and an aperture displacement detector 4 is positioned close to the side surface of the test piece 1. It is brought into contact with the side surface of piece 1. Load signals and displacement signals respectively generated by the load cell 3 and displacement detector 4 are sent to a signal processing device 5 comprising a microprocessor or the like. The signal processing device 5 sends a signal to actuate the actuator 2 to the servo valve 2b, and a signal to print out or display the signal processing result on a cathode ray tube, based on the load signal and displacement signal and the signal from the keyboard 6. is sent to the output device 7.

Conventionally, when determining da/dN using the apparatus shown in Fig. 5, the load ratio R and the variation width ΔK of the stress intensity factor were specified from the beginning for the test piece 1 having an initial crack of a predetermined length. I tried to apply repeated loads. If you use this method of specifying ΔK from the beginning, the crack may progress unexpectedly and quickly reach rupture, or the crack may not progress no matter how long it takes. A situation has arisen that makes it impossible to conduct a smooth test. Particularly when the rate of progress is fast, this is dangerous and undesirable.

The present invention aims to solve the above-mentioned conventional problems and provide a material testing method that allows tests to be carried out smoothly.

The present invention will be explained below with reference to the drawings.

First, a test piece 1 having an initial crack of a predetermined length is set as shown in FIG. Note that this step can be replaced by a step in which a test piece 1 having a crack that has not reached a predetermined length is set, and then the crack is grown to a predetermined length using a method known as the aperture displacement method.

Next, by operating the keyboard 6, signals specifying a constant load ratio R, maximum load Fmax, etc. are applied to the signal processing device 5 to start load control. As a result, a cyclic load of 1 to 20 Hz is applied to the test piece 1, and a load signal and a displacement signal are generated from the load cell 3 and the displacement detector 4, respectively, and these are applied to the signal processing device 5. It becomes like this. The signal processing device 5 first calculates the crack length a using a known method using the load signal and the displacement signal, and then calculates the variation range ΔK of the stress intensity factor using the calculated crack length a and the load signal. Calculated based on the above formula (1). Each calculation result is printed or displayed on the output device 7 for monitoring. This calculation is performed every predetermined number of load repetitions specified by the keyboard 6 and is provided for monitoring.

In the above-mentioned load control, as shown in Fig. 6a, a constant amplitude oscillating load F is applied to the test piece ₁ after the start time t1, and as a result, fatigue of the test piece 1 progresses and its crack length decreases. The phase (a) begins to develop as shown in FIG. 6(c). Along with this, the stress intensity factor ΔK also increases as shown in FIG. 6b. These can be monitored by printing them out, so the operator can check them to determine, for example, the point in time when ΔK has reached the desired value.
At _t2 , it is possible to switch from load control to ΔK control. In ΔK control with this ΔK constant, the maximum load Fmax and minimum load Fmin applied to test piece 1 are
are determined by being sequentially calculated by the signal processing device 5 from the above equations (3) and (4) based on ΔK, R and a, respectively.

In the ΔK control after time _t2 , the increase in the crack length a is linear, as can be seen from FIG. 6a, and the crack growth rate da/dN remains constant. And this da/dN can be obtained by calculating the rate of change of a.

In the above explanation, load control is started at the start of the test to increase ΔK, but
If you start displacement control instead,
ΔK can be reduced as shown by the dashed line in FIG. 6b. In this case as well, it is possible to monitor ΔK and switch to ΔK control when it reaches a predetermined value.

Incidentally, FIG. 7 is a flowchart showing the method of the present invention described above in a simplified manner.

As described above, the present invention monitors changes in ΔK during load control or displacement control, and starts ΔK control when ΔK reaches a predetermined value, so it is smoother than when ΔK control is performed from the beginning. This has the extremely excellent effect of allowing material testing to be carried out.

[Brief explanation of the drawing]

Fig. 1 is a plan view showing a compact tensile standard test piece, Fig. 2 a is a graph showing temporal variation of load,
Figure 2b is a graph showing the temporal variation of the stress intensity factor under the load shown in Figure 2a, Figures 3a to c are graphs showing the relationship between load, crack length and stress intensity factor, and Figure 4. is a graph showing an example of a crack development curve, Fig. 5 is a diagram of the principle of the material testing device used in the method of the present invention, and Figs. Graph showing the relationship between fluctuation width and crack length, and the seventh
The figure is a simplified flowchart of the method of the present invention.

Claims (1)

[Claims] 1. A test piece with an initial crack opened on one side is subjected to a repeated load at a constant load ratio by load control or displacement control, and the displacement of the opening of the test piece at this time and the load applied to the test piece are and calculating a crack length based on the detected displacement and load, calculating a variation range of the stress intensity factor based on the calculated crack length and the detected load, and monitoring this calculated variation range, A material testing method characterized in that when the calculated fluctuation range being monitored reaches an arbitrary value, control is started to keep the fluctuation range constant, and a crack length during this control is calculated.