JPH0658859A - Calibrating method for measurement of crack length - Google Patents

Abstract

PURPOSE:To measure the length of crack accurately by applying load repeatedly by predetermined times on a test piece while keeping constant the variation of stress intensity factor and the ratio between maximum and minimum values thereof thereby setting a constant interval of beach mark. CONSTITUTION:A test piece 7 is set in a test equipment and a computor 4 commands a controller 5 to operate an actuator 1 such that the maximum and minimum values of repeating load and the ratio therebetween are kept constant. When the repeating load is applied by predetermined times, load signal of a load cell 2 and opening dispacement (COD) from a strain gauge 3 are inputted in order to determine a compliance value C and a crack length (a) thus calculating DELTAK, i.e., the difference between maximum and minimum stress intensity factors. Repeating load is applied while keeping the DELTAK constant and the ratio between minimum and maximum loads at 0.1 and the compliance is measured when the load-is applied by predetermined times. Subsequently, Kmax is kept constant and the DELTAK' is kept constant at DELTAK/2 and the load is applied repeatedly on the test piece 7. A beach mark is then inserted, the test piece 7 is separated vertically, crack length (a) from the mark position to each position is measured and combined with the compliance value to determine the coefficients thus calibrating the measurement of crack length.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a fracture toughness test method for obtaining a fatigue crack growth curve, for determining a coefficient of an equation for measuring a crack length introduced into a test piece to measure the crack length. The present invention relates to a calibration method of crack length measurement for calibrating the equation (1).

[0002]

2. Description of the Related Art As one of fracture toughness testing methods, a method of obtaining a fatigue crack growth curve showing a relationship between a crack growth rate da / dN and a stress intensity factor variation width ΔK is known. As a method of obtaining the crack length a, a compliance method, a potential difference method or the like is generally adopted.

In the compliance method, the compliance (the reciprocal of the spring constant) C, which is the slope of the load / displacement curve,
Is calculated by the equation C = Δd / ΔP (d: displacement, P: load), and the crack length a is obtained by substituting this value into a predetermined equation used in the compliance method.

There is an unloading compliance method as one of the compliance methods. In this method, COD (Crac
k Opening Displacement (aperture displacement) is controlled to be constant and a load is repeatedly applied to the compact test piece, and then this load is removed, and then the compliance is calculated from the slope of the load / displacement curve at that time. The crack length is automatically calculated by substituting the value into the successor formula, and ΔK is automatically calculated based on the crack length.

By the way, according to the above-mentioned method, although the actual crack length (actual crack length) and the calculated crack length are substantially coincident with each other, the shape of the material, the treatment, and the actual crack length are accurate. , The calculated value changes depending on the magnitude of the ΔK value, which causes an error in the test result.

Therefore, conventionally, beach marks (shell patterns) M _{1 to} M _n as shown in FIG. 8 are inserted into a compact test piece by a method corresponding to an actual test, and the crack length is measured by this beach mark, From the correspondence between the measured crack length and compliance, the crack length measurement is calibrated by deciding the coefficient of the Sucenna formula used to measure the crack length introduced into the compact test piece. The calibration method of was adopted.

That is, when the crack length is obtained by the conventional method such as the compliance method, the compliance (the reciprocal of the spring constant) is previously obtained by the displacement and load of the COD portion (opening), and the corresponding function of the crack length and the polynomial is obtained, When measuring the crack length, the function is used to determine the crack length from the compliance. At this time, the beach mark method is used to determine each coefficient of the polynomial of the function, the beach mark is inserted, and the curve is determined by obtaining the relationship between the dimension (crack length) of each beach mark part and compliance. Was.

[0008]

However, as described above, conventionally, the COD amplitude was made constant, the operation was performed, the COD amplitude was reduced at each beach mark portion, and the beach mark was inserted. As a result, there is a problem in that the inserted beach marks have a dimension between the beach marks as the crack progresses, that is, the reading of the beach marks is inaccurate and the reading operation becomes difficult.

Therefore, in view of the above-mentioned conventional problems, the present invention enables the beach marks to be read accurately and easily by allowing the beach marks to be inserted at substantially equal intervals as the size of the beach marks does not get boring as the crack progresses. An object of the present invention is to provide a calibration method for measuring the crack length.

[0010]

In order to achieve the above object, a calibration method for crack length measurement according to the present invention calculates a crack length introduced into a test piece by using a measured value of the test piece. The coefficient for the equation for is determined by using the crack length measured by the beach mark inserted in the test piece and the measurement value measured for each insertion of the beach mark, thereby calibrating the equation for measuring the crack length. In the crack length measurement calibration method as described above, the stress intensity factor fluctuation range is kept constant and the ratio of the maximum stress intensity factor and the minimum stress intensity factor is kept constant to apply a predetermined number of repeated loads to the test piece. In addition, each time the predetermined number of repeated loads are applied, the measured value for calculating the crack length is stored using the known equation, and then the repeated load for inserting the beach mark is applied. Is characterized by repeated thereafter until the operation a predetermined number of beach mark is inserted.

In order to achieve the above object, the calibration method of the crack length measurement according to the present invention uses the coefficient of the equation for calculating the crack length introduced into the test piece using the compliance value measured for the test piece. , The crack length measured by the beach mark inserted into the test piece and the compliance value measured for each insertion of the beach mark are used to determine the crack length to calibrate the formula for measuring the crack length. In the calibration method of measurement, the stress intensity factor fluctuation range is kept constant and the ratio of the maximum stress intensity factor and the minimum stress intensity factor is kept constant, and a predetermined number of repeated loads are applied to the test piece, After the repeated load is applied, the compliance value for calculating the crack length is held using the above-mentioned formula, and then the beach mark is inserted. The repeated load added, is characterized by repeated thereafter the operation until a predetermined number of beach mark is inserted.

In order to achieve the above object, the calibration method of the crack length measurement according to the present invention is a method for calculating the crack length introduced into a compact test piece by using a compliance value measured for the compact test piece. Calibrate the successor formula for measuring crack length by determining the coefficient of the formula using the crack length measured by the beach mark inserted in the compact test piece and the compliance value measured for each insertion of the beach mark. In the method for calibrating crack length measurement, the stress intensity factor fluctuation range is kept constant and the ratio between the maximum stress intensity factor and the minimum stress intensity factor is kept constant, and the compact test piece is repeated a predetermined number of times. Each time a load is applied and this predetermined number of repeated loads are finished, the crack length is calculated using the known Saxena equation. Holding the compliance value for, then added repeated load for inserting the beach marks, thereafter the operation a predetermined number of beach mark is characterized by repeated until inserted.

[0013]

According to the above procedure, cracks between beach marks are kept on the test piece for a predetermined number of times while keeping the fluctuation range of the stress intensity factor constant and the ratio of the maximum stress intensity factor and the minimum stress intensity factor constant. Since it is formed by repeatedly applying a load, the amount of crack growth between any beach marks is kept constant, and the intervals between beach marks are made equal.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a test device used for carrying out the method according to the invention, in which 1 is an actuator, 2 is a load cell, 3 is a strain gauge, and 4 is a computer. The actuator 1 is provided with a servo valve 6 controlled by a servo controller 5, and repeatedly applies a load to a compact test piece 7 (see FIG. 2). The load cell 2 detects a load acting on the compact test piece 7 and inputs the detection signal to the computer 4 via the A / D converter 8. The strain gauge 3 is a COD (Crack Opening Displaceme) of the compact test piece 7.
(nt: opening displacement) is detected, and the detection signal is input to the servo controller 5 and also to the computer 4 via the A / D converter 9. The computer 4 inputs and processes the detection signals from the load cell 2 and the strain gauge 3, and inputs the result to the servo controller 5 via the D / A converter 10 to control it. In the above processing, the computer 4 performs the work according to the flowchart shown in FIG.

A compact test piece 7 is set in the above-mentioned test device.
When you start the operation,
Fatigue cracks on the controller 5 through the D / A converter 10
Output the command of introduction, and repeat load P by this command.
Maximum value P of_maxAnd the minimum value P _minIs a constant value, and
The ratio P_min/ P_maxSo that it keeps a constant value of approximately 0.1,
The servo controller 5 operates the actuator 1.
(Step S1). Compact test piece 7 as described above
As shown in Fig. 4, the load P repeatedly applied under the conditions
Predetermined number of times N₀Is detected (step S
2) Then, ΔK is measured (step S3).

In this ΔK measurement, the computer 4 inputs the load signal from the load cell 2 and the COD signal from the strain gauge 3, and the above equation C = Δd / ΔP
The compliance value C is obtained based on (d: displacement, P: load), and the obtained compliance value C is substituted into the following known successor formula (1) to obtain the crack length a,
Substituting the obtained crack length a into the known equation (2), Δ
Find K.

A / W = A₀+ A₁Ux + A₂Ux² + A₃Ux³+ A_FourUx^Four+ A_FiveUx^Five Ux = 1 / [(B ・ E ・ C)^1/2+1] (1) where B is a known plate thickness (mm) and W is a known plate width (m
m) and E are known elastic moduli (kg / mm²), C is compliance
Ans (mm / kg), A when measuring on the load line ₀
Is 1.000196, A₁Is -4.06319, A₂Is 11.242, A₃Is
-106.043, A_FourIs 464.335, A_FiveIs 650.677.

ΔK = [(P _max −P _min ) / B / W ^1/2 ] · f (α) α = a / W f (α) = [(2 + α) · (0.886 + 4.64 · α-13.31) α ² +14.72 α ³ −5.6 α ⁴ )] / (1−α) ^3/2 (2) where P _max is the maximum load and P _min is the minimum load.

Now, in the above formula (2), if the constants determined by the shape of the compact test piece 7 are collectively expressed by k, the formula (2) is expressed as ΔK = (P _max −P _min ) · k · f (a) You can write (3).

[0020] As apparent from the above description, [Delta] K is the difference between the maximum stress intensity factor K _max at the maximum load P _max, and minimum stress intensity factor K _min when the minimum load P _min, K _max And K _min are respectively expressed by K _max = P _max · k · f (a) and K _min = P _min · k · f (a). Then, when these ratios are R and R = K _max / K _min = P _max / P _min , from the above equation (3), ΔK = P _max (1-R) · k · f (a) P _max = [ΔK / (1-R)] · [1 / k · f (a)] (4) P _min = R · P _max = [R / (1-R)] · [ΔK / k · f (a )]… (5).

Thereafter, the measured ΔK is kept constant, and P _max and P _min are controlled so that R is kept at 0.1, for example, and the servo controller 5 operates the actuator 1 to apply a load to the compact test piece 7. It is repeatedly added (step S4). When it is detected that the load P repeatedly applied to the compact test piece 7 under the above-described conditions reaches the predetermined number N _A as shown in FIG. 5 (step S
6) Next, the compliance at that time is measured and output (step S7).

In this compliance measurement, the computer 4 inputs the load signal from the load cell 2 and the COD signal from the strain gauge 3, and the above equation C = Δ.
The compliance value C is calculated based on d / ΔP (d: displacement, P: load).

Then, K _max is kept constant and ΔK '
The [Delta] K / servo controller 5 controls the P _{ma x} and P _min to maintain a constant 2 actuates the actuator 1 is added repeatedly loading a compact specimen 7 (step S8). When it is detected that the load P was repeated added under conditions as described above in a compact specimen 7 has reached a predetermined number N _B (>> N _A) as shown in FIG. 5 (step S9), and then Increment the counter (step S
After 10), the above-described operation is repeated until the counter value reaches the predetermined number X, and when it reaches the predetermined number X, the series of operations is ended. The predetermined number X designates the number of beach marks used to calibrate the coefficient of the above-mentioned Sucenna formula, which is inserted into the compact test piece 7.

As described above, since ΔK is kept constant and the N _A repeated load P is applied a predetermined number of times, ΔK shown in FIG.
From the relationship of da / dN, the crack growth length da when the repeated load P of the predetermined number N _A is applied is kept constant, and the intervals between the beach marks become equal to each other. At the end of each predetermined number of times N _A , the values of COD and load at that time are measured and recorded and stored in order to obtain compliance later. Note that FIG. 7A shows a change in COD when a load is applied as described above, FIG. 7B shows a state of ΔK, and FIG. 7C shows a change in crack length a.

After inserting the predetermined number X of beach marks, the compact test piece 7 is forcibly separated into upper and lower parts, and the crack length a from the predetermined number X of beach marks formed at equal intervals to each position. Is actually measured. Substituting this actual crack length and the compliance value at that time into the above equation (1), the coefficient A
₀ to form a simultaneous equations to unknowns A _5, to determine the coefficients A ₀ to A ₅ the simultaneous equation is solved for A ₀ to A _5.

In the above-mentioned embodiment, the case where the compact test piece is used has been described, but the present invention can be applied to the case where the three-point bending test piece is used. But,
In the case of a three-point bending test piece, the crack length and ΔK must use equations different from the above equations (1) and (2).

Further, in the above-mentioned embodiment, the example in which the compliance method is used has been shown, but it is equally applicable to the case of calibrating the coefficient of the equation for obtaining the crack length by the potential difference method.

[0028]

As described above, according to the present invention, the crack between beach marks is kept constant in the variation range of the stress intensity factor and the ratio of the maximum stress intensity factor to the minimum stress intensity factor is kept constant. Since it is formed by holding and applying a predetermined number of repeated loads to the test piece, the crack growth amount between any beach marks can be kept constant, and the intervals between beach marks are made approximately equal. As a result, the beach mark can be read accurately and easily, and the coefficient of the subsequent equation for calibrating the equation for measuring the crack length can be determined more accurately.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a test apparatus for carrying out a calibration method for crack length measurement according to the present invention.

FIG. 2 is a perspective view showing the structure of a compact test piece.

3 is a flowchart showing work performed by the computer in FIG. 1. FIG.

FIG. 4 is a waveform diagram showing a load for forming a pre-crack.

FIG. 5 is a diagram showing changes in K when crack growth-beach mark formation-crack growth is performed.

FIG. 6 is a graph showing the relationship between ΔK and da / dN.

FIG. 7 is a diagram showing the states of COD, K and crack length a when the method of the present invention was carried out.

FIG. 8 is a diagram for explaining a problem of the conventional method.

[Explanation of symbols]

7 Compact test piece a Crack length ΔK Stress intensity factor fluctuation range K _max Maximum stress intensity factor K _min Minimum stress intensity factor R ratio M _{1 to} M _n Beach mark N _A Predetermined number of times C Compliance value

Claims (3)

[Claims] 1. A crack length measured by a beach mark inserted in a test piece and a beach mark insertion coefficient of an equation for calculating a crack length introduced into the test piece using a measured value measured for the test piece. In the crack length measurement calibration method in which the equation for crack length measurement is calibrated by determining using the measurement values measured for each While maintaining the ratio of the stress intensity factor and the minimum stress intensity factor constant, applying a predetermined number of repeated loads to the test piece, each time the predetermined number of repeated loads are applied, the known crack length is calculated using the known equation above. It is characterized by holding the measured value for calculation, then applying a repeated load for inserting a beach mark, and then repeating the above operation until a predetermined number of beach marks are inserted. Calibration method of length measurement. 2. A coefficient of an equation for calculating the crack length introduced into the test piece using the compliance value measured for the test piece, the crack length measured by the beach mark inserted into the test piece and the insertion of the beach mark. In the crack length measurement calibration method in which the compliance value measured for each is used to calibrate the equation for crack length measurement, the stress intensity factor fluctuation range is kept constant and the maximum While maintaining the ratio of the stress intensity factor and the minimum stress intensity factor constant, applying a predetermined number of repeated loads to the test piece, each time the predetermined number of repeated loads are applied, the known crack length is calculated using the known equation above. Holds the compliance value for calculation, then applies repeated load to insert the beach mark, and then the above operation inserts the specified number of beach marks. Calibration method for crack length measurement Ki and repeating until. 3. The crack length measured by a beach mark inserted in the compact test piece, which is a coefficient of the Sucenna formula for calculating the crack length introduced into the compact test piece using the compliance value measured for the compact test piece. In the calibration method for crack length measurement, the stress intensity factor variation range is determined by using the compliance value measured for each insertion of the Is kept constant and the ratio between the maximum stress intensity factor and the minimum stress intensity factor is kept constant, and a predetermined number of repeated loads is applied to the compact test piece. Hold the compliance value for calculating the crack length using the Saxena formula, and then insert the beach mark. A method for calibrating crack length measurement, characterized in that a repetitive load is applied and the above operation is repeated until a predetermined number of beach marks are inserted.