JPS6156937A - Jic fracture toughness testing method - Google Patents

Abstract

PURPOSE:To obtain an accurate curve, by substituting the amount of shift into the first term Aa of a Saxena's expression, which obtains the length of a crack, when the amount of crack growth DELTAa at the starting point of the R curve, which is obtained by a load removing compliance method, is shifted by DELTAx from the zero value, and correcting the value. CONSTITUTION:A predicted fatigue crack length is set in an operation controller 6. A repeating load is applied to a compact test (CT) piece 1. The load is detected by a load cell 3. The compliance is obtained by an X-Y recorder 4 from a load waveform. The compliance is substituted in a Saxena's expression an= Aa+Baf1(Ks.Cn)+..., where (an) is the length of a crack, n=1,2,3..., Cn is compliance, Aa and Ba are coefficients and Ks is a material constant. Thus the length of a predicted fatigue crack 1b is computed. When the amount of crack growth DELTAa at the starting point of the R curve, which is obtained by a load removing compliance method, is shifted from the zero value, the amount of shift DELTAx is added to or subtracted from the first term Aa in the expression, and the expression is corrected. Thus the accurate R curve is obtained in a short time, and the measuring accuracy can be improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides basic data for determining how much external force a machine, structure, etc. that has defects such as cracks can withstand. . J+' (: Fracture toughness L'<4 regarding test method). [Technical background of the invention and its problems La] When installing machines, structures, etc., strength characteristic data of the 4A° material used (for example, yield strength, tensile strength, fatigue limit, etc., all expressed in terms of stress) Based on this, the stress generated during operation is prevented from exceeding these values.

However, experience has shown that when a fairly large defect or crack already exists in a mechanical or transversely thin member, its strength ml decreases as the size of the defect or crack increases.
known by. The strength properties in such a case do not match the strength property data described above measured using a smooth specimen that does not contain very large defects. For example, even if the tensile strength of a smooth specimen is steel A is higher than steel B, if cracks of the same size exist in each material, the fracture strength of steel B will be higher than steel A. There is.

Defects such as cracks can be determined based on the "fracture toughness value", which is the resistance value of the material when fracture starts from a crack and rapidly progresses without an increase in external force, that is, when true crack failure occurs. A test method has been proposed to determine the fracture strength of materials that have developed.

Methods such as the "R curve method" and the "unloading compliance method" are known as J1c fracture W-J property testing methods for determining the fracture toughness value using the "Jic value."

■? Both the curve method and the unloading compliance method calculate "9 curves", and from these 9 curves and the blunted straight line, the "JHc value" is obtained.
However, the methods for determining the nine curves are different.

However, the R-curve method described in - requires a large number of test pieces to obtain nine curves, which is time-consuming, but the unloading Combra-Iasis method calculates cracks from the load/displacement curve of a single test piece. Since the amount of growth is indirectly measured and nine curves are obtained, the test time can be significantly shortened.

To determine "J+c(i*") using the unloading compliance method, first, a compact test piece (hereinafter abbreviated as CT test piece, see Figure 2) is set in the J1c fracture toughness test device and a "fatigue pre-crack" is performed. In this method, the CT specimen is first subjected to repeated loading as shown in Figure 5 to reduce fatigue. A pre-crack is introduced. At this time, "compliance" is determined from the load waveform acting on the CT specimen, and this compliance is calculated using the "'5axena formula" (a typical formula for calculating the crack length an from compliance). ) a, n = A, a + B a f I (K 5 - Cn)
Calculate the fatigue pre-crack length by substituting 10-...=. When this i lvI+ value reaches the set value, a static load is applied to the CT specimen, and then the operation of slightly unloading the static load is repeated several times, and a series of compliance (see Figure 3).

Then, the crack length an is calculated by similarly substituting this compliance into the formula 5axena, and this crack length a
From n, the amount of crack growth △a △a=an-a.

is calculated, and nine curves are obtained based on this crack growth amount Δa.

However, among the nine curves obtained, the crack growth amount Δa at the starting point does not become zero, and takes, for example, a negative value (see R curve ■ shown in FIG. 6).

There was a problem in that some data could not be used to determine the "ItC value."

As a result of intensive research to elucidate the cause of this situation, the present inventors have found that when fatigue pre-cracking is introduced, work hardening and work softening occur at the tip of the crack, and cracks run in random directions. Therefore, the calculated value of the fatigue pre-crack length may not match the actual length, and even though the actual fatigue pre-crack length is different from the set value, it is treated as equal and the amount of crack growth is calculated. It turned out that the cause was the calculation of Δa.

Therefore, a method was investigated in which the cracks that develop during fatigue pre-cracking are sequentially measured and the repeated loading is stopped when the crack length reaches a set value. With this method, since the crack length is actually measured, there is no possibility of a jlG condition where the bar calculation does not match the actual length.

However, actually measuring the crack length is time-consuming and
Since it is not possible to actually measure the crack length while a cyclic load is applied, it is necessary to stop the cyclic load after each measurement until the set value is reached, which makes the test time extremely long and impractical. isn't it.

[Problems to be solved by the invention] The present invention has been made in view of the above circumstances, and its purpose is to provide a J1c fracture toughness test method that can obtain accurate 8 curves in a short time. That's true.

[Means for solving problems]

In order to achieve the above object, the present invention introduces a "fatigue pre-crack" into a test piece, and then uses the "unloading compliance method" to determine "J+c(i") in the J1c fracture toughness test method. When the amount of crack growth Δa at the starting point of the obtained “8 curve” deviates from the zero value, the amount of deviation ΔX is calculated as the first term “Aa” of the formula % formula % which calculates the crack length from “compliance”. It is characterized in that the calculation formula is corrected by adding or subtracting from , and the eight curves are corrected based on the corrected calculation formula.

(Example) An example of the present invention will be explained below with reference to the drawings. Figure 1 shows an apparatus for carrying out the test method of the present invention. In the figure, reference numeral 1 is a CT test piece, and 2 is a displacement meter. , 3 is a load cell, 4 is an X-Y recorder, 5 is an actuator, and 6 is an arithmetic controller.

An opening 1a is provided at one side edge of the CT test piece 1, and a fatigue pre-crack 1b is introduced into this opening 1a. 1 and 2 show a state in which fatigue pre-crack 1b has already been introduced. Note that a three-point bending test piece may be used instead of the CT test piece 1.

Displacement meter 2 measures COD (see Figure 2).
For example, it consists of a clip gauge.

The load cell 3 detects the load acting on the CT test piece 1 by the actuator 5.

The X-Y recorder 4 inputs displacement signals and load signals from the displacement meter 2 and load cell 3 and records a load/displacement curve.

The arithmetic controller 6 controls the actuator 5 according to the flowchart shown in FIG. 7 and performs arithmetic processing.

Next, an example of the test method of the present invention will be explained using the apparatus described above.

First, a set value for the fatigue pre-crack length is set in the arithmetic controller 6, and then the actuator 5 is operated to apply a repeated load to the CT specimen 1 as shown in FIG. At this time,
The load acting on the CT test piece 1 is detected by the load cell 3, the load waveform is recorded by the X-Y recorder 4, and the compliance is determined from the load waveform. Then, the length of the fatigue pre-crack 1b is calculated by substituting this compliance into the "5axena formula" an=Aa+Ba f+ (Ks-Cn)+.==. The above operation is repeated until this calculated value becomes the set value.

When the set value is reached, a static load is applied to the test piece 1 by the actuator 5, and the operation of removing the static load by about 10% is repeated several times to obtain a series of compliances from one CT test piece 1. (See Figure 3). Then, similarly, this compliance can be expressed as 5axena's formula 1
The crack length an is calculated by substituting into the crack length an, and from this crack length an, the crack growth amount Δa Δa=an−a.

Calculate. Further, Jn is calculated from an and An (An is the area surrounded by the curve in FIG. 3).

Note that Bb is a coefficient.

Thereafter, eight curves are obtained by plotting the relationship between the crack growth amount Δa and the J integral.

The operations up to now are the same as before. Next, it is determined whether Aa at the starting point of the eight curves obtained as described above, which describes the characteristic portion of the present invention, is a zero value. If Aa at the starting point is zero (see R curve l shown in Figure 6), then the ``slowing straight line (J = 2δfs・
Aa and δfs are effective yield strengths. ) is determined, and the "JIC value" is determined from the intersection of this and the 8th curve (see Figure 4).

When Aa at the starting point is not zero (see R curve ■ shown in FIG. 6), the deviation amount ΔX from zero is calculated, and then it is determined whether this ΔX is positive or negative. When △X is negative, “
Add △X to the first term "Aa" of the "5axena formula". Also, when ΔX is positive, add the "5axet + a formula"
ΔX is subtracted from the first item 11Aa″.

After modifying the 5axena formula in this way, the 8th curve is modified based on the modified 5axena formula. That is, in FIG. 6, the R' curve ■ is slid to the right side of the figure by ΔX. Thereafter, the J1c value is determined from the intersection of the blunted straight line and the corrected 8 curves.
・In the above example, the fatigue pre-crack was introduced in the same device and the unloading compliance method was carried out, but the fatigue pre-crack was introduced in another device and this was set in the device shown in Figure 1. may also be used to implement unloading compliance laws.

〔Effect of the invention〕

As explained above, according to the present invention, when the crack growth amount Δa at the starting point of the 8 curves determined by the unloading compliance method deviates from the zero value, the crack length is calculated from the deviation amount ΔX and the compliance. The calculation formula is corrected by adding or subtracting from the first term "Aa", and the 8 curves are corrected based on this corrected calculation formula, so the fatigue pre-crack length becomes the set value. There is no need to actually measure the presence or absence of the 8 curves, and accurate 8 curves can be obtained in a short period of time.Therefore, the test time can be significantly shortened and the "JIC value" can be measured with high accuracy. becomes possible.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram 9 of an apparatus for carrying out the method of the present invention;
Fig. 2 is an enlarged perspective view of the test piece, Fig. 3 is a graph showing load, displacement, and curves, and Fig. 4 is a graph showing “J” from the 8 curves and the blunted straight line.
A graph explaining the method for determining "tc value", Fig. 5 is a graph showing repeated loads, Fig. 6 is a graph showing 8 curves,
・FIG. 7 is a flowchart showing the contents of the operation of the arithmetic controller. 1... Test piece, ■b... Fatigue preelectricity, crack, 2... Displacement meter, 3... Load cell, 4... X-Y recording needle, 5
...Actuator, 6...Arithmetic controller. Patent applicant: Saginomiya Seisakusho Co., Ltd. ζ 2 1 ((

Claims (1)

[Claims] J_1 Introducing "fatigue pre-crack" into a test piece and then determining "J_1c value" by "unloading compliance method"
c In the fracture toughness test method, when the crack growth amount Δa at the starting point of the "R curve" determined by the unloading compliance method deviates from the zero value, the crack length is calculated from the "compliance" using the deviation Δx. The formula an=Aa+Baf_1(Ks・Cn)+...In addition, a
n is the crack length n = 1, 2, 3...Cn is the compliance Aa, Ba is the coefficient Ks is added or subtracted from the first term "Aa" of the material constant to correct the calculation formula, and this corrected A J_1c fracture toughness test method characterized by modifying the R curve based on a calculation formula.