JPH0324979B2 - - Google Patents

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 ₁ c Fracture toughness test method.

[Technical background of the invention and its problems]

When designing machines, structures, etc., the stress generated during operation is calculated based on the strength characteristic data of the materials used (for example, yield strength, tensile strength, fatigue limit, etc., all of which are expressed as stress). I try not to exceed.

However, experience has shown that when fairly large defects or cracks are already present in a mechanical or structural component, its strength decreases as the size of the defect or crack increases. 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 steel A has a higher tensile strength than steel B in a smooth specimen, if a crack of the same size exists in each material, the fracture strength of steel B will be higher than steel A. It may happen.

Therefore, defects such as cracks occur due to the "fracture toughness value", which is the resistance value of the material when fracture progresses rapidly without an increase in external force from a crack etc., that is, when unstable fracture occurs. Test methods have been proposed to determine the fracture strength of materials.

As the J ₁ c fracture toughness test method for determining the fracture toughness value using the “J ₁ c value”, methods such as the “R curve method” and the “unloading compliance method” are known.

Both the R curve method and the unloading compliance method determine the "R curve", and from this R curve and the blunted straight line,
Although this is a method for determining the J ₁ c value, the method for determining the R curve is different.

The above-mentioned R curve method requires a large number of test pieces to obtain the R curve, which is time-consuming, but the unloading compliance method indirectly measures the amount of crack growth from the load/displacement curve of a single test piece. Since the R curve is determined using the same method, the test time can be significantly shortened.

According to this unloading compliance method, after a load is applied to a CT specimen with an ideal crack to a predetermined displacement level, the load is slightly unloaded, and the slope (compliance) of the load-displacement curve at that time is calculated. seek. By repeating this operation multiple times, a series of compliances are obtained from one CT specimen (see Figure 8), and the amount of crack growth is calculated from this compliance.

However, when unloading a test piece after applying a load to a predetermined displacement level, the effect of stress relaxation occurs, so unloading is done in conjunction with this stress relaxation, but the stress relaxation takes a very long time. In addition to the long unloading time, the effects of stress relaxation remain (9th
(see figure), there was a problem that it was not possible to obtain accurate compliance (J ₁ c value).

[Problem that the invention seeks to solve]

The present invention has been made in view of the above circumstances, and its purpose is to provide a J ₁ c fracture toughness test method that can accurately determine compliance in a short time.

[Means for solving problems]

The present invention achieves the above object.
The _J1c fracture toughness test method involves applying a load to a test piece with an ideal crack up to a predetermined displacement level, then slightly unloading the load acting on the test piece, and measuring the load/displacement curve at that time. In the J ₁ c fracture toughness test method, which calculates the J ₁ c value from the slope, the unloading is performed by increasing the load acting on the test piece to such an extent that stress relaxation while maintaining the predetermined displacement level is almost negligible. It is characterized by starting after decreasing.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of an apparatus for carrying out the test method of the present invention. Code 1 in the figure is a CT test piece.
2 is a displacement meter, 3 is a load cell, 4 is an X-Y recorder, 5 is an actuator, and 6 is a controller.

An opening 1a is provided at one side edge of the CT test piece 1, and an ideal crack 1b is inserted into the opening 1a. Note that a three-point bending test piece may be used instead of the CT test piece 1.

The displacement meter 2 measures the opening displacement (COD, see FIG. 6) and is composed of, for example, a clip gauge.

The load cell 3 is controlled by the actuator 5.
Detect the load acting on the test piece 1.

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

The control unit 6 controls the actuator 5 according to the flowchart shown in FIG.

Next, the test method of this example will be explained using the above apparatus.

The actuator 5 is operated to apply a load to the CT specimen 1. At this time, the load cell 3 detects the load, the displacement meter 2 detects the COD, and the X-Y recorder 4 records a load/displacement curve as shown in FIG. When the COD reaches COD ₁ as the load increases, the actuator 5 is controlled by a servo valve to prevent COD ₁ from changing. Then, stress relaxation begins, in which the internal stress caused by the deformation decreases. This stress relaxation is detected by the load cell 3, and as shown in FIG. 2, the load (internal stress) decreases rapidly at first, and gradually decreases as time passes. The rate of decrease of this load (differential value dL/dt) is calculated by the controller 6, and when the rate of decrease is less than a predetermined value, that is, stress relaxation no longer affects the compliance measurement error (ignoring the influence of stress relaxation). actuator 5 by controller 6).
to remove the load all at once. The slope θ of the load/displacement curve obtained by this unloading (see Figure 3)
compliance is required.

Then, operate the actuator 5 again.
By increasing the load acting on CT specimen 1, the COD
When COD ₂ is reached, actuator 5 is controlled by a servo valve to prevent COD ₂ from changing.
Compliance is sought in the same way as in the case above.

By repeating this operation, n=1, 2, 3...
...Find the compliance Cn at each point of n, and calculate these compliances Cn using the "Saxina's formula" an=Aa+Ba・f ₁ (Ks・Cn)+... In addition, Aa, Ba... are coefficients and Ks is substituted into the material constant. The amount of crack growth Δa is calculated based on the computed value an of the crack length.

△a=an−a ₀ (a ₀ is the ideal crack from the point of load application
distance to the tip of 1b) =Aa+Baf ₁ (Ks・Cn)+…… −(Aa+Baf ₁ (Ks・C ₀ )+……) =Baf ₁ Ks(Cn−C ₀ )+…… Also, an, An( (Area surrounded by the curve in Figure 4)
Calculate Jn from.

Jn=An/Bbf(an/W) Note that Bb is a coefficient.

After this, the R curve is obtained by plotting the relationship between the crack growth amount Δa and the J integral, and the blunting straight line is obtained, and the "J ₁ c value" is obtained from the intersection of these R curves and the blunting straight line ( (See Figure 7).

Note that the range in which the blunted straight line is determined is the elastic deformation region, and it may be determined using a smooth test piece with no cracks. In this case, the blunting straight line can be obtained more clearly.

〔Effect of the invention〕

As explained above, according to the present invention, unloading of a load applied to a test piece with an ideal crack up to a predetermined displacement level can be almost ignored by stress relaxation while the predetermined displacement level is maintained. Since the test is started after the load acting on the test piece has decreased to a certain extent, the test time can be shortened compared to the case where the load is unloaded when the stress is relaxed. Accurate J ₁ c values can be obtained without the influence of stress relaxation.

[Brief explanation of drawings]

The drawings show an embodiment of the present invention; FIG. 1 is a block diagram showing an example of an apparatus for carrying out the test method of the present invention, FIG. 2 is a graph explaining stress relaxation, and FIG. is an explanatory diagram explaining the unloading operation,
Fig. 4 is a graph showing the load/displacement curve, Fig. 5 is a flowchart showing the operation of the actuator, Fig. 6 is an enlarged perspective view of the test piece, and Fig. 7 is the J ₁ c value calculated from the R curve and the blunted straight line. FIG. 8 is a graph for explaining the method of obtaining the load, FIG. 8 is a graph for explaining the unloading compliance method, and FIG. 9 is an explanatory diagram of the load/displacement curve including the influence of stress relaxation. DESCRIPTION OF SYMBOLS 1... Test piece, 1b... Crack, 2... Displacement meter, 3... Load cell, 5... Actuator, 6... Control part.

Claims (1)

[Claims] 1. After applying a load to a test piece with ideal cracks up to a predetermined displacement level, the load acting on the test piece is slightly unloaded, and the slope of the load/displacement curve at that time is calculated. In the _J1c fracture toughness test method for determining the _J1c value, the load acting on the test piece is reduced to such an extent that stress relaxation while maintaining the predetermined displacement level is almost negligible. J ₁ C fracture toughness test method, characterized in that it starts after.