JPH0612317B2 - Effective stress intensity factor measuring device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an effective stress intensity factor measuring device for measuring opening / closing behavior of a crack generated in a structure.

[Prior Art] When a crack generated in a structure grows under repeated load, it is important to accurately predict the crack growth rate for safety and maintenance. It is necessary to know the mechanical parameters that govern velocity accurately.

Conventionally, the stress intensity factor range ΔK by fracture mechanics has been used as a mechanical parameter that governs the growth rate of fatigue cracks, and this is used for cracks with a length 2a and stress Δδ acting as shown in FIG. (However, f (a) is given by a correction coefficient determined by the member shape and the like).

[Problems to be Solved by the Invention] Conventional treatment is a method in which the full range ΔK of the stress intensity factor K acting on a crack is considered to be effective for crack growth. Due to deformation, residual stress, and the presence of oxides in the crack, it has been clarified that "the crack tip is closed" in the part where the cyclic stress range is present and it is not involved in crack growth (called crack opening / closing behavior). ). The part of ΔK that corresponds to the range in which the crack tip is open is also called the “effective stress intensity factor” ΔK _eff, and it has been clarified that this is the mechanical parameter that governs crack growth. In general, a crack growth prediction method based on this has been established.

That is, a fatigue crack is a cycle in which stress loading and unloading are repeated as one cycle, and the crack grows every stress cycle, and the shape of the crack tip during one stress cycle is as shown in FIG. Changes to. The stress loading process of FIGS. 7 (a) to 7 (d) is a process in which the crack tip opens as the stress increases, but until the stress reaches a certain level (called the crack opening point), Despite the stress, the crack tip does not open as shown in FIGS. 7 (a) and 7 (b). This state is called "the crack tip is closed". As shown in FIGS. 7 (c) and 7 (d), the crack tip becomes open only when the stress exceeds a certain level. This state is called "the crack tip is open". It is known that the stress fluctuation range under the condition that the crack tip is open is effective for fatigue crack propagation.

The “portion of ΔK corresponding to the range in which the crack tip is open” has the following meaning. In the conventional way of thinking, ignoring the behavior of opening of the crack tip described above, ΔK = Δσ√πaf obtained by the equation (1) using the total stress range Δδ acting.
Although it was thought that (a) governs fatigue crack propagation, it can be obtained by using the "stress range corresponding to the portion where the crack tip is open" Δσ _eff , out of the total stress range Δσ that has recently been applied.

Has really become known to dominate fatigue crack propagation. This ΔK _eff is referred to as “a portion of ΔK corresponding to the range in which the crack tip is open”.

However, it is difficult to apply the research results to the actual machine because there is no practical equipment for measuring ΔK _eff for the actual machine structure.

The present invention has been proposed in view of such circumstances, and it is possible to measure the effective stress intensity factor ΔK _eff that acts on a crack in a structure and to improve the life prediction accuracy of the device. An object is to provide a coefficient measuring device.

[Means for Solving Problems] In the effective stress intensity factor measuring device according to the present invention, a first strain gauge is attached to a crack tip generated in a structure, and a second strain gauge is provided at a position sufficiently distant from the crack. The strain gauge of No. 1 is attached, and the strain Δ (ε ₂ + ε ₄ ) _eff corresponding to the range where the crack tip is open is used by using the two signal sources of these first and second strain gauges and the subtraction circuit. By calculating, the equation ΔK _eff = C ₁ × Δ (ε ₂ + ε ₄ ) _eff, where C ₁ is a constant, and the effective stress intensity factor ΔK _eff is obtained.

[Operation] According to the present invention, the bending point shown in FIG. 2 is obtained from the output signals of the first and second strain gauges, and the subtraction circuit is used from this bending point as shown in FIG. calculated hysteresis with distinct bends in, this strain variation _{Δ (ε 2 + ε 4)} eff ΔK the effective stress intensity factor [delta] K _eff from _{eff = C 1 × Δ (ε} 2 + ε 4) eff ...... (3) Desired. However, C ₁ is a constant.

[Embodiment] FIG. 1 is a block diagram of an embodiment of the present invention, in which 1 is a first strain gauge attached to the tip of the crack, and 7 is a location sufficiently separated from the crack. The second strain gauge, 8 is a measuring device, 9a and 9b are dynamic strain amplifiers, 11
Is a subtraction circuit, 12 is a variable resistor, 13 is an operational amplifier, 1
Reference numeral 4 is a recorder, 15 is the output of the first strain gauge, and 16 is the output of the subtraction circuit.

FIG. 4 is a detailed view of the first strain gauge 1 in FIG. 1 (see Japanese Patent Application No. 61-155200),
4 and 5 are gauge grids, and 6 is a hole.

When the strain gauge shown in FIG. 4 is attached and measured by aligning the center of the gauge with the crack tip as shown in FIG. 5, for example, the relationship between the measured strain and the stress intensity factor ΔK is the strain value of the gauge grids 2 and 4. Assuming Δε ₂ and Δε ₄ , respectively, ΔK = C ₁ × Δ (Δε ₂ + Δε ₄ ) ... (4) C ₁ is a constant related to the gauge grid size r ₁ and is theoretically and experimentally obtained.

The stress intensity factor ΔK is related to the strain of the gauge grids 2 and 4 in FIG. 5, but is not related to the strain of the gauge grids 3 and 5 for the following reason.

Strictly speaking, the stress intensity factor in the two-dimensional crack problem which is the subject of the present invention is ΔK _{1 of the} opening type shown in FIG. 8 (a) according to the “stress state acting on the crack”. , In-plane shear type ΔK ₁₁ shown in FIG.

However, the strain gauges shown in FIGS. 4 and 5 were invented in Japanese Patent Application No. 61-155200 for the purpose of separately measuring ΔK ₁ and ΔK ₁₁ , and the stress component in the direction perpendicular to the crack is dominant. Gauge grids 2 and 4 arranged to measure the typical aperture type ΔK ₁ and gauge grids 3 and 5 arranged to measure the in-plane shear type ΔK _{11 in} which the stress component in the crack direction is dominant. It is configured.

In the present invention, the propagation speed of fatigue cracks is mainly determined by ΔK ₁ (ΔK ₁₁ is mainly related to the propagation direction of cracks), so that the gauge grids 2 and 4 for measuring ΔK ₁ are determined. This is because only the gauge grids 3 and 5 that are not related to the ΔK ₁ measurement are used.

It should be noted that simply speaking ΔK generally refers to ΔK ₁ . Therefore, the description of the present invention will be followed accordingly.

The expressions (1) and (4) representing the stress intensity factor ΔK are different for the following reason. That is, the expression of ΔK according to the expression (1) is an analytical expression and can be used only when Δσ and f (a) are known. On the other hand, the equation of ΔK according to the equation (4) is an empirical expression for obtaining ΔK from strain measurement.

Next, the operation of the embodiment of the present invention will be described.

As shown in Fig. 1, the crack tip in the structure is
The second strain gauge 7 is attached to the strain gauge 1 and the second strain gauge 7 is sufficiently separated from the crack, and the second strain gauge 7 is guided to the measuring device 8. When the gauge output is amplified by the dynamic strain amplifiers 9a and 9b, a waveform with a slight nonlinearity is drawn due to the crack opening / closing behavior as shown in Fig. 2, and the bending point 10 becomes a point where the crack starts to open. Correspond. In order to make this bending point 10 clear, this signal is led to the subtraction circuit 11 of FIG. 1 and the variable resistor 12 is set appropriately, so that hysteresis having a clear bending as shown in FIG. 3 is obtained. The horizontal straight line portion in FIG. 3 is the strain variation Δ (ε ₂ + ε ₄ ) _eff ″ corresponding to the range in which the crack is open, and the effective stress intensity factor ΔK _eff is ΔK _eff = C ₁ × Δ (Ε ₂ + ε ₄ ) _eff (3)

The difference between the outputs of the first and second strain gauges becomes the effective stress intensity factor for the following reason.

The first strain gauge is attached near the crack tip to measure the nonlinear strain output with respect to the external load, which is affected by the crack opening and closing at the crack tip, while the second strain gauge is used. The strain gauge is for sticking at a position far from the crack tip where the effect of the crack opening and closing at the crack tip is attenuated, and for measuring the linear strain output with respect to an external load.

The bending point 10 at which the crack tip begins to open can be determined from the non-linearity of the Lissajous waveforms of both. If the bending point 10 can be determined in this way, the strain range Δ (ε ₂ + ε ₄ ) _eff corresponding to the “range in which the crack tip is open” is determined.
And the effective stress intensity factor ΔK _eff can be calculated by the equation (3).

The output of the subtraction circuit 11 shown in FIG. 1 may be measured by a recorder 14 such as an oscilloscope, or may be digitally processed by a computer.

[Effects of the Invention] Since the present invention is configured as described above, the following effects are achieved.

According to the present invention, the strain variation Δ (ε) corresponding to the horizontal straight line portion of FIG.
By measuring ₂ + ε ₄ ) _eff , the effective stress intensity factor ΔK _eff acting on the crack in the structure can be measured from the equation (3). As a result, the results of the crack opening / closing behavior research, which has been researched in the laboratory up to now, can be applied to the actual machine, and thus excellent effects such as the improvement of the life prediction accuracy of the equipment can be achieved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of the present invention, FIG. 2 is a diagram showing a relation of outputs between strain gauges in FIG. 1, and FIG. 3 is a diagram showing outputs of strain gauges in FIG. FIG. 4 is a detailed view of the strain gauge shown in FIG. 1, FIG. 5 is a diagram showing an example of using the strain gauge of FIG. 4, and FIG. 6 is a crack model. FIG. 7, FIG. 7 is a diagram showing a relationship between a stress state and a crack, and FIG. 8 is an explanatory diagram of stress intensity factors ΔK ₁ and ΔK _{11 in} a two-dimensional crack problem. 1 ... 1st strain gauge, 7 ... 2nd strain gauge, 8 ... measuring device, 11 ... subtraction circuit, 14 ... recorder.

Claims (1)

[Claims] 1. A first strain gauge attached to a tip portion of a crack, a second strain gauge attached to a position separated from the crack, and an opening / closing behavior of the crack is measured. To obtain the strain Δ (ε ₂ + ε ₄ ) _eff corresponding to the range in which the crack tip is open. According to the equation ΔK _eff = C ₁ × Δ (ε ₂ + ε ₄ ) _eff, where C ₁ is a constant, the effective stress intensity factor ΔK _eff is obtained from the constant stress intensity factor measuring device.