JP2003014601A - Method for evaluating progress in fatigue crack - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for performing an accurate fatigue crack progress evaluation for each individual crack by assuming a case where pull load and torsion load are operated simultaneously. SOLUTION: The method for evaluating the progress in fatigue cracks due to the machine fatigue of a rotary shaft comprises a crack angle measurement process, a mode ratio calculation process, a crack progress judgment process, and further a breakdown mode judgment process and a life evaluation process as needed, or a change point calculation process and a life evaluation process.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating fatigue crack growth, and more particularly, to a highly accurate evaluation of crack growth due to mechanical fatigue for a rotary shaft such as a pump shaft on which repeated torsion and rotary bending simultaneously act. It is about how to do it.

[0002]

2. Description of the Related Art In a main shaft or the like of a pump that pumps high-temperature water, thermal fatigue cracks may occur due to temperature fluctuations. It may progress. Generally, in order to apply a rotational force to the rotating shaft, a torsional load and a rotating bending load due to the weight of the rotating shaft and other factors act. In this case, since the rotational force due to the torque and the bending act simultaneously, it is necessary to evaluate the crack considering such conditions. On the other hand, in the conventional evaluation method, when a crack is generated on the circumference of a round bar, for example, the tensile load from the vertical direction is evaluated.

However, even though the crack growth life evaluation against rotational bending fatigue has been established, a crack growth evaluation method considering the superposition of loads including a tensile load and a torsion load has not been developed. In particular, depending on the ratio of the torsional load to the rotational bending load and the level of the torsional load, the fracture morphology may change significantly and the crack growth rate may accelerate rapidly.It is important to predict the change point of the fracture morphology. Despite this, evaluation methods that take these into consideration have not been examined.

[0004]

In view of the above problems, the present inventors have assumed a case in which a tensile load and a torsion load are simultaneously applied, and have a high degree of accuracy in fatigue cracking for each individual crack. We have conducted intensive studies to develop a method that enables evaluation of crack growth or life. As a result, the inventors
It has been found that such a problem can be solved by calculating a mode ratio, which is a ratio of a torsional load to a rotating bending load, and using the mode ratio to predict a crack growth and a change in fracture morphology. The present invention has been completed from this point of view.

[0005]

[Means for Solving the Problems] That is, the present invention is
This is an evaluation method for fatigue crack growth due to mechanical fatigue of the rolling axis.
The crack in the cracked part,
Measuring the inclination angle ψ of the crack with respect to the vertical circumferential surface,
From the angle measurement process and the angle ψ,
The mode ratio, which is the ratio of the torsional load, is calculated.
Using the calculation process and the crack growth lower limit line,
The crack propagation region in the range of mechanical parameters
Calculated and mechanical parameters for the axis of rotation where the crack is generated.
Starter ΔK _IWhether or not belongs to the crack growth area
Fatigue characterized by including a crack growth determination process
It provides a method for evaluating crack growth.

In the fatigue crack growth evaluation method of the present invention, in addition to the above steps, the mode change limit line is used to determine the presence / absence of a fracture mode change point that causes different fracture modes before and after that. Also provided is an aspect including a judgment step and a life evaluation step of evaluating the life until the rotation axis failure due to crack growth from the relationship between the dynamic parameter ΔK _I and the crack growth rate in the judged fracture mode. To do. As another step, in addition to the above steps, a change point calculation step of calculating a fracture form change point at the mode ratio that causes different fracture modes before and after using a mode change limit line, and the fracture form change Of the two failure modes before and after the point, from the relationship with the mechanical parameter ΔK _I and the crack growth rate, the life up to the rotating shaft failure due to crack growth is evaluated by the fracture mode with a faster growth rate. An embodiment including a step is also provided.

According to the present invention, when a crack due to thermal fatigue cracks or the like is found on the rotating shaft, it is predicted what kind of torsional load and tensile load will act on the crack and the mode ratio is determined. To do. Furthermore, by deriving the limit line of crack growth, it becomes possible to determine whether or not the crack mode further propagates or does not progress at the determined mode ratio. Then, when the crack is in the crack growth region where it propagates, the life is calculated by the life evaluation step.
Furthermore, in the case where there is a fracture mode displacement point, it is judged whether the crack growth is of the tensile dominant type or the shear type, and the life is predicted. According to the method of the present invention, it is possible to accurately and accurately predict crack growth as compared with the conventional evaluation method.

The mode ratio is the ratio of the torsional load to the rotational bending load (tensile load) that acts on the crack, and is determined from the inclination of the cracked portion found by inspection or the like. Specifically, the mode ratio can be calculated from the twist angle by the graph as shown in FIG. 1, and the inclination of FIG. 2 is determined by this. Further, the mechanical parameter ΔK _I used in the present invention is a mechanical parameter for a state in which a crack is flat, and in the actually targeted mixed mode, it is necessary to make a prediction by adding a torsional load thereto. . The equation representing the limit line at which crack growth begins when two loads act can be expressed as:

[0009]

[Equation 1]

In the present invention, it is necessary to experimentally determine the growth rates (da / dN) _I and (da / dN) _III in modes I and III for each material. In addition, a point (da / dN) _I = (da / d) at which the mode changes at a different mode ratio (III / I) is experimentally determined for each material.
N) It is necessary to obtain the mode change limit line from _III . Hereinafter, modes for carrying out the method according to the present invention will be described. The present invention is not limited to the embodiments below.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION First, when actually applying the evaluation method of the present invention, the confirmation of different modes from the fracture surface of the rotating shaft and the difference in the propagation speed depending on the modes will be considered. FIG. 3 shows an experimental result when a tensile load and a torsion load act simultaneously on the rotating shaft. Horizontal axis is mode I
It is understood that the action stress of (rotary bending load) and the crack growth rate are plotted on the vertical axis, and the rate of propagation increases as the repeated action stress increases. In the experiment, the ratio of torsional load to tensile load is changed. The crack growth rate tends to increase as the twist ratio increases. And
Under certain conditions, there was a point where the growth rate suddenly increased, and at a point where the slope changed (fracture morphology change point), the growth rate suddenly increased. This is because the destruction mode itself changes.

The fracture mode ratio is represented by ΔK _III / ΔK _I , and the larger the numerical value, the higher the ratio of the torsional load. Observing the fractured surface of the actual rotating shaft, there is almost no change in the propagation speed when the mode ratio is 0 (pure mode I: only tensile load), the mode ratio is 0.5, and the mode ratio is 1.0.
On the other hand, when the mode ratio is 2.5, the growth rate changes greatly. When comparing the fracture surfaces when changing the mode ratio, the mode ratio is 0 to 1.0.
Has a pleated pattern (tensile dominant type). As a failure mechanism, in the case of pure torsional load only, the principal stress direction is 45 °, and a crack is generated perpendicularly thereto. As the ratio of torsional load increases, the direction of stress approaches 45 °.

When the ratio of the torsional load is further increased and the mode ratio is 2.5, the crack does not propagate perpendicularly to the principal stress but propagates by a shearing force (shear type). Due to the difference in the way of fracture due to such different load ratios, in the actual machine, if it is not possible to predict what kind of state and how the crack will propagate, it will not be possible to predict the service life and suddenly fracture. It can happen. Therefore, the method of the present invention is to evaluate such accurate crack growth and predict the life by taking such different loads into consideration.

1) Calculation of Mode Ratio The calculation of the mode ratio is mainly performed on a crack in a portion where a crack occurs, by measuring an inclination angle ψ of the crack with respect to a circumferential surface perpendicular to the rotation axis direction. From the measurement process and the angle ψ,
A mode ratio calculating step of calculating a mode ratio, which is a ratio of a torsion load to a rotating bending load. As for the stress applied to the rotating shaft installed in the actual device, the ratio of torsion to tension is not known. Therefore, the mode ratio ΔK _III / ΔK _I is estimated from the curve shown in FIG. 1 by measuring the inclination angle ψ of the crack with respect to the circumferential surface perpendicular to the rotation axis direction, and the torsional load ratio is determined. Fig. 1 shows the measured values of the fracture angle measured by actually measuring the fracture surface after forming the fracture surface by dividing the ratio of torsion and tensile load into several stages.
The point) is the fitting of the prediction line (solid line) when the principal stress direction is calculated and cracks occur in the direction perpendicular to the stress. The approximate mixed mode ratio can be calculated by measuring the twist angle ψ of the facet, which is the vertical axis. For example, in an actual device, if a crack is found in inspection or the like, the mode ratio is estimated from the crack angle using FIG. If cracks appear on the surface but have not yet been broken, the surface is shaved and the tilt angle with respect to the axis is measured by a sump or the like.

2) Judgment of Crack Growth In the judgment of crack growth, the crack growth lower limit line is used to determine the crack growth region at the above mode ratio by the mechanical parameter Δ.
It is calculated as the range of K _Ith , and it is determined whether or not the mechanical parameter ΔK _I for the rotation axis in which the crack has occurred belongs to the crack growth region. Below a certain load on the rotating shaft, there is a limit value at which the crack does not propagate. This crack growth limit value also differs between the case of tensile load alone and the case of overlapping tensile load and torsion load. In FIG. 2, the plotted points are the measured values of the experiment. here,
The mode I stress intensity factor is a mechanical parameter for tensile loading, and the mode III stress intensity factor is a stress parameter for torsional loading. The intercepts on the ordinate and the abscissa indicate the starting points of crack growth under each load alone. In FIG. 2, ● indicates data in which cracks propagate under the conditions, and ○ indicates data in which cracks do not propagate under the conditions. In the case of pure tensile load only, ΔK _I / ΔK _Ith = 1; in the case of pure torsion load only, ΔK _III / Δ
K _Ith was about 1.35. When two loads in between act, crack growth begins when the limit line in (a) is exceeded. The equation for the crack growth lower limit line can be expressed as follows.

[0016]

[Equation 2] In the equation (1), Y is 0.74 under a pure torsion load (pure mode III), and ν is a Poisson's ratio.

The limit curve according to this equation is in good agreement with the experimental result considering the mode ratio in the actual machine, and the limit in each mode of crack growth can be accurately judged. In the mixed mode of the torsional load and the rotational bending load, a crack is obliquely generated on the circumference. Therefore, the mechanical parameter ΔK _Ith for the oblique crack is calculated from the above formula (1). In the calculation of the normal mechanical parameter ΔK value, it is assumed that a straight crack is formed along the circumference of the rotation axis. However, in the actual crack of the rotating shaft, a crack having an inclination angle ψ is generated from the circumference. In particular, when the tensile load and the torsional load, which are targets of analysis in the present invention, act simultaneously, a crack having an inclination from the circumference occurs due to the ratio of the loads. Therefore, in the present invention, assuming that a crack exists on the circumference and a crack inclined from the tip thereof is generated, the mechanical parameter ΔK value for the crack is expressed by the above formula (1). It is expressed as follows.

Therefore, referring to FIG. 2, the crack propagation lower limit value for mode I (rotary bending load) is determined by using the limit line for crack growth under mixed load with respect to the mode ratio assumed in the actual machine. Calculate ΔK _I th. Assumed in actual machine Δ
For K _I, the case of ΔK _I ≧ ΔK _I th, the crack progress, Δ
When K _I <ΔK _I th, it is determined that the crack does not propagate. If the condition (mode ratio) of the actual machine is known in this way, it can be assumed in what case the crack will start to propagate.

3) Evaluation of Life by Fracture Form (Prediction) When it is judged that a crack grows by the above-mentioned judgment of crack growth, the life of each crack is evaluated by considering the mode ratio. . One of the life evaluations based on the fracture mode is to determine the presence or absence of a fracture mode change point that causes different fracture modes before and after using the mode change limit line, and a destruction mode determination process and a determined fracture mode. Then, there is an evaluation method comprising a life evaluation step of evaluating the life up to fracture of the rotating shaft due to crack growth from the relationship between the mechanical parameter ΔK _I and the crack growth rate. In addition, using a mode change limit line, a change point calculation step of calculating a change point of a change in the form of a fracture that is a different change mode before and after the change point, and two fracture modes before and after the change point of the change in the change point, From the relationship between the mechanical parameter ΔK _I and the crack growth rate, the life up to fracture of the rotating shaft due to crack growth is evaluated by the fracture mode with a higher propagation rate.
There is also an evaluation method consisting of a life evaluation step. Here, an evaluation method of reaching a life evaluation step through a change point calculation step will be described for a mode having a fracture form change point, which is difficult to perform an accurate life evaluation by a normal method.

As shown in FIG. 3, fatigue fatigue under mixed loads
In crack growth, the fracture mode depends on the ΔK level and the mode ratio.
However, the speed of development is
Is different. Predict the point at which this rapid change in growth rate will occur
If it is not possible, it is possible to predict the life due to crack growth.
I can't come. Therefore, first, the fracture form change point ΔK in FIG._I ^tPre
Measure. This breaking mode change point ΔK_I ^tAs shown in Figure 4
It can be predicted from the mode ratio that is assumed in a real machine. here,
ΔK on the horizontal axis_IIs the mechanical parameter for tensile loading,
ΔK on the vertical axis_IIIIs the mechanical parameter for the torsional load
Is. In FIG. 4, in the experiment at each mode ratio,
■ is a shear type, and □ is a tension dominant type. That
The line (b) is the mode change limit line [(da / dN)_I=
(Da / dN)_{II I}] And below the line (b)
Mode I dependent [(da / dN) in the range _I> (Da /
dN)_III], And in the upper range is mode III dependent
[(Da / dN)_I<(Da / dN)_III]. This
From Fig. 4 of Fig. 4, depending on the value of each mode ratio and the magnitude of stress,
Thus, it can be seen that the point of change in fracture mode is different. That is,
If the card ratio is high, mode change will occur even with small stress
On the other hand, if the mode ratio is lower than a certain level,
Even if a large stress is applied, the mode change does not occur. An example
For example, in the case of the mode ratio of the actual machine in Fig. 4, intersect with the line (b).
Delta K_IKnow that there is a breaking point
It

[0021] In the case of Figure 3, progress curves in the case of ^{_{ΔK I ≧ ΔK I t (d}} ), in the case of ΔK _I <ΔK _I ^t with the progress curve (c), the crack growth life calculate. In Figure 3,
A crack propagates according to a curve having a faster propagation speed (vertical axis), so that the propagation speed increases along the line (d) having a large slope after the fracture shape change point. Here, the line shown in FIG. 3 represents the crack growth rate when the mode ratio is 2.5, and has a fracture morphology change point. This breakage form change point can be obtained from ΔK _I at the position where the straight line with the mode ratio of 2.5 intersects the limit line (b) of the mode change in FIG. In addition, in FIG. 3, since the slopes of the straight lines (c) and (d) representing the crack growth rate with respect to ΔK _I differ depending on the type of material, they are obtained by experiments. Further, in FIG. 4, regarding the limit line (b) of the mode change,
It is determined by an experiment in which the mode ratio is changed for each material.

[0022]

EFFECTS OF THE INVENTION According to the evaluation method of the present invention, it is possible to accurately evaluate the possibility of crack growth occurring on a rotating shaft and, when a crack grows, accurately predict the life due to the growth. You can

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a crack angle ψ and a mode ratio.

FIG. 2 is a graph showing a crack growth region with respect to mechanical parameters.

FIG. 3 is a graph plotting the results of crack growth rate against mechanical parameters in Mode I.

FIG. 4 is a graph showing a limit line of mode change with respect to mechanical parameters.

FIG. 5 is a flow chart showing an example of a process for determining the dominant type of crack progress by two mechanical parameters.

[Explanation of symbols]

1 rotation axis 2 cracks

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Claims (3)

[Claims] 1. A method of evaluating fatigue crack growth due to mechanical fatigue of a rotating shaft, comprising measuring an inclination angle ψ of a crack in a cracked portion with respect to a circumferential surface perpendicular to the rotating shaft direction. A crack angle measurement step, and a mode ratio calculation step of calculating a mode ratio, which is a ratio of a torsional load to a rotational bending load, from the angle ψ. The crack propagation region is calculated as the range of the mechanical parameter, and the mechanical parameter ΔK _I about the rotation axis where the crack is generated is calculated.
The method for evaluating fatigue crack growth, comprising: a crack growth determination step of determining whether or not the crack belongs to the crack growth region. 2. In addition to the above-mentioned step, a destruction mode determination step of determining whether or not there is a fracture pattern change point having a different fracture mode before and after using a mode change limit line, and And the mechanical parameter ΔK _I
The fatigue crack growth evaluation method according to claim 1, further comprising: a life evaluation step of evaluating a life up to fracture of a rotating shaft due to crack growth, based on a relationship with a crack growth rate. 3. In addition to the above steps, Using the mode change limit line, different destruction modes are used before and after that.
Calculate the fracture mode change point that becomes the mode at the above mode ratio
Change point calculation process, Of the two destruction modes before and after the point of change in the destruction pattern, the force
Parameter ΔK _IFrom the relationship with the crack growth rate,
The faster propagation rate of fracture mode allows
Life evaluation process to evaluate the life until the rotating shaft is broken,
Of fatigue crack growth according to claim 1, characterized in that
Evaluation methods.