JPS63122928A - Apparatus for measuring effective stress expansion coefficient - Google Patents

Abstract

PURPOSE:To enhance the life estimation accuracy of machinery, by adhering a first strain gauge to the leading end of the crack generated in a structure and further adhering a second strain gauge at a position sufficiently separated from the crack. CONSTITUTION:A first strain gauge 1 is adhered to the leading end of the crack in a structure and a second strain gauge 7 is adhered to the structure at a position sufficiently separated from the crack and both gauges are guided to a measuring apparatus 8. When the outputs of the gauges are amplified by dynamic strain amplifiers 9a, 9b, a waveform having slight non-linearity is drawn because of the opening and closing behavior of a crack and a bent point corresponds to a point where the crack begins to open. When the output signals of both amplifiers are guided to a subtraction circuit 11 in order to identify said bent point and a variable resistor 12 is properly set, a hysteresis having clear bending is obtained. Effective stress expansion coefficient is calculated from this strain change portion. Further, the output of the circuit 11 may be measured by a recorder 14 such as an oscilloscope and digital processing may be performed by a computer.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an effective stress intensity factor measuring device for measuring the opening and closing behavior of cracks generated in structures.

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

Conventionally, the stress intensity factor range ΔK according to fracture mechanics has been used as the mechanical parameter governing the growth rate of fatigue cracks, and as shown in Figure 6, this is Δ to one Δσf dτf (a) (f
(a) is given by a correction coefficient determined by the shape of the member, etc.

[Problems to be solved by the invention] The conventional approach is to consider that the entire range ΔK of the stress intensity factor acting on a crack is effective for crack growth, but recent research has focused on the plasticity of the crack tip. Transformation.

It has been revealed that due to residual stress and the presence of oxides in the crack, the crack tip closes in a certain part of the cyclic stress range and does not participate in crack growth (referred to as crack opening/closing behavior). The part of Δ that corresponds to the open range of the crack tip is called the effective stress intensity factor ΔKof'f', and it has been clarified that this is the mechanical parameter that governs crack growth. A crack growth measurement method based on this has been established. However, for the actual structure, ΔKo
The current situation is that it is difficult to apply research results to actual equipment because there is no practical equipment to actually measure ff.

The present invention was proposed in view of these circumstances, and it is possible to measure the effective stress intensity factor ΔKefr that acts on cracks in structures, thereby improving the accuracy of predicting the life of equipment. Means] The effective stress intensity factor measuring device according to the present invention has a first strain gauge affixed to the tip of a crack that has occurred in a structure, a second strain cage affixed at a position sufficiently distant from the crack, and The present invention is characterized in that it is configured to obtain the effective stress intensity factor θrr using two signal sources, the first and second strain gauges, and a subtraction circuit.

[Operation] According to the present invention, the bending point shown in FIG. 2 is determined from each output signal of the first and second strain gauges, and from this bending point, using a subtraction circuit, the bending point is calculated as shown in FIG. 3. Find the hysteresis that has a clear bend, and calculate the effective stress intensity factor ΔKef'f' from this strain change Δ(ε2+ε4) off by ΔKoff'-01×Δ(ε2+
εa ) off. However, C5 is a constant.

[Example] Fig. 1 is a block diagram of an example of the present invention, where 1 is a first strain gauge attached to the tip of a crack, and 7 is attached at a position sufficiently distant from the crack. 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, 14
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.
4.5 indicates a gauge grid, and 6 indicates a hole.

When the strain gauge shown in Fig. 4 is attached and measured with the center of the gauge aligned with the crack tip as shown in Fig. 5, the relationship between the measured strain and the stress intensity factor Δ is the strain value of gauge grids 2 and 4. are respectively Δε2. Assuming Δε4,
Δ is given by −mc1 (Δε2+Δε4). Note that C1 is a constant related to the gauge grid dimension r1 and is theoretical.

Required experimentally.

The operation of the above embodiment of the present invention will be explained.

As shown in Figure 1, the tip of the crack in the structure is shown in Figure 4.
A second strain gauge 7 is attached at a position sufficiently away from the crack, and then guided to a measuring device 8. When the gauge output is amplified by the dynamic strain amplifiers 9a and 9b, a waveform with slight nonlinearity due to crack opening/closing behavior is drawn as shown in Fig. 2, and the bending point 10 is the point where the crack begins to open. handle. In order to clarify this bending point 10, this signal is led to the subtraction circuit 11 shown in FIG. 1, and the variable resistor 12 is appropriately set, thereby obtaining hysteresis having a clear bend as shown in FIG. In Fig. 3, the horizontal straight line is the strain change Δ(ε2+ε4)aN' corresponding to the range where the crack has opened,
The effective stress intensity factor ΔKef'f is ΔKef'f=CIXΔ('z +e*) of'r
is required.

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

[Effects of the Invention] According to the present invention, it is possible to actually measure the effective stress intensity factor ΔKef'f acting on a crack in a structure.

As a result, the results of research on crack opening and closing behavior, which have been studied in the laboratory up to now, can be applied to actual equipment, resulting in excellent effects such as improving the accuracy of equipment life prediction.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention, FIG. 2 is a diagram showing the output relationship between the strain gauges in FIG. 1, and FIG. 3 is the diagram shown in FIG. 1. Figure 4 is a detailed diagram of the strain gauge in Figure 1, Figure 5 is a diagram showing an example of how the strain gauge in Figure 4 is used, and Figure 6 is a diagram showing the relationship between the output of the strain gauge and the output of the subtraction circuit. The figure shows a crack model. DESCRIPTION OF SYMBOLS 1... First strain gauge, 7... Second strain gauge, 8... Measuring device, 11... Subtraction circuit, 14...
...Recorder. Takehiko Suzue, sub-agent for applicant, patent attorney Figure 1 h4 Output of λ cage 1 4 (ε2+ε4) Figure 2 Figure 3 Figure 5

Claims (1)

[Claims] A first strain gauge affixed to the tip of the crack, a second strain gauge affixed at a position spaced apart from the crack, and a second strain gauge affixed to the tip of the crack, and a second strain gauge affixed to the tip of the crack. An effective stress intensity factor measuring device comprising: a subtraction circuit inputting each output signal of the first and second strain gauges.