JP2007315810A - Repeated stress sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fatigue sensor which is attached to a member for constituting a structure or the like receiving repeated load enabling estimation of the fatigue damage degree of the member, especially a repeated stress sensor with high general-purpose properties, for detecting whether the repeated stress exceeds the fatigue limit of an evaluation target member by adapting a single sensor, regardless of the evaluation target member. <P>SOLUTION: A plurality of short strip-like metal pieces 11, having mutually different stress concentration degrees are arranged in parallelly on a substrate 12 in the order of the magnitude of stress concentration degree, on the basis of a stress concentration shape and both end parts 14 of the metal pieces are mutually fixed, to have the both end parts 14 that are fixed to be further fixed to the substrate 12. Since the metal pieces 11 are broken in the order of higher stress concentration degree, when the metal pieces 11 are bonded to the surface of the target member to be observed for a definite period, repeated stress and the lifetime of the target member can be estimated, on the basis of the breaking degree of the metal pieces 11. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an affixing type repetitive stress sensor that easily detects repetitive stresses generated under actual conditions at various sites where there is a risk of fatigue damage in a structure subjected to repetitive loads.

  By accurately estimating the current strength and remaining life of bridges and other structures, machinery and equipment, vehicles, aircraft, ships, etc. that are actually in service, they can be rebuilt or greatly increased with sufficient remaining life. This makes it possible to eliminate wasteful repair work and to create a maintenance plan accurately. However, in order to accurately estimate the strength and remaining life of structures, vehicles, etc., it is necessary to grasp the state of repetitive stress generated in members that are configured in an actual environment.

As a method conventionally used for estimating a stress state in a material, there is a method in which a strain gauge is attached to a region to be measured and an actual stress generated in the region is measured.
In this method, since a precise measuring device such as a transducer must be used, it is difficult to measure a large number of parts at a time, so it is difficult to grasp the overall stress state of a large structure or the like.

  Patent Document 1 discloses a method for predicting fatigue damage of a structure from the state of fatigue damage caused on the sacrificial test piece by attaching a sacrificial test piece to an actual structure. A sacrificial specimen used in the disclosed method is a thin plate made of the same material as that of a structure for which fatigue damage is to be predicted, and has a length of 70 mm, a width of 20 mm, and a thickness of about 0.25 mm provided with an artificial crack at the center in the longitudinal direction A test piece having a shape and sandwiched between two resin thin plates.

When the SN diagram of the structural member and the sacrificial test piece is obtained in advance, the number of load repetitions when the sacrificial test piece installed on the member is damaged is applied to the SN diagram at that time Or a representative stress amplitude represented by one stress amplitude value is substituted into the SN diagram of the structural member, so that in a hot spot portion such as a welded portion end portion. Lifetime can be estimated.
However, the sacrificial test piece described in Patent Document 1 is an alternative measurement sensor that has a one-to-one correspondence with the evaluation object, and the test result must wait until a fracture or a crack occurs. become.

  Patent Document 2 discloses that a 12 mm long, 5 mm thick, 0.2 mm thick crack extending portion having a width of 2 mm and a thickness of 0.02 mm on a metal foil substrate having a length of 13 mm, a width of 6 mm, and a thickness of 0.05 mm. An extremely small and thin crack type fatigue sensor is disclosed in which a 1 mm fracture piece is formed. In the illustrated embodiment, the crack progressing portion is formed with a slit whose tip is sharpened from the side end, and even if a slight distortion occurs in the member to be measured, a crack is generated immediately from the slit tip and progresses. Such highly sensitive sensors are described.

Since the fatigue sensor disclosed in Patent Document 2 is small and has high sensitivity, it is attached to the very vicinity of the target part, and the fatigue damage degree of the fatigue sensor is measured by the repeated stress at the attached part to determine the fatigue damage degree of the target part. It can be estimated or the actual life can be estimated.
In particular, when estimating the degree of fatigue damage for a stress spot with a large stress concentration rate such as a hot spot in a welded part and having almost no crack initiation period, cracks are generated using a sharp tip formed at the back of the slit. By eliminating the period, it functions as a leading indicator of a hot spot, and a sufficiently reliable estimated value can be obtained.

  However, various members such as structures and transport machines are used in various forms such as machining, extrusion molding, and casting, as well as welding. Let's estimate the fatigue damage and life of these members. Then, since the stress concentration rate differs depending on the measurement target member, a sufficiently correct result cannot be obtained even if a fatigue sensor suitable for measurement of a welded part is used as it is.

  Therefore, the applicant of the present application separately as a fatigue sensor that can perform fatigue evaluation not only on the welded part but also on variously shaped members, machined surfaces, extruded surfaces, cast surfaces, etc., as shown in FIG. A fracture piece that has a fatigue detection part that is formed thinner than the base parts on both sides across the center part, and that has a slit whose tip is the starting point of the crack, and fixes both ends of the fracture piece Fatigue sensor with a foil-like substrate, fatigue detector with a thickness selected according to the degree of crack growth, and fatigue with the slit tip shape selected to adjust the stress concentration rate and crack generation period Trying to disclose a sensor.

In this fatigue sensor, the substrate surface is affixed to the surface of the subject and observed after an appropriate period of time to detect the degree of breakage or crack progress of the fatigue detection part. From the fatigue damage of the obtained fatigue sensor, the actual stress state of the subject is detected. It is used to know the life of a subject and estimate the life of the subject.
JP-A-9-304240 JP 2001-281120 A

  However, these conventional fatigue sensors and the like must be designed, manufactured, or selected and applied to satisfy the appropriate performance for each measurement object. Thus, there is no versatility as a sensor, and specialized knowledge and advanced technology are required to prepare the specifications of the fatigue sensor to be used.

  Therefore, the problem to be solved by the present invention is to apply a repeated load to a member constituting a structure such as a bridge, a mechanical device, a vehicle, an aircraft, a ship, etc. The versatility to detect whether the repeated stress exceeds the fatigue limit of the evaluation target member by applying one sensor, not limited to the target member, in particular. High repetitive stress sensor.

In order to solve the above problems, a repetitive stress sensor according to the present invention includes a plurality of strip-shaped metal pieces having different stress concentrations based on a stress concentration shape, arranged in parallel on the substrate in the order of the stress concentration. In this arrangement, both end portions of the metal piece are fixed to each other, and both the fixed end portions are further fixed to the substrate.
In the repetitive stress sensor, metal pieces having different stress concentration factors are arranged in the order of the stress concentration factors. Since the metal piece is fixed on the foil-like substrate, if the substrate is affixed to the subject, no extra stress is applied to the metal piece, and the metal piece can accurately reflect the distortion of the subject. it can.

When this repetitive stress sensor is attached to the surface of the target member and observed for a certain period of time, the strain generated in that part is transmitted through the substrate, and the strong stress works in descending order of the stress concentration coefficient, so the metal pieces break in order. To do.
Therefore, if it is known which metal piece of the sensor broke after a lapse of a predetermined period, the member in the stress state of the site where the sensor was attached using an SN diagram prepared in advance by analysis or experiment Lifespan can be estimated.

The fatigue limit for each metal piece varies depending on the thickness and the stress concentration shape. Generally, the greater the stress concentration rate, the lower the fatigue limit.
When the repetitive stress sensor of the present invention is placed in a stress field, if the stress amplitude acting between the end portions is lower than the fatigue limit determined according to the shape and other conditions, the metal piece does not cause fatigue damage and does not break. Further, since the metal piece is thin, if the stress amplitude is higher than the fatigue limit, the metal piece breaks in a relatively short period of time.

That is, if the sensitivity of the metal piece is sufficiently high, the upper limit value of the main stress amplitude acting on the member is in the middle of the fatigue limit of the broken metal piece and the non-ruptured metal piece.
When the fatigue limit of a member is known, it is determined whether the stress amplitude of the member is above or below the fatigue limit of the member, and it is estimated whether it will not be permanently damaged or will be damaged in the future it can. Furthermore, it is possible to estimate the fatigue life of the member based on the time when the metal piece breaks using the SN curve of each metal piece.

The plurality of metal pieces arranged on the sensor substrate are equal in length to each other and are fixed to each other at the ends, and the base foil portion and the crack foil portion have the same width, and the length of the crack foil portion is different. Thus, the stress concentration of each metal piece may be adjusted to be different.
The stress concentration factor is substantially proportional to the ratio of the cross-sectional area of the base foil portion and the crack foil portion, but is inversely proportional to the ratio of the length of the crack foil portion to the length of the base foil portion.
Therefore, without changing the combined length of the base foil portion and the crack foil portion, if the metal pieces are arranged so that the length of the crack foil portion increases in order, the metal pieces are arranged according to the magnitude of the stress concentration coefficient. Become. In addition, these are arranged in the order of the fatigue limit height.

When this repetitive stress sensor is attached to a member and observed, the cracked foil portion breaks from the shorter length as time passes, and the metal piece does not break any longer even after a long time. It means that the fatigue limit has reached a metal piece larger than the stress amplitude of the member.
From this data, the upper limit value of the main stress amplitude acting on the member can be easily known, and the life of the member can be estimated. By comparing the measured stress amplitude value with the fatigue limit of the member, it can be determined whether or not fatigue damage will occur in the member in the future.

Further, the plurality of metal pieces arranged on the sensor substrate are adjusted so that the stress concentration degree of each metal piece is different by providing the crack foil portion with a slit having a different curvature radius at the tip for each metal piece. It may be.
You may make it the whole crack foil part be comprised with a slit. Moreover, you may form a crack foil part thinner than a base foil part.
The stress concentration factor can be adjusted because the cross-sectional area of stress transmission is reduced by the depth of the slit. Further, it is well known that the stress at the tip is concentrated if the curvature radius of the slit tip is reduced. As described above, the stress concentration coefficient can be easily adjusted by changing the shape of the slit, particularly the radius of curvature of the tip without changing other conditions.

  It is preferable to arrange the metal pieces so that the width direction of the metal pieces is perpendicular to the substrate surface. Since the thickness of the metal pieces is extremely small, the entire repetitive stress sensor is formed sufficiently small by arranging the width direction to be perpendicular to the substrate surface, and each metal piece is subjected to a stress state very close to each other. Can do. Even when the repetitive stress sensor is affixed to the target site, it is possible to accurately measure all the metal pieces so as to contact the vicinity of the measurement site more accurately.

  In addition, you may make it produce a different stress concentration degree to a some metal piece with the shape difference of a base foil part and a crack foil part, respectively. Adjustment of the stress concentration level according to the shape of the slit can be easily and logically formed so that adjacent metal pieces have slightly different stress concentration levels. However, if the stress concentration is to be changed within a larger range, adjustment of the curvature of the slit tip may be insufficient. Therefore, for example, it is effective to adjust the stress concentration including changing the width of the base foil portion.

Further, by adjusting the ratio of the length of the crack foil portion and the base foil portion, different stress concentrations may be generated in each metal piece. Moreover, you may make it adjust the ratio of the length of a crack foil part and a base foil part, without changing the length of a crack foil part.
The crack foil part is more easily distorted in the crack foil part and the base foil part. That is, the strain generated in the entire metal piece is more concentrated in the crack foil portion as the length of the crack foil portion is smaller. Furthermore, when the size of the entire stress sensor is constant and the length of the crack foil portion is the same, more strain is concentrated on the crack foil portion as the length of the base foil portion is smaller.

Therefore, the stress concentration degree generated in the metal piece can be largely adjusted by adjusting the ratio of the lengths of the crack foil part and the base foil part.
In addition, when changing the shape of a crack foil part, there are many difficulties on manufacture of a sensor. On the other hand, since the base foil part has a simple shape, it is easy to adjust the length ratio of the crack foil part and the base foil part by changing the length of the base foil part, and it is manufactured. Costs will also be reduced.

  The repetitive stress sensor of the present invention can easily apply a plurality of metal pieces having different stress concentration factors by attaching only one piece to a member to be evaluated. By observation, the stress amplitude generated in the member can be estimated, and the lifetime of the member and the presence or absence of future fatigue damage can be predicted.

  Hereinafter, the present invention will be described in detail based on examples with reference to the drawings.

1 is a perspective view of a repetitive stress sensor according to a first embodiment of the present invention, FIG. 2 is a front cross-sectional view, FIG. 3 is a partial plan cross-sectional view, and FIG. 4 is a repetitive embodiment according to another aspect of the present embodiment. It is front sectional drawing of a stress sensor.
As can be seen from FIGS. 1, 2, and 3, the repetitive stress sensor 10 of the present embodiment is such that the end of the detection element block 18 is fixed to the surface of the substrate 12 via the joint portion 13. The detection element block 18 has a large number of strip-shaped metal pieces 11 arranged in parallel, and the metal pieces are fixed to each other at the end 14 of the metal piece 11.

As shown in FIG. 2, the metal piece 11 has a base foil portion 15 having a small thickness extending from the end portions 14 on both sides toward the center, and a crack foil portion 16 having a small width at the center portion. Yes.
The metal pieces 11 of the detection element block 18 are arranged in order from the shortest to the longest crack foil portion 16.
The detection element block 18 is configured such that the positions of the end portions 14 of the strip-shaped metal pieces 11 formed with the same thickness on the base foil portion 15, the crack foil portion 16, and the end portions 14 are spaced from each other. It can be formed by fixing with a spacer having a thickness of a minute.
Further, as shown in FIG. 3, it can be cut out from a rectangular parallelepiped block by a fine processing technique.

The distortion generated with the repetitive stress generated in the evaluation target member 19 is transmitted to the detection element block 18 through the thin substrate 12 and the joint portion 13. Since the end portion 14 of the metal piece 11 is hardly distorted at the portion where the metal pieces are joined to each other, the distortion transmitted from the target member 19 is distributed to the base foil portion 15 and the crack foil portion 16.
When both the foil parts 15 and 16 are formed of the same material with the same thickness, the stress intensity is concentrated on the cracked foil part 16 in inverse proportion to the width of the foil part, and the elongation of the foil parts 15 and 16 is increased. Is proportional to the product of the length of the foil portion and the stress in the foil portion, the elongation δm of the member is the product αLs of the crack foil portion length Ls and the stress concentration factor α to the crack foil portion 16 and the base foil portion 15 and the base foil. It is distributed by the ratio (αLs: Lb) of the part length Lb.
Therefore, when the elongation of the cracked foil portion 16 is δs and k is a constant, the stress σs = kδs / Ls corresponding to the elongation δs of the cracked foil portion can be expressed without fear of error: σs = kαδm / (αLs + Lb)
It can be seen that the stress acting on the cracked foil portion appears stronger as the length of the cracked foil portion 16 is shorter.

The repetitive stress sensor 10 is used for inspecting the actual repetitive stress state in the target member 19 by attaching the back surface of the substrate 12 to the surface of the evaluation target member 19.
After affixing the repeated stress sensor 10, when a sufficient test period has passed, the metal piece 11 of the sensor having the maximum length among the broken pieces of the cracked foil portion 16 is confirmed. Since the sensitivity of the repetitive sensor 10 is sufficiently high, if the test period is sufficiently long, all metal pieces whose actual repetitive load on the target member 19 exceeds the fatigue limit are broken. Therefore, the actual stress amplitude Δσm can be known by examining the fatigue limit of the broken metal piece. However, since the actual repetitive load is distributed instead of a single value, what is obtained here is a representative stress amplitude.

If the representative stress amplitude Δσm obtained in this way exceeds the fatigue limit of the evaluation target member, it is certain that the target member will eventually exhibit fatigue damage, and if it is lower than the fatigue limit, it will not permanently indicate fatigue failure. Can do.
In addition, when the SN curve of the metal piece 11 of the repetitive stress sensor 10 has an inclination substantially equal to the SN curve of the target member, the lifetime of the member can be estimated by measurement using the sensor. .

  The metal piece in the present embodiment may have a configuration as shown in FIG. 4 following the structure of the fatigue sensor shown in FIG. 8 that the applicant of the present application separately discloses. The metal piece 21 in FIG. 4 has a fatigue detection part 26 that is formed thinner than the base parts 25 on both sides across the center part, and a slit 27 whose tip is the starting point of a crack is provided in this fatigue detection part. The end portions 24 are fixed to each other between the metal pieces 21, and both end portions 24 of the metal pieces 21 are fixed to the foil-like substrate 22 by the joint portions 23.

The shape of the slit 27 and the curvature of the tip are slightly different from the adjacent metal piece 21 by adjusting the stress concentration rate and the crack generation period. The metal pieces 21 are arranged in the order of the stress concentration rate, similarly to the repeated stress sensor 10 shown in FIG.
The repetitive stress sensor of the embodiment shown in FIG. 4 has the same function as that of FIG. 1 and can be used by being stuck on the surface of the evaluation target member 29 in the same manner.

FIG. 5 is a plan view showing a part of a repetitive stress sensor according to the second embodiment of the present invention.
The repetitive stress sensor of the second embodiment is a sensor in which the stress concentration shape is changed so that the stress concentration rate sequentially changes for each metal piece, and arranged in order of the magnitude of the stress concentration rate.

  The repetitive stress sensor 30 shown in FIG. 5 is obtained by fixing both end portions 34 of a single metal film, in which a large number of metal pieces 31 are formed in parallel, to a substrate 32. The crack foil part 36 is provided, and the base foil part 35 is provided on both sides of the crack foil part. The crack foil portion 36 has a stress concentration shape having a notch in a circular arc shape from both sides, and the curvature of the notch increases in the order of arrangement, and the stress concentration coefficient increases in accordance with this. The base foil portions 35 are arranged so that the width w becomes larger in order.

The change in the width of the base foil portion 35 and the change in the stress concentration shape of the crack foil portion 36 are combined, so that the stress concentration coefficient can be increased over a large range between the metal pieces 31 formed in one metal film. Can be changed.
In addition, each of the metal pieces 31 shown in FIG. 5 is formed in a separated form, and as shown in FIG. 1, the metal pieces are arranged so that the width direction of the metal pieces is perpendicular to the substrate. Needless to say, it may be a high-density repetitive stress sensor configured to be fixed at the portion.

FIG. 6 is a plan view showing a part of a repetitive stress sensor according to the third embodiment of the present invention.
The repetitive stress sensor of the third embodiment is a sensor in which the lengths of the metal pieces are adjusted in order of length in order to change the stress generated for each metal piece with respect to the distortion of the evaluation target member.
In the repetitive stress sensor 40 shown in FIG. 6, both end portions 44 of a single metal film in which a large number of metal pieces 41 are formed in parallel are fixed to a substrate 42. The crack foil part 46 of the same shape which has a notch in an arc shape is provided. The base foil portions 45 on both sides of the crack foil portion 46 are arranged so that the length L becomes smaller in order.

When affixed to the evaluation target member, strain is transmitted to both end portions 44 of the metal piece 41 fixed to the substrate 42 via the substrate 42 that is in close contact with the target member, but the end portion 44 hardly deforms. This distortion is distributed between the base foil portion 45 and the crack foil portion 46. Therefore, the stress generated in the metal piece 41 increases as the length L of the base foil portion 45 and the crack foil portion 46 decreases.
The repetitive stress sensor 40 of the third embodiment also has the same function as that of the first embodiment, and the target member is obtained by sticking to the surface of the evaluation target member, observing it after time and knowing the number of the broken metal piece. It is possible to predict the possibility of fatigue damage and to estimate the lifetime.

FIG. 7 is a plan view showing a part of a repetitive stress sensor according to the fourth embodiment of the present invention.
The repetitive stress sensor of the fourth embodiment combines the technical idea of the stress adjustment by the effective length of the metal piece in the sensor of the third embodiment and the stress concentration factor change due to the length of the crack foil portion in the sensor of the first embodiment. And it has the feature in the place which expanded the measurement range.
The repetitive stress sensor 50 of the present embodiment is formed by fixing the end portion 54 of the detection element block 58 to the surface of the substrate 52 through a joint portion. The detection element block 58 is formed by arranging a large number of strip-shaped metal pieces 51 in parallel and fixing the metal pieces 51 to each other at an end 54.

The metal piece 51 includes a crack foil portion 56 at the center, and a base foil portion 55 on both sides of the crack foil portion 56. The crack foil portion 56 is narrower than the base foil portion 55 so that stress concentration occurs.
In the repetitive stress sensor 50 of the present embodiment, the metal pieces 51 are arranged in order from the shorter crack foil portion 56 to the longer one. It should be noted that the length L of the metal piece including the crack foil 56 and the base foil 55 on both sides is shortened as the crack foil is shorter, and the change in the stress concentration is increased to ensure a wide measurement range. is doing.

  Accordingly, the ratio of the length of the cracked foil portion 56 and the effective length L of the metal piece in the repetitive stress sensor 50 of the present embodiment varies in a wider range than the repetitive stress sensors of the first and third embodiments, and the metal The stress concentration degree or the increase rate of the stress in the piece 51 can be changed more greatly than those in the first and third embodiments.

1 is a perspective view of a repeated stress fatigue sensor according to a first embodiment of the present invention. It is front sectional drawing of the cyclic stress fatigue sensor of a present Example. It is a plane sectional view of the repetitive stress fatigue sensor of this example. It is front sectional drawing of the cyclic stress sensor which concerns on another aspect of a present Example. It is a partial top view of the cyclic stress fatigue sensor which concerns on 2nd Example of this invention. It is a partial top view of the cyclic stress fatigue sensor which concerns on 3rd Example of this invention. It is a partial top view of the cyclic stress fatigue sensor which concerns on 4th Example of this invention. It is a perspective view of the fatigue sensor indicated separately.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Repeating stress sensor 11 Metal piece 12 Board | substrate 13 Junction part 14 End part 15 Base foil part 16 Crack foil part 18 Detection element block 19 Evaluation object member 21 Metal piece 22 Substrate 23 Junction part 24 End part 25 Base part 26 Fatigue detection part 27 Slit 29 Evaluation target member 30 Repetitive stress sensor 31 Metal piece 32 Substrate 34 End portion 35 Base foil portion 36 Crack foil portion 40 Repetitive stress sensor 41 Metal piece 42 Substrate 44 End portion 45 Base foil portion 46 Crack foil portion 50 Repetitive stress sensor 52 Substrate 54 End 55 Base foil part 56 Crack foil part

Claims (7)

A plurality of strip-shaped metal pieces having different stress concentrations based on the stress concentration shape are arranged in parallel on the substrate in the order of the magnitude of the stress concentration, and both ends of the metal pieces are fixed to each other, A repeated stress sensor in which both end portions are further fixed to the substrate. The plurality of metal pieces are equal in length to each other, and are composed of an end portion, a base foil portion, and a crack foil portion that are fixed to each other, and the base foil portion and the crack foil portion each have the same width between the metal pieces and the crack. 2. The repetitive stress sensor according to claim 1, wherein the stress concentration of each metal piece is made different depending on the length of the foil portion. The plurality of metal pieces are equal in length to each other, and are composed of an end portion, a base foil portion, and a crack foil portion that are fixed to each other, and each of the crack foil portions includes a slit having a different curvature radius at each end of each metal piece. 2. The repetitive stress sensor according to claim 1, wherein the stress concentration of the metal pieces is made different. 4. The repetitive stress sensor according to claim 2, wherein the metal piece is arranged so that a width direction of the metal piece is perpendicular to the substrate surface. The plurality of metal pieces are equal in length to each other, and are composed of an end portion, a base foil portion, and a crack foil portion that are fixed to each other, and the stress concentration shape depends on a shape difference between the base foil portion and the crack foil portion. 2. The repetitive stress sensor according to claim 1, wherein different stress concentrations are generated in the pieces. The plurality of metal pieces are composed of an end portion, a base foil portion, and a crack foil portion to which the metal pieces are fixed to each other, and different stress concentrations are applied to the plurality of metal pieces according to a ratio of lengths of the crack foil portion and the base foil portion. The repetitive stress sensor according to claim 1, wherein: The repetitive stress sensor according to claim 6, wherein the crack foil portions have the same length between the metal pieces.

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