Patent Publication Number: US-2017363487-A1

Title: Structure for strain detection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of International Application No. PCT/JP2016/055517 filed on Feb. 24, 2016, which was published under PCT Article 21(2) in Japanese, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2015-034950 filed on Feb. 25, 2015, and International Application No. PCT/JP2015/066747 filed on Jun. 10, 2015, the contents all of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a structure for detecting strain, and more particularly, relates to a strain detecting structure, which is suitable for detecting a strain, for example, in a metal frame, a pressure vessel, a concrete structure, and a reinforced concrete structure or the like. The term “strain” as used herein includes the meanings of both strain as a phenomenon and an amount of strain as a physical quantity, and in the case that an amount of strain is clearly indicated, the term “strain amount” will be used. 
     Background Art 
     Conventionally, as a displacement detecting device for measuring mechanical strain and displacement of a building or structure, the displacement detecting device disclosed in Japanese Laid-Open Patent Publication No. 2000-065508 is known. Further, as a device for evaluating fatigue and damage of a structure, the fatigue and damage evaluation device disclosed in Japanese Laid-Open Patent Publication No. 2002-014014 is known. In general, buildings and structures of this type are constructed so that principal stresses thereof are supported by structural bodies constituted by a steel material. 
     As a steel material used primarily for construction, mild steel (SS400 or the like) is included, and with respect to a tensile strength 426 Pa thereof, such a steel material is designed with a safety factor of 3 (140 MPa) with a static load over a long period, and a safety factor of 5 (85 MPa) with a pulsating repeated load. Further, the yield point (proof stress) of mild steel is assumed to be 245 MPa. 
     Since the Young&#39;s modulus of mild steel is about 200 GPa, the amount of elastic deformation strain at respective stresses is 0.07%, 0.04%, and 0.12%, and if it were possible to quantitatively detect the occurrence of such amounts of strain, then one could effectively evaluate the degree of damage of a structure. However, since the amount of strain is extremely small, in order to be detected, it has been necessary to use a sophisticated and complicated type of measuring device, such as those described below. 
     The displacement detection device disclosed in Japanese Laid-Open Patent Publication No. 2000-065508 includes a lever mechanism attached to a structural member such as a building or a structure, and which magnifies a strain or displacement amount generated in the structural member, and a displacement detector which detects a displacement amount that is magnified or reduced by the lever mechanism. 
     The fatigue and damage evaluation device disclosed in Japanese Laid-Open Patent Publication No. 2002-014014 includes a deformation amount detecting means for detecting an amount of deformation of a structure to be evaluated, a fatigue and damage rate detecting means for detecting a fatigue and damage rate of the structure to be evaluated in accordance with the deformation amount detected by the deformation amount detecting means, and a fatigue and damage rate integrating means for integrating the fatigue and damage rate detected by the fatigue and damage rate detecting means. 
     SUMMARY OF THE INVENTION 
     It is necessary to provide displacement detectors for the displacement detecting device disclosed in Japanese Laid-Open Patent Publication No. 2000-065508, and due to the fact that switches such as micro-switches or the like are used as such displacement detectors, it is necessary to provide wiring to a power source and to the various sensors, and the detection operations are troublesome to set up and perform. 
     Since the deformation amount detecting means of the fatigue and damage evaluation device disclosed in Japanese Laid-Open Patent Publication No. 2002-014014 is constituted completely by a mechanical structure, no power source or wiring is necessary. However, because the device is made up from a first fixing plate, a second fixing plate, a movable bar, and a rotary shaft, the structure of the device is complicated. 
     The present invention has been devised taking into consideration the aforementioned problems, and has the object of providing a structure for strain detection, which enables confirmation of strains generated in a structure with an inexpensive device and by visual inspection (including visual inspection through use of binoculars or the like), and without requiring a sophisticated, complex, and expensive power source and electrical wiring. 
     Furthermore, another object of the present invention is to easily detect the presence or absence of a history of occurrence of strain amounts exceeding an allowable stress level over a period of time, when a structure is used over a prolonged time period, and an unexpected load is caused by a natural disaster such as a typhoon, an earthquake, or the like. 
     [1] A structure for strain detection according to the present invention is characterized by being made of a material that is elastically deformable without plastic deformation, and which is attached to a target object (an object to be inspected) in which strain is to be detected, whereby the structure is fractured by elastic deformation that is equal to or greater than a predetermined strain. 
     Although ceramic and glass materials serve as materials that are capable of being fractured by an elastic deformation greater than or equal to a predetermined strain without plastic deformation, in the case of glass materials, minute cracks develop therein due to the influence of moisture in the atmosphere, and a deterioration in the strength of such materials tends to occur. Therefore, in order to detect amounts of strain over a prolonged time period, it is preferable to use a ceramic material having excellent durability. The ceramics used herein preferably are fractured with a strain amount that is greater than or equal to a strain amount corresponding to the allowable stress of the object to be inspected. More specifically, a ratio (σ/E) of a strength (σ: MPa) to a Young&#39;s modulus (E: GPa) of the structure for strain detection is preferably greater than or equal to 0.04%, more preferably, is greater than or equal to 0.1%, and particularly preferably, is greater than or equal to 0.3%. 
     Furthermore, in the case that the object to be inspected is used under a fixed temperature condition, although it is unnecessary to give particular consideration to the coefficient of thermal expansion of the structure for strain detection, in the case of buildings and structures that are installed outdoors, changes in temperature occur accompanying changes in the ambient temperature during the measurement period. In such a situation, in order to eliminate the influence of such a temperature change, the difference in the coefficient of thermal expansion between the structure for strain detection and the structure constituting the inspection target building preferably is less than or equal to ±2 ppm/K, and more preferably, is less than or equal to +1 ppm/K. By selecting such a ceramic material, it becomes possible to detect, over a prolonged time period, the amount of strain of a structure that is installed outdoors, without the influence of such a temperature change. For example, in the case that the object to be inspected is a steel material or reinforced concrete, if zirconia or forsterite or the like having the same coefficient of thermal expansion as the object is selected, the amount of strain can be detected without the influence of such a temperature change. 
     [2] A stress concentrated section, which is fractured at a predetermined strain or greater, may further be provided in the main body of the structure for strain detection. In accordance with this feature, when a load is applied to the object to be inspected, and, for example, a predetermined strain occurs in the object to be inspected, a predetermined strain also occurs in the main body of the structure for strain detection, whereby the stress concentrated section is selectively fractured.
 
Consequently, by confirming whether or not the stress concentrated section has been fractured, it can be confirmed whether or not a predetermined strain has taken place in the object to be inspected. When the level of the stress concentration is arbitrarily set upon devising the structure of the stress concentrated section, strain detecting ceramics can be manufactured having different levels for detecting amounts of strain. By disposing a plurality of strain detecting ceramics having different levels for detecting amounts of strain, it is possible to detect an arbitrary amount of strain, and more specifically, an amount of stress generated in the object to be inspected. Furthermore, such a confirmation can be easily performed by the naked eye, since it is merely necessary to confirm the presence or absence of breakage or fracturing in the stress concentrated section.
 
Consequently, using the structure for strain detection of the present invention, it is possible to easily detect and confirm strains cheaply by way of visual inspection (including visual inspection using binoculars or the like), or by the presence or absence of simple electrical signals or the like, even after the strains have occurred in the object to be inspected over a prolonged time period, and without requiring an expensive and complicated power source and electrical wiring.
 
     In the present invention, initially, by selecting materials having different ratios (σ/E) of strength to Young&#39;s modulus, it is possible to manufacture strain detecting ceramics which become fractured at an arbitrary amount of strain. For ceramics that do not undergo plastic deformation, the amount of strain (ε) under a predetermined level of stress is given by the following equation. 
       ε=σ/ E   (1)
 
     Breakage or fracturing takes place when the strength σ reaches the strength of the ceramic, and at this time, the amount of strain (ε) is expressed by equation (1). The values for σ/E for various materials are shown in Table 1, which will be discussed later. Such values are indicative of strain amounts at which respective ceramic or glass materials become fractured. For example, strain detecting ceramics composed of alumina A and which do not have a stress concentrated section therein undergo fracturing at a strain amount of 0.14%. Similarly, the strain detecting ceramics composed of silicon nitride A or mica undergo fracturing at a strain amount of 0.20%. 
     [3] Furthermore, a case in which a stress concentrated section is provided, so as to undergo breakage or fracturing at an arbitrary strain amount, will be explained below. Assuming a dimension of the entire main body in one direction thereof is represented by Lm, and a dimension of the stress concentrated section in the one direction is represented by Lc, then Lc&lt;Lm, and the stress concentrated section may be constituted by a thin-walled portion in the one direction. Consequently, by suitably changing the dimension Lc of the stress concentrated section in the one direction, the main body can be fractured with a predetermined strain. For example, by providing a predetermined stress concentrated section in zirconia B, it becomes possible to design a strain detecting ceramic which is subjected to fracturing at an arbitrary displacement that is less than or equal to 0.56%.
 
[4] In this case, the one direction is a direction which is perpendicular to a longitudinal direction of the main body, as well as being perpendicular to a thickness direction of the main body.
 
[5] In the present invention, the main body preferably includes a structure portion (visualization structure) for visualizing the occurrence of the predetermined strain, by way of a secondary fracture, which is induced by a primary fracture of the stress concentrated section. Consequently, by visually confirming the state of the visualization structure, it is possible to easily confirm whether or not a predetermined strain has occurred in the main body.
 
[6] In this case, the visualization structure may include a thin-walled region that causes a portion of the main body to drop off due to the secondary fracture. In accordance with this feature, when a strain occurs in the main body and the stress concentrated section experiences a fracture (primary fracture), then taking this fracture as a starting point, fracturing (secondary fracturing) of the thin-walled region is induced, and a portion of the main body drops off. Consequently, by confirming whether or not the portion of the main body has fallen off, it can be confirmed whether or not a predetermined strain has taken place in the object to be inspected. Such a confirmation can easily be carried out with the naked eye.
 
[7] In this case, a length La of the main body is preferably greater than or equal to 10 mm and less than or equal to 300 mm, a width Lm of the main body is preferably greater than or equal to 5 mm and less than or equal to 100 mm, a thickness ta of a central portion of the main body is preferably greater than or equal to 0.3 mm and less than or equal to 3 mm, a thickness tae of each of both end portions of the main body is preferably greater than or equal to 1 mm and less than or equal to 10 mm and is thicker than the thickness ta of the central portion, and a thickness tb of the thin-walled region is preferably greater than or equal to 0.01 mm and less than or equal to 0.5 mm and is thinner than the thickness ta of the central portion.
 
[8] Furthermore, the thin-walled region may be provided in a frame shape, and one part of the main body may be a portion that is surrounded by the thin-walled region. In accordance with this feature, when a strain occurs in the main body and the stress concentrated section experiences a fracture (primary fracture), then taking this fracture as a starting point, a crack occurs in the thin-walled region. The crack expands in a frame shape along the thin-walled region due to the presence of the one part of the main body, whereupon breakage or fracturing (secondary fracturing) of the thin-walled region is induced.
 
[9] Further, at least one through hole may be formed in the thin-walled region. In this case, when the stress concentrated section experiences a fracture (primary fracture) and a crack occurs in the thin-walled region, development of the crack is accelerated due to the presence of the through hole, and the one part of the main body can assuredly be made to drop off at an early stage.
 
[10] In any of features [5] to [9] discussed above, the visualization structure may include a visible member that is exposed by the secondary fracture. In accordance with this feature, by the one part of the main body dropping off, the visible member becomes exposed, and thus, by confirming the exposure of the visible member, an observer can easily realize that a predetermined strain has occurred in the main body.
 
[11] In any of features [5] to [9] discussed above, the visualization structure may include a conductive ceramic, the electrical characteristics of which are changed by the secondary fracture.
 
[12] In any of features [2] to [4] discussed above, one through hole may be included in the main body, and a curved portion of the through hole may constitute a part of the stress concentrated section.
 
[13] In this case, the through hole may be rectangular, and two apex portions thereof that constitute a part of the stress concentrated section may be formed respectively in a curved shape.
 
[14] In the present invention, the ceramic constituting the main body preferably contains zirconia.
 
[15] In the present invention, the predetermined strain preferably is a strain in a range within which the target object is elastically deformed.
 
[16] In the present invention, both end portions of the main body may be formed respectively to be thick-walled, and steps may be formed respectively between the central portion of the main body and both of the end portions. In this case, boundary portions between each of the steps and the central portion of the main body are preferably formed in a curved shape, whereby concentration of stresses can be alleviated by the boundary portions.
 
[17] In this case, the boundary portions are preferably formed in a curved shape having a radius of curvature of 0.5 mm R or greater. The term 0.5 mm R represents the radius of curvature of the curved shape.
 
[18] In either of features [16] or [17] above, the main body preferably is fixed to the object to be inspected using respective thick-walled sections of both of the end portions.
 
[19] In feature [18] above, the respective thick-walled sections of both of the end portions preferably are bonded and fixed to the target object. Further, assuming that a length of each of the thick-walled sections at both of the end portions along a lengthwise direction of the main body represents a length Lae of the thick-walled sections, and a length of the thick-walled sections along a widthwise direction of the main body represents a width Lme of the thick-walled sections, then concerning each of the thick-walled sections, the areas of each of the thick-walled sections, which are obtained respectively by multiplying the length Lae of the thick-walled sections times the width Lme of the thick-walled sections, preferably are equivalent to each other. In addition, the areas of each of the thick-walled sections are areas sufficient to support a load generated in the structure for strain detection when the target object reaches a predetermined amount of strain.
 
[20] In this case, assuming that a tensile shear adhesive strength of an adhesive by which the respective thick-walled sections of both of the end portions are bonded and fixed to the target object is represented by F (N/m 2 ), the area of each of the respective thick-walled sections is represented by A (mm 2 ), and the load generated in the structure for strain detection when the target object reaches the predetermined amount of strain is represented by L, then preferably, the inequality A&gt;L/F is satisfied.
 
     In accordance with the structure for strain detection according to the present invention, it is possible to confirm the presence of strains generated in a structure with an inexpensive device and by visual inspection (including visual inspection through use of binoculars or the like), and without requiring an expensive and complicated power source and electrical wiring. 
     Furthermore, it is possible to easily detect the presence or absence of a history of occurrence of strain amounts exceeding an allowable stress level over a period of time, when a structure is used over a prolonged time period, and an unexpected load is caused by a natural disaster such as a typhoon, an earthquake, or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a plan view showing a structure for strain detection (a first structure for strain detection) according to a first embodiment as viewed from above,  FIG. 1B  is a cross-sectional view taken along line IB-IB in  FIG. 1A , and  FIG. 1C  is a cross-sectional view taken along line IC-IC in  FIG. 1A ; 
         FIG. 2A  is a plan view showing a structure for strain detection (a second structure for strain detection) according to a second embodiment as viewed from above,  FIG. 2B  is a cross-sectional view taken along line IIB-IIB in  FIG. 2A , and  FIG. 2C  is a cross-sectional view taken along line IIC-IIC in  FIG. 2A ; 
         FIG. 3A  is a plan view showing a structure for strain detection (a third structure for strain detection) according to a third embodiment as viewed from above,  FIG. 3B  is a cross-sectional view taken along line IIIB-IIIB in  FIG. 3A , and  FIG. 3C  is a cross-sectional view taken along line IIIC-IIIC in  FIG. 3A ; 
         FIG. 4A  is a cross-sectional view showing one example of a formation position of a thin-walled region constituting a visualization structure, and  FIG. 4B  is a cross-sectional view showing another example of a formation position for the thin-walled region; 
         FIG. 5  is a cross-sectional view showing an example in which a visible member is disposed between a main body of the third structure for strain detection, and a target object to be inspected (indicated by the two-dot chain line); 
         FIG. 6A  is a plan view showing a structure for strain detection (a fourth structure for strain detection) according to a fourth embodiment as viewed from above,  FIG. 6B  is a cross-sectional view taken along line VIB-VIB in  FIG. 6A , and  FIG. 6C  is a cross-sectional view taken along line VIC-VIC in  FIG. 6A ; 
         FIG. 7A  is a plan view showing a structure for strain detection (a fifth structure for strain detection) according to a fifth embodiment as viewed from above,  FIG. 7B  is a cross-sectional view taken along line VIIB-VIIB in  FIG. 7A , and  FIG. 7C  is a cross-sectional view taken along line VIIC-VIIC in  FIG. 7A ; 
         FIG. 8A  is a plan view showing a structure for strain detection (a sixth structure for strain detection) according to a sixth embodiment as viewed from above,  FIG. 8B  is a cross-sectional view taken along line VIIIB-VIIIB in  FIG. 8A , and  FIG. 8C  is a cross-sectional view taken along line VIIIC-VIIIC in  FIG. 8A ; 
         FIG. 9A  is a plan view showing another example of the second structure for strain detection as viewed from above,  FIG. 9B  is a cross-sectional view taken along line IXB-IXB in  FIG. 9A , and  FIG. 9C  is a cross-sectional view taken along line IXC-IXC in  FIG. 9A ; 
         FIG. 10A  is a plan view showing another example of the third structure for strain detection as viewed from above,  FIG. 10B  is a cross-sectional view taken along line XB-XB in  FIG. 10A , and  FIG. 10C  is a cross-sectional view taken along line XC-XC in  FIG. 10A ; 
         FIG. 11A  is a cross-sectional view showing a first example in which both ends of the main body are formed respectively to be thick-walled, and  FIG. 1  is a plan view of the first example as viewed from above; and 
         FIG. 12A  is a cross-sectional view showing a second example in which both ends of the main body are formed respectively to be thick-walled, and  FIG. 12B  is a cross-sectional view showing a third example. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of a structure for strain detection according to the present invention will be explained below with reference to  FIGS. 1A through 12B . It should be noted that, in the present specification, a numerical range of “A to B” includes both the numerical values A and B, respectively, as the lower limit and upper limit values thereof. 
     Initially, as shown in  FIGS. 1A to 1C , the structure for strain detection according to the first embodiment (hereinafter referred to as a first structure for strain detection  10 A) includes a ceramic main body  12  which is attached to a target object (object to be inspected: not shown) in which strain is to be detected. Further, both end portions  18   a  and  18   b  excluding a central portion  12   c  of the main body  12  constitute attachment sections to be attached to the object to be inspected using, for example, tightening of bolts, or an adhesive or the like. 
     A ratio (σ/E) of a strength (σ: MPa) and a Young&#39;s modulus (E: GPa) of the first structure for strain detection  10 A is preferably greater than or equal to 0.04%, more preferably, is greater than or equal to 0.1%, and particularly preferably, is greater than or equal to 0.3%. Further, the difference in the coefficient of thermal expansion between the structure for strain detection and the structure constituting the inspection target building preferably is less than or equal to ±2 ppm/K, and more preferably, is less than or equal to ±1 ppm/K. 
     An experimental example of the first structure for strain detection  10 A will be described. In the experimental example, the size of the main body  12  was kept constant, and the change in the strain in the case that the material thereof was changed was confirmed. More specifically, in relation to Exemplary Embodiments 1 to 24 and Comparative Examples 1 and 2, a tensile load was applied in the longitudinal direction of the main body  12 , and the amount of strain (distortion) at the time that the main body  12  underwent fracturing was confirmed. As shown in  FIGS. 1A to 1C , in all of the Exemplary Embodiments 1 to 24 and in the Comparative Examples 1 and 2, a dimension in one direction (y-direction), and more specifically, a width Lm (see  FIG. 1A ), of the main body  12  was 20 mm. In this instance, the one direction is a direction perpendicular to the longitudinal direction (x-direction), as well as being perpendicular to the thickness direction (z-direction) of the main body  12 . Further, a thickness ta (see  FIG. 1B ) of the main body  12  was 0.5 mm. The results are shown in the following Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Fracture 
                 Fracture Strain 
                   
               
               
                   
                   
                   
                   
                 Strain 
                 Amount 
                 Thermal 
               
               
                   
                   
                   
                 Young&#39;s 
                 Amount 
                 Actual Measured 
                 Expansion 
               
               
                   
                   
                 Strength σ 
                 Modulus E 
                 σ/E 
                 Value 
                 Coefficient α 
               
               
                   
                 Material 
                 (MPa) 
                 (GPa) 
                 (%) 
                 (%) 
                 (ppm/K) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Exp. Example 1 
                 Alumina A 
                 380 
                 280 
                 0.14 
                 0.12 to 0.15 
                 8 
               
               
                 Exp. Example 2 
                 Alumina B 
                 350 
                 320 
                 0.11 
                 0.10 to 0.12 
                 8 
               
               
                 Exp. Example 3 
                 Alumina C 
                 300 
                 400 
                 0.08 
                 0.07 to 0.09 
                 8 
               
               
                 Exp. Example 4 
                 Alumina D 
                 350 
                 400 
                 0.09 
                 0.08 to 0.10 
                 8 
               
               
                 Exp. Example 5 
                 Zirconia A 
                 700 
                 200 
                 0.35 
                 0.31 to 0.39 
                 10 
               
               
                 Exp. Example 6 
                 Zirconia B 
                 1,000 
                 180 
                 0.56 
                 0.50 to 0.61 
                 10 
               
               
                 Exp. Example 7 
                 Silicon Nitride A 
                 600 
                 300 
                 0.20 
                 0.18 to 0.22 
                 3 
               
               
                 Exp. Example 8 
                 Silicon Nitride B 
                 1,000 
                 300 
                 0.33 
                 0.30 to 0.36 
                 3 
               
               
                 Exp. Example 9 
                 Silicon Nitride C 
                 1,200 
                 320 
                 0.38 
                 0.34 to 0.41 
                 3 
               
               
                 Exp. Example 10 
                 Aluminum Nitride A 
                 250 
                 320 
                 0.08 
                 0.07 to 0.09 
                 5 
               
               
                 Exp. Example 11 
                 Aluminum Nitride B 
                 350 
                 320 
                 0.11 
                 0.10 to 0.12 
                 5 
               
               
                 Exp. Example 12 
                 Silicon Carbide A 
                 400 
                 450 
                 0.09 
                 0.08 to 0.10 
                 4 
               
               
                 Exp. Example 13 
                 Silicon carbide B 
                 600 
                 450 
                 0.13 
                 0.12 to 0.14 
                 4 
               
               
                 Exp. Example 14 
                 SiSiC A 
                 250 
                 340 
                 0.07 
                 0.07 to 0.08 
                 2.4 
               
               
                 Exp. Example 15 
                 SiSiC B 
                 150 
                 340 
                 0.04 
                 0.03 to 0.05 
                 2.4 
               
               
                 Exp. Example 16 
                 Mullite 
                 280 
                 210 
                 0.13 
                 0.12 to 0.15 
                 5 
               
               
                 Exp. Example 17 
                 Cordierite A 
                 150 
                 140 
                 0.11 
                 0.10 to 0.12 
                 0 
               
               
                 Exp. Example 18 
                 Cordierite B 
                 240 
                 137 
                 0.18 
                 0.16 to 0.19 
                 0 
               
               
                 Exp. Example 19 
                 Aluminum Titanate 
                 40 
                 6 
                 0.67 
                 0.60 to 0.73 
                 1 
               
               
                 Exp. Example 20 
                 Steatite 
                 200 
                 130 
                 0.15 
                 0.14 to 0.17 
                 9 
               
               
                 Exp. Example 21 
                 Forsterite 
                 200 
                 150 
                 0.13 
                 0.12 to 0.15 
                 10 
               
               
                 Exp. Example 22 
                 Titania 
                 300 
                 260 
                 0.12 
                 0.10 to 0.13 
                 12 
               
               
                 Exp. Example 23 
                 Mica 
                 100 
                 50 
                 0.20 
                 0.18 to 0.22 
                 11.7 
               
               
                 Exp. Example 24 
                 LTCC 
                 240 
                 125 
                 0.19 
                 0.17 to 0.21 
                 6.3 
               
               
                 Comp. Example 1 
                 Quartz Glass 
                 48 
                 72 
                 0.07 
                 Unmeasurable 
                 0.6 
               
               
                 Comp. Example 2 
                 Soda Glass 
                 150 
                 71 
                 0.21 
                 Unmeasurable 
                 9 
               
               
                   
               
            
           
         
       
     
     Next, as shown in  FIGS. 2A to 2C , the structure for strain detection according to a second embodiment (hereinafter referred to as a second structure for strain detection  10 B) includes a ceramic main body  12  which is attached to a target object (object to be inspected: not shown) in which strain is to be detected, and stress concentrated sections  14  formed in the main body  12 , and which are fractured at a predetermined strain or greater. Concerning attachment of the main body  12  to the object to be inspected, it can be attached by a known method, and attachment thereof can be performed for example by bolt tightening, or through use of an adhesive or the like. 
     Any arbitrary shape can be adopted for the shape of the main body  12 , however, assuming that the mounting surface of the object to be inspected is planar, for example, as shown in  FIGS. 2A to 2C , a flat plate shape (typically, a rectangular parallelepiped shape) may be adopted. In this case, a ridge line portion thereof may be chamfered (a chamfered surface or a rounded surface). Hereinafter, cases will primarily be described in which the main body  12  is of a flat plate shape. 
     The second structure for strain detection  10 B includes a circular through hole  16  at the center of the main body  12  as viewed from a planar surface (upper surface) thereof. Accordingly, the stress concentrated sections  14  are portions which are thin-walled owing to the presence of the through hole  16  formed within the main body  12 . More specifically, assuming that a dimension in one direction (y-direction), and more specifically a width, of the main body  12  is represented by Lm (see  FIG. 2A ), and a dimension in the one direction of each of the stress concentrated sections  14  is represented by Lc (see  FIG. 2C ), then the inequality Lc&lt;Lm is satisfied. Stated otherwise, the stress concentrated sections  14  are constituted by thin-walled regions in the one direction. 
     In addition to a circular shape, for the shape of the through hole  16  as viewed from the upper surface, there can be adopted an elliptical shape, a track shape, a rectangular shape, or the like. Further, both end portions  18   a  and  18   b  of the main body  12  constitute attachment sections to be attached to the object to be inspected using, for example, tightening of bolts, or an adhesive or the like. 
     The predetermined strain is a strain lying within a range that enables determination of whether or not the object to be inspected has been deformed by an amount in excess of an allowable stress, and for example, a deformation amount of 0.1%, 0.2%, or the like is selected. In this case, as objects to be inspected, there are included, for example, a pressure vessel, a frame made of metal (a frame of heavy machinery, a frame of a press machine, a frame of a device for applying a hydrostatic pressure, etc.), a utility pole, a steel tower, a concrete structure, a reinforced concrete structure, and the like. However, if the object to be inspected is an object having a yield point that clearly appears within a stress strain diagram, the amount of strain can be selected as lying within a range before and after the yield point and between which the yield point is sandwiched. In the case of an object to be inspected having a yield point that does not clearly appear in such a stress strain diagram, it is possible to select the amount of strain to lie within a range before and after the strain amount at a time of generated stress corresponding to a 0.2% proof stress. 
     One reason for selecting, as the predetermined strain, a strain as lying within a range of elastic deformation of the object to be inspected and which is less than the yield point is as follows. More specifically, even if a strain within the range of elastic deformation occurs in the structure, since the structure will return to its original position, it is difficult to comprehend if such a strain has occurred, that is, whether or not a load has been applied. Thus, for example, by periodically confirming whether fracturing of the stress concentrated sections  14  in the second structure for strain detection  10 B has occurred, and if it has become fractured, by repeatedly performing an operation to replace it with a new second structure for strain detection  10 B, it is possible to know how many times a strain of about 0.1% has occurred, and such knowledge can be used in analysis of aging of the object to be inspected. Of course, by shortening the inspection period, it is possible to know with greater accuracy the number of times that strains on the order of 0.1% have occurred. 
     As the ceramic that constitutes the main body  12 , a ceramic containing zirconia is preferred. The strain when fracturing takes place is 0.56%, and by providing the stress concentrated sections  14 , it is possible to cause the main body  12  to undergo fracturing at a strain within a range in which the object to be inspected is elastically deformed, for example, a strain of 0.1% or 0.2%, or the like. In addition, due to the fact that the coefficient of thermal expansion of zirconia is substantially the same as the coefficient of thermal expansion of carbon steel (mild steel) or reinforced concrete, it is possible to compensate for changes in temperature. This is connected with being able to detect strains without being influenced by changes in temperature, which is also advantageous in terms of improving detection accuracy. 
     The size of the main body  12  is limited from the visibility of the fracture and the size to which a ceramic member of a desired shape can be manufactured. More specifically, in order to confirm with a simple method such as visual inspection whether or not fracturing has occurred in the strain detecting ceramic, from the standpoint of visibility from a distance or the like, it is preferable for the width Lm of the main body  12  to be greater than or equal to 5 mm, and for the length La of the main body  12  to be greater than or equal to 10 mm. On the other hand, concerning the manufacturing process of the ceramic member which is constituted by ceramics, the ceramic member is manufactured by molding a ceramic powder and then firing the molded ceramic powder. In this case, since the strength of the molded body is small and is accompanied by a large amount of firing shrinkage on the order of several 10% during firing, in order to manufacture the main body  12  with a small amount of deformation and with dimensions as designed, there is naturally a limit to how large the main body  12  can be. More specifically, it is preferable for the width Lm of the main body  12  to be less than or equal to 100 mm, and for the length La of the main body  12  to be less than or equal to 300 mm. Furthermore, in relation to the thickness ta of the main body  12 , although a large thickness ta thereof has a tendency to simplify manufacturing, the load generated at the time that strains are detected increases, which makes the method of fixing the main body  12  to the object to be inspected more difficult. Therefore, the thickness ta of the main body  12  is preferably less than or equal to 3 mm. Further, if the thickness ta thereof is too small, since cracking or deformation occurs during molding and firing, it is preferable for the thickness ta to be equal to or greater than 0.3 mm. 
     First Experimental Example 
     A first experimental example of the second structure for strain detection  10 B will now be shown. Zirconia B (see Table 1 above) was used as the ceramic thereof. In the experimental example, the change in strain, the possibility of visibility of fracturing, and the propriety of manufacturing the main body  12  were confirmed for cases in which the size of the main body  12  and the diameter Da of the through hole  16  were changed. Concerning the strain, a tensile load was applied in the longitudinal direction of the main body  12 , and the strain therein at the time that the main body  12  experienced fracturing was confirmed. 
     (Samples 1 to 7) 
     As shown in  FIGS. 2A to 2C , in each of Samples 1 to 7, the length La of the main body  12  was 100 mm, the width Lm (the length in one direction of the main body  12 ) was 30 mm, and the thickness ta (see  FIG. 2B ) of the main body  12  was 1 mm. Concerning the diameter Da of the through hole  16 , the diameter thereof was 4 mm in Sample 1, the diameter thereof was 8 mm in Sample 2, the diameter thereof was 9 mm in Sample 3, the diameter thereof was 11 mm in Sample 4, the diameter thereof was 15 mm in Sample 5, the diameter thereof was 19 mm in Sample 6, and the diameter thereof was 26 mm in Sample 7. The length of each of both end portions  18   a  and  18   b , and more specifically, the length Lae along the longitudinal direction of the main body  12  was 20 mm. Using both of the end portions  18   a  and  18   b , Samples 1 to 7 were fixed to a target object in which strain was to be detected. 
     (Sample 8) 
     In Sample 8, the main body  12  had a width Lm of 5 mm, a length La of 10 mm, and a thickness ta of 0.3 mm. The diameter Da of the through hole  16  was 0.67 mm. The respective lengths Lae of both end portions  18   a  and  18   b  were 2.5 mm. Using both of the end portions  18   a  and  18   b , Sample 8 was fixed to a target object in which strain was to be detected. 
     (Sample 9) 
     In Sample 9, the main body  12  had a width Lm of 100 mm, a length La of 300 mm, and a thickness ta of 3 mm. The diameter Da of the through hole  16  was 87 mm. The respective lengths Lae of both end portions  18   a  and  18   b  were 50 mm. Using both of the end portions  18   a  and  18   b , Sample 9 was fixed to a target object in which strain was to be detected. 
     Comparative Example 3 
     In Comparative Example 3, the main body  12  had a width Lm of 100 mm, a length La of 300 mm, and a thickness ta of 0.2 mm. The diameter Da of the through hole  16  was 63 mm. The respective lengths Lae of both end portions  18   a  and  18   b  were 50 mm. Using both of the end portions  18   a  and  18   b , Comparative Example 3 was fixed to a target object in which strain was to be detected. 
     Comparative Example 4 
     In Comparative Example 4, the main body  12  had a width Lm of 3 mm, a length La of 7 mm, and a thickness ta of 0.3 mm. The diameter Da of the through hole  16  was 1.9 mm. The respective lengths Lae of both end portions  18   a  and  18   b  were 2 mm. Using both of the end portions  18   a  and  18   b , Comparative Example 4 was fixed to a target object in which strain was to be detected. 
     Comparative Example 5 
     In Comparative Example 5, the main body  12  had a width Lm of 120 mm, a length La of 350 mm, and a thickness ta of 1 mm. The diameter Da of the through hole  16  was 76 mm. The respective lengths Lae of both end portions  18   a  and  18   b  were 50 mm. Using both of the end portions  18   a  and  18   b , Comparative Example 5 was fixed to a target object in which strain was to be detected. 
     &lt;Evaluation Results&gt; 
     Evaluation results of Samples 1 to 9 and Comparative Examples 3 to 5 are shown in the following Table 2 together with a breakdown of items shown therein. In Table 2, the lengths Lae of both end portions  18   a  and  18   b  are expressed as “end portion length”. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Main Body Dimensions 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 End 
                   
                   
                   
                 Strain at 
                   
                   
               
               
                   
                 Length 
                 Portion 
                 Width 
                 Thickness 
                 Through Hole 
                 Time of 
                 Visibility 
               
               
                   
                 La 
                 Length Lae 
                 La 
                 ta 
                 Diameter Da 
                 Fracturing 
                 of 
               
               
                   
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (%) 
                 Fracturing 
                 Manufacturability 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Sample 1 
                 100 
                 20 
                 30 
                 1 
                 4 
                 0.179 
                 ◯ 
                 Possible 
               
               
                 Sample 2 
                 100 
                 20 
                 30 
                 1 
                 8 
                 0.166 
                 ◯ 
                 Possible 
               
               
                 Sample 3 
                 100 
                 20 
                 30 
                 1 
                 9 
                 0.161 
                 ◯ 
                 Possible 
               
               
                 Sample 4 
                 100 
                 20 
                 30 
                 1 
                 11 
                 0.161 
                 ◯ 
                 Possible 
               
               
                 Sample 5 
                 100 
                 20 
                 30 
                 1 
                 15 
                 0.126 
                 ◯ 
                 Possible 
               
               
                 Sample 6 
                 100 
                 20 
                 30 
                 1 
                 19 
                 0.096 
                 ◯ 
                 Possible 
               
               
                 Sample 7 
                 100 
                 20 
                 30 
                 1 
                 26 
                 0.04 
                 ◯ 
                 Possible 
               
               
                 Sample 8 
                 10 
                 2.5 
                 5 
                 0.3 
                 0.67 
                 0.179 
                 ◯ 
                 Possible 
               
               
                 Sample 9 
                 300 
                 50 
                 100 
                 3 
                 87 
                 0.04 
                 ◯ 
                 Possible 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Comparative 
                 300 
                 50 
                 100 
                 0.2 
                 63 
                 Evaluation Impossible 
                 Impossible 
               
               
                 Example 3 
                   
                   
                   
                   
                   
                 Because of Cracking 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Comparative 
                 7 
                 2 
                 3 
                 0.3 
                 1.9 
                 0.1 
                 Difficult 
                 Possible 
               
               
                 Example 4 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Comparative 
                 350 
                 50 
                 120 
                 1 
                 76 
                 Evaluation Impossible 
                 Impossible 
               
               
                 Example 5 
                   
                   
                   
                   
                   
                 Due to Large 
               
               
                   
                   
                   
                   
                   
                   
                 Deformation 
               
               
                   
               
            
           
         
       
     
     From Table 2, it can be understood that Sample 1 to 9 exhibit good visibility of fracturing, and manufacturing thereof also is possible. On the other hand, in Comparative Example 3, cracks were generated during the manufacturing process, and visibility of strain at the time of fracturing could not be evaluated. In Comparative Example 4, although manufacturing thereof was possible, since the size was small, visibility of fracturing was poor, and it was difficult to visually recognize such fracturing. In Comparative Example 5, deformation due to the manufacturing process was significant, and since manufacturing thereof was not possible, strains occurring at the time of fracturing and visibility of such fracturing could not be evaluated. 
     Second Experimental Example 
     In the second experimental example, the length La of the second structure for strain detection  10 B (the distance from one end of the end portion  18   a  to one end of the end portion  18   b ) was 100 mm, the width Lm thereof was 30 mm, and the thickness ta thereof was 0.5 mm, and under such conditions, a change in strain upon changing the diameter Da of the through hole  16  was confirmed. The respective lengths Lae of both end portions  18   a  and  18   b  were 20 mm. More specifically, concerning Samples 11 to 13 shown in the following Table 3, using both of the end portions  18   a  and  18   b , each of the samples was fixed to a target object in which strain was to be detected. A tensile load was applied in a longitudinal direction of the main body  12 , and the strain therein at the time of fracturing of the main body  12  was confirmed. The diameter Da of the through hole  16  was 4 mm in the case of Sample 11, 11 mm in the case of Sample 12, and 19 mm in the case of Sample 13. The results are shown in the following Table 3. In Table 3, the lengths Lae of both end portions  18   a  and  18   b  are expressed as “end portion length”. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Main Body Dimensions 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 End Portion 
                   
                   
                   
                 Strain at 
               
               
                   
                 Length 
                 Length 
                 Width 
                   
                 Through Hole 
                 Time of 
               
               
                   
                 La 
                 Lae 
                 Lm 
                 Thickness ta 
                 Diameter Da 
                 Fracturing 
               
               
                   
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Sample 11 
                 100 
                 20 
                 30 
                 0.5 
                 4 
                 0.184 
               
               
                 Sample 12 
                 100 
                 20 
                 30 
                 0.5 
                 11 
                 0.172 
               
               
                 Sample 13 
                 100 
                 20 
                 30 
                 0.5 
                 19 
                 0.142 
               
               
                   
               
            
           
         
       
     
     It can be understood from Table 3 that by changing the diameter Da of the through hole  16 , the main body  12  can be made to undergo fracturing with a predetermined level of strain. Such a feature is also apparent from the results of Samples 1 to 7 of the first experimental example (see Table 2). More specifically, by suitably changing the dimension Lc in the one direction of the stress concentrated sections  14 , the main body  12  can be fractured with a predetermined strain. 
     In this manner, in the second structure for strain detection  10 B, when a load is applied to the object to be inspected, and, for example, a predetermined strain takes place in the object to be inspected, a predetermined strain is also generated in the main body  12  of the second structure for strain detection  10 B, and the stress concentrated sections  14  thereof are then fractured. For example, cracks enter into the stress concentrated sections  14 , and fracturing thereof occurs. Consequently, by confirming whether or not the stress concentrated sections  14  have been fractured, it can be confirmed whether or not a predetermined strain has taken place in the object to be inspected. Such a confirmation can easily be carried out with the naked eye. 
     Accordingly, by using the second structure for strain detection  10 B, it is possible to confirm the presence of strains generated in the object to be inspected inexpensively, by visual inspection (including visual inspection through use of binoculars or the like), and without requiring a power source or electrical wiring. 
     Next, a structure for strain detection according to a third embodiment (hereinafter referred to as a third structure for strain detection  10 C) will be explained with reference to  FIGS. 3A to 3C . 
     As shown in  FIGS. 3A to 3C , the third structure for strain detection  10 C has substantially the same configuration as the above-described second structure for strain detection  10 B, but differs therefrom in that a structure portion (hereinafter referred to as a visualization structure  20 ) is included for visualizing the occurrence of the predetermined strain by way of a secondary fracture, which is induced by a fracture (primary fracture) of the stress concentrated sections  14 . 
     The visualization structure  20  has a disk-shaped thin-walled region  22  formed integrally at the center of the main body  12 , and which is thinner than the thickness of the main body  12 . More specifically, the visualization structure  20  has a structure in which the through hole  16  (see  FIG. 2A ) of the second structure for strain detection  10 B is closed by the thin-walled region  22 . 
     Therefore, when a strain occurs in the main body  12  and the stress concentrated sections  14  experience a fracture (primary fracture), then taking this fracture as a starting point, fracturing (secondary fracturing) of the thin-walled region  22  is induced, and the totality or a portion of the thin-walled region  22  drops off. 
     Consequently, by confirming whether or not the totality or a portion of the thin-walled region  22  has fallen off, it can be confirmed whether or not a predetermined strain has taken place in the object to be inspected. Such a confirmation can easily be carried out with the naked eye. 
     Positions where the thin-walled region  22  may be formed are the positions shown in  FIGS. 3B, 4A, and 4B . 
     (a) As shown in  FIG. 3B , one main surface  22   a  of the thin-walled region  22  may be formed so as to be the same as one main surface  12   a  of the main body  12 . 
     (b) As shown in  FIG. 4A , the other main surface  22   b  of the thin-walled region  22  may be formed so as to be the same as the other main surface  12   b  of the main body  12 . 
     (c) As shown in  FIG. 4B , the thin-walled region  22  may be formed at the center in the thickness direction of the main body  12 . 
     Of course, the thin-walled region  22  may also be located between the position shown in (a) and the position shown in (b), or between the position shown in (b) and the position shown in (c). It is desirable that the wall thickness tb (see  FIG. 3B ) of the thin-walled region  22  is less than or equal to such a wall-thickness as not to alleviate or lessen the concentration of stress on the main body  12 . However, if the thin-walled region  22  is too thin, there is a concern that deformation or cracking thereof may take place in the ceramic manufacturing processes such as molding and firing. Therefore, preferably, the wall thickness tb of the thin-walled region  22  is greater than or equal to 0.01 mm and less than or equal to 0.5 mm. 
     Further, as shown in  FIG. 5 , a visible member  24  preferably is disposed with an adhesive or the like on at least a portion facing toward the thin-walled region  22 , between the main body  12  and the object to be inspected (indicated by the two-dot chain line). In this case, by the totality or a portion of the thin-walled region  22  after undergoing secondary fracturing dropping off, the visible member  24  becomes exposed, and thus, by confirming the exposure of the visible member  24 , an observer can easily realize that a predetermined strain has occurred in the main body  12 . 
     A metal film such as Al (aluminum) or the like, a fluorescent coating material, or a coloring agent or the like can be used as the visible member  24 . The visible member  24  may be attached through an adhesive or the like to the object to be inspected, or may be attached through an adhesive or the like to a portion of the third structure for strain detection  10 C facing toward the object to be inspected. 
     Next, a structure for strain detection according to a fourth embodiment (hereinafter referred to as a fourth structure for strain detection  10 D) will be explained with reference to  FIGS. 6A to 6C . 
     As shown in  FIGS. 6A to 6C , the fourth structure for strain detection  10 D is of substantially the same configuration as the above-described third structure for strain detection  10 C, however, differs therefrom in that the thin-walled region  22  constituting the visualization structure  20  is provided in a frame shape. A portion surrounded by the frame-shaped thin-walled region  22  is thicker than the thin-walled region  22  and functions as a weight  26 . The thickness of the portion that functions as a weight (hereinafter referred to as a “weighted region  26 ”) is thicker than the thin-walled region  22 , and preferably is less than or equal to the thickness of the main body  12 . 
     Therefore, when a strain is generated in the main body  12  and the stress concentrated sections  14  experience a fracture (primary fracture), then taking this fracture as a starting point, a crack occurs in the thin-walled region  22 . The crack expands in a frame shape along the thin-walled region  22  due to the presence of the weighted region  26 , whereupon breakage or fracturing (secondary fracturing) of the thin-walled region  22  is induced. By the thin-walled region  22  undergoing such fracturing, the weighted region  26  falls off from the main body  12 . Consequently, by confirming whether or not the weighted region  26  has fallen off, it can be confirmed whether or not a predetermined strain has taken place in the object to be inspected. Such a confirmation can easily be carried out with the naked eye. 
     In this case as well, the visible member  24  (see  FIG. 5 ) preferably is disposed with an adhesive or the like on at least a portion facing toward the weighted region  26 , between the main body  12  and the object to be inspected. Consequently, by the weighted region  26  dropping off due to secondary fracturing of the thin-walled region  22 , the visible member  24  becomes exposed, and thus, by confirming the exposure of the visible member  24 , an observer can easily realize that a predetermined strain has occurred in the main body  12 . 
     Next, a structure for strain detection according to a fifth embodiment (hereinafter referred to as a fifth structure for strain detection  10 E) will be explained with reference to  FIGS. 7A to 7C . 
     As shown in  FIGS. 7A to 7C , the fifth structure for strain detection  10 E is of substantially the same configuration as the above-described fourth structure for strain detection  10 D, however, differs therefrom in that the thin-walled region  22 , which is provided in a frame shape, is formed with at least one small-diameter through hole  28  therein. In the example of  FIG. 7A , a plurality of through holes  28  are formed at equal intervals along the thin-walled region  22 . Of course, it is not necessary that the through holes  28  be equally spaced, and the sizes of the diameters thereof may all be the same or may be different from each other. 
     In this case, when the stress concentrated sections  14  experience a fracture (primary fracture) and a crack occurs in the thin-walled region  22 , development of the crack is accelerated due to the presence of the plurality of through holes  28 , and the weighted region  26  can assuredly be made to drop off from the main body  12  at an early stage. 
     Next, a structure for strain detection according to a sixth embodiment (hereinafter referred to as a sixth structure for strain detection  10 F) will be explained with reference to  FIGS. 8A to 8C . 
     As shown in  FIGS. 8A to 8C , the sixth structure for strain detection  10 F is of substantially the same configuration as the above-described second structure for strain detection  10 B, however, the shape of the through hole  16  thereof differs in the following ways. 
     More specifically, the shape of the through hole  16  is not a circular shape, but rather is a rectangular shape as viewed from above. Further, among the four apex portions  30   a  to  30   d , two of the apex portions  30   a  and  30   b , which constitute parts of the stress concentrated sections  14 , are formed with curved shapes, respectively. The other two apex portions  30   c  and  30   d  may be formed with curved shapes, or may be of shapes having corners formed therein. 
     In the sixth structure for strain detection  10 F, since the stress concentration factors of the stress concentrated sections  14  are changed by modifying the radius of curvature of the two apex portions  30   a  and  30   b  that constitute parts of the stress concentrated sections  14 , the size of the through hole  16  can be kept substantially constant, and the main body  12  can be fractured with a predetermined level of strain while ensuring visibility of the fracture. 
     Moreover, the above-described shape in the sixth structure for strain detection  10 F, and more specifically, the rectangular shape thereof as viewed from above, wherein among the four apex portions  30   a  to  30   d  thereof, the shapes of the two apex portions  30   a  and  30   b , which constitute parts of the stress concentrated sections  14 , are formed respectively in a curved shape, may also be applied to the visualization structures  20  of the third structure for strain detection  10 C through the fifth structure for strain detection  10 E which were described above. 
     In the above-described  FIGS. 2A to 2C  and  FIGS. 3A to 3C , the shape of the through hole  16  of the second structure for strain detection  10 B and the shape of the visualization structure  20  of the third structure for strain detection  10 C, and in particular, the shapes thereof as viewed from above, are circular. However, apart therefrom, as was described above, the shapes thereof may also be elliptical. 
     In this case, as shown in  FIGS. 9A to 9C , in the second structure for strain detection  10 B, a ratio (Dax/Day) of a diameter (axis in the x-direction) Dax of the through hole  16  in the x-direction, to a diameter (axis in the y-direction) Day of the through hole  16  in the y-direction may be less than 1, or alternatively, may be greater than 1. With the example of  FIG. 9A , an example is shown in which the ratio (Dax/Day) is less than 1. 
     Similarly, as shown in  10 A to  10 C, in the third structure for strain detection  10 C, a ratio (Dax/Day) of a diameter Dax of the visualization structure  20  in the x-direction, to a diameter Day of the visualization structure  20  in the y-direction may be less than 1, or alternatively, may be greater than 1. 
     Experimental examples (a third experimental example and a fourth experimental example) in relation to the second structure for strain detection  10 B and the third structure for strain detection  10 C will now be described. Zirconia B (see Table 1 above) was used as the ceramic thereof. 
     Third Experimental Example 
     In the third experimental example, the length La of the second structure for strain detection  10 B shown in  FIGS. 9A to 9C  was 50 mm, the width Lm thereof was 30 mm, and the thickness ta thereof was 0.5 mm, and for a case in which the diameter Day of the through hole  16  in the y-direction was 19 mm, a change in strain upon changing the diameter Dax of the through hole  16  in the x-direction was confirmed. The respective lengths Lae of both end portions  18   a  and  18   b  were 10 mm. More specifically, concerning Samples 21 to 23 shown in the following Table 4, using both of the end portions  18   a  and  18   b , the samples were fixed to a target object in which strain was to be detected. A tensile load was applied in a longitudinal direction of the main body  12 , and the strain therein at the time of fracturing of the main body  12  was confirmed. The diameter Dax in the x-direction of the through hole  16  was 19 mm in the case of Sample 21, 9.5 m in the case of Sample 22, and 2.85 mm in the case of Sample 23. The results are shown in the following Table 4. In Table 4, the lengths Lae of both end portions  18   a  and  18   b  are expressed as “end portion length”. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
             
            
               
                   
                   
               
               
                   
                 Main Body Dimensions 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 End Portion 
                   
                   
                 Through Hole 
                 Strain at 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Length 
                 Length 
                 Width 
                 Thickness 
                 Diameter 
                 Diameter 
                 Time of 
               
               
                   
                 La 
                 Lae 
                 Lm 
                 ta 
                 Day 
                 Dax 
                 Fracturing 
               
               
                   
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Sample 21 
                 50 
                 10 
                 30 
                 0.5 
                 19 
                 19 
                 0.188 
               
               
                 Sample 22 
                 50 
                 10 
                 30 
                 0.5 
                 19 
                 9.5 
                 0.119 
               
               
                 Sample 23 
                 50 
                 10 
                 30 
                 0.5 
                 19 
                 2.85 
                 0.048 
               
               
                   
               
            
           
         
       
     
     Fourth Experimental Example 
     In the fourth experimental example, the length La of the third structure for strain detection  10 C shown in  FIGS. 10A to 10C  was 50 mm, the width Lm thereof was 30 mm, and the thickness ta of the main body  12  was 0.5 mm, the thickness tb of the thin-walled region  22  of the visualization structure  20  was 0.1 mm, and for a case in which the diameter Day of the visualization structure  20  in the v-direction was 19 mm, a change in strain upon changing the diameter Dax of the visualization structure  20  in the x-direction was confirmed. The respective lengths Lae of both end portions  18   a  and  18   b  were 10 mm. More specifically, concerning Samples 31 to 33 shown in the following Table 5, using both of the end portions  18   a  and  18   b , the samples were fixed to a target object in which strain was to be detected. A tensile load was applied in a longitudinal direction of the main body  12 , and the strain therein at the time of fracturing of the main body  12  was confirmed. The diameter Dax in the x-direction of the visualization structure  20  was 19 mm in the case of Sample 31, 9.5 mm in the case of Sample 32, and 2.85 mm in the case of Sample 33. The results are shown in the following Table 5. In Table 5, the lengths Lae of both end portions  18   a  and  18   b  are expressed as “end portion length”. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
             
            
               
                   
                   
               
               
                   
                 Main Body Dimensions 
                 Visualization Structure 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 End Portion 
                   
                   
                 Thin-Walled 
                   
                   
                 Strain at 
               
               
                   
                 Length 
                 Length 
                 Width 
                 Thickness 
                 Region 
                 Diameter 
                 Diameter 
                 Time of 
               
               
                   
                 La 
                 Lae 
                 Lm 
                 ta 
                 Thickness tb 
                 Day 
                 Dax 
                 Fracturing 
               
               
                   
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Sample 31 
                 50 
                 10 
                 30 
                 0.5 
                 0.1 
                 19 
                 19 
                 0.204 
               
               
                 Sample 32 
                 50 
                 10 
                 30 
                 0.5 
                 0.1 
                 19 
                 9.5 
                 0.123 
               
               
                 Sample 33 
                 50 
                 10 
                 30 
                 0.5 
                 0.1 
                 19 
                 2.85 
                 0.048 
               
               
                   
               
            
           
         
       
     
     From Table 4 and Table 5, it can be understood that even if the shapes of the through hole  16  and the visualization structure  20  (the shapes thereof as viewed from above) are elliptical, it is possible for the main body  12  to be fractured with a predetermined level of strain by modifying the diameters of the through hole  16  and the visualization structure  20 , for example, by modifying only the diameter Dax in the x-direction, only the diameter Day in the y-direction, or both the diameter Dax in the x-direction and the diameter Day in the y-direction. More specifically, by suitably changing the dimension Lc in the one direction of the stress concentrated sections  14 , the main body  12  can be fractured with a predetermined strain. 
     Moreover, as shown in the above examples, it is necessary to set the two diameters (axes) of the elliptical shape in the x-direction and the y-direction, respectively. If Dax and Day are equal (i.e., in the case of a circle), the diameters Dax and Day may be set in any direction. 
     Further, the elliptical shape described above may also be applied to the visualization structure  20  of the fourth structure for strain detection  10 D and the fifth structure for strain detection  10 E. 
     Incidentally, the main body  12  of the above-described first structure for strain detection  10 A through the sixth structure for strain detection  10 F may be constituted from both end portions  18   a  and  18   b  and the central portion  12   c.    
     With the first structure for strain detection  10 A, as shown in  FIGS. 1A to 1C , both end portions  18   a  and  18   b  and the central portion  12   c  of the main body  12  have the same thickness, respectively, and one main surface  32   a  of both of the end portions  18   a  and  18   b , and the one main surface  12   a  of the central portion  12   c  of the main body  12  are flush with each other, and further, the other main surface  32   b  of both of the end portions  18   a  and  18   b  and the other main surface  12   b  of the central portion  12   c  of the main body  12  are flush with each other. 
     With the second structure for strain detection  10 B and the third structure for strain detection  10 C, within the central portion  12   c  of the main body  12 , a portion thereof other than the through hole  16  or the visualization structure  20 , and both end portions  18   a  and  18   b  have the same thickness, respectively, and the one main surface  32   a  of both of the end portions  18   a  and  18   b , and the one main surface  12   a  of the central portion  12   c  of the main body  12  are flush with each other, and further, the other main surface  32   b  of both of the end portions  18   a  and  18   b  and the other main surface  12   b  of the central portion  12   c  of the main body  12  are flush with each other. 
     Although the structures described above are acceptable, apart therefrom, as shown in  FIG. 11A to 12B , the thickness tae of both of the end portions  18   a  and  18   b  may be made greater than the thickness ta of the central portion  12   c  of the main body  12 . More specifically, both end portions  18   a  and  18   b  may be formed to be thick-walled respectively, and steps  34  may be formed respectively between the central portion  12   c  and both end portions  18   a  and  18   b  of the main body  12 .  FIGS. 11A to 12B  show examples of being applied to the third structure for strain detection  10 C. In the examples shown in  FIGS. 1A to 10C , the thickness of the central portion  12   c  is the same as the thickness of both of the end portions  18   a  and  18   b , and therefore, the thickness of the main body  12  is expressed as “ta”. However, in the examples of  FIGS. 11A to 12B , since the thickness of both end portions  18   a  and  18   b  is greater than the thickness of the central portion  12   c  of the main body  12 , the thickness of the central portion  12   c  is expressed as “ta”, whereas the thickness of both end portions  18   a  and  18   b  is expressed as “tae”. 
     According to the examples shown in  FIGS. 11A to 12B , using thick-walled sections  40   a  and  40   b  of both of the end portions  18   a  and  18   b , it is possible to easily fix the main body  12  to the object to be inspected. It is desirable for the thickness tae of both end portions  18   a  and  18   b  to be greater than or equal to 1 mm, in order to prevent interference between the central portion  12   c  and the object to be inspected. On the other hand, if the thickness tae of both end portions  18   a  and  18   b  is too thick, since the difference in wall-thickness from the central portion  12   c  becomes excessively large at the time of manufacturing the structure for strain detection, the central portion  12   c  becomes deformed, or cracks are generated between both of the end portions  18   a  and  18   b . Therefore, it is preferable for the thickness tae of both end portions  18   a  and  18   b  to be less than or equal to 10 mm. 
     In the case of using surfaces of the thick-walled sections  40   a  and  40   b  of both end portions  18   a  and  18   b , and furthermore, fixing them to the object to be inspected with an adhesive or the like, it is desirable that the following conditions (a) and (b) are satisfied. The surfaces of the thick-walled sections  40   a  and  40   b  make up the other main surface  32   b  in the examples of  FIGS. 11A and 11B , the one main surface  32   a  in the example of  FIG. 12A , and the one main surface  32   a  or the other main surface  32   b  in the example of  FIG. 12B . 
     (a) The respective shapes of both end portions  18   a  and  18   b  are equivalent with each other.
 
(b) The areas of the surfaces of the thick-walled sections  40   a  and  40   b  of both end portions  18   a  and  18   b  are sufficiently large to support the load generated in the structure for strain detection at a time of reaching a predetermined amount of strain in the object to be inspected. Moreover, as shown in  FIG. 11B , the surface areas of the thick-walled sections  40   a  and  40   b  are obtained by multiplying the length Lae along the lengthwise direction of the main body  12  by the length (width Lme) along the widthwise direction of the main body  12 .
 
     Further, it is preferable for the boundary portions between each of the steps  34  and the central portion  12   c  of the main body  12  to be formed in a curved shape. Owing to this feature, concentration of stresses at the boundary portions can be alleviated. In this case, the radius of curvature of the boundary portions is preferably 0.5 mm R or greater. 
     Fifth Experimental Example 
     In the fifth experimental example, the length La of the main body  12  of the structure for strain detection shown in  FIGS. 11A and 11B  was 100 mm, the width Lm thereof was 30 mm, the thickness (thickness ta of the central portion  12   c ) of the main body  12  was 0.5 mm, the lengths Lae (lengths along the lengthwise direction of the main body  12 ) of the thick-walled sections of both end portions  18   a  and  18   b  were 25 mm, the widths Lme (lengths along the widthwise direction of the main body  12 ) of the thick-walled sections of both end portions  18   a  and  18   b  were 30 mm, respectively, the thickness tb of the thin-walled region  22  of the visualization structure  20  was 0.1 mm, and for a case in which the diameter Day of the visualization structure  20  in the y-direction was 19 mm, a change in strain was confirmed upon changing the thicknesses tae of both end portions  18   a  and  18   b , the radius of curvature (indicated as “boundary portion” in Table 6) of the boundary portions between the central portion  12   c  and both end portions  18   a  and  18   b , as well as changing the diameter Dax in the x-direction of the visualization structure  20 . More specifically, concerning Samples 41 to 43 shown in the following Table 6, a tensile load was applied in the longitudinal direction of the main body  12 , and the strain therein at the time that the main body  12  experienced fracturing was confirmed. The thickness tae of both end portions  18   a  and  18   b  was 10 mm in the case of Sample 41, 3 mm in the case of Sample 42, and 1 mm in the case of Sample 43. The diameter Dax in the x-direction of the visualization structure  20  was 19 mm in the case of Sample 41, 7.26 mm in the case of Sample 42, and 2.85 mm in the case of Sample 43. The results are shown in the following Table 6. Moreover, the structure for strain detection that was used in the fifth experimental example was constituted by zirconia B (see Table 1 above) as a ceramic. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 6 
               
             
            
               
                   
                   
               
               
                   
                 Main Body Dimensions 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Central 
                   
                 Visualization Structure 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Portion 
                 End Portions 
                 Thin-Walled 
                   
                   
                 Strain at 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Length 
                 Width 
                 Thickness 
                 Length 
                 Width 
                 Thickness 
                 Boundary 
                 Region 
                 Diameter 
                 Diameter 
                 Time of 
               
               
                   
                 La 
                 La 
                 ta 
                 Lae 
                 Lme 
                 tae 
                 Portions 
                 Thickness tb 
                 Day 
                 Dax 
                 Fracturing 
               
               
                   
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm R) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Sample 41 
                 100 
                 30 
                 0.5 
                 25 
                 30 
                 10 
                 3 
                 0.1 
                 19 
                 19 
                 0.204 
               
               
                 Sample 42 
                 100 
                 30 
                 0.5 
                 25 
                 30 
                 3 
                 1 
                 0.1 
                 19 
                 7.25 
                 0.100 
               
               
                 Sample 43 
                 100 
                 30 
                 0.5 
                 25 
                 30 
                 1 
                 0.5 
                 0.1 
                 19 
                 2.85 
                 0.048 
               
               
                   
               
            
           
         
       
     
     From Table 6, it can be understood that by changing the thickness tae of the thick-walled sections of both end portions  18   a  and  18   b , the radius of curvature of the boundary portions between the central portion  12   c  and both end portions  18   a  and  18   b , and the diameter of the visualization structure  20 , e.g., only the diameter Dax in the x-direction or only the diameter Day in the y-direction, or alternatively, both the diameter Dax in the x-direction and the diameter Day in the y-direction, it is possible for the main body  12  to be fractured with a predetermined level of strain. More specifically, by suitably changing the thickness tae of the thick-walled sections of both end portions  18   a  and  18   b , and the dimension Lc in the one direction of the stress concentrated sections  14 , the main body  12  can be fractured with a predetermined strain. 
     In addition, in the case that the tensile shear adhesive strength of the adhesive for attaching the main body  12  to the object to be inspected is 20 N/mm 2 , since the thick-walled sections  40   a  and  40   b  of both end portions  18   a  and  18   b  are of the same shape, and the area that can be used for bonding can be assured to be 750 mm 2  (25 mm×30 mm) on each of the respective sides, it is possible to support a load of 15,000 N. The loads at which fracturing occurs of Samples 41, 42 and 43 are values between about 5,500 N and 1,300 N, respectively, and sufficient adhesive strength can be secured. 
     As shown in  FIG. 11A , the one main surface  32   a  of both of the end portions  18   a  and  18   b , and the one main surface  12   a  of the central portion  12   c  of the main body  12  may be flush with each other, and further, the steps  34  may be formed between the other main surface  32   b  of both of the end portions  18   a  and  18   b  and the other main surface  12   b  of the central portion  12   c  of the main body  12 . 
     Alternatively, as shown in  FIG. 12A , the steps  34  may be formed between the one main surface  32   a  of both of the end portions  18   a  and  18   b  and the one main surface  12   a  of the central portion  12   c  of the main body  12 , and further, the other main surface  32   b  of both of the end portions  18   a  and  18   b , and the other main surface  12   b  of the central portion  12   c  of the main body  12  may be flush with each other. 
     Alternatively, as shown in  FIG. 12B , the steps  34  may be formed between the one main surface  32   a  of both of the end portions  18   a  and  18   b  and the one main surface  12   a  of the central portion  12   c  of the main body  12 , and further, the steps  34  may be formed between the other main surface  32   b  of both of the end portions  18   a  and  18   b  and the other main surface  12   b  of the central portion  12   c  of the main body  12 . 
     Furthermore, boundary portions  36  between each of the steps  34  and the central portion  12   c  of the main body  12  are preferably formed in a curved shape, whereby concentration of stresses on the boundary portions  36  can be alleviated. In this case, the boundary portions  36  are preferably formed in a curved shape having a radius of curvature of 0.5 mm R or greater. 
     Next, there will briefly be described below a method for manufacturing the above-described first structure for strain detection  10 A through the sixth structure for strain detection  10 F. The term “structures for strain detection” will be used when referring collectively to the first structure for strain detection  10 A through the sixth structure for strain detection  10 F. 
     First, it should be noted that the method of manufacturing the first structure for strain detection  10 A through the sixth structure for strain detection  10 F is not particularly limited, and any of a doctor blade method, an extrusion method, a gel casting method, a powder pressing method, or an imprint method, etc., may be used arbitrarily. With respect to a complex shape, such as in the third structure for strain detection  10 C through the fifth structure for strain detection  10 E, it is particularly preferable for such structures to be manufactured using a gel cast method. In a preferred embodiment, the third structure for strain detection  10 C through the fifth structure for strain detection  10 E can be obtained by casting a slurry containing a ceramic powder, a dispersion medium and a gelling agent, allowing the slurry to gel to thereby obtain a molded body, and then subjecting the molded body to sintering (see Japanese Laid-Open Patent Publication No. 2001-335371). With respect to a simple shape, such as in the first structure for strain detection  10 A and the second structure for strain detection  10 B, a tape forming method such as a doctor blade method or the like is preferred. 
     As the material of the structures for strain detection, it is particularly preferable to use a raw material in which a 3 mol % yttria (Y 2 O 3 ) auxiliary agent is added to a zirconia powder. Although yttria is preferred as the auxiliary agent, calcia (CaO), magnesia (MgO), and the like, can also be offered as examples. 
     The following methods may be mentioned as suitable techniques for the gel casting method. 
     (1) Together with an inorganic powder, a prepolymer, such as polyvinyl alcohol, epoxy resin, phenolic resin or the like, which serves as a gelling agent, is dispersed in a dispersion medium along with a dispersing agent, to thereby prepare a slurry, and after casting, the slurry is solidified by three-dimensional crosslinking using a crosslinking agent and gelatinization thereof. 
     (2) A slurry is solidified by chemically bonding a gelling agent and an organic dispersion medium having a reactive functional group. This method is the method disclosed in Japanese Laid-Open Patent Publication No. 2001-335371 of the present applicant. 
     It is a matter of course that the structures for strain detection according to the present invention are not limited to the embodiments described above, and various additional or modified configurations can be adopted therein without departing from the scope of the present invention. 
     For example, in the third structure for strain detection  10 C through the fifth structure for strain detection  10 E, the thin-walled region  22  may be constituted by a conductive ceramic. In this case, since the electrical characteristics of the conductive ceramic are changed by fracturing (secondary fracturing) of the thin-walled region  22 , by perceiving such a change as an electrical signal and displaying it on a display or the like, the fact that a predetermined strain has occurred can be visualized.