JP4685541B2 - Bolt tension test method - Google Patents

Description

  The present invention relates to a bolt tensile test method, and more particularly, to a method that enables accurate measurement of bolt elongation.

  The bolt tension test is standardized by ISO898-1 and JIS B 1051 “Mechanical properties of carbon steel and alloy steel fastening parts—Part 1: Screws and studs”. At present, the tensile test of bolts in the state of the product only measures the maximum load, but from the viewpoint that quality assurance at a higher level should be realized, it depends on the bolt product to investigate the yield characteristics and ductility A revision to add a tensile test method is underway, and a draft committee (CD) is currently being prepared and technical feasibility is being discussed.

  In the new bolt tension test defined by ISO / CD898-1, as shown in FIG. 4A, the first jig A for screwing the thread portion Wa of the bolt W to be tested and the bolt W Using a tensile testing machine having a second jig B that receives the head Wb, the length of the idle screw portion Wa ′ (the portion of the screw portion Wa that does not screw into the first jig A) is the nominal size of the bolt W The bolt W is set so that the diameter is equal to or larger than d, for example, 1.2d, and the second jig B is moved relative to the first jig A in the axial direction of the bolt W to apply a tensile load to the bolt W. .

  Here, the first jig and the second jig are respectively connected to the testing machine main body and the cross head via a universal joint, and the second jig is moved relative to the first jig by the movement of the cross head. The Then, by using a universal testing machine having a function of recording the load acting on the cross head and the displacement of the cross head, the tensile load F acting on the bolt as shown in FIG. If a load-elongation curve showing the relationship is prepared, the mechanical properties including the yield characteristics and ductility of the bolt such as the nominal tensile strength Rmf, 0.4% proof stress Rpf, and elongation at break Af of the bolt can be obtained from this load-elongation curve. The value to be expressed can be obtained. Rmf is a value obtained by dividing the maximum load Fmt by the effective sectional area of the bolt, Rpf is a value obtained by dividing the load Fpt that causes a permanent elongation of 0.4% by the effective sectional area of the bolt, and Af is a value at the time of fracture. Is obtained by dividing the permanent elongation ΔLp by the length (= 1.2d) of the idle screw portion Wa ′. Fpt is the load at the intersection of the straight line L2 and the load-elongation curve, which are offset in parallel by 0.4% of the elongation (= 0.0048d) with respect to the straight line L1 applied to the elastic region of the load-elongation curve. Yes, ΔLp is the amount of elongation between the straight line L3 parallel to the straight line L1 passing through the breaking point and the straight line L1. The straight line L1 is a straight line passing through the upper reference point and the lower reference point in the elastic region of the load-elongation curve, and the upper reference point and the lower reference point are respectively assumed to have a guaranteed load defined by ISO898-1 as Fp. The point on the load-elongation curve is set to a point where the load becomes 0.7 Fp and the point where the load becomes 0.4 Fp.

  By the way, in the load-elongation curve created by the universal testing machine, non-linearity occurs in the elastic region due to the influence of the connecting portion of the jig, and the fitting accuracy of the straight line L1 is deteriorated. That is, in order to improve the fitting accuracy of the straight line L1, it is desirable to lower the load at the lower reference point to about 0.2 Fp. However, at such a low load, the load-elongation curve becomes non-linear. The load of the lower reference point has to be set as high as 0.4 Fp, and the accuracy of fitting the straight line L1 becomes worse. In addition, the deformation of the testing machine main body and jigs are superimposed, the inclination in the elastic region becomes gentle, the crossing angle at the intersection of the straight line L2 and the load-elongation curve becomes small, and Fpt reading error occurs. It becomes easy and it becomes difficult to obtain | require the mechanical property of a volt | bolt with high precision combined with the fitting precision of the straight line L1 worsening.

  In order to solve such a problem, as shown in FIG. 5, a stroke type extensometer C is attached to the outer surface of the second jig B, and a contact plate C2 in which the rod C1 of the extensometer C is attached to the first jig A. It is also conceivable that the displacement of the second jig B in the bolt axis direction with respect to the first jig A is measured by an extensometer C and the elongation of the bolt W is obtained based on this. The load-elongation curve created by this method is as shown by line b in FIG. 3, and the slope in the elastic region becomes steeper than the load-elongation curve by the universal testing machine shown by line c in FIG. However, in this method, since the extensometer C is laterally separated from the bolt W, the distance between the extensometer C and the contact plate C2 changes due to the inclination and deflection of the jigs A and B. The elongation of the bolt W cannot be measured accurately. For example, as indicated by line b, the elongation may swing in the negative direction immediately after the start of the test.

As a general tensile test method, a method of measuring the elongation of a test piece with an extensometer set on the test piece is known (see, for example, Patent Document 1). This extensometer includes a pair of arms having sharp tip portions and detection means for detecting a change in the distance between both arms, and the tip portions of both arms abut against two test points on the test piece. In this way, an extensometer is set on the test piece, and the elongation between the test specimens is measured. It is conceivable to set such an extensometer on the bolt in the bolt tensile test. Here, most of the parts extending in the tensile test of the bolt are free screw parts, and in order to measure the extension of the free screw part, the tip of one arm of the extensometer matches the surface of the first jig. It is necessary to set so as to abut against the part of the idle screw part. However, in order to avoid interference with the first jig, the tip of the arm must butt against the part of the free screw part away from the surface of the first jig. It is not possible to accurately measure the elongation of In addition, it takes time to set the extensometer on the bolt, and the bolt may be damaged by hitting the tip of the arm against the bolt. May also occur.
JP-A-11-83419

  In view of the above points, an object of the present invention is to provide a bolt tension test method that can accurately measure the elongation of the bolt and improve the measurement accuracy of the mechanical properties of the bolt.

In order to solve the above-mentioned problems, the present invention uses a tensile testing machine comprising a first jig for screwing a threaded portion of a bolt to be tested and a second jig for receiving the head of the bolt, In the bolt tensile test method for measuring the tensile load acting on the bolt and the elongation of the bolt by moving the second jig relative to the first jig relative to the first jig,
The elongation of the bolt is measured by using an extensometer disposed between the first jig and the second jig, and this extensometer is seated on the surface of the first jig facing the second jig. A first seating portion in which a hole for inserting a bolt is formed, and a second seating portion in which a hole for inserting the bolt is seated on a surface of the second jig facing the first jig, The extensometer is configured to detect the axial displacement of the bolt of the second seating portion relative to the first seating portion, and the extensometer is formed in an oval annular shape having a major axis direction perpendicular to the axial direction of the bolt. And a detecting means for detecting distortion of curved portions on both sides in the long axis direction of the strain generating body, and one central portion and the other of the pair of long side portions along the long axis direction of the strain generating body The first seating portion and the second seating portion are respectively provided in the central portion of the first seating portion .

  According to the present invention, since the bolt is inserted through the holes of the first and second seating portions, the seating portions are seated substantially concentrically with the bolts on the first and second jigs. . Accordingly, the elongation of the bolt portion including the free screw portion located between the first jig and the second jig can be accurately measured without being affected by the inclination or bending of each jig, or the deformation of the main body of the testing machine. Can be measured. As a result, the linearity in the elastic region of the load-elongation curve is ensured, and the gradient in the elastic region becomes steep, and various values representing the mechanical properties of the bolt can be obtained from the load-elongation curve with high accuracy. .

  Moreover, since it is not necessary to set an extensometer on the bolt, the tensile test can be performed efficiently, and there is no possibility that the bolt is damaged or the extensometer is damaged when the bolt is broken.

  It should be noted that the displacement of the second seat relative to the first seat can be detected in a non-contact manner with a laser or the like, but in terms of cost, the strain is connected to both the first and second seats. It is advantageous to obtain a displacement of the second seating portion relative to the first seating portion by providing a body and detecting the strain of the strain generating body with a detecting means such as a strain gauge. In particular, the extensometer detects the distortion of the ellipse formed in the shape of an ellipse whose major axis is the direction perpendicular to the axial direction of the bolt, and the distortion of the curved portions on both sides of the major axis of the strain generator. It is desirable that a first seating portion and a second seating portion are provided respectively at one central portion and the other central portion of the pair of long side portions along the longitudinal direction of the strain generating body. . According to this, even if the distortion of the bending portion on one side in the major axis direction of the strain generating body is different from the distortion of the bending portion on the other side due to the inclination of each jig, the distortion of the bending portions on both sides is averaged. Thus, the elongation of the bolt can be accurately measured.

  In addition, the strain body has an elastic force in a direction that widens the distance between the pair of long side portions, and the first and second seating portions are pressed against the first and second jigs by the elastic force. By doing so, it is not necessary to fix each seating part to each jig with a screw or the like, and the work of setting the extensometer can be facilitated and the efficiency can be improved.

  Referring to FIG. 1, reference numeral 1 denotes a main body of a tensile testing machine, and a first jig 3 is connected to a lower end portion of the testing machine main body 1 via a universal joint 2. A second jig 6 is connected to the cross head 4 that is movably supported by a universal joint 5.

  The first jig 3 includes a jig body 31 formed in a rectangular frame shape, and an adapter 32 that is detachably fitted to the upper side of the jig body 31 from below. Is composed of a jig body 61 formed in a rectangular frame shape and an adapter 62 detachably fitted to the lower side of the jig body 61 from above. As shown in FIG. 2, the adapter 32 of the first jig 3 is formed with a screw hole 32a that is long in the vertical direction for screwing the threaded portion Wa of the bolt W to be tested. The adapter 62 of the two jigs 6 is formed with a horizontal seating surface 62a that receives the head portion Wb of the bolt W, and an insertion hole 62b through which the shaft portion of the bolt W extending downward from the seating surface 62a is inserted.

  In the tensile test of the bolt W, the bolt W is inserted from above into the insertion hole 62b of the adapter 62 of the second jig 6, and the head Wb of the bolt W is seated on the seating surface 62a of the adapter 62. The screw portion Wa is screwed into the screw hole 32a of the adapter 32 of the first jig 3 so as to produce an idle screw portion Wa ′ having a length 1.2 times the nominal diameter of the bolt W. After the bolt W is set in the testing machine in this way, the cross head 4 is moved upward to move the second jig 6 away from the first jig 3 in the axial direction of the bolt W, that is, upward. Then, a tensile load is applied to the bolt W via the adapter 62 of the second jig 6.

  Here, an extensometer 7 for measuring the elongation of the bolt W is disposed between the first jig 3 and the second jig 6. As shown in FIG. 2, the extensometer 7 includes a strain generating body 71 formed in an oval ring having a major axis in a direction perpendicular to the axial direction (vertical direction) of the bolt W, A pair of detection means 72 and 72 for detecting distortion of the curved portions 71a and 71a on both sides in the long axis direction of the distortion body 71 are provided. Each detecting means 72 includes a strain gauge 72a attached to the front and back of each bending portion 71a.

  A first seat 73 that seats on the surface of the first jig 3 facing the second jig 6 is provided at the center of the lower long side 71b along the major axis direction of the strain generating body 71. A second seating portion 74 that seats on the surface of the second jig 6 facing the first jig 3 is provided at the center of the upper long side portion 71b along the major axis direction of the strain generating body 71. . The first and second seating portions 73, 74 are fixed blocks 73a, 74a composed of two upper and lower plates screwed to the central portion of each long side portion 71b of the strain body 71, and the fixed blocks 73a, The collars 73c and 74c are screwed into the screw holes 73b and 74b formed in the 74a, and the bolts W are inserted into the inner holes 73d and 74d of the collars 73c and 74c. Thus, the first seating portion 73 is seated concentrically with the bolt W on the upper end surface of the adapter 32 of the first jig 3 at the lower end of the collar 73c, and the second seating portion 74 is secondly seated at the upper end of the collar 74c. The bolt 6 is seated concentrically on the lower end surface of the adapter 62 of the tool 6. Here, the collars 73c and 74c are exchanged in accordance with the diameter of the bolt W. In addition, a hole through which the collars 73 c and 74 c are inserted is formed at the center of each long side portion 71 b of the strain generating body 71. In addition, the strain body 71 is divided at the central portion of the long side portion 71b, and one half portion and the other half portion of the strain body 71 in the long axis direction are connected via the fixing blocks 73a and 74a. It is also possible to constitute an ellipsoidal strain generating body 71.

  By the way, the first and second seats 73 and 74 may be fastened to the first and second jigs 3 and 6 with screws, but this requires time and labor to set the extensometer 7. . Therefore, in the present embodiment, the strain body 71 is formed of a spring plate so as to have an elastic force in the direction of widening the interval between the upper and lower long side portions 71b, 71b, and the first and second are caused by this elastic force. The respective seating portions 73 and 74 are pressed against the first and second jigs 3 and 6. According to this, it takes less time to set the extensometer 7, and the efficiency of the tensile test can be improved.

  According to this embodiment, the first and second seats 73 and 74 of the extensometer 7 are seated substantially concentrically with the bolt W on the first and second jigs 3 and 6 as described above. become. Therefore, the axial displacement of the bolt W of the second seating portion 74 relative to the first seating portion 73 measured from the strain of the curved portions 71a on both sides in the longitudinal direction of the strain generating body 71 detected by the detecting means 72 of the extensometer 7 Accurately represents the elongation of the portion of the bolt W including the free screw portion Wa ′ located between the first jig 3 and the second jig 6. Accordingly, it is possible to accurately measure the elongation of the bolt W without being affected by the inclination and bending of the jigs 3 and 6 and the deformation of the testing machine main body 1. Even if the distortion of the bending portion 71a on one side in the long axis direction of the strain generating body 71 is different from the distortion of the bending portion 71a on the other side due to the inclination of the jigs 3 and 6, the bending portions 71a, The elongation of the bolt W can be accurately measured by averaging the distortion of 71a.

  The load-elongation curve created based on the axial displacement of the bolt W of the second seating portion 74 relative to the first seating portion 73 is as shown by line a in FIG. Linearity is ensured and the gradient in the elastic region becomes steep. Accordingly, the load at the lower reference point for determining the straight line (straight line L1 in FIG. 4B) to be applied to the elastic region of the load-elongation curve can be reduced, and the fitting accuracy of the straight line L1 is improved. Since the slope of the straight line L1 becomes steep, the intersection angle of the intersection of the straight line L2 and the load-elongation curve offset by 0.4% of the straight line L1 increases, and the reading accuracy of this intersection also improves. To do. As a result, various values such as the nominal tensile strength Rmf, the 0.4% proof stress Rpf, and the elongation at break Af that express the mechanical properties of the bolt W can be obtained with high accuracy.

  In addition, unlike the extensometer used in the tensile test of test pieces other than bolts, the extensometer 7 of the present embodiment is not set on the bolt W. Therefore, the tensile test can be performed efficiently. There is no risk of scratching or causing damage to the extensometer 7 when the bolt W is broken.

  As mentioned above, although embodiment of this invention was described with reference to drawings, this invention is not limited to this. For example, the extensometer 7 can be configured to detect the displacement of the second seating portion 74 with respect to the first seating portion 73 in a non-contact manner using a laser or the like. In the above embodiment, the second jig 6 is moved. However, the first jig 3 may be moved. In short, the second jig 6 is bolted to the first jig 3. What is necessary is just to make it relatively move so that it may leave in the axial direction of W.

The whole front view which shows embodiment of the tensile tester used for implementation of the method of this invention. The cutting front view of the principal part of the tensile testing machine of FIG. A graph showing a load-elongation curve of a bolt. (a) The figure which shows the outline | summary of the tension test of the bolt prescribed | regulated by ISO / CD898-1, (b) The figure which shows the calculation method of the mechanical property of a bolt based on the load-elongation curve of a bolt. The figure which shows the outline | summary of the tension test of the volt | bolt using a stroke type extensometer.

Explanation of symbols

  W ... Bolt, Wa ... Screw part, Wb ... Head, 3 ... First jig, 6 ... Second jig, 7 ... Extensometer, 71 ... Strain body, 71a ... Curved part, 71b ... Long side part, 72 ... detecting means, 73 ... first seating portion, 74 ... second seating portion, 73d, 74d ... holes through which bolts are inserted.

Claims (2)

Using a tensile testing machine comprising a first jig for screwing a threaded portion of a bolt to be tested and a second jig for receiving the head of the bolt, the second jig is bolted to the first jig. In the bolt tensile test method of measuring the tensile load acting on the bolt and the elongation of the bolt by relatively moving in the axial direction of the bolt,
The elongation of the bolt is measured by using an extensometer disposed between the first jig and the second jig, and this extensometer is seated on the surface of the first jig facing the second jig. A first seating portion in which a hole for inserting a bolt is formed, and a second seating portion in which a hole for inserting the bolt is seated on a surface of the second jig facing the first jig, Configured to detect axial displacement of the bolt of the second seating portion relative to the first seating portion ,
Further, the extensometer detects strains of an ellipse formed in an elliptical shape having a major axis direction perpendicular to the axial direction of the bolt, and strains of curved portions on both sides in the major axis direction of the strain generator. The first seating portion and the second seating portion are provided at one central portion and the other central portion of the pair of long sides along the longitudinal direction of the strain generating body . A bolt tensile test method characterized by the above. The strain body has an elastic force in a direction to widen a distance between the pair of long side portions, and the first and second seating portions are applied to the first and second jigs by the elastic force. The bolt tension test method according to claim 1 , wherein the bolt is pressed.

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