JP5644436B2 - Deformation state evaluation method of cold-formed square steel pipe - Google Patents

Description

  The present invention relates to a deformation state evaluation method for determining whether or not a cold-formed square steel pipe having a small width-to-thickness ratio that has reached the end when it is bent reaches a final deformation state. Cross-section (width-to-thickness ratio) belonging to the FA rank described in the Building Guidance Division, Structure-Related Technical Standards for Buildings 2007, National Gazette Sales Cooperative (ISBN: 978-4-915392-09-2), 200805 The present invention relates to a material suitable for application to a cold-formed square steel pipe having 33 × √ {square root over (235 / (material design standard strength))}.

  In the structural calculation such as Chapter 6 possessed horizontal strength calculation, the structural related technical standard manual for buildings (2007 version), the deformation capacity of the frame is determined by buckling as long as the joint does not break. It is described as being limited, and the plastic deformation capacity of the member in the case where the end of the member is buckled is represented by the structural characteristic coefficient Ds. Based on the relationship between the member size and the deformation capacity, the presence or absence of the brace, the type, The numerical value is determined according to the type of pillar and beam.

  However, to determine whether a building or member is actually in a limit state against the horizontal force of an earthquake, the limit plasticity ratio (the plasticity ratio at break (ratio of the deformation angle of the member to the deformation angle at yield)) ) And where the interlaminar deformation angle of the building must be compared with the limit value, making accurate determination difficult.

  The reason for this is that the Ministry of Land, Infrastructure, Transport and Tourism has announced a mathematical formula for determining the critical deformation angle of a member (Hei 12 No. 1457 No. 3), but in general, it is difficult to determine the critical plasticity ratio and the deformation angle. Even if it is obtained, it does not become a stable value. There is also an idea that it is not always appropriate to determine the limit state based only on the deformation angle and plasticity factor of the member.

  On the other hand, there is no clear guideline for determining whether the critical state of a column-beam rigid frame is not a buckling of a member but ends with a fracture of a beam-column joint. It is mentioned that FEM analysis, which is an important method for obtaining a value approximating the value, is not necessarily suitable for predicting the fracture phenomenon of a member.

  FEM analysis is used for stress analysis of various structures as an important technique for obtaining values approximate to experimental values as stress and strain of structures (for example, Non-Patent Documents 1 and 2). There is no prior literature dealing with the analysis of. For example, Patent Documents 1 to 3 describe a method for estimating stress and strain of a desired part without performing FEM analysis in order to solve the fact that FEM analysis requires a large amount of calculation and takes a long time to obtain a result. This is not intended to suggest application to the analysis of the fracture phenomenon of members.

JP 2002-81134 A JP 2008-31818 A JP 2008-31819 A

Listed Teruyasu, short column compression behavior of high strength round steel pipe with small diameter-thickness ratio, Architectural Institute of Japan (507), 123-129, 199880530 Naoto Shimono et al., Monotonic Tensile Test of Long Stiffener-Shaped Circular Steel Pipe Concrete Column / Beam Flange Joints, Annual Conference of Architectural Institute of Japan, September 1998

  When trying to simulate the fracture phenomenon by FEM analysis, it is extremely difficult to reproduce “behavior during negative gradient after stress reaches its peak due to stress-strain relationship” and “abrupt cross-sectional change”. It is difficult to simulate the occurrence of a crack that precedes the fracture phenomenon, and it is difficult to theoretically estimate the stress state and propagation path of the crack tip and the influence of material toughness. Yes.

  Therefore, the present invention does not directly obtain the limit deformation capacity by directly simulating the fracture phenomenon of a cold-formed square steel pipe by FEM analysis, but obtains the stress-strain relationship of the part where fracture is expected by FEM analysis. It is an object of the present invention to provide a method for determining whether or not a final deformation state exists from a quantity.

  The present inventors consider that a member (cold forming) is a member formed of a cold-formed square steel pipe having a width-to-thickness ratio of 33 × √ (235 / (design standard strength of material)) or less, which is considered to end with a break. Square steel pipe) FEM analysis is performed by faithfully reproducing the shape of the welded part in the whole and the member, reproducing the stress-strain relationship of the base material part of the member, the weld heat affected zone of the base material and the welding material, and cold It was compared with the bending fracture test result of the member made of the formed square steel pipe.

  As a result, in the stress-strain relationship obtained for each of the base material part of the member, the weld heat-affected part of the base material and the weld metal part, the part where the strain proceeds most rapidly is the most dangerous part, and the equivalent strain is If it is judged that the point of time when the uniform elongation in the stress-strain relationship in the material test that constitutes the most dangerous part is reached is the end of the deformation of the member, the knowledge that it corresponds to the fracture position in the bending fracture experiment of the actual machine member with high accuracy is obtained. It was.

The present invention has been made by further study based on the above knowledge, that is, the present invention,
1. This is a method for determining the deformation state of a cold-formed square steel pipe that breaks by bending. The stress-strain relationship of each part of the cold-formed square steel pipe when bending acts is obtained by FEM analysis, and the local strain progresses the fastest. Determining the deformation state of a cold-formed square steel pipe, considering that the part is regarded as the most dangerous part, and determining when the equivalent strain at the most dangerous part reaches the uniform elongation of the material as the ultimate deformation state Method.
2. When performing FEM analysis, the stress-strain relationship in the weld heat affected zone of the cold-formed square steel pipe is determined using the hardness test result of the weld heat affected zone and the stress-strain relationship of the base metal. 2. The method for determining a deformation state of a cold-formed square steel pipe according to 1, wherein
3. A structure which includes a cold-formed square steel pipe and whose deformation performance is evaluated based on the deformation state determination method described in 1 or 2.

  INDUSTRIAL APPLICABILITY According to the present invention, the deformation state of a cold-formed square steel pipe when bending acts can be determined without performing a test using a full-scale test body that requires a lot of time and cost, and is extremely useful industrially. .

The figure explaining a bending tester. It is a figure which shows the structure of the test body 1 typically, (a) is a side view of the test body 1, (b) is sectional drawing of the center part of the test body 1 (AA sectional drawing of (a)). The figure which shows the FEM analysis model of the test body 1 shown in FIG. FIG. 4 is an enlarged view of a cold-formed square steel pipe and a welded portion of a through diaphragm in the FEM analysis model shown in FIG. 3. The figure which shows the stress-strain relationship in each element of a steel pipe base material (a flat plate part and a corner | angular part) and the welding part DEPO. The figure which shows an example of the hardness test result of a welding part. The figure explaining the setting method of the stress-strain relationship of a welding heat affected zone based on the strength estimation result using the hardness test result of a weld. The figure which shows the most dangerous part 14 specified in the test body 1 of FIG. The enlarged view of the most dangerous part 14 shown in FIG. 8 in a steel pipe cross section. The figure which shows the correlation of the actual machine test result of an Example, and a FEM analysis result. The flowchart of this invention.

  In the present invention, when a bending stress is applied to a cold-formed square steel pipe by FEM analysis, a site where the local strain proceeds most quickly is designated as the most dangerous site, and the equivalent strain in the most dangerous site is The point in time when the uniform elongation is reached is determined as the ultimate deformation state. Hereinafter, the present invention will be described in detail with reference to a test body simulating a column.

  FIG. 11 is a flowchart showing the procedure of the present invention. First, a member to be subjected to FEM analysis is divided into elements including a welded portion shape (STEP 1).

  FIG. 2 schematically shows the structure of the test body 1 used for explaining the present invention. FIG. 2A is a side view of the test body 1, and FIG. 2B is a cross-sectional view of the central portion of the test body 1 (A- A sectional view) is shown. The test body 1 has a cold-formed square steel pipe as a main body and welds two diaphragms through its central portion to reproduce a shape similar to that of an actual structure column.

  3 shows an FEM analysis model of the test body 1 shown in FIG. 2, FIG. 4 shows an enlarged view of a cold-formed square steel pipe and a welded portion of the through diaphragm in the FEM analysis model shown in FIG. A diaphragm 5 is attached to the welded portion, and the welded portion is composed of a welded portion (DEPO) and a weld heat affected zone 6. The base material and the attachment part of the diaphragm are welded with a backing metal 7.

  The FEM analysis model may be created for a part of the specimen using the symmetry of the specimen. The FEM analysis model of the specimen 1 shown in FIG. 3 is a 1/4 analysis model because the specimen 1 is vertically and horizontally symmetrical. When modeling a welded part with FEM analysis, three-dimensional laser shape measurement is performed on the shape of the welded part in the actual specimen so that the stress-concentrated part in the welded part, for example, the undercut is accurately reproduced as an element (mesh). Measure and model with equipment.

  Next, by FEM analysis, the progress of strain with increasing bending stress is compared with each element (mesh) from the stress-strain relationship for each element (mesh) when bending stress is applied (STEP2). The area where the increase of the deformation is large, that is, the place where the deformation speed is the fastest is set as the most dangerous place (STEP 3). FIG. 8 shows the most dangerous part 14 specified in the test body 1 of FIG. 2, and FIG. 9 shows an enlarged view of the most dangerous part 14 in the steel pipe cross section.

  The stress-strain relationship in each element when determining the most dangerous part is obtained in advance by conducting a tensile test on the material constituting the element. Note that the stress-strain relationship used in the FEM analysis is not the nominal stress-nominal strain relationship, but the true stress-true strain relationship obtained by conversion from the nominal stress-nominal strain relationship.

  FIG. 5 is a view showing a stress-strain relationship in each element of the steel pipe base material (flat plate portion / corner portion) and the welded portion DEPO. In the figure, the stress-strain line 9 is the steel pipe base material (flat plate portion), the stress-strain line. Reference numeral 10 denotes a steel pipe base material (corner portion), and a stress-strain line 11 denotes a stress-strain relationship in each element of the welded portion DEPO.

  Since the weld heat-affected zone has a narrow area in the actual specimen and cannot obtain the stress-strain relationship by collecting a tensile specimen, the stress-strain relationship is determined from the correlation between hardness and tensile strength.

  FIG. 6 shows an example of the hardness test result of the weld. The hardness test results in the figure show the results normalized by the hardness of the base material. Due to the linearity specified in the hardness conversion table (SAE-J-417) (3 times the Vickers hardness is the strength of the material), the "Vickers hardness" of the weld heat affected zone is 16% lower than the base metal on average. Therefore, it can be considered that the “strength” of the weld heat affected zone is 16% lower than the base material strength on average.

FIG. 7 shows a method for setting the stress-strain relationship of the weld heat affected zone based on the strength estimation result.
In the description, the rate of decrease in hardness at the weld heat affected zone is defined as α, the yield strength is YS, the strength is TS, and the stress-strain relationship curve is called the SS curve.
(1) The base material SS curve 9 is used up to the base material YS × (1-α) × 0.6 (point A in the figure).
(2) The distortion of the base material SS curve is left as it is, and a curve 12 with a low TS is created by stress × (1−α).
(3) A point where a straight line 13 with a 0.2% offset intersects the curve 12 with the slope of the base material SS curve is defined as a point B.
(4) Interpolate point A and point B by spline interpolation.
(5) The origin-point A-point B is connected, and after point B, the curve overlapping the curve 12 is taken as the SS curve of the welding heat affected zone.

  Regardless of the method described above, a tensile test may be performed using a reproducible thermal cycle specimen of the weld heat affected zone to directly obtain the stress-strain relationship.

  Next, for the element (mesh) identified as the most dangerous part, it is determined whether or not the strain has reached a uniform elongation in the tensile test of the material by FEM analysis (STEP 4). Judgment is over.

  The actual machine experiment and FEM analysis were performed on the specimen 1 of the cold-formed square steel pipe shown in FIG. 2, and the deformation at the end was compared. The cold-formed square steel pipe used as the test body 1 has the dimensions of each part, which has a width-to-thickness ratio of 33 × √ {square root over (235 / (material design standard strength))}, which ends with fracture.

  FIG. 1 shows the structure of the bending tester 3 used in the actual machine test. In the bending test, both ends of the test body 1 were supported by the pins 2, and the deformation performance until the center portion was loaded and broken was evaluated.

  Table 1 shows the schematic dimensions of the test body 1 and the test results, and FIG. 10 shows the correlation between the experimental test results and the FEM analysis results. From the figure, it can be seen that according to the present invention, the ultimate deformation performance of a cold-formed square steel pipe having a small width-to-thickness ratio, which ends with fracture, can be accurately predicted.

DESCRIPTION OF SYMBOLS 1 Test body 2 Pin 3 Bending test machine 4 Steel pipe base material 5 Diaphragm 6 Welding part (DEPO)
7 Welding heat affected zone 8 Backing metal 9, 10, 11, 12, Stress-strain line 13 Straight line 14 Most dangerous part

Claims (3)

A method for determining a deformation state of a cold-formed square steel pipe that is broken by bending, wherein the cold-formed square steel pipe has a through-diaphragm weld and a cross-sectional width-thickness ratio of 33 × √ (235 / (material design criteria Strength)) The following is a stress-strain relationship between the cold-formed square steel pipe and the through-diaphragm welded portion when bending is applied by FEM analysis, and the portion where the local strain on the tensile side proceeds the fastest The deformation state determination method for a cold-formed square steel pipe, characterized in that the time when the equivalent strain in the most dangerous region reaches the uniform elongation of the material is determined as the ultimate deformation state . When performing FEM analysis, the stress-strain relationship in the weld heat-affected zone of the cold-formed square steel pipe is as follows (1) using the hardness test result of the weld heat-affected zone and the stress-strain relationship of the base metal. The method of determining a deformation state of a cold-formed square steel pipe according to claim 1 , wherein the determination is made according to the procedure of (5) to (5) .
Here, the rate of decrease in hardness at the weld heat affected zone is defined as α, the yield strength is called YS, the strength is TS, and the stress-strain relationship curve is called the SS curve.
(1) The base material SS curve is used up to the point A where the stress is the base material YS × (1-α) × 0.6.
(2) The distortion of the base material SS curve is left as it is, and a curve with a low TS is created by stress × (1−α).
(3) A point where a straight line with a 0.2% offset in the inclination of the base material SS curve intersects the curve with a low TS is defined as a point B.
(4) Interpolate point A and point B by spline interpolation.
(5) The origin-point A-point B are connected, and after the point B, the curve that overlaps with the low TS curve is taken as the SS heat affected zone SS curve. A structure including a cold-formed square steel pipe whose deformation performance is evaluated based on the method for determining a deformation state of a cold-formed square steel pipe according to claim 1 or 2.