Multi-axial fatigue test method for welding structure details in steel structure

Abstract

The invention discloses a multi-axial fatigue test method for welding structure details in a steel structure, and relates to a multi-axial fatigue test method for welding structure details. The invention aims to solve the problems of complexity and high cost of the existing steel structure welding structure detail multi-axial fatigue test. The method comprises the steps that firstly, a welding structure detail piece is prepared, then the prepared welding structure detail piece is cut to obtain a plurality of welding structure detail test pieces, and each welding structure detail test piece is composed of a clamping section, a transition section, a straight section and a gusset plate; verifying through finite element simulation to ensure the length of a straight section and determine the main stress direction, so as to determine the setting direction of a strain gauge in the multi-axial fatigue test process of welding structure details in the steel structure; according to the invention, from the design angle of the structural member, the multi-axial fatigue test of structural details is realized through the single-axial fatigue test equipment, the cost is reduced, and meanwhile, the method is not limited by the size of the member and can be suitable for wider engineering structures and members.

Description

Technical Field

[0001] The invention relates to a multi-axial fatigue test method for welding structure details. Background Art

[0002] The fatigue resistance of steel structures refers to the ability of steel to resist crack initiation, expansion and even fracture under long-term cyclic loads. Fatigue failure is one of the common failure forms of steel structures, which usually occurs in areas with concentrated stress or cyclic stress changes. The fatigue performance of steel structures is affected by many factors, including material properties, stress levels, structural details, environmental conditions, etc. For steel structural members and their connections that are directly subjected to repeated dynamic loads, when the number of stress change cycles n is equal to or greater than 5×10 4 Fatigue verification should be carried out every time.

[0003] The fatigue resistance of steel structures is controlled by structural details that are prone to fatigue failure. Among various typical fatigue details, fatigue problems of welded structural details are often more prominent, which is related to the initial defects that inevitably occur in the structural details during the welding process. The types of defects include inclusions, cold shuts, dents, microcracks, etc. The existence of initial defects greatly reduces the fatigue life of steel structures. Accurately predicting the fatigue life of welded structural details has great engineering significance.

[0004] The current fatigue performance tests on structural details are mainly focused on unidirectional stress conditions. However, in actual engineering, structures often face more complex multi-axial stress conditions. How to accurately evaluate the fatigue life of welded structural details under multi-axial conditions is an urgent problem to be solved. In the existing technology, multi-axial fatigue tests are all based on the perspective of test equipment. One method is to directly use a tension-torsion composite fatigue testing machine, which can apply tensile stress and shear stress to the specimen at the same time to simulate a planar stress state. However, this type of equipment is expensive and has a high unit time charge. At the same time, due to the problem of load-bearing capacity, this type of experimental device is only suitable for small rod-shaped or tubular components to study the principle of multi-axial fatigue problems, and is not suitable for studying large structural components or details in engineering. The other method is to use a combination of multiple actuators to apply tension in different directions to simulate the stress conditions of structures or components in engineering. However, this type of test technology requires a larger site area and requires on-site assembly, which increases the complexity and cost of the test. Since the actuator loading frequency is low and very time-consuming, both the tension-torsion composite fatigue testing machine and the combination of multiple actuators have a high cost problem, which limits their use in extensive research and engineering applications. Summary of the invention

[0005] In order to solve the problems of complexity and high cost of existing multi-axial fatigue tests on steel structure welding structural details, the present invention starts from the design perspective of structural components and implements multi-axial fatigue tests on structural details through uniaxial fatigue testing equipment, thereby reducing costs and making research more economical and efficient. At the same time, it is not limited by the size of the components and can be applied to a wider range of engineering structures and components.

[0006] The multiaxial fatigue test method for welded structural details in steel structures of the present invention is carried out according to the following steps:

[0007] 1. Preparation of welding structural details

[0008] According to the structural details of the weld of the steel structure to be evaluated, a welded structural detail is prepared, the welded structural detail is composed of two steel plates (1) and a node plate (2), the two steel plates (1) are symmetrically arranged on both sides of the node plate (2), the steel plates (1) and the node plate (2) are perpendicular, and the steel plates (1) and the node plate (2) are welded to obtain a weld (3);

[0009] 2. Preparation of welding structure detail test specimens (4)

[0010] A plurality of welding structure detail test specimens (4) are cut from the welding structure detail pieces prepared in step 1; an angle θ is formed between the length direction of the welding structure detail test specimens (4) and the length direction of the gusset plate (2);

[0011] The θ is calculated by formula 1;

[0012]

[0013] In formula 1, θ is the angle between the length direction of the welded structure detail test specimen (4) and the length direction of the gusset plate (2), and the unit is rad; σ max is the design tensile stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; τ max is the design shear stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa;

[0014] The welding structure detail test specimen (4) is composed of a clamping section (5), a transition section (6), a straight section (7) and a node plate (2); the welding structure detail test specimen (4) is in the shape of a strip, the clamping section (5) and the straight section (7) are in the shape of a rectangle, the center of the welding structure detail test specimen (4) is the node plate (2), the two straight sections (7) are symmetrically welded on both sides of the node plate (2), the transition section (6) is arranged between the clamping section (5) and the straight section (7), the width of the clamping section (5) is greater than the width of the straight section (7), and the two side edges of the transition section (6) are concave arc surfaces; the two side edges of the transition section (6) adopt the form of arc transition to ensure that there is no sudden change in the cross-sectional shape, so as to reduce stress concentration;

[0015] The width W of the straight section (7) is calculated by formula 2;

[0016]

[0017] In formula 2, W is the width of the straight section (7), in mm; F max is the maximum load capacity allowed by the test equipment, in N; σ max is the design tensile stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; τ max is the design shear stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; t is the thickness of the straight section (7), in mm;

[0018] The lengths of the two sides of the straight section (7) are respectively 1 to 3 times the width of the straight section (7). After selecting the lengths of the two sides of the straight section (7), finite element simulation is used to verify that the length of the straight section (7) does not contain stress concentration in the transition section (6) during the multi-axial fatigue test. If stress concentration exists, a larger length of the two sides of the straight section (7) is reselected within the range of 1 to 3 times the width of the straight section (7) and re-verified. The principal stress direction is determined by finite element simulation. The principal stress direction is used to determine the setting direction of the strain gauge during the multi-axial fatigue test of the welded structural details in the steel structure. The lengths of the two sides of the straight section (7) should ensure uniform force transmission, that is, the stress should be evenly distributed before being transmitted from the horizontal section to the structural detail position, so as to reduce unexpected damage caused by stress concentration.

[0019] The method for verifying the lengths of the two sides of the straight section (7) and determining the direction of the principal stress by finite element simulation is as follows:

[0020] (1) Establishing a 3D deformable entity unit; the 3D deformable entity unit includes: plate thickness, width of the clamping section, length of the clamping section, width of the straight section, length of two sides of the straight section, diameter of the arc surface of the side of the transition section, and an angle θ between the length direction of the welding structure detail test specimen (4) and the length direction of the node plate (2);

[0021] (2) Define the cross section and assign material properties;

[0022] (3) Assembling the joint structure detail test specimen (4);

[0023] (4) Set the uniaxial stretching process analysis step;

[0024] (5) Set boundary conditions;

[0025] (6) applying a tensile force to the unfixed clamping section (5) of the joint structure detail test specimen (4);

[0026] (7) Perform mesh division on the structural detail test specimen (4);

[0027] (8) Submit the work and conduct finite element analysis to obtain the stress nephogram of the joint detail test specimen (4), and determine whether there is stress concentration in the transition section (6) through the stress nephogram; at the same time, extract the tension and shear force of the section at the weld toe position through the stress nephogram, and determine the direction of the principal stress through the tension and shear force.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The welded structural detail test specimen (4) prepared by the present invention is used to evaluate the multiaxial fatigue performance of the structural details of the steel structure, and a uniaxial fatigue test device is used to implement the multiaxial fatigue test of the structural details. Among them, in the welded structural detail test specimen (4) cut from the welded structural detail prepared in step 1, the node plate (2) has an angle with the clamping section (5), the transition section (6) and the straight section (7). During the uniaxial tensile fatigue test, the weld toe position is simultaneously subjected to the principal stress, the nominal tension and the nominal shear force, and the application of the multiaxial fatigue load is realized. Therefore, the angle at the key position of the test specimen of the welded structural detail is designed, and the principle of the equivalent resultant force is used to implement the multiaxial fatigue test of the welded structural detail. It does not rely on the expensive tension-torsion composite fatigue test machine, and the multiaxial fatigue test can be completed only by the fatigue test machine with unidirectional tension and compression functions, which reduces the cost, makes the research more economical and efficient, and is not limited by the size of the component. The method of the present invention can be applied to a wider range of engineering structures and components, such as cross-butt non-force-transmitting welds, cross-butt force-transmitting fillet welds, cross-butt non-force-transmitting fillet welds and other different forms of welded structural details in steel structures.

[0030] The present invention only conducts experiments in the direction of the principal stress, and no longer needs to use a complex strain gauge device. The fatigue process can be monitored only by a unidirectional strain gauge. The unidirectional strain gauge is simpler than a multi-axis strain gauge and is easy to install and adjust. The time and complexity of the test preparation are reduced. In addition, the amount of data generated by the unidirectional strain gauge is small, which simplifies the data collection and processing process. When the direction of the principal stress is known, the unidirectional strain gauge can directly measure the strain in this direction, avoiding the error that may be introduced by the multi-axis strain gauge. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a structural schematic diagram of welding structural details;

[0032] Figure 2 is a schematic diagram of the structure of a welding structure detail test specimen (4);

[0033] Figure 3 The stress analysis diagram of the welded structure detail test specimen (4) during the multi-axial fatigue test;

[0034] Figure 4 The mesh division diagram of the test specimen (4) for the construction details;

[0035] Figure 5 Apply diagrams for finite element loads and boundary conditions;

[0036] Figure 6 This is the finite element simulation result diagram. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is not limited to the specific implementation modes listed below, but also includes any reasonable combination of the specific implementation modes.

[0038] Specific implementation method 1: The multiaxial fatigue test method for welding structural details in steel structures in this implementation method is carried out according to the following steps:

[0039] 1. Preparation of welding structural details

[0040] According to the structural details of the weld of the steel structure to be evaluated, a welded structural detail is prepared, the welded structural detail is composed of two steel plates (1) and a node plate (2), the two steel plates (1) are symmetrically arranged on both sides of the node plate (2), the steel plates (1) and the node plate (2) are perpendicular, and the steel plates (1) and the node plate (2) are welded to obtain a weld (3);

[0041] 2. Preparation of welding structure detail test specimens (4)

[0042] A plurality of welding structure detail test specimens (4) are cut from the welding structure detail pieces prepared in step 1; an angle θ is formed between the length direction of the welding structure detail test specimens (4) and the length direction of the gusset plate (2);

[0043] The θ is calculated by formula 1;

[0044]

[0045] In formula 1, θ is the angle between the length direction of the welded structure detail test specimen (4) and the length direction of the gusset plate (2), and the unit is rad; σ max is the design tensile stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; τ max is the design shear stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa;

[0046] The welding structure detail test specimen (4) is composed of a clamping section (5), a transition section (6), a straight section (7) and a node plate (2); the welding structure detail test specimen (4) is in the shape of a strip, the clamping section (5) and the straight section (7) are in the shape of a rectangle, the center of the welding structure detail test specimen (4) is the node plate (2), the two straight sections (7) are symmetrically welded on both sides of the node plate (2), the transition section (6) is arranged between the clamping section (5) and the straight section (7), the width of the clamping section (5) is greater than the width of the straight section (7), and the two side edges of the transition section (6) are concave arc surfaces; the two side edges of the transition section (6) adopt the form of arc transition to ensure that there is no sudden change in the cross-sectional shape, so as to reduce stress concentration;

[0047] The width W of the straight section (7) is calculated by formula 2;

[0048]

[0049] In formula 2, W is the width of the straight section (7), in mm; F max is the maximum load capacity allowed by the test equipment, in N; σ max is the design tensile stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; τ max is the design shear stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; t is the thickness of the straight section (7), in mm;

[0050] The lengths of the two sides of the straight section (7) are respectively 1 to 3 times the width of the straight section (7). After selecting the lengths of the two sides of the straight section (7), finite element simulation is used to verify that the length of the straight section (7) does not contain stress concentration in the transition section (6) during the multi-axial fatigue test. If stress concentration exists, a larger length of the two sides of the straight section (7) is reselected within the range of 1 to 3 times the width of the straight section (7) and re-verified. The principal stress direction is determined by finite element simulation. The principal stress direction is used to determine the setting direction of the strain gauge during the multi-axial fatigue test of the welded structural details in the steel structure. The lengths of the two sides of the straight section (7) should ensure uniform force transmission, that is, the stress should be evenly distributed before being transmitted from the horizontal section to the structural detail position, so as to reduce unexpected damage caused by stress concentration.

[0051] The method for verifying the lengths of the two sides of the straight section (7) and determining the direction of the principal stress by finite element simulation is as follows:

[0052] (1) Establishing a 3D deformable entity unit; the 3D deformable entity unit includes: plate thickness, width of the clamping section, length of the clamping section, width of the straight section, length of two sides of the straight section, diameter of the arc surface of the side of the transition section, and an angle θ between the length direction of the welding structure detail test specimen (4) and the length direction of the node plate (2);

[0053] (2) Define the cross section and assign material properties;

[0054] (3) Assembling the joint structure detail test specimen (4);

[0055] (4) Set the uniaxial stretching process analysis step;

[0056] (5) Set boundary conditions;

[0057] (6) applying a tensile force to the unfixed clamping section (5) of the joint structure detail test specimen (4);

[0058] (7) Perform mesh division on the structural detail test specimen (4);

[0059] (8) Submit the work and conduct finite element analysis to obtain the stress nephogram of the joint detail test specimen (4), and determine whether there is stress concentration in the transition section (6) through the stress nephogram; at the same time, extract the tension and shear force of the section at the weld toe position through the stress nephogram, and determine the direction of the principal stress through the tension and shear force.

[0060] This embodiment has the following beneficial effects:

[0061] The welded structural detail test specimen (4) prepared in this embodiment is used to evaluate the multiaxial fatigue performance of the steel structure structural detail, and a uniaxial fatigue test device is used to implement a multiaxial fatigue test of the structural detail. In this embodiment, in the welded structural detail test specimen (4) cut from the welded structural detail prepared in step 1, the node plate (2) has an angle with the clamping section (5), the transition section (6) and the straight section (7). During the uniaxial tensile fatigue test, the weld toe position is simultaneously subjected to the principal stress, the nominal tension and the nominal shear force, thereby implementing the application of a multiaxial fatigue load. Therefore, the angles at key positions of the test specimens of the welding structural details are designed, and the principle of equivalent resultant force is used to realize multi-axial fatigue testing of the welding structural details. This does not rely on expensive tension-torsion composite fatigue testing machines, but can be completed by fatigue testing machines with unidirectional tension and compression functions, thereby reducing costs and making the research more economical and efficient. At the same time, it is not limited by the size of the components. The method of this embodiment can be applied to a wider range of engineering structures and components, such as cross-butt non-force-transmitting welds, cross-butt force-transmitting fillet welds, cross-butt non-force-transmitting fillet welds and other different forms of welding structural details in steel structures.

[0062] This embodiment only conducts experiments in the direction of the principal stress, and no longer needs to use a complex strain gauge device. The fatigue process can be monitored only by a unidirectional strain gauge, which is simpler than a multi-axis strain gauge and is easy to install and adjust. The time and complexity of test preparation are reduced. In addition, the amount of data generated by the unidirectional strain gauge is small, which simplifies the data collection and processing process. When the direction of the principal stress is known, the unidirectional strain gauge can directly measure the strain in this direction, avoiding the error that may be introduced by the multi-axis strain gauge.

[0063] Specific implementation method 2: The difference between this implementation method and specific implementation method 1 is that the weld (3) in step 1 is a butt joint weld, a T-joint weld or a cross joint weld.

[0064] Specific implementation method three: This implementation method is different from specific implementation methods one or two in that: the steel plate (1) in step one is a steel plate designed and used in the steel structure to be evaluated.

[0065] Specific implementation method 4: This implementation method is different from any one of specific implementation methods 1 to 3 in that: in step 2, the two side edges of the transition section (6) are concave arc surfaces, and the diameter thereof is not less than 2 times the width W of the straight section (7).

[0066] Specific implementation method five: This implementation method is different from any one of specific implementation methods one to four in that: the material properties in step two (2) are material properties in the elastic stage, including the material, the modulus of the material, and the modulus Poisson's ratio of the material.

[0067] Specific implementation method 6: This implementation method is different from any one of specific implementation methods 1 to 5 in that: the uniaxial stretching process analysis step in step 2 (4) is: selecting Static and General.

[0068] Specific implementation method seven: This implementation method is different from any one of specific implementation methods one to six in that: the boundary condition set in step two (5) is to couple part of the clamping segment (5) to the center point and keep one segment completely fixed; after the boundary condition is set, the other end of the clamping segment (5) can only move along the length direction of the construction detail.

[0069] Specific implementation eight: This implementation differs from any one of specific implementations one to seven in that: the grid division in step two (7) adopts the C3D8R unit type.

[0070] Specific implementation method 9: This implementation method is different from any one of specific implementation methods 1 to 8 in that the global size of the grid divided in step 2 (7) is 2 mm.

[0071] Specific implementation method ten: This implementation method is different from any one of specific implementation methods one to nine in that: the grid division in step two (7) is divided into six layers of cells along the thickness direction.

[0072] Embodiment 1:

[0073] The multiaxial fatigue test method for welding structural details in the steel structure of this embodiment is carried out according to the following steps:

[0074] 1. Preparation of welding structural details

[0075] According to the structural details of the weld of the steel structure to be evaluated, a welded structural detail is prepared, the welded structural detail is composed of two steel plates (1) and a node plate (2), the two steel plates (1) are symmetrically arranged on both sides of the node plate (2), the steel plates (1) and the node plate (2) are perpendicular, and the steel plates (1) and the node plate (2) are welded to obtain a weld (3);

[0076] The weld (3) is a butt joint weld, a T-joint weld or a cross joint weld;

[0077] The steel plate (1) is a steel plate designed and used in the steel structure to be evaluated;

[0078] 2. Preparation of welding structure detail test specimens (4)

[0079] A plurality of welding structure detail test specimens (4) are cut from the welding structure detail pieces prepared in step 1; an angle θ is formed between the length direction of the welding structure detail test specimens (4) and the length direction of the gusset plate (2);

[0080] The θ is calculated by formula 1;

[0081]

[0082] In formula 1, θ is the angle between the length direction of the welded structure detail test specimen (4) and the length direction of the gusset plate (2), and the unit is rad; σ max is the design tensile stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; τ max is the design shear stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa;

[0083] The welding structure detail test specimen (4) is composed of a clamping section (5), a transition section (6), a straight section (7) and a node plate (2); the welding structure detail test specimen (4) is in the shape of a strip, the clamping section (5) and the straight section (7) are in the shape of a rectangle, the center of the welding structure detail test specimen (4) is the node plate (2), the two straight sections (7) are symmetrically welded on both sides of the node plate (2), the transition section (6) is arranged between the clamping section (5) and the straight section (7), the width of the clamping section (5) is greater than the width of the straight section (7), and the two side edges of the transition section (6) are concave arc surfaces; the two side edges of the transition section (6) adopt the form of arc transition to ensure that there is no sudden change in the cross-sectional shape, so as to reduce stress concentration;

[0084] The two sides of the transition section (6) are concave arc surfaces with a diameter of 125 mm;

[0085] The width W of the straight section (7) is calculated by formula 2;

[0086]

[0087] In formula 2, W is the width of the straight section (7), in mm; F max is the maximum load capacity allowed by the test equipment, in N; σ max is the design tensile stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; τ max is the design shear stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; t is the thickness of the straight section (7), in mm;

[0088] The length of the two sides of the straight section (7) is 75 mm. After selecting the length of the two sides of the straight section (7), finite element simulation is used to verify that the selection of the length of the straight section (7) does not have stress concentration in the transition section (6) during the multi-axial fatigue test. If stress concentration exists, a larger length of the two sides of the straight section (7) is reselected within the range of 1 to 3 times the width of the straight section (7) and re-verified; and the principal stress direction is determined by finite element simulation. The principal stress direction is used to determine the setting direction of the strain gauge during the multi-axial fatigue test of the welded structural details in the steel structure; the length of the two sides of the straight section (7) should ensure uniform force transmission, that is, the stress should be evenly distributed before being transmitted from the horizontal section to the structural detail position, so as to reduce unexpected damage caused by stress concentration.

[0089] The method for verifying the lengths of the two sides of the straight section (7) and determining the direction of the principal stress by finite element simulation is as follows:

[0090] (1) Establishing a 3D deformable entity unit; the 3D deformable entity unit includes: a plate thickness of 12 mm, a clamping section width of 100 mm, a clamping section length of 104 mm, a straight section width of 50 mm, two side lengths of the straight section of 75 mm and 125 mm respectively, a transition section side arc surface diameter of 125 mm, and an angle θ between the length direction of the welded structure detail test specimen (4) and the length direction of the node plate (2) of 45°; Figure 4 As shown;

[0091] (2) defining a cross section and assigning material properties; the material properties are material properties in the elastic stage, including that the material is Q355 steel, the material modulus is 210 GPa, and the material modulus Poisson's ratio is Q355 steel;

[0092] (3) Assembling the joint structure detail test specimen (4);

[0093] (4) Set the uniaxial stretching process analysis step; the uniaxial stretching process analysis step is: select Static and General,

[0094] (5) Setting boundary conditions: coupling part of the clamping section (5) to the center point and keeping one section completely fixed; after setting the boundary conditions, the other end of the clamping section (5) can only move along the length direction of the construction detail;

[0095] (6) Apply a tensile force of 1 kN to the unfixed clamping section (5) of the joint structure detail test specimen (4).

[0096] (7) Mesh the structure detail test specimen (4); the mesh is divided into: using C3D8R unit type, the global mesh size is 2 mm, and it is divided into 6 layers of cells along the thickness direction; Figure 5 As shown;

[0097] (8) Submit the assignment and conduct finite element analysis to obtain the stress nephogram of the weld detail test specimen (4), and determine whether there is stress concentration in the transition section (6) through the stress nephogram; at the same time, extract the tension and shear force of the cross section at the weld toe position through the stress nephogram, and determine the direction of the principal stress through the tension and shear force; Figure 6 As shown, it is found that there is no obvious stress concentration at the transition section (6), indicating that the length of the straight section (7) is reasonably selected. At the same time, the absence of obvious stress concentration at the transition section (6) in the stress cloud diagram also indicates that the setting of the arc diameter of the transition section (6) is reasonable; the tension and shear force of the cross section at the weld toe position are extracted from the stress cloud diagram, and the setting direction of the strain gauge during the multi-axial fatigue test of the welding structure details in the steel structure is further determined;

[0098] In this embodiment, the clamping section (5) in the welded structure detail test specimen (4) must ensure matching with the test machine fixture to achieve stable clamping. The clamping section (5) should have a sufficient size to withstand the maximum expected load during the test while avoiding damage to the specimen due to improper clamping. The thickness of the clamping section (5) should be consistent with the thickness of the steel structure of the actual engineering design. The width of the clamping section (5) should be consistent with the width of the fatigue test machine fixture. The length of the clamping section (5) should be able to ensure that the test machine fixture clamps the clamping end of the test specimen.

[0099] The welded structural detail test specimen (4) prepared in this embodiment is used to evaluate the multiaxial fatigue performance of the structural details of the steel structure. The multiaxial fatigue test of the structural details is realized by using a uniaxial fatigue test device, which does not rely on an expensive tension-torsion composite fatigue test machine. The multiaxial fatigue test can be completed by using only a fatigue test machine with uniaxial tension and compression functions, which reduces the cost and makes the research more economical and efficient. At the same time, it is not limited by the size of the component. In this embodiment, it is no longer necessary to use a complex strain rosette device. The fatigue process can be monitored only by a uniaxial strain gauge. The uniaxial strain gauge is simpler than the multiaxial strain rosette and is easy to install and adjust. The time and complexity of the test preparation are reduced. In addition, the uniaxial strain gauge generates less data, which simplifies the data collection and processing process. When the direction of the principal stress is known, the uniaxial strain gauge can directly measure the strain in this direction, avoiding the error that may be introduced by the multiaxial strain rosette.

Claims

1. A multiaxial fatigue test method for welded structural details in steel structures, characterized in that: The multiaxial fatigue test method for welded construction details in steel structures is carried out according to the following steps:

1. Preparation of welding structural details According to the structural details of the weld of the steel structure to be evaluated, a welded structural detail is prepared, the welded structural detail is composed of two steel plates (1) and a node plate (2), the two steel plates (1) are symmetrically arranged on both sides of the node plate (2), the steel plates (1) and the node plate (2) are perpendicular, and the steel plates (1) and the node plate (2) are welded to obtain a weld (3); 2. Preparation of welding structure detail test specimens (4) A plurality of welding structure detail test specimens (4) are cut from the welding structure detail pieces prepared in step 1; an angle θ is formed between the length direction of the welding structure detail test specimens (4) and the length direction of the gusset plate (2); The θ is calculated by formula 1; In formula 1, θ is the angle between the length direction of the welded structure detail test specimen (4) and the length direction of the gusset plate (2), and the unit is rad; σ max is the design tensile stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; τ max is the design shear stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; The welding structure detail test specimen (4) is composed of a clamping section (5), a transition section (6), a straight section (7) and a node plate (2); the welding structure detail test specimen (4) is in the shape of a strip, the clamping section (5) and the straight section (7) are in the shape of a rectangle, the center of the welding structure detail test specimen (4) is the node plate (2), the two straight sections (7) are symmetrically welded on both sides of the node plate (2), the transition section (6) is arranged between the clamping section (5) and the straight section (7), the width of the clamping section (5) is greater than the width of the straight section (7), and the two side edges of the transition section (6) are concave arc surfaces; the two side edges of the transition section (6) adopt the form of arc transition to ensure that there is no sudden change in the cross-sectional shape, so as to reduce stress concentration; The width W of the straight section (7) is calculated by formula 2; In formula 2, W is the width of the straight section (7), in mm; F max is the maximum load capacity allowed by the test equipment, in N; σ max is the design tensile stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; τ max is the design shear stress at the weld toe of the welded structural detail in the steel structure to be evaluated, in MPa; t is the thickness of the straight section (7), in mm; The lengths of the two sides of the straight section (7) are respectively 1 to 3 times the width of the straight section (7). After selecting the lengths of the two sides of the straight section (7), a finite element simulation is used to verify that the length of the straight section (7) does not contain stress concentration in the transition section (6) during the multi-axial fatigue test. If stress concentration exists, a larger length of the two sides of the straight section (7) is reselected within the range of 1 to 3 times the width of the straight section (7) and the verification is performed again. The principal stress direction is determined by the finite element simulation. The principal stress direction is used to determine the setting direction of the strain gauge during the multi-axial fatigue test of the welding structure details in the steel structure. The method for verifying the lengths of the two sides of the straight section (7) and determining the direction of the principal stress by finite element simulation is as follows: (1) Establish 3D deformable entity element; The 3D deformable entity unit includes: plate thickness, width of the clamping section, length of the clamping section, width of the straight section, length of two sides of the straight section, diameter of the arc surface of the side of the transition section, and angle θ between the length direction of the welding structure detail test specimen (4) and the length direction of the node plate (2); (2) Define the cross section and assign material properties; (3) Assembling the joint structure detail test specimen (4); (4) Set the uniaxial stretching process analysis step; (5) Set boundary conditions; (6) applying a tensile force to the unfixed clamping section (5) of the joint structure detail test specimen (4); (7) Perform mesh division on the structural detail test specimen (4); (8) Submit the work and conduct finite element analysis to obtain the stress nephogram of the joint detail test specimen (4), and determine whether there is stress concentration in the transition section (6) through the stress nephogram; at the same time, extract the tension and shear force of the section at the weld toe position through the stress nephogram, and determine the direction of the principal stress through the tension and shear force.

2. The multiaxial fatigue test method for welded structural details in steel structures according to claim 1, characterized in that: The weld (3) in step 1 is a butt joint weld, a T-joint weld or a cross joint weld.

3. The multiaxial fatigue test method for welded structural details in steel structures according to claim 1, characterized in that: Step 1: The steel plate (1) is a steel plate designed and used in the steel structure to be evaluated.

4. The multiaxial fatigue test method for welded structural details in steel structures according to claim 1, characterized in that: In step 2, the two side edges of the transition section (6) are concave arc surfaces, the diameter of which is not less than twice the width W of the straight section (7).

5. The multiaxial fatigue test method for welded structural details in steel structures according to claim 1, characterized in that: The material properties in step 2 (2) are the material properties in the elastic stage, including the material, the modulus of the material, and the modulus Poisson's ratio of the material.

6. The multiaxial fatigue test method for welded structural details in steel structures according to claim 1, characterized in that: The uniaxial stretching process analysis step in step 2 (4) is: select Static and General.

7. The multiaxial fatigue test method for welded structural details in steel structures according to claim 1, characterized in that: The boundary condition set in step 2 (5) is to couple part of the clamping section (5) to the center point and keep one section completely fixed; after the boundary condition is set, the other end of the clamping section (5) can only move along the length direction of the construction detail.

8. The multiaxial fatigue test method for welded structural details in steel structures according to claim 1, characterized in that: The meshing in step 2 (7) adopts C3D8R unit type.

9. The multiaxial fatigue test method for welded structural details in steel structures according to claim 1, characterized in that: The global size of the grid divided in step 2 (7) is 2 mm.

10. The multiaxial fatigue test method for welded structural details in steel structures according to claim 1, characterized in that: The grid division in step 2 (7) is divided into 6 layers of cells along the thickness direction.

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