Measurement method based on local stress release and strain feedback

Abstract

The invention discloses a method for measuring longitudinal residual stress at the joint of a rail bottom and a rail web of a steel rail, and belongs to the technical field of material mechanical property testing. Comprising the following steps of sample preparation, measurement point positioning, surface treatment, strain gauge arrangement, blind hole stress release and data acquisition and calculation. According to the measurement method based on local stress release and strain feedback, accurate measurement of the longitudinal residual stress at the joint of the rail bottom and the rail web is achieved by optimizing a surface treatment process and a strain gauge arrangement scheme.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of material mechanical property testing, and particularly relates to a measuring method based on local stress release and strain feedback. BACKGROUND

[0002] During the rolling, straightening and welding processes of a rail, high-amplitude residual stress is prone to be generated at the connecting position between the rail bottom and the rail waist due to geometric shape mutation and uneven cooling. The stress concentration in this area significantly reduces the fatigue strength of the rail, which may lead to crack initiation and even fracture, thereby seriously threatening the safety of train operation. The existing residual stress measurement methods have the following limitations:

[0003] 1. Traditional strain gauge method: needs to rely on external loading and cannot directly measure static residual stress;

[0004] 2. Drilling method: limited by the complex geometric structure of the connecting position, the drilling positioning accuracy is low and the rail integrity is damaged;

[0005] 3. X-ray diffraction method: can only detect surface stress and is difficult to reflect the internal stress gradient at the connecting position.

[0006] Therefore, there is an urgent need for a non-destructive and high-precision residual stress detection method for the connecting position between the rail bottom and the rail waist. SUMMARY

[0007] The purpose of the application is to provide a measuring method based on local stress release and strain feedback, which realizes accurate measurement of the longitudinal residual stress at the connecting position between the rail bottom and the rail waist by optimizing the surface treatment process and the strain gauge layout scheme.

[0008] To solve the above technical problems, the application adopts the following technical solutions:

[0009] The measuring method based on local stress release and strain feedback of the application has the following specific steps:

[0010] 1. Sample preparation: cut a 1m-long rail sample;

[0011] 2. Positioning mark: at the midpoint of the rail length direction, vertically downward along the rail waist side to the center point of the rail bottom and rail waist circular arc transition zone, marked as measurement point P;

[0012] 3. Surface treatment:

[0013] a. Use an angle grinder with a grinding wheel to roughen the area around the measurement point P;

[0014] b. Using electrolytic polishing equipment to polish the rough grinding area, the surface roughness Ra≤1.0μm;

[0015] 4). Strain gauge layout:

[0016] a. Determine the position of point P at the center of the polishing area, and paste unidirectional strain gauge 2 at 0.5mm to the right of point P along the longitudinal line connecting the rail waist and the rail low, for measuring the longitudinal residual stress at this point; paste unidirectional strain gauge 1 at 0.5mm above strain gauge 2 along the vertical direction, for measuring the vertical residual stress of the rail waist; paste strain gauge 3 at 0.5mm below strain gauge 2 along the direction perpendicular to strain gauge 2, for measuring the horizontal residual stress of the rail low;

[0017] b. Fix and ensure that the strain gauges are tightly attached to the substrate;

[0018] 5). Data acquisition and calculation:

[0019] a. Connect the static strain gauge and record the initial strain value ε0;

[0020] b. Use the drilling device to drill a hole at point P, with a diameter of 2mm and a depth of 1mm;

[0021] c. The distance between the three unidirectional strain gauges and the blind hole is 0.5mm;

[0022] d. Collect the strain value ε1 after release, and calculate the longitudinal residual stress σ according to Δε=ε1-ε0 and the elasticity mechanics formula.

[0023] Further, in step 1), ensure that there are no visible defects at the connection between the rail bottom and the rail waist.

[0024] Further, in step 3), the grain size of the grinding wheel is ≥600#.

[0025] Further, in step 3), the area around the measurement point P within 20mm×20mm is rough ground.

[0026] Further, in step 4), epoxy resin glue is used for fixation.

[0027] Further, in step 4), apply a pressure of 0.2-0.3MPa for solidification, to ensure that the strain gauges are tightly attached to the substrate.

[0028] Further, use a drilling device of model JY-RSD1 to drill a hole at point P

[0029] Compared with the prior art, the beneficial technical effects of the present application are:

[0030] 1. Positioning optimization: According to the geometric characteristics of the connection between the rail bottom and the rail waist, a positioning method is proposed based on the center of the circular arc transition area, with an error of ≤±0.1mm;

[0031] 2. Composite polishing process: coarse grinding-electrolytic polishing combined technology eliminates surface work hardening layer, and the error of strain gauge response to true residual stress is reduced to within 3%;

[0032] 3. Micro blind hole release method: local stress release is realized through micron level drilling, and the damage of traditional drilling to the structure of the rail is avoided.

[0033] 4. The method can simultaneously measure the rail waist vertical residual stress, the rail waist and rail bottom connection longitudinal residual stress and the rail low transverse residual stress at one time. BRIEF DESCRIPTION OF DRAWINGS

[0034] The application will be further described below in combination with the description of the drawings.

[0035] Figure 1 Positioning diagram for measuring point at rail bottom and rail waist connection;

[0036] Figure 2 Planing diagram for patch position. DETAILED DESCRIPTION

[0037] A measuring method based on local stress release and strain feedback, the specific steps are as follows:

[0038] 1. Sample preparation: cut a steel rail sample with a length of 1m, and ensure that there is no visible defect at the rail bottom and rail waist connection;

[0039] 2. Positioning mark: at the midpoint of the steel rail length direction, position vertically downward along the rail waist side to the center point of the rail bottom and rail waist arc transition area (marked as measuring point P);

[0040] 3. Surface treatment:

[0041] a. Use an angle grinder with a grinding wheel (particle size ≥600#) to coarsely grind a 20mm×20mm area around the measuring point P;

[0042] b. Use electrolytic polishing equipment to finely polish the coarsely ground area, and the surface roughness Ra≤1.0μm;

[0043] 4. Strain gauge layout:

[0044] a. Determine the position of point P at the center of the polished area, and paste unidirectional strain gauge 2 at 0.5mm to the right of point P along the rail waist and rail low connecting line for measuring the longitudinal residual stress at the point; paste unidirectional strain gauge 1 at 0.5mm above strain gauge 2 along the vertical direction for measuring the rail waist vertical residual stress; paste strain gauge 3 at 0.5mm below strain gauge 2 along the direction perpendicular to strain gauge 2 for measuring the rail low transverse residual stress; (see Figure 1 、 Figure 2 )

[0045] b. Adopt epoxy resin glue to fix and apply 0.2-0.3MPa pressure to cure, ensure that the strain gauge is closely attached to the base;

[0046] 5. Data acquisition and calculation:

[0047] a. Connect the static strain gauge and record the initial strain value ε0;

[0048] b. Use the JY-RSD1 drilling device to drill a hole at P point, with a diameter of 2mm and a depth of 1mm (drilling stress release) ;

[0049] c. The distance between the three one-way strain gauges and the blind hole is 0.5mm;

[0050] d. Collect the released strain value ε1, and calculate the longitudinal residual stress σ according to Δε=ε1-ε0 and the elastic mechanics formula.

[0051] Example: Select 60kg / m U71Mn rail, length 1m. Surface treatment parameters: electrolytic polishing solution is perchloric acid-ethanol mixed solution (volume ratio 1:9), voltage 20V, time 90s. Strain gauge type KFG-5-120-C1-11L3M3R, blind hole drilling depth control accuracy ±0.02mm.

[0052] Actual measurement

[0053] Δ ε1 =108με;

[0054] Δ ε2 =96με;

[0055] Δ ε3 =78με;

[0056] Calculated

[0057] σ1=22.4MPa;

[0058] σ2=19.9MPa;

[0059] σ3=16.1MPa.

[0060] The above-described examples are only to describe the preferred mode of the present application, and not to limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. A measurement method based on local stress release and strain feedback, characterized in that: The specific steps are as follows: 1) Sample preparation: Cut a rail sample of 1m in length; 2) Positioning mark: At the midpoint of the rail length, locate vertically downward along the side of the rail waist to the center of the transition area between the rail bottom and the rail waist arc, which is recorded as measuring point P; 3).Surface treatment: a. Use an angle grinder with a grinding wheel to roughly grind the area around the measuring point P; b. Use electrolytic polishing equipment to fine-polish the rough grinding area, with a surface roughness of Ra ≤ 1.0 μm; 4). Strain gauge layout: a. Locate point P at the center of the polishing area. Attach unidirectional strain gauge 2 longitudinally along the line connecting the rail waist and rail base 0.5 mm to the right of point P to measure the longitudinal residual stress at that point. Attach unidirectional strain gauge 1 vertically 0.5 mm above strain gauge 2 to measure the vertical residual stress in the rail waist. Attach strain gauge 3 perpendicular to strain gauge 2 0.5 mm below strain gauge 2 to measure the transverse residual stress in the rail base. b. Fix and ensure that the strain gauge fits tightly to the substrate; 5) Data collection and calculation: a. Connect the static strain gauge and record the initial strain value ε0; b. Use a drilling device to drill a blind hole at point P with a diameter of 2 mm and a depth of 1 mm; c. The distance between the three unidirectional strain gauges and the blind hole is 0.5mm; d. Collect the strain value ε1 after release and calculate the longitudinal residual stress σ according to Δε=ε1-ε0 and the elastic mechanics formula.

2. The measurement method based on local stress release and strain feedback according to claim 1, characterized in that: In step 1), ensure that there are no visible defects at the connection between the rail bottom and the rail waist.

3. The measurement method based on local stress release and strain feedback according to claim 1, characterized in that: In the step 3), the grinding wheel particle size is ≥600#.

4. The measurement method based on local stress release and strain feedback according to claim 1, characterized in that: In step 3), a 20 mm×20 mm area around the measuring point P is coarsely ground.

5. The measurement method based on local stress release and strain feedback according to claim 1, characterized in that: In the step 4), epoxy resin glue is used for fixing.

6. The measurement method based on local stress release and strain feedback according to claim 5, characterized in that: In step 4), a pressure of 0.2-0.3 MPa is applied to solidify the strain gauge to ensure that the strain gauge is tightly fitted to the substrate.

7. The measurement method based on local stress release and strain feedback according to claim 1, characterized in that: Use the drilling device model JY-RSD1 to drill a hole at point P.

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