Fatigue life prediction method and system for arc threaded connection pair bolt

Abstract

The invention relates to a fatigue life prediction method and system for an arc threaded connection pair bolt, and belongs to the technical field of fatigue life prediction. The method comprises the following steps that a prediction result for predicting the fatigue life of the bolt is obtained by combining a correction relational expression of the total strain amplitude and the fatigue life of the arc threaded connection pair bolt with a fatigue load spectrum and a fatigue cumulative damage rule. According to the method, a fitting relational expression of total strain amplitude and stress amplitude, a hysteresis loop curve expression and a modified Neuber hyperbolic expression are combined to obtain local stress-strain response in combination with a fatigue load spectrum and a fatigue notch coefficient at the root of the bolt; a size effect coefficient, a surface processing strengthening coefficient and an SWT damage parameter selected through the local stress-strain response are used for correcting an elastic line of a fitting relational expression of the total strain amplitude and the fatigue life to obtain a corrected relational expression; the correction relation is combined with a fatigue load spectrum and a fatigue accumulated damage rule to realize high-precision prediction of the fatigue life of the bolt.

Description

Technical Field

[0001] The invention relates to a fatigue life prediction method and system for arc thread connection pair bolts, belonging to the technical field of fatigue life prediction. Background Art

[0002] In actual engineering, fatigue failure of mechanical parts is the most common form of failure, accounting for 50% to 90% of the total failure (total failure) of mechanical parts. Taking the arc thread connection pairs, metric thread connection pairs, and MJ thread connection pairs commonly used in large wind turbines as an example, the fatigue failure of the bolts in the connection pairs accounts for as much as 80.6% of the total bolt failure.

[0003] The fatigue life of bolts is usually divided into two stages: fatigue crack initiation life and fatigue crack propagation life. Fatigue life is affected by many factors, such as notch effect, average stress, size effect, surface processing method (or thread processing method), surface strengthening and anti-corrosion coating, etc. Generally, the fatigue life of bolts will be shortened in working environments with high stress levels and large stress amplitudes, while the fatigue life of bolts will be relatively long in working environments with low stress levels and small stress amplitudes; under the same working environment, bolts made of high-toughness and high-strength materials have relatively longer fatigue life.

[0004] The development of fatigue failure is a gradual process in which fatigue damage gradually accumulates. Usually, fatigue failure will occur after the bolt has worked for a period of time, and there will be no obvious signs before failure occurs. Therefore, fatigue failure occurs suddenly.

[0005] The sudden occurrence of fatigue failure makes it difficult to predict the fatigue fracture of the bolted parts of the bolts that have fatigue failure, and the risk is relatively high. Therefore, sufficient attention must be paid to the fatigue resistance design of the bolts. For the bolt structure design of important connection parts of wind power equipment (such as bolting of towers, bolting of blade hubs, bolting of yaw bearings, etc.), first of all, its quality must be guaranteed to be stable and reliable. During the design process, it is also necessary to reasonably predict the fatigue life of the bolts used in the design, and continuously optimize the feasibility of the design scheme to ensure the safety and reliability of the product.

[0006] Arc threaded connection bolts have been widely used in wind power equipment, such as anchor bolt assemblies for prestressed wind turbine foundations and blade root connection bolt assemblies. Figure 1As shown, the arc thread bolt 1 is threaded with the nut 2, which has great advantages in processing, manufacturing, structural strength and fatigue resistance compared with ordinary metric threads and MJ threads. For example, the Chinese invention patent application with application publication number CN109967982A discloses a method for preparing an arc thread anchor assembly for a wind turbine generator set, wherein the external thread on the anchor is a continuous arc thread, including an upper tooth top arc, a straight line segment, a lower tooth bottom arc, and a straight line segment connected in sequence, and the straight line segment is tangent to the upper tooth top arc and the lower tooth bottom arc respectively. Since the arc thread adopts a large pitch and non-uniform tooth thickness, not only the axial fit clearance is large, but also stress concentration can be reduced and fatigue resistance can be improved. The Chinese utility model patent with authorization announcement number CN209818480U discloses a high-strength arc thread connection pair, the arc thread connection pair adopts an arc thread tooth shape, the internal thread of the arc thread connection pair nut and the external thread of the arc thread connection pair bolt are in surface contact, the force is relatively uniform, and the stress concentration when the thread is loaded can be effectively reduced, and the fatigue resistance is good. The utility model patent discloses two arc thread connection substructures with thread major diameters of about 42 mm and 48 mm, with pitches of 6.35 mm and 8.5 mm respectively, and the ratios of arc thread pitch to thread major diameter are 0.151 and 0.177 respectively. The stress cross-sectional area of ​​the arc thread is higher than that of the ordinary thread. The Chinese invention patent with authorization announcement number CN116877555B discloses a stud for connecting the root of a wind turbine blade and a processing method thereof, wherein the external thread of the stud thread section is an arc thread, and the nominal diameter d of the arc thread is 0.151 and 0.177 respectively. 公称 =36mm~40mm, pitch P of arc thread r =(0.153~0.192)d 公称 , arc thread tooth height h r =(0.37~0.44)P r , the minor diameter d of the arc thread 1r =d 公称 -0.8746P r .

[0007] In summary, the arc thread connection pair improves the fatigue resistance in the application of wind turbine tower bolt connection and anchor bolt connection. However, the application environment of the stud bolts used for the root connection of wind turbine blades is more complex, and they are subjected to impact and vibration alternating stress loads. Due to the limited installation space and the overall strength of the blade, the major diameter of the external thread will not be too large, generally within the range of 36mm to 42mm, which puts higher requirements on the fatigue resistance of the bolts.

[0008] However, the arc threaded bolts, metric threaded bolts and MJ threaded bolts currently used in wind power equipment design are based on infinite life design methods, and the fatigue life of the bolts is not reliably predicted. Some bolts are designed for fatigue resistance based on nominal stress. However, since it ignores the complex situation of the high stress area at the bottom of the tooth, the basic assumptions of the nominal stress method are inconsistent with the fatigue mechanism of fatigue damage in actual bolts, the fatigue characteristics equivalent relationship between the bolt specimen and the standard test piece is difficult to determine, and a large amount of material fatigue performance data (SN curve (stress-life curve) or pSN curve (survival rate-stress-life curve)) is required. As a result, when predicting the fatigue life of bolts based on the nominal stress method, there are generally problems such as low prediction accuracy, unstable prediction results, reliance on experience, high cost and low efficiency. Compared with the test results, the fatigue life prediction results usually differ by more than 3 times, and sometimes by 10 times or even more than 10 times. A research result of Smith et al. in fatigue research in other structural fields: the maximum stress σ at a given fatigue life is max and strain amplitude ε a will change, but the product of the two will remain unchanged, so he proposed the product of maximum stress and strain amplitude ε a σ max The SWT model is used as the damage parameter. Compared with other models, for materials with tensile cracking as the main cracking mode, this model can better describe the role of tensile stress and strain in low-cycle fatigue, and has good life prediction results for most materials. However, it does not take into account the influence of stress gradient, size effect, surface strengthening effect, etc. according to the characteristics of arc threaded connection bolts, so the fatigue life prediction is inaccurate. Therefore, it is urgent to study and develop a new fatigue life prediction method for arc threaded connection bolts to improve the prediction accuracy and efficiency of bolt fatigue life and reduce the cost of fatigue life prediction. Summary of the invention

[0009] The purpose of the present invention is to provide a fatigue life prediction method and system for arc threaded connection bolts, so as to solve the problem that the fatigue life prediction results of arc threaded connection bolts obtained based on existing prediction methods are inaccurate.

[0010] To achieve the above object, the solution of the present invention includes:

[0011] A fatigue life prediction method for a circular arc thread connection pair bolt of the present invention comprises the following steps:

[0012] The modified relationship between the total strain amplitude and fatigue life of the arc thread connection bolt based on the SWT damage parameter is combined with the fatigue load spectrum and the fatigue cumulative damage law to obtain the prediction result for predicting the fatigue life of the arc thread connection bolt.

[0013] The correction relationship is obtained by elastically correcting the fitting relationship between the total strain amplitude and fatigue life of the arc threaded bolts by using the size effect coefficient of the arc threaded bolts, the surface processing strengthening coefficient and the SWT damage parameter selected by the local stress-strain response at the tooth bottom of the arc threaded bolts.

[0014] The local stress-strain response is obtained by combining the fitting relationship between the total strain amplitude and the stress amplitude of the arc thread connection bolt, the hysteresis loop curve expression and the modified Neuber hyperbola expression, and then combining the fatigue load spectrum and the fatigue notch coefficient at the tooth bottom of the arc thread connection bolt.

[0015] Furthermore, the expression of the SWT damage parameter is as follows:

[0016] ε a σ max =σ max (ε max -ε min ) / 2

[0017] In the formula, ε a σ max is the SWT damage parameter, ε a is the strain amplitude, σ max is the maximum local stress at the thread notch of the arc thread connection bolt, ε max is the maximum strain, ε min is the minimum strain.

[0018] Furthermore, the expression of the modified fatigue strength index adopted by the elastic line correction is as follows:

[0019]

[0020] Where b * is the modified fatigue strength index, b is the fatigue strength index, K t is the stress concentration factor, ε is the size effect factor, β3 is the surface processing strengthening factor, K f is the fatigue notch factor, and N0 is the number of cycles corresponding to the fatigue limit.

[0021] Furthermore, the expression of the size effect coefficient is as follows:

[0022]

[0023] Wherein, ε is the size effect coefficient, ε0 is the reference size effect coefficient, d is the nominal diameter of the arc thread connection bolt, the nominal diameter ranges from 30 mm to 120 mm, A1 and A2 are both size effect fitting parameters; ε0, A1 and A2 are constants related to the value range of the nominal diameter.

[0024] Furthermore, the fitting relationship between the total strain amplitude and fatigue life of the arc threaded connection bolts is obtained by least square method, using the total strain amplitude and corresponding fatigue life of the arc threaded connection bolts at each strain level to perform power function fitting in a double logarithmic coordinate system.

[0025] Furthermore, the fitting relationship between the total strain amplitude and stress amplitude of the arc threaded connection bolt is obtained by least square method by fitting the total strain amplitude and stress amplitude of the arc threaded connection bolt at each strain level in a Cartesian rectangular coordinate system.

[0026] Furthermore, the fatigue notch coefficient at the tooth root of the arc thread connection bolt is obtained by using the Peterson method based on point stress and sensitivity coefficient to calculate the stress concentration coefficient at the tooth root of the arc thread connection bolt, or by using the Neuber method based on average stress to calculate the stress concentration coefficient at the tooth root of the arc thread connection bolt, or by using the Siebel-Stieler method based on relative stress gradient to calculate the stress concentration coefficient at the tooth root of the arc thread connection bolt, or by using the Zhao Shaobian method based on relative stress gradient to calculate the stress concentration coefficient at the tooth root of the arc thread connection bolt.

[0027] Furthermore, the stress concentration coefficient at the root of the arc thread connection bolt is obtained by utilizing finite element simulation technology and an axisymmetric bolt model. An axial average stress corresponding to the nominal stress at the thread notch is loaded on the end face of the arc thread connection bolt away from the thread engagement portion, and the size of the finite element mesh is gradually reduced, and the finite element mesh of the area near the root of the bolt is encrypted. The condition for stopping the encryption is that the relative error of the stress concentration coefficient obtained by this encryption and the previous encryption solution is less than or equal to 0.1%.

[0028] Furthermore, in the process of gradually reducing the finite element mesh size, the finite element mesh size after each step of reduction is γ times the finite element mesh size before the step of reduction, γ∈[0.3,0.6].

[0029] A fatigue life prediction system for arc threaded connection bolts of the present invention comprises a processor, and the processor is used to execute a computer program to implement the steps of the fatigue life prediction method for arc threaded connection bolts as described above.

[0030] Beneficial effects of the present invention:

[0031] The present invention is a pioneering invention, which provides a fatigue life prediction method for arc thread connection pair bolts. Specifically, a correction relationship between the total strain amplitude and fatigue life of the arc thread connection pair bolts based on the SWT damage parameter is combined with the fatigue load spectrum and the fatigue cumulative damage law to obtain a prediction result for predicting the fatigue life of the arc thread connection pair bolts. The present invention obtains a local stress-strain response that can reflect the complex situation of fatigue strength reduction in the high stress area of ​​the tooth bottom of the arc threaded connection pair bolt by combining a fitting relationship between the total strain amplitude and the stress amplitude of the arc threaded connection pair bolt, a hysteresis loop curve expression of the arc threaded connection pair bolt and a modified Neuber hyperbola expression of the arc threaded connection pair bolt, and combines the fatigue load spectrum and the fatigue notch coefficient at the tooth bottom that can reflect the complex situation of fatigue strength reduction in the high stress area of ​​the tooth bottom of the arc threaded connection pair bolt. The fitting relationship between the total strain amplitude and the fatigue life of the arc threaded connection pair bolt is elastically corrected by using the size effect coefficient of the arc threaded connection pair bolt, the surface machining strengthening coefficient of the arc threaded connection pair bolt and the SWT damage parameter selected through the local stress-strain response to obtain a modified relationship between the total strain amplitude and the fatigue life based on the SWT damage parameter. The modified relationship is combined with the fatigue load spectrum and the fatigue cumulative damage law to achieve high-precision prediction of the fatigue life of the arc threaded connection pair bolt, and the prediction result obtained is accurate and the prediction accuracy is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the structural coordination of an existing arc thread connection pair;

[0033] Figure 2 It is a fatigue life prediction flow chart of arc thread connection bolts;

[0034] Figure 3 is the fitting curve graph of total strain amplitude and cycle reversal number;

[0035] Figure 4 is a fitted curve diagram of the stress amplitude as the cyclic stress and the total strain amplitude as the cyclic strain;

[0036] Figure 5 It is a schematic diagram reflecting the local mesh encryption in the area near the bottom of the arc threaded connection bolt.

[0037] Figure 6 It is the stress distribution cloud diagram of the area near the bottom of the arc thread connection bolt finite element simulation;

[0038] Figure 7 This is a comparison chart between the fatigue life prediction results and the test results of the arc thread connection bolts. DETAILED DESCRIPTION

[0039] In order to solve the problems in the background technology, the present invention takes into account the complex situation of fatigue strength reduction in the high stress area at the bottom of the thread of the arc threaded connection bolt, uses correction measures and appropriate damage parameters, and based on the modified local stress-strain method, only one material fatigue performance test is performed on the sample to obtain the corresponding fatigue parameters, so that the fatigue life of the arc thread high-strength bolt can be predicted with high accuracy. The prediction result obtained by this prediction method is accurate and the prediction accuracy is high.

[0040] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0041] An embodiment of a fatigue life prediction method for arc thread connection bolts:

[0042] A fatigue life prediction method for arc thread connection bolts comprises the following steps:

[0043] Through the fitting relationship between the total strain amplitude and stress amplitude of the arc threaded bolt, the hysteresis loop curve expression of the arc threaded bolt and the modified Neuber hyperbola expression of the arc threaded bolt, the fatigue load spectrum and the fatigue notch coefficient at the tooth bottom that can reflect the complex situation of fatigue strength reduction in the high stress area of ​​the tooth bottom of the arc threaded bolt are combined to obtain the local stress-strain response that can reflect the complex situation of fatigue strength reduction in the high stress area of ​​the tooth bottom of the arc threaded bolt.

[0044] The size effect coefficient of the arc threaded bolt, the surface processing strengthening coefficient of the arc threaded bolt, and the SWT damage parameter selected by the local stress-strain response that can reflect the complex situation of fatigue strength reduction in the high stress area at the bottom of the arc threaded bolt are used to perform elastic line correction on the fitting relationship between the total strain amplitude and fatigue life of the arc threaded bolt, and the corrected relationship between the total strain amplitude and fatigue life based on the SWT damage parameter is obtained;

[0045] The modified relationship is combined with the fatigue load spectrum and the fatigue cumulative damage law to obtain a prediction result for predicting the fatigue life of the arc thread connection pair bolts, thereby achieving a high-precision prediction of the fatigue life of the arc thread connection pair bolts. The obtained prediction result is accurate and has a high prediction accuracy.

[0046] Among them, the corrected relationship between the total strain amplitude and fatigue life of the arc thread connection bolt based on the SWT damage parameters takes into account the influence of the bolt size effect and the bolt surface strengthening effect. The size effect coefficient of the arc thread connection bolt, the surface processing strengthening coefficient and the SWT damage parameters selected by the local stress-strain response at the tooth bottom are used to perform elastic line correction on the fitting relationship between the total strain amplitude and the fatigue life.

[0047] Among them, the modified relationship between the total strain amplitude and fatigue life of the arc thread connection bolt based on the SWT damage parameter is as follows:

[0048]

[0049] In the formula, ε a σ max is the SWT damage parameter, ε a is the strain amplitude, σ max is the maximum stress (maximum local stress at the thread notch), σ f ′ is the fatigue strength coefficient, E is the cyclic elastic modulus, N f is fatigue life (cycle life), b * is the modified fatigue strength index, ε f ′ is the fatigue ductility coefficient, b is the fatigue strength index, and c is the fatigue ductility index.

[0050] Specifically, the expression of the SWT damage parameter is as follows:

[0051] ε a σ max =σ max (ε max -ε min ) / 2

[0052] In the formula, ε a σ max is the SWT damage parameter, ε a is the strain amplitude, σ max is the maximum local stress at the thread notch of the arc thread connection bolt, ε max is the maximum strain, ε min is the minimum strain.

[0053] Specifically, the expression of the modified fatigue strength index used in the elastic line correction is as follows:

[0054]

[0055] Where b * is the modified fatigue strength index, b is the fatigue strength index, K t is the stress concentration factor, ε is the size effect factor, β3 is the surface processing strengthening factor, K f is the fatigue notch factor, and N0 is the number of cycles corresponding to the fatigue limit.

[0056] Specifically, the size effect coefficient is determined by the following functional relationship with the nominal diameter of the arc thread connection bolt:

[0057]

[0058] In the formula, ε is the size effect coefficient, ε0 is the reference size effect coefficient, d is the nominal diameter of the arc thread connection bolt, the nominal diameter d of the arc thread connection bolt ranges from 30 mm (millimeter) to 120 mm, that is, d∈[30mm,120mm], A1 and A2 are both size effect fitting parameters; ε0, A1 and A2 are constants related to the value range of the nominal diameter of the arc thread connection bolt.

[0059] Specifically, the fitting relationship between the total strain amplitude and fatigue life of the arc threaded connection bolts is obtained by least square method by fitting the total strain amplitude and the corresponding fatigue life of the arc threaded connection bolts at each strain level in a double logarithmic coordinate system.

[0060] Among them, the fitting relationship between the total strain amplitude and fatigue life of the arc thread connection bolt is as follows:

[0061]

[0062] In the formula, Δε t / 2 is the total strain amplitude, σ f ′ is the fitting coefficient (fatigue strength coefficient), E is the cyclic elastic modulus, N f is fatigue life (cycle life), b is fitting index (fatigue strength index), ε f ′ is the fitting coefficient (fatigue ductility coefficient), and c is the fitting index (fatigue ductility index).

[0063] Specifically, the fitting relationship between the total strain amplitude and stress amplitude of the arc threaded connection bolt is obtained by least square method by fitting the total strain amplitude and stress amplitude of the arc threaded connection bolt at each strain level in a Cartesian rectangular coordinate system (plane rectangular coordinate system).

[0064] Among them, the fitting relationship between the total strain amplitude and stress amplitude of the arc thread connection bolt is as follows:

[0065]

[0066] In the formula, Δε t / 2 is the total strain amplitude, Δσ / 2 is the stress amplitude, E is the cyclic elastic modulus, K′ is the fitting coefficient (cyclic strength coefficient), and n′ is the fitting exponent (cyclic strain hardening exponent).

[0067] Specifically, the fatigue notch coefficient at the tooth root of the arc threaded connection bolt is obtained by using the stress concentration coefficient at the tooth root of the arc threaded connection bolt using the Peterson method based on point stress and sensitivity coefficient, or by using the stress concentration coefficient at the tooth root of the arc threaded connection bolt using the Neuber method based on average stress, or by using the Siebel-Stieler method based on relative stress gradient, or by using the Zhao Shaobian method.

[0068] Specifically, the stress concentration coefficient at the root of the arc threaded connection bolt is converted by using finite element simulation technology and an axisymmetric bolt model through the conversion relationship between the distal load and the nominal stress. The axial average stress corresponding to the nominal stress at the thread notch is loaded on the end face of the arc threaded connection bolt away from the thread engagement portion, and the size of the finite element mesh is gradually reduced, and the finite element mesh of the area near the bolt root is encrypted. The condition for stopping the encryption is that the relative error of the stress concentration coefficient obtained by this encryption and the previous encryption solution is less than or equal to 0.1%.

[0069] The number of times the finite element mesh is encrypted near the bolt tooth bottom varies with the bolt structure. The mesh is encrypted only when the relative error of the stress concentration factor obtained from the two encrypted solutions is ≤0.1%.

[0070] Specifically, in the process of gradually reducing the finite element mesh size, the finite element mesh size after each step of reduction is γ times the finite element mesh size before the step of reduction, γ∈[0.3,0.6].

[0071] Specifically, the local stress-strain response at the tooth bottom of the arc thread connection bolt is obtained by the random load treatment method of the bolt, and the number of complete cycles and half cycles in the load spectrum after random load treatment and the corresponding cyclic response characteristics are obtained. The load spectrum after random load treatment is used, and the modified Neuber method is used to combine the cyclic stress-strain curve and the hysteresis loop curve to obtain it.

[0072] Among them, the local stress-strain response at the root of the arc thread connection bolt is determined by the following expression:

[0073]

[0074] In the formula, σ i is the local stress response of the notch corresponding to the i-th data point in the load spectrum, ε i is the local strain response of the notch corresponding to the i-th data point in the load spectrum, σ i+1 is the local stress response of the notch corresponding to the i+1th data point in the load spectrum, ε i+1is the local strain response of the notch corresponding to the i+1th data point in the load spectrum, σ i and ε i is the local response of the notch corresponding to the i-th data point in the load spectrum, σ i+1 and ε i+1 is the local response of the notch corresponding to the i+1th data point in the load spectrum; the local response includes stress and strain.

[0075] Specifically, a strain fatigue test is performed on a smooth small specimen by a strain control method to obtain the total strain amplitude and corresponding fatigue life at each strain level, and the total strain amplitude and stress amplitude at each strain level.

[0076] As another embodiment, a fatigue life prediction method for arc thread connection bolt pair is provided. Figure 2 , including the following steps:

[0077] (1) Obtain strain data.

[0078] The strain data is obtained through strain fatigue test records. Specifically, the strain fatigue test (axial constant amplitude low cycle fatigue test of metal materials) is carried out on a smooth small specimen through the strain control method. At each strain level (strain level or strain amplitude level), the cycle life (fatigue test life) N corresponding to the strain amplitude is obtained. f Valid data points, at the same time, the corresponding total strain amplitude Δε at each strain amplitude level is also obtained t / 2, plastic strain amplitude Δε p / 2, elastic strain amplitude Δε e / 2 and stress amplitude Δσ / 2.

[0079] The strain data include the corresponding total strain amplitude Δε at different strain levels: t / 2, plastic strain amplitude Δε p / 2, elastic strain amplitude Δε e / 2 and stress amplitude Δσ / 2, and the corresponding cycle life (fatigue test life) N corresponding to the strain amplitude f Valid data points.

[0080] (2) Obtain the fitting relationship between total strain amplitude and fatigue life.

[0081] The total strain amplitude Δε t / 2, elastic strain amplitude Δε e / 2, plastic strain amplitude Δε p / 2 respectively corresponds to the cycle life N f The curve is plotted in a double logarithmic coordinate system to obtain the total strain life curve ( Figure 2The total strain life curve is shown in the figure), and the total strain life equation (the fitting relationship between the total strain amplitude and fatigue life) is fitted based on the total strain life curve.

[0082] Specifically, the least square method is used in the double logarithmic coordinate system, with the total strain amplitude Δε at each strain level t / 2 and corresponding cycle life N f The effective data points were fitted with a power function in a double logarithmic coordinate system to obtain the fitting relationship between the total strain amplitude and the corresponding cycle life (fatigue life).

[0083] (3) Obtain the fitting relationship between the total strain amplitude and the stress amplitude.

[0084] The least square method is used to calculate the total strain amplitude Δε at each strain level in the Cartesian rectangular coordinate system. t / 2 and the stress amplitude Δσ / 2 are fitted with a power function to obtain the fitting relationship between the total strain amplitude and the stress amplitude.

[0085] (4) Obtain the corrected relationship between total strain amplitude and fatigue life based on SWT damage parameters.

[0086] Based on the total strain life equation, the size effect coefficient (size coefficient) ε and the surface strengthening coefficient (surface processing strengthening coefficient) β3 of the arc thread connection bolt are introduced. Combined with the bolt size effect and the bolt surface strengthening effect, the total strain life equation is elastically corrected by the SWT damage parameter selected through the local stress-strain response at the tooth bottom, and the corrected total strain life equation (the corrected relationship between the total strain amplitude and fatigue life based on the SWT damage parameter) is obtained.

[0087] Among them, the expression of the modified fatigue strength index used in the elastic line correction is as follows:

[0088]

[0089] Where b * is the modified fatigue strength index, b is the fatigue strength index, K t is the stress concentration factor, ε is the size effect factor (size factor), β3 is the surface processing strengthening factor, K f is the fatigue notch factor, and N0 is the number of cycles corresponding to the fatigue limit.

[0090] The calculation formula of the modified total strain life equation is as follows:

[0091]

[0092] In the formula, ε a σ max is the SWT damage parameter, ε a is the strain amplitude, σmax is the maximum stress (maximum local stress at the thread notch), σ f ′ is the fatigue strength coefficient, E is the cyclic elastic modulus, N f is fatigue life (cycle life), b * is the modified fatigue strength index, ε f ′ is the fatigue ductility coefficient, b is the fatigue strength index, and c is the fatigue ductility index.

[0093] Through the local stress-strain response, select the SWT damage parameter and select ε a σ max is the SWT damage parameter, where the strain amplitude is given by ε a =(ε max -ε min ) / 2 is calculated.

[0094] The expression of SWT damage parameter is as follows:

[0095] ε a σ max =σ max (ε max -ε min ) / 2

[0096] In the formula, ε a σ max is the SWT damage parameter, ε a is the strain amplitude, σ max is the maximum local stress at the thread notch, ε max is the maximum strain, ε min is the minimum strain.

[0097] (5) The stress concentration factor K at the bottom of the bolt teeth of the arc thread connection pair is obtained through finite element simulation analysis with high solution accuracy and convergence results. t .

[0098] Specifically, through finite element simulation technology, an axisymmetric bolt model is adopted, and the load conversion is performed through the conversion relationship between the distal load and the nominal stress. The axial average stress corresponding to the nominal stress at the thread notch is loaded at the distal end of the arc thread connection bolt (that is, the end face of the arc thread connection bolt away from the thread engagement part), and the finite element grid size is gradually reduced. The finite element grid near the bolt tooth bottom is encrypted multiple times to obtain a stress concentration factor (stress concentration factor) at the tooth bottom of the arc thread connection bolt with high accuracy and convergence.

[0099] Among them, the stress concentration factor at the root of the arc thread connection bolt is determined by the following expression:

[0100]

[0101] In the formula, K t is the stress concentration factor at the bottom of the arc thread connection bolt, σ max is the maximum local stress at the thread notch, σ n It is the nominal stress or average stress at the root of the thread notch.

[0102] Among them, the thread gap refers to the area range of the entire space of the screw tooth gap; the thread gap root refers to the vertex of the arc at the bottom of the screw tooth gap, which is a specific position.

[0103] The condition for stopping encryption is that the relative error of the stress concentration factor obtained by the current encryption and the previous encryption is less than or equal to 0.1%.

[0104] In the process of gradually reducing the finite element mesh size, the finite element mesh size after each step of reduction is γ times the finite element mesh size before the step of reduction, γ∈[0.3,0.6].

[0105] (6) Obtain the fatigue notch factor K at the bottom of the bolt of the arc threaded connection pair by the Neuber method of average stress, the Peterson method based on point stress and sensitivity method (sensitivity coefficient), the Siebel-Stieler method based on stress gradient, or the Zhao Shaobian method f (fatigue notch factor).

[0106] (7) Through the random load treatment method of the bolt, the number of complete cycles and half cycles in the load spectrum after random load treatment and the corresponding cyclic response characteristics are obtained. Using the load spectrum after random load treatment, the modified Neuber method is used to combine the cyclic stress-strain curve and the hysteresis loop curve to obtain the local stress-strain response at the bottom of the arc thread of the arc thread connection bolt.

[0107] (8) Finally, the fitting relationship between the total strain amplitude and fatigue life of the arc threaded connection bolts, the fitting relationship between the total strain amplitude and stress amplitude of the arc threaded connection bolts, the hysteresis loop curve expression, the modified Neuber hyperbola expression, the fatigue notch coefficient at the tooth root of the arc threaded connection bolts, the local stress-strain response at the tooth root of the arc threaded connection bolts and the modified relationship between the total strain amplitude and fatigue life of the arc threaded connection bolts based on the SWT damage parameter are combined as a nonlinear equation group. The nonlinear equation group is solved and combined with the fatigue load spectrum and the fatigue cumulative damage law to obtain the predicted fatigue life of the arc threaded connection bolts.

[0108] In step (1), the cycle life of the sample is less than 10 5The strain control was performed using an extensometer with a test frequency of 0.25 Hz to 2.5 Hz.

[0109] In step (1), during the fatigue test of the sample using the strain control method, 5 to 7 strain levels are selected, and the gradient of adjacent strain levels is 0.01% to 0.3%; the total strain amplitude (total strain amplitude) Δε obtained at each strain level t / 2 and the corresponding cycle life N f The number of valid data points is 2 to 4.

[0110] In step (4), the introduced size effect coefficient ε gradually decreases with the increase of the nominal diameter of the arc thread connection bolt. When the nominal diameter d satisfies 30mm≤d≤60mm, the value range of the size effect coefficient ε is 0.800≤ε≤0.900. When the nominal diameter d satisfies 60mm≤d≤120mm, the value of the size effect coefficient ε is 0.750≤ε≤0.800.

[0111] Specifically, the introduced size effect coefficient ε is determined by the value calculated according to the following formula:

[0112]

[0113] In the formula, ε is the size effect coefficient, ε0 is the reference size effect coefficient, d is the nominal diameter of the arc thread connection bolt, the nominal diameter d of the arc thread connection bolt ranges from 30 mm (millimeter) to 120 mm, that is, d∈[30mm,120mm], A1 and A2 are both size effect fitting parameters; ε0, A1 and A2 are constants related to the value range of the nominal diameter of the arc thread connection bolt.

[0114] Specifically, when the nominal diameter d of the arc threaded connection bolt satisfies 30mm≤d≤60mm, ε0=23 / 30, A1=8 / 15, A2=-21.64043; when the nominal diameter d of the arc threaded connection bolt satisfies 60mm≤d≤120mm, ε0=22 / 30, A1=4 / 15, A2=-43.28085.

[0115] Specifically, the nominal diameter of the arc thread connection auxiliary bolt is 36mm, and the calculated value of the size effect coefficient ε is 0.870; the nominal diameter of the arc thread connection auxiliary bolt is 39mm, and the calculated value of the size effect coefficient ε is 0.855; the nominal diameter of the arc thread connection auxiliary bolt is 42mm, and the calculated value of the size effect coefficient ε is 0.840.

[0116] In step (4), for the arc threaded connecting bolt formed by rolling, the introduced surface strengthening coefficient β3 has a value range of 1.15 to 1.25.

[0117] In step (5), the local mesh size of the area near the bolt tooth bottom in the subsequent simulation is reduced to 0.3 to 0.6 times the mesh size of the previous simulation, until the relative error of the stress concentration factor obtained from the two simulations is no more than 0.1%, and it is considered that a converged solution with high accuracy can be obtained.

[0118] In step (5), the converged stress concentration factor K t The expression is as follows:

[0119]

[0120] In the formula, K t is the stress concentration factor at the bottom of the arc thread connection bolt, σ max is the maximum local stress at the thread notch, σ n It is the nominal stress or average stress at the root of the thread notch.

[0121] In step (6), the fatigue notch factor (bolt tooth bottom fatigue notch factor) is defined as:

[0122]

[0123] In the formula, K f is the fatigue notch factor of the bolt tooth bottom, S e is the fatigue strength of the smooth specimen (smooth specimen), S N is the fatigue strength of the notched specimen.

[0124] Specifically, the fatigue notch factor K f Calculated by any of the following formulas ① to ④:

[0125] ① The fatigue notch coefficient K at the bottom of the bolt of the arc thread connection obtained by the Peterson method based on the sensitivity method (sensitivity coefficient) f The calculation formula is as follows:

[0126]

[0127] In the formula, K f is the fatigue notch coefficient of the bolt tooth bottom, q is the sensitivity coefficient, K t is the stress concentration factor, α is the Peterson parameter of the bolt, and ρ is the bolt tooth bottom radius (in mm).

[0128] Among them, the Peterson parameter of tempered steel bolts is 0.0635mm, and the Peterson parameter of normalized steel bolts is 0.254mm.

[0129] For medium and high strength steel (limit stress u>560MPa (megapascals) for bolts, the approximate calculation formula for α is as follows:

[0130]

[0131] Where α is the Peterson parameter, σ b For tensile strength.

[0132] ② The fatigue notch coefficient K at the bottom of the arc thread connection bolt obtained by Neuber method f The calculation formula is as follows:

[0133]

[0134] In the formula, K f is the fatigue notch coefficient of the bolt tooth bottom; A is the Neuber parameter, which is a material constant and is related to the strength limit of the bolt material. A=f(σ b );K t is the stress concentration factor, σ b For tensile strength.

[0135] ③ The fatigue notch coefficient K at the bottom of the bolt of the arc thread connection obtained by the Siebel-Stieler method based on stress gradient f The calculation formula is as follows:

[0136]

[0137] In the formula, K f is the fatigue notch factor of the bolt tooth bottom, K t is the stress concentration factor; S q is a material constant, a function of the yield limit, defined as S q =f(σ s ), σ s is the yield limit; χ is the relative stress gradient.

[0138] ④ The fatigue notch coefficient K at the bottom of the arc thread connection bolt obtained by Zhao Shaobian method f The calculation formula is as follows:

[0139]

[0140] In the formula, K f is the fatigue notch factor of the bolt tooth bottom, K t is the stress concentration factor, Q is the relative stress gradient (in mm -1 ), A and b are constants related to material heat treatment, etc.

[0141] For example: when the steel is in the hot-rolled state and has not been heat treated, A is taken as 0.336 and b is taken as 0.345.

[0142] The raw materials used for high-strength bolts are generally quenched and then tempered. In this case, A is taken as 0.290 and b is taken as 0.152. If it is normalized, A is taken as 0.423 and b is taken as 0.279.

[0143] In step (7), the random load processing method includes a cycle counting method and a power spectrum method.

[0144] Preferably, the random load processing method uses the rain flow counting method in the cycle counting method, which can be easily implemented through a computer program when the amount of random load data is large.

[0145] In step (7), the steps of obtaining the local stress-strain response at the bottom of the arc thread by combining the cyclic stress-strain curve and the hysteresis loop curve using the modified Neuber method are as follows:

[0146] Step I: Given the nominal stress S1 or nominal strain e1, obtain the corresponding e1 or S1 from the cyclic stress-strain curve.

[0147] Specifically, the modified Neuber curve and the cyclic stress-strain curve (σ a -ε a Curve), the following relationship is used to obtain the nonlinear equation system:

[0148]

[0149] Solve the nonlinear equations to obtain the local gap σ1 and ε1.

[0150] e1 or S1 can also be obtained from the cyclic stress-strain curve.

[0151] Step II: After the load is reversed, if ΔS or Δe is known, the corresponding Δe or ΔS can be obtained from the hysteresis loop curve.

[0152] Specifically, the nonlinear equations are obtained by combining the modified Neuber curve and the hysteresis loop curve (Δσ-Δε curve) using the following relationship:

[0153]

[0154] Solve the nonlinear equations to obtain the local Δσ and Δε of the notch.

[0155] Step III: After the subsequent load reversal, the local response of the notch corresponding to the i-th data point in the load spectrum is σ i and ε i , the local response of the notch corresponding to the i+1th data point in the load spectrum is σ i+1and ε i+1 , the nonlinear equations are obtained by combining the following relations:

[0156]

[0157] Solve the nonlinear equations to obtain the notch local response σ corresponding to the i+1th data point in the load spectrum i+1 and ε i+1 .

[0158] In step (7), the expression of the cyclic stress-strain curve is as follows:

[0159]

[0160] In the formula, ε a is the strain amplitude, σ a is the stress amplitude, E is the cyclic elastic modulus, K′ is the cyclic strength coefficient, and n′ is the cyclic strain hardening exponent.

[0161] In step (7), the expression of the hysteresis loop curve is as follows:

[0162]

[0163] Where Δε is the local strain range of the notch, Δσ is the local stress range of the notch, E is the cyclic elastic modulus, K′ is the cyclic strength coefficient, and n′ is the cyclic strain hardening exponent.

[0164] In step (8), the fatigue cumulative damage law is based on the modified linear cumulative damage theory, that is, the modified Palmgren-Miner cumulative damage theory, which is expressed by the following formula:

[0165]

[0166] Where n i is the number of actual load cycles, N i n i The failure cycle life under the stress level, a is a constant.

[0167] A large number of studies have shown that under constant amplitude load, a is often taken as 1, and under random load, when a is taken as 0.7, its life estimation result is safer than Miner's law, and the accuracy of life estimation is generally improved compared with Miner's law.

[0168] The present invention belongs to the field of fatigue life prediction of bolt structures, and provides a fatigue life prediction method for arc thread connection bolts, which can predict the fatigue life of large-size arc thread high-strength bolts used in professional fields such as wind power. The method is based on a modified local stress-strain method, and adopts a method combining material testing and structural simulation analysis. Considering the complex situation of the high stress area at the bottom of the arc thread high-strength bolt tooth, by taking certain correction measures, selecting appropriate damage parameters, and only performing a material fatigue performance test on the sample once to obtain the corresponding fatigue parameters, the fatigue life prediction of the bolt with high accuracy is realized. The prediction method has high prediction accuracy, a wide range of application, and greatly reduced costs. The parameters in the prediction method have realistic physical meanings, and the prediction results have certain scientificity, accuracy and effectiveness. The present invention only needs to use a small number of samples to perform low-cycle fatigue tests to obtain fatigue parameters. Compared with the existing traditional (classical) bolt life prediction method based on the nominal stress method and requiring a large number of bolt fatigue tests to obtain SN curves, the economic cost and time cost of the test are greatly reduced. The process is easy to implement, the cost is low, and the accuracy of the life prediction results is also greatly improved, which can effectively solve the problem of inaccurate fatigue life prediction results of bolt components. In practical applications, the fatigue life prediction results of the arc thread connection pair bolts are usually used as the fatigue life prediction results of the arc thread connection pair to achieve the prediction of the fatigue life of the arc thread connection pair.

[0169] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0170] The present invention obtains fatigue parameters through material strain fatigue test, combines finite element simulation technology to perform systematic simulation analysis on the bolt structure, and realizes fatigue life prediction of arc thread connection pair bolts under large-size service status.

[0171] The present invention has good prediction accuracy, especially for arc threaded connection bolts with specifications of 36 mm and above. The prediction accuracy of the fatigue life of the prediction method of the present invention is much better than the prediction accuracy of the fatigue life based on the SN curve.

[0172] The present invention has good universality and is applicable to bolts with other thread forms that have high loads and large local deformations.

[0173] The present invention is based on material strain fatigue test, finite element simulation technology and local stress-strain solution, and can obtain life prediction of large-sized in-service bolts of engineering equipment. The method is simple, accurate and efficient, and can save labor costs, test costs and test time to a great extent.

[0174] The prediction results of the prediction method of the present invention can be used to guide the operation and maintenance monitoring and detection of bolts, improve the bearing capacity and service safety of bolt fasteners, and have important scientific value and engineering significance for optimizing bolt parameters and preventing fatigue fracture.

[0175] To facilitate understanding, here is an example:

[0176] Fatigue life prediction method for arc thread connection bolts, such as Figure 2 As shown, the following steps are included:

[0177] (1) A strain fatigue test (axial constant amplitude low cycle fatigue test of metal materials) is performed on a smooth small specimen by a strain control method to obtain the cycle life (fatigue test life) N corresponding to the strain amplitude at each strain level (strain level). f Valid data points, at the same time, the corresponding total strain amplitude Δε at each strain amplitude level is also obtained t / 2, plastic strain amplitude Δε p / 2, elastic strain amplitude Δε e / 2 and stress amplitude Δσ / 2.

[0178] According to the specimen requirements of GB / T 15248-2008 "Method for axial constant amplitude low cycle fatigue test of metallic materials", the specimens with equal cross-section were processed, and the surface roughness of the specimen test section met the requirements of the national standard. The fatigue test was completed on the MTS hydraulic fatigue tester, and the strain levels were 0.4%, 0.5%, 0.55%, 0.7%, 0.8%, 0.9% and 0.12%, respectively. The cycle life N corresponding to the strain amplitude was obtained at each strain level. f The number of valid data points is 2. The test frequencies are 1.25Hz, 1.00Hz, 0.909Hz, 0.714Hz, 0.625Hz, 0.556Hz and 0.417Hz respectively. The test ends when the cyclic stress decreases by 20% to 30%.

[0179] (2) The total strain amplitude Δε t / 2, elastic strain amplitude Δε e / 2, plastic strain amplitude Δε p / 2 respectively corresponds to the cycle life N f The curve is plotted in a double logarithmic coordinate system to obtain the total strain life curve ( Figure 2 (where is the strain-life curve), such as Figure 3 shown.

[0180] Based on the total strain-life curve, the total strain-life equation (the fitting relationship between the total strain amplitude and the fatigue life, i.e., the Manson-Coffin equation) is fitted. Specifically, the least squares method is used to fit the total strain amplitude Δε at each strain level. t / 2 and corresponding cycle life N f The power function is fitted in the double logarithmic coordinate system to obtain the following total strain amplitude Δε t / 2 and corresponding cycle life N f The fitting relationship between:

[0181]

[0182] In the formula, Δε t / 2 is the total strain amplitude, σ f ′ is the fitting coefficient (fatigue strength coefficient), E is the cyclic elastic modulus, N f is fatigue life (cycle life), b is fitting index (fatigue strength index), ε f ′ is the fitting coefficient (fatigue ductility coefficient), and c is the fitting index (fatigue ductility index).

[0183] Among them, the fatigue strength coefficient σ f ′=1.33772GPa (gigaPa), fatigue ductility coefficient ε f ′=0.30835, fatigue strength index b=-0.07040, fatigue ductility index c=-0.55703, cyclic elastic modulus E=194530MPa (megapascals).

[0184] (3) The total strain amplitude Δε t / 2 and stress amplitude Δσ / 2 are plotted in the Cartesian rectangular coordinate system (plane rectangular coordinate system) to obtain the total strain stress curve ( Figure 2 (where ) is the cyclic stress-strain curve, such as Figure 4 shown.

[0185] The least square method is used to calculate the total strain amplitude Δε at each strain level. t / 2 and stress amplitude Δσ / 2 are fitted by power function, and the fitting relationship between the total strain amplitude and stress amplitude is obtained as follows:

[0186]

[0187] In the formula, Δε t / 2 is the total strain amplitude, Δσ / 2 is the stress amplitude, E is the cyclic elastic modulus, K′ is the fitting coefficient (cyclic strength coefficient), and n′ is the fitting exponent (cyclic strain hardening exponent).

[0188] Among them, the cyclic strength coefficient K′=1343.231MPa, the cyclic strain hardening exponent n′=0.10235, and the cyclic elastic modulus E=194530MPa.

[0189] (4) Based on the total strain life equation, the size effect coefficient (size coefficient) ε and the surface strengthening coefficient (surface processing strengthening coefficient) β3 of the arc thread connection bolt are introduced. Combined with the bolt size effect and the bolt surface strengthening effect, the total strain life equation is elastically corrected by the SWT damage parameter selected through the local stress-strain response at the tooth bottom to obtain the corrected total strain life equation.

[0190] Among them, the SWT damage parameter selected by local stress-strain response is ε a σ max , the expression of SWT damage parameter is as follows:

[0191] ε a σ max =σ max (ε max -ε min ) / 2

[0192] In the formula, ε a σ max is the SWT damage parameter, ε a is the strain amplitude, σ max is the maximum local stress at the thread notch, ε max is the maximum strain, ε min is the minimum strain.

[0193] The calculation formula of the modified total strain life equation is as follows:

[0194]

[0195] In the formula, ε a σ max is the SWT damage parameter, ε a is the strain amplitude, σ max is the maximum stress (maximum local stress at the thread notch), σ f ′ is the fatigue strength coefficient, E is the cyclic elastic modulus, N f is fatigue life (cycle life), b * is the modified fatigue strength index, ε f ′ is the fatigue ductility coefficient, b is the fatigue strength index, and c is the fatigue ductility index.

[0196] Among them, the expression of the modified fatigue strength index used in the elastic line correction is as follows:

[0197]

[0198] Where b * is the modified fatigue strength index, b is the fatigue strength index, K tis the stress concentration factor, ε is the size effect factor (size factor), β3 is the surface processing strengthening factor, K f is the fatigue notch factor, and N0 is the number of cycles corresponding to the fatigue limit.

[0199] Among them, the surface processing strengthening coefficient β3 = 1.20.

[0200] The calculation formula of the size effect coefficient is:

[0201]

[0202] In the formula, ε is the size effect coefficient, ε0 is the reference size effect coefficient, d is the nominal diameter of the arc thread connection bolt, the nominal diameter d of the arc thread connection bolt ranges from 30 mm (millimeter) to 120 mm, that is, d∈[30mm,120mm], A1 and A2 are both size effect fitting parameters; ε0, A1 and A2 are constants related to the value range of the nominal diameter of the arc thread connection bolt.

[0203] For arc threaded connecting bolts made of alloy structural steel and high-quality carbon structural steel, when the nominal diameter d of the arc threaded connecting bolts meets 30mm≤d≤60mm, ε0=23 / 30, A1=8 / 15, A2=-21.64043; when the nominal diameter d of the arc threaded connecting bolts meets 60mm≤d≤120mm, ε0=22 / 30, A1=4 / 15, A2=-43.28085.

[0204] Substituting the values ​​of ε0, A1, and A2 into the calculation formula of the size effect coefficient, the calculation formula of the size effect coefficient is:

[0205]

[0206] Substituting the nominal diameter data of the arc thread connection bolt, we can calculate:

[0207] When the nominal diameter of the arc thread connection bolt is 36mm, the size effect coefficient ε=0.870, and the modified fatigue strength index b * =-0.06620;

[0208] When the nominal diameter of the arc thread connection bolt is 39mm, the size effect coefficient ε=0.855, and the modified fatigue strength index b * =-0.06727;

[0209] When the nominal diameter of the arc thread connection bolt is 42mm, the size effect coefficient ε=0.840, and the modified fatigue strength index b * =-0.06834.

[0210] (5) Through finite element simulation technology, an axisymmetric bolt model is used to convert the load through the conversion relationship between the distal load and the nominal stress. The corresponding nominal stress is applied to the end face of the arc threaded connection bolt away from the thread engagement portion. The finite element mesh size is gradually reduced, and the finite element mesh near the bolt tooth bottom is repeatedly encrypted. The mesh encryption area is as follows: Figure 5 As shown, multiple simulations are performed until the relative error between the two results is less than 0.1%. At this time, the local stress of the thread bottom area converges, as shown in Figure 6 As shown, the stress concentration factor K at the bottom of the bolt teeth of the arc thread connection pair can be calculated with high accuracy and convergence. t .

[0211] The calculation example of the arc thread connection bolt pair was performed with an initial finite element mesh size of 0.4 mm, and four encryptions were performed. Each time the finite element mesh size was reduced to 0.5 times the previous finite element mesh size, the mesh size was 0.025 mm when the result converged.

[0212] According to the finite element simulation results, when the nominal diameter of the arc thread connection bolt is 36mm, the stress concentration factor K t =2.95.

[0213] (6) The fatigue notch coefficient K of the bolt tooth bottom of the arc thread connection pair is obtained by using the Peterson method based on point stress and sensitivity coefficient, the Neuber method based on average stress, the Siebel-Stieler method based on relative stress gradient, or the Zhao Shaobian method f .

[0214] Using the Peterson method, when the nominal diameter of the arc thread connection bolt is 36mm, the fatigue notch coefficient K f =2.87.

[0215] Using the Neuber method, when the nominal diameter of the arc thread connection bolt is 36mm, the fatigue notch coefficient K f =2.72.

[0216] According to Zhao Shaobian's method, when the nominal diameter of the arc thread connection bolt is 36mm, the fatigue notch coefficient K f =2.50.

[0217] (7) The number of complete cycles and half cycles and the corresponding cyclic response characteristics are obtained through the random load treatment method of the bolt. The load spectrum after random load treatment is used to obtain the local stress-strain response at the bottom of the arc thread by combining the cyclic stress-strain curve and the hysteresis loop curve through the modified Neuber method.

[0218] Taking the fatigue test of arc threaded connection bolts with a nominal diameter of 36mm as an example, according to the requirements of GB / T 13682-92 "Axial load fatigue test method for threaded fasteners", the lifting method is used to measure the conditional fatigue limit. Bolt specimens are randomly selected from products with qualified dimensions and mechanical properties. After cleaning, the mating surface is lubricated and installed on a high-frequency fatigue testing machine for fatigue testing. It first undergoes a 1 / 4 cycle and a complete cycle, and then repeats the cycle until the bolt is fatigued and fractured. The solution steps are as follows:

[0219] Step I: Given the nominal stress S1=550MPa, calculate the local notch σ1=850.7333MPa and ε1=0.015907.

[0220] Step II: Load reversal, ΔS known 1-2 =200MPa, and the local notch Δσ=589.9282MPa and Δε=0.0030333 are obtained.

[0221] Step III: Calculate the local gap σ2=260.8051 and ε2=0.0128737.

[0222] The above formulas are combined to obtain a nonlinear equation group, and the predicted fatigue life of the arc thread connection bolt is obtained by solving it as follows:

[0223] S1=550MPa,ΔS 1-2 = Bolt fatigue life at 200MPa N f is 27.9804×10 4 (The upper and lower limits of its 2-time lifespan band are 55.9608×10 4 and 13.9902×10 4 ), the corresponding bolt fatigue test results are 39.2900×10 4 and 41.2500×10 4 , the results are all located in the 2 times lifetime dispersion band.

[0224] Since there are many result data for fatigue life prediction, only some of the results are listed here as examples, and the rest of the prediction results are not listed one by one. Figure 7 The comparison chart of life prediction results and test results of Rd36, Rd39 and Rd42 arc thread connection bolts is shown. The prediction results are evenly distributed on both sides of the middle line. All life prediction results are distributed within the 3 times life dispersion band, among which most of the data are distributed within the 2 times life dispersion band.

[0225] in, Figure 7 The anti-corrosion coating types of the Rd36 arc thread connection bolts are Derken and Zincmet. Figure 7 The anti-corrosion coating type of the Rd39 arc thread connection auxiliary bolt and the anti-corrosion coating type of the Rd42 arc thread connection auxiliary bolt are both ZINCMETE.

[0226] The above examples further prove that the bolt fatigue life prediction method proposed in the present invention has high accuracy, and the results are effective, scientific and reasonable.

[0227] An embodiment of a fatigue life prediction system for arc thread connection bolts:

[0228] A fatigue life prediction system for arc threaded connection bolts includes a processor, the processor is used to execute a computer program to implement the steps of a fatigue life prediction method for arc threaded connection bolts. The fatigue life prediction method for arc threaded connection bolts has been described in detail in an embodiment of a fatigue life prediction method for arc threaded connection bolts, and will not be repeated here.

Claims

1. A fatigue life prediction method for arc thread connection bolts, characterized in that: The steps include: The correction relationship between the total strain amplitude and fatigue life of the arc thread connection bolt based on the SWT damage parameter is combined with the fatigue load spectrum and the fatigue cumulative damage law to obtain a prediction result for predicting the fatigue life of the arc thread connection bolt; The correction relationship is obtained by elastically correcting the fitting relationship between the total strain amplitude and fatigue life of the arc threaded connection bolt by using the size effect coefficient of the arc threaded connection bolt, the surface processing strengthening coefficient and the SWT damage parameter selected by the local stress-strain response at the tooth bottom of the arc threaded connection bolt; The local stress-strain response is obtained by combining the fitting relationship between the total strain amplitude and the stress amplitude of the arc thread connection bolt, the hysteresis loop curve expression and the modified Neuber hyperbola expression, and then combining the fatigue load spectrum and the fatigue notch coefficient at the tooth bottom of the arc thread connection bolt.

2. The fatigue life prediction method for arc thread connection bolt according to claim 1, characterized in that: The expression of the SWT damage parameter is as follows: e a s max =s max (′ max -′ min ) / 2 In the formula, ′ a σ max is the SWT damage parameter, ε a is the strain amplitude, σ max is the maximum local stress at the thread notch of the arc thread connection bolt, ε max is the maximum strain, ε min is the minimum strain.

3. The fatigue life prediction method for arc thread connection bolt according to claim 1, characterized in that: The expression of the modified fatigue strength index used in the elastic line correction is as follows: Where b * is the modified fatigue strength index, b is the fatigue strength index, K t is the stress concentration factor, ε is the size effect factor, β3 is the surface processing strengthening factor, K f is the fatigue notch factor, and N0 is the number of cycles corresponding to the fatigue limit.

4. The fatigue life prediction method for arc thread connection bolt according to claim 1 or 3, characterized in that: The expression of the size effect coefficient is as follows: In the formula, ε is the size effect coefficient, ε0 is the reference size effect coefficient, d is the nominal diameter of the arc thread connection bolt, the nominal diameter ranges from 30 mm to 120 mm, A1 and A2 are both size effect fitting parameters; ε0, A1 and A2 are constants related to the value range of the nominal diameter.

5. The fatigue life prediction method for arc thread connection bolt according to claim 1, characterized in that: The fitting relationship between the total strain amplitude and fatigue life of the arc threaded connection bolt is obtained by least square method by fitting a power function in a double logarithmic coordinate system using the total strain amplitude and corresponding fatigue life of the arc threaded connection bolt at each strain level.

6. The fatigue life prediction method for arc thread connection bolt according to claim 1, characterized in that: The fitting relationship between the total strain amplitude and stress amplitude of the arc threaded connection bolt is obtained by least square method by fitting the total strain amplitude and stress amplitude of the arc threaded connection bolt at each strain level in a Cartesian rectangular coordinate system.

7. The fatigue life prediction method for arc thread connection bolt according to claim 1, characterized in that: The fatigue notch coefficient at the tooth root of the arc threaded connection bolt is obtained by using the Peterson method based on point stress and sensitivity coefficient for the stress concentration coefficient at the tooth root of the arc threaded connection bolt, or by using the Neuber method based on average stress for the stress concentration coefficient at the tooth root of the arc threaded connection bolt, or by using the Siebel-Stieler method based on relative stress gradient for the stress concentration coefficient at the tooth root of the arc threaded connection bolt, or by using the Zhao Shaobian method based on relative stress gradient for the stress concentration coefficient at the tooth root of the arc threaded connection bolt.

8. The fatigue life prediction method for arc thread connection bolts according to claim 7, characterized in that: The stress concentration coefficient at the thread bottom of the arc thread connection pair bolt is obtained by using finite element simulation technology and an axisymmetric bolt model, loading an axial average stress corresponding to the nominal stress at the thread notch at the end face of the arc thread connection pair bolt away from the thread engagement portion, gradually reducing the size of the finite element grid, and encrypting the finite element grid of the area near the bolt thread bottom; the condition for stopping the encryption is that the relative error of the stress concentration coefficient obtained by the current encryption and the previous encryption solution is less than or equal to 0.1%.

9. The fatigue life prediction method for arc thread connection bolt according to claim 8, characterized in that: In the process of gradually reducing the finite element mesh size, the finite element mesh size after each step of reduction is γ times the finite element mesh size before the step of reduction, γ∈[0.3,0.6].

10. A fatigue life prediction system for arc thread connection bolts, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of the fatigue life prediction method for a circular arc thread connection pair bolt as described in any one of claims 1 to 9.

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