Method for evaluating fastening effect of fastening process considering internal stress
By obtaining the back stress of fasteners through simulated fastening process tests, the shortcomings of fastening effect evaluation under high temperature conditions are solved, and the accurate evaluation of fastening effect is achieved, ensuring the safety and reliability of equipment.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- EAST CHINA UNIV OF SCI & TECH
- Filing Date
- 2023-08-28
- Publication Date
- 2026-06-26
Smart Images
Figure CN117309579B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of fastening effect evaluation, and more specifically to a fastening process fastening effect evaluation method that takes into account internal stress. Background Technology
[0002] Bolts, as a representative fastener, are widely used in various fields such as machinery, civil engineering, and aerospace due to their high maintainability, high load-bearing capacity, and high reliability, making them one of the most widely used connection methods. High-strength bolts, in addition to their connecting function, also transmit force and torque. The tightness of bolted connections is a prerequisite for ensuring the safe and economical operation of equipment, and sufficient preload is the fundamental condition for guaranteeing the bolt's tightness. In actual service, especially under high-temperature conditions, bolted connections are prone to insufficient bolt preload due to factors such as vibration, impact, alternating loads, or prolonged operation. This can lead to loosening between connected components, structural slippage, and affect the normal use of components. Conversely, excessive preload increases the risk of bolt breakage and failure. Therefore, accurately evaluating the fastening effect of fasteners under high-temperature conditions ensures their safe and reliable operation throughout their service life.
[0003] In the prior art, the fastening effect is usually evaluated by measuring the initial preload after the fastener is tightened. For example, if the initial preload reaches the preset value, the fastening effect is considered to meet the requirements and the fastener installation is qualified; conversely, if the initial preload is less than the preset value, the fastening effect is considered to not meet the requirements and the fastener installation is unqualified.
[0004] However, stress relaxation tests on fasteners revealed that even with the same initial strain or initial stress after pre-tightening, stress relaxation behavior is affected by factors such as the strain rate during the initial loading stage, the load history before relaxation, and dynamic strain aging, leading to differences in stress relaxation results. In other words, even with the same initial pre-tightening force, different tightening processes may result in different tightening effects. Therefore, to accurately evaluate the tightening effect of fasteners, the influence of the tightening process must be considered. Summary of the Invention
[0005] The purpose of this invention is to provide a method for evaluating the fastening effect of a fastening process that considers internal stress. This method uses the back stress of the fastener sample to evaluate the fastening effect, thereby taking into account the influence of the fastening process on the fastening effect and accurately assessing the fastening effect of the fastener.
[0006] To achieve the above objectives, the present invention provides a method for evaluating the fastening effect of a fastening process that considers internal stress, comprising:
[0007] The fastener specimen is fastened to a preset strain using the fastening process to be evaluated, and then unloaded and reverse-fastened at the same fastening rate as the fastening process to be evaluated. The stress and strain of the fastener specimen at different times during the unloading and reverse-fastening process are obtained.
[0008] With strain as the abscissa and stress as the ordinate, the stress and strain of the fastener specimen at different times are plotted in the stress-strain coordinate system to obtain multiple stress-strain points in the stress-strain coordinate system.
[0009] The elastic segment region of the unloading and reverse tightening process is obtained from multiple stress and strain points in the stress-strain coordinate system.
[0010] Obtain the maximum and minimum stress values at each stress-strain point in the elastic segment region during the unloading and reverse tightening processes. Take the maximum stress value in the elastic segment region during the unloading and reverse tightening processes as the upper limit of the elastic segment, and take the minimum stress value in the elastic segment region during the unloading and reverse tightening processes as the lower limit of the elastic segment.
[0011] The back stress of the fastener specimen is determined based on the upper limit and the lower limit of the elastic segment.
[0012] The fastening effect evaluation result of the fastening process to be evaluated is obtained based on the back stress of the fastener sample.
[0013] Furthermore, the elastic segment region of the unloading and reverse tightening process is obtained based on multiple stress-strain points in the stress-strain coordinate system, specifically including:
[0014] Several stress-strain points that are roughly on a straight line are selected from the multiple stress-strain points in the unloading and reverse tightening process under the stress-strain coordinate system, and are taken as the stress-strain points of the elastic segment of the unloading and reverse tightening process.
[0015] Linear fitting is performed on each stress-strain point of the elastic segment during the unloading and reverse tightening processes to obtain the elastic segment during the unloading and reverse tightening processes.
[0016] The elastic segment of the unloading and reverse tightening process is translated by a preset offset in the stress-strain coordinate system in the positive X and negative X directions, respectively, to obtain the right offset line and the left offset line. The area between the right offset line and the left offset line is taken as the elastic segment area of the unloading and reverse tightening process.
[0017] Furthermore, the preset offset value ranges from 5 × 10⁻⁶. -6 Up to 5×10 -3 .
[0018] Furthermore, the preset offset is 2×10 -5 .
[0019] Furthermore, the back stress of the fastener specimen satisfies the following relationship:
[0020]
[0021] Where R is the back stress of the fastener specimen. The upper limit of the elastic segment, This is the lower limit of the elastic segment.
[0022] Furthermore, the evaluation result of the fastening effect of the fastening process to be evaluated is obtained based on the back stress of the fastener sample, specifically including:
[0023] The back stress of the fastener sample is compared with a preset threshold. If the back stress of the fastener sample is greater than or equal to the preset threshold, the fastening effect of the fastening process is qualified; if the back stress of the fastener sample is less than the preset threshold, the fastening effect of the fastening process is unqualified.
[0024] Furthermore, the preset threshold is the back stress value of fasteners that have met service requirements in historical service.
[0025] Furthermore, the evaluation result of the fastening effect of the fastening process to be evaluated is obtained based on the back stress of the fastener sample, specifically including:
[0026] Determine the tightening effect level and the corresponding back stress range for each tightening effect level;
[0027] Based on the back stress of the fastener sample and the back stress range corresponding to each fastening effect level, the fastening effect level of the fastening process to be evaluated is determined, and this is used as the evaluation result of the fastening effect of the fastening process to be evaluated.
[0028] Furthermore, the various fastening effect levels and the corresponding back stress ranges are determined based on engineering experience.
[0029] The present invention provides a method for evaluating the fastening effect of a fastening process that considers internal stress. This method simulates a fastening process by performing fastening, unloading, and reverse fastening tests on a fastener sample. The back stress of the fastener sample is obtained based on the stress and strain of the sample during the unloading and reverse fastening processes. The fastening effect of the fastening process is evaluated by using the back stress of the sample, thus taking into account the influence of the fastening process on the fastening effect and accurately assessing the fastening effect of the fastener. Attached Figure Description
[0030] Figure 1 A flowchart of a method for evaluating the fastening effect of a fastening process considering internal stress according to an embodiment of the present invention;
[0031] Figure 2This is a schematic diagram of multiple stress-strain points of a fastener specimen in the stress-strain coordinate system during the fastening, unloading, and reverse fastening processes according to an embodiment of the present invention.
[0032] Figure 3 This is a schematic diagram of the elastic segment, left offset line, right offset line, and elastic segment region during the unloading and reverse tightening process according to an embodiment of the present invention.
[0033] Figure 4 The graph shows the change of relaxation stress over time in two sets of stress relaxation tests according to an embodiment of the present invention. Detailed Implementation
[0034] The preferred embodiments of the present invention are given below with reference to the accompanying drawings and described in detail.
[0035] like Figure 1 As shown, this embodiment of the invention provides a method for evaluating the fastening effect of a fastening process considering internal stress, comprising the following steps:
[0036] S100: The fastener specimen is fastened to a preset strain using the fastening process to be evaluated, and then unloaded and reverse-fastened at the same fastening rate as the fastening process to be evaluated, and the stress and strain of the fastener specimen at different times during the unloading and reverse-fastening process are obtained.
[0037] Different fastening processes have different temperatures and fastening rates (e.g., strain rates), thus resulting in different fastening effects. When evaluating the fastening effect of a fastening process, the fastener specimen can be loaded by tension, torsion, or any other method to tighten it, followed by unloading and reverse tightening. Reverse tightening refers to loading the fastener specimen in the opposite manner to the fastening method (e.g., compression or reverse torsion).
[0038] A fastener specimen is a specimen made of the same material as the actual fastener. In some embodiments, the fastener specimen may be a round bar specimen to better facilitate tensile and unloading tests.
[0039] In some embodiments, the preset strain needs to be greater than the elastic limit strain of the fastener specimen material; that is, during the fastening process, the stress applied to the fastener specimen needs to exceed the elastic limit. The elastic limit and elastic modulus of the fastener specimen can be obtained by consulting existing design manuals, and the elastic limit strain can be calculated from the elastic limit and elastic modulus.
[0040] During the tightening, unloading, and reverse tightening of fastener samples, stress sensors can be used to measure the stress of the fastener samples at different times, and strain sensors can be used to measure the strain of the fastener samples at different times. The time interval can be set as needed; for example, stress and strain can be measured every 0.05 seconds, thus obtaining stress and strain at multiple time points. It is understandable that the more time points measured, the more accurate the results will be.
[0041] S200: With strain as the abscissa and stress as the ordinate, the stress and strain of the fastener specimen at different times are plotted on the stress-strain coordinate system to obtain multiple stress-strain points in the stress-strain coordinate system.
[0042] like Figure 2 As shown, by plotting the stress and strain at different times on a stress-strain coordinate system, multiple stress-strain points can be obtained. These stress-strain points reflect the stress-strain relationship of the fastener specimen under the fastening process being evaluated. For easier viewing, Figure 2 The text also describes the stress and strain points during the fastening process, from... Figure 2 As can be seen, each stress and strain point roughly forms a ring, which includes an ascending segment (i.e., the segment where the strain gradually increases) and a descending segment (i.e., the segment where the strain gradually decreases). The ascending segment represents the stress and strain during the tightening process, and the descending segment represents the stress and strain during the unloading and reverse tightening processes.
[0043] S300: Obtain the elastic segment region of the unloading and reverse tightening process based on multiple stress and strain points in the stress-strain coordinate system.
[0044] Step S300 specifically includes:
[0045] S310: Select several stress and strain points that are approximately on a straight line from the multiple stress and strain points of the unloading and reverse tightening process in the stress and strain coordinate system, and use them as the stress and strain points of the elastic segment of the unloading and reverse tightening process.
[0046] like Figure 2 As shown, in the descent segment, some stress-strain points are roughly located on the same straight line. These points can be selected as the stress-strain points of the elastic segment during the unloading and reverse tightening processes.
[0047] S320: Linear fitting is performed on each stress-strain point of the elastic segment during the unloading and reverse tightening process to obtain the elastic segment during the unloading and reverse tightening process.
[0048] In some embodiments, the least squares method or other fitting methods can be used to fit the stress-strain points of the elastic segment during the unloading and reverse tightening processes to obtain the elastic segment during the unloading and reverse tightening processes, such as... Figure 3Line A in the diagram represents the elastic segment during the unloading and reverse tightening process.
[0049] S330: The elastic segment of the unloading and reverse tightening process is translated by a preset offset in the stress-strain coordinate system in the positive X direction and the negative X direction, respectively, to obtain the right offset line and the left offset line. The area between the right offset line and the left offset line is taken as the elastic segment area of the unloading and reverse tightening process.
[0050] In some embodiments, the preset offset value can be 5×10. -6 Up to 5×10 -3 Between, for example, can be 2×10 -5 .like Figure 3 As shown, after shifting the elastic segment A in the positive X direction, we can obtain the right offset line B. After shifting the elastic segment A in the negative X direction, we can obtain the left offset line C. The region D between the left offset line C and the right offset line B is the elastic segment region.
[0051] S400: Obtain the maximum and minimum stress values at each stress-strain point in the elastic segment region during the unloading and reverse tightening processes. Take the maximum stress value in the elastic segment region during the unloading and reverse tightening processes as the upper limit of the elastic segment, and take the minimum stress value in the elastic segment region during the unloading and reverse tightening processes as the lower limit of the elastic segment.
[0052] The elastic segment region during the unloading and reverse tightening process is a quadrilateral region between the right offset line and the left offset line. Multiple stress-strain points are distributed in this region. The stress at the highest stress-strain point in the coordinate system is the maximum stress value of this region, i.e., the upper limit of the elastic segment; the stress at the lowest stress-strain point in the coordinate system is the minimum stress value of this region, i.e., the lower limit of the elastic segment.
[0053] S500: Determine the back stress of the fastener specimen based on the upper limit and lower limit of the elastic segment.
[0054] Internal stress is the stress that exists within an object and remains in equilibrium without external force or torque acting on it, arising from uneven deformation of the material. Based on the Cottrell internal stress classification method, the initial preload after tightening can be divided into two parts: back stress and effective stress. Back stress is the anisotropic component of the flow stress, generated by long-range interactions between dislocations due to deformation inconsistencies; effective stress is the isotropic component of the flow stress, generated by short-range interactions experienced by dislocations during movement, and serves as the threshold stress for dislocation movement. The formula for calculating back stress is as follows:
[0055]
[0056] Where R is the back stress. The upper limit of the elastic segment, This is the lower limit of the elastic segment.
[0057] S600: The fastening effect evaluation result of the fastening process to be evaluated is obtained based on the back stress of the fastener sample.
[0058] Stress relaxation tests revealed that even when fastener specimens have the same initial strain or initial preload after pre-tightening, stress relaxation behavior is influenced by factors such as the strain rate during the initial loading stage, the load history before relaxation, and dynamic strain aging, leading to differences in stress relaxation results. In other words, even with consistent initial preload, different fastening processes can result in varying fastening effects. The deformation behavior of materials is intricately linked to microstructure evolution, accompanied by the evolution of internal stress at different scales during deformation. Therefore, the stress relaxation process is not only related to preload but also to the microstructure introduced during fastening and its distribution, as well as the composition of internal stress at different scales.
[0059] With a consistent initial preload, different fastening processes (such as different temperatures and strain rates) will result in varying back stresses in the fasteners, leading to different fastening effects. At the same initial preload, a higher back stress in the fastener results in a better fastening effect. Therefore, the fastening effect of the fastening process can be evaluated based on the magnitude of the back stress in the fastener sample.
[0060] In some embodiments, step S600 specifically includes:
[0061] The back stress of the fastener sample is compared with a preset threshold. If the back stress of the fastener sample is greater than or equal to the preset threshold, the fastening effect of the fastening process is qualified; if the back stress of the fastener sample is less than the preset threshold, the fastening effect of the fastening process is unqualified.
[0062] In some embodiments, the preset threshold can be obtained based on experience. For example, the back stress value of a fastener that meets the service requirements can be obtained based on historical data of a certain working condition, and the back stress value or the back stress value multiplied by a preset safety factor can be used as the preset threshold.
[0063] In some other embodiments, step S600 specifically includes:
[0064] The fastening effectiveness grade of the fastening process to be evaluated is obtained based on the back stress of the fastener sample, and this grade is used as the evaluation result of the fastening effectiveness of the fastening process. For example, based on engineering experience, each fastening effectiveness grade and the corresponding back stress range can be obtained; then, it can be determined which back stress range the back stress of the fastener sample falls into, and the fastening effectiveness grade corresponding to that back stress range is used as the fastening effectiveness grade of the fastening process to be evaluated.
[0065] In some embodiments, for a fastener under a certain working condition, the fastening effect evaluation method of the present invention, which considers internal stress, can be used to evaluate the fastening effect of different fastening processes, obtain the evaluation results of different fastening processes, and then use the fastening process with qualified evaluation results to tighten the fastener under the working condition to a preset preload. The fastening effect of the fastener obtained in this way is considered to meet the service requirements under the working condition. Alternatively, the fastening process with the largest back stress among different fastening processes can be selected as the optimal fastening process, and then the optimal fastening process can be used to tighten the fastener under the working condition to a preset preload. The fastening effect of the fastener obtained in this way is considered to be the best fastening effect, which can meet the service requirements under the working condition.
[0066] The fastening effect evaluation method of the fastening process considering internal stress in this invention simulates the fastening process and performs fastening, unloading and reverse fastening tests on the fastener sample. The back stress of the fastener sample is obtained based on the stress and strain of the fastener sample during the unloading and reverse fastening processes. The fastening effect of the fastening process is evaluated by the back stress of the fastener sample, thereby considering the influence of the fastening process on the fastening effect and accurately evaluating the fastening effect of the fastener.
[0067] To verify the relationship between back stress and fastening effect, this invention uses 316L stainless steel as the research material, and performs tensile unloading compression tests and stress relaxation tests on round bar specimens. 316L stainless steel is a commonly used high-temperature structural material, widely used in high-temperature structural fastening systems due to its excellent fatigue resistance, creep resistance, and high-temperature corrosion resistance. To adapt to the tensile testing machine, the diameter of the round bar specimen was 8 mm and the gauge length was 16 mm during the tensile unloading test, with the gauge length section mechanically polished to ensure a roughness of 0.2 mm. To adapt to the creep testing machine, the diameter of the round bar specimen was 10 mm and the gauge length was 100 mm during the stress relaxation test, with a roughness of 0.8 μm in the gauge length section. Both the tensile unloading test and the stress relaxation test employed axial strain control.
[0068] The specific experimental steps are as follows:
[0069] (1) Two sets of tensile unloading tests were conducted on the round bar specimens, with strain rates of 0.1% / s and 0.4% / s respectively. The round bar specimens were stretched to the initial strain of 0.4%, and then unloaded and compressed at the same rate to obtain the stress and strain data of the two sets of tests.
[0070] (2) Based on the stress and stress data of the two sets of tests, the internal stress (including back stress and effective stress) of the two sets of tests were obtained respectively; the stress, strain and internal stress data of the two sets of tests are shown in Table 1 below:
[0071] Table 1 Tensile unloading test data
[0072]
[0073] (3) Two sets of stress relaxation tests were conducted, with strain rates of 0.1% / s and 0.4% / s in the tensile section, respectively. The specimens were stretched to 0.4% of the initial strain and then relaxed. The relaxation stress was calculated to obtain the evolution law of the relaxation stress over time for the two sets of tests; where the relaxation stress σ cr The calculation formula is:
[0074]
[0075] in and These represent stress data at subsequent and initial times during the relaxation process, respectively. The curves showing the change in relaxation stress over time for the two sets of stress relaxation tests are shown below. Figure 4 As shown.
[0076] The relaxation test data of the two sets of stress relaxation tests are shown in Table 2:
[0077] Table 2: Relaxation Test Data
[0078]
[0079] As shown in Tables 1 and 2, with an initial strain of 0.4%, the back stress X of the sample with a strain rate of 0.1% / s is 78.49 MPa, and the final relaxation stress is -30.365 MPa. The back stress X of the sample with a strain rate of 0.4% / s is 48.78 MPa, and the final relaxation stress is -46.134 MPa. Furthermore, the stress relaxation amount of the sample with a strain rate of 0.1% / s is consistently less than that of the sample with a strain rate of 0.4% / s. This indicates that, under the same preload strain / stress, the back stress contributes more to the fastening effect of the fastener. The greater the back stress, the smaller the relaxation amount of the sample during the entire relaxation process, resulting in a better fastening effect. In other words, with the same initial preload, the greater the back stress, the better the fastening effect of the fastener. Since the initial preload equals the sum of the back stress and the effective stress, the greater the back stress, the greater its ratio to the initial preload, meaning a larger proportion of back stress results in a better fastening effect.
[0080] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention. Various variations can be made to the above embodiments of the present invention. That is, all simple and equivalent changes and modifications made based on the claims and description of this invention fall within the protection scope of the claims of this patent. All aspects not described in detail in this invention are conventional technical content.
Claims
1. A method for evaluating the fastening effect of a fastening process considering internal stress, characterized in that, include: The fastener specimen is fastened to a preset strain using the fastening process to be evaluated, and then unloaded and reverse-fastened at the same fastening rate as the fastening process to be evaluated. The stress and strain of the fastener specimen at different times during the unloading and reverse-fastening process are obtained. With strain as the abscissa and stress as the ordinate, the stress and strain of the fastener specimen at different times are plotted in the stress-strain coordinate system to obtain multiple stress-strain points in the stress-strain coordinate system. The elastic segment region of the unloading and reverse tightening process is obtained from multiple stress and strain points in the stress-strain coordinate system. Obtain the maximum and minimum stress values at each stress-strain point in the elastic segment region during the unloading and reverse tightening processes. Take the maximum stress value in the elastic segment region during the unloading and reverse tightening processes as the upper limit of the elastic segment, and take the minimum stress value in the elastic segment region during the unloading and reverse tightening processes as the lower limit of the elastic segment. The back stress of the fastener specimen is determined based on the upper limit and the lower limit of the elastic segment. The fastening effect evaluation result of the fastening process to be evaluated is obtained based on the back stress of the fastener sample.
2. The method for evaluating the fastening effect of a fastening process considering internal stress according to claim 1, characterized in that, The elastic segment region of the unloading and reverse tightening process is obtained from multiple stress-strain points in the stress-strain coordinate system, specifically including: Several stress-strain points that are roughly on a straight line are selected from the multiple stress-strain points in the unloading and reverse tightening process under the stress-strain coordinate system, and are taken as the stress-strain points of the elastic segment of the unloading and reverse tightening process. Linear fitting is performed on each stress-strain point of the elastic segment during the unloading and reverse tightening processes to obtain the elastic segment during the unloading and reverse tightening processes. The elastic segment of the unloading and reverse tightening process is translated by a preset offset in the stress-strain coordinate system in the positive X and negative X directions, respectively, to obtain the right offset line and the left offset line. The area between the right offset line and the left offset line is taken as the elastic segment area of the unloading and reverse tightening process.
3. The method for evaluating the fastening effect of a fastening process considering internal stress according to claim 2, characterized in that, The preset offset value ranges from 5 × 10. -6 Up to 5×10 -3 .
4. The method for evaluating the fastening effect of a fastening process considering internal stress according to claim 3, characterized in that, The preset offset is 2×10 -5 .
5. The fastening effect evaluation method according to claim 1, characterized in that, The back stress of the fastener specimen satisfies the following relationship: Where R is the back stress of the fastener specimen. The upper limit of the elastic segment, This is the lower limit of the elastic segment.
6. The method for evaluating the fastening effect of a fastening process considering internal stress according to claim 1, characterized in that, The evaluation result of the fastening effect of the fastening process to be evaluated is obtained based on the back stress of the fastener sample, specifically including: The back stress of the fastener sample is compared with a preset threshold. If the back stress of the fastener sample is greater than or equal to the preset threshold, the fastening effect of the fastening process is qualified; if the back stress of the fastener sample is less than the preset threshold, the fastening effect of the fastening process is unqualified.
7. The method for evaluating the fastening effect of a fastening process considering internal stress according to claim 6, characterized in that, The preset threshold is the back stress value of fasteners that have met service requirements in historical service.
8. The method for evaluating the fastening effect of a fastening process considering internal stress according to claim 1, characterized in that, The evaluation result of the fastening effect of the fastening process to be evaluated is obtained based on the back stress of the fastener sample, specifically including: Determine the tightening effect level and the corresponding back stress range for each tightening effect level; Based on the back stress of the fastener sample and the back stress range corresponding to each fastening effect level, the fastening effect level of the fastening process to be evaluated is determined, and this is used as the evaluation result of the fastening effect of the fastening process to be evaluated.
9. The method for evaluating the fastening effect of a fastening process considering internal stress according to claim 6, characterized in that, The various fastening effect levels and the corresponding back stress ranges are determined based on engineering experience.