A test method for evaluating the heat aging resistance of cable sheath materials

Abstract

The present invention relates to the field of cable sheath testing technology, and specifically to a method for evaluating the heat aging resistance of cable sheath materials, comprising: marking the obtained cable sheath, recording the size and shape of the cable sheath, and determining the performance parameters of the cable sheath; the performance parameters include tensile strength, elongation at break, elastic modulus, and expansion coefficient; subjecting the cable sheath to hot air aging treatment to obtain the aged cable sheath; testing the aged cable sheath to determine the performance parameters of the cable sheath after aging; determining the stability index of the performance parameters at different temperatures and times according to the corresponding temperature and time of the aging treatment; determining the correlation between the performance parameters at various temperatures and times based on the stability index of the performance parameters, and determining the performance degradation time of the cable sheath according to the correlation between the performance parameters at various temperatures and times. Accurate prediction of the performance degradation time of the cable sheath is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of cable sheath testing, in particular to a method for evaluating the heat aging resistance performance of a cable sheath material. Background Art

[0002] As an important part of power transmission and communication, the quality and performance of wires and cables are directly related to the reliability and safety of the power system. With the continuous development and upgrading of power equipment, the requirements for cable materials are getting higher and higher, especially in terms of heat aging resistance. As an important part of the cable, the cable sheath not only protects the inner core of the cable, but also directly affects the service life and overall performance of the cable.

[0003] However, in actual applications, cable sheath materials are affected by various external factors, such as high temperature, oxygen, ultraviolet light, mechanical stress, etc. These factors will cause the sheath material to gradually age and deteriorate in performance. The aged sheath material may crack, harden, discolor, etc., which will affect the insulation performance and overall safety of the cable. Especially in high temperature environments, the heat aging resistance of the sheath material is particularly important.

[0004] For example, Chinese patent publication number CN114187348A discloses a method for measuring the elongation at break of a cable sheath based on computer vision, which includes: drawing circular markers at both ends of the cable sheath and clamping the sheath vertically with a clamp; stretching the sheath and using a camera to shoot the stretching process; obtaining the marker position information through image processing, tracking the marker using the inter-frame difference method, and recording the marker spacing in real time.

[0005] However, when the existing technology tests the heat aging resistance of cables, the test standards are too single. It is necessary to determine the performance changes of the cable sheath under different temperature tests and the size of the corresponding performance changes based on its appearance and mechanical properties to improve the accuracy of the test. Summary of the invention

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] A test method for evaluating the heat aging resistance of cable sheath materials, comprising:

[0008] S100, marking the obtained cable sheath, recording the size and shape of the cable sheath, and determining the performance parameters of the cable sheath; the performance parameters include tensile strength, elongation at break, elastic modulus, and expansion coefficient.

[0009] S200, performing hot air aging treatment on the cable sheath to obtain an aged cable sheath.

[0010] S300, testing the cable sheath after aging treatment to determine the performance parameters of the cable sheath after aging; determining the stability index of the performance parameters under different temperatures and times according to the obtained performance parameters according to the temperature and time corresponding to the aging treatment.

[0011] S400, based on the stability index of the performance parameters, determines the correlation between the performance parameters at various temperatures and times, and determines the performance degradation time of the cable sheath.

[0012] The beneficial effects of the present invention are as follows: by analyzing the stability indicators of performance parameters at different temperatures and times, as well as the correlation between the performance parameters, the present invention can accurately predict the performance degradation time of the cable sheath, providing an important reference for the use and maintenance of the cable; by means of comprehensive scoring, the aging degree of the cable sheath is divided into different levels, so that a more scientific understanding and judgment of the aging state of the cable sheath is achieved; not only the aging performance of the cable sheath is paid attention to, but also its recovery ability and fracture condition are carefully evaluated, providing a more comprehensive cable sheath performance evaluation; the heat aging resistance of the cable sheath is comprehensively and accurately evaluated, and the accurate prediction of the performance degradation time is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0014] Figure 1 The present invention is a flow chart of a test method for evaluating the heat aging resistance of cable sheath materials.

[0015] Figure 2 The present invention is a flow chart of step S400 of a method for evaluating the heat aging resistance of cable sheath materials. DETAILED DESCRIPTION

[0016] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention. If no specific techniques or conditions are specified in the embodiments, the techniques or conditions described in the literature in the art or the product specifications are used.

[0017] See also Figure 1 , a test method for evaluating the heat aging performance of cable sheath materials, comprising:

[0018] S100, marking the obtained cable sheath, recording the size and shape of the cable sheath, and determining the performance parameters of the cable sheath; the performance parameters include tensile strength, elongation at break, elastic modulus, and expansion coefficient.

[0019] S200, performing hot air aging treatment on the cable sheath to obtain an aged cable sheath.

[0020] S300, testing the cable sheath after aging treatment to determine the performance parameters of the cable sheath after aging; determining the stability index of the performance parameters under different temperatures and times according to the obtained performance parameters according to the temperature and time corresponding to the aging treatment.

[0021] S400, based on the stability index of the performance parameters, determines the correlation between the performance parameters at various temperatures and times, and determines the performance degradation time of the cable sheath.

[0022] At this time, it is preferred to hang the specimens vertically in the middle of the oven, ensuring that the distance between each specimen and other specimens is at least 20 mm to avoid mutual influence.

[0023] If test pieces are required for weight loss tests, the volume of the oven occupied by these test pieces should not exceed 0.5%.

[0024] According to the cable product standards, set the temperature and time in the oven. The temperature and time should be selected to simulate the heating conditions of the cable sheath material in actual use.

[0025] Start the oven and begin the test. During the test, check regularly whether the temperature in the oven is stable and record the relevant data.

[0026] After the specified time, remove the specimen from the oven and place it at ambient temperature for at least 16 hours, away from direct sunlight. This step helps the specimen return to normal temperature and reduces the impact of temperature on subsequent test results.

[0027] In this step, the cable sheath can be treated at multiple times and temperatures. The temperature can be set to 70°C, 90°C, 125°C, etc., and the time can be set to 24 hours, 144 hours, 168 hours, 336 hours, etc.; to test the percentage of degradation of the tensile performance of the current cable sheath after aging treatment at different times and temperatures, and the reduction ratio within the corresponding time under the corresponding degradation trend, so as to predict the current usable time of the cable, so as to know the time required for its performance degradation.

[0028] The tensile strength of the cable sheath is expressed as: the maximum tensile force and the original cross-sectional area corresponding to the tensile strength are obtained; the tensile strength of the cable sheath is determined according to the ratio of the maximum tensile force of the cable sheath to the cross-sectional area of ​​the cable sheath; that is, Where F is the maximum tensile force (N), A is the original cross-sectional area of ​​the specimen (mm 2 ), σ b is the tensile strength; and determine the elongation at break corresponding to the cable sheath at the maximum tensile force, obtain the length increment when the cable sheath breaks and the original length of the cable sheath, and determine the elongation at break of the cable sheath. Among them, ε b is the elongation at break, ΔL1 is the length increment when the cable sheath breaks, and L0 is the original length of the cable sheath.

[0029] Determining the stability index of the performance parameters at different temperatures and times includes respectively determining the stability index of the tensile strength, the stability index of the elongation at break, the stability index of the elastic modulus, and the stability index of the expansion coefficient among the performance parameters.

[0030] The stability index of tensile strength in the performance parameters is expressed as the tensile strength of the cable sheath before and after aging. The stability index of tensile strength is obtained according to the ratio of the tensile strength of the cable sheath before and after aging:

[0031]

[0032] Among them, TSI1 is the stability index corresponding to the tensile strength, σ b after is the tensile strength after aging, σ b before is the tensile strength before aging.

[0033] For the elongation at break, the stability index of the elongation at break can be expressed as follows: the elongation at break of the cable sheath before and after aging is obtained, and the stability index of the elongation at break is obtained according to the ratio of the elongation at break of the cable sheath before and after aging:

[0034]

[0035] Among them, TSI2 is the stability index corresponding to the elongation at break, ε b_after is the elongation at break after aging, ε b_before is the elongation at break before aging.

[0036] The method for obtaining the stability index of the elastic modulus is to obtain the stress and strain of the cable sheath during the test, calculate the elastic modulus based on the stress and strain, and determine the stability index of the elastic modulus:

[0037]

[0038] Where E is the elastic modulus, σ is the stress, and ε is the strain.

[0039]

[0040] Among them, TSI3 is the stability index corresponding to the elastic modulus, E after is the elastic modulus after aging, E before is the elastic modulus before aging.

[0041] The expansion coefficient is obtained by obtaining the change in length and temperature of the cable sheath at each temperature during the cable sheath test, and determining the expansion coefficient at the corresponding temperature according to the change in length and temperature of the cable sheath at each temperature:

[0042]

[0043] Among them, α is the expansion coefficient at the corresponding temperature, L0 is the original length of the cable sheath, ΔL2 is the change in the length of the cable sheath at the corresponding temperature, and ΔT is the change in temperature. The stability index corresponding to the expansion coefficient represents the average value of the ratio of the expansion coefficient to the preset expansion coefficient at the same temperature.

[0044] The cable will change in length due to thermal expansion and contraction of the material at different temperatures. When the temperature rises, the cable will become longer due to material expansion. Conversely, when the temperature drops, the cable will shorten due to material contraction. This length change is generally related to the material's expansion coefficient. Materials with large expansion coefficients also have large changes in thermal expansion and contraction. For general cables, their length changes are usually between 0.1% and 0.4%, but this change may accumulate and cause significant effects under extreme conditions or long-term effects. Therefore, tensile strength, elongation at break, elastic modulus, and expansion coefficient are obtained as identification parameters to determine the changes in the values ​​related to the appearance and mechanical properties of the cable sheath under what circumstances, and to determine the degree of aging. The main test is whether the cable sheath as a whole can maintain its value integrity for a long time under the corresponding environment to improve the protection of the inside of the cable.

[0045] like Figure 2 As shown, in step S400, the method of determining the performance degradation time of the cable sheath according to the correlation between the performance parameters at various temperatures and times includes:

[0046] S401, based on the acquired performance parameters, the performance parameters are sorted according to the size of the corresponding values, the collected temperatures and times are generated, a target feature sequence is generated, and a change trend of the target feature sequence at each time and temperature is determined.

[0047] S402, based on the obtained change trend of the target feature sequence, determine the inflection point corresponding to the performance parameter in the change trend, determine the slope between the point near each inflection point and the inflection point, and draw a distribution diagram of the time and slope value at different temperatures according to the time and slope value corresponding to the inflection point at different temperatures, and cluster the performance parameters of the distribution diagram, and determine the aging level of the cable sheath based on the clustered performance parameters.

[0048] S403, determining the recovery capability and fracture condition of the cable sheath under the corresponding aging level.

[0049] S404, determining the performance degradation time of the cable sheath according to the recovery capability and the fracture condition of the cable sheath.

[0050] The distribution diagram set at this time is used to indicate the trend of performance changes of the tested cable sheath caused by different heat treatment times when the same temperature or different temperatures are set, so as to identify the relationship between the specific degradation value and time when the performance of each cable sheath degrades during the adjustment time.

[0051] The processing method for clustering the performance parameters in the distribution diagram is as follows:

[0052] Determine the similarity of multiple groups of performance parameters at different times and temperatures, check whether the similarity between all pairs of performance parameter groups exceeds the preset threshold, if there are two groups of performance parameters whose similarity is greater than the preset threshold, determine whether the stability index of the performance index is the same, if the stability index of the performance index is the same, mark the time and temperature of the two groups of performance parameters and the corresponding slope value, at this time, under different temperatures and times, if the curves corresponding to the two inflection points are highly correlated and have the same values, it means that there is a strong correlation, at this time, it can be understood at what temperature and time period consistency changes are likely to occur; if the stability index of the performance index is different, determine the slope of the corresponding performance parameter, compare the slopes of the two groups of performance indexes separately, and output the parts with the same slope, and the difference is to determine the same The correlation between the points of the trend is used to determine the time and temperature values ​​corresponding to the performance parameters when the same trend appears; if there are two groups of performance parameters whose similarity is less than the preset threshold, the change rate of the two groups of performance indexes at the corresponding temperature is determined, and the change rate of the two adjacent groups of performance parameters is compared. If the change rate of the two adjacent groups of performance parameters is the same, the situation where the change rate is the same is recorded; if the change rate of the two adjacent groups of performance parameters is different, the time period and temperature range corresponding to the different change rates are identified, and the performance parameters in the corresponding time period and temperature range are compared with the two groups of performance indexes with the greatest similarity among the multiple groups of performance parameters, the mean of the performance parameters in the corresponding time period and temperature range is calculated, and the mean of the performance parameters in the corresponding time period and temperature range is used as the preset threshold for judging the similarity between the performance parameters.

[0053] The first-order derivative is used to identify sudden changes in the slope of the curve in the changing trend, identify the corresponding inflection point, and select a certain number of points near the inflection point to calculate the slope to quantify the rate at which the performance parameter changes with time or temperature; at the same time, in order to determine the relative change of the performance parameter in a specific time period or temperature range, and whether the relative change of the performance parameter in a specific time period and temperature range is the same, the rate of change is used to measure the relative change of the performance parameter in a specific time period or temperature range under specific conditions. It is more similar and can show consistent changes.

[0054] The above-mentioned time period indicates the time period corresponding to the same rate of change of the stability index during the aging treatment. For example, if the changes corresponding to the aging treatment time of 60 hours and 70 hours are the same, the marked time period indicates the setting of 60 hours to 70 hours. At the same time, the corresponding temperature range indicates that the settings of 70 degrees and 80 degrees are in the same time period, or there is a corresponding time period that makes the corresponding change rate of the current cable sheath the same. At this time, the corresponding 70 degrees and 80 degrees, as well as the corresponding time period, are marked to identify the time period of the corresponding test in which the same or similar results are likely to occur.

[0055] At the same time, when processing the obvious inflection points in the cable sheath test, its tensile strength and elongation at break are determined to determine the degree of degradation of the mechanical properties corresponding to the tensile strength of the cable sheath when the cable sheath changes accordingly, and based on this degree of degradation, the mechanical aging grade is determined.

[0056] The aging level of the cable sheath can also be achieved by:

[0057] Determine the stable indicators of tensile strength, elongation at break, elastic modulus and expansion coefficient corresponding to each inflection point in the changing trend, determine the comprehensive score of the cable sheath based on the stable indicators of tensile strength, elongation at break, elastic modulus and expansion coefficient, and set the aging grade according to the comprehensive score.

[0058] The aging grade is determined by determining the reduction ratio of tensile strength and elongation at break to determine the aging grade at the corresponding temperature. At the same time, during the aging treatment, the corresponding tensile strength will first drop sharply, and then rise after dropping to a certain ratio, and then drop again after rising to a certain value. At this time, the aging time and temperature corresponding to these values ​​are marked, and the aging grade is set for the performance parameters according to the reduced ratio; at this time, the elastic modulus and expansion coefficient of the cable sheath will also be affected. If the elastic modulus and expansion coefficient of the aged cable sheath are also evaluated, the aging grade related to the mechanical properties of the cable sheath can be obtained, thereby improving the recognition effect of the corresponding aging grade.

[0059] The comprehensive score is expressed as:

[0060]

[0061] Among them, S is the comprehensive score, β i is the weight corresponding to the i-th performance parameter, x iIt represents the value of the i-th performance parameter, which is expressed as a stability index of tensile strength, elongation at break, elastic modulus and expansion coefficient. a is an adjustment parameter used to adjust the value of the corresponding performance parameter. e is an exponential constant. x1, x2, x3 and x4 represent tensile strength, elongation at break, elastic modulus and expansion coefficient respectively. β1, β2, β3 and β4 represent the weights corresponding to tensile strength, elongation at break, elastic modulus and expansion coefficient respectively.

[0062] According to the obtained comprehensive score value, assuming that there is a standard score value, the calculated comprehensive score is compared with the standard score value, and the comprehensive score value is converted into a corresponding score range of 0 to 100; when the score is 0-10, it means that the current cable shows brand new performance without any obvious signs of aging, and it is set to the first level; when the score is 11-30, it means that the cable has some slight signs of aging, such as slight surface wear or slight hardening of the insulation material, but the overall performance is still close to that of a new cable, and it is set to the second level; when the score is 31-60, it means that the cable shows more signs of aging, such as cracks in the insulation layer or slight current leakage, but the cable can still be used normally and needs to be monitored, and it is set to the third level; when the score is 61-80, it means that the cable is seriously aged and there may be major safety hazards, such as severe hardening of the insulation material, crack expansion, or significant increase in contact resistance. It is recommended to conduct a detailed evaluation and consider replacement, and it is set to the fourth level; when the score is 81-100, it means that the cable has reached the end of its service life and there are serious safety hazards, such as insulation damage and high risk of short circuit. It must be replaced immediately and set to the fifth level. The test performed at this time is to determine the state of the cable sheath at what temperature and set aging time through limit testing, so as to evaluate the performance of the cable sheath.

[0063] At this time, the set aging level is mainly used to measure the temperature and time at which the cable sheath's bearing capacity will be reduced when a certain performance aging occurs, and after what time period, the performance of the cable sheath will recover to a certain extent, and the level of this part of the recovery will be measured to find out under what circumstances the cable is more dangerous. At the same time, the set danger level will be used as a part of the influencing factors to determine the degradation time of the cable sheath, and verify the direction and size of the break when the cable breaks, to determine the degradation time of the cable sheath.

[0064] The recovery ability and fracture treatment methods of the cable sheath are determined at the corresponding aging level:

[0065] Verify the direction and size of the cable breakage, obtain the corresponding images of the cable sheath during the test, and determine the recovery capacity and breakage condition of the cable sheath during the test based on the images between the cable sheaths. The recovery capacity is expressed as the ability of the cable sheath to recover its deformation after bending, winding and other operations, and the change in the length of time required to recover the deformation. The breakage condition indicates the length, depth and direction of the crack when cracking occurs, as well as the crack severity score after the combination of the length, depth and direction of the crack.

[0066] The recovery capacity assessment is to determine the deformation recovery ratio and recovery time constant, obtain the original area of ​​the cable sheath before deformation and the area after deformation, determine the deformation recovery ratio; obtain the deformation recovery time of the cable sheath during deformation and the area changing with time, and determine the recovery time constant:

[0067]

[0068] Among them, A g is the original area before deformation, A f is the area after deformation, R p is the deformation recovery ratio.

[0069]

[0070] Where A(t) is the area changing with time t, τ is the recovery time constant, which reflects the speed of recovery to the final state, t is the time of deformation recovery, and e is the exponential constant.

[0071] At this time, the current cable sheath recovery time constant can be determined according to the area that changes with time to determine the recovery capacity of the cable sheath. The closer the deformation recovery ratio is to zero, the higher the degree to which the material has recovered to its original state; the smaller the recovery time constant is, the faster the material recovers. The corresponding deformation recovery time is recorded for subsequent evaluation of cable sheath aging.

[0072] The fracture condition is assessed by obtaining the length, depth and direction of the crack and determining a crack severity score after combining the length, depth and direction of the crack.

[0073] The crack depth and length are determined by analyzing the corresponding pixels and pixel distribution in the image, calculating the position and number of pixels related to the crack edge to determine the length of the crack, and determining the crack depth based on the similarity between the corresponding pixel value of the crack and the pixel value of the preset depth. That is, the length of the crack is determined by the length from the starting point to the end point of the crack, and by analyzing the grayscale value of the pixel in the crack, the corresponding grayscale value is compared with the grayscale value of the crack sample set in the database, and the depth corresponding to the crack sample with the largest grayscale value similarity is used as the current detection depth.

[0074] The direction of the crack is determined based on the coordinates of the pixel at the crack start point and the coordinates of the pixel corresponding to the end point, expressed as:

[0075]

[0076] Among them, x start and start is the coordinate of the crack starting point, x end and x start is the coordinate of the crack endpoint, and θ is the angle of the crack direction relative to the horizontal.

[0077] Then the crack severity score is expressed as:

[0078] S=ω L L C +ω D D C +ω θ cos(θ-θ0);

[0079] Where S is the crack severity score, L C is the normalized crack length, D C is the normalized crack depth, θ is the crack direction angle, θ0 is the reference angle of the crack direction, ω L is the crack length weight, ω D is the weight of the crack depth, ω θ is the weight of the crack orientation angle.

[0080] According to the recovery ability and fracture condition of the cable sheath, the performance degradation time of the cable sheath is determined as follows:

[0081] Determine the deformation recovery time of the cable sheath. If the deformation recovery time is greater than the preset recovery time and the crack severity score is greater than the preset crack score, the corresponding hot air aging treatment time is used as the performance degradation time of the current cable sheath.

[0082] If the deformation recovery time is less than the preset recovery time and the crack severity score is greater than the preset crack score, the time of the inflection point closest to the current time point in the corresponding change trend is taken as the performance degradation time of the current cable sheath.

[0083] If the deformation recovery time is less than the preset recovery time and the crack severity score is less than the preset crack score, the time point with the largest slope decrease in the change trend is taken as the performance degradation time of the current cable sheath.

[0084] By measuring the area before and after deformation, the deformation recovery ratio is calculated; by monitoring the area changing with time, the recovery time constant is determined, reflecting the speed at which the sheath recovers to the final state; if the recovery time is greater than the preset value and the crack severity score is also higher than the preset value, it means that the sheath has poor recovery ability and severe cracks. At this time, the aging treatment time is used as the performance degradation time.

[0085] The length, depth and direction of the crack are determined by image analysis; the crack length is determined by calculating the position and number of pixels at the crack edge; the crack depth is determined by analyzing the grayscale value of the pixels within the crack and comparing it with the crack samples in the database; the crack direction is calculated based on the coordinates of the crack start and end points; the crack severity score is calculated based on the length, depth and direction of the crack.

[0086] The performance degradation time of the cable sheath is determined based on the comparison between the recovery time and the crack severity score and the preset value; if the recovery time is less than the preset value, but the crack severity score is higher than the preset value, the inflection point time closest to the current time point in the change trend is taken as the performance degradation time; if the recovery time and the crack severity score are both lower than the preset value, the time point with the largest slope decrease value in the change trend is taken as the performance degradation time.

[0087] Assume that there is a set of cable sheath samples, and their recovery and cracking are monitored regularly during aging treatment; for one of the samples:

[0088] Recovery time assessment: The measured recovery time is 10 hours, and the preset recovery time is 8 hours.

[0089] Crack condition assessment: The crack severity score is 0.7 and the preset crack score is 0.5.

[0090] Since the recovery time is greater than the preset value and the crack severity score is also higher than the preset value, according to the plan, the performance degradation time of the sample is judged to be the time point of aging treatment; this means that at this time point, the recovery ability of the cable sheath has significantly decreased, and the crack situation is serious, indicating that the performance of the sheath has begun to degrade.

[0091] Through this method, the time point when the cable sheath performance deteriorates can be effectively identified, providing an accurate basis for cable maintenance and replacement; at the same time, the comprehensive evaluation method that considers recovery ability and crack conditions also more comprehensively reflects the actual use status of the cable sheath.

[0092] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. A person skilled in the art may make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention and they are still covered by the protection scope of the present invention.

Claims

1. A test method for evaluating the heat aging resistance of cable sheath materials, characterized in that: include: Mark the obtained cable sheaths, record the size and shape of the cable sheaths, and determine the performance parameters of the cable sheaths; The performance parameters include tensile strength, elongation at break, elastic modulus, and expansion coefficient; Performing hot air aging treatment on the cable sheath to obtain an aged cable sheath; Test the cable sheath after aging treatment to determine the performance parameters of the cable sheath after aging; determine the stability index of the performance parameters under different temperatures and times according to the corresponding temperature and time of aging treatment; Based on the stability index of the performance parameters, the correlation between the performance parameters at various temperatures and times is determined, and based on the correlation between the performance parameters at various temperatures and times, the performance degradation time of the cable sheath is determined; The tensile strength of the cable sheath is expressed as, obtaining the maximum tensile force and original cross-sectional area corresponding to the tensile strength; The tensile strength of the cable sheath is determined based on the ratio of the maximum tensile force of the cable sheath to the cross-sectional area of ​​the cable sheath; that is, ;in, is the maximum pulling force, is the original cross-sectional area of ​​the specimen, is the tensile strength; and determine the elongation at break corresponding to the cable sheath at the maximum tensile force, obtain the length increment when the cable sheath breaks and the original length of the cable sheath, and determine the elongation at break of the cable sheath. ;in, is the elongation at break, is the length increment when the cable sheath breaks, is the original length of the cable sheath; Determining the stability index of the performance parameters at different temperatures and times includes respectively determining the stability index of the tensile strength, the stability index of the elongation at break, the stability index of the elastic modulus, and the stability index of the expansion coefficient among the performance parameters; The stability index of tensile strength in the performance parameters is expressed as the tensile strength of the cable sheath before and after aging. The stability index of tensile strength is obtained according to the ratio of the tensile strength of the cable sheath before and after aging: ； in, It is a stability index corresponding to tensile strength. is the tensile strength after aging, is the tensile strength before aging; For the elongation at break, the stability index of the elongation at break is expressed as follows: the elongation at break of the cable sheath before and after aging is obtained, and the stability index of the elongation at break is obtained according to the ratio of the elongation at break of the cable sheath before and after aging: ； in, It is a stable index corresponding to the elongation at break. is the elongation at break after aging, is the elongation at break before aging; The method for obtaining the stability index of the elastic modulus is as follows: obtaining the stress and strain of the cable sheath during the test, calculating the elastic modulus based on the stress and strain, and determining the stability index of the elastic modulus: ； in, represents the elastic modulus, is stress, It is strain; ； in, is the stability index corresponding to the elastic modulus, is the elastic modulus after aging, is the elastic modulus before aging; The expansion coefficient is obtained by obtaining the change in length and temperature of the cable sheath at each temperature during the cable sheath test, and determining the expansion coefficient at the corresponding temperature according to the change in length and temperature of the cable sheath at each temperature: ； in, is the expansion coefficient at the corresponding temperature, is the original length of the cable sheath, is the change in length of the cable sheath at the corresponding temperature, is the change in temperature; the stability index corresponding to the expansion coefficient represents the average value of the ratio of the expansion coefficient to the preset expansion coefficient at the same temperature; The method of determining the performance degradation time of the cable sheath according to the correlation between the performance parameters at various temperatures and times includes: Based on the various performance parameters obtained, the performance parameters are sorted according to the corresponding values, the collected temperatures and times are sorted, a target feature sequence is generated, and the change trend of the target feature sequence at each time and temperature is determined; Based on the change trend of the acquired target feature sequence, the inflection point corresponding to the performance parameter in the change trend is determined, the slope between the point near each inflection point and the inflection point is determined, and according to the time and slope value corresponding to the inflection point at different temperatures, the distribution diagram of the time and slope value at different temperatures is drawn, and the performance parameters of the distribution diagram are clustered. Based on the clustered performance parameters, the aging level of the cable sheath is determined; Determine the recovery capacity and fracture condition of the cable sheath under the corresponding aging level; Determine the performance degradation time of the cable sheath based on the recovery capacity and fracture conditions of the cable sheath; The aging level of the cable sheath is achieved by: Determine the stable indexes of tensile strength, elongation at break, elastic modulus, and expansion coefficient corresponding to each inflection point in the change trend, determine the comprehensive score of the cable sheath based on the stable indexes of tensile strength, elongation at break, elastic modulus, and expansion coefficient, and set the aging grade according to the comprehensive score; The comprehensive score is expressed as: ； in, is a comprehensive rating. is the weight corresponding to the ith performance parameter, represents the value of the i-th performance parameter, expressed as a stable index of tensile strength, elongation at break, elastic modulus, and expansion coefficient, is the adjustment parameter, is the exponential constant, They represent tensile strength, elongation at break, elastic modulus, and expansion coefficient, respectively. They represent the weights corresponding to tensile strength, elongation at break, elastic modulus, and expansion coefficient respectively; The method for determining the recovery capacity and fracture condition of the cable sheath under the corresponding aging level is as follows: The recovery capacity assessment is to determine the deformation recovery ratio and recovery time constant, obtain the original area of ​​the cable sheath before deformation and the area after deformation, determine the deformation recovery ratio; obtain the deformation recovery time of the cable sheath during deformation and the area changing with time, and determine the recovery time constant: ； in, is the original area before deformation, is the area after deformation, is the deformation recovery ratio; ； in, Over time The area of ​​change, is the recovery time constant, It is the time for deformation recovery. is the exponential constant; The fracture condition is assessed by obtaining the length, depth and direction of the crack and determining the crack severity score after combining the length, depth and direction of the crack. The crack severity score is expressed as: ； in, is the crack severity score, is the normalized crack length, is the normalized crack depth, is the angle of the crack direction, is the reference angle of the crack direction, is the crack length weight, is the weight of the crack depth, is the weight of the crack orientation angle; The implementation method of determining the performance degradation time of the cable sheath according to the recovery ability and fracture condition of the cable sheath is: Determine the deformation recovery time of the cable sheath. If the deformation recovery time is greater than the preset recovery time and the crack severity score is greater than the preset crack score, the corresponding hot air aging treatment time is used as the performance degradation time of the current cable sheath. If the deformation recovery time is less than the preset recovery time, and the crack severity score is greater than the preset crack score, the time of the inflection point closest to the current time point in the corresponding change trend is taken as the performance degradation time of the current cable sheath; If the deformation recovery time is less than the preset recovery time and the crack severity score is less than the preset crack score, the time point with the largest slope decrease in the change trend is taken as the performance degradation time of the current cable sheath.

2. A cable sheath material heat aging performance evaluation test method according to claim 1, characterized in that: The processing method for clustering the performance parameters in the distribution diagram is as follows: Determine the similarity of multiple groups of performance parameters at different times and temperatures, check whether the similarity between all pairs of performance parameter groups exceeds the preset threshold, if there are two groups of performance parameters whose similarity is greater than the preset threshold, determine whether the stability indicators of the performance parameters are the same, if the stability indicators of the performance parameters are the same, mark the time and temperature of the two groups of performance parameters and the corresponding slope values, at this time, under different temperatures and times, if the curves corresponding to the two inflection points are highly correlated and have the same values, it means that there is a strong correlation, at this time, it can be understood at what temperature and time period consistency changes are likely to occur; if the stability indicators of the performance parameters are different, determine the slope of the corresponding performance parameter, compare the slopes of the two groups of performance parameters separately, and output the parts with the same slope, and the difference is to determine the same The correlation between the points of the trend is used to determine the time and temperature values ​​corresponding to the performance parameters when the same trend appears; if there are two groups of performance parameters whose similarity is less than the preset threshold, the change rate of the two groups of performance parameters at the corresponding temperature is determined, and the change rate of the two adjacent groups of performance parameters is compared. If the change rate of the two adjacent groups of performance parameters is the same, the situation where the change rate is the same is recorded; if the change rate of the two adjacent groups of performance parameters is different, the time period and temperature interval corresponding to the different change rates are identified, and the performance parameters in the corresponding time period and temperature interval are compared with the two groups of performance parameters with the greatest similarity among the multiple groups of performance parameters, the mean of the performance parameters in the corresponding time period and temperature interval is calculated, and the mean of the performance parameters in the corresponding time period and temperature interval is used as the preset threshold for judging the similarity between the performance parameters.

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