CN108572134B - Method and system for testing residual life of pipe - Google Patents
Method and system for testing residual life of pipe Download PDFInfo
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Abstract
The invention provides a method and a system for testing the residual life of a pipe, wherein the method for testing the residual life of the pipe comprises the following steps: obtaining strain hardening modulus values G for at least three polyethylene materials of known design service lives TPThe at least three polyethylene materials are polyethylene materials with different types; according to the strain hardening modulus value GPAnd establishing a function with the design service life value T: a is T ═ a0+a1×<Gp>+a2×<Gp>2+……+an×<Gp>nWherein a is0、a1、a2……anTaking a positive integer as a preset coefficient, wherein n is the preset maximum number of times of the function; obtaining a test sample from an in-service pipe; measuring the strain hardening modulus value G of the test specimenP'; according to the function and the strain hardening modulus value G of the test sampleP', estimating the remaining life of the test specimen. The invention can obtain the residual life of the test sample to be used as the reference value of the residual life of the in-service pipe which fails due to slow crack propagation, and timely discover the risk of the in-service pipe which fails due to slow crack propagation.
Description
Technical Field
The invention relates to a material testing technology, in particular to a method and a system for testing the residual life of a pipe.
Background
Polyethylene (PE) gas pipelines have gradually replaced steel pipes due to the advantages of strong corrosion resistance, good mechanical properties, long service life, environmental protection and the like, and are widely used in urban gas pipeline networks as common means for natural gas transportation. At present, in-service polyethylene gas pipes paved in China are mainly third-generation PE 100-grade pipes, the design life of the pipes is 58 years, and the pipes are about 20 years old to use; as the gas pipe is buried in the ground throughout the year and continuously affected by various external factors such as soil pressure, thermal-oxidative aging, point load, corrosion and the like, the pipe is easy to form local stress concentration along with the increase of service time, and finally the pipe is easy to have slow crack failure, so that the residual service life of the PE gas pipe in service is directly affected. The in-service PE gas pipe is special pressure-bearing equipment, is transported as flammable and explosive products, and is fatal once gas leakage occurs; therefore, whether the residual life of the PE gas pipe in service can reach the pre-designed life directly influences the normal operation of the urban gas pipeline network and the life and property safety of people is an important technical index. However, at present, a rapid and effective evaluation method aiming at the residual life of the PE gas pipe in service is not available at home and abroad, and due to the lack of an accurate and effective residual life effective prediction method, the gas pipe network in many cities cannot timely and effectively replace the failed pipe, so that gas leakage accidents and quality safety accidents occur, even explosion accidents are caused, and serious hidden dangers are caused to urban public safety.
Therefore, the residual service life of the in-service PE pipe is accurately and effectively analyzed, and the failed pipe is replaced in time, so that accidents such as gas leakage and the like can be effectively reduced; moreover, the method for predicting and evaluating the residual life of the in-service PE gas pipe is short in test period, simple in test condition and low in cost, can provide powerful safety guarantee for quality and cost control of the PE gas pipe, can shorten the development period of special resin for the novel gas pipe, and can also guarantee safe operation of the existing gas pipeline system.
Disclosure of Invention
The object of the present invention is to solve at least one of the above technical drawbacks, in particular with respect to the problem of difficult estimation of the residual life of the piping system.
The invention provides a method for testing the residual life of a pipe, which comprises the following steps:
obtaining values of Strain hardening modulus < G for at least three polyethylene materials of known design service Life Tp>, the at least three polyethylene materials being different types of polyethylene materials;
according to said strain hardening modulus value < GpAnd > and the design service life T, constructing a function:
T＝a0+a1×<Gp>+a2×<Gp>2+……+an×<Gp>nwherein a is0、a1、a2……anTaking a positive integer as a preset coefficient, wherein n is the preset maximum number of times of the function;
obtaining a test sample from an in-service polyethylene pipe;
measuring the strain hardening modulus value of the test specimen<GP'>；
According to the function and the strain hardening modulus value of the test sample<GP'>And calculating the residual life of the test sample.
Preferably, after estimating the remaining life of the test specimen, the method further includes:
and judging whether to replace the in-service polyethylene pipe or not according to the residual service life.
Preferably, the judging whether to replace the in-service polyethylene pipe comprises the following steps:
when the residual life is not more than a first preset value, replacing the in-service polyethylene pipe;
and when the residual life is greater than the first preset value and less than a second preset value, monitoring the in-service polyethylene pipe.
Preferably, the strain hardening modulus values of at least three polyethylene materials of known design service life T are obtained<GP>The method comprises the following steps:
respectively manufacturing at least five standard samples of each polyethylene material in the at least three polyethylene materials;
obtaining the strain hardening modulus value of each standard sample through a high-temperature tensile test<GPi>Wherein i is the number of the standard sample;
according to the strain hardening modulus value of each standard sample<GPi>Calculating the root mean square average value of the strain hardening modulus value of each polyethylene material, and taking the root mean square average value as the strain hardening modulus value<GP>。
Preferably, said separately making at least five standard specimens of each of said at least three polyethylene materials comprises:
preparing the at least five standard samples of each polyethylene material in a die-pressing device, wherein the standard samples are dumbbell-shaped sheets comprising a middle testing part and two end clamping parts, and the thickness of each dumbbell-shaped sheet is 0.3mm-1 mm;
and annealing the at least five standard samples at 115-125 ℃ for one hour, and cooling the samples to room temperature at a cooling speed of less than 2 ℃/min.
Preferably, the strain hardening modulus value of each standard sample is obtained through a high-temperature tensile test<GPi>The method comprises the following steps:
placing the dumbbell-shaped sheet in a thermostat at 80 ℃ for 30-60 min;
clamping the two end clamping parts on a high-temperature tensile testing machine, and applying 0.4Mpa prestress at a strain rate of 5 mm/min;
stretching the dummy at a constant moving speed of 20mm/minBell-shaped flakes, collecting data values for a stretch ratio lambda between 8 and 12 for said middle test portion, said stretch ratio lambda being
Wherein L is0Is the initial length of the middle test portion, and the L is the stretched length of the middle test portion;
recording the draw ratio λ and the true stress σ during the drawtrueThe corresponding relation of (1):
wherein F is a tensile force corresponding to the data value of the tensile ratio, and A is0The initial cross-sectional area of the middle test part of the dumbbell-shaped sheet is shown;
determining the strain hardening modulus value of each standard sample according to the Neo-Hookean constitutive model and the slope K of the corresponding relation when the lambda is 12 and the lambda is 8<GPi>Comprises the following steps:
<GPi>＝20K。
preferably, the method for obtaining the test sample from the in-service polyethylene pipe comprises the following steps:
obtaining a test material from the in-service polyethylene pipe, and processing the test material into the test sample in a sampling machine;
the test sample is a dumbbell-shaped sheet comprising a middle test part and two end clamping parts, and the thickness of the dumbbell-shaped sheet is 0.3mm-1mm
Preferably, the measurement of the strain hardening modulus value G of the test specimenP', includes:
placing the test sample in a thermostat at 80 ℃ for 30-60 min;
clamping the clamping parts at the two ends of the test sample on a high-temperature tensile testing machine, and applying 0.4Mpa prestress at a strain rate of 5 mm/min;
stretching the test specimen at a constant moving speed of 20mm/min, and collecting data values of a stretching ratio lambda' between 8 and 12 at a middle test part of the test specimenThe draw ratio
Wherein L is0'is the initial length of the middle test portion of the test specimen, and L' is the stretched length of the middle test portion of the test specimen;
recording the draw ratio λ' and the true stress σ during the drawtrue' the correspondence relationship:
wherein F 'is a stretching force corresponding to the data value of the stretching ratio lambda', and A0' is the initial cross-sectional area of the middle test portion of the test specimen;
determining the strain hardening modulus value of the test sample according to the Neo-Hookean constitutive model and the slope K ' of the corresponding relation when the lambda ' is 12 and the lambda ' is 8<GP'>Is composed of
<GP'>＝20K’。
Preferably, the test sample is in at least two pieces; determining the strain hardening modulus value of the test sample according to the Neo-Hookean constitutive model and the slope K ' of the corresponding relation when the lambda ' is 12 and the lambda ' is 8<GP'>The method comprises the following steps:
determining the strain hardening modulus value of each of the test specimens<GPi'>；
Calculating the strain hardening modulus value<GPi'>The root mean square average value is taken as the strain hardening modulus value of the test sample<GP'>。
The invention also provides a system for testing the residual life of the pipe, which comprises:
a sample preparation device: the test sample is obtained from the in-service polyethylene pipe;
a tensile test device: for measuring the strain hardening modulus value of the test specimen<GP'>；
A central processing unit: at least three for obtaining a known design service life THard-to-harden modulus value of polyethylene material<GP>The at least three polyethylene materials are polyethylene materials with different types; hardening modulus value according to the strain<GP>And establishing a function with the design service life T:
T＝a0+a1×<Gp>+a2×<Gp>2+……+an×<Gp>nwherein a is0、a1、a2……anTaking a positive integer as a preset coefficient, wherein n is the preset maximum number of times of the function; according to the function and the strain hardening modulus value of the test sample<GP'>And calculating the residual life of the test sample.
The invention has the following beneficial effects:
1. the present invention can be used to analyze a polyethylene material of known design service life T to construct a strain hardening modulus value<GP>A function of the design life T; then obtaining a test sample from the in-service polyethylene pipe, and measuring the strain hardening modulus value of the in-service polyethylene pipe<GP'>Finally, combining the function to obtain the residual life of the test sample, wherein the residual life is used as a reference value of the residual life of the in-service polyethylene pipe which fails due to slow crack propagation, and the risk of the in-service polyethylene pipe which fails due to slow crack propagation can be found in time; the method can effectively evaluate the residual service life of different in-service polyethylene pipes so as to take different treatment measures for the in-service polyethylene pipes according to the residual service life.
2. The invention can correct the strain hardening modulus value of the polyethylene material known in the prior art by measuring the strain hardening modulus values of a plurality of standard samples and taking the mean root mean square value; the manufacturing parameters and the measurement parameters of the standard sample are used as the measurement parameters of a subsequent test sample, so that the calculation result of the residual life can be further improved; the invention is particularly suitable for analyzing the residual service life of the gas pipeline.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
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The foregoing and/or additional aspects and advantages of the present invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
fig. 1 is a schematic flow chart of a first embodiment of the testing method of the present invention.
FIG. 2 is a schematic structural diagram of an embodiment of the tensile testing apparatus of the present invention;
FIG. 3 is a schematic structural diagram of a tensile testing apparatus according to another embodiment of the present invention;
FIG. 4 is a schematic structural diagram of another embodiment of the tensile testing apparatus of the present invention.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the drawings are illustrative only and should not be construed as limiting the invention.
As used herein, the singular forms "a", "an", "the" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It will be understood that when an element is referred to as being "connected" to another element, it can be directly connected to the other element or intervening elements may also be present. Further, "connected" as used herein may include wirelessly connected. As used herein, the term "and/or" includes all or any element and all combinations of one or more of the associated listed items.
It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
As shown in the flowchart of fig. 1, the present invention provides a first embodiment of a method for testing the remaining life of a pipe, including the following steps:
step S10: obtaining strain hardening modulus values for at least three polyethylene materials of known design service life T<GP>The at least three polyethylene materials are polyethylene materials with different types;
various material performance parameters have been disclosed in the prior art, such as the modulus of elasticity, poisson's ratio, density, coefficient of thermal expansion, yield strength, tensile strength, elongation, strain hardening modulus values, etc. of the material. During the material testing process, through correlation analysis, the strain hardening modulus value in the performance parameters can be found to be highly correlated with the slow crack propagation of the material, so that the slow crack propagation failure service life of the material can be estimated through the strain hardening modulus value of the tested material. Strain hardening modulus values for different types of polyethylene materials of known design service life T<GP>Performing correlation analysis to obtain the strain hardening modulus value<GP>And the design service life T. The different types of polyethylene materials include pipes made of different brands of polyethylene materials named by various polyethylene manufacturers, such as: the design life T of the pipe made of 100-grade high-density polyethylene is 58 years, or the design life T of the pipe made of the anti-cracking polyethylene PE100-RC is 100 years and the like.
The strain hardening modulus value<GP>The service life T can be measured by the brand name of each factory or by self-test to obtainMore accurate values.
Step S20: hardening modulus value according to the strain<GP>And the design service life T, a function is constructed:
T＝a0+a1×<Gp>+a2×<Gp>2+……+an×<Gp>nwherein a is0、a1、a2……anTaking a positive integer as a preset coefficient, wherein n is the preset maximum number of times of the function;
in the present invention, the design service life T and strain hardening modulus values of at least three different types of polyethylene materials are known<GP>The equation can be solved by an analysis method such as a Newton iteration method to obtain the numerical value of each preset coefficient, so that the strain hardening modulus value is obtained<GP>And the design service life T. The strain hardening modulus value of a polyethylene material of a known design life<GP>The design service life T is used as an array, when two arrays exist, a linear equation with an error function of zero can be obtained, but the error is larger; in order to improve the iteration precision, the invention can take more arrays to obtain a unitary n-order fitting equation and improve the accuracy of constructing the function. The number of the arrays can exceed the positive integer n, and then a function of minimizing errors is fitted by a Newton iteration method so as to further reduce the error of the fitted function; some of the values in the array may lie on the fitted function curve, and the other part is evenly distributed on both sides of the function curve. The greater the number of arrays, the lower the error rate of the function built.
Step S30: obtaining a test sample from an in-service polyethylene pipe;
the in-service polyethylene pipe is a pipe in use, such as a gas pipe buried underground. Fitting the strain hardening modulus values in a previous step<GP>After the function of the service life T is designed, the strain hardening modulus value of the in-service polyethylene pipe can be directly obtainedAnd (5) service life. The test sample is obtained from the in-service polyethylene pipe, a part of pipe sample can be directly cut from the in-service polyethylene pipe, and the sample is processed into a proper test sample shape.
Step S40: measuring the strain hardening modulus value of the test specimen<GP'>；
Measuring the strain hardening modulus value of the test specimen<GP'>The measurement can be carried out by adopting the existing tensile test equipment, and the proper tensile test equipment can also be designed or assembled according to the specific conditions of the test sample. Measuring the strain hardening modulus value of the test specimen<GP'>Preferably with strain hardening modulus values measured for said at least three polyethylene materials of known design life<GP>The parameters are consistent; the parameters comprise the size of the test sample, the heating temperature and time of the test sample, the stretching force and the stretching time in the stretching process and the like.
Step S50: according to the function and the strain hardening modulus value of the test sample<GP'>And estimating the residual life of the test sample.
Obtaining the strain hardening modulus value of the test sample<GP'>Then, the remaining life of the test sample is obtained by substituting the function constructed in step S20. Since the material of the test sample is taken from the polyethylene pipe in service, the material has changed in the service environment and the change is the same as that of the polyethylene pipe in service, the residual life of the polyethylene pipe in service can be estimated through the residual life of the test sample. The residual life of the in-service polyethylene pipe can be the same as that of the test sample, and a certain proportion relation can exist.
The defects possibly caused in the production, transportation, construction and other processes of the existing polyethylene pipe are influenced by external factors such as temperature, pressure, point load and the like in subsequent use, and various failure modes such as creep deformation, stress relaxation, rapid crack propagation, slow crack propagation, material aging and the like can exist; of these, slow crack propagation is the predominant failure mode. Although the design service life of the existing polyethylene pipe is estimated in the design process, the actual service life of the pipe does not necessarily accord with the design service life due to the influence of factors such as different actual use parameters and different use environments. As mentioned above, slow crack propagation is the most important failure mode of the pipe, and the service life of slow crack propagation failure is highly related to the value of the strain hardening modulus, so the invention can firstly establish the functional relationship between the value of the strain hardening modulus and the service life by analysis, and then obtain the test sample from the in-service polyethylene pipe so as to obtain the test sample according to the value of the strain hardening modulus of the test sample<GP'>Calculating the residual life of the test sample; and (4) calculating the residual service life of the in-service polyethylene pipe due to slow crack propagation according to the residual service life of the test sample.
Therefore, the method can find the failure risk of the in-service polyethylene pipe due to slow crack propagation in time, and avoid the danger caused by the failure of the in-service polyethylene pipe before the designed service life, such as gas leakage accidents caused by slow crack propagation of the polyethylene gas pipe; by the method, the residual life of different in-service polyethylene pipes can be effectively evaluated, so that different treatments can be performed on the in-service polyethylene pipes according to the residual life.
Based on the remaining life of the test specimen described in the previous embodiment, the present invention proposes another embodiment: after estimating the remaining life of the test sample, the method further comprises:
and judging whether to replace the in-service polyethylene pipe or not according to the residual service life.
Although the test sample is taken from the polyethylene pipe in service, the subsequent processes of processing and forming into a shape suitable for testing and the like can exist in the test sample, so that the residual service life of the test sample and the residual service life of the actual polyethylene pipe in service possibly have a difference, the specific conditions of the service environment, the safety factor, the processing process of the test sample and the like of the polyethylene pipe in service can be referred to, and whether the polyethylene pipe in service needs to be replaced or not is judged according to the residual service life. For example: because the use environment is severe, the service life of the in-service pipe is 20 years from the original design service life, but the residual life calculated by the testing method is only 3 years, and the in-service polyethylene pipe relates to the transportation of environmental pollutants, so that a user can comprehensively judge whether the in-service polyethylene pipe needs to be replaced immediately or at the latest or other materials are replaced.
Based on the above embodiment, the present invention provides another embodiment: the judgment of whether to replace the in-service polyethylene pipe comprises the following steps:
when the residual life is not more than a first preset value, replacing the in-service polyethylene pipe;
and when the residual life is greater than the first preset value and less than a second preset value, monitoring the in-service polyethylene pipe.
In practical applications, corresponding measures can be taken according to the value of the remaining life, such as immediate replacement of the in-service polyethylene pipe or periodic inspection of the in-service polyethylene pipe, so as to provide a reference for user decision. Different parameters can be set according to the service conditions of the in-service polyethylene pipe. For example, the following examples: for the water supply pipeline, if the residual service life is more than 5 years, the normal use is continued; if the residual life is more than 2 years and less than 5 years, detecting once every year to monitor the in-service polyethylene pipe and prevent the water pipe from bursting; for the gas pipeline, if the residual life is more than 5 years and less than 8 years, the detection is performed every year at regular intervals to monitor the in-service polyethylene pipe and prevent gas leakage.
Based on the first embodiment, the present invention also proposes a second embodiment: obtaining the strain hardening modulus values of at least three polyethylene materials of known design life T<GP>The method comprises the following steps:
respectively manufacturing at least five standard samples of each polyethylene material in the at least three polyethylene materials;
obtaining the strain hardening modulus value of each standard sample through a high-temperature tensile test<GPi>Wherein i is the composition of the standard sampleNumber;
according to the strain hardening modulus value of each standard sample<GPi>Calculating the root mean square average value of the strain hardening modulus value of each polyethylene material, and taking the root mean square average value as the strain hardening modulus value<GP>。
Although the strain-hardening modulus values of various polyethylene materials are known in the prior art<GP>The design service life T is equal to the design service life T, but due to the improvement of the processing technology of the material, the improvement of the testing environment, the testing equipment and the like, in order to improve the precision of estimating the residual life, the known strain hardening modulus value can be matched<GP>The values of (A) were determined again. This example was carried out by reforming at least five standard specimens of each polyethylene material to obtain the strain-hardening modulus values of each of the standard specimens, respectively<GPi>The root mean square average value of the strain hardening modulus of each polyethylene material is taken as<GP>Further increase the strain hardening modulus value<GP>The accuracy of (2).
Based on the second embodiment, the present invention also proposes another embodiment: the separately making at least five standard specimens of each of the at least three polyethylene materials comprises:
preparing the at least five standard samples of each polyethylene material in a die-pressing device, wherein the standard samples are dumbbell-shaped sheets comprising a middle testing part and two end clamping parts, and the thickness of each dumbbell-shaped sheet is 0.3mm-1 mm;
and annealing the at least five standard samples at 115-125 ℃ for one hour, and cooling the samples to room temperature at a cooling speed of less than 2 ℃/min.
When the standard samples are manufactured, the number of the preset standard samples can be two times or three times of the actual target number so as to avoid the defect or defective products generated in the manufacturing process of the standard samples from causing insufficient number, and reserve enough test number for the subsequent tensile test so as to prevent the tensile test from failing for too many times and needing more standard samples. The shape of the standard sample can be referred to the shape of the existing tensile test, and the thickness of the standard sample can be determined according to the thickness of the in-service polyethylene pipe.
Based on the above embodiment, the present invention further provides the following embodiments: obtaining the strain hardening modulus value of each standard sample through high-temperature tensile test<GPi>The method comprises the following steps:
placing the dumbbell-shaped sheet in a thermostat at 80 ℃ for 30-60 min;
clamping the two end clamping parts on a high-temperature tensile testing machine, and applying 0.4Mpa prestress at a strain rate of 5 mm/min;
stretching the dumbbell sheets at a constant moving speed of 20mm/min, and collecting data values of the stretching ratio lambda of the middle test part between 8 and 12, the stretching ratio lambda being
Wherein L is0Is the initial length of the middle test portion, and the L is the stretched length of the middle test portion;
recording the draw ratio λ and the true stress σ during the drawtrueThe corresponding relation of (1):
wherein F is a tensile force corresponding to the data value of the tensile ratio, and A is0The initial cross-sectional area of the middle test part of the dumbbell-shaped sheet is shown;
determining the strain hardening modulus value of each standard sample according to the Neo-Hookean constitutive model and the slope K of the corresponding relation when the lambda is 12 and the lambda is 8<GPi>Comprises the following steps:
<GPi>＝20K。
in the present embodiment, the<GPi>The specific estimation process for 20K is as follows:
wherein K is lambda epsilon (lambda)1,λ2) The slope of the corresponding relation, C is a constant; when taking lambda1＝8、 λ2When the number is equal to 12, the number is as follows,
then there are:
obtaining the strain hardening modulus value of each piece of the standard sample<GPi>Then, according to the strain hardening modulus value of each piece of standard sample<GPi>Calculating the root mean square average value of the strain hardening modulus of each polyethylene material, and taking the root mean square average value as the strain hardening modulus value<GP>Namely:
Based on the second embodiment, the present invention also proposes another embodiment: the test sample is obtained from an in-service polyethylene pipe, and the method comprises the following steps:
obtaining a test material from the in-service polyethylene pipe, and processing the test material into the test sample in a sampling machine; the test sample is a dumbbell-shaped sheet comprising a middle test part and two end clamping parts, and the thickness of the dumbbell-shaped sheet is 0.3mm-1 mm.
In this embodiment, the test sample may be obtained from the in-service polyethylene pipe, and then the material is cut in a sample preparation machine to be processed into a dumbbell-shaped sheet having a shape identical to that of the standard sample.
Further, the strain hardening modulus value G of the test sample is measuredP' may also be the same as the parameters for measuring the standard sample, so that the strain hardening modulus value of the test sample is measured<GP'>The method comprises the following steps:
placing the test sample in a thermostat at 80 ℃ for 30-60 min;
clamping the clamping parts at the two ends of the test sample on a high-temperature tensile testing machine, and applying 0.4Mpa prestress at a strain rate of 5 mm/min;
stretching the test specimen at a constant moving speed of 20mm/min, and collecting data values of a stretching ratio lambda 'between 8 and 12 at a middle test part of the test specimen, wherein the stretching ratio lambda' is
Wherein L is0'is the initial length of the middle test portion of the test specimen, and L' is the stretched length of the middle test portion of the test specimen;
recording the draw ratio λ' and the true stress σ during the drawtrue' the correspondence relationship:
wherein F 'is a stretching force corresponding to the data value of the stretching ratio lambda', and A0' is the initial cross-sectional area of the middle test portion of the test specimen;
determining the strain hardening modulus value G of the test sample according to the Neo-Hookean constitutive model and the slope K ' of the corresponding relation when the lambda ' is 12 and the lambda ' is 8P' is
<GP'>＝20K’。
The above-mentioned<GP'>The calculation process of 20K' is the same as that of the standard sample, and is not described again.
In order to improve the accuracy of the test, the test samples are at least two pieces; determining the strain hardening modulus value of the test sample according to the Neo-Hookean constitutive model and the slope K ' of the corresponding relation when the lambda ' is 12 and the lambda ' is 8<GP'>The method comprises the following steps:
determining the strain hardening modulus value of each of the test specimens<GPi’>；
Calculating the strain hardening modulus value<GPi’>The root mean square average value is taken as the strain hardening modulus value of the test sample<GP'>。
This example Strain hardening modulus values for a plurality of the test specimens<GPi’>Taking the mean root mean square value can further improve the strain hardening modulus value<GP'>The reliability of (2).
Further, the test samples can be taken from different positions of the in-service polyethylene pipe to obtain the residual life of the test samples at different positions, so that a user can know the residual life difference of the in-service polyethylene pipe at different positions to perform different treatments; or strain hardening modulus values of test specimens at different positions<GPi’>Taking the mean root mean square value as the integral hardening modulus value of the position of the test sample<GP’>So as to reduce accidental errors caused by a single test sample. Of course, the material can be obtained from the same position of the in-service polyethylene pipe to be made into a plurality of test samples to be used as a group of test samples; the materials are also obtained at the other position, and a plurality of test samples are manufactured to be used as another group of test samples, so as to further improve the test accuracy.
According to the testing method, the invention also provides a system for testing the residual life of the pipe, which comprises the following steps:
a sample preparation device: the test sample is obtained from the in-service polyethylene pipe;
tensile testThe device comprises the following steps: for measuring the strain hardening modulus value of the test specimen<GP'>；
A central processing unit: hard-to-harden modulus values for at least three polyethylene materials for obtaining a known design service life T<GP>The at least three polyethylene materials are polyethylene materials with different types; hardening modulus value according to the strain<GP>And establishing a function with the design service life T:
T＝a0+a1×<Gp>+a2×<Gp>2+……+an×<Gp>nwherein a is0、ax、a2……anTaking a positive integer as a preset coefficient, wherein n is the preset maximum number of times of the function; according to the function and the strain hardening modulus value of the test sample<GP'>And calculating the residual life of the test sample.
The sample preparation device can be an existing sample preparation machine, and the tensile test device can be a high-temperature tensile test machine. The high-temperature tensile testing machine can adopt a tensile structure shown in fig. 2-4, and comprises:
a frame 1;
a pair of clamps 2, namely a clamp 2A and a clamp 2B in the figure, wherein the clamp 2A and the clamp 2B are fixed on the frame 1, clamping grooves for clamping one end of the sample 9 are arranged on the clamp 2A and the clamp 2B, and the opening directions of the clamping grooves of the clamp 2A and the clamp 2B are opposite to each other so as to respectively clamp two ends of the sample 9 and fix the sample 9;
a moving device 3, at least one clamp 2 is fixed on the frame 1 through the moving device 3, so that the clamp 2 moves in the direction far away from or close to the other clamp 2; in fig. 1, the clamp 2A is fixed by the moving device 3, and the moving device 3 can drive the clamp 2A to move upward or downward in the figure, so that the distance between the clamp 2A and the clamp 2B is increased or decreased, and the purpose of stretching the sample 9 between the clamp 2A and the clamp 2B is achieved;
the auxiliary locking device is fixed on the clamp 2 and comprises a locking station and an avoiding station, and when the auxiliary locking device is positioned at the locking station, auxiliary clamping force is applied to the clamping groove to increase the clamping force on the sample 9, so that the sample 9 is not easy to slide or shift in the stretching process, and the accuracy of the stretching test is increased;
the force measuring device 4 is connected with at least one clamp 2 and is used for measuring the tensile force generated when the moving device 3 moves; taking fig. 1 as an example, since the sample 9 is clamped between the clamp 2A and the clamp 2B, when the moving device 3 drives the clamp 2A to move upward, the sample 9 will generate a certain resistance; when the tensile force exceeds the resistance force, the distance between the grips 2A and 2B increases, and the test specimen 9 is elongated; the force measuring device 4 is used for measuring the tensile force when the test sample 9 is elongated;
and the length measuring device is fixed on the machine frame 1 and comprises a measuring head, wherein the measuring range of the measuring head is larger than the moving range of the clamp 2 so as to measure the elongated real-time length of the test sample 9 in the moving process of the clamp 2.
When the device is used, the auxiliary locking device is positioned at the avoidance station in advance, so that two ends of a sample 9 are respectively inserted into the clamping groove of the clamp 2A and the clamping groove of the clamp 2B and clamped; after clamping, the auxiliary locking device is adjusted to the locking station to increase the clamping force on the sample 9; then, the moving device 3 is started, so that the moving device 3 drives the clamp 2A to move towards a direction away from the clamp 2B, for example, the clamp moves upwards in the figure 1, and the sample 9 is stretched; in the stretching process, the force measuring device 4 measures the stretching force of the stretching sample 9, and the length measuring device measures the real-time stretching length of the sample 9; the strain hardening modulus value can be calculated according to the stretching force and the stretching length. The auxiliary locking device can strengthen the friction force between the sample 9 and the clamp 2, so that the sample 9 is not easy to slide in the stretching process, the precision of the tensile test is improved, and the failure probability of the tensile sample is reduced.
Referring to fig. 2 and 3, each of the clamps includes a pair of clamping panels 21 and a fixing base 22 having a positioning groove formed in a middle portion thereof, and the clamping panels 21 are fixed to opposite sides of the positioning groove; one surface of each clamping panel 21 is a fixing surface provided with anti-skid grains, and the other surface is an adjusting surface connected with a screw rod; the clamping groove is formed between the fixing surfaces of the pair of clamping panels 21, one end of the screw rod is rotatably fixed on the adjusting surface, the other end of the screw rod penetrates through the fixing seat and is connected with the adjusting knob 23, and the rotating shaft of the screw rod is perpendicular to the adjusting surface so as to convert the rotation of the adjusting knob 23 into the linear movement of the clamping panels 21 along the rotating shaft direction of the screw rod, so that the distance between the pair of clamping panels 21 is adjusted, and therefore samples 9 with different thicknesses are clamped; after the test is finished, the distance between the clamping panels 21 is increased through the adjusting knob 23, and the sample 9 is convenient to take out.
The adjusting knobs 23 on the two clamps can be arranged on the same side to adjust the clamping panels 21 on the same side of the two clamps 2, so that the clamping grooves are prevented from being dislocated, the clamped sample 9 is ensured to keep a vertical or horizontal position, and the testing accuracy is improved; moreover, the adjusting space is convenient to reserve on one side of the testing equipment.
As shown in fig. 3, the auxiliary locking device includes a first locking mechanism and a second locking mechanism;
clamping grooves 211 are respectively formed in two sides of each clamping panel 21, the first locking mechanism and the second locking mechanism respectively comprise two convex edges, and the two convex edges are respectively matched with the clamping grooves 211 on the same side of the pair of clamping panels 21; when the auxiliary locking device is positioned at the locking station, the two convex edges are respectively inserted into the clamping grooves 211 at the same side of the pair of clamping panels.
The engagement between the locking groove 211 and the rib in this embodiment serves to maintain the clamping force between the two clamping panels 21; especially when the first and second locking means are means for applying an auxiliary clamping force, the distance between the two ribs of each locking means is adjustable, and by reducing the distance between the two ribs, the clamping force between the two clamping panels 21 is increased, so that the specimen 9 clamped between the two clamping panels 21 does not slip.
When the positions of the clamping groove and the convex rib are exchanged, the same effect can be achieved; namely: the auxiliary locking device comprises a first locking mechanism and a second locking mechanism; the two sides of each clamping panel 21 are respectively provided with a convex rib, the first locking mechanism and the second locking mechanism respectively comprise two clamping grooves, and the two clamping grooves are respectively matched with the convex ribs on the same side of the pair of clamping panels 21; when the auxiliary locking device is located at the locking station, the protruding ribs on the same side of the pair of clamping panels 21 are respectively inserted into the two clamping grooves. When the first locking mechanism and the second locking mechanism are mechanisms capable of applying auxiliary clamping force, the distance between the two slots of each locking mechanism is adjustable, and the clamping force between the two clamping panels 21 can be increased by reducing the distance between the two slots.
At least one of the pair of holding panels 21 may be provided with a recess having a shape corresponding to the shape of one end of the held sample 9, or may have a partial shape having a size larger than the outline size of one end of the sample 9, so long as the recess is capable of holding the sample 9 and preventing it from slipping.
The recess may be provided on only one retainer panel 21; more preferably: the pair of clamping panels 21 are each provided with the recess, and the depth of the recess is less than half of the thickness of the sample 9, namely: the thickness of the sample 9 is greater than the sum of the depths of the two recesses to balance the forces applied to the two retainer panels 21, thereby improving the reliability of the retention and the life of the retainer panels 21. When the concave parts on the two clamping panels 21 are buckled and abutted on the test sample 9, a part of gap is still left between the two clamping panels 21 so as to apply clamping force to the test sample 9; the recess also serves to position the sample 9 so that the sample 9 is held in an optimal position. The depth of the recess may be 0.1mm to 3mm depending on the thickness of the sample 9.
To further improve the clamping effect, the surface of the recess is provided with anti-slip threads to increase the friction between the sample 9 and the recess when the sample 9 is stretched, thereby increasing the clamping force.
In order to hold the sample 9, each of the clamps 2 further comprises a sample positioning device, as shown in fig. 3, the sample positioning device comprises a fixing component 51 and a positioning component 52, the fixing component 51 is fixed on the side of one of the clamping panels 21, and the positioning component 52 is perpendicular to the clamping panel 21 and faces the other clamping panel 21. When holding the sample 9, as shown in fig. 2, one side of the sample 9 can abut against the positioning assembly 52 to use the positioning assembly 52 as a reference for positioning the sample 9, so that the sample 9 can be held in place. A slide slot may be provided in the fixed member 51 so that the sample positioning device can slide along the slide slot to adjust the position of the positioning member 52 and thus the positioning reference position of the sample 9.
In another embodiment, the measuring head of the length measuring device comprises a first measuring rod 61 which can be aligned with one end of the length to be measured of the test sample 9, and a second measuring rod 62 which is aligned with the other end of the length to be measured of the test sample 9; the first measuring rod 61 moves synchronously with one of said clamps 2 and the second measuring rod 62 moves synchronously with the other of said clamps to ensure that the distance between the first measuring rod 61 and the second measuring rod 62 is synchronized with the length of the test specimen 9. Of course, one or both of the holders 2 may also be in a stationary state, and then the corresponding first measuring rod 61 and/or second measuring rod 62 are also in a stationary state. The two measuring rods in this embodiment may be mounted on the frame 1 or on a slide rail 8, and may be separated along with the stretching of the sample 9 to identify the stretching length of the sample 9 in real time.
More preferably, the measuring head comprises a non-contact video optical measuring assembly. The non-contact video optical measuring assembly is an instrument for optically measuring the line deformation between two points of a target component or an object, and generally comprises an illumination field, a camera and a control chip. The deformation process is directly recorded by the camera, and the measured signal does not need to be amplified and subjected to digital-to-analog conversion, so that the measuring speed and accuracy are improved, and the measuring precision is higher than that of the conventional extensometer; moreover, the non-contact measurement can not cause any damage to the test sample, thereby avoiding the influence on the tensile test; in the test process, the measuring head does not need to be taken down, and the strain of the sample can be tracked in the whole process; also avoided current tensile equipment in the sample fracture, lead to the extensometer to drop and the danger of falling bad.
In the present invention, the pair of clamps 2, i.e., the clamp 2A and the clamp 2B, may be disposed vertically or horizontally, and in order to avoid the influence of gravity on the test, it is preferable that the clamps 2A and 2B are disposed vertically. When the pair of clamps 2 are an upper clamp and a lower clamp which are arranged up and down, the moving device 3 is fixed above the upper clamp, as shown in fig. 2, the force measuring device 4 is fixed at the bottom of the lower clamp, and the tensile force of the tensile sample 9 is obtained by detecting the tensile force applied to the lower clamp.
The tensile test device of the invention can also comprise a heating device 7 to heat the sample 9 and improve the activity of the high polymer material so as to obtain tensile test results under different parameter conditions.
The foregoing is only a partial embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (9)
1. A method for testing the residual life of a pipe is characterized by comprising the following steps:
obtaining strain hardening modulus values for at least three polyethylene materials of known design service life T<GP>The at least three polyethylene materials are polyethylene materials with different types;
hardening modulus value according to the strain<GP>And establishing a function with the design service life T:
T＝a0+a1×<Gp>+a2×<Gp>2+……+an×<Gp>n，
wherein, a0、a1、a2……anTaking a positive integer as a preset coefficient, wherein n is the preset maximum number of times of the function;
obtaining a test specimen from an in-service polyethylene pipe, comprising: intercepting part of the pipe from the in-service polyethylene pipe to be used as the test sample;
measuring the strain hardening modulus value of the test specimen<GP'>；
According to the function and the testStrain hardening modulus value of test specimen<GP'>Calculating the residual life of the test sample;
the strain hardening modulus values of at least three polyethylene materials of known design service life T are obtained<GP>The method comprises the following steps:
respectively manufacturing at least five standard samples of each polyethylene material in the at least three polyethylene materials;
obtaining the strain hardening modulus value of each standard sample through a high-temperature tensile test<GPi>Wherein i is the number of the standard sample;
according to the strain hardening modulus value of each standard sample<GPi>Calculating the root mean square average value of the strain hardening modulus values of each polyethylene material, and taking the root mean square average value as the strain hardening modulus value<GP>。
2. The method of claim 1, wherein after estimating the remaining life of the test specimen, further comprising:
and judging whether to replace the in-service polyethylene pipe or not according to the residual service life.
3. The method of claim 2, wherein the determining whether to replace in-service polyethylene pipe comprises:
when the residual life is not more than a first preset value, replacing the in-service polyethylene pipe;
and when the residual life is greater than the first preset value and less than a second preset value, monitoring the in-service polyethylene pipe.
4. The test method according to claim 1, wherein said separately making at least five standard specimens of each of said at least three polyethylene materials comprises:
preparing the at least five standard test samples of each polyethylene material in a die-pressing device, wherein the standard test samples are dumbbell-shaped sheets, each dumbbell-shaped sheet comprises a middle testing part and two end clamping parts, and the thickness of each dumbbell-shaped sheet is 0.3mm-1 mm;
and annealing the at least five standard samples at 115-125 ℃ for one hour, and cooling the samples to room temperature at a cooling speed of less than 2 ℃/min.
5. The method according to claim 4, wherein the strain hardening modulus value of each of the standard samples is obtained by a high temperature tensile test<GPi>The method comprises the following steps:
placing the dumbbell-shaped sheet in a thermostat at 80 ℃ for 30-60 min;
clamping the two end clamping parts on a high-temperature tensile testing machine, and applying 0.4Mpa prestress at a strain rate of 5 mm/min;
stretching the dumbbell sheet at a constant moving speed of 20mm/min, and collecting data values of the middle test portion having a stretch ratio lambda of 8 to 12, the stretch ratio lambda being
Wherein L is0Is the initial length of the middle test portion, and the L is the stretched length of the middle test portion;
recording the draw ratio λ and the true stress σ during the drawtrueThe corresponding relation of (1):
wherein F is a tensile force corresponding to the data value of the tensile ratio, and A is0The initial cross-sectional area of the middle test part of the dumbbell-shaped sheet is shown;
determining the strain hardening modulus value of each standard sample according to the Neo-Hookean constitutive model and the slope K of the corresponding relation when the lambda is 12 and the lambda is 8<GPi>Comprises the following steps:
<GPi>＝20K。
6. the method of claim 4, wherein the obtaining of test specimens from in-service polyethylene pipe comprises:
obtaining a test material from the in-service polyethylene pipe, and processing the test material into the test sample in a sampling machine;
the test sample is a dumbbell-shaped sheet, the dumbbell-shaped sheet comprises a middle test part and two end clamping parts, and the thickness of the dumbbell-shaped sheet is 0.3mm-1 mm.
7. The method of claim 6, wherein the measuring the strain hardening modulus value of the test specimen<GP'>The method comprises the following steps:
placing the test sample in a thermostat at 80 ℃ for 30-60 min;
clamping the clamping parts at the two ends of the test sample on a high-temperature tensile testing machine, and applying 0.4Mpa prestress at a strain rate of 5 mm/min;
stretching the test specimen at a constant moving speed of 20mm/min, and collecting data values of a stretching ratio lambda 'between 8 and 12 at a middle test part of the test specimen, wherein the stretching ratio lambda' is
Wherein L is0'is the initial length of the middle test portion of the test specimen, and L' is the stretched length of the middle test portion of the test specimen;
recording the draw ratio λ' and the true stress σ during the drawtrue' the correspondence relationship:
wherein F 'is a stretching force corresponding to the data value of the stretching ratio lambda', and A0' is the initial cross-sectional area of the middle test portion of the test specimen;
determining the strain hardening of the test sample according to the Neo-Hookean constitutive model and the slope K ' of the corresponding relation when the lambda ' is 12 and the lambda ' is 8Modulus value<GP'>Is composed of
<GP'>＝20K’。
8. The test method of claim 7, wherein the test sample is in at least two pieces; determining the strain hardening modulus value of the test sample according to the Neo-Hookean constitutive model and the slope K ' of the corresponding relation when the lambda ' is 12 and the lambda ' is 8<GP'>The method comprises the following steps:
determining the strain hardening modulus value of each of the test specimens<GPi'>；
Calculating the strain hardening modulus value<GPi'>The root mean square average value is taken as the strain hardening modulus value of the test sample<GP'>。
9. A system for testing the remaining life of a pipe, comprising:
a sample preparation device: for obtaining test specimens from in-service polyethylene pipe, comprising: intercepting part of the pipe from the in-service polyethylene pipe to be used as the test sample;
a tensile test device: for measuring the strain hardening modulus value of the test specimen<GP'>；
A central processing unit: strain hardening modulus values for at least three polyethylene materials for obtaining a known design service life T<GP>The at least three polyethylene materials are polyethylene materials with different types; hardening modulus value according to the strain<GP>And establishing a function with the design service life T:
T＝a0+a1×<Gp>+a2×<Gp>2+……+an×<Gp>nwherein a is0、a1、a2……anTaking a positive integer as a preset coefficient, wherein n is the preset function highest frequency; according to the function and the strain hardening modulus value of the test sample<GP'>Push awayCalculating the residual life of the test sample;
the strain hardening modulus values of at least three polyethylene materials of known design service life T are obtained<GP>The method comprises the following steps:
respectively manufacturing at least five standard samples of each polyethylene material in the at least three polyethylene materials;
obtaining the strain hardening modulus value of each standard sample through a high-temperature tensile test<GPi>Wherein i is the number of the standard sample;
according to the strain hardening modulus value of each standard sample<GPi>Calculating the root mean square average value of the strain hardening modulus values of each polyethylene material, and taking the root mean square average value as the strain hardening modulus value<GP>。
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| 基于应变硬化和循环载荷的ＰＥ管材慢速裂纹增长试验新方法评述;陈国华等;《高分子通报》;20180228;22-30 * |
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