WO2019061156A1 - Service life evaluation method for plastic piping - Google Patents

Abstract

A service life evaluation method for plastic piping. The method comprises the following steps: holding a sample (100) at two ends, the sample (100) being cylindrical and having a preformed circumferential notch (110) on a peripheral surface; pulling the sample (100) at a predetermined frequency f, wherein the tensile stress is periodically and gradually varied between a predetermined maximum stress Fmax and a predetermined minimum stress Fmin during the pulling process, and the duration of the variation of tensile stress from one extreme to the other is one period; observing a surface of the sample (100), and recording an iteration number Nini of the tensile stress variation and a stress range △σ when a new crack forms proximate to the preformed notch (110); converting the variation iteration number Nini to a pulling duration T; repeating the above steps by using different stress ranges △σ to obtain multiple pulling durations T; and plotting a log-log graph according to the multiple stress ranges △σ and the corresponding pulling durations T. The method can be used to greatly reduce the time period of an evaluation process.

Description

Plastic pipe life evaluation method Technical field

The invention relates to the technical field of plastic pipe manufacturing, in particular to a method for evaluating the life of a plastic pipe.

Background technique

At present, the evaluation methods for slow crack growth resistance of plastic pipes mainly include notched tube test (NPT), full notched tensile creep test (FNCT), Pennsylvania notched tensile test (PENT), notched ring test (NRT) and points. Load test (PLT), etc. Evaluation of the above evaluation method is too long period, typically one year or ¹⁰⁴ hours (about 13 months). The long evaluation period also hinders the development speed of the special resin for plastic pipes. Moreover, in the evaluation cycle, it is difficult to observe the failure of the sample, and thus the microscopic morphology after the failure of the sample cannot be observed, resulting in inaccurate evaluation results.

Summary of the invention

The object of the present invention is to overcome the deficiencies of the prior art and provide a method for evaluating the life of a plastic pipe, by which the evaluation period can be significantly reduced, and the microscopic morphology after the failure of the sample can be observed during the evaluation period, and the evaluation result is improved. The accuracy.

Embodiments of the present invention are implemented by the following technical solutions:

Plastic pipe life evaluation methods, including:

Holding the two ends of the sample, the sample is cylindrical and the circumferential surface is provided with a ring-shaped pre-formed notch;

The sample is stretched by the preset frequency f, and the tensile stress gradually changes between the maximum preset stress F _max and the minimum preset stress F _min during the stretching process, and the tensile stress changes once for one cycle;

Observing the surface of the sample, when a new crack is generated near the pre-formed gap, the number of changes in the tensile stress N _ini and the stress range _{Δσ are} recorded;

Converting the number of changes N _ini to the stretching time T;

Repeating the above steps using different stress ranges Δσ to obtain a plurality of stretching times T;

Drawing a double logarithmic coordinate map according to the plurality of stress ranges Δσ and the corresponding stretching time T;

Where Δσ=F _max —F _min , T=N _ini /(3600*f);

The units of Δσ, F _max and F _min are all Mpa; the unit of T is hour; the unit of f is Hz; the unit of N _ini is second.

Further, 5 Hz ≤ f ≤ 15 Hz.

Further, the ratio of the minimum preset stress F _min to the maximum preset stress F _max is R; 0.1 ≤ R ≤ 1.0.

Further, the plastic pipe life evaluation method is performed in an environment of 21 ° C to 25 ° C.

Further, the surface of the sample is observed with a microscope; a plurality of microscopes are evenly arranged around the sample, and the microscope is vertically aligned with the pre-formed notch.

Further, the depth of the pre-cut notch is 1.49 mm - 1.51 mm; the depth direction of the pre-formed notch is perpendicular to the axial direction of the sample.

Further, the diameter of the sample is 13 mm to 15 mm, and the length of the sample is 70 mm to 110 mm.

Further, the diameter of the sample is 13.5 mm - 14.5 mm, and the length of the sample is 90 mm - 100 mm.

Further, the method for evaluating the life of the plastic pipe further includes the steps of preparing the sample, and the steps of preparing the sample include:

Injection molding a cylindrical bar;

Cutting a cylindrical bar to obtain a semi-finished product;

A ring-shaped pre-formed notch was machined on the circumferential surface of the semi-finished product at a blade feed speed of 0.03 mm to 0.04 mm/turn to obtain a sample.

Further, the shortest distance between the clamping position of the two ends of the sample and the pre-formed notch is 5 mm to 10 mm.

The technical solution of the present invention has at least the following advantages and beneficial effects:

The method for evaluating the life of the plastic pipe provided by the embodiment of the invention can observe the crack initiation of the sample in several tens of hours or even several hours, and the double drawn by the plurality of stress ranges Δσ and the corresponding stretching time T The logarithmic graph is used to evaluate the service life of plastic pipes. The method for evaluating the life of the plastic pipe provided by the embodiment of the invention greatly reduces the evaluation period and helps to improve the development speed of the special resin for the plastic pipe. Moreover, the plastic pipe life evaluation method provided by the embodiment of the invention can observe the microscopic shape of the sample after the failure in the evaluation period, and improve the accuracy of the evaluation result.

DRAWINGS

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following drawings for the embodiments need to be briefly introduced. It is understood that the following drawings are merely illustrative of certain embodiments of the invention and are not intended to Other drawings can be obtained from those skilled in the art without departing from the drawings.

1 is a schematic structural view of a sample according to an embodiment of the present invention;

2 is a schematic view showing a sample in a stretched state according to an embodiment of the present invention;

3 is a graph showing changes in tensile stress during a process of a sample according to an embodiment of the present invention;

4 is a first double logarithmic coordinate diagram in an embodiment of the present invention;

FIG. 5 is a second double logarithmic coordinate diagram in the embodiment of the present invention.

In the figure: 100-sample; 110-preformed notch; 200-electron stretching machine; 210-clamping member; 300-microscope.

Detailed ways

In order to make the objects, technical solutions and advantages of the embodiments of the present invention more clear, the present invention will be The technical solutions in the examples are clearly and completely described. It is apparent that the described embodiments are part of the embodiments of the invention, and not all of the embodiments.

Therefore, the following detailed description of the embodiments of the invention is not intended to All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts are within the scope of the present invention.

It should be noted that the features and technical solutions in the embodiments and the embodiments of the present invention may be combined with each other without conflict.

It should be noted that similar reference numerals and letters indicate similar items in the following figures. Therefore, once an item is defined in one figure, it is not necessary to further define and explain it in the subsequent figures.

Example 1:

This embodiment provides a method for evaluating the life of a plastic pipe, which is realized based on the sample 100 shown in FIG. Referring to FIG. 1, the sample 100 is cylindrical and has an annular pre-formed notch 110 on the circumferential surface. It should be noted that the diameter of the sample 100 can be selected between 13 mm and 15 mm, and the length of the sample 100 can be selected between 70 mm and 110 mm. Further, the diameter of the sample 100 can be controlled between 13.5 mm and 14.5 mm, and the length of the sample 100 can be controlled between 90 mm and 100 mm. In this way, it is possible to avoid the problem that the test result cannot be accurately displayed because the size of the sample 100 is too small, and the problem that the evaluation period is too long due to the excessive size of the sample 100 can be avoided. In the present embodiment, the diameter of the sample 100 is 14 mm, and the length of the sample 100 is 95 mm. The prefabricated notch 110 is disposed at an intermediate position in the axial direction of the sample 100. In addition, the depth of the pre-formed notch 110 is 1.49 mm - 1.51 mm, and the depth direction of the pre-formed notch 110 is perpendicular to the axial direction of the sample 100. The depth of the prefabricated notch 110 is controlled to be 1.49 mm-1.51 mm, which can avoid that the new crack is difficult to be initiated due to the too small depth of the pre-formed notch 110, thereby prolonging the evaluation period and avoiding the sample due to the excessive depth of the pre-formed notch 110. The 100 fractured under repeated stretching, and thus the problem of the test purpose could not be achieved.

The method for evaluating the life of a plastic pipe includes the following steps:

S01: Both ends of the sample 100 are clamped.

Please refer to FIG. 2. FIG. 2 is a schematic view showing the sample 100 in a stretched state in the embodiment. In the present embodiment, the sample 100 is stretched using an electron stretching machine 200. The upper and lower clamping members 210 of the electronic stretching machine 200 respectively hold the two ends of the sample 100. The shortest distance between the clamping position of the sample 100 and the pre-formed notch 110 is 5 mm to 10 mm. That is, the shortest distance L _min holder between the notch 210 and the preform 110 is 5mm-10mm. L _{min is} set within the above range, and it can be avoided that the distance between the holding member 210 and the pre-formed notch 110 during the stretching process is too large, so that the crack cannot be generated near the pre-formed notch 110, and the clip during the stretching process can be avoided. The distance between the holder 210 and the pre-formed notch 110 is too small, causing the sample 100 to be broken and the test purpose cannot be achieved. In the present embodiment, the shortest distance L _min clamp member 210 between the notches 110 and the preform is 7.5mm.

S02: The sample 100 is stretched by a preset frequency f, and the tensile stress gradually changes between the maximum preset stress F _max and the minimum preset stress F _min during the stretching, and the tensile stress changes once for one cycle.

Please refer to FIG. 3. FIG. 3 is a graph showing changes in tensile stress during stretching of the sample 100. It can be seen from the figure that the tensile stress gradually increases from the minimum preset stress F _min to the maximum preset stress F _max during the stretching process, and then gradually decreases from the maximum preset stress F _max to the minimum preset stress. F _min . Thus, the sample 100 was stretched. The time during which the tensile stress gradually increases from the minimum preset stress F _min to the maximum preset stress F _{max is the same} as the time when the tensile stress maximum preset stress F _max gradually decreases to the minimum preset stress F _min . The tensile stress changes between the minimum preset stress _Fmin and the maximum preset stress _Fmax once for one cycle.

The unit of the minimum preset stress F _min and the maximum preset stress F _max is Mpa, and the unit of the preset frequency f is Hz.

The preset frequency f can be selected between 5 Hz and 15 Hz, so as to avoid the problem that the evaluation period is prolonged due to the preset frequency f being too small, and the problem that the life of the plastic pipe cannot be accurately reflected due to the excessively large preset frequency f can be avoided. . In this embodiment, the preset frequency f = 10 Hz.

Further, in the present embodiment, the maximum preset stress F _{max is set in} accordance with the density of the material of the sample 100. If the density of the material of the sample 100 is large, the maximum preset stress F _max is increased; if the density of the material of the sample 100 is small, the maximum preset stress F _max is decreased.

It should be further noted that the ratio R of the minimum preset stress F _min to the maximum preset stress F _max is greater than or equal to 0.1 and less than or equal to 1.0. In this way, it can be ensured that the minimum predetermined stress _Fmin can also achieve the effect of applying a sufficient load to the sample 100 to cause crack initiation, and the evaluation period is lowered. In the present embodiment, the ratio of the minimum preset stress _Fmin to the maximum preset stress _Fmax is R=0.5.

S03: observing the surface of the sample 100, when a new crack is generated near the pre-formed notch 110, the number of changes in the tensile stress N _ini and the stress range _{Δσ are} recorded;

Converting the number of changes N _ini to the stretching time T;

Where Δσ=F _max —F _min , T=N _ini /(3600*f);

The units of Δσ, F _max and F _min are all Mpa; the unit of T is hour; the unit of f is Hz; the unit of N _ini is second.

Referring again to FIG. 2, in the present embodiment, the surface of the sample 100 is observed by the microscope 300. The surface of the sample 100 was observed to determine whether a new crack was generated near the pre-formed notch 110. Since the size of the new crack generated near the pre-formed notch 110 is very small, in order to immediately grasp the crack initiation in the vicinity of the pre-formed notch 110, the surface of the sample 100 is observed by the microscope 300 in this embodiment. Further, in order to be able to comprehensively observe the surface of the sample 100, in the present embodiment, a plurality of microscopes 300 are uniformly arranged around the sample 100. Further The four microscopes 300 are uniformly arranged around the sample 100, and the angle between the adjacent microscopes 300 is 90°. Thus, a comprehensive observation of the sample 100 indicates that the crack initiation near the pre-formed notch 110 is immediately grasped. Further, in the present embodiment, the microscope 300 is vertically aligned with the pre-formed notch 110, that is, the microscope 300 is perpendicular to the axis of the sample 100. In this way, new cracks generated above and below the pre-formed notch 110 can be accurately collected by the microscope 300, and new cracks generated above and below the pre-formed notch 110 can be clearly displayed, which is advantageous for the staff to grasp the prefabricated notch 110. Crack initiation in the vicinity.

When the worker observed a new crack near the pre-formed notch 110, the number of changes in the tensile stress N _ini and the stress range _{Δσ were} recorded. The unit of the number of changes N _ini is the second, which corresponds to the number of cycles of the tensile stress. The stress range Δσ refers to the difference between the maximum preset stress F _max and the minimum preset stress F _min , that is, Δσ=F _max —F _min , and the unit of Δσ is Mpa. Then, the number of changes N _{ini is} converted into the stretching time T. T = N _ini / (3600 * f), the unit of T is hour.

Draw a double logarithmic graph. Referring to FIG. 4, FIG. 4 is a drawing of a double logarithmic coordinate diagram in the embodiment. In the double logarithmic graph, the abscissa is the stretching time T and the ordinate is the stress range Δσ. The T and Δσ obtained above are plotted as a coordinate point in the double logarithmic graph in the double logarithmic graph.

S04: Steps S01 to S03 are repeated with different stress ranges Δσ to obtain a plurality of stretching times T. According to the plurality of stress ranges Δσ and the corresponding stretching time T, coordinate points in the double logarithmic graph are drawn, and the coordinate points are drawn in the double logarithmic graph. Then, according to the coordinate points in the double logarithmic graph, a line representing the relationship between T and Δσ is drawn, thereby completing the drawing of the double logarithmic graph. According to the double logarithmic graph, the relationship between the service life of the material constituting the sample 100 and the stress range Δσ after the plastic pipe is fabricated can be determined. Among them, the larger the stretching time T, the longer the service life of the plastic pipe produced by the material constituting the sample 100.

The method for evaluating the life of the plastic pipe provided in this embodiment is carried out in an environment of 21 ° C to 25 ° C to ensure that crack initiation can be observed in several tens of hours or even several hours, thereby greatly reducing the evaluation period and helping To improve the development speed of special resins for plastic pipes. At the same time, the plastic pipe life evaluation method provided by the embodiment can observe the microscopic morphology of the sample 100 after the failure in the evaluation period, and improve the accuracy of the evaluation result. Evaluation methods such as the existing Notch Tube Test (NPT), Full Notch Tensile Creep Test (FNCT), Pennsylvania Notched Tensile Test (PENT), Notched Ring Test (NRT), and Point Load Test (PLT) are required. The test is carried out at about 80 ° C. The life evaluation method of the plastic pipe provided in this embodiment has low requirements on the ambient temperature and can effectively reduce the evaluation cost. Further, in the present embodiment, the plastic pipe life evaluation method is performed in an environment of 23 ° C.

It should be further noted that the method for evaluating the life of the plastic pipe provided by the embodiment further includes the step of preparing the sample 100, and the steps of preparing the sample 100 include:

(1) injection molding cylindrical rods;

(2) cutting the cylindrical bar material to obtain a semi-finished product;

(3) An annular pre-formed notch 110 was machined on the circumferential surface of the semi-finished product at a blade feed speed of 0.03 mm to 0.04 mm/turn to obtain a sample 100.

Processing the pre-formed notch 110 at a blade feed speed of 0.03 mm to 0.04 mm/turn ensures that the surface of the pre-formed notch 110 is smooth. In this way, it is possible to avoid the occurrence of burrs on the surface of the pre-formed notch 110 caused by the slow feed rate of the blade, thereby affecting the problem of new crack initiation; and also avoiding cracks occurring around the pre-formed notch 110 during processing due to excessive blade feed speed. problem. Make the evaluation results more accurate.

Next, the method for evaluating the life of the plastic pipe provided by the embodiment is further described:

Table 1 shows the case where the plastic pipe life evaluation method is performed using different preset frequencies f under the same conditions (the conditions employed in the present embodiment).

Table 1

It can be seen from Table 1 that when f is less than 5 Hz, the evaluation period is too long; when 5 Hz ≤ f ≤ 15 Hz, the evaluation period is greatly shortened; when f is greater than 15 Hz, a large area of strain is caused in the sample 100 in a short time. This makes it impossible to evaluate the life of plastic pipes.

Table 2 shows the case where the plastic pipe life evaluation method is performed using R of different values under the same conditions (the conditions employed in the present embodiment).

Table 2

It can be seen from Table 2 that when R is less than 0.1, since the minimum preset stress F _{min is} too small, it is impossible to apply a sufficiently large load to the sample 100, thereby prolonging the new crack initiation time and increasing the evaluation period; when 0.1≤R≤0.6, since the preset minimum F _min stress increases, so that the new crack initiation time is shortened, thereby shortening the evaluation played back; when R is greater than 0.6, due to the preset minimum F _min and a maximum pre-stress It is _assumed that the difference between the stresses F _max becomes small, which causes the alternating load to be withstood by the sample 100 to be reduced, which also prolongs the new crack initiation time and increases the evaluation period.

Example 2:

In this embodiment, the plastic pipe life evaluation method provided in Embodiment 1 is used to test four different materials of PE100-1, PE100-2, PE100RC-3 and PE100RC-4, and the above four materials are represented by T and △. The lines of the σ relationship are drawn in the same double logarithmic graph. Figure 5 is a double logarithmic coordinate diagram in the present embodiment. From Fig. 5, it is possible to compare the difference in the stretching time T of the above four materials under the same stress range Δσ. Under the same stress range Δσ, the longer the stretching time T, the longer the service life of the finished plastic pipe. In this way, the life of plastic pipes made of different materials can be compared and evaluated, so as to provide reference for the development of special resin for plastic pipes.

It is apparent that the above-described embodiments of the present invention are merely illustrative of the present invention and are not intended to limit the embodiments of the present invention. Other variations or modifications of the various forms may be made by those skilled in the art in light of the above description. There is no need and no way to exhaust all of the implementations. Any modifications, equivalent substitutions and improvements made within the spirit and scope of the invention are intended to be included within the scope of the appended claims.

Industrial applicability

The method for evaluating the life of the plastic pipe provided by the embodiment of the invention can observe the crack initiation of the sample in several tens of hours or even several hours, and the double drawn by the plurality of stress ranges Δσ and the corresponding stretching time T The logarithmic graph is used to evaluate the service life of plastic pipes. The method for evaluating the life of the plastic pipe provided by the embodiment of the invention greatly reduces the evaluation period and helps to improve the development speed of the special resin for the plastic pipe. Moreover, the plastic pipe life evaluation method provided by the embodiment of the invention can observe the microscopic shape of the sample after the failure in the evaluation period, and improve the accuracy of the evaluation result.

Claims (10)

1. A method for evaluating the life of a plastic pipe, characterized in that it comprises:

   Holding the two ends of the sample, the sample is cylindrical and the circumferential surface is provided with an annular pre-formed notch;

   The sample is stretched by a preset frequency f, and the tensile stress gradually changes between the maximum preset stress F _max and the minimum preset stress F _min during the stretching, and the tensile stress is changed once for each cycle. ;

   Observing the surface of the sample, when a new crack is generated in the vicinity of the pre-formed notch, recording the number of changes in the tensile stress N _ini and the stress range Δσ;

   Converting the number of changes N _ini to a stretching time T;

   Repeating the above steps using different stress ranges Δσ to obtain a plurality of stretching times T;

   Drawing a double logarithmic coordinate map according to the plurality of stress ranges Δσ and the corresponding stretching time T;

   Where Δσ=F _max —F _min , T=N _ini /(3600*f);

   The units of Δσ, F _max and F _min are all Mpa; the unit of T is hour; the unit of f is Hz; the unit of N _ini is second.
2. The method for evaluating the life of a plastic pipe according to claim 1, wherein:

   5 Hz ≤ f ≤ 15 Hz.
3. The method for evaluating the life of a plastic pipe according to claim 1, wherein:

   The ratio of the minimum preset stress F _min to the maximum preset stress F _max is R;

   0.1 ≤ R ≤ 1.0.
4. The method for evaluating the life of a plastic pipe according to claim 1, wherein:

   The method is carried out in an environment of 21 ° C to 25 ° C.
5. The method for evaluating the life of a plastic pipe according to claim 1, wherein:

   The surface of the sample is observed with a microscope; a plurality of the microscopes are evenly arranged around the sample, the microscope being vertically aligned with the pre-formed notch.
6. A method for evaluating the life of a plastic pipe according to any one of claims 1 to 5, characterized in that:

   The depth of the pre-formed notch is 1.49 mm - 1.51 mm; the depth direction of the pre-formed notch is perpendicular to the axial direction of the sample.
7. A method for evaluating the life of a plastic pipe according to any one of claims 1 to 5, characterized in that:

   The sample has a diameter of 13 mm to 15 mm, and the sample has a length of 70 mm to 110 mm.
8. The method for evaluating the life of a plastic pipe according to claim 7, wherein:

   The sample has a diameter of 13.5 mm to 14.5 mm, and the sample has a length of 90 mm to 100 mm.
9. A method for evaluating the life of a plastic pipe according to any one of claims 1 to 5, characterized in that:

   The method also includes the step of making the sample, the step of making the sample comprising:

   Injection molding a cylindrical bar;

   Cutting the cylindrical bar to obtain a semi-finished product;

   The preformed notch was formed on the circumferential surface of the semi-finished product at a blade feed speed of 0.03 mm to 0.04 mm/turn to obtain the sample.
10. A method for evaluating the life of a plastic pipe according to any one of claims 1 to 5, characterized in that:

    The shortest distance between the clamping position of the two ends of the sample and the pre-formed notch is 5 mm - 10 mm.

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