CN113466042A - Gas pipeline corrosion fatigue crack propagation trend fractal dimension characterization method and system - Google Patents
Gas pipeline corrosion fatigue crack propagation trend fractal dimension characterization method and system Download PDFInfo
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Abstract
A fractal dimension characterization method and a fractal dimension characterization system for corrosion fatigue crack propagation trend of a gas pipeline comprise the following steps: cutting and sampling the L360-set pipeline, prefabricating fatigue cracks and cleaning; before the fatigue test, testing and collecting the low-frequency noise of the CT sample; calculating a fractal dimension D of low-frequency noise before fatigue; carrying out corrosion fatigue test on the CT sample; measuring the crack noise of the CT sample after corrosion fatigue; and calculating the fractal dimension D of the low-frequency noise after fatigue, and forecasting and characterizing the fatigue crack propagation degree of the CT sample through the change of the fractal dimension value. The method utilizes the fractal dimension of the electrical noise in the measured fatigue crack propagation process to represent the propagation behavior of the pipeline crack, the measuring and calculating time is less, and the pipeline body cannot be damaged in the measuring process. In combination with the above analysis, it was found that in the three stages of crack propagation, the microcracks gradually grow and grow with the initiation, propagation, and coalescence of the microcracks, and then gradually decrease, and the main cracks gradually grow and grow.
Description
Technical Field
The invention belongs to the technical field of fatigue monitoring of oil and gas pipelines, and particularly relates to a fractal dimension characterization method and a fractal dimension characterization system for a corrosion fatigue crack propagation trend of a gas pipeline.
Background
PipelineTransportation is the primary mode of oil and gas transportation. In recent years, with the increasing demand of natural gas in China, high-pressure and large-diameter transportation and digital management are development trends of natural gas pipeline construction at present. However, natural gas contains some amount of H2S (e.g. Hongkong oil field H)2S content 1.4%, inner Mongolia Changqing gas field 1.5%), H2S is not only toxic, but also strongly corrosive. Containing H2Natural gas of S acid gas can corrode pipelines during transportation. Especially, when a new pipeline is built, more water is left in the pipeline pressing test, and the existence of the water can accelerate H2And S, corrosion of acid gas. The fatigue damage of the pipeline is accelerated to the phenomenon of damage due to corrosion at home and abroad, and the research on the corrosion fatigue of the pipeline has certain practical significance on the normal operation and safe production of the pipeline.
It is believed that fatigue damage gradually accumulates within the pipe, and when a certain threshold is reached, an initial fatigue crack forms. Then, the initial fatigue crack is continuously opened and closed under the combined action of the cyclic stress and the environment, and gradually expands and enlarges, namely, subcritical expansion occurs. When the crack length reaches its critical crack length, it is difficult to withstand the applied load, and the crack rapidly propagates to break. It can be seen that this pipe failure is a progressive and cumulative failure process and is irreversible. For natural gas transmission pipelines, pipeline leakage occurs once cracks occur, which causes an economic loss which is difficult to estimate for oil and gas fields. Therefore, predicting the service life of the pipeline and avoiding the overdimensioning of the pipeline with the fault is a key direction of the current pipeline research.
At home and abroad, a plurality of researches on fatigue crack propagation life prediction methods are carried out, and the method which is most widely applied in engineering at present is a fatigue crack propagation formula which is proposed by Paris and Erdogan in 1963 on the basis of tests, namely a famous Paris formula. The formula establishes the relation between the stress intensity factor and the crack propagation rate, and is the theoretical basis for predicting the fatigue crack propagation life in the current engineering application. Paris's formula is shown below:
where a is the crack length, N is the cycle number, Δ K is the stress intensity factor variation amplitude, and C and m are constants associated with the material, which can be obtained by fitting experimental data. However, with the progress of research, people find that although the Paris formula has better universality, certain defects exist, and the main reasons for the defects are two points: first, the material constant C, m generally varies with changes in parameters such as stress ratio; second, the Pairs's formula only applies to the steady-state phase of fatigue crack propagation. In addition, environmental factors such as temperature, humidity, medium, loading frequency, etc. may also affect the material constant C, m. Therefore, C and m are rarely found to be constant in practical experiments. The assumption of Paris formula is too simple and has more influence factors. Therefore, the parameters extracted from the method have certain uncertainty, and the propagation information of the fatigue crack cannot be accurately analyzed.
Noise, a phenomenon of random fluctuation occurring in nature. 1/f noise is a type of low frequency noise whose power spectral density S (f) is proportional to 1/f α, which is typically around 1. Noise research has been easily overlooked, and the use of low frequency noise in electronic devices, which began in 1989 by the discovery by the national laboratory of oak ridge, Fleetwood et al, low frequency 1/f noise being extremely sensitive to radiation damage in MOSFET devices, opened the high tide of low frequency noise research. The research shows that: low frequency noise contains a lot of important information. When defects (such as dislocation, crack and the like) exist in the material and the device, low-frequency 1/f noise is dominant, and the fluctuation information of the low-frequency 1/f noise has strong correlation with the defects of the material. Since low frequency 1/f noise belongs to the class of fractal noise, the fractal dimension D has the following relationship with α:
α＝5-2D (2)
thus, studying changes in fractal dimension can predict defect changes within the material. Since the pipeline crack is a defect in the material, the fractal dimension of the pipeline crack can be calculated by measuring low-frequency 1/f noise, and the crack propagation degree can be judged.
The conventional Pairs characterization method is to perform destructive fatigue tests on a fatigue testing machine and determine parameters related to the propagation of fatigue cracks. To obtain accurate and reliable results, the Paris method requires a large number of samples to perform the measurement over a long period of time, which is time and labor intensive. It is also mentioned that some of the measured parameter values have a certain uncertainty due to environmental influences. The method for collecting low-frequency noise of a pipeline and calculating the fractal dimension of the noise is a sensitive nondestructive testing method. The method has the advantages of high detection speed and no damage to the sample. And in the process of fatigue crack propagation, from crack initiation, crack propagation to crack breakage, the formation and change information of the internal defects are related to the fractal dimension of crack low-frequency noise. Based on the analysis, the noise signal can be acquired through the data acquisition card, the fractal dimension of the noise signal is calculated, and the expansion trend of the fatigue crack of the gas pipeline can be reflected by the change of the fractal dimension.
Disclosure of Invention
The invention aims to solve the technical problem that the fractal dimension characterization method and the fractal dimension characterization system for the corrosion fatigue crack propagation trend of the gas transmission pipeline are provided aiming at the defects in the prior art.
The invention adopts the following technical scheme:
the fractal dimension characterization method for the corrosion fatigue crack propagation trend of the gas pipeline comprises the following steps of:
cutting and sampling the L360-set pipeline, prefabricating fatigue cracks and cleaning; before the fatigue test, testing and collecting the low-frequency noise of the CT sample; calculating a fractal dimension D of low-frequency noise before fatigue; carrying out corrosion fatigue test on the CT sample; measuring the crack noise of the CT sample after corrosion fatigue; and calculating the fractal dimension D of the low-frequency noise after fatigue, and forecasting and characterizing the fatigue crack propagation degree of the CT sample through the change of the fractal dimension value.
Further, cutting and sampling, prefabricating fatigue cracks and cleaning the concrete steps of:
s101, sampling a test sample in a CR direction, wherein C represents the normal direction of the surface of the crack, namely the circumferential direction; r represents that the crack extends along the wall thickness direction, the pipe wall of the pipeline does not cut a sample according to the CR orientation, the sample is cut on the pipe according to the CL orientation, L is the crack extending direction, and 3 base material samples are cut according to the CL orientation;
s102, manufacturing a CT sample according to GB/T6398-2000 standard, directly processing a flat plate structure after taking down the sample, wherein the sample is a standard compact tensile sample containing penetrating cracks, namely the CT sample, the CT sample W is 65mm, the thickness B is 15mm, and a crack source with the length of 5mm is cut by using a molybdenum wire with the length of 0.2 mm;
s103, performing fatigue crack prefabrication on the sample before testing, wherein the pre-crack load ratio is 0.1, the maximum load is 10kN, and the length of the prefabricated fatigue crack is about 1.5-2 mm;
s104, firstly, removing rust on the surface of the CT sample, and soaking the CT sample in a rust removing solution for 20-30 minutes; and then taking out the CT sample, washing with deionized water for 2-3 minutes, removing redundant rust removing liquid attached to the surface, finally hanging the sample with a clamp, and drying.
Further, the low frequency noise of the CT sample is tested and collected:
respectively welding A, B two leads on two sides of a prefabricated crack of a CT sample, connecting the lead A to a bias circuit of a low-frequency noise test platform, connecting a 100 omega slide rheostat in series, and then adjusting the slide rheostat to enable the voltage value to be 2.5V; the B lead is connected with a preposed low-noise amplifier on a noise test platform, the noise test platform comprises a bias circuit, the low-noise amplifier and a computer acquisition system, and the whole noise test platform is built in a Cu network electromagnetic shielding room.
Further, calculating a fractal dimension D of low-frequency noise before fatigue;
a box counting method is selected to calculate the fractal dimension, and the main method is as follows:
covering the noise signal graph by using a square grid with the side length r, counting the number N (r) of grids containing noise points in the covered grid, and changing the side length r of the square grid every time to obtain the noise signal graphDifferent N (r), namely obtaining a group of data (r)i，N(ri) Meet the following requirements:
N(r)∝r-D
wherein N (r) is the number of lattices containing noise points, r is the side length of a square lattice, and D is a fractal dimension; for data ln (1/r)i) And lnN (r)i) The fitting was performed, and the calculation result was D1.907.
Further, the CT samples were subjected to corrosion fatigue testing:
taking out the CT sample from the low-frequency noise test system, adopting special leak-proof glue to construct a closed space around the crack of the CT sample, and using a medical injector to inject H2Injecting an S saturated aqueous solution into the cavity, placing the sample on an MTS 810-250 type hydraulic servo testing machine, automatically recording the information of load, displacement and crack length in the testing process by the testing machine, and adopting a load ratio R-Pmin/PmaxMaximum load P of 0.1maxThe loading frequency is 5Hz and the loading waveform is a sine wave under the condition of 10 kN;
in fatigue crack propagation, the crack size a grows along with the increase of the load cycle number N, the change amplitude delta K of the stress intensity factor at the crack tip also changes continuously, and a series of data (a) is recorded by recording the data of a certain crack propagation length a and the corresponding cycle number N in the testi,Ni) Data, fatigue test was stopped when the crack propagated to 10-15 mm.
Further, measuring the crack noise of the CT sample after corrosion fatigue;
taking the CT sample from the fatigue machine, and quickly cleaning the crack of the sample by using acetone; then, washing the acetone residual solution for 2-3 minutes by using deionized water, wherein the surface of the CT sample is parallel to the water flow direction when the acetone residual solution is washed by using the deionized water; finally, hanging the sample by using a clamp and drying the sample; and (3) carrying out low-frequency noise test on the CT sample after fatigue test on a low-frequency noise test platform, wherein the applied bias voltage is the same as that of the previous step.
Further, calculating a fractal dimension D of the low-frequency noise after fatigue, and forecasting and characterizing the fatigue crack propagation degree of the CT sample through the change of the fractal dimension:
the method comprises the following steps of finding out the change of a fractal dimension value through the calculation of the fractal dimension of low-frequency noise before and after a fatigue test, and further judging the condition of crack propagation of a pipeline, wherein the fractal dimension calculation comprises the following main steps:
1) selecting a group of proper square side lengths to form an array r ═ r1,r2,...rn}；
2) Each time using side length of riThe squares of (a) form a grid covering the noise signal. Counting the total number N (r) of square lattices covered with noise signalsi) After N times, a set of data N (r) ═ N (r) is obtained1),N(r2),...N(rn)}；
3) For data ln (1/r)i) And lnN (r)i) Fitting is carried out, and the obtained slope is the fractal dimension D;
according to the relation between the fatigue crack expansion rate da/dN and the change amplitude delta K of the stress intensity factor, fatigue damage is gradually accumulated in the pipeline, and when a certain critical value is reached, an initial fatigue crack is formed; then, the initial fatigue crack gradually expands under the combined action of the cyclic stress and the environment, namely, the subcritical expansion occurs, when the crack length reaches the critical crack length, the external load is difficult to bear, and the crack rapidly expands to be broken.
Further, the fractal dimension characterization system for the corrosion fatigue crack propagation trend of the gas pipeline comprises the following steps:
the sampling module is used for cutting and sampling the L360 casing pipeline, prefabricating fatigue cracks and cleaning;
the low-frequency noise acquisition module is used for testing and acquiring the low-frequency noise of the CT sample before the fatigue test;
the fractal dimension D calculation module is used for calculating the fractal dimension D of the low-frequency noise before fatigue;
the corrosion fatigue test module is used for carrying out corrosion fatigue test on the CT sample;
the CT sample crack noise acquisition module is used for measuring the CT sample crack noise after corrosion fatigue;
and the fatigue crack propagation degree forecasting and characterizing module is used for calculating the fractal dimension D of the low-frequency noise after fatigue and forecasting and characterizing the fatigue crack propagation degree of the CT sample according to the change of the fractal dimension value.
Compared with the prior art, the invention has at least the following beneficial effects:
the method utilizes the fractal dimension of the electrical noise in the measured fatigue crack propagation process to represent the propagation behavior of the pipeline crack, the measuring and calculating time is less, and the pipeline body cannot be damaged in the measuring process. In combination with the above analysis, it was found that in the three stages of crack propagation, the microcracks gradually grow and grow with the initiation, propagation, and coalescence of the microcracks, and then gradually decrease, and the main cracks gradually grow and grow. The roughness of the crack changes in the process, because the fractal dimension of the low-frequency noise is strongly related to the roughness of the material crack.
Further, the sampling position: welding, also known as fusion bonding, is a process and technique for joining metals or other thermoplastic materials, such as plastics, in a heated, high temperature or high pressure manner, and is a method of forming long distance oil and gas pipelines. During the welding process, the workpiece and the solder are melted to form a molten zone, and the molten pool is cooled and solidified to form the connection between the materials. The quality of the weld, and therefore the properties of the heat affected zone, is therefore closely related to the service life of the pipe. Therefore, the sampling location is representative and is typically selected to be near the location of the weld at the pipe joint. During the forming process, residual stress is inevitably introduced into the oil and gas transmission pipeline. Cracks will initiate at stress-concentrated sites in the material, including inclusions, vacancies, defects, dislocations, and cracks. Under the action of external force or alternating load, the crack starts to grow, and the propagation direction of the crack is determined by the nature of the stress field at the tip of the crack. In experiments, in order to ensure that the crack propagates in a predetermined direction and the accuracy of calculating the crack propagation rate, it is generally required to perform fatigue crack preparation during cutting and sampling.
Further, low frequency noise has been widely used in the fields of diagnosis, prediction, and evaluation of quality, reliability, and service life of materials and electronic components as an effective and sensitive characterization method. The low frequency band is generally considered to be in the range of 0.1 to 1000 Hz. On the one hand, in this frequency band range, the power spectral density of low frequency 1/f noise is very sensitive to fluctuations in the number of defects in the material; on the other hand, in the fatigue crack test experiment, the frequency of the alternating load is 10Hz, and is just in the low-frequency band.
Further, the low-frequency noise is caused by the fluctuation of the number of the defects and the cracks in the material, and the following processes are carried out in the data processing process: (1) collecting electrical signals such as voltage, current and the like in a sample in real time by using a data acquisition card; (2) carrying out autocorrelation processing on the acquired time domain signal; (3) the power spectral density of the low-frequency noise can be obtained by performing Fourier Transform (Fourier Transform) on the obtained autocorrelation signal. It has been found that the characteristics of the electrical signals, the autocorrelation function and the noise power spectral density thereof have natural fractal characteristics. Therefore, it is feasible to further analyze the propagation behavior of the fatigue crack by adopting a fractal theory.
Further, the corrosion fatigue test is a phenomenon in which cracks are formed and propagated under the interaction of an alternating load and a corrosive medium. The sample firstly generates fatigue damage on the surface under the alternating load and finally generates fracture under the action of a continuous corrosive medium environment. This is also accompanied by three processes of crack propagation.
Furthermore, the purpose of the corrosion fatigue test is to further expand the application field of the invention. On one hand, the fatigue crack propagation behavior of the oil and gas transmission pipeline in the atmospheric environment can be represented by a fractal dimension method provided by the invention; on the other hand, in the fatigue test in the corrosion medium, due to the synergistic effect of the corrosion medium, the test time can be shortened and the same purpose can be achieved.
Furthermore, the fractal dimension of the noise signal has a natural connection with the propagation of fatigue cracks in the oil and gas pipelines, on one hand, the frequency applied to the sampling is usually 10Hz (or lower) from the experimental process, and belongs to a low-frequency band, which is consistent with the range of low-frequency 1/f noise; on the other hand, no matter the electric signal or the noise signal (namely, noise power spectral density), the correlation and the similarity of the signals are high, and the signals have the characteristic of fractal; finally, low frequency 1/f noise is a sensitive and effective non-destructive inspection method. The method has the advantages of high detection speed and no damage to the sample. And in the process of fatigue crack propagation, from crack initiation, crack propagation to crack breakage, the formation and change information of the internal defects are related to the fractal dimension of crack low-frequency noise. Based on the analysis, the noise signal can be acquired through the data acquisition card, the fractal dimension of the noise signal is calculated, and the expansion trend of the fatigue crack can be reflected by the change of the fractal dimension.
In conclusion, the invention is efficient and sensitive, realizes nondestructive detection, can predict the service life of the pipeline, and can greatly reduce the cost in the aspects of pipeline detection, maintenance and the like.
Drawings
FIG. 1 is a sampling view of the L360 pipeline of the present invention;
FIG. 2 is a graph of a standard compact tensile CT sample of the present invention;
FIG. 3 is a block diagram of a low frequency noise test platform according to the present invention;
fig. 4 is a flow chart of fractal dimension calculation according to the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
It will be understood that the terms "comprises" and/or "comprising," when used in this specification and the appended claims, 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 is also to be understood that the terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in the specification of the present invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.
It should be further understood that the term "and/or" as used in this specification and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.
Various structural schematics according to the disclosed embodiments of the invention are shown in the drawings. The figures are not drawn to scale, wherein certain details are exaggerated and possibly omitted for clarity of presentation. The shapes of various regions, layers and their relative sizes and positional relationships shown in the drawings are merely exemplary, and deviations may occur in practice due to manufacturing tolerances or technical limitations, and a person skilled in the art may additionally design regions/layers having different shapes, sizes, relative positions, according to actual needs.
The invention provides a fractal dimension characterization method for corrosion fatigue crack propagation trend of a gas pipeline, which comprises the steps of firstly cutting an L360 pipe wall material and preparing a CT sample; then carrying out noise test and fractal dimension calculation on the pipeline cracks before fatigue; then carrying out corrosion fatigue test on the CT sample on a fatigue machine; finally, carrying out noise test on the corrosion fatigue crack and calculating a fractal dimension; performing fractal dimensional comparison on signals according to low-frequency noise obtained by calculation before and after fatigue, and representing the expansion trend of fatigue cracks of the oil-gas pipeline; the invention is based on low-frequency noise test and subsequent signal processing on the sample, and has the advantages of high sensitivity, wide application range, high detection speed, easy realization and the like.
Referring to fig. 4, according to the fractal dimension characterization method for the corrosion fatigue crack propagation trend of the gas transmission pipeline, firstly, an arc-shaped test sample is cut out from the pipeline, and a flat plate structure is directly processed to be a standard compact tensile test sample (CT sample) containing a through crack. And measuring the low-frequency 1/f noise of the cracks before and after the corrosion fatigue, and calculating the fractal dimension of the noise. By comparing and analyzing fractal dimension values before and after fatigue crack propagation, the L360 pipeline fatigue crack propagation prediction and characterization can be accurately realized, and the method specifically comprises the following steps:
s1, cutting and sampling the L360-set pipeline, prefabricating fatigue cracks and cleaning;
s101, cutting a pipe;
referring to fig. 1, a test material is taken from a certain pipe of a plain gas field, the design pressure is 11MPa, the wall thickness is 17.5mm, and according to the specification, a test sample is sampled in the CR direction, wherein C represents the normal direction of the surface of a crack, i.e., the circumferential direction; r represents the direction of the crack along the wall thickness expansion, because the pipe wall of the pipeline is very thin, the sample is not cut according to the CR orientation, the sample is cut on the pipe according to the CL orientation, L is the crack expansion direction, and 3 base material samples are cut according to the CL orientation.
S102, preparing a CT sample;
referring to fig. 2, a CT sample is prepared according to the GB/T6398-2000 standard, and after the arc sample is taken off, the flat plate structure is directly processed, and the CT sample is a standard compact tensile sample containing a through crack, that is, the CT sample, wherein the CT sample W is 65mm, the thickness B is 15mm, and a crack source with a length of 5mm is cut out from a molybdenum wire with a thickness of 0.2 mm.
S103, prefabricating fatigue cracks;
in order to facilitate crack initiation and reduce the time for prefabricating fatigue cracks, fatigue crack prefabrication is carried out on a sample before a test, the pre-crack load ratio is 0.1, the maximum load is 10kN, and the length of the prefabricated fatigue crack is about 1.5-2 mm.
And S104, cleaning.
The formula of the rust removing liquid is as follows:
| formulation of | Weight percent of |
| Hydrochloric acid (mass fraction concentration 15%) | 55％ |
| Hexamethylenetetramine | 10％ |
| Water (W) | 35％ |
Firstly, derusting the surface of a CT sample, and soaking the CT sample in a derusting solution for 20-30 minutes; then taking out the CT sample, washing with deionized water for 2-3 minutes, and removing redundant rust removing liquid attached to the surface (note that the surface of the CT sample is parallel to the water flow direction when the CT sample is washed with the deionized water); and finally, hanging the sample by using a clamp and drying.
S2, testing and collecting the low-frequency noise of the CT sample before fatigue test;
referring to fig. 3, A, B two leads are respectively welded on two sides of the pre-crack of the CT sample, the lead a is connected to the bias circuit of the low-frequency noise testing platform and is connected in series with a 100 ohm rheostat, and then the rheostat is adjusted to have a voltage value of 2.5V; the B lead wire is connected with a preposed low-noise amplifier on the noise test platform, the noise test platform comprises a bias circuit, the low-noise amplifier and a computer acquisition system, the whole noise test platform is built in a Cu network electromagnetic shielding room, and each part of the noise test platform has the following main functions:
1) a bias circuit: providing required voltage for the tested CT sample;
2) a low noise amplifier: because the low-frequency noise signal is extremely weak, the signal must be fully amplified, so that the data acquisition card can effectively acquire and quantize the signal;
3) computer acquisition system: the module converts the analog noise signal output by the low noise amplifier into a digital signal and sends the digital signal to a computer for storage;
4) electromagnetic shielding room: because the low-frequency noise signal is very weak and is easily interfered by external electromagnetic waves, the whole test system needs to be placed in an electromagnetic shielding environment.
S3, calculating the fractal dimension D of the low-frequency noise before fatigue;
referring to fig. 4, a box counting method is used to calculate the fractal dimension, and the main method is as follows:
covering the noise signal graph by using a square grid with the side length r, counting the number N (r) of grids containing noise points in the covered grid, and changing the side length r of the square grid each time to obtain different N (r), namely obtaining a group of data (r)i，N(ri) Meet the following requirements:
N(r)∝r-D (3)
wherein N (r) is the number of lattices containing noise points, r is the side length of a square lattice, and D is a fractal dimension; for data ln (1/r)i) And lnN (r)i) The fitting was performed, and the calculation result was D1.907.
S4, carrying out corrosion fatigue test on the CT sample;
taking out the CT sample from the low-frequency noise test system, adopting special leak-proof glue to construct a closed space around the crack of the CT sample, and using a medical injector to inject H2Injecting an S saturated aqueous solution into the cavity, placing the sample on an MTS 810-250 type hydraulic servo testing machine, automatically recording the information of load, displacement and crack length in the testing process by the testing machine, and adopting a load ratio R-Pmin/PmaxMaximum load P of 0.1max10kN, loading frequency 5Hz, and loading waveform is sine wave.
In fatigue propagation, the crack size a increases with the number of fatigue cycles NThe change amplitude delta K of the stress intensity factor of the crack tip is changed continuously along with the growth, and a series of (a) is recorded by recording the data of a certain crack propagation length a and the corresponding cycle number N in the testi,Ni) Data, fatigue test was stopped when the crack propagated to 10-15 mm.
S5, measuring the crack noise of the CT sample after corrosion fatigue;
taking the CT sample from the fatigue machine, and quickly cleaning the crack of the sample by using acetone; then, washing the acetone residual solution with deionized water for 2-3 minutes (note that the surface of the CT sample needs to be parallel to the water flow direction when the acetone residual solution is washed with deionized water); and finally, hanging the sample by using a clamp and drying.
And carrying out low-frequency noise test after fatigue test on the CT sample on a low-frequency noise test platform, wherein the applied bias voltage is the same as that in the second step, so as to ensure the comparability of the experimental data.
S6, calculating the fractal dimension D of the low-frequency noise after fatigue, and forecasting and characterizing the fatigue crack propagation degree of the CT sample through the change of the fractal dimension value.
Because the fractal dimension of the low-frequency noise is related to cracks and defects in the material, the change of the fractal dimension value is found out by calculating the fractal dimension of the low-frequency noise before and after a fatigue test, and then the condition of crack propagation of the pipeline is judged, wherein the fractal dimension calculation comprises the following main steps:
1) selecting a group of proper square side lengths to form an array r ═ r1,r2,...rn}；
2) Each time using side length of riThe squares of (a) form a grid covering the noise signal. Counting the total number N (r) of square lattices covered with noise signalsi) After N times, a set of data N (r) ═ N (r) is obtained1),N(r2),...N(rn)}；
3) For data ln (1/r)i) And lnN (r)i) And fitting to obtain a slope which is the fractal dimension D.
According to the relation between the fatigue crack propagation rate da/dN and the change amplitude delta K of the stress intensity factor, fatigue damage is gradually accumulated in the pipeline, and when a certain critical value is reached, an initial fatigue crack is formed. Then, the initial fatigue crack gradually expands under the combined action of the cyclic stress and the environment, namely, the subcritical expansion occurs, when the crack length reaches the critical crack length, the external load is difficult to bear, and the crack rapidly expands to be broken. Fatigue crack propagation can be divided into the following three phases:
the first stage is the fatigue crack initiation stage, and a threshold stress intensity factor amplitude delta K existsthWhen the variation range of the stress intensity factor is lower than the threshold value, namely delta K is less than or equal to delta KthFatigue cracks basically do not spread, and a plurality of tiny cracks exist on the surface of a smooth sample; when the micro cracks start to initiate and expand, the initiation and expansion processes are also random in time, the main cracks are not easy to form, and the crack texture is rough.
The second stage is a stable crack propagation stage (subcritical crack propagation stage) with a stress intensity factor greater than Δ KthAt this time, the crack growth behavior gradually flattens. However, as a plurality of micro cracks are expanded and combined, main cracks are generated, the main cracks are rapidly formed and expanded, and the roughness of the cracks is reduced; during this phase, the crack propagation rate da/dN and the stress intensity factor amplitude obey the Paris equation, also known as the Paris region.
The third stage is a rapid crack propagation stage, da/dN is large, the macroscopic main cracks are already strong, and the fatigue crack propagation life is short. It can be seen that, in the change process, the roughness of the crack also changes, in the fatigue crack initiation stage, because the number of the microcracks is large and dense, the roughness of the crack texture is large, along with the progress of fatigue damage, in the second stage and the third stage, the microcracks are merged and generate a main crack, the main crack grows gradually, the microcracks are reduced gradually, and the roughness of the crack texture is reduced. Since the pipeline crack belongs to a defect of the material, the origin of the low-frequency 1/f noise fluctuation information has strong correlation with the defect of the material, and the fractal dimension is a measure of the irregularity of a signal complex system. Thus, the roughness variation during crack propagation can then be characterized by the fractal dimension.
The noise signal can be represented in a two-dimensional coordinate system, the fractal dimension D of the noise signal is generally larger than 1 and smaller than 2, and if the noise curve is relatively straight (reflecting that cracks are relatively smooth), the fractal dimension of the noise signal is smaller; on the contrary, the more tortuous the noise curve is (the crack texture is rough), the larger the fractal dimension value is, so that the roughness of the pipeline crack can be known by extracting the fractal dimension value of the low-frequency 1/f noise, and the crack expansion condition can be represented by the change of the fractal dimension value.
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. The components of the embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example 1
A fractal dimension characterization method for a corrosion fatigue crack propagation trend of a gas pipeline mainly comprises the following steps:
(1) cutting and preparing a CT sample: cutting the arc-shaped pipe material as shown in figure 1, and then manufacturing the arc-shaped pipe material into a CT sample (shown in figure 2);
(2) prefabricating fatigue cracks, wherein the ratio of the pre-crack load is 0.1, the maximum load is 10kN, and the length of the prefabricated fatigue cracks is about 1.5-2 mm;
(3) cleaning: firstly, carrying out sample surface rust removal work, wherein the formula of the rust removal liquid is as follows:
| formulation of | Weight percent of |
| Hydrochloric acid (mass fraction concentration of 15%) | 55％ |
| Hexamethylenetetramine | 10％ |
| Water (W) | 35％ |
Soaking the sample in the rust removing liquid to ensure that the surface of the CT sample piece is fully contacted with the rust removing liquid for 20-30 minutes; then taking out the sample and washing the sample with deionized water for about 2 to 3 minutes to remove redundant rust removing liquid attached to the surface (note that the surface of the sample is parallel to the water flow direction when the sample is washed with the deionized water); and finally, hanging the sample by using a clamp and drying.
Step 2, before fatigue test, testing and collecting low-frequency noise of CT sample
Respectively welding two lead wires A and B on two sides of a prefabricated crack of a CT sample, connecting the lead wire A to a bias circuit, connecting a 100-ohm slide rheostat in series, and then adjusting the slide rheostat to enable the voltage value to be 2.5V; the lead B is connected with a preposed low noise amplifier on the noise test platform, and the structure of the whole noise test platform is shown in figure 3; lack of description of computer acquisition system, data acquisition card and memory
Step 3, calculating the fractal dimension D of low-frequency noise before fatigue
According to the flow shown in fig. 4, a program is written to realize fractal dimension calculation, and the formula is as follows:
N(r)∝r-D (3)
for data ln (1/r)i) And lnN (r)i) Fitting was performed, and D was calculated to be 1.892.
Step 4, carrying out corrosion fatigue test on the CT sample
Taking out the CT sample from the low-frequency noise test platform, adopting special leak-proof glue to construct a closed space around the crack of the sample, and using a medical injector to inject H2And S saturated aqueous solution is injected into the closed cavity. The samples were then placed on a hydraulic servo testing machine of the MTS 810-250 type for fatigue testing. The tester automatically records the load, displacement, crack length and other related information in the test process. The load ratio R ═ P usedmin/PmaxMaximum load P of 0.1max10kN, loading frequency 5Hz, and loading waveform is sine wave.
Recording data of each crack propagation length a and the corresponding cycle number N in the test, namely recording a series of (a)i,Ni) Data, fatigue test was stopped when the crack propagated to 12 mm.
Step 5, measuring the noise of the CT sample cracks after corrosion fatigue
Taking the CT sample out of the fatigue machine, and quickly cleaning a fracture of the sample by using acetone; then, washing the acetone residual solution by deionized water for about 2-3 minutes (note that the surface of the sample needs to be parallel to the water flow direction when the acetone residual solution is washed by the deionized water); and finally, hanging the sample by using a clamp and drying.
And (4) carrying out low-frequency noise test after corrosion fatigue on the sample on a low-frequency noise test platform, wherein the applied bias voltage is the same as that in the step two, so that the front and back comparability of the experimental data is ensured.
Step 6, calculating the fractal dimension D of the low-frequency noise after fatigue
And (3) calculating the noise dimension D of the sample after fatigue to be 1.753 by using a fractal dimension calculation program.
Comparing with the fractal dimension calculated in the step 3, the fractal dimension has a tendency of decreasing with the progress of corrosion fatigue, which reflects that there are more microcracks at the beginning, larger overall roughness of cracks and higher fractal dimension value. After the corrosion fatigue has progressed for a certain period of time, a majority of the microcracks propagate and coalesce to form main cracks, and the main cracks rapidly form and propagate, and the microcracks gradually decrease. The crack roughness is reduced, and the fractal dimension value is reduced. Therefore, the fatigue crack propagation degree of the CT sample can be predicted and characterized through the change of the fractal dimension value.
Example 2
A fractal dimension characterization method for a corrosion fatigue crack propagation trend of a gas pipeline mainly comprises the following steps:
1) Cutting and preparing a CT sample: the arc tube was cut as shown in fig. 1 and then made into CT samples (as shown in fig. 2).
2) And (3) prefabricating fatigue cracks, wherein the ratio of the pre-crack load to the maximum load is 0.1 and the maximum load is 10 kN. The length of the prefabricated fatigue crack is about 1.5-2 mm.
3) Cleaning: firstly, carrying out sample surface rust removal work, wherein the formula of the rust removal liquid is as follows:
| formulation of | Weight percent of |
| Hydrochloric acid (mass fraction concentration of 15%) | 55％ |
| Hexamethylenetetramine | 10％ |
| Water (W) | 35％ |
Soaking the sample in the rust removing liquid to ensure that the surface of the CT sample piece is fully contacted with the rust removing liquid for 20-30 minutes; then taking out the sample and washing the sample with deionized water for about 2 to 3 minutes to remove redundant rust removing liquid attached to the surface (note that the surface of the sample is parallel to the water flow direction when the sample is washed with the deionized water); finally, hanging the sample by using a clamp and drying the sample;
step 2, testing the low-frequency noise of the CT sample before fatigue
Respectively welding two lead wires A and B on two sides of a prefabricated crack of a CT sample, connecting the lead wire A to a bias circuit, connecting a 100-ohm slide rheostat in series, and then adjusting the slide rheostat to enable the voltage value to be 2.5V; the lead B is connected with a front low noise amplifier on the noise test platform. The whole noise test platform structure is shown in FIG. 3;
step 3, calculating the fractal dimension D of low-frequency noise before fatigue
According to the process shown in fig. 4, a square side length array r is selected as r ═ r1,r2,...rnAnd calculating a fractal dimension, wherein the main process is as follows: (1) inputting a noise figure, and using the side length of the noise figure as riCovering the square grid, and calculating the side length r containing the noise pointiNumber of square lattices N (r)i) (2) if i is less than array length n, repeating the operation of the process (1) to obtain a new pair riAnd N (r)i) The value is obtained. (3) Let i equal i +1, if i is greater than array length n, then data ln (1/r)i) And lnN (r)i) And fitting, wherein the obtained slope value is the fractal dimension value. And realizing fractal dimension calculation by using the formula:
N(r)∝r-D (3)
for data ln (1/r)i) And lnN (r)i) The fitting was performed and the result calculated was D-1.913.
Step 4, carrying out corrosion fatigue test on the CT sample
Taking out the CT sample from the low-frequency noise test platform, adopting special leak-proof glue to construct a closed space around the crack of the sample, and using a medical injector to inject H2And S saturated aqueous solution is injected into the closed cavity. The samples were then placed on a hydraulic servo testing machine of the MTS 810-250 type for fatigue testing. The tester automatically records the load, displacement, crack length and other related information in the test process. The load ratio R ═ P usedmin/PmaxMaximum load P of 0.1max10kN, loading frequency 5Hz, and loading waveform is sine wave.
Recording data of each crack propagation length a and the corresponding cycle number N in the test, namely recording a series of (a)i,Ni) Data, fatigue test was stopped when the crack propagated to 15 mm.
Step 5, measuring the noise of the CT sample cracks after corrosion fatigue
Taking the CT sample out of the fatigue machine, and quickly cleaning a fracture of the sample by using acetone; the acetone residue was then rinsed with deionized water for approximately 2-3 minutes, noting that: when the deionized water is used for washing, the surface of the sample is parallel to the water flow direction; and finally, hanging the sample by using a clamp and drying.
And (3) carrying out low-frequency noise test after corrosion fatigue on the sample on a low-frequency noise test platform, wherein the applied bias voltage is the same as that in the step (2) so as to ensure the comparability of the experimental data.
Step 6, calculating the fractal dimension D of the low-frequency noise after fatigue
And (3) calculating the noise dimension of the sample after fatigue to be D-1.702 by using a fractal dimension calculation program.
Comparing with the fractal dimension calculated in the step 3, the fractal dimension is found to have a tendency of decreasing along with the progress of corrosion fatigue. This also reflects a higher initial microcrack, a higher overall crack roughness and a higher fractal dimension value. After the corrosion fatigue has progressed for a certain period of time, the propagation of many micro cracks and the formation of a main crack occur, and the main crack rapidly forms and propagates, and the micro cracks gradually decrease. The crack roughness is reduced, and the fractal dimension value is reduced. Therefore, the fatigue crack propagation degree of the CT sample can be predicted and characterized through the change of the fractal dimension value.
The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.
Claims (8)
1. The fractal dimension characterization method for the corrosion fatigue crack propagation trend of the gas pipeline is characterized by comprising the following steps of:
cutting and sampling a gas pipeline to obtain a CT sample, prefabricating and cleaning fatigue cracks;
before the fatigue test, testing and collecting the low-frequency noise of the CT sample; calculating a fractal dimension D of low-frequency noise before fatigue;
carrying out corrosion fatigue test on the CT sample; measuring the crack noise of the CT sample after corrosion fatigue; calculating a fractal dimension D of the low-frequency noise after fatigue;
and predicting and characterizing the fatigue crack propagation degree of the CT sample through the change of the fractal dimension value.
2. The fractal dimension characterization method for the corrosion fatigue crack propagation trend of the gas pipeline according to claim 1, wherein the specific steps of sample preparation, fatigue crack prefabrication and cleaning are as follows:
s101, sampling a test sample in a CR direction, wherein C represents the normal direction of the surface of the crack; r represents the direction of the crack along the wall thickness expansion, the pipe wall of the gas transmission pipeline is cut on the pipe according to the CL orientation to obtain a sample, L is the crack expansion direction, and a base material sample is cut according to the CL orientation;
s102, manufacturing a CT sample, taking down the sample, processing a flat plate structure which is a standard compact tensile sample, and cutting a prefabricated crack by using a molybdenum wire;
s103, performing fatigue crack prefabrication on the sample before testing, wherein the length of the prefabricated fatigue crack is 1.5-2 mm;
s104, firstly, removing rust on the surface of the CT sample, and soaking the CT sample in a rust removing solution for 20-30 minutes; and then taking out the CT sample, washing with deionized water for 2-3 minutes, and finally drying the sample.
3. The gas pipeline corrosion fatigue crack propagation trend fractal dimension characterization method according to claim 1, wherein the low frequency noise of the CT sample is tested and collected:
respectively welding A, B two leads on two sides of a prefabricated crack of a CT sample, connecting the lead A to a bias circuit of a low-frequency noise test platform and connecting a slide rheostat in series, and then adjusting the slide rheostat to enable the voltage value to be 2.5V; the B lead is connected with a preposed low-noise amplifier on a noise test platform, the noise test platform comprises a bias circuit, the low-noise amplifier and a computer acquisition system which are sequentially connected, and the noise test platform is built in a Cu network electromagnetic shielding room.
4. The gas pipeline corrosion fatigue crack propagation trend fractal dimension characterization method according to claim 1, wherein the fractal dimension D for calculating the low-frequency noise before fatigue is specifically:
the box counting method is selected to calculate the fractal dimension as follows:
covering the noise signal graph by using a square grid with the side length r, counting the number N (r) of grids containing noise points in the covered grid, changing the side length r of the square grid each time to obtain different N (r), and obtaining a group of data (r)i，N(ri) ) as follows:
N(r)∝r-D
wherein N (r) is the number of lattices containing noise points, r is the side length of a square lattice, and D is a fractal dimension; for data ln (1/r)i) And lnN (r)i) And (6) fitting.
5. The gas pipeline corrosion fatigue crack propagation trend fractal dimension characterization method according to claim 1, wherein the corrosion fatigue test performed on the CT sample specifically comprises:
taking out the CT sample from the low-frequency noise test system, adopting special leakage-proof glue to construct a closed space around the crack of the CT sample, and taking out the H sample2Injecting S saturated aqueous solution into the cavity, recording the information of load, displacement and crack length of the CT sample in the test process, wherein the load ratio R is Pmin/PmaxMaximum load P of 0.1maxThe loading frequency is 5Hz and the loading waveform is sine wave under the condition of 10 kN;
in fatigue crack propagation, data of a certain crack propagation length a and the corresponding number of cycles N in the test are recorded as the next series (a)i,Ni) And (3) stopping the fatigue test when the crack is expanded to 10-15 mm.
6. The gas pipeline corrosion fatigue crack propagation trend fractal dimension characterization method according to claim 1, wherein the CT sample crack noise after the corrosion fatigue measurement specifically comprises:
taking the CT sample out of the fatigue machine, and cleaning the crack of the sample by using acetone; then, washing the acetone residual solution for 2-3 minutes by using deionized water, wherein the surface of the CT sample is parallel to the water flow direction when the acetone residual solution is washed by using the deionized water; finally, drying the CT sample; and (3) carrying out low-frequency noise test on the CT sample after fatigue test on a low-frequency noise test platform, wherein the applied bias voltage is the same as that of the previous step.
7. The gas pipeline corrosion fatigue crack propagation trend fractal dimension characterization method as claimed in claim 1, wherein the fractal dimension D of the low-frequency noise after fatigue is calculated, and the fatigue crack propagation degree of the CT sample is predicted and characterized by the change of the fractal dimension value, specifically:
the method comprises the following steps of finding out the change of a fractal dimension value through the calculation of the fractal dimension of low-frequency noise before and after a fatigue test, and judging the condition of pipeline crack propagation, wherein the fractal dimension calculation comprises the following steps:
1) selecting a group of square side lengths to form an array r ═ r1,r2,...rn}；
2) Each time using side length of riCovering the noise signal, and counting the total number N (r) of square grids covered with the noise signali) After N times, a set of data N (r) ═ N (r) is obtained1),N(r2),...N(rn)}；
3) For data ln (1/r)i) And ln N (r)i) Fitting is carried out, and the obtained slope is the fractal dimension D;
according to the relation between the fatigue crack expansion rate da/dN and the change amplitude delta K of the stress intensity factor, forming an initial fatigue crack when the fatigue damage reaches a critical value; then, the initial fatigue crack gradually propagates under the combined action of cyclic stress and environment, and breaks when the crack length reaches a critical crack length.
8. Gas pipeline corrosion fatigue crack propagation trend fractal dimension characterization system, its characterized in that includes:
the sampling module is used for cutting and sampling the gas transmission pipeline to obtain a CT sample, prefabricating fatigue cracks and cleaning;
the low-frequency noise acquisition module is used for testing and acquiring the low-frequency noise of the CT sample before the fatigue test;
the fractal dimension D calculation module is used for calculating the fractal dimension D of the low-frequency noise before fatigue;
the corrosion fatigue test module is used for carrying out corrosion fatigue test on the CT sample;
the CT sample crack noise acquisition module is used for measuring the CT sample crack noise after corrosion fatigue;
and the fatigue crack propagation degree forecasting and characterizing module is used for calculating the fractal dimension D of the low-frequency noise after fatigue and forecasting and characterizing the fatigue crack propagation degree of the CT sample according to the change of the fractal dimension value.
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