Method for obtaining circumferential mechanical property parameters of pipe

A method for obtaining circumferential mechanical property parameters of a pipe, comprising the following steps:

• step (1):
• adopting a flexible tensile test fixture used for testing circumferential mechanical property parameters of a pipe to conduct a tensile test on a circumferential tensile test sample of the pipe, and making the circumferential tensile test sample subjected to pulling forces until fracture occurs;
• during the tensile test, a deformation amount in a whole tension process of the circumferential tensile test sample is obtained in real time based on a digital image correlation method by using a non-contact video extensometer, thus to obtain an engineering strain of the circumferential tensile test sample, and then obtain time of the tensile test and a force of the tensile test in combination with a universal tensile testing machine;
• the deformation amount of the circumferential tensile test sample, the engineering strain of the circumferential tensile test sample, the time of the tensile test and the force of the tensile test are collectively referred to as tensile measurement data of the pipe;
• step (2):
• preprocessing the tensile measurement data of the pipe;
• preprocessing means sequentially conducting:
• all abnormal data clearing before starting sampling, force check and convert, data cleaning and data screening;
• the preprocessed data includes only two types of data:
• the force of the tensile test and the engineering strain of the circumferential tensile test sample;
• the specific process is as follows:
• using the tensile measurement data of the pipe as an output data set, including the force F, the deformation amount ΔL, the engineering strain εeng and the time t;
• reading a data column of the force F and denoting as column A0=[A01,A02, . . . , A0n]T, reading a data column of the deformation amount ΔL and denoting as column B0=[B01,B02, . . . , B0n]T, reading a data column of the engineering strain εeng and denoting as column C0=[C01,C02, . . . , C0n]T, and reading a data column of the time t and denoting as column D0=[D01,D02, . . . , D0n]T;
• (2.1) all abnormal data clearing before starting sampling;
• reading the data column D0, D0(p+1)−D0p=Tp, and p∈{1,2,3,4, . . . , n−1};
• Tp represents a time difference between row p+1 and row p;
• when
• 0
• <
• T
• p
• <
• k
• =
• 1
• f
• ,
• data of row p+1 is marked as normal;
• otherwise, row p+1 and row p are marked as abnormal, wherein 1.5<k<2, and f is a sampling frequency;
• When 5 normal marks appears continuously, the reading is stopped, and all data in rows with abnormal marks is cleared;
• obtaining a new data set:
• denoting a data column of the force F as column A1=[A11,A12, . . . , A1m]T, denoting a data column of the deformation amount ΔL as column B1=[B11,B12, . . . , B1m]T, denoting a data column of the engineering strain εeng as column C1=[C11,C12, . . . , C1m]T, and denoting a data column of the time t as column D=[D11,D12, . . . , D1m]T;
• m<n;
• (2.2) force check and convert;
• reading all data [A11,A12, . . . , A1m]T in column A1, and obtaining a maximum force Fmax;
• if Fmax≤200, [A11,A12, . . . , A1m]T×1000=[A′11,A′12, . . . , A′1m]T, and [A11,A12, . . . , A1m]T is overwritten with [A′11,A′12, . . . , A′1m]T to obtain a new column A1;
• if Fmax>200, a next step is conducted directly;
• (2.3) data cleaning;
• reading all data [C11,C12, . . . , C1m]T in column C1, and judging whether C1q≤0, wherein q∈{1,2,3, . . . , m};
• recording the last position where C1q is zero or negative, clearing all data [A1,B1,C1,D1] before the position, deleting the data column of the deformation amount ΔL and the data column of the time t, and finally forming a new data set [A2,C2];
• (2.4) data screening;
• conducting periodic sampling and data screening to the data set [A2,C2] to reduce data and reduce a calculation amount on the premise of ensuring that a true stress-true strain curve can transition smoothly;
• step (3):
• obtaining true stress-true strain data of the circumferential tensile test sample of the pipe by calculation according to the data obtained in step (2);
• step (4):
• obtaining a strength coefficient K and a strain hardening index β in a plastic section by calculation through a method of linear regression fitting by setting starting and ending strains of the plastic section according to the true stress-true strain data of the circumferential tensile test sample;
• the strength coefficient K and the strain hardening index β in the plastic section as well as the true stress-true strain data of the circumferential tensile test sample are used for finite element analysis of a pipeline system.