A method for obtaining crack growth resistance curve based on unnotched tensile specimen

Abstract

The invention discloses a method for obtaining a crack propagation resistance curve based on an unnotched tensile specimen, comprising: determining an unnotched thin steel pipe specimen, carrying out a tensile test, and obtaining load-load line displacement curve F-LLD data; using an acoustic emission device and a scanning electron microscope to obtain the specimen crack initiation position, crack tip micro-void nucleation, growth and aggregation data during the tensile process; taking the maximum load point as the starting point, calculating the crack propagation length and the corresponding J integral value according to the load-load line displacement curve F-LLD data, and obtaining a crack propagation resistance curve; establishing a Gurson constitutive model of a non-uniform stress field around a hole and the interaction between adjacent holes, and obtaining the crack tip constraint level of the specimen during the tensile process through the Gurson constitutive model; calibrating the crack tip constraint level; and correcting the crack propagation resistance curve through the calibrated crack tip constraint level and the tensile test data, so as to provide key experimental data for the safety evaluation of metal structures.

Description

Technical Field

[0001] The invention relates to the technical field of metal fracture toughness testing, and in particular to a method for obtaining a crack extension resistance curve based on a non-notched tensile specimen. Background Art

[0002] To conduct fracture evaluation on structures made of metal materials, we must first test their crack propagation resistance curve (Resistance Curve, referred to as R curve). In order to obtain the geometrically independent R curve, the traditional fracture test standard needs to ensure a high level of constraint at the crack tip, which results in a thicker specimen. However, some structures, such as high-strength pipeline steel pipes, have thinner walls, but traditional test standards require high constraint at the crack tip, and only sufficiently thick specimens can ensure high constraint at the crack tip. Therefore, it may not be possible to continue traditional fracture testing for the wall thickness of high-strength pipelines, and thus it is impossible to conduct fracture evaluation on structures made of metal materials.

[0003] There is a certain correlation between the tensile properties of metals and their fracture properties. The size requirements of tensile specimens are relatively loose, and the uniaxial tensile test is a conventional material performance test method, and its data is easy to obtain. Therefore, from the perspective of practicality, convenience and rapidity, inventing a method for obtaining a crack propagation resistance curve of a metal material through a uniaxial tensile test has important practical significance for the fracture evaluation of metal structures. Summary of the invention

[0004] The technical problem to be solved by the present invention is to solve the problem that the traditional fracture test standard cannot perform fracture evaluation on steel pipes with thin wall thickness.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0006] A method for obtaining a crack growth resistance curve based on an unnotched tensile specimen, comprising:

[0007] S10, determine the thin steel pipe specimen without notch;

[0008] S20, a tensile test is carried out on a thin steel tube sample without a notch to obtain the load-load line displacement curve F-LLD data; at the same time, an acoustic emission device and a scanning electron microscope are used to obtain the sample crack initiation position, crack tip microvoid nucleation, growth and aggregation data during the tensile process;

[0009] S30, taking the maximum load point as the starting point, calculating the crack extension length and the corresponding J integral value according to the load-load line displacement curve F-LLD data, and obtaining the crack extension resistance curve;

[0010] S40, establish the Gurson constitutive model of the non-uniform stress field around the hole and the interaction between adjacent holes, and obtain the crack tip constraint level of the unnotched thin steel pipe specimen during the tensile process through the Gurson constitutive model;

[0011] S50, the crack tip constraint level is calibrated by finite element analysis;

[0012] S60, the crack propagation resistance curve is corrected by the calibrated crack tip constraint level and the specimen crack initiation position, crack tip microvoid nucleation, growth and aggregation data.

[0013] In one embodiment of the present invention, the modified crack growth resistance curve is used to perform fracture evaluation on a thin steel pipe.

[0014] In one embodiment of the present invention, the J integral value is obtained by the following formula:

[0015]

[0016] Where K is the stress intensity factor, v is the Poisson's ratio, E is the elastic modulus, η pl is the shape parameter, A pl is the area enclosed by the F-LLD curve, B 0 is the thickness of the unnotched thin steel pipe specimen, b 0 is the remaining ligament length of the thin steel pipe specimen without notch, J pl is an intermediate variable.

[0017] In one embodiment of the present invention, the Gurson constitutive model is the following formula:

[0018]

[0019] Where Φ is the Gurson constitutive model, σ e is the Mises equivalent stress, is the flow stress, f * is the effective volume of the hole, q 1 ,q 2 and q 3 is the correction coefficient, σ m is the average normal stress of the hole, and cosh is the hyperbolic cosine function.

[0020] In one embodiment of the present invention, the effective volume of the hole is obtained by the following formula:

[0021]

[0022] Where f is the volume percentage of the pores, f c is the volume percentage of the pores when the pores begin to aggregate, is the reference parameter, f F is the critical void volume percentage when the material fractures.

[0023] In one embodiment of the present invention, during the stretching process, the growth rate of the pore volume fraction is expressed by the following formula:

[0024] df=df growth +df nucleation ;

[0025] Where df is the growth rate of the pore volume fraction, df growth is the growth rate of the pore volume fraction caused by the pore growth, df nucleation is the growth rate of the pore volume fraction caused by pore nucleation;

[0026] Among them, the growth rate of the pore volume fraction caused by the pore growth is df growth Obtained by the following formula;

[0027]

[0028] In the formula, is the plastic strain rate;

[0029] The growth rate of the void volume fraction caused by void nucleation df nucleation , obtained by the following formula;

[0030]

[0031] Where A and B are the intensity factors of strain-controlled nucleation and stress-controlled nucleation, respectively. is the equivalent plastic strain.

[0032] In one embodiment of the present invention, the intensity factor A of strain-controlled nucleation is obtained by the following formula:

[0033]

[0034] In the formula, f N is the volume percentage of all the two-phase particles where microvoid nucleation occurs, S N is the corresponding standard deviation, ε N is the average strain of hole nucleation, π is pi, and exp is the exponential function.

[0035] In one embodiment of the present invention, the strength factor B of stress-controlled nucleation is obtained by the following formula:

[0036]

[0037] In the formula, σ n is the average stress for hole nucleation.

[0038] In one embodiment of the present invention, the crack tip constraint level is calibrated by finite element analysis, including: taking the crack tip unit void volume ratio and stress triaxiality as characteristics to obtain the crack tip constraint level.

[0039] In one embodiment of the present invention, the stress triaxiality is obtained by the following formula:

[0040]

[0041] In the formula, A m is the stress triaxiality, σ h is the hydrostatic pressure, σ e is the Mises equivalent stress.

[0042] Compared with the prior art, the beneficial effects of the present invention are as follows: using tensile specimens to test the crack propagation resistance curve of metal materials requires solving three problems: (1) determining the crack initiation moment of the metal material tensile specimen by acoustic emission technology; (2) determining the crack propagation length by establishing the intrinsic energy law rule of the metal material tensile specimen; (3) obtaining the crack tip constraint level of different specimens by numerical calculation, and correcting the influence of different constraints caused by specimen size and load on crack propagation behavior. In the context of China's vigorous construction of "large-volume" long-distance natural gas pipelines, the fracture toughness test method for metal materials such as high-strength pipes has great practical significance. This method can solve the three key problems faced by the study of intrinsic crack propagation behavior in metal material tensile experiments, thereby developing a method for testing crack propagation resistance curves through uniaxial tensile specimens, which can provide key experimental data for safety evaluation and fracture assessment of metal structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 The present invention is a flowchart of a method for obtaining a crack growth resistance curve based on an unnotched tensile specimen according to an embodiment of the present invention.

[0044] Figure 2 Schematic diagram of a thin steel tube sample without a notch according to an embodiment of the present invention.

[0045] Figure 3 A of the embodiment of the present invention pl Schematic diagram.

[0046] Figure 4 Schematic diagram of the tensile test process of an embodiment of the present invention. DETAILED DESCRIPTION

[0047] In order to facilitate those skilled in the art to understand the technical solution of the present invention, the technical solution of the present invention is further described in conjunction with the accompanying drawings of the specification.

[0048] The terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0049] See also Figure 1 This embodiment discloses a method for obtaining a crack growth resistance curve based on an unnotched tensile specimen, comprising the following steps:

[0050] S10, determine the unnotched thin steel pipe specimen.

[0051] In one embodiment of the present invention, Figure 2 As shown, according to the provisions of the national standard GB / T 228.1-2021 "Tensile test of metallic materials Part 1: Room temperature test method", prepare a tensile specimen of a certain shape.

[0052] S20, a tensile test is carried out on a thin steel pipe sample without a notch to obtain the load-load line displacement curve F-LLD data; at the same time, an acoustic emission device and a scanning electron microscope are used to obtain the sample crack initiation position, crack tip microvoid nucleation, growth and aggregation data during the tensile process.

[0053] In one embodiment of the present invention, according to the universal material testing machine, during the stretching process of the thin steel pipe sample without a notch, the load F and the load line displacement LLD, that is, the load-load line displacement curve F-LLD data can be obtained.

[0054] S30, taking the maximum load point as the starting point, calculating the crack extension length and the corresponding J integral value according to the load-load line displacement curve F-LLD data, and obtaining the crack extension resistance curve.

[0055] See also Figure 3 and Figure 4 As shown, in one embodiment of the present invention, based on the energy method assumption, there is a cluster of F-LLD curves for metal specimens with different crack lengths. Therefore, there will be countless intersections between this cluster of F-LLD curves and the F-LLD curve of the tensile specimen with continuous crack extension, and each intersection corresponds to a crack extension length Δa. The crack extension length Δa is obtained through regression analysis and regularization methods.

[0056] The J integral value is a parameter that characterizes fracture toughness, and its calculation formula is as follows:

[0057]

[0058] Where K is the stress intensity factor, v is the Poisson's ratio, E is the elastic modulus, ηpl is the shape parameter, A pl is the area enclosed by the F-LLD curve, B 0 is the thickness of the unnotched thin steel pipe specimen, b 0 is the remaining ligament length of the thin steel pipe specimen without notch, J pl is an intermediate variable.

[0059] S40, the Gurson constitutive model of the non-uniform stress field around the hole and the interaction between adjacent holes is established, and the crack tip constraint level of the unnotched thin steel pipe specimen during the tensile process is obtained through the Gurson constitutive model.

[0060] In one embodiment of the present invention, during crack initiation and propagation evolution, the size of micropores can be determined by scanning electron microscopy, thereby determining the scale of representative volume units. Based on the representative volume unit, a Gurson constitutive model is established that takes into account the influence of the non-uniform stress field around the pores and the interaction between adjacent pores:

[0061]

[0062] Where Φ is the Gurson constitutive model, σ e is the Mises equivalent stress, is the flow stress, f * is the effective volume of the hole, q 1 ,q 2 and q 3 is the correction coefficient, σ m is the average normal stress of the hole, and cosh is the hyperbolic cosine function.

[0063] The effective volume of the hole is obtained by the following formula:

[0064]

[0065] Where f is the volume percentage of the pores, f c is the volume percentage of the pores when the pores begin to aggregate, is the reference parameter, whose value is 1 / q 1 , f F is the critical void volume percentage when the material breaks. Assuming that when the void volume reaches the critical value f c When the pore volume fraction reaches the limit value f F When the pores are completely polymerized, the load-bearing capacity of the material is zero.

[0066] The growth rate of the pore volume fraction can be expressed as:

[0067] df=df growth +df nucleation ;

[0068] Where df is the growth rate of the pore volume fraction, df growth is the growth rate of the pore volume fraction caused by the pore growth, df nucleation is the growth rate of the pore volume fraction caused by pore nucleation.

[0069] Since the matrix material is assumed to be incompressible, df growth Depends on macroscopic plastic volume deformation, therefore:

[0070]

[0071] In the formula, is the plastic strain rate. Void growth is considered to be independent of the material. However, void nucleation is a highly material-dependent process. In general, void nucleation depends on the particle strength, size and shape and the hardening exponent of the matrix material. The void nucleation mechanism is mainly divided into two types: stress-controlled and strain-controlled. In numerical simulations, the void volume fraction growth rate caused by void nucleation can be expressed as:

[0072]

[0073] Where A and B are the intensity factors of strain-controlled nucleation and stress-controlled nucleation, respectively. is the equivalent plastic strain.

[0074] Among them, the intensity factor A of strain-controlled nucleation is obtained by the following formula:

[0075]

[0076] In the formula, f N is the volume percentage of all the two-phase particles where microvoid nucleation occurs, which controls the intensity of nucleation, S N is the corresponding standard deviation, ε N is the average strain of hole nucleation, which controls the speed of nucleation, π is pi, and exp is the exponential function.

[0077] The intensity factor B of stress-controlled nucleation is obtained by the following formula:

[0078]

[0079] In the formula, σ n is the average stress for hole nucleation.

[0080] The Gurson constitutive model mainly includes 9 parameters: constitutive parameters (q 1 ,q 2 ,q 3 ), failure parameters (f, f N、f c 、f F ) and the hole nucleation parameter (ε N 、s N ). Numerical simulation is mainly based on the plastic strain deformation core mechanism to simulate the damage process of the material. Therefore, accurate calibration of the values ​​of these parameters is a prerequisite for accurately simulating the crack propagation behavior of the specimen in the experiment. The nine hole model parameters are determined by the test results of the scanning electron microscope and the F-LLD curve of the numerical finite element simulation.

[0081] S50, the crack tip constraint level is calibrated by finite element analysis.

[0082] In one embodiment of the present invention, the stress triaxiality characterizes the effect of the constraint on the stress field at the crack tip. The stress triaxiality A m The parameter is proportional to the hole growth. Among them, the stress triaxiality is obtained by the following formula:

[0083]

[0084] In the formula, A m is the stress triaxiality, σ h is the hydrostatic pressure, σ e is the Mises equivalent stress.

[0085] S60, the crack propagation resistance curve is corrected by the calibrated crack tip constraint level and the specimen crack initiation position, crack tip microvoid nucleation, growth and aggregation data.

[0086] It is obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be regarded as exemplary and non-limiting from any point of view, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention, and any reference numerals in the claims should not be regarded as limiting the claims involved.

[0087] The above-described embodiments merely represent implementation methods of the invention. The protection scope of the present invention is not limited to the above-described embodiments. For those skilled in the art, several modifications and improvements may be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for obtaining a crack growth resistance curve based on an unnotched tensile specimen, characterized in that: include: S10, determine the thin steel pipe specimen without notch; S20, a tensile test is carried out on a thin steel tube sample without a notch to obtain the load-load line displacement curve F-LLD data; at the same time, an acoustic emission device and a scanning electron microscope are used to obtain the sample crack initiation position, crack tip microvoid nucleation, growth and aggregation data during the tensile process; S30, taking the maximum load point as the starting point, calculating the crack extension length and the corresponding J integral value according to the load-load line displacement curve F-LLD data, and obtaining the crack extension resistance curve; S40, establish the Gurson constitutive model of the non-uniform stress field around the hole and the interaction between adjacent holes, and obtain the crack tip constraint level of the unnotched thin steel pipe specimen during the tensile process through the Gurson constitutive model; S50, the crack tip constraint level is calibrated by finite element analysis; S60, the crack propagation resistance curve is corrected by the calibrated crack tip constraint level and the specimen crack initiation position, crack tip microvoid nucleation, growth and aggregation data.

2. The method for obtaining a crack growth resistance curve based on an unnotched tensile specimen according to claim 1, characterized in that: The modified crack growth resistance curve is used to evaluate the fracture of thin steel pipes.

3. The method for obtaining a crack growth resistance curve based on an unnotched tensile specimen according to claim 1, characterized in that: The J integral value is obtained by the following formula: Where K is the stress intensity factor, v is the Poisson's ratio, E is the elastic modulus, η pl is the shape parameter, A pl is the area enclosed by the F-LLD curve, B0 is the thickness of the thin steel tube specimen without a notch, b0 is the remaining ligament length of the thin steel tube specimen without a notch, J pl is an intermediate variable.

4. The method for obtaining a crack growth resistance curve based on an unnotched tensile specimen according to claim 1, characterized in that: Gurson constitutive model is the following formula: Where Φ is the Gurson constitutive model, σ e is the Mises equivalent stress, is the flow stress, f * is the effective volume of the hole, q1, q2 and q3 are correction coefficients, σ m is the average normal stress of the hole, and cosh is the hyperbolic cosine function.

5. The method for obtaining a crack growth resistance curve based on an unnotched tensile specimen according to claim 4, characterized in that: The effective volume of the hole is obtained by the following formula: Where f is the volume percentage of the pores, f c is the volume percentage of the pores when the pores begin to aggregate, is the reference parameter, f F is the critical void volume percentage when the material fractures.

6. The method for obtaining a crack growth resistance curve based on an unnotched tensile specimen according to claim 5, characterized in that: During the stretching process, the growth rate of the pore volume fraction is expressed by the following formula: df=df growth +df nucleation ; Where df is the growth rate of the pore volume fraction, df growth is the growth rate of the pore volume fraction caused by the pore growth, df nucleation is the growth rate of the pore volume fraction caused by pore nucleation; Among them, the growth rate of the pore volume fraction caused by the pore growth is df growth Obtained by the following formula; In the formula, is the plastic strain rate; The growth rate of the void volume fraction caused by void nucleation df nucleation , obtained by the following formula; Where A and B are the intensity factors of strain-controlled nucleation and stress-controlled nucleation, respectively. is the equivalent plastic strain.

7. The method for obtaining a crack growth resistance curve based on an unnotched tensile specimen according to claim 6, characterized in that: The intensity factor A of strain-controlled nucleation is obtained by the following formula: In the formula, f N is the volume percentage of all the two-phase particles where microvoid nucleation occurs, S N is the corresponding standard deviation, ε N is the average strain of hole nucleation, π is pi, and exp is the exponential function.

8. The method for obtaining a crack growth resistance curve based on an unnotched tensile specimen according to claim 7, characterized in that: The intensity factor B of stress-controlled nucleation is obtained by the following formula: In the formula, σ n is the average stress for hole nucleation.

9. The method for obtaining a crack growth resistance curve based on an unnotched tensile specimen according to claim 1, characterized in that: The crack tip constraint level is calibrated through finite element analysis, including: the crack tip constraint level is obtained by characterizing the crack tip unit pore volume ratio and stress triaxiality.

10. The method for obtaining a crack growth resistance curve based on an unnotched tensile specimen according to claim 9, characterized in that: The stress triaxiality is obtained by the following formula: In the formula, A m is the stress triaxiality, σ h is the hydrostatic pressure, σ e is the Mises equivalent stress.

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