Method for testing small size fracture toughness of metal material based on fracture cross-section characteristics

Abstract

The application belongs to the technical field of fracture toughness evaluation, and specifically discloses a small-size fracture toughness testing method for metal materials based on fracture cross-section characteristics, which utilizes the fracture cross-section shape of a test sample, considers the elastic energy distribution of a normal fracture zone dominated by I-type fracture and a shear fracture zone dominated by II-type fracture, establishes a linear relationship between G and J, carries out a fracture test by using a multi-sample method with different thicknesses, determines different groups of data, obtains and estimates values by combining a least square method, forms a new method for evaluating the small-size fracture toughness of metal materials based on fracture cross-section characteristics, and avoids the difficulty of directly using standard pieces for evaluation for materials such as high-strength steel, and has significant engineering application value.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of fracture toughness evaluation, and specifically discloses a small-size fracture toughness testing method for metal materials based on fracture cross-section characteristics. BACKGROUND

[0002] Fracture toughness is a key mechanical parameter for evaluating the resistance of a material in the process of crack propagation instability, and is an important safety evaluation index and structural component design index, playing a crucial role in damage tolerance design of key components. Therefore, experimental measurement and standardization of fracture toughness have an important role in integrity evaluation, damage tolerance design, applicability evaluation and residual strength analysis of various engineering components. In addition, fracture toughness values not only provide a basis for material performance evaluation, but also are key elements for ensuring the quality of typical engineering structures, including important industrial fields such as nuclear pressure vessels, oil pipelines, automobiles, ships and aircraft structures. At present, in the fracture toughness testing method, parameters such as stress field intensity factor K C (including the critical value K I of the opening stress intensity factor K IC and the critical value K II of the sliding stress intensity factor K IIC , or the equivalent form of energy release rate G), J integral, crack tip opening displacement (CTOD) and crack tip opening angle (CTOA) are mainly used for evaluation; wherein K IC and K IIC are intrinsic mechanical parameters that describe the fracture performance of materials and are independent of size and shape, and are most widely used in engineering.

[0003] According to the “GB / T 4161-2007 Metal Material Plane Strain Fracture Toughness K IC Test Method”, K IC is the crack propagation resistance exhibited by an I-type crack at the tip under slow loading in the linear elastic and negligible yield state; the measurement result has strict requirements on the size of the test piece to meet the plane strain condition, i.e. However, for high-strength steel, the part size thickness designed according to the standard will become extremely large, which not only makes it difficult to obtain materials, but also makes it difficult to load the test; it is difficult to meet the standard requirements, on the other hand, in the calculation of the metal material evaluation K IC , the corresponding F Q (F Q is a certain condition value, see Figure 2 ) and K Q (K Q is the condition value of K IC ) values are calculated from the loading curve, which meet the effectiveness criterion, at this time, the K Q value calculated is K ICValue. At this time, in actual engineering, the J-R resistance curve of the material is often further tested by using J integral according to and GB / T 21143-2014 Unified Test Method for Quasi-static Fracture Toughness of Metallic Materials, and the fracture toughness J IC Substitution is carried out to evaluate the ability of the material to resist fracture.

[0004] Therefore, developing a small size test method for evaluating fracture toughness has important engineering application value. SUMMARY

[0005] To solve the above technical problems, the application discloses a small-size fracture toughness test method for metal materials based on fracture cross-section characteristics, which is based on the load displacement curves of test pieces with different thicknesses to construct a data group corresponding to K I 2 C and K I 2 IC linear relationship, and based on the least square method to calculate the data group to obtain the estimated value of K I 2 C and K I 2 IC For high-strength steel and other materials, the difficulty of directly using standard pieces for evaluation is avoided, and the method has significant engineering application value.

[0006] To achieve the above purpose, the application adopts the following technical scheme:

[0007] The small-size fracture toughness test method for metal materials based on fracture cross-section characteristics comprises the following steps:

[0008] Step 1: Obtain the load displacement curves of test pieces with different thicknesses through the test method;

[0009] Step 2: Based on the load displacement curves of test pieces with different thicknesses, a data group corresponding to K I 2 C and K I 2 IC linear relationship is constructed;

[0010] Step 3: Based on the least square method, the data group is calculated to obtain the estimated value of K I 2 C and K I 2 IC .

[0011] Further, the metal material small-size fracture toughness test method based on fracture cross-section characteristics, the step 1 obtains the specimen load displacement curve of different thicknesses through a test method, and the steps are as follows: a plurality of samples of different thicknesses are designed, the corresponding relationship of the loading line displacement q, the crack opening displacement V and the corresponding loading load F of the samples is obtained respectively, and the specimen load displacement curve is generated.

[0012] Further, the metal material small-size fracture toughness test method based on fracture cross-section characteristics, the step 1 obtains the specimen load displacement curve of different thicknesses through a test method, and the steps are as follows: a plurality of samples of different thicknesses are designed, the corresponding relationship of the loading line displacement q, the crack opening displacement V and the corresponding loading load F of the samples is obtained respectively, and the specimen load displacement curve is generated.

[0013] Based on the specimen load displacement curve, the determination condition value F Q is extracted respectively, and the corresponding K Q is calculated. IC The effectiveness of the result K IC is judged, and the cross-section shear fracture area ratio λ is measured.

[0014] Further, the metal material small-size fracture toughness test method based on fracture cross-section characteristics, the step 2 constructs the expression of the linear relationship of K and The expression of the linear relationship is as follows:

[0015]

[0016] Wherein, K is the fracture correction dimensionless coefficient related to the material type, A C is the shear fracture area ratio, A S is the total area of the two side shear fracture zones, K Q is the cross-section average fracture toughness, K I is the square of the critical value of the fracture toughness required to be solved for the normal fracture zone dominated by I-type fracture, K II is the square of the critical value of the fracture toughness required to be solved for the shear fracture zone dominated by II-type fracture.

[0017] Further, the metal material small-size fracture toughness test method based on fracture cross-section characteristics, the expression of the cross-section average fracture toughness K Q is as follows:

[0018]

[0019] Wherein, K is the stress intensity factor coefficient, a0 is the initial crack length, a is the nominal crack length, B is the sample thickness, S is the specimen support span, and W is the effective width of the sample.

[0020] Further, the metal material small-size fracture toughness test method based on fracture cross-section characteristics, the expression of the stress intensity factor coefficient is:

[0021]

[0022] Further, the metal material small-size fracture toughness test method based on fracture cross-section characteristics, The expression of the linear relationship between the stress intensity factor coefficient and the partition area ratio is:

[0023] Further, the metal material small-size fracture toughness test method based on fracture cross-section characteristics, the minimum thickness of the sample should ensure that the macroscopic fracture surface can distinguish the normal fracture zone and the shear fracture zone.

[0024] Compared with the prior art, the present application has the following advantages and technical effects:

[0025] The metal material small-size fracture toughness test method based on fracture cross-section characteristics disclosed by the present application utilizes the fracture cross-section shape of the test sample, considers the elastic energy distribution of the normal fracture zone dominated by I-type fracture and the shear fracture zone dominated by II-type fracture, establishes the linear relationship between the stress intensity factor coefficient and the partition area ratio, carries out the fracture test by using the multi-sample method with different thicknesses, determines different groups of data, obtains the estimated values of the stress intensity factor coefficient and the partition area ratio by combining the least square method, forms a new method for evaluating the small-size fracture toughness of the metal material based on the fracture cross-section characteristics, avoids the difficulty of directly using the standard piece for evaluation for the high-strength steel and other materials, and has significant engineering application value. And The metal material small-size fracture toughness test method based on fracture cross-section characteristics disclosed by the present application utilizes the fracture cross-section shape of the test sample, considers the elastic energy distribution of the normal fracture zone dominated by I-type fracture and the shear fracture zone dominated by II-type fracture, establishes the linear relationship between the stress intensity factor coefficient and the partition area ratio, carries out the fracture test by using the multi-sample method with different thicknesses, determines different groups of data, obtains the estimated values of the stress intensity factor coefficient and the partition area ratio by combining the least square method, forms a new method for evaluating the small-size fracture toughness of the metal material based on the fracture cross-section characteristics, avoids the difficulty of directly using the standard piece for evaluation for the high-strength steel and other materials, and has significant engineering application value. And The metal material small-size fracture toughness test method based on fracture cross-section characteristics disclosed by the present application utilizes the fracture cross-section shape of the test sample, considers the elastic energy distribution of the normal fracture zone dominated by I-type fracture and the shear fracture zone dominated by II-type fracture, establishes the linear relationship between the stress intensity factor coefficient and the partition area ratio, carries out the fracture test by using the multi-sample method with different thicknesses, determines different groups of data, obtains the estimated values of the stress intensity factor coefficient and the partition area ratio by combining the least square method, forms a new method for evaluating the small-size fracture toughness of the metal material based on the fracture cross-section characteristics, avoids the difficulty of directly using the standard piece for evaluation for the high-strength steel and other materials, and has significant engineering application value. BRIEF DESCRIPTION OF DRAWINGS

[0026] The accompanying drawings, which form a part of this application, are included to provide a further understanding of the application and are incorporated in and constitute a part of this application. The embodiments illustrated in the drawings are provided and described solely for purposes of explaining the present application and are not intended to limit the present application in any way. In the drawings:

[0027] Figure 1 The size of the fracture of the embodiment of the present application and the fracture partition map;

[0028] Figure 2 The displacement-load curve diagram of the embodiment of the present application;

[0029] Figure 3 The test piece processing size design diagram of the embodiment of the present application;

[0030] Figure 4 The three-point bending test piece test result diagram (B=12mm) of the embodiment of the present application. DETAILED DESCRIPTION

[0031] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other in the case of no conflict. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0032] Embodiment one

[0033] As Figure 1 shown, a metal material small size fracture toughness testing method based on fracture cross section characteristics is provided in the embodiment, comprising:

[0034] According to the analysis results of the port macro-morphology characteristics of the fracture test piece in the existing literature, the fracture zone of the material can be divided into a normal fracture zone and a shear fracture zone, as Figure 1 shown, the energy of the normal fracture zone and the shear fracture zone is directly related to the corresponding area, wherein the shear lip width is defined as the width of the stable and constant shear area formed on both sides of the crack during the crack propagation process.

[0035] The physical mechanism of the formation of the shear lip during the fracture process is a complex and multi-faceted process involving material mechanics, fracture mechanics, and microstructure changes, etc. The formation of the shear lip usually occurs in the process of ductile fracture. Ductile fracture refers to the fracture of a material after a large plastic deformation, which is characterized by obvious plastic deformation and necking phenomenon before fracture. In this process, the appearance of the shear lip is due to the uneven stress distribution on the fracture surface. From the perspective of microstructure, the grains, phase boundaries, inclusions, etc. in the microstructure of the material will affect the mechanical properties and fracture behavior of the material. When a crack or a micro-hole propagates in the material, it may encounter these microstructure features, thereby changing its propagation path. In some cases, the crack or micro-hole may propagate along a specific grain boundary or phase boundary to form a shear lip. In addition, factors such as the processing history, heat treatment state, and environmental temperature of the material also affect the formation of the shear lip.

[0036] To ensure the feasibility of the estimated fracture toughness K IC and K IIC method, the minimum thickness of the fracture specimen should ensure that the macroscopic fracture surface can distinguish the normal fracture zone and the shear fracture zone, i.e. the shear lip width is separated into two zones. Further, the relationship between the nominal crack length a and the shear lip width s is represented by the function s(a) using the measured port morphology; since the s(a) function is relatively smooth, a polynomial fitting or other means can be used to characterize it in actual application.

[0037] Three-point bending specimens are often the first choice for specimen form in the field of metal material fracture mechanics property testing due to their simple form, fast processing, and strong representation. It is assumed that the fracture zone of a three-point bending standard specimen is divided into zones as Figure 1 shown, wherein A CA is the area of the central normal region S The total area of the two shear fracture regions;

[0038] The energy release relationship of the interface region during the crack propagation process will be derived below. The energy release rate G is equal to the fracture resistance, i.e. the G-R curve, which represents the energy consumption during the crack propagation process, including plastic deformation, crack initiation and micro-pore growth. The actual shear fracture region is not in the same plane as the normal region, and forms a spatial angle of about 45° with the normal region, so the actual shear fracture area is The elastic part of the total energy release from the crack propagation to the critical instability state U Q satisfies the following relationship

[0039] U Q = U C + U S (1)

[0040] wherein U C = G C · A C and U Q are the energy consumed by the normal region and the shear fracture region respectively, and φ is a fracture correction dimensionless coefficient related to the material type.

[0041] The crack propagation follows the energy release rate criterion, and the difference between the surface and the internal three-dimensional stress of the sample will lead to uneven distribution of the energy release rate in the thickness direction of the whole sample. In order to facilitate analysis, it is assumed that the unit area energy release rate of the normal region and the shear fracture region in the vertical thickness plane is constant, and at this time

[0042]

[0043] wherein G C , G S and G S are the average energy release rates of the whole cross section, the normal region and the shear region respectively; A IC = ∫s(a)da. Then the above formula is further rewritten as

[0044]

[0045] Based on the theory of fracture mechanics, the energy release rate G and the stress intensity factor K of the normal region (approximately plane strain) and the shear region (approximately plane stress) types satisfy different energy relationships. Substituting formula (3) can obtain

[0046]

[0047] wherein E is the elastic modulus of the material, μ is the Poisson's ratio of the material, K IIC and K Q are the required fracture toughness to be solved, KQ KIC is the plane strain fracture toughness.

[0048] When the loading reaches F Q , F Q and K Q satisfy the following linear relationship

[0049]

[0050] where the stress intensity factor coefficient K has

[0051]

[0052] For small size specimens, equation (5) can be further modified as

[0053]

[0054] where, is a dimensionless constant coefficient related to material properties, and for steel It can be seen that, K and present a linear relationship with λ.

[0055] When A S << A C , the shear fracture zone energy U S will also be much smaller than the positive fracture zone U C , at this time equation (8) can be degenerated as the following relationship

[0056]

[0057] That is, at this time the K Q value is the effective K IC , which meets the K IC test standard.

[0058] For specimens with obvious positive fracture and shear zone, ignoring the effect of the shear zone will lead to the obtained fracture toughness K I being larger than the effective K IC value, equation (8) gives the linear relationship between K and the partition area ratio λ; using specimens of different thicknesses for evaluation, the K values under different area ratios λ are obtained, and using this relationship can fit to determine K and K The specific process is as follows:

[0059] (1) as Figure 3As shown, the test method is used to obtain the load displacement curve of the test piece: different thickness of the sample is designed, and the corresponding relationship of the loading line displacement q, the crack opening displacement V and the corresponding loading load F of different samples is obtained, and F Q (F Q To determine the condition value, see Figure 2 ), and calculate the corresponding K Q , judge the effectiveness of the result K IC , and measure the cross section shear fracture area projection area ratio λ;

[0060] (2) Multiple sets of sample construction data set: using multiple sample measurement results, constructing corresponding and linear relationship:

[0061]

[0062] (3) Linear fitting extraction result: based on the data set, the least square method is used to determine and value, combined with the material correction coefficient to determine

[0063] Example two:

[0064] The embodiment provides a metal material small size fracture toughness testing method based on the fracture cross section characteristics, which comprises the following steps:

[0065] The fracture toughness K IC of a certain type of steel is estimated by using the method, the yield strength of the material is 1070.5 MPa, the tensile strength is 1368 MPa, and the elastic modulus is 184.5 GPa. In the experiment, the standard three-point bending sample (code SE (B)) is used, and the sample is determined to be the standard three-point bending sample according to the "GB / T 4161-2007 Metal Material Plane Strain Fracture Toughness K IC Test method". It is predicted that the value of K IC is 100 MN·m -3 / 2 , and at this time, the thickness of the test piece needs to meet B≥2.5 (K IC / R p0.2 ) 2 = 25 mm. However, the actual material is a steel plate with a thickness of only 15 mm, which cannot meet the requirements of the test standard.

[0066] According to the thickness of the actual raw material plate and the "GB / T 4161-2007 Metal Material Plane Strain Fracture Toughness K ICTest method, the standard sample with thickness of 4mm, 8mm, 12mm and 15mm is designed; the straight-through notch is adopted, the nominal crack length a is 12.00mm, the pre-crack notch width is 2mm, the notch root radius is less than 0.1mm, and the notch tip angle is less than 90°; the support span S is 4W. The crack starting notch should be perpendicular to the sample surface, and the deviation is within ±2°, and the pre-crack notch width is 2mm. The size and tolerance requirements are as shown in Figure 3 .

[0067] The MTS material mechanics testing machine three-point bending test system is calibrated and normally operated. The prepared sample is subjected to the fracture test on the material mechanics testing machine. For the three-point bending sample, the collected test data can be stored in the computer in the form of a file, and the P-V curve is drawn by the recorder, the cross-section partition geometry size of the sample is measured by the tool microscope, and the test result is as shown in Figure 4 .

[0068] According to the test curve, K Q and K IC are calculated and determined, as shown in Table 1; it can be seen that the calculated K Q value cannot be used as the effective plane fracture toughness K IC value.

[0069] Table 1

[0070]

[0071]

[0072] It can be seen from the data in Table 1 that the four groups of measurement results cannot meet the K IC determination condition, and the obtained K Q value cannot be used as the effective K IC result.

[0073] By using the method of the application, the cross-section partition size is further measured, the shear fracture area ratio λ is calculated, and the data are listed in Table 2.

[0074] Table 2

[0075]

[0076] By using formula (10), the following relationship is obtained

[0077]

[0078] The least square fitting is performed on the above measurement results, and the final K and K are calculated and determined. According to the test measurement experience, The value is 5.45, so K is obtained. IC =78.67 MPa 0.5 K IIC =107.34 MPa 0.5 .

[0079] This invention discloses a method for testing the small-size fracture toughness of metallic materials based on fracture section characteristics. Utilizing the fracture section shape of the test specimen, and considering the elastic energy distribution between the normal fracture zone dominated by Mode I fracture and the shear fracture zone dominated by Mode II fracture, a method is established... and The linear relationship between them; fracture tests were conducted using a multi-sample method with different thicknesses to determine different sets of data, and the least squares method was used to obtain... and The estimated values ​​have led to a new method for evaluating the small-size fracture toughness of metallic materials based on fracture section characteristics. For materials such as high-strength steel, this method avoids the difficulties of directly using standard parts for evaluation and has significant engineering application value.

[0080] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for testing small size fracture toughness of a metallic material based on fracture surface characteristics, characterized by, The method comprises the following steps: Step 1: obtaining the load-displacement curve of the test piece with different thicknesses through a test method; Step 2: Based on the load displacement curves of the different thickness specimens, construct a data set corresponding to and a linear relationship, construct an expression corresponding to and a linear relationship as: wherein, is a fracture correction non-dimensional coefficient related to the material type, is the area ratio of the shear fracture zone, A C is the area of the central normal fault zone, A S is the total area of the shear fracture zones on both sides, K Q is the average fracture toughness of the cross section, is the fracture toughness K required to be solved for a normal fault zone dominated by mode I fracture I is the square of the critical value, is the fracture toughness K required to be solved for a shear fracture zone dominated by mode II fracture II is the square of the critical value; the cross-sectional average fracture toughness K Q is expressed by the formula: wherein, K is the stress intensity factor, a0is the initial crack length, a is the nominal crack length, B is the specimen thickness, B N is the net specimen thickness between the side notches, S is the support span of the specimen, and W is the effective width of the specimen. The expression of the stress intensity factor coefficient is: The The linear relationship between the ratio of the area of the partition and the expression is: Step 3: Calculate the data set based on the least square method to obtain and estimated values.

2. The method of testing the small-size fracture toughness of a metallic material based on the fracture surface characteristics according to claim 1, characterized in that, The process of obtaining the load-displacement curve of the test piece with different thicknesses through a test method comprises: designing multiple test pieces with different thicknesses, respectively obtaining the corresponding relationship of the loading line displacement q, the crack opening displacement V and the corresponding loading load F of different test pieces, and generating the test piece load-displacement curve.

3. The method for testing the small-size fracture toughness of a metal material based on the fracture cross-section characteristics according to claim 1 or 2, characterized in that, The process of obtaining the load-displacement curve of the test piece with different thicknesses through a test method further comprises: F is extracted based on the load-displacement curve of the specimen. Q And calculate the corresponding K Q The judgment result is K. IC The effectiveness of the method was determined, and the area ratio λ of the cross-section was measured.

4. The method for testing the small-size fracture toughness of a metallic material based on the fracture cross-section characteristics according to claim 1, characterized in that, The minimum thickness of the test piece should ensure that the macroscopic fracture surface can distinguish the normal fracture zone and the shear fracture zone.

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