CN105651603A - TC18 titanium alloy basketweave microstructure fracture toughness prediction method - Google Patents

Abstract

A TC18 titanium alloy basketweave microstructure fracture toughness prediction method comprises the following steps: establishing a fracture toughness prediction model based on the tensile performance and the crack expansion path of titanium alloy through comprehensively considering influences of the intrinsic fracture resistance and external fracture resistance on the fracture toughness of the titanium alloy, and working out concrete enforcement standards for using the model to carry out titanium alloy fracture toughness prediction. Obtaining of the sinuosity of the crack expansion path of the titanium alloy is an important link in the fracture toughness predication of the titanium alloy in the invention. The method allows the fracture toughness of the TC18 titanium alloy to be successfully predicated, and the predication error is not greater than 8%. The method is suitable for any kind of titanium alloys with any microstructure.

Description

TC18 titanium alloy basket structure fracture toughness prediction method

Technical Field

The invention relates to the field of materials, in particular to a method capable of effectively predicting fracture toughness of a titanium alloy.

Background

Since the beginning of the twentieth and forty years, due to frequent crashes of passenger aircraft, aircraft structural members have been transformed from a pure static strength design concept to a safety-life design concept, a breakage-safety design concept, to modern damage tolerance design criteria. Fracture toughness is an important indicator in damage tolerance design criteria, and characterizes the ability of a material to resist crack propagation, and therefore, ensuring fracture toughness of an aerospace structure is critical to its safe flight. Due to the importance of fracture toughness, researchers at home and abroad have been trying to establish a method for predicting fracture toughness of materials. Much work has been done abroad in the prediction of fracture toughness. In 1964, Krafft published an article entitled "CrackToughnend and StrainHardeningof steels" in journal applied materials and research, which pioneered the relationship between plane strain fracture toughness and strain hardening index for low, medium and high strength steels. Gokhale et al published an article entitled "relationship between the fracture toughness of7050aluminum alloy and the fracture morphology," in Metallurgicalalandmaterials transaction sA, and established a quantitative relationship between the fracture toughness of7050aluminum alloy and the fracture morphology based on a microscopic fracture mechanism. Baron published an article named "relationship between fracture elongation zone size and fracture toughness" in Strength hof materials, and established a quantitative relationship between fracture elongation zone size and fracture toughness of 15Kh2NMFA steel, and the prediction result is better. It should be noted that, because the fracture mechanisms of different alloys are different, and the fracture mechanisms of different structures of the same alloy are also different, the application range of the prediction model is limited.

Titanium and titanium alloys have excellent properties such as low density, high specific strength, and good corrosion resistance, and are widely used in the fields of aviation and the like. As is known, the fracture toughness of titanium alloy is particularly sensitive to microstructure, however, the description of the fracture toughness change rule of the titanium alloy under different microstructure characteristics at home and abroad only stays in a qualitative category, and a quantitative prediction method and corresponding specifications for the fracture toughness of different structures of the titanium alloy are not established.

Disclosure of Invention

In order to overcome the defects that a quantitative prediction method for fracture toughness of different structures of titanium alloy and corresponding specifications are not available in the prior art, the invention provides a method for predicting fracture toughness of a TC18 titanium alloy basket structure.

The specific process of the invention is as follows:

and step 1, deducing a fracture toughness prediction model.

Derivation of fracture toughness prediction model by equation (6)

$K_{IC}\≈{10}^{-2}\sqrt{\frac{4\[(2{\mathrm{E\δ}}_e-2{\mathrm{\σ}}_y)({\mathrm{\σ}}_y+2{\mathrm{\σ}}_b)+3{\mathrm{\σ}}_y^2\]\[L\left(\mathrm{\ϵ}\right)/L_0\left(\mathrm{\ϵ}\right)\]}{3(1-{\mathrm{\υ}}^2)}}---\left(6\right)$

Wherein, K_ICIn order to be the fracture toughness, the alloy is,_eis the uniform elongation of the alloy, i.e. the elongation of the tensile specimen before necking, σ_bAnd σ_yTensile strength and yield strength, respectively; e is the modulus of elasticity, L () is the true length of the crack propagation path, L₀() The projection length of the crack propagation path along the opening direction of the fracture toughness test sample is shown, and ν is the Poisson ratio.

And 2, predicting the fracture toughness of the TC18 titanium alloy and implementing the method. And (3) predicting the fracture toughness of the TC18 titanium alloy by adopting a formula (6), wherein the four structures are named as A, B, C and D respectively.

To predict the fracture toughness of each structure of the TC18 titanium alloy, the mechanical property parameters and the crack propagation path tortuosity L ()/L of each structure of the TC18 titanium alloy must be obtained respectively₀The specific process comprises the following steps:

and I, obtaining mechanical property parameters. The mechanical property parameters include tensile properties and fracture toughness. Tensile properties parameters are yield strength, breaking strength, uniform elongation, elastic modulus and poisson's ratio. Three tensile property specimens and one fracture toughness specimen were cut at a given texture.

The dimensions and test method of the tensile property test specimens are carried out according to GB/T228.1-2010. The size of the fracture toughness test sample and the testing method are strictly carried out according to the national standard GB/T4161-2007. All parameters of yield strength, breaking strength, uniform elongation and elastic modulus are the average value of tensile results measured by the TC18 titanium alloy under a certain tissue.

The modulus of elasticity is 110GPa and the Poisson's ratio is 0.35.

II crack propagation path tortuosity L ()/L₀And (4) obtaining. And obtaining a crack propagation path through the side metallographic phase of the damaged titanium alloy fracture toughness sample, and randomly selecting five view fields on the crack propagation path, wherein the width of each view field is not less than 200 mu m. Specifically, firstly, grinding the side metallographic phase of the damaged titanium alloy fracture toughness sample, and taking a picture by using an optical microscope for observation to obtain a crack propagation path.

For the obtained L ()/L in each field₀The calculation process is carried out as follows: and carrying out binarization, corrosion expansion and opening and closing operation processing through image processing software to obtain a clear crack propagation path. And (3) counting the actual crack length L () in the field of view by using a divdermethod method. To ensure accuracy, the length of the code scale is 10 μm. Projected crack length L₀Obtained by a scale in the field of view. Calculating L ()/L₀And obtaining the crack propagation path tortuosity under a certain visual field.

L ()/L of the rest 4 visual fields of the TC18 titanium alloy under the given structure according to the steps₀Respectively calculating, and finally carrying out L ()/L of 5 fields₀And averaging the sum of the values, namely obtaining the tortuosity of the crack propagation path of the TC18 titanium alloy under the given structure.

And repeating the process until obtaining the mechanical property parameters of the four tissues A, B, C and D and the tortuosity of a crack propagation path.

And 3, verifying the effectiveness of the TC18 titanium alloy fracture toughness prediction method. The tensile property parameter and the crack propagation path tortuosity L ()/L obtained in the step 2 are processed₀The fracture toughness value of the TC18 titanium alloy in a given structure can be calculated by substituting the formula (6). The fracture toughness of the TC18 titanium alloy under the four tissue forms of A, B, C and D is respectively determined according to the steps 1 and 2. Finally, the actual measured value and the predicted value of the fracture toughness of the TC18 titanium alloy in the four tissue forms are compared. The comparison shows that the formula (6) can well predict the fracture toughness of the TC18 titanium alloy, and the maximum error is not more than 8%.

The invention establishes a fracture toughness prediction model based on the tensile property and the crack propagation path of the titanium alloy by comprehensively considering the influence of the intrinsic fracture resistance and the extrinsic fracture resistance on the fracture toughness of the titanium alloy, and establishes a specific implementation standard for predicting the fracture toughness of the titanium alloy by using the model. In the invention, the obtained tortuosity of the crack propagation path of the titanium alloy is an important link for predicting the fracture toughness of the titanium alloy, and the attached diagram 1 is a schematic diagram of the crack propagation path of the fracture toughness sample. In the embodiment 1, the invention introduces the method for obtaining the tortuosity of the crack propagation path of the TC18 titanium alloy in detail, and fig. 2 and fig. 3 show the extraction flow of the crack propagation path of the TC18 titanium alloy. Based on the invention, the fracture toughness of the TC18 titanium alloy is successfully predicted, and the prediction error is within 8 percent. Based on the invention, the fracture toughness of the TC17 titanium alloy is successfully predicted, and the prediction error is within 8 percent. In conclusion, the present invention is effective in predicting the fracture toughness of a given titanium alloy.

The invention is applicable to titanium alloys of any kind and any structure.

Drawings

FIG. 1 is a schematic view of the crack propagation path in the present invention. Wherein: 1. a crack propagation path; 2. fracture surface of fracture toughness specimen.

FIG. 2 is a graph showing the crack propagation paths of the four structures of the TC18 titanium alloy of the present invention; wherein: FIG. 2a is the crack propagation path of structure A, FIG. 2B is the crack propagation path of structure B, FIG. 2C is the crack propagation path of structure C, and FIG. 2D is the crack propagation path of structure D.

FIG. 3 shows the crack propagation paths of the TC18 titanium alloy after the four structures are clearly treated in the invention; wherein: FIG. 3a is the crack propagation path for structure A, FIG. 3B is the crack propagation path for structure B, FIG. 3C is the crack propagation path for structure C, and FIG. 3D is the crack propagation path for structure D.

Fig. 4 is a flow chart of the present invention.

Detailed Description

Example 1:

the embodiment is a method for predicting fracture toughness of a TC18 titanium alloy basket structure. The specific process of this embodiment is:

and step 1, deducing a fracture toughness prediction model.

According to the theory of linear elastic fracture mechanics, fracture toughness is represented by the following formula:

$K_{IC}\≈\sqrt{\frac{G_{IC}E}{1-{\mathrm{\υ}}^2}}---\left(1\right)$

in the formula K_ICFor fracture toughness, G_ICThe critical strain energy release rate is E, the elastic modulus is E, and v is Poisson's ratio.

Critical strain energy release rate G in equation (1) at a small range of yield conditions_ICCan be represented by the following formula:

G_IC＝2γ_eff(2)

γ_effrepresenting the effective surface energy, is the resistance to crack propagation forward.

In 1998, Charkalukukukk et al published an article entitled "FractalsandFractrechanics" on engineering FractureMs, believing that the critical strain energy release rate G_ICOnly the effect of the effective surface energy is taken into account, whereas the effect of the crack propagation path is not taken into account. According to the study, the amplification factor L ()/L is included in equation (2)₀Thereby taking into account the influence of the tortuosity of the crack propagation path. L () represents the true length of the crack propagation path, L₀The projection length of the crack propagation path along the opening direction of the fracture toughness test sample is represented, and the length of a code scale for measuring the crack propagation path is represented. The corrected critical strain energy release rate is shown in formula (3).

G_IC＝2γ_effL()/L₀()(3)

Finally, combining equations (1) and (3) yields:

$K_{IC}\≈\sqrt{\frac{2{\mathrm{E\γ}}_{eff}\[L\left(\mathrm{\ϵ}\right)/L_0\left(\mathrm{\ϵ}\right)\]}{1-{\mathrm{\υ}}^2}}---\left(4\right)$

as can be seen from the formula (4), the fracture toughness is the modulus of elasticity E, and the crack propagation path tortuosity L ()/L₀And material effective surface energy gamma_effIs used as the multivariate function of (1). Of these independent variables, only effective surface energy is difficult to obtain. However, in 1984, Ragozin et al published a name "MethodfAcceleostedFractureToughnessK" on Strength hofmaterials_ICThe article by testingof metallic materials "established the calculation of the effective surface energy γ of a material by tensile properties_effThe formula (2) is shown in formula (5).

$2{\mathrm{\γ}}_{eff}=8\×{10}^{-4}\[\frac{({\mathrm{\δ}}_e-\frac{{\mathrm{\σ}}_y}{E})({\mathrm{\σ}}_y+2{\mathrm{\σ}}_b)}{3}+\frac{{\mathrm{\σ}}_y^2}{2E}\]---\left(5\right)$

Wherein,_eis the uniform elongation of the alloy, i.e. the elongation of the tensile specimen before necking, σ_bAnd σ_yTensile strength and yield strength, respectively; e is the modulus of elasticity, L () is the true length of the crack propagation path, L₀() Is the projected length of the crack propagation path in the direction of the fracture toughness specimen opening.

Combining the formulas (4) and (5), a titanium alloy fracture toughness prediction model is obtained:

$K_{IC}\≈{10}^{-2}\sqrt{\frac{4\[(2{\mathrm{E\δ}}_e-2{\mathrm{\σ}}_y)({\mathrm{\σ}}_y+2{\mathrm{\σ}}_b)+3{\mathrm{\σ}}_y^2\]\[L\left(\mathrm{\ϵ}\right)/L_0\left(\mathrm{\ϵ}\right)\]}{3(1-{\mathrm{\υ}}^2)}}---\left(6\right)$

and 2, predicting the fracture toughness of the TC18 titanium alloy and implementing the method. The invention adopts a formula (6) to predict the fracture toughness of the TC18 titanium alloy in four tissue forms, wherein the four tissues are respectively named as A, B, C and D, and are shown in figure 2. The fracture toughness prediction of the TC18 titanium alloy is carried out on each structure of the TC18 titanium alloy.

To predict the fracture toughness of each structure of the TC18 titanium alloy, the mechanical property parameters and the crack propagation path tortuosity L ()/L of each structure of the TC18 titanium alloy must be obtained₀Taking a single organization as an example, the specific process is as follows:

and I, obtaining mechanical property parameters. The mechanical property parameters include tensile properties and fracture toughness. Tensile properties parameters are yield strength, breaking strength, uniform elongation, elastic modulus and poisson's ratio. Three tensile property specimens and one fracture toughness specimen were cut at a given texture. The dimensions of the tensile properties test specimens and the test method were carried out in accordance with GB/T228.1-2010. The size of the fracture toughness test sample and the testing method are strictly carried out according to the national standard GB/T4161-2007. The four parameters of yield strength, breaking strength, uniform elongation and elastic modulus are the average value of the tensile results of the TC18 titanium alloy under a given structure, and are shown in Table 1. Because the elastic modulus difference between different tissues is not large, the elastic modulus is uniformly taken as 110 GPa. The poisson ratio is taken to be 0.35.

II crack propagation path tortuosity L ()/L₀And (4) obtaining. In order to obtain the crack propagation path tortuosity L ()/L of TC18 titanium alloy fracture toughness test piece under given structure₀Firstly, grinding the side metallographic phase of the damaged titanium alloy fracture toughness sample, and photographing and observing the side metallographic phase by using an OLYMPUSPMG3 optical microscope to obtain a crack propagation path. To guarantee the parameter L ()/L₀The accuracy of the method is that five visual fields are randomly selected in the crack propagation direction, and the width of the visual field is not less than 200 mu m.

Illustrating L ()/L in the view field by taking a single view field as an example₀The calculation process is carried out as follows: since the crack propagation paths are not necessarily in the same plane in space, the observed paths may be blurred and have unclear outlines, which seriously affects the accuracy of statistics. In view of this, it is necessary to perform a sharpening process on the crack propagation path, and a specialized image processing software imageproplus6.0 is used to perform binarization, erosion expansion and opening/closing operation processes, so as to obtain a sharpened crack propagation path, as shown in fig. 3. The true crack length L () within the field of view is counted using a divdermethod. To ensure accuracy, the length of the code scale is 10 μm. Projected crack length L₀Directly from the scale in the field of view. Calculating L ()/L₀And obtaining the crack propagation path tortuosity under a certain visual field.

According to the aboveThe steps are carried out on the L ()/L of the rest 4 visual fields of the TC18 titanium alloy under the given structure₀Respectively calculating, and finally carrying out L ()/L of 5 fields₀The sum of the values is averaged to obtain the tortuosity of the crack propagation path of the TC18 titanium alloy in the given structure, as shown in Table 1.

And 3, verifying the effectiveness of the TC18 titanium alloy fracture toughness prediction method. The tensile property parameter and the crack propagation path tortuosity L ()/L obtained in the step 2 are processed₀The fracture toughness value of the TC18 titanium alloy at a given structure can be calculated by substituting the formula (6). And respectively calculating the fracture toughness of the TC18 titanium alloy under other tissue forms according to the steps 1 and 2. Finally, the fracture toughness measured values and the fracture toughness predicted values of the TC18 titanium alloy in the four tissue forms are compared, and are shown in Table 2. The comparison shows that the formula (6) can well predict the fracture toughness of the TC18 titanium alloy, and the maximum error is not more than 8%.

TABLE 1TC18 titanium alloy tensile parameters and crack propagation path tortuosity

TABLE 2 comparison of measured and predicted values of fracture toughness of TC18 Ti alloy

Claims (4)

1. The method for predicting the fracture toughness of the TC18 titanium alloy basket structure is characterized by comprising the following specific steps:

step 1, derivation of a fracture toughness prediction model:

derivation of fracture toughness prediction model by equation (6)

$K_{IC}\≈{10}^{-2}\sqrt{\frac{4\[(2{\mathrm{E\δ}}_e-2{\mathrm{\σ}}_y)({\mathrm{\σ}}_y+2{\mathrm{\σ}}_b)+3{\mathrm{\σ}}_y^2\]\[L\left(\mathrm{\ϵ}\right)/L_0\left(\mathrm{\ϵ}\right)\]}{3(1-{\mathrm{\υ}}^2)}}---\left(6\right)$

Wherein, K_ICIn order to be the fracture toughness, the alloy is,_eis the uniform elongation of the alloy, i.e. the elongation of the tensile specimen before necking, σ_bAnd σ_yTensile strength and yield strength, respectively; e is the modulus of elasticity, L () is the true length of the crack propagation path, L₀() The projection length of a crack propagation path along the opening direction of a fracture toughness sample is shown, and ν is Poisson's ratio;

step 2, predicting the fracture toughness of the TC18 titanium alloy and implementing the method: predicting the fracture toughness of the four tissue forms of the TC18 titanium alloy by adopting a formula (6), wherein the four tissues are named as A, B, C and D respectively;

to predict the fracture toughness of each structure of the TC18 titanium alloy, the mechanical property parameters and the crack propagation path tortuosity L ()/L of each structure of the TC18 titanium alloy must be obtained respectively₀The specific process comprises the following steps:

i, acquiring mechanical property parameters; the mechanical property parameters comprise tensile property and fracture toughness; the tensile property parameters include yield strength, breaking strength, uniform elongation, elastic modulus and Poisson's ratio; cutting three tensile property samples and a fracture toughness sample under a given tissue;

II crack propagation path tortuosity L ()/L₀Obtaining; obtaining a crack propagation path through a side metallographic phase of the damaged titanium alloy fracture toughness sample, and randomly selecting five view fields on the crack propagation path, wherein the width of each view field is not less than 200 mu m;

for the obtained L ()/L in each field₀The calculation process is carried out as follows: carrying out binarization, corrosion expansion and opening and closing operation processing through image processing software to obtain a clear crack propagation path; counting the real crack length L () in the field of view by adopting a divdermethod method; in order to ensure the accuracy, the length of the code ruler is 10 mu m; projected crack length L₀Obtained by a scale in the field of view; calculating L ()/L₀Obtaining the crack propagation path tortuosity under a certain view field;

l ()/L of the rest 4 visual fields of the TC18 titanium alloy under the given structure according to the steps₀Respectively calculating, and finally carrying out L ()/L of 5 fields₀The sum of the values is taken as an average value, namely the tortuosity of the crack propagation path of the TC18 titanium alloy under the given tissue;

repeating the process until obtaining the mechanical property parameters of the four tissues A, B, C and D and the tortuosity of a crack propagation path;

step 3, verifying the effectiveness of the TC18 titanium alloy fracture toughness prediction method: the tensile property parameter and the crack propagation path tortuosity L ()/L obtained in the step 2 are processed₀Substituting the formula (6) into the formula (6), the fracture toughness value of the TC18 titanium alloy under a given structure can be calculated; respectively carrying out fracture toughness on TC18 titanium alloy under the four tissue forms of A, B, C and D according to the steps 1 and 2; finally, comparing the actual measured value and the predicted value of the fracture toughness of the TC18 titanium alloy in the four tissue forms; the comparison shows that the formula (6) can well predict the fracture toughness of the TC18 titanium alloy, and the maximum error is not more than 8%.

2. The method for predicting the fracture toughness of the TC18 titanium alloy mesh basket structure in claim 1, wherein the size and the test method of the tensile property sample in the step 2 are performed according to GB/T228.1-2010, and the size and the test method of the fracture toughness sample are strictly performed according to the national standard GB/T4161-2007; the yield strength, the breaking strength, the uniform elongation and the elastic modulus are all the average values of the tensile results of the TC18 titanium alloy under a certain tissue.

3. The method for predicting the fracture toughness of the TC18 titanium alloy basket structure according to claim 1, wherein the elastic modulus is 110GPa and the Poisson's ratio is 0.35.

4. The method for predicting the fracture toughness of the TC18 titanium alloy basket structure as claimed in claim 1, wherein the crack propagation path of the TC18 titanium alloy fracture toughness test sample is obtained by grinding the side metallographic phase of the damaged titanium alloy fracture toughness test sample, and taking a picture by using an optical microscope for observation.