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

Abstract

A prediction method for fracture toughness of TC18 titanium alloy basket structure. By comprehensively considering the influence of intrinsic fracture resistance and external fracture resistance on the fracture toughness of titanium alloy, a fracture toughness prediction model based on tensile properties and crack propagation path of titanium alloy is established , and formulate specific implementation rules for predicting the fracture toughness of titanium alloys using this model. In the present invention, obtaining the tortuosity of the crack propagation path of the titanium alloy is an important link in predicting the fracture toughness thereof. Based on the invention, the fracture toughness of the TC18 titanium alloy is successfully predicted, and the prediction error is within 8%. The present invention is applicable to titanium alloys of any kind and any structure.

Description

A Prediction Method for Fracture Toughness of TC18 Titanium Alloy Basket Structure

technical field

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

Background technique

Since the 1940s, due to the frequent accidents of passenger planes, the concept of aircraft structural parts has changed from the concept of pure static strength design to the concept of safety-life design, damage-safety design, and the modern damage tolerance design criterion. In the damage tolerance design criteria, fracture toughness is an important indicator, which characterizes the ability of materials to prevent crack propagation, so ensuring the fracture toughness of aerospace structural parts is crucial to its safe flight. Due to the importance of fracture toughness, domestic and foreign scholars have been trying to establish a prediction method for fracture toughness of materials. A lot of work has been done abroad on the prediction of fracture toughness. In 1964, Krafft published an article called "Crack Toughness and Strain Hardening of Steels" in the journal Applied Materials and Research, pioneering the establishment of the relationship between the plane strain fracture toughness and the strain hardening exponent of low-strength, medium-strength and high-strength steels. Gokhale et al. published an article titled "Relationship Between Fracture Toughness, Fracture Path, and Microstructure of 7050 Aluminum Alloy: Part II. Multiple Micro mechanisms-based Fracture Toughness Model" in the journal Metallurgical and Materials Transactions A. Based on the microscopic fracture mechanism, the quantitative relationship between fracture toughness and fracture morphology of 7050 aluminum alloy was established. Baron published an article called "Relationship between fracture toughness and deformation a head of the crack tip" in the journal Strength of Materials, and established a quantitative relationship between the fracture extension zone size and fracture toughness of 15Kh2NMFA steel, and the prediction results are good. It should be noted that due to the different fracture mechanisms of different alloys, the fracture mechanisms of different structures of the same alloy are also different, so the application range of the above prediction model is limited.

Titanium and titanium alloys are widely used in aviation and other fields due to their excellent properties such as low density, high specific strength, and good corrosion resistance. It is well known that the fracture toughness of titanium alloys is particularly sensitive to the microstructure. However, the description of the fracture toughness variation law of titanium alloys under different microstructure characteristics at home and abroad is only in the qualitative category, and no fracture properties applicable to different microstructures of titanium alloys have been established. Resilience quantitative prediction method and corresponding specification.

Contents of the invention

In order to overcome the deficiency that there is no quantitative prediction method for fracture toughness and corresponding specifications applicable to different structures of titanium alloys in the prior art, the present invention proposes a method for prediction of fracture toughness of TC18 titanium alloy basket structure.

Concrete process of the present invention is:

Step 1, the derivation of the fracture toughness prediction model.

Derivation of fracture toughness prediction model by formula (6)

${\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; I</span>}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; \&ap;</span>{\mathrm{<span\; class="notranslate">\; 10</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; \&lsqb;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; E\&delta;</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 3</span>}{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&rsqb;</span><span\; class="notranslate">\; \&lsqb;</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&rsqb;</span>}{\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&upsi;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

Among them, K _IC is the fracture toughness, δ _e is the uniform elongation of the alloy, that is, the elongation of the tensile specimen before necking, σ _b and σ _y are the tensile strength and yield strength, respectively; E is the elastic modulus, L(ε) is the real length of the crack propagation path, L ₀ (ε) is the projected length of the crack propagation path along the opening direction of the fracture toughness specimen, and ν is Poisson's ratio.

Step 2, prediction and implementation method of fracture toughness of TC18 titanium alloy. Using formula (6) to predict the fracture toughness of the four microstructures of TC18 titanium alloy, the four microstructures are named A, B, C, and D respectively.

To predict the fracture toughness of each structure of TC18 titanium alloy, the mechanical property parameters and crack propagation path tortuosity L(ε)/L ₀ of each structure of TC18 titanium alloy must be obtained respectively. The specific process is as follows:

Ⅰ Acquisition of mechanical property parameters. Mechanical property parameters include tensile properties and fracture toughness. The tensile properties parameters are yield strength, breaking strength, uniform elongation, modulus of elasticity and Poisson's ratio. Three tensile properties samples and one fracture toughness sample were cut from a given tissue.

The size and test method of the tensile performance sample are carried out according to GB/T228.1-2010. The size and test method of the fracture toughness sample are strictly in accordance with the national standard GB/T4161-2007. The parameters of yield strength, breaking strength, uniform elongation and elastic modulus are the summed average value of the tensile results measured for TC18 titanium alloy under a certain structure.

The elastic modulus is 110 GPa, and Poisson's ratio is 0.35.

Ⅱ Acquisition of the tortuosity L(ε)/L ₀ of the crack propagation path. Obtain the crack propagation path through the metallography of the fracture toughness of the titanium alloy sample after destruction, and randomly select five fields of view on the crack propagation path, and the width of each field of view is not less than 200 μm. Specifically, it is first necessary to grind the metallographic phase of the fracture toughness of the titanium alloy specimen on the side surface, and use an optical microscope to take pictures and observe to obtain the crack propagation path.

The process of calculating the obtained L(ε)/L ₀ in each field of view: binarization, corrosion expansion, and opening and closing operations are performed by image processing software to obtain a clear crack propagation path. The real crack length L(ε) in the field of view is counted by dividermethod. In order to ensure the accuracy, the code length is taken as 10 μm. The crack projected length L ₀ is obtained by the scale bar in the field of view. Calculate L(ε)/L ₀ to obtain the tortuosity of the crack propagation path in a certain field of view.

According to the above steps, the L(ε)/L ₀ of the remaining 4 fields of view of the TC18 titanium alloy under the given structure were calculated respectively, and finally the L(ε)/L ₀ values of the 5 fields of view were summed and averaged , which is the tortuosity of the crack propagation path of TC18 titanium alloy under a given structure.

Repeat the process until the mechanical property parameters and crack propagation path tortuosity of the four tissues A, B, C, and D are obtained.

Step 3, Validation of the prediction method for fracture toughness of TC18 titanium alloy. Substituting the tensile property parameters obtained in step 2 and the crack propagation path tortuosity L(ε)/L ₀ into formula (6), the fracture toughness value of TC18 titanium alloy under a given structure can be calculated. Fracture toughness of TC18 titanium alloy under the four microstructures of A, B, C, and D according to steps 1 and 2, respectively. Finally, the measured and predicted values of fracture toughness of TC18 titanium alloy under four microstructures were compared. By comparison, it is found that formula (6) can predict the fracture toughness of TC18 titanium alloy very well, and the maximum error does not exceed 8%.

The present invention comprehensively considers the influence of intrinsic fracture resistance and external fracture resistance on the fracture toughness of titanium alloys, establishes a fracture toughness prediction model based on the tensile properties of titanium alloys and crack propagation paths, and formulates a fracture toughness prediction model for titanium alloys using the model. Specific implementation specifications for resilience prediction. In the present invention, obtaining the tortuosity of the crack propagation path of the titanium alloy is an important part of predicting its fracture toughness, and Figure 1 is a schematic diagram of the crack propagation path of the fracture toughness sample. In Example 1, the present invention introduces in detail the method of obtaining the tortuosity of the crack propagation path of TC18 titanium alloy, and the accompanying drawings 2 and 3 show the extraction process of the crack propagation path of 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%. Based on the invention, the fracture toughness of the TC17 titanium alloy is successfully predicted, and the prediction error is within 8%. In summary, the present invention is effective in predicting the fracture toughness of a given titanium alloy.

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

Description of drawings

Fig. 1 is a schematic diagram of the crack propagation path in the present invention. Among them: 1. Crack propagation path; 2. Fracture toughness specimen fracture surface.

Fig. 2 is the crack propagation path of four kinds of structures of TC18 titanium alloy in the present invention; Wherein: Fig. 2a is the crack propagation path of organization A, Fig. 2b is the crack propagation path of organization B, Fig. 2c is the crack propagation path of organization C, Fig. 2d is the crack propagation path of tissue D.

Fig. 3 is the crack propagation path after four kinds of clearing treatment of TC18 titanium alloy in the present invention; Wherein: Fig. 3 a is the crack propagation path of structure A, Fig. 3 b is the crack propagation path of structure B, Fig. 3 c is the crack growth path of structure C Path, Figure 3d is the crack propagation path of tissue D.

Fig. 4 is a flowchart of the present invention.

detailed description

Example 1:

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

Step 1, the derivation of the fracture toughness prediction model.

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

${\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; I</span>}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; \&ap;</span>\sqrt{\frac{{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; I</span>}\mathrm{<span\; class="notranslate">\; C</span>}}\mathrm{<span\; class="notranslate">\; E.</span>}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&upsi;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

where K _IC is the fracture toughness, G _IC is the critical strain energy release rate, E is the modulus of elasticity, and ν is Poisson's ratio.

Under small-scale yield conditions, the critical strain energy release rate G _IC in formula (1) can be expressed by the following formula:

G _IC =2γ _eff (2)

γ _eff represents the effective surface energy, which is the resistance to crack propagation.

In 1998, Charkaluk et al. published an article called "Fractals and Fracture" on Engineering Fracture Mechanics, arguing that only the effect of effective surface energy was considered in the critical strain energy release rate G _IC , but the effect of crack propagation path was not considered. According to research, the amplification factor L(ε)/L ₀ is included in the formula (2), so as to take into account the influence of the tortuosity of the crack propagation path. L(ε) represents the real length of the crack propagation path, L ₀ represents the projected length of the crack propagation path along the opening direction of the fracture toughness sample, and ε represents the yardstick length for measuring the crack propagation path. The corrected critical strain energy release rate is shown in formula (3).

G _IC ＝2γ _eff L(ε)/L ₀ (ε)(3)

Finally, combining formulas (1) and (3), we get:

${\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; I</span>}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; \&ap;</span>\sqrt{\frac{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; E\&gamma;</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; \&lsqb;</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&rsqb;</span>}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&upsi;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

It can be known from formula (4) that the fracture toughness is a multivariate function of the elastic modulus E, the tortuosity of the crack propagation path L(ε)/L ₀ and the effective surface energy γ _eff of the material. Among these independent variables, only the effective surface energy is difficult to obtain. However, in 1984, Ragozin et al. published an article called "Method of Accelerated Fracture Toughness K _IC Testing of Metallic Materials" on Strength of Materials, and established a formula for calculating the effective surface energy γ _eff of materials through tensile properties, as shown in formula (5).

$\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; 10</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; \&lsqb;</span>\frac{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; \&rsqb;</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

Among them, δ _e is the uniform elongation of the alloy, that is, the elongation of the tensile specimen before necking, σ _b and σ _y are the tensile strength and yield strength, respectively; E is the modulus of elasticity, and L(ε) is the crack The true length of the propagation path, L ₀ (ε) is the projected length of the crack propagation path along the opening direction of the fracture toughness specimen.

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

${\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; I</span>}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; \&ap;</span>{\mathrm{<span\; class="notranslate">\; 10</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; \&lsqb;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; E\&delta;</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 3</span>}{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&rsqb;</span><span\; class="notranslate">\; \&lsqb;</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&rsqb;</span>}{\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&upsi;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

Step 2, prediction and implementation method of fracture toughness of TC18 titanium alloy. The present invention uses the formula (6) to predict the fracture toughness of the four microstructures of the TC18 titanium alloy. The four microstructures are named A, B, C, and D respectively, as shown in Figure 2. The fracture toughness prediction of the TC18 titanium alloy is to predict the fracture toughness of each structure of the TC18 titanium alloy.

To predict the fracture toughness of each structure of TC18 titanium alloy, it is necessary to obtain the mechanical property parameters and crack propagation path tortuosity L(ε)/L ₀ of each structure of TC18 titanium alloy. Taking a single structure as an example, the specific process is as follows:

Ⅰ Acquisition of mechanical property parameters. Mechanical property parameters include tensile properties and fracture toughness. The tensile properties parameters are yield strength, breaking strength, uniform elongation, modulus of elasticity and Poisson's ratio. Three tensile properties samples and one fracture toughness sample were cut from a given tissue. The size and test method of the tensile performance sample are carried out according to GB/T228.1-2010. The size and test method of the fracture toughness sample are strictly in accordance with the national standard GB/T4161-2007. The four parameters of yield strength, breaking strength, uniform elongation, and elastic modulus are the summed average of the tensile results measured for TC18 titanium alloy under a given structure, as shown in Table 1. Since there is little difference in elastic modulus among different tissues, the elastic modulus is uniformly taken as 110GPa. Poisson's ratio is uniformly taken as 0.35.

Ⅱ Acquisition of the tortuosity L(ε)/L ₀ of the crack propagation path. In order to obtain the crack propagation path tortuosity L(ε)/L ₀ of the TC18 titanium alloy fracture toughness sample under a given structure, it is first necessary to grind the metallographic phase of the fracture toughness of the titanium alloy sample after grinding, and use OLYMPUSPMG3 optical microscope Take pictures and observe to obtain the crack propagation path. In order to ensure the accuracy of the parameter L(ε)/L ₀ , five fields of view are randomly selected in the direction of crack propagation, and the width of the field of view is not less than 200 μm.

Taking a single field of view as an example to illustrate the calculation process of L(ε)/L ₀ in the field of view: since the crack propagation path is not necessarily in the same plane in space, the path illuminated will produce blurring, outline Unclear situations seriously affect the accuracy of statistics. In view of this, it is necessary to clarify the crack propagation path, and use the professional image processing software ImageProPlus6.0 to perform binarization, corrosion expansion, and opening and closing operations to obtain a clear crack propagation path, as shown in Figure 3. The real crack length L(ε) in the field of view is counted by dividermethod. In order to ensure the accuracy, the code length is taken as 10 μm. The crack projected length L ₀ is directly calculated according to the scale in the field of view. Calculate L(ε)/L ₀ to obtain the tortuosity of the crack propagation path in a certain field of view.

According to the above steps, the L(ε)/L ₀ of the remaining 4 fields of view of the TC18 titanium alloy under the given structure were calculated respectively, and finally the L(ε)/L ₀ values of the 5 fields of view were summed and averaged , which is the tortuosity of the crack propagation path of TC18 titanium alloy under a given structure, as shown in Table 1.

Step 3, Validation of the prediction method for fracture toughness of TC18 titanium alloy. Substituting the tensile property parameters obtained in step 2 and the crack propagation path tortuosity L(ε)/L ₀ into formula (6), the fracture toughness value of TC18 titanium alloy under a given structure can be calculated. According to steps 1 and 2, the fracture toughness of TC18 titanium alloy under other microstructures was calculated respectively. Finally, the measured and predicted values of the fracture toughness of TC18 titanium alloy under the four microstructures were compared, as shown in Table 2. By comparison, it is found that formula (6) can predict the fracture toughness of TC18 titanium alloy very well, and the maximum error does not exceed 8%.

Table 1 Tensile parameters and crack propagation path tortuosity of TC18 titanium alloy

Table 2 Comparison of measured and predicted values of fracture toughness of TC18 titanium 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)

$K_{IC}\&ap;{10}^{-2}\sqrt{\frac{4\&lsqb;(2{\mathrm{E\&delta;}}_e-2{\mathrm{\&sigma;}}_y)({\mathrm{\&sigma;}}_y+2{\mathrm{\&sigma;}}_b)+3{\mathrm{\&sigma;}}_y^2\&rsqb;\&lsqb;L\left(\mathrm{\&epsiv;}\right)/L_0\left(\mathrm{\&epsiv;}\right)\&rsqb;}{3(1-{\mathrm{\&upsi;}}^2)}}---\left(6\right)$

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.