Quantitative evaluation method for effect of residual stress on fatigue crack propagation

Abstract

The invention relates to the technical field of metal fatigue, in particular to a quantitative evaluation method for the effect of residual stress on fatigue crack propagation, which comprises the following steps: carrying out surface strengthening treatment on a sample; residual stress distribution in the depth direction of the surface strengthening treatment area of the sample is measured; performing a crack propagation test on the sample, and measuring and calculating crack propagation rates under different crack lengths; determining crack tip extension energy density according to a metal fatigue theory; wherein the crack tip extension energy density is crack extension energy provided for the crack tip by a far-field load in unit square millimeter; and according to the crack tip extension energy density, the crack extension driving energy density and the crack extension inhibition energy density are determined. The quantitative evaluation method for the action of the residual stress on the fatigue crack propagation aims to solve the problem that the quantitative evaluation of the action of the residual stress on the fatigue crack propagation is difficult.

Description

Technical Field

[0001] The invention relates to the technical field of metal fatigue, and in particular to a quantitative evaluation method for the effect of residual stress on fatigue crack extension. Background Art

[0002] Residual stress is the self-balanced internal stress that remains inside the material after eliminating external forces or uneven temperature fields. Its value can be positive or negative, corresponding to the stress state. A positive stress value represents residual tensile stress, and a negative value represents residual compressive stress. Common residual stress induction methods include: mechanical method, physical method, chemical method, etc. Among them, surface deformation strengthening processes represented by shot peening, rolling, laser shock, and hole extrusion are the most widely used. For load conditions dominated by tensile stress, the introduction of residual compressive stress can offset local loads and obtain improved service performance such as fatigue resistance, stress corrosion resistance, and wear resistance.

[0003] In terms of anti-fatigue requirements, residual stress is the basis of anti-fatigue manufacturing of metal materials. It can not only inhibit the initiation of fatigue cracks, but also reduce the crack propagation rate. However, "residual stress inhibits fatigue crack initiation and reduces the crack propagation rate" is a qualitative description. How to quantitatively evaluate the effect of residual stress on crack propagation and use residual stress to assist structural design and support structure optimization is also a difficult problem that needs to be solved in the field of residual stress and fatigue.

[0004] Therefore, the inventors provide a quantitative evaluation method for the effect of residual stress on fatigue crack growth. Summary of the invention

[0005] (1) Technical issues to be solved

[0006] The embodiment of the present invention provides a method for quantitatively evaluating the effect of residual stress on fatigue crack growth, which solves the technical problem that it is difficult to quantitatively evaluate the effect of residual stress on crack growth.

[0007] (2) Technical solution

[0008] The present invention provides a quantitative evaluation method for the effect of residual stress on fatigue crack growth, comprising the following steps:

[0009] Perform surface strengthening treatment on the sample;

[0010] Measuring the residual stress in the depth direction of the surface strengthening treatment area of ​​the sample;

[0011] Performing a crack growth test on the sample, and measuring and calculating the crack growth rate at different crack lengths;

[0012] According to metal fatigue theory, the crack tip propagation energy density is determined; wherein the crack tip propagation energy density is the crack propagation energy provided to the crack tip by the far-field load per square millimeter;

[0013] The crack extension driving energy density and the crack extension inhibition energy density are determined according to the crack tip extension energy density.

[0014] Furthermore, the surface strengthening treatment of the sample is specifically performed as follows:

[0015] The processed sample is treated by using a surface treatment process.

[0016] Furthermore, the sample is subjected to a crack extension test, and the crack extension rates at different crack lengths are measured and calculated, specifically:

[0017] An extensometer is applied to measure and calculate the crack growth rate of the sample at different crack lengths by the compliance method.

[0018] Furthermore, the determination of the crack tip propagation energy density according to the metal fatigue theory specifically includes the following steps:

[0019] Determine a third expression for the applied stress based on the first expression for the crack growth intensity factor range and the second expression for the crack growth rate;

[0020] The third expression is integrated within the thickness range of the sample to obtain the crack tip propagation energy density.

[0021] Furthermore, the crack extension intensity factor range is positively correlated with the shape factor, the applied stress, and the crack length.

[0022] Furthermore, the crack growth rate is exponentially related to the crack growth intensity factor range.

[0023] Furthermore, the determining of the third expression of the applied stress based on the first expression of the crack growth intensity factor range and the second expression of the crack growth rate specifically includes the following steps:

[0024] Substituting the first expression of the crack growth intensity factor range into the second expression of the crack growth rate to obtain a fourth expression;

[0025] Based on the fourth expression, a third expression of the applied stress is determined.

[0026] Furthermore, the crack tip extension energy density includes crack extension driving energy density and crack extension inhibition energy density.

[0027] (3) Beneficial effects

[0028] In summary, the present invention quantitatively characterizes the crack propagation energy by the crack tip propagation energy density, and can solve the residual stress separately, thereby realizing the separate solution of the effect of the external field driving load and the residual stress on the crack propagation. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments of the present invention are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0030] Figure 1 It is a flow chart of a quantitative evaluation method of the effect of residual stress on fatigue crack growth provided by an embodiment of the present invention;

[0031] Figure 2 This is a curve diagram of residual stress influence depth provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The following detailed description of the embodiments of the present invention is provided in conjunction with the accompanying drawings and examples. The following detailed description of the embodiments and the accompanying drawings are used to illustrate the principles of the present invention, but cannot be used to limit the scope of the present invention, that is, the present invention is not limited to the described embodiments.

[0033] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0034] Figure 1 is a flow chart of a quantitative evaluation method for the effect of residual stress on fatigue crack growth provided by an embodiment of the present invention, see Figure 1 , the method may include the following steps:

[0035] S100, performing surface strengthening treatment on the sample.

[0036] Specifically, CT and MT specimens of specific thickness are designed and processed according to relevant standards, and the processed specimens are treated by surface treatment processes such as mechanical shot peening, ultrasonic rolling, and laser shock strengthening.

[0037] S200. Measure the residual stress in the depth direction of the surface strengthening treatment area of ​​the sample.

[0038] Specifically, X-ray diffraction, drilling method, etc. are used to measure the residual stress distribution in the depth direction of the surface strengthening treatment area.

[0039] S300. Carry out crack extension test on the specimen, and measure and calculate the crack extension rate under different crack lengths.

[0040] Specifically, a standard testing machine is used to perform a crack growth test on the sample in step S200. The applied parameters include: load, frequency and stress ratio. An extensometer is applied, and the crack growth rate under different crack lengths is measured and calculated by the compliance method, and is given by the corresponding software.

[0041] S400. Determine the crack tip propagation energy density according to metal fatigue theory; wherein the crack tip propagation energy density is the crack propagation energy provided to the crack tip by the far-field load per square millimeter.

[0042] Specifically, step S400 includes the following steps:

[0043] S401, determining a third expression for applied stress according to a first expression for a crack growth intensity factor range and a second expression for a crack growth rate;

[0044] S402. Integrate the third expression within the thickness range of the sample to obtain the crack tip propagation energy density.

[0045] According to metal fatigue theory, the crack growth intensity factor range (ΔK) can be expressed as formula (1):

[0046]

[0047] Where Y is the shape factor, which can be obtained by equation (2), Δσ is the magnitude of the applied stress, and a is the crack length.

[0048]

[0049] Where W is the width of the specimen and a is the crack length.

[0050] The Paris formula in metal fatigue theory stipulates that there is an exponential relationship between the crack growth rate (da / dN, a is the crack length, N is the number of cycles) and the crack intensity factor range (ΔK), as shown in formula (3):

[0051] da / dN=C(ΔK) m (3)

[0052] Where a is the crack length, N is the number of cycles; C and m are material constants and can be obtained by linear fitting of test data.

[0053] Substituting formula (1) into formula (3) yields formula (4):

[0054]

[0055] After taking the inverse function on both sides of equation (4), we can get equation (5):

[0056]

[0057] Integrating equation 5 over the thickness h yields equation 6:

[0058]

[0059] At this time, the physical quantity on the left side of equation (6) has the characteristics of energy density, which we define as a new physical quantity: crack tip extension energy density (ρ crack tip ), whose physical meaning is the crack extension energy provided by the far-field load to the crack tip per square millimeter. Furthermore, it mainly provides the driving energy density (ρ) for the forward propagation of the crack. crack tip,drive ), can be expressed as, that is, formula (6) can be expressed as formula (7):

[0060]

[0061] S500, determining crack extension driving energy density and crack extension inhibition energy density according to the crack tip extension energy density.

[0062] Specifically, when the crack tip is subjected to the driving force Δσ applied by the remote field, it is also subjected to the hindering residual stress Δσ introduced by the surface strengthening method. rs When , it mainly provides the crack extension inhibition energy density, which can be expressed as formula (8):

[0063]

[0064] Then, for the case of both driving force and hindering force, the crack extension energy density can be expressed as formula (9):

[0065]

[0066] In summary, the present invention proposes for the first time the physical quantity of crack tip propagation energy density, which can quantitatively reveal the effects of external force and residual stress field on crack propagation, thereby realizing the separate solution of the effects of external field driven load and residual stress on crack propagation.

[0067] Example 1

[0068] Step 1: Sample design and processing. According to the National Standard of the People's Republic of China GB / T 6398-2000 "Test Method for Fatigue Crack Growth Rate of Metallic Materials", a TC17 titanium alloy compact tensile C (T) specimen with a thickness of 5 mm was designed and processed.

[0069] Step 2: Sample surface strengthening treatment. The TC17 titanium alloy C (T) sample is subjected to double-sided impact strengthening treatment using nanosecond pulse laser. Laser parameters: single pulse energy is 20 J, impact times are 3 times, overlap rate is 50%, and impact method is double-sided impact.

[0070] Step 3: Residual stress test in the depth direction of the surface strengthening treatment area. The residual stress distribution in the depth direction of the strengthening area was tested using the drilling residual stress test device of StressTech. The results are as follows: Figure 2 shown.

[0071] Step 4: Crack extension test. The Instron 8801 servo hydraulic test system was used to perform a crack extension test on the C(T) specimen after laser shock strengthening. The crack extension test parameters were: load 1500N, stress ratio 0.1, frequency 20Hz.

[0072] Step 5: Measurement of crack growth rate at a specific crack length. The crack opening displacement is measured using the compliance method in combination with an extensometer. To ensure the accuracy of the measurement, the stable crack growth stage is selected as the object. In this embodiment, the crack growth rate at a crack length of 27.15 mm is 9×10 -5 mm / cycle.

[0073] Step 6: Inversely solve the far-field load stress. The stress value corresponding to the external field load is 437.1MPa according to equation (5).

[0074] Step 7: Solving the crack propagation driving energy density. The crack propagation energy density of the applied load at the laser shock strengthening influence depth of 1 mm is obtained by formula (7):

[0075]

[0076] Step 8: Solve the crack growth inhibition energy density. Figure 2 The residual stress equation is fitted and its function equation is shown in formula (11):

[0077] σ rs =-7362h 4 +2.11e4h 3 +2.078e4h 2 +7246h-123.9 (11)

[0078] Similarly, by integrating it over the residual stress influence depth of 1 mm, the residual stress-induced crack growth inhibition energy density can be obtained as shown in formula (12):

[0079]

[0080] Step 9: Solve the crack extension energy density. Then the crack extension energy density at a crack length of 27.5 mm is:

[0081] ρ crack tip =∫(Δσ+Δσ rs )dh=437.1-387.5=49.6mJ / mm 2 (13)

[0082] It should be clear that the various embodiments in this specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. The present invention is not limited to the specific steps and structures described above and shown in the figures. In addition, for the sake of brevity, a detailed description of known methods and technologies is omitted here.

[0083] The above are only embodiments of the present application and are not limited to the present application. For those skilled in the art, the present application may have various changes and variations without departing from the scope of the present invention. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present application should be included in the scope of the claims of the present application.

Claims

1. A quantitative evaluation method for the effect of residual stress on fatigue crack growth, characterized in that: The method comprises the following steps: Perform surface strengthening treatment on the sample; Measuring the residual stress in the depth direction of the surface strengthening treatment area of ​​the sample; Performing a crack growth test on the sample, and measuring and calculating the crack growth rate at different crack lengths; According to metal fatigue theory, the crack tip propagation energy density is determined; wherein the crack tip propagation energy density is the crack propagation energy provided to the crack tip by the far-field load per square millimeter; The crack extension driving energy density and the crack extension inhibition energy density are determined according to the crack tip extension energy density.

2. The quantitative evaluation method of residual stress on fatigue crack growth according to claim 1, characterized in that: The surface strengthening treatment of the sample is specifically as follows: The processed sample is treated by using a surface treatment process.

3. The quantitative evaluation method of residual stress on fatigue crack growth according to claim 1, characterized in that: The crack extension test is performed on the sample, and the crack extension rate under different crack lengths is measured and calculated, specifically: An extensometer is applied to measure and calculate the crack growth rate of the sample at different crack lengths by the compliance method.

4. The quantitative evaluation method of residual stress on fatigue crack growth according to claim 1, characterized in that: The method of determining the crack tip propagation energy density according to metal fatigue theory specifically includes the following steps: Determine a third expression for the applied stress based on the first expression for the crack growth intensity factor range and the second expression for the crack growth rate; The third expression is integrated within the thickness range of the sample to obtain the crack tip propagation energy density.

5. The quantitative evaluation method of residual stress on fatigue crack growth according to claim 4, characterized in that: The crack extension intensity factor range is positively correlated with the shape factor, the applied stress, and the crack length.

6. The quantitative evaluation method of residual stress on fatigue crack growth according to claim 4, characterized in that: The crack growth rate is exponentially related to the crack growth intensity factor range.

7. The quantitative evaluation method of residual stress on fatigue crack growth according to claim 4, characterized in that: Determining the third expression of the applied stress based on the first expression of the crack growth intensity factor range and the second expression of the crack growth rate specifically includes the following steps: Substituting the first expression of the crack growth intensity factor range into the second expression of the crack growth rate to obtain a fourth expression; Based on the fourth expression, a third expression of the applied stress is determined.

8. The quantitative evaluation method of residual stress on fatigue crack growth according to claim 1, characterized in that: The crack tip extension energy density includes crack extension driving energy density and crack extension inhibition energy density.

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