Load marking method under random spectrum of laser directional energy deposition modeling Ti-6Al-4V material

Abstract

The invention belongs to the field of alloy material testing, and discloses a method for identifying a load under a random spectrum of a laser-directed energy deposition modeling Ti-6Al-4V material, and the method comprises the following steps: obtaining historical data; the equivalent initial defect size of the Ti-6Al-4V material obtained through laser directed energy deposition modeling is determined on the basis of historical data; the equivalent initial defect size serves as the initial crack size, and identification load parameters are determined through pseudo-vacuum environment crack propagation numerical simulation under the random spectrum; generating a random load spectrum containing the identification load based on the identification load parameter; and performing a fatigue test based on the random load spectrum containing the identification load until the test piece is fractured, and performing fracture interpretation to obtain a data set. According to the method, a numerical simulation method of three-dimensional crack propagation and an actual fatigue test are combined, the technical problem that three-dimensional crack detection in an internal pseudo-vacuum environment cannot be carried out by an acoustic emission method, a potential drop method and a flexibility method is solved, meanwhile, compared with the high cost of CT, the detection cost is greatly reduced, and expensive professional equipment is not needed.

Description

Technical Field

[0001] The invention belongs to the field of alloy material testing and relates to a marking load method for Ti-6Al-4V material, in particular to a marking load method under a random spectrum of Ti-6Al-4V material formed by laser directed energy deposition. Background Art

[0002] Laser-directed energy deposition (LDED) is a metal additive manufacturing technology with advantages such as fast molding, high performance, and high efficiency. It is particularly advantageous in the production of large, high-density structures. Currently, this technology is widely used in the manufacture of large structures with complex topologies in aerospace, mechanical manufacturing, biomedicine, and other fields. During manufacturing, the powder undergoes a rapid melting and solidification process, resulting in unique microstructures and pore defects, and thus unique fatigue failure behavior. Porosity-induced fatigue failure and microstructure-induced failure are common. This restricts the application of this technology in primary load-bearing structures, and in-depth research on its fatigue failure behavior and patterns is urgently needed.

[0003] The defects of Ti-6Al-4V materials manufactured using LDED technology are mainly pores. As the protective gas enters the interior of the material during the powder melting process, it forms relatively regular spheres or ellipsoids under uniform pressure in all directions. The crack propagation process of the material is in an internal pseudo-vacuum environment for a considerable period of time, and its crack propagation rate is significantly different from that in an air environment. The fatigue process of Ti-6Al-4V materials formed by laser directed energy deposition is divided into two stages: crack initiation and propagation. Due to the influence of the complex stress and strain state and uneven microstructure inside the material, it is still a great challenge to accurately simulate the three-dimensional crack propagation behavior. Accurately monitoring the three-dimensional crack front morphology through experimental means and obtaining the historical state of three-dimensional crack propagation have important theoretical and application value for improving crack propagation simulation methods and accurately predicting fatigue life.

[0004] However, internal pseudo-vacuum cracks cannot be detected through surface observation. Other internal crack detection methods such as acoustic emission, potential drop, and compliance are not suitable for three-dimensional cracks with complex shapes. Although CT can effectively detect the shape and size of internal cracks, it is expensive. Therefore, the marker load method remains the most effective method for monitoring internal cracks. However, due to the different failure modes and crack propagation environments of LDED Ti-6Al-4V materials (the presence of a pseudo-vacuum environment), the parameter determination method for the marker load is different. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for identifying loads under random spectra of Ti-6Al-4V materials formed by laser directed energy deposition, so as to solve the problem that existing internal crack detection methods are not applicable to internal three-dimensional crack monitoring with more complex shapes.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: The identification load method for Ti-6Al-4V material formed by laser directed energy deposition under random spectrum includes: Acquire historical data; determine the equivalent initial defect size of Ti-6Al-4V material formed by laser directed energy deposition based on the historical data; Taking the equivalent initial defect size as the initial crack size, the signature load parameters are determined through numerical simulation of crack growth in a pseudo-vacuum environment under a random spectrum. Based on the signature load parameters, a random load spectrum containing the signature load is generated. Based on the random load spectrum containing the identification load, fatigue test is carried out until the specimen breaks, and the fracture surface is interpreted to obtain the data set.

[0007] A further improvement of the present invention is that: in the step of obtaining historical data, the historical data is the critical pore size of the laser directed energy deposition formed Ti-6Al-4V material obtained based on CT scanning and fatigue testing; The step of determining the equivalent initial defect size of the laser directed energy deposition formed Ti-6Al-4V material based on historical data specifically includes: Based on historical data, the lognormal distribution is used to describe the equivalent size distribution of critical pores; The median size is obtained based on the equivalent size distribution of critical pores, and the equivalent initial defect size is determined based on the median size; the equivalent initial defect size is used as the initial crack size; the initial crack is preset at the center of the specimen section based on the size of the initial crack; the median size is √ area ; The shape of the initial crack is circular with a diameter of D; the diameter D is equal to the median size√ area The relationship is: √ area 2 =πD 2 / 4.

[0008] A further improvement of the present invention is that: in the step of determining the identification load parameter by numerically simulating crack growth in a pseudo-vacuum environment under a random spectrum using the equivalent initial defect size as the initial crack size, a crack growth model that considers stress ratio and crack closure effect is used to perform numerical simulation of crack growth to obtain crack growth simulation results; Obtaining the insertion interval and insertion position of the identification load based on the crack growth simulation results; Determine peak and signature load stress ratios based on random spectra Rml ; Based on insertion interval, insertion position, peak value, stress ratio R ml , a new round of crack growth simulation is performed to determine the number of cycles N ml ; Finally determine all the identification load parameters; the identification load parameters include the identification load insertion interval, insertion position, identification load stress ratio R ml and number of cycles N ml .

[0009] A further improvement of the present invention is that in the step of performing numerical simulation of crack propagation using a crack propagation model that takes into account stress ratio and crack closure effect, the crack propagation model used is the NASGRO v3 crack propagation model.

[0010] A further improvement of the present invention is that: in the step of numerically simulating crack propagation using a crack propagation model that takes into account stress ratio and crack closure effect, formula (4) is substituted into formula (3) to simulate crack propagation for each cycle in the stress spectrum. When the crack propagates to K max ≥ K C , the specimen breaks and the crack growth simulation results are obtained: a - T curve; (3) in, is the crack growth rate, a is the crack length, T is the flight time, C, n, p, q is the material parameter, K max The peak stress intensity factor for each loading cycle, Δ K th is the crack extension threshold, Δ K is the stress intensity factor range, which includes the minimum value and maximum value , is the stress intensity factor variation range Δ K = - ; R is the stress ratio per cycle, K C is the fracture toughness, f is the Newman closed function; Stress intensity factor Take Newman's solution; (4) Where S is the nominal stress at the far end, a e , c e , is the minor axis radius, major axis radius and polar angle of the crack front of the elliptical crack surface, Fe is the geometric correction function, Q is the shape factor, b , t It is half of the length and width of the specimen cross section.

[0011] A further improvement of the present invention is that in the step of generating a random load spectrum containing an identification load based on the identification load parameters, identification lines are arranged at intervals of 1 / 15 of the total life below 60% of the life; and an identification line is inserted every 1 / 30 of the total life cycle above 60% of the life.

[0012] A further improvement of the present invention is that the insertion position of the identification line is in front of the nearest high load of the same level or above. A further improvement of the present invention is that: the peak value and the mark load stress ratio are determined based on the random spectrum. R ml In the steps, the peak value is selected as any one of the three high loads before the reference spectrum; the load stress ratio is marked R ml Select any value from 0.6 to 0.9.

[0013] A further improvement of the present invention is that: the method based on the insertion interval, insertion position, peak value, stress ratio R ml , a new round of crack growth simulation is performed to determine the number of cycles N ml In the steps: The number of identified load cycles corresponding to the i-th insertion position for: (6) Among them, Δ a ml,i =min[0.05Δ a bl,i ,0.02mm], where Δ a ml,i is the width of the i-th identification line, Δ a bl,i is the difference in crack length between the i-th and i-1-th identification lines.

[0014] A further improvement of the present invention is that: in the step of performing fatigue test on the basis of random load spectrum containing identification load until the specimen breaks and performing fracture interpretation to obtain a data set, the obtained data set is ( a , T );in a is the crack length, T For flight time.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a method for applying a marking load to a Ti-6Al-4V material formed by laser directed energy deposition under a random spectrum, comprising: obtaining historical data based on CT scanning and fatigue testing; the historical data being critical pore sizes of a plurality of test pieces obtained in the historical tests; using a log-normal distribution to describe the equivalent size distribution of the critical pores based on the historical data; obtaining a median size based on the equivalent size distribution of the critical pores, and determining the size of an initial crack based on the median size; presetting an initial crack at the center of a cross section of the test piece based on the size of the initial crack; performing a crack propagation numerical simulation using a crack propagation model that considers a stress ratio and a crack closure effect to obtain a crack propagation simulation result; obtaining a marking load insertion interval and an insertion position based on the crack propagation simulation result; and determining a peak value and a marking load stress ratio based on a random spectrum. R ml ; Based on insertion interval, insertion position, peak value, stress ratio R ml , a new round of crack growth simulation is performed to determine the number of cycles N ml Finally determine all the identification load parameters. The identification load parameters include the identification load insertion interval, insertion position, identification load stress ratio R ml and number of cycles N ml The method of the present invention combines the numerical simulation method of three-dimensional crack propagation with actual fatigue testing, solving the technical problem that the acoustic emission method, potential drop method, and compliance method cannot perform three-dimensional crack detection in an internal pseudo-vacuum environment. At the same time, compared with the high cost of CT, the method greatly reduces the detection cost and does not require expensive professional equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The specimen size diagram; Figure 2 Schematic diagram of the statistical results of critical pore equivalent size; (a) is the probability density function of the parameter estimation results, and (b) is the cumulative distribution function; Figure 3 The geometric dimensions of the specimen and crack are shown in Figure 2; (a) is the geometric dimensions of the specimen, and (b) is the geometric dimensions of the crack; Figure 4Schematic diagram of crack propagation simulation results; Figure 5 is a schematic diagram of the identification load spectrum; Figure 6 The number of loops for each insertion position N ml Schematic diagram of calculation results; Figure 7 This is an optical microscope photo of the fracture; Figure 8 This is a schematic diagram of the identification line interpretation results; Figure 9 The figure is a flow chart of a method for marking loads under a random spectrum of Ti-6Al-4V material formed by laser directed energy deposition according to an embodiment of the present invention. DETAILED DESCRIPTION

[0017] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments. It should be noted that, in the absence of conflict, the embodiments and features of the embodiments of the present invention can be combined with each other.

[0018] The following detailed description is an exemplary description and is intended to provide further detailed description of the present invention. Unless otherwise indicated, all technical terms used in the present invention have the same meaning as those generally understood by those skilled in the art to which the present invention belongs. The terms used in the present invention are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention.

[0019] The present invention provides a method for marking loads under random spectrum of Ti-6Al-4V material formed by laser directed energy deposition, and the technical approach is as follows: (1) Crack propagation analysis within the life span Using the equivalent initial defect size (EIDS) and fracture mechanics methods, the life of the target object under random spectra is simulated and analyzed. It includes the following two parts: 1) Identify EIDS; 2) Crack propagation analysis.

[0020] (2) Determination of the insertion interval and insertion position of the marker load To clarify the fatigue failure process and analyze crack propagation patterns, it is generally required that there be at least 10 identification traces within the crack length range from 0.1 mm to fracture failure. The insertion position is before the nearest high load of the same level or higher in the reference spectrum.

[0021] (3) Determination of peak load, stress ratio, and number of cycles 1) Peak value determination method Take any one of the first three high loads in the reference spectrum as the identification load peak.

[0022] 2) Selection of stress ratio Stress ratio R ml It should be between 0.6-0.9.

[0023] 3) Determination of the number of cycles The number of cycles determines the width of the identification line, and the calculated extension step length of each identification line and random spectrum is A set of identification line cycles N ml,i , where Δ a ml is the width of the identification line, Δ a bl The interval between adjacent identification lines.

[0024] (4) Interpretation of fracture Interpret the fracture identification line and establish ( a , T ) dataset; a is the crack length, T For flight time.

[0025] See also Figure 9 As shown, an embodiment of the present invention provides a method for marking loads under a random spectrum of Ti-6Al-4V material formed by laser directed energy deposition, comprising: S1. Crack growth analysis within the life span; S11. Determine EIDS; Equivalent initial damage size (EIDS) is a quantitative representation of the initial damage of materials and structures, which equates the initial defect to a crack.

[0026] The failure of Ti-6Al-4V material processed by LDED process is mainly caused by pores. Therefore, when the identification load is initially applied, only the failure induced by pores is considered.

[0027] This embodiment uses the following method to determine EIDS and initial cracks: S111, obtain historical data; the historical data is the critical pore size of each specimen in previous work (see Figure 2 and Table 1); wherein, the number of specimens collecting the critical pore size is greater than or equal to 5; preferably, greater than 20. Figure 1 shown.

[0028] Table 1 Critical pore statistics

[0029] S112. Use log-normal distribution (base 10) to describe the equivalent size distribution of critical pores. The statistical results are shown in Figure 2 (The probability density function and mean variance parameter estimation methods are shown in Eq. (1) and Eq. (2) respectively); (1) (2) in, D is the diameter of the critical pore, in μm; D i For the i The diameter of the critical pore of each specimen; m is the total number of specimens.

[0030] S113, critical pore size is selected as the median size√ area=65.8 μm, median size√ area 2 =πD 2 / 4, the initial crack diameter D = 74.2 μm (equivalent pore size corresponding to 50% of the cumulative distribution); S114. The equivalent initial crack is preset at the center of the specimen cross section. The initial shape is circular with a diameter of D.

[0031] S12, crack growth analysis; The NASGRO v3 crack growth model considering the stress ratio and crack closure effect was used for numerical simulation to obtain the crack growth process.

[0032] (3) in, is the crack growth rate, a is the crack length, N is the number of cycles, C, n, p, q is the material parameter, K max The peak stress intensity factor for each loading cycle, Δ K th is the crack extension threshold, Δ K is the stress intensity factor range, which includes the minimum value and maximum value The difference between the two is the stress intensity factor range Δ K ; R is the stress ratio per cycle, K C is the fracture toughness, f is the Newman closed function.

[0033] For a crack located at the center of a finite plate, the stress intensity factor Take Newman's solution.

[0034] (4) Where S is the nominal stress at the far end, a e , c e , is the minor axis radius, major axis radius and polar angle of the crack front of the elliptical crack surface, Fe is the geometric correction function, Q is the shape factor, b , t Half the length and width of the specimen cross section (see Figure 3 ).

[0035] The NASGRO model involves a total of 15 parameters. The parameter values ​​used in this example are all taken from the NASGRO material parameter library (see Table 2).

[0036] Substituting Eq. (4) into Eq. (3) can simulate the crack growth in each cycle of the stress spectrum. K max ≥ K C , the specimen breaks and the whole life process is obtained a - T Curves such as Figure 4 shown.

[0037] Table 2 Crack extension parameters of NASGRO v3 model

[0038] S2. Determine the insertion interval and insertion position of the marker load; S21. Determine the insertion interval According to the crack growth simulation results, 14,000 flight hours ( T ) below, a marking line is inserted every 1000 flight hours. Above 14000 flight hours, the crack growth rate increases, so a marking line is inserted every 500 flight hours (see Figure 4 The load spectrum after the identification load is inserted into the reference load is as follows: Figure 5 shown.

[0039] S22, insert position determination The insertion position is before the nearest high load of the same level or above in the reference spectrum.

[0040] S3, mark the load peak and determine the stress ratio; S31, peak value; The second level high load in the reference spectrum is taken as the identification load peak.

[0041] S32, stress ratio R ml Select; Take the stress ratio R ml =0.75.

[0042] S4. Number of cycles N ml Sure From Eq. (3), the number of cycles corresponding to each insertion position is (Δ K >Δ K th , K max < K C ): (5) in N ml,i For the i The number of identified load cycles corresponding to the insertion position, Δ a ml,i is the width of the i-th identification line, a i For the i Crack length corresponding to the insertion position.

[0043] For the identification load with a fixed stress ratio, due to the identification line width Δ a ml,i << a , so from the integral mean value theorem we get: (6) The width of the marking line should not be greater than 0.05 times the interval between adjacent marking lines, and should not exceed 0.02mm, that is, Δ a ml,i =min[0.05Δ a bl,i ,0.02mm]. Select R ml = 0.75, the number of load cycles for each insertion position is calculated by Eq. (6) as follows: Figure 6 shown.

[0044] S5, fracture interpretation; Optical microscope photos of the fracture surface Figure 7 As shown, the identification line is clear and obvious. a , T ) Datasets such as Figure 8 shown.

[0045] See also Figure 9As shown, an embodiment of the present invention provides a method for marking loads under a random spectrum of Ti-6Al-4V material formed by laser directed energy deposition, comprising the following steps: S100, acquiring historical data; determining an equivalent initial defect size of a Ti-6Al-4V material formed by laser directed energy deposition based on the historical data; S200, using the equivalent initial defect size as the initial crack size, determining a signature load parameter through numerical simulation of crack growth in a pseudo-vacuum environment under a random spectrum; generating a random load spectrum containing the signature load based on the signature load parameter; S300, performing a fatigue test based on a random load spectrum containing an identification load until the specimen breaks, and performing fracture interpretation to obtain a data set.

[0046] In a specific implementation, step S100 specifically includes: The equivalent initial flaw size is determined based on historical data from CT scans and fatigue tests, including: Acquiring historical data; the historical data is the critical pore size of a plurality of test pieces obtained in historical tests; Based on historical data, the lognormal distribution is used to describe the equivalent size distribution of critical pores; The median size is obtained based on the equivalent size distribution of critical pores, and the equivalent initial defect size is determined based on the median size; the equivalent initial defect size is used as the initial crack size; the initial crack is preset at the center of the specimen section based on the size of the initial crack; the median size is √ area ; The shape of the initial crack is circular with a diameter of D; the diameter D is equal to the median size√ area The relationship is: √ area 2 =πD 2 / 4.

[0047] In a specific embodiment, in the step of determining the identification load parameter by numerically simulating crack growth in a pseudo-vacuum environment under a random spectrum in step S200 using the equivalent initial defect size as the initial crack size, a crack growth model that considers stress ratio and crack closure effect is used to perform numerical simulation of crack growth to obtain crack growth simulation results; Obtaining the insertion interval and insertion position of the identification load based on the crack growth simulation results; Determine the peak value of the marker load and the stress ratio of the marker load based on the reference spectrum R ml ; Based on the marker load insertion interval, insertion position, peak value, and stress ratio, a new crack growth simulation is carried out to determine the number of marker load cycles. N ml ; A random load spectrum containing the marker load is generated based on the marker load insertion interval, insertion position, peak value, stress ratio, and number of cycles.

[0048] In a specific embodiment, in the step of performing numerical simulation of crack growth using a crack growth model that takes into account stress ratio and crack closure effect, the crack growth model used is the NASGRO v3 crack growth model.

[0049] In a specific embodiment, in the step of numerically simulating crack growth using a crack growth model that considers stress ratio and crack closure effect, formula (4) is substituted into formula (3) to simulate crack growth for each cycle in the stress spectrum. When the crack grows to K max ≥ K C , the specimen breaks and the crack growth simulation results are obtained: a - T curve; (3) in, is the crack growth rate, a is the crack length, T is the flight time, C, n, p, q is the material parameter, K max The peak stress intensity factor for each loading cycle, Δ K th is the crack extension threshold, Δ K is the stress intensity factor range, which includes the minimum value and maximum value , is the stress intensity factor variation range Δ K = - ; R is the stress ratio per cycle, K C is the fracture toughness, f is the Newman closed function; Stress intensity factor Take Newman's solution; (4) Where S is the nominal stress at the far end, a e , c e , is the minor axis radius, major axis radius and polar angle of the crack front of the elliptical crack surface, Fe is the geometric correction function, Q is the shape factor,b , t It is half of the length and width of the specimen cross section.

[0050] In one specific embodiment, in the step of generating a random load spectrum containing an identification load based on the identification load parameters, identification lines are arranged at intervals of 1 / 15 of the total life cycle for cycles below 60% of the life cycle; and an identification line is inserted every 1 / 30 of the total life cycle for cycles above 60% of the life cycle. The insertion position is before the nearest high load of the same level or higher.

[0051] In a specific embodiment, the load peak value and the load stress ratio are marked based on the reference spectrum. R ml In the steps: Determine the peak value and identify the load-stress ratio based on the random spectrum R ml The peak value of the identification load is selected as any one of the first three high loads of the reference spectrum; the stress ratio of the identification load is R ml Select any value from 0.6 to 0.9.

[0052] In a specific embodiment, the new crack growth simulation is carried out based on the insertion interval, insertion position, peak value, and stress ratio of the marker load to determine the number of marker load cycles. N ml In the steps: Number of identified load cycles corresponding to each insertion position for: (6) Among them, Δ a ml,i =min[0.05Δ a bl,i ,0.02mm].

[0053] In a specific embodiment, in the step S300 of performing a fatigue test based on a random load spectrum containing an identification load until the specimen breaks and performing fracture interpretation to obtain a data set, the obtained data set is ( a , T );in a is the crack length, T For flight time.

[0054] It is understood from common technical knowledge that the present invention may be implemented by other embodiments that do not depart from its spirit or essential features. Therefore, the embodiments disclosed above are, in all respects, merely illustrative and not exclusive. All modifications within the scope of the present invention or equivalent to the scope of the present invention are intended to be encompassed by the present invention.

Claims

1. Laser Directed Energy Deposition Ti-6Al-4V material random spectrum identification load method, characterized in that: include: Get historical data; Determine the equivalent initial defect size of Ti-6Al-4V material formed by laser directed energy deposition based on historical data; Taking the equivalent initial defect size as the initial crack size, the signature load parameters are determined through numerical simulation of crack growth in a pseudo-vacuum environment under a random spectrum. Based on the signature load parameters, a random load spectrum containing the signature load is generated. Based on the random load spectrum containing the identification load, fatigue test is carried out until the specimen breaks, and the fracture surface is interpreted to obtain the data set.

2. The marking load method for laser directed energy deposition forming Ti-6Al-4V material under random spectrum according to claim 1 is characterized in that: In the step of obtaining historical data, the historical data is the critical pore size of the laser directed energy deposition formed Ti-6Al-4V material obtained based on CT scanning and fatigue testing; The step of determining the equivalent initial defect size of the laser directed energy deposition formed Ti-6Al-4V material based on historical data specifically includes: Based on historical data, the lognormal distribution is used to describe the equivalent size distribution of critical pores; The median size is obtained based on the equivalent size distribution of critical pores, and the equivalent initial defect size is determined based on the median size; the equivalent initial defect size is used as the initial crack size; the initial crack is preset at the center of the specimen section based on the size of the initial crack; the median size is √ area ; The shape of the initial crack is circular with a diameter of D; the diameter D is equal to the median size√ area The relationship is: √ area 2 =πD 2 / 4.

3. The marking load method for Ti-6Al-4V material under random spectrum of laser directed energy deposition according to claim 1 is characterized in that: In the step of determining the identification load parameter by numerically simulating crack growth in a pseudo-vacuum environment under a random spectrum using the equivalent initial defect size as the initial crack size, a crack growth model that considers stress ratio and crack closure effect is used to perform numerical simulation of crack growth to obtain crack growth simulation results; Obtaining the insertion interval and insertion position of the identification load based on the crack growth simulation results; Determine peak and signature load stress ratios based on random spectra R ml ; Based on insertion interval, insertion position, peak value, stress ratio R ml , a new round of crack growth simulation is performed to determine the number of cycles N ml ; Finally determine all the identification load parameters; the identification load parameters include the identification load insertion interval, insertion position, identification load stress ratio R ml and number of cycles N ml .

4. The marking load method for laser directed energy deposition forming Ti-6Al-4V material under random spectrum according to claim 3 is characterized in that: In the step of performing numerical simulation of crack growth using a crack growth model that takes into account stress ratio and crack closure effect, the crack growth model used is the NASGRO v3 crack growth model.

5. The marking load method for laser directed energy deposition forming Ti-6Al-4V material under random spectrum according to claim 3, characterized in that: In the step of numerically simulating crack growth using the crack growth model that considers stress ratio and crack closure effect, formula (4) is substituted into formula (3) to simulate crack growth for each cycle in the stress spectrum. When the crack grows to K max ≥ K C , the specimen breaks and the crack growth simulation results are obtained: a - T curve; （3） in, is the crack growth rate, a is the crack length, T is the flight time, C, n, p, q is the material parameter, K max The peak stress intensity factor for each loading cycle, Δ K th is the crack extension threshold, Δ K is the stress intensity factor range, which includes the minimum value and maximum value , is the stress intensity factor variation range Δ K = - ; R is the stress ratio per cycle, K C is the fracture toughness, f is the Newman closed function; Stress intensity factor Take Newman's solution; （4） Where S is the nominal stress at the far end, a e , c e , is the minor axis radius, major axis radius and polar angle of the crack front of the elliptical crack surface, Fe is the geometric correction function, Q is the shape factor, b , t It is half of the length and width of the specimen cross section.

6. The marking load method for laser directed energy deposition forming Ti-6Al-4V material under random spectrum according to claim 1, characterized in that: In the step of generating a random load spectrum containing an identification load based on the identification load parameter, identification lines are arranged at intervals of 1 / 15 of the total life below 60% of the life; and an identification line is inserted every 1 / 30 of the total life cycle above 60% of the life.

7. The marking load method for laser directed energy deposition forming Ti-6Al-4V material under random spectrum according to claim 6, characterized in that: The insertion position of the identification line is in front of the nearest high load of the same level or above.

8. The marking load method for laser directed energy deposition forming Ti-6Al-4V material under random spectrum according to claim 3, characterized in that: Determine the peak value and identify the load-stress ratio based on the random spectrum R ml In the steps, the peak value is selected as any one of the three high loads before the reference spectrum; the load stress ratio is marked R ml Select any value from 0.6 to 0.

9.

9. The marking load method for laser directed energy deposition forming Ti-6Al-4V material under random spectrum according to claim 5, characterized in that: The insertion interval, insertion position, peak value, stress ratio R ml , a new round of crack growth simulation is performed to determine the number of cycles N ml In the steps: The number of identified load cycles corresponding to the i-th insertion position for: （6） Among them, Δ a ml,i =min[0.05Δ a bl,i ,0.02mm], where Δ a ml,i is the width of the i-th identification line, Δ a bl,i is the difference in crack length between the i-th and i-1-th identification lines.

10. The marking load method for laser directed energy deposition forming Ti-6Al-4V material under random spectrum according to claim 1, characterized in that: In the step of performing fatigue test on the basis of random load spectrum containing identification load until the specimen breaks and performing fracture interpretation to obtain data set, the obtained data set is ( a , T );in a is the crack length, T For flight time.

Images