CN113588378B - Preparation and calculation method of brazed joint fracture toughness sample - Google Patents
Preparation and calculation method of brazed joint fracture toughness sample Download PDFInfo
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- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N1/00—Sampling; Preparing specimens for investigation
- G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
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- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
- G01N3/36—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces generated by pneumatic or hydraulic means
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/40—Investigating hardness or rebound hardness
- G01N3/42—Investigating hardness or rebound hardness by performing impressions under a steady load by indentors, e.g. sphere, pyramid
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- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0001—Type of application of the stress
- G01N2203/0005—Repeated or cyclic
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0001—Type of application of the stress
- G01N2203/0012—Constant speed test
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0014—Type of force applied
- G01N2203/0016—Tensile or compressive
- G01N2203/0017—Tensile
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0014—Type of force applied
- G01N2203/0016—Tensile or compressive
- G01N2203/0019—Compressive
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/003—Generation of the force
- G01N2203/0042—Pneumatic or hydraulic means
- G01N2203/0048—Hydraulic means
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0058—Kind of property studied
- G01N2203/006—Crack, flaws, fracture or rupture
- G01N2203/0067—Fracture or rupture
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/02—Details not specific for a particular testing method
- G01N2203/026—Specifications of the specimen
- G01N2203/0296—Welds
Abstract
The invention provides a preparation method of a soldered joint fracture toughness sampleAnd a calculation method. The samples were prepared by applying a solder resist during the brazing process so that the initial crack tips were within the braze joint micro-zone. In the aspect of calculating the fracture toughness, the nanometer indentation size effect is comprehensively considered, and the elastic modulus, the yield strength and the hardening index of a diffusion influence area (on a fracture path) of the brazing joint are calculated. And a loading linear displacement control mode is adopted, and a J-delta a resistance curve is obtained through multiple times of loading/unloading according to an unloading flexibility method. Determination of fracture resistance J based on passivation line equation of power hardening elastoplasticity constitutive 0.2BL . The invention solves the problems that in the conventional fracture toughness experiment, the initial crack size of the current processing mode is far larger than that of a brazed joint, the crack tip is difficult to form at the brazed joint, and reliable performance parameters for resisting crack propagation are difficult to obtain. The invention solves the problems of experimental and analytical methods for measuring the fracture toughness of the soldered joint and is of great importance for further researching the fracture mechanism of the soldered joint.
Description
Technical Field
The invention relates to the technical field of fracture toughness testing, in particular to a method for preparing and calculating a sample of the fracture toughness of a soldered joint.
Background
The brazing technology has the advantages of small welding deformation, high size precision, smooth welding structure and the like. Therefore, the welding method is widely applied to the welding process of the compact components. A large number of experimental researches show that the component is easy to lose effectiveness at the position of a soldered joint under extreme service conditions such as high temperature and high pressure. The braze joint is the weakest part of the overall component, which determines the safety and reliability of the component over long periods of operation. During the brazing process, the wettability of the brazing filler metal, the limitation of the brazing process and the Kirkendall phenomenon generate defects such as micro cracks, holes and the like, which are main inducers of the failure of the brazed joint. In a brazed structure, once such defects occur, the brazed joint develops cracks until the component is broken. Therefore, the method of testing for fracture toughness is critical to understanding the fracture mechanism of braze joints.
Brazing is a thermal joining technique. In the heating process, the liquid brazing filler metal is wetted and spread on the gaps or the surfaces of the base metals, and generates dissolution, diffusion and solidification reactions with the base metals to connect the base metals together. The braze joint can be divided into an isothermal solidification zone, a non-isothermal solidification zone, a diffusion affected zone, and a parent metal according to microstructure distribution. The diffusion influence area contains a large amount of boride, which is formed by the boron element in the brazing filler metal diffusing into the base material and reacting with elements such as chromium, molybdenum and the like in the base material. Failure occurred in the diffusion affected zone in all of the tensile, shear and peel tests. Thus, the diffusion-affected zone is the weakest point in the joint. In addition, the diffusion reaction is only carried out at the interface of tens of microns below the base material, and generally does not involve the deep structure of the base material, so that the micro-area normal temperature mechanical property is difficult to obtain. In conventional fracture toughness experiments, the wire-cut kerf size is much larger than the braze joint, it is difficult to form crack tips at the braze joint, and it is more difficult to obtain reliable performance parameters that resist crack propagation.
Disclosure of Invention
The invention aims to solve the problems of the prior art, provides a sample preparation and calculation method for the fracture toughness of a brazed joint, and aims to solve the technical problems that the brazed joint has crack propagation in a brazed structure until a component is fractured, the micro-area normal-temperature mechanical property is difficult to obtain, a crack tip is difficult to form at the brazed joint, and reliable performance parameters for resisting crack propagation are difficult to obtain.
The application also provides a preparation method of the brazed joint fracture toughness test sample, which comprises the following steps:
aligning and pressing the sample blocks, preparing an upper sample block and a lower sample block which are matched with each other up and down, wherein the butting surfaces of the upper sample block and the lower sample block are rectangles with the same shape and size, the length of one side of each rectangle is 32mm, a first rectangular area and a second rectangular area which are adjacently arranged are arranged in the extending direction of the side of each rectangle, the boundary line of the first rectangular area and the second rectangular area is parallel to the other side, and the area ratio range of the first rectangular area and the second rectangular area is 9: 10 to 1: 1, placing an amorphous foil brazing filler metal in the first rectangular area, coating a brazing flux inhibitor in the second rectangular area, aligning the butt joint surfaces of the upper sample block and the lower sample block, and pressing the upper sample block and the lower sample block; and
and heating in a vacuum brazing furnace, wherein the upper sample block and the lower sample block are pressed and then placed in the vacuum brazing furnace for heating, and a brazed joint fracture toughness sample is prepared.
Further, an adhesive is coated around the amorphous foil brazing filler metal.
Further, the braze joint prepared and formed had initial cracks in the fracture toughness specimens, the initial cracks being located within the braze joint.
Further, the parent material of the block is austenitic stainless steel 316L; the amorphous foil brazing filler metal is made of nickel-based BNi-2 brazing filler metal.
Furthermore, the heating temperature in the vacuum brazing furnace is 1065 ℃, the heating rate is 10 ℃/min, and the heat preservation time is 60min at 1065 ℃.
Further, the heating process in the vacuum brazing furnace comprises the following steps: (1) in the vacuum pumping stage, the vacuum degree is less than 0.008 Pa; (2) heating to 850 deg.C at a heating rate of 10 deg.C/min; (3) keeping the temperature at 850 ℃ for 30 min; (4) heating to 1065 deg.C at 10 deg.C/min; (5) keeping the temperature at 1065 ℃ for 60min to ensure that the amorphous foil brazing filler metal is fully diffused and reacted; (6) and cooling the vacuum brazing furnace to room temperature.
The application also provides a method for calculating the fracture toughness sample of the brazed joint, which is characterized by comprising the following steps of:
preparing a soldered joint fracture toughness test sample by adopting the preparation method of the soldered joint fracture toughness test sample of any one of claims 1 to 6;
a step of processing a brazing joint compact tensile sample, wherein the brazing sample is processed into the compact tensile sample, and a brazing seam and an initial crack are positioned in the middle; specimen thickness B12.7 mm, width W25.4 mm, and initial crack length to specimen width W ratio of 0.5; pre-restricting a fatigue crack of 1.72-8.08mm by using a high-frequency fatigue testing machine, wherein the fatigue crack is expanded in an isothermal solidification area;
nano indentation experiment step of diffusion influence area of the soldered joint, selecting the diffusion influence area in the soldered joint as an action point; loading to a maximum load of 30mN/60mN/90mN/120mN/150mN/180mN at a loading rate of 5 mN/s; carrying for 10s to eliminate the creep effect; unloading to 10% of the maximum load at an unloading rate of 5mN/s to eliminate the influence of temperature; the load is saved for 100S; completely unloading;
calculating the yield strength and the hardening index of the diffusion affected zone of the braze joint, and calculating the yield strength sigma of the diffusion affected zone of the braze joint y And a hardening index n;
a resistance curve experiment step of the soldered joint by a single sample method is carried out on an Instron 8801 hydraulic fatigue testing machine, a loading linear displacement control mode is adopted in the test process, and the loading/unloading speed is 0.5 mm/min; measuring the crack length delta a and the fracture resistance J integral by using an unloading flexibility method through multiple times of loading/unloading to obtain a J-delta a resistance curve; and
calculation of fracture resistance J 0.2BL Step, obtaining a calculation offset passivation line according to the yield strength and the hardening index to determine the fracture resistance J 0.2BL 。
Further, the yield strength σ of the diffusion affected zone of the braze joint was calculated y And a hardening index n, comprising:
(1) fitting parameters according to an unloading curve in a load p-displacement h curve in the unloading process, wherein the fitting range is 20% of the upper part of the unloading curve;
P＝α(h-h f ) m (formula 1)
Wherein: alpha, h f And m is a fitting parameter;
(2) the elastic contact stiffness S and the contact area a are calculated,
wherein, P max Is the maximum load; h is max Is the maximum indentation depth; ε is the indenter shape related constant, ε is 0.75 for the Berkovich indenter; c is a constant of the revised area function, C for Berkovich indenter i A value of about 150 nm;
(3) calculating the modulus of elasticity E and the reduced modulus E r ，
Wherein β is an indenter geometry-related constant, β is 1.034 for the Berkovich indenter; e and v are the modulus of elasticity and Poisson's ratio of the sample material; e i And v i The elastic modulus and the Poisson ratio of the pressure head material are shown, and the values of the diamond are 1140GPa and 0.07;
(4) according to the hardness values under different indentation loads, the hardness value H and the yield strength sigma which are irrelevant to the indentation depth are calculated y ，
H 0 ＝4.15σ y (formula 8)
(5) Calculating the plastic strain epsilon p Stress σ at 0.033 0.033 ，
Wherein the C value is the loading phase P ═ Ch 2 Fitting the obtained loading curvature;
(6) the hardening index n is calculated and the hardening index,
further, a J-delta a resistance curve is obtained according to an unloading flexibility method.
Further, the calculated fracture resistance J 0.2BL The method specifically comprises the following steps:
judging the effectiveness of the J-delta a resistance curve;
the passivation line equation is obtained from the yield strength and the hardening index:
D n ＝0.787+1.554n-2.45n 2 +16.952n 3 -38.206n 4 +33.13n 5 (formula 13)
Obtaining fracture resistance J according to the intersection point of the 0.2mm offset passivation line and the J-delta a resistance curve 0.2BL 。
The invention has the beneficial effects that the preparation and calculation method of the fracture toughness sample of the brazed joint is provided, the sample is prepared by smearing the solder resist agent in the brazing process, and the initial crack tip is positioned in the range of the micro-area of the brazed joint. In the aspect of calculating the fracture toughness, the nanometer indentation size effect is comprehensively considered, and the elastic modulus, the yield strength and the hardening index of a diffusion influence area (on a fracture path) of the brazing joint are calculated. Using a loading linear displacement control mode, by multiple loading/unloading, rootAnd obtaining a J-delta a resistance curve according to an unloading flexibility method. Determination of fracture resistance J based on passivation line equation of power hardening elastoplasticity constitutive 0.2BL . The invention solves the problems that in the conventional fracture toughness experiment, the initial crack size of the current processing mode is far larger than that of a brazed joint, the crack tip is difficult to form at the brazed joint, and reliable performance parameters for resisting crack propagation are difficult to obtain. The invention solves the problems of experimental and analytical methods for measuring the fracture toughness of the soldered joint and is of great importance for further researching the fracture mechanism of the soldered joint.
Drawings
FIG. 1 is a schematic flow chart of a method for preparing a fracture toughness test sample of a brazed joint according to the present application;
FIG. 2 is a schematic flow chart of a calculation method for a fracture toughness sample of a brazed joint according to the present application;
FIG. 3 is a schematic illustration of the brazing sample preparation of the present application;
FIG. 4 is a cross-sectional optical microscope image of a brazed sample of the present application;
FIG. 5 is a graph of the dimensions of a compact tensile specimen of a brazed joint according to the present application;
FIG. 6 is a nano-indentation load-depth curve of the present application;
FIG. 7 is a fracture path of fracture toughness of a braze joint of the present application;
FIG. 8 is a J-R resistance curve for a braze joint according to the present application;
FIG. 9 is an overall schematic diagram of the fracture toughness test of the brazed joint according to the present invention.
In the figure: 1. a sample block is put; 2. brazing filler metal; 3. a solder resist; 4. and (4) loading a sample block.
Detailed Description
The invention is further illustrated below with reference to the accompanying drawings:
the invention provides a method for preparing a fracture toughness sample suitable for a brazing joint micro brazing seam, a resistance curve and fracture resistance J 0.2BL Calculating values, wherein a flow chart of the preparation and calculation method of the brazing joint fracture toughness test piece is respectively shown in fig. 1 and fig. 2.
As shown in FIG. 1, the preparation method of the fracture toughness test sample of the braze welding joint adopts a mode of re-processing the pre-fabricated initial cracks of the solder resist, and can simultaneously prepare q fracture toughness test samples which specifically comprise S11-S12.
S11, a sample block aligning and pressing step, in conjunction with fig. 3, preparing two upper and lower sample blocks 4 and 1 fitted to each other, wherein the abutting surfaces of the upper and lower sample blocks 4 and 1 are rectangles having the same shape and size, the rectangle has a long side and a short side, the length of the short side is 32mm, a first rectangular region (refer to a region corresponding to reference numeral 2 in fig. 3) and a second rectangular region (refer to a region corresponding to reference numeral 3 in fig. 3) are provided adjacently along the extending direction of the short side, the borderlines of the first rectangular region and the second rectangular region are parallel to the long side, and the length ratio range of the first rectangular region and the second rectangular region at the position along the short side is 9: 10 to 1: 1, placing amorphous foil brazing filler metal 2 (hereinafter simply referred to as brazing filler metal) in the first rectangular area, coating solder resist 3 in the second rectangular area, aligning the butt joint surfaces of the upper sample block 4 and the lower sample block 1, so that the amorphous foil brazing filler metal 2 in the first rectangular area and the solder resist 3 in the second rectangular area can be adjacently aligned respectively, and pressing the upper sample block 4 and the lower sample block 1; an adhesive is also applied around the amorphous foil strip solder 2. The parent metal of the block is austenitic stainless steel 316L; the amorphous foil brazing filler metal is made of nickel-based BNi-2 brazing filler metal.
Referring to fig. 3, two 50mm by 16mm by 32mm blocks were prepared, 16.3mm by 32mm size solder resist (Nicrobraz Stop-Off Materials) was applied to the 32mm by 50mm surface side of the blocks, 15.7mm by 50mm BNi-2 amorphous foil strip solder was placed on the surface side, and the two blocks were aligned and placed in a vacuum brazing furnace for heating. In fig. 3, 1 denotes a lower sample block, 2 denotes a brazing material, 3 denotes a solder resist, and 4 denotes an upper sample block. The 32mm 50mm surface of block is the butt joint face, is the rectangle, and the long limit of rectangle is 50mm, and the minor face of rectangle is 32 mm. It will be appreciated that 16mm is the thickness of the block. The short side of the rectangle is preferably 32mm, so that the welding quality can be ensured, and the size of the long side of the rectangle and the thickness of the block body can be set according to actual requirements. The length proportion range of the first rectangular area and the second rectangular area at the position along the short side is 9: 10 to 1: 1, this embodiment is preferably 15.7: 16.3.
the specific process of step S11 is: two blocks of 16mm by 32mm cross section were prepared, q times the thickness of a single fracture toughness specimen. In addition, in order to reduce the solder wettability and the lack of penetration due to the caulking property, the thickness direction needs to be increased by 5 mm. Thus, the overall thickness d (mm) ═ q 12.7+ 5. BNi-2 amorphous foil brazing filler metal with a size of 15.7mm x thickness D was placed on the block surface (32mm x D) side, and an adhesive was added around the brazing filler metal to fix the brazing filler metal in order to prevent the brazing filler metal from slipping. The other side was coated with 16.3mm x D size solder resist (Nicrobraz Stop-Off Materials). And respectively aligning the solder resist and the solder in the two blocks, and after the solder resist and the solder are compressed, putting the blocks into a vacuum brazing furnace for heating.
And S12, heating in a vacuum brazing furnace, wherein the upper sample block 4 and the lower sample block 1 are pressed and then placed in the vacuum brazing furnace for heating, and a brazed joint fracture toughness sample is prepared. The braze joint fracture toughness test specimens prepared and formed had initial cracks that were within the braze joint range. The heating temperature in the vacuum brazing furnace is 1065 ℃, the heating rate is 10 ℃/min, and the heat preservation time is 60min at 1065 ℃.
The heating process of the vacuum brazing furnace comprises the following specific processes: (1) in the vacuum pumping stage, the vacuum degree is less than 0.008 Pa; (2) heating to 850 deg.C at a heating rate of 10 deg.C/min; (3) keeping the temperature at 850 ℃ for 30min to ensure the temperature in the furnace to be stable; (4) heating to 1065 deg.C at 10 deg.C/min; (5) keeping the temperature at 1065 ℃ for 30min to ensure that the brazing filler metal is fully diffused and reacted; (6) cooling to room temperature along with the furnace.
As shown in FIG. 2, the method for calculating the fracture toughness test sample of the braze joint comprises the steps of S1-S6.
S1, preparing a brazing joint fracture toughness test sample, namely preparing the brazing joint fracture toughness test sample by adopting the preparation method of the brazing joint fracture toughness test sample, and specifically comprising the steps S11-S12.
Taking a brazing joint part from a brazing sample and observing the brazing joint part microscopicallyGrinding and polishing the sample by using corrosive liquid (10ml HNO) 3 -10ml C 2 H 4 O 2 -15ml HCl) corrosion, observed under an optical microscope. As shown in fig. 4, the brazed joint is divided into an isothermal solidification zone, a diffusion-affected zone, and a parent metal. Furthermore, the initial cracks are located within the micro-zones of the braze joint.
And S2, a brazing joint compact tensile sample processing step, wherein the brazing sample is processed into a Compact Tensile (CT) sample, so that a brazing seam and an initial crack are positioned in the middle, and the geometric dimension of the specific sample is shown in the attached figure 5. The specimen thickness B was 12.7mm, the width W was 25.4mm, and the ratio of the initial crack length to the specimen width a0/W was about 0.5. The high-frequency fatigue testing machine pre-restricts fatigue cracks of 1.72-8.08mm, and the fatigue cracks are expanded in an isothermal solidification area.
S3, a soldering joint diffusion influence area nano indentation experiment step, a specific step of obtaining the routine mechanical property of the soldering joint diffusion influence area by a nano indentation method: (1) selecting a diffusion influence area in a brazed joint as an action point; (2) loading to a maximum load of 30mN/60mN/90mN/120mN/150mN/180mN at a loading rate of 5 mN/s; (3) carrying for 10s to eliminate the creep effect; (4) unloading to 10% of the maximum load at an unloading rate of 5mN/s to eliminate the influence of temperature; (5) carrying out 100S; (6) and (4) completely unloading. The load-displacement curve obtained by nanoindentation is shown in fig. 6.
S4, calculating the yield strength and the hardening index of the diffusion-affected zone of the braze joint, and calculating the yield strength sigma of the diffusion-affected zone of the braze joint y And a hardening index n.
Wherein the yield strength sigma of the diffusion affected zone of the braze joint is calculated y And a hardening index n, comprising:
(1) fitting parameters according to an unloading curve in a load p-displacement h curve in the unloading process, wherein the fitting range is 20% of the upper part of the unloading curve;
P＝α(h-h f ) m (formula 1)
Wherein: alpha, h f And m is a fitting parameter;
(2) the elastic contact stiffness S and the contact area a are calculated,
wherein, P max Is the maximum load; h is max Is the maximum indentation depth; ε is the indenter shape related constant, ε is 0.75 for the Berkovich indenter; c is a constant of the revised area function, C for Berkovich indenter i A value of about 150 nm;
(3) calculating the modulus of elasticity E and the reduced modulus E r ，
Wherein β is an indenter geometry-related constant, β is 1.034 for the Berkovich indenter; e and v are the modulus of elasticity and Poisson's ratio of the sample material; e i And v i The elastic modulus and Poisson's ratio of the pressure head material are shown, and the values of diamond are 1140GPa and 0.07;
(4) according to the hardness values under different indentation loads, the hardness value H and the yield strength sigma which are irrelevant to the indentation depth are calculated y ，
H 0 ＝4.15σ y (formula 8)
(5) Calculating the plastic strain epsilon p When it is 0.033Stress σ of 0.033 ，
Wherein the C value is the loading phase P ═ Ch 2 Fitting the obtained loading curvature;
(6) the hardening index n is calculated and the result is obtained,
in summary, the yield strength σ in the above-described calculated braze joint diffusion affected zone y And a hardening index n, the step S4 including:
H 0 ＝4.15σ y ；
s5, carrying out resistance curve experiment on the soldered joint by a single sample method, wherein the experiment is carried out in an Instron 8801 hydraulic fatigue testing machine, the test process adopts a loading linear displacement control mode, and the loading/unloading speed is 0.5 mm/min; and measuring the crack length delta a and the integral of the breaking resistance J by using an unloading flexibility method through multiple times of loading/unloading, and obtaining a J-delta a resistance curve according to the unloading flexibility method.
FIG. 7 shows the fracture toughness fracture path where the fatigue pre-crack propagates in the isothermally solidified zone and the fracture toughness test fractures in the diffusion affected zone.
S6, calculating fracture resistance J 0.2BL Step of obtaining a calculated offset passivation line from the yield strength and the hardening index to determine the fracture resistance J 0.2BL 。
Wherein the calculated fracture resistance J 0.2BL Step S6 specifically includes:
firstly, judging the effectiveness of a J-delta a resistance curve;
then, the passivation line equation is obtained by the yield strength and the hardening index:
D n ＝0.787+1.554n-2.45n 2 +16.952n 3 -38.206n 4 +33.13n 5 (formula 13).
As shown in FIG. 8, fracture resistance J was obtained from the intersection of the 0.2mm offset passivation line and the J- Δ a resistance curve 0.2BL 。
FIG. 9 is a schematic diagram of the fracture toughness test of the brazed joint. The invention solves the problems of experimental and analytical methods for measuring the fracture toughness of the soldered joint and is of great importance for further researching the fracture mechanism of the soldered joint; the invention adopts the solder resist to prepare the sample, so that the initial crack tip is positioned in the range of the micro-area of the soldered joint; the method adopts nano indentation to measure normal-temperature mechanical parameters of a diffusion influence area of the soldered joint, including elastic modulus, yield strength and hardening index; the invention adopts a passivation line equation based on a power hardening elastoplasticity constitutive structure to obtain more accurate fracture resistance J 0.2BL 。
The invention has the beneficial effects that the preparation and calculation method of the fracture toughness sample of the brazed joint is provided, the sample is prepared by smearing the solder resist agent in the brazing process, and the initial crack tip is positioned in the range of the micro-area of the brazed joint. In the aspect of calculating the fracture toughness, the nano indentation size effect is comprehensively considered, and the elastic modulus, the yield strength and the hardening index of a diffusion influence area (on a fracture path) of the brazing joint are calculated. And a loading linear displacement control mode is adopted, and a J-delta a resistance curve is obtained through multiple times of loading/unloading according to an unloading flexibility method. Determination of fracture resistance J based on passivation line equation of power hardening elastoplasticity constitutive 0.2BL . The invention solves the problems that in the conventional fracture toughness experiment, the initial crack size of the current processing mode is far larger than that of a brazed joint, and cracks are difficult to form at the brazed jointTip, it is more difficult to obtain reliable performance parameters against crack propagation. The invention solves the problems of experimental and analytical methods for measuring the fracture toughness of the soldered joint and is of great importance for further researching the fracture mechanism of the soldered joint.
Claims (9)
1. A method for calculating a fracture toughness test sample of a brazed joint is characterized by comprising the following steps:
preparing a brazed joint fracture toughness test sample by adopting a preparation method of the brazed joint fracture toughness test sample; the preparation method of the soldered joint fracture toughness test sample comprises the following steps: aligning and pressing the sample blocks, preparing an upper sample block and a lower sample block which are matched with each other up and down, wherein the abutting surfaces of the upper sample block and the lower sample block are rectangles with the same shape and size, the length of one side of each rectangle is 32mm, a first rectangular area and a second rectangular area which are adjacently arranged are arranged in the extending direction of the side, the boundary line of the first rectangular area and the second rectangular area is parallel to the other side, and the area ratio range of the first rectangular area and the second rectangular area is 9: 10 to 1: 1, placing an amorphous foil brazing filler metal in the first rectangular area, coating a brazing flux inhibitor in the second rectangular area, aligning the butt joint surfaces of the upper sample block and the lower sample block, and pressing the upper sample block and the lower sample block; and a heating step of a vacuum brazing furnace, wherein the upper sample block and the lower sample block are pressed and then placed into the vacuum brazing furnace for heating, so as to prepare a brazing joint fracture toughness sample;
a step of processing a brazing joint compact tensile sample, wherein the brazing sample is processed into a Compact Tensile (CT) sample, and a brazing seam and an initial crack are positioned in the middle; the thickness B of the specimen was 12.7mm, the width W was 25.4mm, and the ratio of the initial crack length to the specimen width W was 0.5; pre-restricting fatigue cracks of 1.72-8.08mm by using a high-frequency fatigue testing machine, wherein the fatigue cracks are expanded in an isothermal solidification area;
nano indentation experiment of a diffusion influence area of the soldered joint, wherein the diffusion influence area in the soldered joint is selected as an action point; loading to a maximum load of 30mN/60mN/90mN/120mN/150mN/180mN at a loading rate of 5 mN/s; carrying for 10s to eliminate the creep effect; unloading to 10% of the maximum load at an unloading rate of 5mN/s to eliminate the influence of temperature; carrying out protection for 100 s; completely unloading;
calculating the yield strength and hardening index of the diffusion influence area of the braze joint, and calculating the yield strength sigma of the diffusion influence area of the braze joint y And a hardening index n;
the method comprises the steps of a resistance curve experiment of the soldered joint by a single sample method, wherein the experiment is carried out in an Instron 8801 hydraulic fatigue testing machine, the test process adopts a loading linear displacement control mode, and the loading/unloading speed is 0.5 mm/min; measuring the crack length delta a and the fracture resistance J integral by using an unloading flexibility method through multiple times of loading/unloading to obtain a J-delta a resistance curve; and calculating fracture resistance J 0.2BL Step of obtaining a calculated offset passivation line from the yield strength and the hardening index to determine the fracture resistance J 0.2BL 。
2. The method for calculating the fracture toughness test sample of the brazed joint according to claim 1, wherein an adhesive is further coated around the amorphous foil brazing filler metal.
3. The method for calculating the fracture toughness test sample of the brazing joint as recited in claim 1, wherein the prepared fracture toughness test sample of the brazing joint has an initial crack, and the initial crack is located in the range of the brazing joint.
4. The method for calculating the fracture toughness sample of the braze joint according to claim 1, wherein the base materials of the upper sample block and the lower sample block are austenitic stainless steel 316L; the amorphous foil brazing filler metal is made of nickel-based BNi-2 brazing filler metal.
5. The method for calculating the fracture toughness sample of the brazing joint as recited in claim 1, wherein the heating temperature in the vacuum brazing furnace is 1065 ℃, the heating rate is 10 ℃/min, and the holding time at 1065 ℃ is 60 min.
6. The method for calculating the fracture toughness sample of the brazing joint as recited in claim 1, wherein the heating process in the vacuum brazing furnace comprises the following steps: (1) in the vacuum pumping stage, the vacuum degree is less than 0.008 Pa; (2) heating to 850 deg.C at a heating rate of 10 deg.C/min; (3) keeping the temperature at 850 ℃ for 30 min; (4) heating to 1065 deg.C at 10 deg.C/min; (5) keeping the temperature at 1065 ℃ for 60min to ensure that the amorphous foil brazing filler metal is fully diffused and reacted; (6) and cooling the vacuum brazing furnace to room temperature.
7. The method of calculating the fracture toughness specimen of the braze joint according to claim 1, wherein the yield strength σ of the diffusion affected zone of the braze joint is calculated y And a hardening index n, comprising:
(1) fitting parameters according to an unloading curve in a load p-displacement h curve in the unloading process, wherein the fitting range is 20% of the upper part of the unloading curve;
P＝α(h-h f ) m (formula 1)
Wherein: alpha, h f And m is a fitting parameter;
(2) the elastic contact stiffness S and the contact area a are calculated,
wherein, P max Is the maximum load; h is max Is the maximum indentation depth; ε is the indenter shape related constant, ε is 0.75 for the Berkovich indenter; c is a constant of the revised area function, C for Berkovich indenter i A value of about 150 nm;
(3) calculating the modulus of elasticity E and the reduced modulus E r ，
Wherein β is an indenter geometry-related constant, β is 1.034 for the Berkovich indenter; e and v are the modulus of elasticity and Poisson's ratio of the sample material; e i And v i The elastic modulus and Poisson's ratio of the pressure head material are shown, and the values of diamond are 1140GPa and 0.07;
(4) calculating to obtain a hardness value H irrelevant to the indentation depth according to hardness values under different indentation loads 0 And yield strength σ y ，
H 0 ＝4.15σ y (formula 8)
Wherein h is the indentation depth; h is a hardness value; h is * Describing the relation of the hardness value along with the depth of the indentation as the characteristic length;
(5) calculating the plastic strain epsilon p Stress σ at 0.033 0.033 ，
Wherein the C value is the loading phase P ═ Ch 2 Fitting the obtained loading curvature;
(6) the hardening index n is calculated and the hardening index,
8. the method for calculating the fracture toughness specimen of the brazed joint according to claim 1, wherein a J- Δ a resistance curve is obtained according to an unloading compliance method.
9. The method of calculating the braze joint fracture toughness specimen of claim 1, wherein the calculated fracture resistance J 0.2BL The method specifically comprises the following steps:
judging the effectiveness of the J-delta a resistance curve;
the passivation line equation is obtained from the yield strength and the hardening index:
D n ＝0.787+1.554n-2.45n 2 +16.952n 3 -38.206n 4 +33.13n 5 (formula 13)
Obtaining fracture resistance J according to the intersection point of the 0.2mm offset passivation line and the J-delta a resistance curve 0.2BL 。
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