CN110849739A - Mixed type fatigue test method - Google Patents
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Abstract
The invention relates to a fatigue life test method of a material, in particular to a hybrid fatigue test method capable of simultaneously carrying out common fatigue and fretting fatigue of two different contact types (conformal contact and non-conformal contact). The mixed type fatigue test device comprises a chuck, a test piece, a pin shaft and a clamp spring, wherein a dovetail-shaped through hole and a circular through hole are respectively formed in two ends of the test piece in the length direction; the invention integrates a common fatigue test piece and two fretting fatigue test pieces with different contact types, and completes a mixed fatigue test through a single test; the auxiliary correction device is used for accurately positioning and correcting the test piece; the invention can simulate the bending load born by the actual component in the service process, more accurately reproduce the common fatigue and fretting fatigue phenomena of the component, reduce the dispersion of the fatigue test result and improve the rationality and scientificity of the test.
Description
Technical Field
The invention relates to a fatigue life test method of a material, in particular to an integral hybrid fatigue (common fatigue and fretting fatigue) test and a method capable of simultaneously carrying out common fatigue and fretting fatigue of two different contact types (conformal contact and non-conformal contact).
Background
Fatigue causes the component to break and fail at stress levels far below its structural strength, seriously affecting the service life and service safety of the component, and is the main reason for the greatly reduced service life of the component, which is called "industrial cancer". Before the train of clothes is in service, the critical parts need to be subjected to fatigue life tests of materials, and the fatigue life tests are used for guiding the structural design of the component, the formulation of a machining process and the selection of a surface strengthening technology and accurately predicting the fatigue life of the component. For a common fatigue test, the test equipment and method are relatively fixed, and corresponding national standards are provided for reference, such as GB/T6398-. In addition, a common fatigue testing machine for bending-twisting multi-axis loading is disclosed in patent CN103900916A, wherein a stretching component, a bending component and a twisting component are combined together to realize the bending-twisting multi-axis fatigue test; patent CN103471843A discloses a bending multi-axis fatigue test mechanism, which applies axial tensile load to a sample, applies bending vibration load to a test sample by a vibration exciter at the same time, realizes the loading of the bending load, and can only perform common fatigue test under the bending load; patent US6718833B2 discloses a high frequency fatigue tester, but using three separate actuators to generate the tension-bending-torsion loads. However, for the fretting fatigue test, a standardized test method is not available, so that the development of the fretting fatigue test lacks a basis, and the test result has great dispersity. For fretting fatigue tests, the test devices widely used in the existing documents can be classified into three types according to the contact type of the fretting block and the test piece: bridge (plane-to-plane contact or cylinder-to-plane contact), single-contact (cylinder-to-plane contact or cylinder-to-cylinder contact), and grab (cylinder-to-plane contact). The contact types in the three fretting fatigue test devices described above are simplified in order to satisfy the hertzian contact theory. However, in practice, most of the component connections are conformal contacts that do not satisfy the hertzian theory of contact, such as press-fit or shrink-fit joints, rivet joints, bolted joints, axle assemblies, gear shafts, and spline shafts. The patent CN104297046A discloses a multi-axis fretting fatigue test device for steel wires, which can perform multi-axis fretting fatigue tests of the steel wires in composite motion modes of pulling-pulling fatigue, torsion, variable cross angle swing and the like; patent CN108627389A discloses a gear driven load-loaded fatigue tester capable of independently applying bending and torsion loads. However, in the existing fretting fatigue testing device and the related patents, before the test, the contact state between the fretting block and the test piece needs to be carefully aligned and adjusted to ensure good contact between the fretting block and the test piece. In addition, in the existing literature, the research on the fatigue failure problem of the component is mainly to respectively and independently research the common fatigue problem and the fretting fatigue problem after the load borne by the component in the actual service process is stripped and simplified. However, in the real service process of the component, the common fatigue problem and the fretting fatigue problem of the same component are frequently faced. However, there are few studies on such a hybrid fatigue combining ordinary fatigue and fretting fatigue and developments of corresponding test devices.
In summary, the prior art disclosed in the above patent and literature has disadvantages that ① existing fretting fatigue test researches are mostly only directed at simplified contact types, such as cylinder-plane contact, plane-plane contact and cylinder-cylinder contact, and do not conform to the common contact form of members widely existing in practical application, ② does not have a more specific testing device and testing method for researching conformal contact fretting fatigue, and is also lack of a fretting fatigue device for comparing different contact types, ③ ordinary fatigue test and fretting fatigue test are separately studied, neglecting the problems of ordinary fatigue and fretting fatigue of the same member in service of an actual member, resulting in a larger difference between the test result and the actual result, ④ existing fretting fatigue device needs to carefully adjust the contact surface state of a fretting block and a test piece before fatigue test, and the dispersibility of the test result is large due to the difference and the reliability of the test device.
Disclosure of Invention
The invention aims to solve the technical problem of providing a mixed fatigue test device and a method, which utilize a load vibration exciter of a common fatigue test machine to realize common fatigue and fretting fatigue of two different contact forms of a component under the loading of bending load, simulate the loading condition of the component in a service environment, simultaneously obtain the mixed fatigue test result of the combination of the common fatigue and the two fretting fatigue through single loading, and reveal which type of fatigue and which type of contact can cause the component to firstly generate fatigue failure under the same test environment, the same test condition and the same test parameters.
In order to solve the technical problems, the invention adopts the following technical scheme:
① constructing a hybrid fatigue testing device;
the hybrid fatigue test apparatus includes: the fatigue testing machine comprises a fatigue testing machine body, two chucks, a test piece, a pin shaft with a clamp spring groove and two clamp springs, wherein the fatigue testing machine body is provided with an upper fixing groove and a lower fixing groove, two ends of each chuck are respectively arranged between the upper fixing groove and the lower fixing groove of the fatigue testing machine body, and the number of the chucks is 2 and the chucks are symmetrically distributed;
the upper end of the test piece is provided with a dovetail hole, and the lower end of the test piece is provided with a round hole; the upper end of the test piece is arranged in a U-shaped groove of a chuck, and the pin shaft penetrates through a through hole and a dovetail hole in a lug of the chuck and is arranged on the chuck; the lower end of the test piece is arranged in a U-shaped groove of the other chuck, and the pin shaft penetrates through a through hole and a round hole in a lug of the chuck and is arranged on the other chuck; the clamp spring is arranged in the clamp spring groove along the axial direction;
the test piece is a plate-shaped part, a connecting handle is arranged between the dovetail hole and the round hole, the width of the middle part of the connecting handle is smaller than the width of two ends of the test piece, the connecting handle is respectively connected with the dovetail hole and the round hole and a smooth circular arc, and the dovetail hole and the round hole are both positioned on the center line of the test piece in the width direction;
② establishing an auxiliary correction device and a crack initiation and propagation monitoring system;
③ determining a hybrid fatigue test method based on the established hybrid fatigue test device.
The step ② includes that the auxiliary correction device comprises a base, two upright seats, a leaf spring, a correction block and a screw, the bottoms of the upright seats are arranged on the base through the screw, the upright seats are oppositely arranged, a first rectangular groove is formed in the inner end face of each upright seat, two third rectangular grooves are formed in the bottom of each first rectangular groove, a second rectangular groove is formed in the outer end face of each upright seat, the first rectangular groove, the second rectangular groove and the third rectangular grooves are intersected in the upright seats, the leaf spring penetrates through the first rectangular groove and is respectively arranged in the two third rectangular grooves, the correction block penetrates through the second rectangular groove and is arranged in the intersection area of the second rectangular groove and the first rectangular groove, the micro-mirror crack initiation and propagation monitoring system comprises a high-multiple digital display and a computer, and the auxiliary correction device, the high-multiple micro-mirror and the computer jointly form a monitoring system.
Said step ③ includes:
selecting an electromagnetic vibration type fatigue testing machine and setting testing conditions, wherein the maximum peak value of an externally-applied cyclic load cannot exceed the material strength limit of a weak part of a test piece, and setting the load ratio to be greater than 0;
secondly, starting a test and setting a test stopping standard, and stopping the test when the fatigue fracture of the test piece occurs or the maximum test cycle number is exceeded;
thirdly, analyzing the micro-crack morphology and the fatigue crack macro-morphology by using a white light interferometer, shooting a fatigue fracture by using a scanning electron microscope, analyzing the fracture morphology characteristics and determining the fatigue fracture type;
fourthly, considering the internal stress of the test piece and the load distribution of the contact area, analyzing the load distribution characteristics of the common fatigue stress, the dovetail hole contact area and the round hole contact area, and establishing the connection with the expansion of the fatigue crack of the test piece and the formation of the fracture morphology, wherein the stress and the load distribution are carried out according to the following mathematical models:
the connecting handle of the test piece is a common fatigue test section, the cross section of the test piece bears uniformly distributed axial load, the uniformly distributed load on the unit area is the internal stress sigma value, and the value is expressed as follows:
σ＝P/SA(A)
In the formula (I), P is an external load, SAThe cross section area of the middle narrow part of the test piece;
for fretting fatigue of two different contact types, the load distribution of the dovetail holes and the contact area of the round hole and the pin shaft is as follows:
in the dovetail hole region, the contact type of round pin axle and dovetail hole both sides surface is cylinder-plane contact, satisfies the hertz's contact condition, and the load in contact region can simplify:
the contact peak stress was found to be:
in the formulas (II) and (III), α is the half cone angle value of the dovetail hole, E1V and v1Young's modulus and Poisson's ratio, E, respectively, of the material of the test piece2V and v2Young's modulus and Poisson's ratio, P, of the material of the pin shaft, respectivelynAnd PtNormal and tangential loads, R, respectively, on the contact surfaces of the dovetail holes0The radius of the middle part of the pin shaft;
in the round hole region, round pin axle and round hole are conformal contact, do not satisfy the hertz contact condition, and the contact load distribution in the contact region is:
in the formula (IV), y is an unknown number related to the spatial position of the conformal contact region, and B and B are respectively:
in the formula (VI), the half contact angle of the epsilon conformal contact region, E*For effective modulus of elasticity, in plane strain
Under plane stress of
Wherein i is 1 and 2, which respectively represent the material property parameters of the test piece and the pin shaft, and △ R is the difference between the radii of the round hole and the pin shaft.
Compared with the prior art, the invention has the following advantages: according to the invention, through the proposed hybrid fatigue test device, bending load borne, micro-motion phenomenon and fatigue phenomenon generated in the actual component service process are simulated, the common fatigue and micro-motion fatigue phenomenon of the component can be accurately reproduced, the test precision of the fatigue life is improved, and the dispersity of the test result is reduced; the requirements of a common fatigue test on a test piece are met, two fretting fatigue tests in different contact forms can be realized, the common fatigue test piece and the fretting fatigue test piece are integrated, a mixed type fatigue test can be completed through a single test, and the different types of fatigue failure problems of the component and which type of fatigue is the main cause of fatigue failure are conveniently researched; the invention meets the standard common fatigue test requirement through the design of the connecting handle on the test piece, and can realize the comparison between common fatigue and two fretting fatigue problems (dovetail holes and round holes); through the dovetail hole and round hole design on the test piece, a cylindrical-plane contact form which meets Hertz contact and a pin-hole conformal contact form which does not meet Hertz contact are simulated, and two types of fretting fatigue problems are introduced into the same test to be used for comparing the fretting fatigue problems of different contact types, so that which type of contact type of fretting fatigue is the main cause of fretting fatigue failure of the component is determined; according to the invention, through the design of the dovetail holes and the round holes, the reliable self-centering contact of the pin shafts with the dovetail holes and the round holes is realized by utilizing the special geometric self-centering characteristic of the dovetail holes and the round holes, the contact state does not need to be carefully adjusted before the test, the dispersity of the test result is reduced, and the reliability of the test is improved; according to the invention, through the design of the clamp spring groove and the clamp spring, the precision is ensured, and meanwhile, the reliable limit is realized; according to the invention, through the design of the auxiliary correction device, the accurate positioning and shape correction of the test piece in the axial direction of the hole are ensured, and the accuracy of the fatigue life is improved; according to the invention, through a crack initiation and propagation monitoring system, the crack initiation and propagation is observed in real time, and dynamic test evidence is provided for later analysis and research on the initiation and propagation of fretting fatigue cracks through computer video recording; the invention starts from the internal stress of the test piece and the load distribution of the contact area, analyzes the characteristics of the common fatigue stress, the load distribution of the dovetail hole contact area and the circular hole contact area, and establishes the connection with the fatigue failure of the test piece, thereby obtaining the fatigue type and the influence factors thereof which lead the mixed fatigue fracture of the test piece, and providing test and theoretical support for the research and prediction of the fatigue life, the optimization of the process parameters and the selection of the component surface strengthening method.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
FIG. 1 is a schematic view of the installation of the device of the present invention and a fatigue testing machine;
FIG. 2 is an overall exploded view of the apparatus of the present invention;
FIG. 3 is an overall installation view of the apparatus of the present invention;
FIG. 4 is a general half-section and partial cross-sectional view of the apparatus of the present invention;
FIG. 5 is a schematic diagram of a test piece of the apparatus of the present invention;
FIG. 6 is a schematic view of the Y-clamp of the apparatus of the present invention;
FIG. 7 is a schematic view of the pin structure of the device of the present invention;
FIG. 8 is a schematic view of an auxiliary calibration device of the apparatus of the present invention;
FIG. 9 is a cross-sectional view of an auxiliary alignment device of the apparatus of the present invention;
FIG. 10 is a flow chart of a test method corresponding to the apparatus of the present invention;
FIG. 11 is a graph of contact stress distribution for an embodiment of the apparatus of the present invention;
in the figure: a fatigue testing machine, B invention device, A1 upper fixed groove, A2 lower fixed groove, 1 chuck2 test pieces, 3 pin shafts, 4 clamp springs, 5 auxiliary correction devices, 501 bases, 502 vertical seats, 502a first rectangular groove, 502b second rectangular groove, 502c third rectangular groove, 503 leaf springs, 504 correction blocks, 505 screws, 6 high-magnification digital microscope, 1a through hole, 2a dovetail hole, 2b connecting handle, 2c round hole, 3a pin shaft middle part, 3b clamp spring groove, α dovetail hole half-cone angle, P external loadnContact normal load, PtContact tangential load, R0The diameter of the pin shaft.
Detailed Description
The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without any inventive step, are within the scope of the present invention.
① hybrid fatigue test device:
as shown in fig. 1, the device part B of the present invention is directly installed between the upper fixing groove a1 and the lower fixing groove a2 of the fatigue testing machine a. As shown in fig. 2, 3, 4, 5, 6 and 7, the hybrid fatigue test device comprises a chuck 1, a test piece 2, a pin shaft 3 and a clamp spring 4, wherein the chuck 1 is Y-shaped and is provided with two, and two lugs of the Y-shaped chuck 1 are coaxially provided with through holes 1 a; the test piece 2 is a plate-shaped part, two ends of the test piece in the length direction are respectively provided with a through dovetail hole 2a and a through round hole 2c, the middle part of the test piece 2 in the width direction is thin and narrow, the connecting handle 2b is connected with a smooth circular arc, and the dovetail hole 2a and the round hole 2c are both positioned on the center line of the test piece 2 in the width direction; the pin shaft 3 is an optical shaft, the diameter of the middle part 3a of the pin shaft is smaller than that of the two end parts, and two ends of the pin shaft 3 are respectively provided with a clamp spring groove 3 b; the clamp spring 4 is a standard part; the head of the chuck 1 is respectively clamped between an upper fixing groove A1 and a lower fixing groove A2 of a fatigue testing machine A, and the test piece 2 is respectively coaxial with a through hole 1a on a lug of the chuck 1 through a dovetail hole 2a and a round hole 2c on the test piece and is arranged in a U-shaped groove of the chuck 1; after the pin shaft 3 sequentially passes through the left lug-dovetail hole 2a or the round hole 2 c-right lug of the chuck 1, the clamp springs 4 are respectively arranged in the clamp spring grooves 3a from the two end faces of the pin shaft 3 along the axial direction of the pin shaft; in the test process, the middle part 3a of the pin shaft is respectively contacted with the dovetail hole 2a and the round hole 2c to form a micro-motion matching pair.
② auxiliary correction and crack initiation and propagation monitoring system:
as shown in fig. 8 and 9, the device of the present invention further includes a set of test piece centering auxiliary correction device 5 and a crack initiation and propagation monitoring system, where the auxiliary correction device 5 includes a base 501, two upright seats 502, a leaf spring 503, a correction block 504 and screws 505, the bottom of the upright seat 502 is disposed on the base 501 through the screws 505; two opposite surfaces of the two vertical seats 502 are respectively provided with a first rectangular groove 502a after being installed, the bottom of the first rectangular groove 502a is provided with two third rectangular grooves 502c, and the outer end surface of the vertical seat 502 is provided with a second rectangular groove 502 b; the first rectangular groove 502a, the second rectangular groove 502b and the third rectangular groove 502c intersect inside the vertical seat 502, and the leaf spring 503 is respectively arranged in the two third rectangular grooves 502c after passing through the first rectangular groove 502 a; the correcting block 504 penetrates into the second rectangular groove 502b and is arranged in the intersection area of the second rectangular groove 502b and the first rectangular groove 502 a; the crack initiation and propagation monitoring system comprises a high-power digital microscope 6 and a computer; the auxiliary correction device, the high-multiple digital microscope 6 and the computer together form a monitoring system.
The auxiliary correcting device 5 is used for accurately positioning and correcting the test piece 2 in the axial direction of the hole before applying the fatigue load, and removing the auxiliary correcting device 5 after the application of the preload is finished; the auxiliary correcting device 5 can realize the correction of the test pieces 2 with different thicknesses through the deformation control of the leaf spring 503; the high-power digital microscope 6 can observe crack initiation and expansion in real time and record video in real time through a computer.
③ A hybrid fatigue test method is proposed based on the established hybrid fatigue test apparatus, and the influence of the load distribution on the fatigue life is studied based on the load distribution in the contact region:
the method and flow of the present invention are shown in FIG. 10. The working principle of the method is as follows: the test piece 2 and the pin shaft 3 are processed through different processing parameters, the processing quality of each plane of the test piece 2 meets the requirement of the national standard on a common fatigue test piece, and the processing parameters of the dovetail hole 2a, the round hole 2c and the pin shaft 3 of the test piece 2 are selected according to different surface qualities in a fretting fatigue test. After the test piece 2 and the pin shaft 3 are processed, the testing device is installed on a fatigue testing machine A through the chuck 1 and the clamp spring 4, corresponding testing environment, testing temperature and testing conditions are configured, fatigue load, load ratio and fatigue testing frequency are set, and fatigue testing is conducted. And observing the fatigue fracture failure characteristics of the test piece through given test conditions, including the fatigue fracture position and the fatigue fracture type. The method is used for simulating the fatigue failure problem and hidden danger of the mapping component in the real service environment. And (3) an improvement guide is selected for structural design, optimized improvement of the processing process and surface strengthening process of the component.
Firstly, the hybrid fatigue testing device and corresponding detection and measurement equipment provided by the invention are installed in place, the resonance frequency of the testing device can reach 70Hz, and an electromagnetic vibration type fatigue testing machine is selected to shorten the testing time;
secondly, setting test conditions, wherein the maximum peak value of the applied cyclic load cannot exceed the material strength limit of the weak part of the test piece, and the load ratio is larger than 0 to prevent the device from generating impact vibration;
thirdly, starting the test, and stopping the test after the test piece 2 is subjected to fatigue fracture or exceeds the set maximum test cycle number;
fourthly, analyzing the micro-crack morphology and the fatigue crack macro-morphology by using a white light interferometer, shooting the fatigue crack by using a scanning electron microscope, analyzing the fracture morphology characteristics and determining the fatigue fracture type;
and fifthly, analyzing a fatigue fracture mechanism of the mixed fatigue, starting from the internal stress of the test piece 2 and the load distribution of the contact area, analyzing the load distribution characteristics of the common fatigue stress, the contact area of the dovetail hole 2a and the contact area of the round hole 2c, establishing a connection with the expansion of the fatigue crack of the test piece 2 and the formation of the fracture morphology, wherein the stress and the load distribution are carried out according to the following mathematical model:
for ordinary fatigue, the stress value σ of the region of the shank 2b of the test piece 2 is:
σ＝P/SA(A)
In the formula (I), P is an external load, SAThe cross-sectional area of the narrow portion.
For fretting fatigue of two different contact types, the load distribution of the dovetail hole 2a and round hole 2c contact area is:
in the forked tail hole 2a region, the contact type of round pin axle 3 and forked tail hole 2a both sides surface is cylinder-plane contact, satisfies the hertz's contact condition, and the load in contact area can simplify:
to find, contact peak stress:
in the formulas (II) and (III), α represents the half cone angle value of the dovetail hole 2a, E1V and v1Young's modulus and Poisson's ratio, E, respectively, of the material of test piece 22V and v2Young's modulus and Poisson's ratio, P, of the material of the pin shaft 3, respectivelynAnd PtNormal and tangential loads, R, respectively, on the contact surfaces of the dovetail holes 2a0The radius of the middle part of the pin shaft 3;
in the circular hole 2c area, the pin shaft 3 is in conformal contact with the circular hole 2c, and does not meet the Hertz contact condition. The contact load distribution in the contact area is:
in the formula (IV), y is an unknown number related to the spatial position of the conformal contact region, and B and B are respectively:
in the formula (VI), the half contact angle of the epsilon conformal contact region, E*For effective modulus of elasticity, in plane strain
Under plane stress of
Where i is 1 and 2, which respectively represent the material property parameters of the test piece 2 and the pin 3, and △ R is the difference between the radii of the circular hole 2c and the pin 3.
Fifthly, verifying the action mechanism of stress and load distribution on crack propagation by combining an extended Finite element method XEFEM (extended finish Elements method) and simulating the crack propagation phenomenon through numerical values;
and sixthly, establishing the influence of different load distributions on the fatigue fracture behavior according to the results of the fifth step and the sixth step, and obtaining the fatigue type and the influence factors of the mixed fatigue fracture of the leading test piece 2.
The specific embodiment is as follows:
in order to verify the test device and the method provided by the invention, six sets of fatigue tests under different milling parameters are carried out in a room temperature laboratory environment, the obtained results are shown in table 1, the material of the test piece 2 is TC4, the material of the pin shaft 3 is PH13-8Mo, the half cone angle α of the dovetail hole 2a is 20 degrees, the diameter of the round hole 2c is 13mm, the diameter of the pin hole 3 is 12mm, and the fatigue test parameters are that the maximum external load is 5kN, the stress ratio is 0.1 and the frequency is 73 Hz.
TABLE 1 fatigue test results
Comparing the experimental results of three different fatigue types, all the fatigue samples with fracture positions are located at the round hole at the lower end, namely, the fatigue fracture occurs in the area of the conformal contact (the round hole 2 c).
In the axial direction of conformal contact, fretting fatigue cracks are located within and throughout the partial slip zone of fretting wear. From this, it is understood that the test piece finally fractured due to fretting fatigue. Microcracks initiate in the partial slip region of fretting wear and gradually propagate until complete fracture occurs. That is, in the hybrid fatigue, the fretting motion is a main cause of fatigue fracture of the test piece. As can be seen from the fatigue life results in table 1, the number ratio of the fatigue lives does not exceed 1 order of magnitude, the fatigue life distribution is concentrated, and the degree of dispersion is low.
The novel fretting fatigue crack characteristics are revealed by the test device and the test method provided by the invention. The crack initiates near the lowermost point of contact and propagates in the direction of the applied load until finally breaking. The propagation path of the crack is generally spoon-shaped and can be divided into three stages: a first stage in which the crack propagates in a direction at about 45 ° to a tangential plane on the circumference of the initiation point and has a length of about 250 μm; in the second stage, after the crack propagation direction is gradually rotated by an angle along the counterclockwise direction, the crack is continuously expanded by about 300 mu m; and in the third stage, the crack propagation direction is changed again, and the crack is rotated by another angle gradually in the clockwise direction and continues to propagate until the fatigue fracture occurs. The crack propagation direction at this stage is perpendicular to the tangent plane at the lowest point of the conformal contact location.
In the contact area between the dovetail hole 2a and the circular hole 2c and the pin shaft 3, the contact stress distribution is as shown in fig. 11. It can be found from the test results that cracks are not generated in the region of the non-conformal contact (dovetail hole 2a) where the contact load and stress value are large, but are initiated and propagated in the conformal contact region (round hole 2c) where the contact load and stress value are relatively small. The reason for this is that the large contact stress at the dovetail hole 2a causes severe plastic deformation of the contact surface of the test piece 2, and strain hardening occurs, so that the hardness of the contact area increases, and initiation and expansion of microcracks are hindered. In addition, at the dovetail hole 2a, the contact area is mainly subjected to a compressive load due to the bending load, so that the microcracks in the contact area are closed, and therefore the microcracks do not spread.
According to the embodiment, when the component is subjected to multiple fatigue types in the service process, the component mainly generates fretting fatigue fracture failure at the conformal contact part, and the effect of strain hardening on preventing crack initiation and crack propagation is revealed. Meanwhile, the hybrid fatigue test device and the hybrid fatigue test method provided by the invention effectively reduce the dispersity of fretting fatigue test results, and have important significance and application value for the research on the fatigue failure problem of the component.
The embodiments described above are only preferred embodiments of the invention and are not exhaustive of the possible implementations of the invention. Any obvious modifications to the above would be obvious to those of ordinary skill in the art, but would not bring the invention so modified beyond the spirit and scope of the present invention.
Claims (3)
1. A mixed fatigue test method is characterized by comprising the following steps:
① constructing a hybrid fatigue testing device;
the hybrid fatigue test apparatus includes: the device comprises a fatigue testing machine (A), two chucks (1), a test piece (2), a pin shaft (3) provided with a clamp spring groove (3b) and a clamp spring (4), wherein the chucks (1) are Y-shaped and provided with two clamp spring grooves (3b), the fatigue testing machine (A) is provided with an upper fixing groove (A1) and a lower fixing groove (A2), two ends of each chuck (1) are respectively arranged between the upper fixing groove (A1) and the lower fixing groove (A2) of the fatigue testing machine (A), and the number of the chucks (1) is 2 and the chucks are symmetrically distributed;
the upper end of the test piece (2) is provided with a dovetail hole (2a), and the lower end is provided with a round hole (2 c); the upper end of the test piece (2) is arranged in a U-shaped groove of the chuck (1), and the pin shaft (3) penetrates through a through hole (1a) and a dovetail hole (2a) on a lug of the chuck (1) and is arranged on the chuck (1); the lower end of the test piece (2) is arranged in a U-shaped groove of the other chuck (1), and the pin shaft (3) penetrates through a through hole (1a) and a round hole (2c) on a lug of the chuck (1) and is arranged on the other chuck (1); the clamp spring (4) is arranged in the clamp spring groove (3b) along the axial direction;
the test piece (2) is a plate-shaped part, a connecting handle (2b) is arranged between the dovetail hole (2a) and the round hole (2c), the width of the middle part of the connecting handle (2b) is smaller than the width of the two ends of the test piece (2), the connecting handle (2b) is respectively connected with the dovetail hole (2a) and the round hole (2c) and a smooth circular arc, and the dovetail hole (2a) and the round hole (2c) are both positioned on the central line of the test piece (2) in the width direction;
② establishing an auxiliary correction device (5) and a crack initiation and propagation monitoring system;
③ determining a hybrid fatigue test method based on the established hybrid fatigue test device.
2. The method of claim 1, wherein the step ② is performed by:
the auxiliary correction device (5) comprises: the correcting device comprises a base (501), a vertical seat (502), a leaf spring (503), a correcting block (504) and screws (505), wherein the bottom of the vertical seat (502) is arranged on the base (501) through the screws (505), and the two vertical seats (502) are arranged oppositely; a first rectangular groove (502a) is formed in the inner end face of the vertical seat (502), two third rectangular grooves (502c) are formed in the bottom of the first rectangular groove (502a), and a second rectangular groove (502b) is formed in the outer end face of the vertical seat (502); the first rectangular groove (502a), the second rectangular groove (502b) and the third rectangular grooves (502c) are intersected in the vertical seat (502), and the leaf spring (503) penetrates through the first rectangular groove (502a) and then is respectively arranged in the two third rectangular grooves (502 c); the correcting block (504) penetrates into the second rectangular groove (502b) and is arranged in the intersection area of the second rectangular groove (502b) and the first rectangular groove (502 a); the crack initiation and propagation monitoring system comprises a high-power digital microscope (6) and a computer; the auxiliary correction device (5), the high-multiple digital microscope (6) and the computer jointly form a monitoring system.
3. The method of claim 1, wherein said step ③ comprises:
firstly, selecting an electromagnetic vibration type fatigue testing machine and setting testing conditions, wherein the maximum peak value of an externally-applied cyclic load cannot exceed the material strength limit of a weak part of a test piece (2), and setting the load ratio to be more than 0;
secondly, starting the test and setting a test stopping standard, and stopping the test when the fatigue fracture of the test piece (2) occurs or the number of test cycles exceeds a set maximum test cycle number;
thirdly, analyzing the micro-crack morphology and the fatigue crack macro-morphology by using a white light interferometer, shooting the fatigue fracture by using a scanning electron microscope, analyzing the fracture morphology characteristics and determining the fatigue fracture type;
fourthly, the internal stress of the test piece (2) and the load distribution of the contact area are considered, the common fatigue stress, the load distribution characteristics of the contact area of the dovetail hole (2a) and the contact area of the round hole (2c) are analyzed, the connection between the common fatigue stress and the expansion of the fatigue crack of the test piece (2) and the formation of the fracture morphology is established, and the stress and the load distribution are carried out according to the following mathematical model:
the connecting handle (2b) of the test piece (2) is a common fatigue test section, the cross section of the test piece bears uniformly distributed axial load, the uniformly distributed load on the unit area is the internal stress sigma value, and the value is expressed as follows:
σ＝P/SA(A)
In the formula (I), P is an external load, SAThe cross section area of the middle narrow part of the test piece;
for fretting fatigue of two different contact types, the load distribution of the contact area of the dovetail hole (2a) and the round hole (2c) with the pin shaft (3) is as follows:
in the dovetail hole (2a) area, the contact type of the pin shaft (3) and the surfaces on the two sides of the dovetail hole (2a) is cylindrical-plane contact, the Hertz contact condition is met, and the load of the contact area can be simplified as follows:
the contact peak stress was found to be:
in the formulas (II) and (III), α is the half cone angle value of the dovetail hole (2a),E1v and v1Young's modulus and Poisson's ratio, E, respectively, of the material of the test piece (2)2V and v2Young's modulus and Poisson's ratio, P, of the material of the pin shaft (3), respectivelynAnd PtNormal and tangential loads, R, respectively on the contact surfaces of the dovetail holes (2a)0Is the radius of the middle part of the pin shaft (3);
in round hole (2c) region, round pin axle (3) and round hole (2c) are conformal contact, do not satisfy the hertz's contact condition, and the contact load distribution in the contact region is:
in the formula (IV), y is an unknown number related to the spatial position of the conformal contact region, and B and B are respectively:
in the formula (VI), the half contact angle of the epsilon conformal contact region, E*For effective modulus of elasticity, in plane strain
Under plane stress of
Wherein i is 1 and 2, which respectively represent the material property parameters of the test piece (2) and the pin shaft (3), and △ R is the difference between the radiuses of the round hole (2c) and the pin shaft (3).
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