CN110967267A - A Test Method for Determining Fatigue Crack Initiation Life - Google Patents
A Test Method for Determining Fatigue Crack Initiation Life Download PDFInfo
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Abstract
本发明公开了一种判定疲劳裂纹萌生寿命的试验方法，为确定材料在疲劳试验过程中的裂纹萌生寿命提供了一种解决方案。本发明包括一种判定疲劳裂纹萌生寿命的试验方法和一种疲劳试验过程中标准试件的预处理方法。本发明采用逆向推理方法实现疲劳裂纹萌生寿命的预测，其优点是仅用少数样本数据即可完成微观裂纹萌生寿命的预估，为实现疲劳裂纹萌生寿命预测提供了一种简单、易行、可操作的试验方法，在疲劳试验过程中可节约大量的人力、物力和财力。
The invention discloses a test method for determining the fatigue crack initiation life, and provides a solution for determining the crack initiation life of a material in the fatigue test process. The invention includes a test method for judging the initiation life of fatigue cracks and a pretreatment method for standard test pieces in the fatigue test process. The invention adopts the reverse reasoning method to realize the prediction of the initiation life of fatigue cracks, and has the advantage that the prediction of the initiation life of microscopic cracks can be completed with only a few sample data, and provides a simple, feasible and feasible method for realizing the prediction of the initiation life of fatigue cracks. The operational test method can save a lot of manpower, material and financial resources in the fatigue test process.
Description
Technical Field
The invention relates to the technical field of fatigue performance testing of materials, in particular to a test method for judging fatigue crack initiation life.
Background
Fatigue is an important cause of fracture failure of mechanical structural materials, and statistically, more than 80% of fracture failures of mechanical structures are caused by fatigue failure. Even under low stress cyclic loading, the material still suffers from fatigue fracture. Fatigue-induced accidents are often catastrophic because the stresses experienced by a material during fatigue failure are much less than the yield strength of the material, and there is no significant deformation or damage before fatigue failure occurs. During fatigue fracture failure of a material, the fatigue crack usually needs to go through an initiation stage, a propagation stage and a transient fracture stage of the crack, wherein the transient fracture stage has a very small and negligible fatigue life during the fatigue failure, so that the fatigue life of the material is mainly determined by the initiation and propagation life of the crack. A great deal of research shows that the initiation process of the fatigue crack usually occupies most of the fatigue life, but how to determine the initiation life of the fatigue crack still lacks an effective test method for quantitative verification. In order to solve the problems, the invention provides a test method for judging fatigue crack initiation life.
Disclosure of Invention
In the process of fatigue fracture failure of the material, the formation and evolution processes of fatigue cracks are extremely difficult to monitor in real time through a test method, so that the fatigue life consumed in different crack formation stages is obtained. The invention adopts a reverse reasoning method to realize the prediction of the fatigue crack initiation life, and provides a solution for determining the crack initiation life of the material in the fatigue test process.
The invention is realized by the following technical scheme:
generally, the fatigue fracture failure process of a material can be divided into a crack initiation stage, a crack propagation stage and an instant fracture stage, different crack formation stages show different micro-topography characteristics on the fracture morphology, so that different stages of fatigue crack formation are determined, and the different crack formation stages can be divided into a crack initiation area, a crack propagation area and an instantaneous fracture area on the fracture morphology.
The fatigue cracks of the porous test piece are initiated and expanded from the vicinity of the prefabricated hole type cracks, although the porous test piece also undergoes the initiation process of the fatigue cracks in the fatigue test process, the initiation process of the cracks is extremely short, most of the fatigue cracks are directly expanded from the periphery of the hole type cracks without undergoing the initiation process of the cracks, the prefabricated hole type cracks exist as initial initiated cracks, and the fatigue life is mainly consumed in the expansion process of the fatigue cracks. Under the action of the same tension-compression cyclic load, the standard test piece and the perforated test piece present similar water flow-shaped cracks and fatigue stripe crack characteristics in the crack propagation stage, and the standard test piece and the perforated test piece have similar stress field environments in the crack propagation stage.
And carrying out fatigue tests under different stress level conditions in similar stress field environments of the two, and measuring that under the same stress condition, the fatigue life of the porous test piece is only 0.03% -0.15% of that of the standard test piece, fracture analysis obtains that most of fatigue cracks of the porous test piece directly expand from the periphery of the prefabricated hole type crack without undergoing a crack initiation process, the hole type crack only exists as an initial initiation crack, and the fatigue life is mainly consumed in the expansion process of the fatigue crack. The method is characterized in that the initiation life of the micro-cracks obtained through analysis can account for more than 99% of the total fatigue life, the fatigue life of the standard test piece comprises the crack initiation and propagation life under the same stress condition in an S-N curve chart of the complete relationship between the fatigue life and the fatigue strength, and the crack initiation life of the standard test piece can be obtained by subtracting the fatigue life of the crack initiation and propagation from the average value of the total fatigue life of the standard test piece under the same stress condition.
A test method for judging fatigue crack initiation life comprises the following steps:
s1: performing hole-type crack prefabrication on a standard test piece;
s2: after pretreatment, carrying out a fatigue test of cyclic stress control on the perforated test piece on an QBG-100 electro-hydraulic servo high-frequency fatigue testing machine;
s3: generating an S-N curve of the test piece with the hole and comparing and analyzing the S-N curve with an S-N curve of a conventional high-cycle fatigue failure behavior of a standard test piece;
further, in the step S2, six test pieces with holes S1 are selected to carry out fatigue tests under six different stress levels, wherein the six different stress levels are distributed between 220MPa and 280 MPa;
further, the comparative analysis in S3 shows that the fatigue life of the standard test piece is composed of two parts, one part is the initiation life of the micro-crack, and the other part is the fatigue life to which the micro-crack is propagated to cause the fracture failure of the material. Life of micro-crack initiationWith NfIIndicates that the pore size microcracks propagate to the fatigue life experienced by the failure of the material to fracturefholeThe mean value of the fatigue life obtained under the same load as that of the S2 is shown for the standard test piece
Is represented byfholeThe data obtained in S2, the life of microcracks (N)fI) By using
Calculating to obtain the initiation life N of the microcracksfI。
Further, the standard test piece test material is an aluminum lithium alloy AA2198 bar, the diameter of the bar is 16mm, and the heat treatment state of the bar is T8. The AA2198 alloy contains about 1 percent of Li element by mass fraction, and is added with Mg, Zn, Zr and other improved elements, so that the material performance of the alloy is greatly improved, and the AA2198 alloy is widely used as a key structural material in the field of aerospace.
Further, in S1, the standard test piece is processed from a bar-shaped material with a diameter of 16mm along the rolling direction, the diameter of the minimum diameter portion of the standard test piece is 5mm, the diameter of the clamping ends at the two ends is 14mm, and the minimum diameter portion of the standard test piece and the clamping ends at the two ends are in smooth arc transition.
A pretreatment method for a fatigue test standard test piece comprises the steps of pretreating a standard test piece and a hole type crack of the standard test piece;
the standard test piece is formed by processing a rod-shaped material with the diameter of 16mm along the rolling direction, the diameter of the minimum diameter part of the standard test piece is 5mm, the diameter of the clamping ends at the two ends is 14mm, and the minimum diameter part of the standard test piece and the clamping ends at the two ends are in smooth arc transition;
further, the pretreatment method of the standard test piece is that the rod-shaped material with the diameter of 16mm is firstly subjected to rotary cutting to finish the initial processing of the test piece, after the initial processing, the rod-shaped material with the diameter of 16mm is in a shape that the diameter of the part with the smallest diameter is 5mm, the diameter of the clamping ends at the two ends is 14mm, and the part with the smallest diameter and the clamping ends at the two ends are in smooth circular arc transition;
further, polishing the surface of the standard test piece step by step from thick to thin by using abrasive paper along the rolling direction, and eliminating annular cutting tool marks formed in the cutting process and surface residual stress formed by annular cutting;
further, after grinding, adopting 2.0 μm carborundum polishing paste to carry out fine polishing treatment on the surface of the standard test piece;
further, finally, drilling is carried out on the center of the transition arc of the standard test piece by using drilling equipment, the diameter of the processed hole is 0.6 +/-0.02 mm, the depth of the hole is 0.5 +/-0.02 mm, and then the fine polishing treatment of the standard test piece is carried out again.
Further, the preprocessing in the step S1 is a drilling process, the drilling position is the center of the transition arc of the standard test piece, the hole diameter is 0.6 ± 0.02mm, and the hole depth is 0.5 ± 0.02 mm.
In S2, the load stress is controlled to be a cyclic load sine waveform, and the stress ratio R is-1.
Further, in the S2, the vibration frequency is 100Hz, and the test environment is room temperature.
Further, in the step S2, when the fatigue life of the holed test piece reaches 107The number of times of the test piece is regarded as an infinite cycle, and if the number of the test pieces with holes reaches 107When the fracture does not occur in the week, the QBG-100 electro-hydraulic servo high-frequency fatigue testing machine is stopped manually.
Further, the conventional high cycle fatigue test of the standard test piece is the prior art, an S-N curve of a tested material is obtained through a standard test method, the fatigue test method of the perforated test piece is required to be the same as that of the standard test piece in the fatigue test process, the load stress level of the perforated test piece is required to be consistent with that of the standard test piece as far as possible, and data comparison between the perforated test piece and the standard test piece is facilitated.
The invention has the following advantages and beneficial effects:
the invention adopts the fatigue test method of the prefabricated crack to find the fatigue of most of the test pieces with holesThe fatigue life does not undergo a crack initiation process and directly propagates from the periphery of the hole, and the microscopic hole exists as an initial initiation crack. The total fatigue life of the standard test piece consists of two parts, one part is the initiation life of the microcracks, and the other part is the fatigue life which is endured by the propagation of the microcracks to the failure of the material due to fracture. Crack initiation life N from nucleation to formation of pore size microcracksfIMeans for expressing the mean fatigue life of the standard test pieces under the same stress conditions
Indicates the fatigue life N experienced by the propagation of a pore-sized microcrack until failure of the material by fracturefholeIndicating crack initiation life (N) for formation of pore size microcracksfI) Can be used
And (6) calculating. The invention adopts a reverse reasoning method to realize the prediction of the fatigue crack initiation life, has the advantages that the prediction of the microcrack initiation life can be completed only by using a few sample data, provides a simple, feasible and operable test method for realizing the prediction of the fatigue crack initiation life, and can save a large amount of manpower, material resources and financial resources in the fatigue test process.
Drawings
The accompanying drawings, which are included to provide a further understanding of the embodiments of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principles of the invention. In the drawings:
FIG. 1 is a table of the chemical compositions of the AA2198 alloy of the present invention.
Fig. 2a is a diagram of the location of the hole pattern crack of the present invention on a standard test piece.
FIG. 2b is a micro-topography of a hole pattern crack of the present invention.
FIG. 3 is an S-N curve of a standard test piece and a test piece with a hole of the present invention.
FIG. 4 is a table comparing the fatigue lives of the standard test pieces and the test pieces with holes according to the present invention.
FIG. 4a is a full view of a fatigue fracture of a holed test piece with a stress level of 260MPa according to the present invention.
FIG. 4b is an enlarged view of the hole area of the present invention.
FIG. 5a is a fracture morphology characteristic diagram near the preformed hole type crack under the 220MPa stress state of the invention.
FIG. 5b is a fracture morphology characteristic diagram near the preformed hole type crack under the 230MPa stress state.
FIG. 5c is a fracture morphology characteristic diagram near the preformed hole type crack under the 240MPa stress state.
FIG. 5d is a fracture morphology characteristic diagram near the preformed hole type crack under the 280MPa stress state of the invention.
FIG. 6 is a life ratio of the present invention
And fatigue strength.
Detailed Description
Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any inventive changes, are within the scope of the present invention.
A test method for judging fatigue crack initiation life comprises the following steps:
s1: performing hole-type crack prefabrication on a standard test piece;
s2: after pretreatment, carrying out a fatigue test of cyclic stress control on the perforated test piece on an QBG-100 electro-hydraulic servo high-frequency fatigue testing machine;
s3: generating an S-N curve of the test piece with the hole and comparing and analyzing the S-N curve with an S-N curve of a conventional high-cycle fatigue failure behavior of a standard test piece;
preferably, in the step S2, six test pieces with holes S1 are selected to perform fatigue tests under six different stress levels, wherein the six different stress levels are distributed between 220MPa and 280 MPa;
preferably, the comparison analysis in S3 shows that the total fatigue life of the standard test piece is composed of two parts, one part is the microcrack initiation life, and the other part is the fatigue life experienced by the microcrack propagation to material failure at fracture. Microcrack initiation life NfIIndicates that the pore size microcracks propagate to the fatigue life experienced by the failure of the material to fracturefholeThe mean value of the fatigue life obtained under the same load as that of the S2 stress condition for the standard test piece is shown
Is represented byfholeThe data obtained in S2, the life of microcracks (N)fI) By using
Calculating to obtain the initiation life N of the microcracksfI. Preferably, the standard test piece test material is an aluminum lithium alloy AA2198 bar, the diameter of the bar is 16mm, and the heat treatment state of the bar is T8. The AA2198 alloy contains about 1 percent of Li element by mass fraction, and is added with Mg, Zn, Zr and other improved elements, so that the material performance of the alloy is greatly improved, and the AA2198 alloy is widely used as a key structural material in the field of aerospace. The AA2198 alloy has the chemical composition shown in figure 1.
Preferably, in S1, the standard test piece is processed from a bar-shaped material with a diameter of 16mm along the rolling direction, the diameter of the minimum diameter portion of the standard test piece is 5mm, the diameter of the clamping ends at both ends is 14mm, and the minimum diameter portion of the standard test piece and the clamping ends at both ends are in smooth arc transition.
A pretreatment method for a standard test piece of a fatigue test comprises the pretreatment method for a standard test piece and a hole type crack of the standard test piece;
the standard test piece is formed by processing a rod-shaped material with the diameter of 16mm along the rolling direction, the diameter of the minimum diameter part of the standard test piece is 5mm, the diameter of the clamping ends at two ends is 14mm, and the minimum diameter part of the standard test piece and the clamping ends at two ends are in smooth arc transition;
preferably, the pretreatment method of the standard test piece is that the rod-shaped material with the diameter of 16mm is firstly subjected to rotary cutting to finish the initial processing of the test piece, after the initial processing, the rod-shaped material with the diameter of 16mm is in a shape that the diameter of the part with the smallest diameter is 5mm, the diameters of the clamping ends at the two ends are 14mm, and the part with the smallest diameter and the clamping ends at the two ends are in smooth circular arc transition;
preferably, the surface of the standard test piece is polished step by step from thick to thin by using sand paper along the rolling direction, so that annular cutting tool marks formed in the cutting process and surface residual stress formed by annular cutting are eliminated;
preferably, after grinding, adopting 2.0 μm carborundum polishing paste to carry out fine polishing treatment on the surface of the standard test piece;
preferably, finally, drilling is carried out on the transition arc center of the standard test piece by using drilling equipment, the diameter of the processed hole is 0.6 +/-0.02 mm, the depth of the hole is 0.5 +/-0.02 mm, and then the fine polishing treatment of the test piece with the hole is carried out again.
Preferably, the preprocessing in S1 is a drilling process, the drilling position is the center of the transition arc of the standard test piece, the hole diameter is 0.6 ± 0.02mm, the hole depth is 0.5 ± 0.02mm, the drilling position is shown in fig. 2a, and the micro-topography of the hole is shown in fig. 2 b.
Preferably, in S2, the load stress is controlled to be a cyclic load sine waveform, and the stress ratio R is-1.
Preferably, in S2, the vibration frequency is 100Hz, and the test environment is room temperature.
Preferably, in the step S2, when the fatigue life of the perforated test piece reaches 107The number of times of the test piece is regarded as an infinite cycle, and if the number of the test pieces with holes reaches 107When the fracture does not occur in the week, the QBG-100 electro-hydraulic servo high-frequency fatigue testing machine is stopped manually.
Preferably, as shown in fig. 3, the relationship between the load stress and the fatigue life of the perforated test piece and the standard test piece is represented by black dotted squares and black solid circles in fig. 3, respectively. Black arrowIndicating that the fatigue life reached or exceeded 107Test pieces which failed to break down on a weekly basis.
Preferably, as shown in the relation between the load stress and the fatigue life of the standard test piece with black solid dots in FIG. 3, when the load stress is 240-320 MPa, the fatigue life of the AA2198 alloy is mainly distributed at 105～107In the course of the week, the fatigue life tends to increase gradually as the fatigue strength decreases. At 105～107The fatigue strength of the AA2198 alloy is relatively reduced by nearly 50MPa between weeks. In the fatigue test process, when the fatigue strength is less than 240MPa, the fatigue life of 4 test pieces exceeds 107Weekly without fatigue fracture. Thus, it was judged that the fatigue life reached 107The fatigue strength of the AA2198 alloy is about 245MPa at week time, the test result shows that the fatigue strength corresponds to the traditional fatigue limit, and meanwhile, the fatigue data of the AA2198 alloy presents certain dispersion under the condition of lower stress level.
Preferably, as shown in the relationship between the load stress and the fatigue life of the perforated test piece of the black virtual-center square in fig. 3, the fatigue life of the perforated test piece gradually increases with the decrease of the fatigue strength, but the increase of the fatigue life is not obvious and is mainly distributed at 103～105Within the range of weeks. Compared with the fatigue test data of the standard test piece, the fatigue life of the test piece with the hole is obviously attenuated compared with the standard test piece under the condition of the same stress level. It can be judged that the damage caused by the pre-pass cracks has a great influence on the fatigue performance of the AA2198 alloy. The fatigue life data for the two test methods (standard test piece and perforated test piece) are compared as shown in table 4,
and NfholeRespectively representing the average fatigue life of the standard test piece and the fatigue life of the perforated test piece, and the fatigue life of the perforated test piece is 10 orders of magnitude when the average fatigue life is smaller than or close to the conventional fatigue limit (245MPa) of AA2198 alloy4Weeks (FIG. 3), and the average fatigue life of the standard test piece corresponds to 107The times of the week. At low levelThe fatigue life of the holed test pieces was only 0.25% and 0.14% of that of the standard test pieces at stress levels (220 and 240 MPa). It is to be noted that most of the standard test pieces do not undergo fatigue fracture under the condition of less than 245MPa, which means that the ratio of the two is theoretically less than the above-mentioned value. Along with the increase of the fatigue strength, the fatigue life ratio of the perforated test piece and the standard test piece is not obviously improved, and the fatigue life of the perforated test piece is only 0.03-0.15% of that of the standard test piece under the same stress condition.
Preferably, in the conventional high-cycle fatigue test process, the loading stress is often far less than the yield strength of the material, the fatigue fracture failure of the material is not caused by macroscopic plastic deformation, and the fatigue data can be fitted by adopting a Basquin formula to obtain a prediction formula between the loading stress and the fatigue life. The Basquin equation is described in the form: sigmaa＝σb′(2Nf)b. In the formula sigmaaTo apply stress, NfIs fatigue life, σ'bAnd b is a fatigue strength index or a Basquin index and is related to the change trend of an S-N curve. Based on the test data of the standard test piece, the obtained prediction formula of the loading stress and the fatigue life is calculated as follows: sigmaa＝590.74(2Nf)-0.0539. The prediction formula expression of the test piece with the hole is as follows: sigmaa＝559.50(2Nf)-0.4028. In fig. 3, the fatigue life prediction curves of the standard test piece and the test piece with a hole under the condition of 50% survival rate are shown by a black dotted line and a black solid line, respectively.
Preferably, a JSM-6510LV type Scanning Electron Microscope (SEM) is used for microscopic observation of the fatigue fracture of the hole test piece, and visual criteria are provided for researching a fatigue crack initiation and expansion mechanism under the effect of hole type crack influence. According to the fatigue performance analysis of the perforated test piece, the fatigue life attenuation of the perforated test piece is extremely larger than that of a standard test piece under the same stress condition, which is caused by the damage of the prefabricated hole type crack to the test piece. Due to the existence of microscopic hole defects, fatigue cracks are easy to initiate and propagate at holes under the action of tension-compression cyclic load. Fatigue cracks of all the porous test pieces are initiated and expanded from the hole defects, and finally the fatigue fracture phenomenon is caused.
Preferably, FIG. 4a is a fatigue fracture overview chart of the holed test piece with stress of 260MPa, corresponding to a fatigue life of 6.90X 103The times of the week. The continuous expansion of the fatigue crack takes the appearance of a 'water flow-shaped' crack as a main appearance characteristic, and the reverse convergence of the 'water flow-shaped' crack points to the initiation area of the fatigue crack. As can be seen in fig. 4a, fatigue cracks initiate and propagate from the periphery of the hole, which is more evident in the enlarged view of the hole area (fig. 4 b). Due to the influence of the prefabricated hole type cracks, the fatigue crack initiation process of the hole-shaped test piece is extremely short, and the crack initiation area formed by the fatigue crack initiation process is not obvious. Most fatigue cracks do not undergo a crack initiation process and directly propagate from the periphery of the hole, and the prefabricated hole exists as an initial initiation crack, which is also the main reason that the fatigue life of the perforated test piece is far shorter than that of the standard test piece.
Preferably, the fracture morphology characteristics near the pre-hole type crack are respectively shown in stress states of 220MPa, 230MPa, 240MPa and 280MPa in fig. 5a, 5b, 5c and 5 d. The common characteristic of different stress states is that the water-flow crack propagation paths point to the prefabricated hole type cracks in opposite directions, namely the fatigue cracks are initiated and propagated from the prefabricated holes initially. It can be seen from the fatigue fracture that the fatigue crack is not obvious in the crack initiation area formed in the crack initiation stage due to the influence of the prefabricated hole type crack, and even if the fatigue crack is existed, the crack initiation area of the test piece with the hole is extremely small. According to fatigue fracture analysis under different stress conditions, the influence of the prefabricated hole type cracks on the porous test piece is utilized, so that the fatigue life consumed in the crack initiation stage in the fatigue test process is extremely short, and the fatigue life of the porous test piece is mainly consumed in the fatigue crack propagation stage. Therefore, the phenomenon that the fatigue life of the perforated test piece and the fatigue life of the standard test piece are greatly different under the same stress condition can be explained, and the hole-type crack becomes the initial form of the fatigue crack nucleation.
Preferably, due to the existence of the hole-shaped cracks, the porous test piece generates a fatigue fracture phenomenon in a very short life cycle, fatigue cracks are all initiated and expanded from the vicinity of the hole-shaped cracks, although the porous test piece also experiences the initiation process of the fatigue cracks in the fatigue test process, the crack initiation process is very short, most of the fatigue cracks are directly expanded from the periphery of the prefabricated hole-shaped cracks without undergoing the crack initiation process, the hole-shaped cracks exist as initial initiation cracks, and the fatigue life is mainly consumed in the expansion process of the fatigue cracks. Under the action of the same tension-compression cyclic load, the standard test piece and the perforated test piece present similar water flow-shaped cracks and fatigue stripe crack characteristics in the crack propagation stage, and the standard test piece and the perforated test piece have similar stress field environments in the crack propagation stage.
Preferably, assuming that the 0.6mm × 0.5mm hole pattern crack in the perforated test piece has a similar load stress environment to the i-type open crack of the same size as the standard test piece (fatigue crack usually generates an opening effect and propagates under the tensile stress of a cyclic load), according to the above analysis, the obtained fatigue data of the perforated test piece can represent the fatigue life required for the micro-crack of 0.6mm × 0.5mm to continuously propagate and cause the material to fail at break. Accordingly, the fatigue life of the standard test piece may be composed of two parts: one part is fatigue life for forming a micro crack with a size of 0.6mm x 0.5mm, and the other part is fatigue life for extending from the micro crack with a size of 0.6mm x 0.5mm to failure of the material by fracture, and the fatigue life of the two parts is respectively NfIAnd NfholeIs shown in which N isfholeCan be obtained from fatigue data of a perforated test piece. N under different stress conditionsfholeThe data are shown in Table 2, and the microcrack initiation life NfIThen is available
Is obtained by calculation in the formula
The average value of the fatigue life of the standard test piece under the same load stress condition is shown. The data statistics and calculation show that the microcrack has the initiation lifeSpecific gravity in total fatigue life
The relationship with the load stress is shown in fig. 6. The figure shows that the specific gravity of the microcrack initiation life in the total fatigue life varies weakly under different stress conditions. It was determined that the microcrack initiation life (N) was observed under different stress conditionsfI) Can account for over 99% of fatigue life.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are merely exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention, and any modifications, equivalent substitutions, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (8)
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