CN109163990B - Method for measuring initiation life of axial-loading high-cycle fatigue crack - Google Patents
Method for measuring initiation life of axial-loading high-cycle fatigue crack Download PDFInfo
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Abstract
The invention discloses a method for measuring the initiation life of an axial-loading high-cycle fatigue crack, and belongs to the technical field of fatigue performance tests. According to the method, a fatigue experiment is carried out by using a plurality of samples containing cracks with different lengths to simulate the expansion rule of a single crack, finally, the fatigue crack initiation life is obtained by subtracting the total life of a smooth sample from the expansion life of the crack, and further, a test means and a corresponding theoretical basis are provided for the safety and reliability of the service life of the material in engineering design.
Description
Technical Field
The invention relates to the technical field of fatigue performance testing, in particular to a method for measuring the initiation life of an axial-loading high-cycle fatigue crack.
Background
When the component is subjected to fatigue load, microcracks can be initiated firstly, then the cracks can continue to expand to macroscopic dimensions, and finally rapid failure fracture occurs, so that the fatigue process can be divided into two parts, namely crack initiation and crack expansion. The fatigue life prediction method generally divides the fatigue process into crack initiation and propagation life for prediction respectively. It is important to consider the crack initiation process and the crack propagation process separately, since some use conditions may have a large effect on the crack initiation process, but a small effect on crack propagation. For the crack propagation part, since the macro cracks are easy to observe and characterize, the test of the macro cracks is easy to carry out, and a plurality of research results are obtained at present. The test method for crack initiation is still in an exploration stage, and no unified standard exists. This is because there are differences in the knowledge of people in different fields and the limitations of measurement techniques also present obstacles to establishing test standards.
It is generally accepted that more than 90% of the high cycle fatigue life is the crack initiation life, and it is therefore common practice to simply consider the entire fatigue life of the high cycle fatigue test specimen as the initiation life. However, this method causes problems in practical use. In high cycle fatigue, the number of fatigue cycles for a smooth specimen to initiate a major crack accounts for a significant portion of the total fatigue life, which is considered to be a minor error in initiating life. However, if the percentage of the initiation life is significantly reduced under high stress or if internal defects in the material affect the fatigue process, the total life cannot be considered as the initiation life. In addition, small samples, due to their small size, propagate to break very quickly after a crack is initiated, while the actual component will eventually fail after a crack has been initiated by a longer propagation process. The total life of the component is therefore likewise not considered as the initial life. Overall, considering the entire fatigue life of the specimen as the initiation life is inaccurate.
With regard to the testing and the initiation life of small cracks, there are currently the following problems:
1. small crack propagation is uncontrollable, randomness is high:
the propagation of small cracks is determined by the interaction between the microstructural features of the material and the resolved stresses of the load at the crack tip as a driving force. However, the microstructure of the material is not uniform, so that the crack propagation rate and propagation direction are not controllable, random, from a measurement point of view. At the same time, small cracks have a spatially three-dimensional nature, the shape of which has a great influence on the propagation, which results in a more uncontrolled crack propagation. This brings difficulty in studying crack propagation rules, making it difficult to describe the propagation rules of small cracks using mathematical formulas.
2. Crack length is not easy to measure:
at present, people generally adopt a replica method, a potential method and a long-focus microscope observation method for measuring cracks. While the length of the small cracks is in the order of micrometers, accurate observation is difficult due to the limitations of the measurement techniques described above. In addition, since the crack length is usually different between the sample surface and the core, it is not reasonable to use the length of the crack on the sample surface as the length of the crack.
3. Necessity to acquire the life of germination:
various methods are used to predict the life of a component under a given load, where a common method used by engineering is to calculate the total life using S-N curves. However, this method is insufficient in that the life calculated using the S-N curve is the total life of the specimen, which is the life of the specimen to failure fracture, while the engineering safe life is the life at which a visible crack appears, excluding the subsequent crack propagation life. And the S-N curve is usually obtained by testing small-sized specimens, which will rapidly break after crack initiation, with the total life being dominated by the initiation life. However, the actual member may have a large percentage of crack propagation life. In this case, the life measured by the small-sized test piece is different from the life of the real member. Therefore, in predicting the life of a component, the total life of the component cannot be simply regarded as the lifetime of the component, and an accurate measurement of the lifetime of the component is necessary.
4. There is no good way to predict germination life:
when the load is larger than the fatigue limit of the material, the material can generate cracks and expand to fracture, and the cracks are widely used as a criterion for judging whether the cracks can generate, but no good theory and method are provided for predicting a specific generation life value at present. Existing methods define crack initiation life by defining a crack initiation size. In practical applications, detection means vary according to the applicable problems, and various lengths have been proposed to define the size of the crack at the time of crack initiation. These defined crack initiation sizes are based on the means of detection, not on the mechanism of fatigue. In summary, there is currently no good way to predict the lifetime of an germination.
Therefore, the invention tries to provide a simple method for measuring the crack initiation life of the material, the expansion life of the material is predicted by obtaining the expansion rule of the small crack through test, and then the fatigue crack initiation life is obtained by subtracting the crack expansion life from the total life of the smooth sample. The current measuring means for small crack propagation include a replica method, a potential method and a long-focus microscope observation method. Wherein the replica method is complicated to operate and can only measure surface cracks; the potentiometric method needs a special measuring system, is complex in early calibration, and has measurement precision influenced by various factors; the long-focus microscope observation method can only measure surface cracks and has high labor intensity. Therefore, the core of the invention is to develop a method for simply and rapidly measuring the small crack propagation law.
Disclosure of Invention
The invention aims to provide a method for measuring the initiation life of an axially-loaded high-cycle fatigue crack, which obtains a crack propagation rule by utilizing the characteristic that a through-thickness crack can be stably propagated under a fatigue load, solves the problem of difficult measurement of the propagation of a short crack by using a method of simulating a single crack by using a plurality of samples containing cracks with different lengths, solves the problem of difficult measurement of the initiation life of the high-cycle fatigue crack, and provides a means for measuring the initiation life for the whole-life design in engineering.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a method for measuring the initiation life of an axially-loaded high-cycle fatigue crack comprises the following steps:
(1) preparing fatigue samples containing penetrating short cracks, wherein the number of the fatigue samples is 10-20, and the penetrating short cracks in the fatigue samples are different in length;
(2) fatigue life testing including penetration of short crack specimens;
(3) measuring the length of the pre-crack from the fracture of the fatigue specimen: measuring the fracture to obtain the length a of the prefabricated crack on each sample; the body type microscope is adopted for measurement;
(4) fitting a plurality of data points [ fatigue life, prefabricated crack length ] formed by the measured fatigue life of each sample and the corresponding prefabricated crack length according to a formula (1) to obtain values of fitting parameters P and k, so as to establish the relation between the fatigue life of the sample and the prefabricated crack length;
N＝Pak (1)；
in the formula (1), N is the fatigue life of the sample, a is the length of the penetration crack contained in the sample, and P and k are fitting parameters;
(5) calculating the minimum crack length d for the transition from fatigue crack initiation to fatigue crack propagation1And fatigue crack propagation life Np；
(6) Calculating the fatigue crack initiation life: total life N with smooth specimensfReduced fatigue crack propagation life NpNamely, the fatigue crack initiation life Ni。
In the step (1), the geometric shapes and the sizes of all fatigue samples need to be kept consistent; the fatigue test sample is of a plate-shaped structure, and the cross section of the parallel section is rectangular; each specimen contained through-cracks of different lengths, uniformly distributed from 50 μm to 500 μm. In the step (1), the preparation process of the fatigue test piece containing the penetrating short cracks comprises the following steps:
(a) prefabricating fatigue cracks on the three-point bending test sample;
(b) processing of uniaxially loaded fatigue specimens: performing linear cutting along a direction perpendicular to the line before the fatigue crack to obtain a fatigue test sample containing the penetrating crack and provided with a clamping section, a transition arc and a parallel section;
(c) measuring crack length a1: removing the wire-cut traces by sanding, observing the cracks at the parallel sections of the test specimen using an optical microscope, and measuring the length of the cracks as a1；
(d) For the step (3), the crack length a is tested1Carrying out secondary cutting on the sample to remove a notch;
(e) measuring crack length a2: removing the wire-cut traces by sanding, observing the cracks at the parallel sections of the test specimen using an optical microscope, and measuring the length of the cracks as a2；
(f) And finely grinding to obtain the target short crack length, thus obtaining the metal material fatigue sample containing the penetrating short crack.
In the step (2), the fatigue life testing process is as follows: fatigue loading is carried out on the sample until the sample is broken to obtain the service life N; the load of fatigue loading is above the fatigue limit of the material, and the load is symmetrical in tension and compression; the parameters of the fatigue load include the stress amplitude.
In the step (3), the specific process of measuring the length of the pre-crack is as follows: after fatigue life testing, two fractures are obtained after each sample fails and fractures; the fracture is photographed by using a body type microscope, and the prefabricated crack area and the subsequent crack propagation area can be clearly distinguished in the fracture; the area A of the pre-crack area and the thickness B of the sample are measured, and then the area A is divided by the thickness B of the sample by using an area equivalent method to obtain the length a of the penetration crack of the sample.
The minimum crack length d at which the fatigue crack initiation is shifted to the fatigue crack propagation in the above step (5)1Calculating according to the formula (2); d to be obtained1The fatigue crack propagation life N is obtained by substituting the formula (1)pThe calculation formula (3);
Np＝Pd1 k (3)；
in formulae (2) to (3), d1Critical value for the transition of crack size from initiation to propagation, Y is crack shape factor, Δ Keff,thIs the intrinsic crack propagation threshold, σ, of the materialaStress amplitude for applied fatigue load, NpFatigue crack propagation life.
In the above step (6), the fatigue crack initiation life NiCalculating according to the formula (4);
Ni＝Nf-Np (4)；
in the formula (4), NiFor fatigue crack initiation life, NfFor the total life of the smooth specimen, NpFatigue crack propagation life.
In the step (6), the loading conditions of the smooth specimen and the specimen containing the short cracks are completely the same in the life test of the smooth specimen.
The invention has the following beneficial effects:
the method can accurately measure the germination life through tests, the general small cracks have irregular expansion, the expansion life is difficult to predict, and the germination life cannot be determined by a method of subtracting the total life and the expansion life of the sample. The fatigue test is carried out on a plurality of samples containing the cracks with different lengths and penetrating thicknesses to simulate the expansion rule of a single crack, the difficulty in measuring the small crack expansion rate is overcome, and finally the fatigue crack initiation life is obtained by subtracting the total life of the smooth sample from the crack expansion life.
Drawings
FIG. 1 is a schematic representation of a pre-fatigue crack on a three-point bend specimen.
Fig. 2 is a schematic diagram of wire cutting.
FIG. 3 is a schematic view of a notch portion removed using wire cutting.
Fig. 4 is a schematic view of a sample being polished using a polishing machine.
Fig. 5 is a schematic view of sample clamping.
Fig. 6 is a graph showing the measurement of the length of a pre-crack by an area equivalent method using a body microscope.
FIG. 7 shows the relationship between the fatigue life and the length of the through crack contained in the sample.
Fig. 8 is a photograph of a scan of the fracture morphology of two different samples.
FIG. 9 is a schematic representation of the relationship of through-thickness cracks to material texture.
FIG. 10 is a graph of stress amplitude versus critical non-propagating crack length.
Fig. 11 is a graph showing the measurement of the length of a pre-crack by an area equivalent method using a body microscope.
In the figure: 1-three point bending fatigue test sample; 2-supporting rolls; 3-a notch; 4-cracking; 5-linear cutting trajectory I; 6-line cutting trajectory II; 7-grinding the surface; 8-fatigue machine upper clamping head; 9-fatigue machine lower chuck; 10-uniaxial fatigue test specimens; 11-short crack penetration.
Detailed Description
For further understanding of the present invention, the present invention will be described with reference to the following examples, which are provided for the purpose of further illustrating the features and advantages of the present invention and are not intended to limit the scope of the present invention as claimed.
The invention relates to a method for measuring the initiation life of an axial loading high cycle fatigue crack, wherein the samples used by the method are a group of samples containing a penetrating short crack and a smooth sample, and the method comprises the following specific processes:
1. preparing a set of fatigue test pieces containing different lengths of penetrating cracks, wherein the preparation of the fatigue test pieces containing the penetrating short cracks comprises the following steps (a) to (f):
(a) prefabricating fatigue cracks on a three-point bending test sample: the specific process of prefabricating the fatigue crack comprises the following steps: firstly, a three-point bending sample is processed, a notch is processed by a linear cutting method, and then an alternating cyclic load is added on a fatigue machine to prefabricate a fatigue crack.
(b) Processing of uniaxially loaded fatigue specimens: and carrying out linear cutting along the direction perpendicular to the line before the fatigue crack, wherein the position of the cut sample is at least 1mm away from the edge of the three-point bending sample, and the plastic zone at two ends of the crack is ensured not to be included. Obtaining a fatigue test sample containing a penetrating crack and having a clamping section, an excessive arc and a parallel section after cutting;
(c) measuring crack length a1: removing the wire-cut traces by sanding, observing the cracks at the parallel sections of the test specimen using an optical microscope, and measuring the length of the cracks as a1(ii) a The geometric shape of the sample cannot be damaged when sand paper is used for polishing; meanwhile, the polishing direction needs to be vertical to the crack cracking direction; the crack length measured using optical microscopy observation is the length from the crack tip to the edge where the notch is located.
(d) For the step (3), the crack length a is tested1Carrying out secondary cutting on the sample to remove a notch; the secondary cutting means that the cutting width is a through linear cutting1-1mm of portion, the notch is removed, leaving a real crack; the cutting path is ensured to be parallel to the other side relative to the notch (i.e. the cutting direction is perpendicular to the crack direction).
(e) Measuring crack length a2: and the wire cutting traces are removed by sanding through sand paper, so that the cross section of the sample is ensured to be rectangular during sanding, and the geometric shape of the sample cannot be damaged. Meanwhile, the direction of sanding is perpendicular to the direction of crack opening. The crack length measured using optical microscopy observation is the length from the crack tip to the edge where the crack penetrated. The crack at the parallel section of the specimen was observed using an optical microscope, and the length of the crack was measured as a2；
(f) And (3) finely grinding to obtain the target short crack length: in the fine grinding process, firstly determining the length of a required short crack as a; the width of the sample at this time was measured as W using a micrometer; polishing the surface of the sample penetrated by the crackTo a width of W- (a) of the sample2-a), the direction of sanding is perpendicular to the direction of crack propagation; and grinding to obtain the metal material fatigue sample containing the penetrating short cracks. The prepared metal material fatigue test sample containing the penetrating short cracks is a plate-shaped uniaxial fatigue test sample containing the penetrating short cracks with any length, and comprises a clamping section, a transition arc and a parallel section, wherein the short cracks are all positioned on a flat surface of a surface where the non-transition arc is positioned.
A group of fatigue samples are prepared by the method, the number of the fatigue samples is 10-20, and the geometric shapes and the sizes of the fatigue samples need to be kept consistent. The test specimen is a plate-shaped test specimen, and the cross section of the parallel section is rectangular. Each specimen contained through-cracks of different lengths, uniformly distributed from 50 μm to 500 μm. Because the through-thickness cracks have an average effect, the problem of uncontrollable and high randomness of the cracks during propagation can be overcome.
2. Each fatigue test piece containing the penetrating short cracks is subjected to fatigue loading until the test piece is broken, and the service life N is obtained. The fatigue loading load is above the fatigue limit of the material and the load is tension-compression symmetric. The various parameters of the fatigue loading, including but not limited to stress amplitude, frequency, etc., once determined, will not change throughout the test.
3. The length a of the pre-crack contained in each specimen was obtained by fracture measurement: and (3) obtaining two fractures after the fatigue-loaded sample fails and fractures in the step (2), and photographing the fractures by using a body microscope to clearly distinguish a prefabricated crack region and a subsequent crack propagation region in the fractures. The area a of the precracked area and the thickness B of the test specimen were measured, as shown in fig. 6. The area a is divided by the thickness B of the sample using an area equivalent method to obtain the length a of the through crack contained in the sample. The method for measuring the equivalent crack length on the fracture solves the problem that the short crack length is difficult to measure.
4. The formula is obtained by fitting the relation of N-a. The formula used for the fitting is formula (1):
N＝Pak (1)；
where N is the fatigue life of the sample, a is the length of the through crack contained in the sample obtained in step 3, and P, k is a fitting parameter. The relation between the fatigue life and the length of the penetrating crack contained in the sample is shown in FIG. 7, and it can be seen that N and a have a good power function relation, so that the power function can be used for fitting to predict the propagation life of the crack. The problem that the short crack propagation rate is difficult to measure is solved by using a method of simulating one crack by using a plurality of samples and matching with the method for measuring the equivalent crack length in the step 3.
5. Calculating the minimum crack length d for the transition from fatigue crack initiation to fatigue crack propagation1And fatigue crack propagation life Np；
Substituting d1 into equation (1) yields the extended lifetime Np。d1Is calculated by equation (2):
in the formula (2), d1Critical value for the transition of crack size from initiation to propagation, Y is crack shape factor, Δ Keff,thIs the intrinsic crack propagation threshold, σ, of the materialaThe stress amplitude of the applied fatigue load. Wherein Δ Keff,thAnd Y can be queried from the literature. Wherein d is1Minimum crack length for the transition from fatigue crack initiation to fatigue crack propagation, i.e. crack length less than d1Equation (1) will no longer apply. D obtained by calculation1Substituting into equation (1) yields the following equation (3):
Np＝Pd1 k (3)；
in the formula NpFatigue crack propagation life. By analyzing the fatigue mechanism, a method for defining the fatigue crack initiation size and a calculation method are obtained, and the problem that the unified fatigue crack initiation size is not available is solved.
6. Total life N with smooth specimensfReduced fatigue crack propagation life NpObtaining fatigue crack initiation life Ni. Wherein a smooth specimen refers to a specimen that does not contain cracks and whose geometry and dimensions are completely the same as those of the specimen in step 1The same is true. The total life N of the sample is obtained after fatigue loading to fracturef. The fatigue load is exactly the same as in step 2. N is a radical ofpThe fatigue crack propagation life obtained in step 5.
The scientific principle on which the invention is based is as follows:
1. multiple samples containing different length cracks can be used to simulate the propagation behavior of a crack:
according to the method, a plurality of samples containing cracks with different lengths are used for simulating the expansion behavior of one crack, and finally the relationship between the crack length and the residual service life is obtained. By observing the fatigue fracture of all samples, as shown in fig. 8, scanning photographs of fracture morphology of different samples are given, wherein two left columns and two right columns are different samples. It can be seen that the fracture surface morphology left by the crack propagation of the two samples containing cracks of different lengths is the same, and is a river-like pattern, the direction of which is approximately along the propagation direction of the crack. It is therefore possible to use a plurality of specimens containing different length cracks to fit the propagation law of a single crack.
2. Through-thickness cracks can propagate stably:
the traditional theory holds that the linear elastic fracture mechanics theory applicable to long cracks is not applicable to short cracks any more, the propagation behavior of short cracks cannot be described by Paris formula, and the propagation of short cracks shows completely different rules from that of long cracks, and has a much higher propagation rate than that of long cracks under the same nominal driving force. The theory of linear elastic fracture mechanics states that the crack tip stress intensity factor Δ K can be used to describe the driving force for crack propagation. The calculation formula of Δ K is formula (5).
Wherein Y is the crack shape factor, σaA is the stress amplitude of the applied fatigue load, and a is the length of the crack. When the value of Δ K is the same, the load on the long cracks is smaller, and the load on the short cracks is larger, even close to the yield limit of the material. Should be reused at this timeIt becomes unreasonable to compare the force intensity factors to their expansion laws. Therefore, a new theory and formula are needed to describe the propagation law of the short crack. According to the invention, by researching the propagation rule of the short crack, the relation between the length of the prefabricated crack and the residual life can be fitted by using a power function under the condition that the fatigue load is not changed, as shown in FIG. 7.
As can be seen from the figure, the crack grew steadily from short to long. This is because the leading edge of the through short crack passes through a plurality of grains, as shown in fig. 9. The figure shows a schematic representation of the relationship between through-thickness cracks and the material structure. It can be seen from the figure that the crack front penetrating the thickness passes through a plurality of grains, and although the crack propagation rate differs within each grain, the crack front stably propagates forward in a substantially straight line as a whole. The through-thickness short crack exhibits a characteristic of stably propagating without being affected by the microstructure.
3. Stable propagation of cracks exists with a lower crack length limit:
as shown in FIG. 10, a graph showing the relationship between crack propagation and crack propagation for a given fatigue load stress amplitude for a test piece containing a crack of a given length is shown. By reducing the load, a propagating crack will eventually stop propagating. Similarly, by increasing the load, a crack that does not propagate will eventually begin to propagate. That is, at a fixed load, there is a critical crack size that does not propagate. As the load stress amplitude increases, the critical non-propagating crack size decreases.
When the stress amplitude increases to the fatigue limit σeAt the time of critical non-propagating crack, the size of critical non-propagating crack is d1. When the stress amplitude continues to increase beyond the fatigue limit, there are theoretically no cracks that will not propagate. However, when the crack is very short and has a similar size to the microstructure, the crack propagation is greatly affected by the microstructure, the crack propagation no longer exhibits the characteristic of stable propagation, and the propagation rate fluctuates. At this time d1The lower limit of the length as a stable propagation of the crack is conservative. Wherein d is1The calculation formula of (2) is as follows:
ΔKeff,this the intrinsic crack propagation threshold of the material, Y is the crack shape factor, σaThe stress amplitude of the applied fatigue load.
Example 1:
defining the initiation life of T250 (cobalt-free) maraging steel when the applied fatigue load is 630MPa and a measuring process thereof, and specifically comprising the following steps:
1. a set of 10 fatigue specimens in total containing different length through cracks was prepared. Each sample contained a different length of through-crack, varying from 45 μm to 459 μm. The fatigue test piece containing the penetrating short cracks is prepared by the following steps:
first, fatigue cracks were preformed in a three-point bending specimen:
the heat treated raw blank was processed into a three-point bend sample 1 with a notch 3 having a shape as shown in fig. 1. The loading schematic is shown in fig. 1, and the crack 4 termination length is tested while supported by the support roll 2.
Secondly, carrying out linear cutting processing and slicing on the three-point bending sample containing the cracks:
and cutting and slicing the three-point bending sample with cracks after fatigue, wherein a linear cutting track I is shown in figure 2, and obtaining an initial blank of the uniaxial fatigue sample with a clamping section, an excessive arc and a parallel section.
Thirdly, cutting the uniaxial fatigue test piece blank:
firstly, using sand paper to polish all the surfaces of the blank except the notch, and removing the linear cutting trace. Then, the distance a from the crack tip to the edge where the notch is located was measured using an optical microscope1. Excising by wire cutting with width a1A 1mm portion, the wire cutting trajectory line cutting trajectory II is shown in FIG. 3. And a real crack is left after the notch is removed.
And finally, fine grinding:
first, the crack was polished with sandpaperThe surface (i.e., the polished surface 7) to remove the wire-cut traces. Then, the distance a from the crack tip to the edge where the crack is located was measured using an optical microscope2. Then, determining the length of the required short crack as a; the width of the sample at this time was measured as W using a micrometer; finally, the surface of the crack is polished until the width of the sample is W- (a)2-a)。
2. Fatigue loading to specimen fracture to obtain life N: the clamping of the sample is schematically shown in fig. 5. During the test, the prepared uniaxial fatigue test specimen 10 with the penetrating short crack 11 is fixed between the fatigue machine upper chuck 8 and the fatigue machine lower chuck 9. The fatigue loading load was 630MPa and the load was tension-compression symmetric. Other parameters of the fatigue load are: the frequency is 30Hz, and the waveform is a sine wave. The fatigue load parameters were unchanged during the testing of this set of 10 specimens in total.
3. The length a of the pre-crack contained in each specimen was obtained by fracture measurement. The fracture was photographed using a split microscope, as shown in FIG. 11, from which it can be seen that the pre-crack region clearly differed from the subsequent crack propagation region. The area of the pre-crack zone can thus be measured directly in the figure. The area a of the precracked area, and the thickness B of the test specimen were measured using software. The length a of the through short crack can be obtained by an area equivalent method.
4. Obtaining a relation of N-a by fitting, wherein the fitting result is as the formula:
N＝4.98×106a-1.18 (6)；
the results of the sample testing and the fitting are shown in fig. 7.
5. Will d1Substituting into the above formula to obtain the extended lifetime Np. Obtaining the delta K of the material through literature queryeff,thI.e. intrinsic crack propagation threshold of 2.5MPa.m1/2. The crack shape factor Y was taken to be 1.12. D is calculated from the formula (2)14 μm. Then d is1Substituting equation (6) results in Np＝9.7×105。
6. Total life N with smooth specimensfReduced extended lifetime NpTo obtain Ni. Wherein the fatigue load of the smooth sample is completely the same as the fatigue load of the sample containing the penetrating short crack in the step 2. Test results Nf＝2.68×106Week, therefore Ni＝2.68×106-9.7×105＝1.71×106And (4) week. That is, the T250 (cobalt-free) maraging steel had an initiation life of 1.71X 10 at an applied fatigue load of 630MPa6And (4) week.
In this example, FIG. 7 shows the relationship between the residual life of specimens containing through-thickness cracks of different lengths under the same fatigue load. Calculating to obtain the lower limit of the crack length of the stable crack propagation through a mathematical expression of the crack propagation rule fitted by the formula (1) and the formula (2), and finally obtaining the fatigue crack propagation life of the sample of 1.71 multiplied by 106And (4) week. The fatigue crack initiation life of the sample was obtained by performing a fatigue test on smooth fatigue samples of the same size and shape under the same load to obtain the total life of the sample, and the results are shown in table 1.
TABLE 1 predicted extended Life and Germination Life
By adopting the method provided by the invention, the fatigue crack initiation life of the metal material in an axial loading mode can be obtained through simple tests, and the requirement of people on the research on the fatigue crack initiation life can be met.
Claims (6)
1. The method for measuring the initiation life of the axially-loaded high-cycle fatigue crack is characterized by comprising the following steps of: the method comprises the following steps:
(1) preparing fatigue samples containing penetrating short cracks, wherein the number of the fatigue samples is 10-20, and the penetrating short cracks in the fatigue samples are different in length;
(2) fatigue life testing including penetration of short crack specimens;
(3) measuring the length of the pre-crack from the fracture of the fatigue specimen: measuring the fracture to obtain the length a of the prefabricated crack on each sample; the body type microscope is adopted for measurement;
(4) fitting a plurality of data points formed by the measured fatigue life of each sample and the corresponding length of the prefabricated crack according to a formula (1) to obtain values of fitting parameters P and k, so as to establish the relationship between the fatigue life of the sample and the length of the prefabricated crack;
N＝Pak (1)；
in the formula (1), N is the fatigue life of the sample, a is the length of the penetration crack contained in the sample, and P and k are fitting parameters;
(5) calculating the minimum crack length d for the transition from fatigue crack initiation to fatigue crack propagation1And fatigue crack propagation life Np(ii) a Minimum crack length d for transition from fatigue crack initiation to fatigue crack propagation1Calculating according to the formula (2); d to be obtained1Substituting into formula (1) to obtain fatigue crack propagation life NpThe calculation formula (3);
Np＝Pd1 k (3)；
in formulae (2) to (3), d1Critical value for the transition of crack size from initiation to propagation, Y is crack shape factor, Δ Keff,thIs the intrinsic crack propagation threshold, σ, of the materialaStress amplitude for applied fatigue load, NpFatigue crack propagation life;
(6) calculating the fatigue crack initiation life: total life N with smooth specimensfReduced fatigue crack propagation life NpNamely, the fatigue crack initiation life Ni(ii) a In the smooth specimen life test, the loading conditions of the smooth specimen and the specimen containing the short cracks were identical.
2. The method of claim 1, wherein the method comprises the following steps: in the step (1), the geometric shapes and sizes of all fatigue samples need to be kept consistent; the fatigue test sample is of a plate-shaped structure, and the cross section of the parallel section is rectangular; each specimen contained through-cracks of different lengths, uniformly distributed from 50 μm to 500 μm.
3. The method for measuring the axial loading high cycle fatigue crack initiation life according to claim 1 or 2, wherein: in the step (1), the preparation process of the fatigue test piece containing the penetrating short cracks comprises the following steps:
(a) prefabricating fatigue cracks on the three-point bending test sample;
(b) processing of uniaxially loaded fatigue specimens: performing linear cutting along a direction perpendicular to the line before the fatigue crack to obtain a fatigue test sample containing the penetrating crack and provided with a clamping section, a parallel section and an excessive arc;
(c) measuring crack length a1: removing the wire-cut traces by sanding, observing the cracks at the parallel sections of the test specimen using an optical microscope, and measuring the length of the cracks as a1；
(d) Testing the crack length a for step (c)1Carrying out secondary cutting on the sample to remove a notch;
(e) measuring crack length a2: removing the wire-cut traces by sanding, observing the cracks at the parallel sections of the test specimen using an optical microscope, and measuring the length of the cracks as a2；
(f) And finely grinding to obtain the target short crack length, thus obtaining the metal material fatigue sample containing the penetrating short crack.
4. The method of claim 1, wherein the method comprises the following steps: in the step (2), the fatigue life testing process is as follows: fatigue loading is carried out on the sample until the sample is broken to obtain the service life N; the load of fatigue loading is above the fatigue limit of the material, and the load is symmetrical in tension and compression; the parameters of the fatigue load include the stress amplitude.
5. The method of claim 1, wherein the method comprises the following steps: in the step (3), the specific process of measuring the length of the prefabricated crack is as follows: after fatigue life testing, two fractures are obtained after each sample fails and fractures; the fracture is photographed by using a body type microscope, and the prefabricated crack area and the subsequent crack propagation area can be clearly distinguished in the fracture; the area A of the pre-crack area and the thickness B of the sample are measured, and then the area A is divided by the thickness B of the sample by using an area equivalent method to obtain the length a of the penetration crack of the sample.
6. The method of claim 1, wherein the method comprises the following steps: in step (6), fatigue crack initiation life NiCalculating according to the formula (4);
Ni＝Nf-Np (4)；
in the formula (4), NiFor fatigue crack initiation life, NfFor the total life of the smooth specimen, NpFatigue crack propagation life.
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