CN107631935B - Crack propagation threshold value testing method for prefabricating cracks by adopting pressure-pressure cyclic load - Google Patents
Crack propagation threshold value testing method for prefabricating cracks by adopting pressure-pressure cyclic load Download PDFInfo
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Abstract
The invention belongs to the technical field of fatigue crack propagation tests, and relates to a fatigue crack propagation threshold value test method for prefabricating cracks by adopting pressure-pressure cyclic load. The method comprises the following steps: designing a test piece which is easy to generate fatigue cracks under the compression-compression cyclic compression load; estimating a fatigue crack cyclic compression load and applying the load to generate a crack; calculating the size of the residual tensile stress area by adopting a finite element method; establishing a crack expansion scheme to enable the crack tip to expand a residual tensile stress area generated by cyclic compression load; the crack growth was started by increasing the load stepwise, and the crack growth threshold was measured. The test method is simple and efficient, has short test time compared with the conventional threshold value test method, can avoid the load history effect caused by load reduction, and has outstanding test efficiency and economical efficiency.
Description
Technical Field
The invention belongs to the technical field of fatigue crack propagation tests, and relates to a fatigue crack propagation threshold value test method for prefabricating cracks by adopting pressure-pressure cyclic load.
Background
Fatigue crack propagation threshold value delta KthRepresents: when crack propagation driving force DeltaK is lower than DeltaKthWhen it is used, the crack cannot be caused to propagate. The fatigue crack propagation threshold value is an important parameter in the evaluation of the mechanical properties of airplane materials, and a measurement procedure of the fatigue crack propagation threshold value is given in GB/T6398-. The procedure adopts a pulling-pulling cyclic load to prefabricate the crack, and further tests the fatigue crack expansion threshold value by reducing the load step by step, when the threshold value is measured by adopting the method, the plastic region of residual compressive stress exists at the tip of the crack, generally, in order to rapidly initiate the crack, the crack needs to be prefabricated by adopting a larger load, at the moment, the residual compressive stress is very large, the influence on the threshold value measurement is correspondingly larger, and in order to eliminate the influence, the gradual load reducing process is very long. When the crack is prefabricated by adopting a small load, the crack initiation itself consumes time and labor in actual tests, and usually 1 fatigue crack propagation threshold value data curve needs at least 15-20 days. The threshold value is used as important data of mechanical performance for airplane material design, a large number of test requirements exist, and the efficiency of the current test method is difficult to meet the design requirements.
Disclosure of Invention
The invention aims to solve the technical problem that the efficiency of the existing fatigue crack propagation threshold value testing method is too low, and the crack propagation threshold value testing method for prefabricating cracks by adopting pressure-pressure cyclic load is designed.
The technical solution of the invention is as follows:
(1) prefabricated crack
Designing a test piece capable of generating fatigue cracks under a pressure-pressure cyclic load, estimating the pressure-pressure cyclic load for prefabricating the fatigue cracks according to the elastic modulus of the material of the test piece and applying the load to generate cracks;
(2) propagating cracks out of the residual tensile stress region
Calculating the size of a residual tensile stress area generated by a compression-compression cyclic load by adopting a finite element method, applying the tensile-compression cyclic load on the test piece according to the typical value of the fatigue crack propagation threshold value of the material type of the test piece, wherein the load value is 50% -80% of the typical value of the fatigue crack propagation threshold value of the material type of the test piece, and applying the load to propagate the crack until the crack tip does not propagate under 1000000 cycles, namely considering that the crack tip propagates out of the residual tensile stress area;
(3) testing fatigue crack propagation threshold
And (3) gradually increasing the load, wherein the load increased at each stage is 5% -10% of the load of the upper stage until the crack starts to expand, continuously adjusting the load to enable the crack to expand by 0.1mm under 1000000 cycles, and measuring the crack expansion threshold value.
The adopted test piece is a central crack tensile M (T) test piece, a hexagonal notch is arranged at the central position of the test piece, the notch penetrates through the whole thickness direction, the upper side and the lower side of the hexagon are parallel to the width direction of the test piece, the distance between the upper side and the lower side is 1-5 mm, the left top point and the right top point are located on a central line in the horizontal direction, the distance between the two top points is 5-20 mm, the included angle between the two sides connecting each top point is 30-90 degrees, and the two sides are symmetrical relative to the central line in the horizontal direction.
The maximum compression load adopted by the compression-compression cyclic load is that the absolute value of the stress intensity factor of the crack tip is ensured
Wherein sigmayIs the yield strength of the material.
The stress ratio adopted by the pressure-pressure cyclic load is 10-40.
Applying 100000-200000 cycles of pressure-pressure cyclic load to make crack expand to 0.1-0.3 mm.
Applying a load of 50-80% of the fatigue crack propagation threshold value of the same material to expand the crack by utilizing the residual tensile stress area
The invention has the advantages and effects that: the crack is prefabricated by adopting the pressure-pressure cyclic load, the crack can be rapidly initiated by only applying the larger pressure-pressure cyclic load, the residual stress at the tip end of the crack is tensile stress, the size of the residual tensile stress area caused by the compressive load is smaller than that of the residual compressive stress area caused by the tensile load, after the crack is prefabricated, the crack can be rapidly expanded out of the residual tensile stress area by directly adopting the smaller tension-tension cyclic load, the tip end of the crack is not influenced by the residual stress area at the moment, and the proper load can be directly applied to measure the fatigue crack expansion threshold value. The method avoids a very time-consuming step-by-step load reduction process, can directly measure the fatigue crack expansion threshold value, and only 3-5 days are needed for testing 1 fatigue crack expansion threshold value data curve, so that the time for testing the threshold value is greatly saved, and meanwhile, the influence of a residual stress area on the threshold value test is effectively avoided.
Drawings
FIG. 1 is a test flow chart of the test method;
FIG. 2 is a drawing of the dimensions of a test piece designed in the test method.
Detailed Description
The invention is described in further detail below with reference to the figures and examples. As shown in fig. 1:
the test method comprises the following steps: the test piece which is easy to generate fatigue cracks under the pressure-pressure cyclic load is designed, and the notch part in the center of the test piece is provided with a sharp corner, so that a residual tensile stress area is easy to generate around the notch when the test piece is subjected to the compressive load. Estimating the pressure-pressure cyclic load of the fatigue crack and applying the load to generate the crack, wherein when estimating the compressive load, the load is calculated according to the yield strength of the material, and the compressive load is enough to generate the residual tensile stress at the notch so as to initiate the crack. Limited by the size of the tensile residual stress region, the residual tensile stress will be insufficient to continue crack propagation after a certain cycle of application of the load, at which point the crack will stop propagating. And calculating the size of the residual tensile stress area by adopting a finite element method. And (3) formulating a crack expansion scheme to enable the tip of the crack to expand out of a residual tensile stress area generated by the compression-compression cyclic load, applying a smaller tension-tension cyclic load in the process, selecting the tension-tension cyclic load, wherein the load is not too large or too small, the crack with too small load cannot continue to expand, and the crack cannot stop expanding after expanding out of the residual tensile stress area when the load is too large, so that the load needs to be reduced again, and the test efficiency is reduced. And after the crack expands out of the residual tensile stress area, gradually increasing the load to ensure that the crack expands and begins to expand, and measuring the crack expansion threshold value.
The adopted test sample is a central crack tensile M (T) test piece, a recommended central position of the test sample is provided with a hexagonal notch, the notch penetrates through the whole thickness direction, the upper side and the lower side of the hexagon are parallel to the width direction of the test piece, the distance between the upper side and the lower side is 1-5 mm, the left top point and the right top point are located on a central line in the horizontal direction, the distance between the two top points is 5-20 mm, the included angle between the two sides connecting each top point is 30-90 degrees, and the two sides are symmetrical relative to the central line in the horizontal direction.
The maximum compression load adopted by the pressure-pressure cyclic load is that the absolute value of the stress intensity factor of the crack tip is enabled
Wherein sigmayIs the yield strength of the material.
The stress ratio adopted by the pressure-pressure cyclic load is 10-40.
Limited by the size of the residual tensile stress area, after a certain cycle of pressure-pressure cyclic load, the residual tensile stress is not enough to continue the crack to propagate, and the crack stops propagating. Applying 100000-200000 cycles under normal pressure-pressure cyclic load to make crack expand to 0.1-0.3 mm.
The crack can be expanded by applying a load which is 50 to 80 percent of the fatigue crack expansion threshold value of the same material in the residual tensile stress area.
After the crack expands out of the residual tensile stress area, the crack stops expanding because the applied load is lower than the crack expansion threshold value of the material, at the moment, the load is added by 5-10% according to each stage of load, and when the crack can be observed to expand by 0.1mm after every 1000000 cycles, the fatigue crack expansion threshold value can be measured.
Examples
1. Test pieces capable of producing fatigue cracks under pressure-pressure cyclic loads were designed. The test piece is designed in such a manner that a notch capable of generating a crack is formed at the center of the test piece in order to facilitate the center crack tension m (t) of the test piece to which a compressive load is applied. One test piece size that can be achieved is given here, as shown in fig. 2: a hexagonal notch is processed in the center of the sample, the notch penetrates through the whole thickness direction, the upper side and the lower side of the hexagon are parallel to the width direction of the test piece, the distance between the upper side and the lower side is 3.5mm, the left top point and the right top point are positioned on the central line in the horizontal direction, the distance between the two top points is 10mm, the included angle between the two sides connecting each top point is 60 degrees, and the two sides are symmetrical relative to the central line in the horizontal direction;
2. estimating the fatigue crack pressure-pressure cyclic load and applying the load to generate the crack, wherein the maximum compression load is selected so that the absolute value of the stress intensity factor of the crack tip is selectedWherein sigmayThe stress ratio is 40 for the yield strength of the material. Applying 100000 cycles of pressure-pressure cyclic load to expand the crack to 0.2mm, wherein the crack stops expanding because the size of a residual tensile stress area generated by the compression load is limited;
3. calculating the size of the residual tensile stress area by adopting a finite element method;
4. applying a pull-pull cyclic load on the test piece, wherein the load value is 80% of the typical value of the fatigue crack propagation threshold value of the material type of the test piece, applying the load to propagate the crack until the crack tip does not propagate under 1000000 cycles, namely considering that the crack tip extends out of a residual pull stress area, and the crack does not propagate any more when the stress intensity factor of the crack tip is lower than the crack propagation threshold value due to the applied load;
5. and (3) gradually increasing the load, wherein the load increased in each step is 10% of the load of the upper step until the crack begins to expand, continuously adjusting the load to enable the crack to expand by 0.1mm under 1000000 cycles, and measuring the crack expansion threshold value.
Claims (6)
1. A crack propagation threshold value test method for prefabricating cracks by adopting pressure-pressure cyclic load is characterized in that,
(1) prefabricated crack
Designing a test piece capable of generating fatigue cracks under a pressure-pressure cyclic load, estimating the pressure-pressure cyclic load for prefabricating the fatigue cracks according to the elastic modulus of the material of the test piece and applying the load to generate cracks;
(2) propagating cracks out of the residual tensile stress region
Calculating the size of a residual tensile stress area generated by a compression-compression cyclic load by using a finite element method, applying the tensile-compression cyclic load on the test piece according to the typical value of the fatigue crack propagation threshold value of the material type of the test piece, wherein the load value is 50-80% of the typical value of the fatigue crack propagation threshold value of the material type of the test piece, and applying the load to propagate the crack until the crack tip does not propagate under 1000000 cycles, namely, the crack tip is considered to propagate out of the residual tensile stress area;
(3) testing fatigue crack propagation threshold
And (3) gradually increasing the load, wherein the load increased at each stage is 5-10% of the load of the upper stage until the crack starts to expand, continuously adjusting the load to enable the crack to expand by 0.1mm under 1000000 cycles, and measuring the crack expansion threshold value.
2. The method for testing the crack propagation threshold of the pre-fabricated crack by the aid of the compression-compression cyclic loads as claimed in claim 1, wherein the adopted test piece is a central crack tensile M (T) test piece, a hexagonal notch is formed in the center of the test piece and penetrates through the whole thickness direction, the upper side and the lower side of the hexagon are parallel to the width direction of the test piece, the distance between the upper side and the lower side is 1-5 mm, the left vertex and the right vertex are located on a central line in the horizontal direction, the distance between the two vertexes is 5-20 mm, and the included angle between the two sides connecting each vertex is 30-90 degrees and is symmetrical relative to the central line in the horizontal direction.
3. The method as claimed in claim 1, wherein the maximum compressive loading for the compressive-compressive cyclic loading is such that the absolute value of the stress intensity factor of the crack tip is
Wherein sigmayIs the yield strength of the material.
4. The crack propagation threshold value test method for the pre-crack by the compressive-compressive cyclic load as claimed in claim 1, wherein the stress ratio of the compressive-compressive cyclic load is 10-40.
5. The method as claimed in claim 1, wherein the pressure-pressure cyclic load is applied for 100000-200000 cycles to make the crack propagate to 0.1-0.3 mm.
6. The method for testing the crack propagation threshold value of the prefabricated crack by using the pressure-pressure cyclic load as claimed in claim 1, wherein the crack is propagated by applying a load of 50-80% of the fatigue crack propagation threshold value of the same type of material by using the residual tensile stress region.
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