CN108910081B - Method for determining relationship between metal structure defect tolerance stress and cycle number - Google Patents

Method for determining relationship between metal structure defect tolerance stress and cycle number Download PDF

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CN108910081B
CN108910081B CN201810676634.7A CN201810676634A CN108910081B CN 108910081 B CN108910081 B CN 108910081B CN 201810676634 A CN201810676634 A CN 201810676634A CN 108910081 B CN108910081 B CN 108910081B
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curve
stress
relation curve
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cycle number
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喻溅鉴
陈亚萍
张君男
胡磊
邹静
熊欣
高雅
崔韦
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China Helicopter Research and Development Institute
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B64—AIRCRAFT; AVIATION; COSMONAUTICS
    • B64F—GROUND OR AIRCRAFT-CARRIER-DECK INSTALLATIONS SPECIALLY ADAPTED FOR USE IN CONNECTION WITH AIRCRAFT; DESIGNING, MANUFACTURING, ASSEMBLING, CLEANING, MAINTAINING OR REPAIRING AIRCRAFT, NOT OTHERWISE PROVIDED FOR; HANDLING, TRANSPORTING, TESTING OR INSPECTING AIRCRAFT COMPONENTS, NOT OTHERWISE PROVIDED FOR
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Abstract

The invention provides a method for determining the relation between the tolerance stress and the cycle number of a metal structure defect, and relates to the field of helicopter structure fatigue design. The method comprises the steps of making a first tangent of a high-cycle defect tolerance average stress-cycle number curve through a starting point, intersecting the first tangent with the curve to form a first relation curve, and multiplying the stress of the first relation curve by a stress reduction coefficient of a metal structure to obtain a second relation curve; dividing the cycle number of the first relation curve by the service life dispersion coefficient of the metal structure to obtain a third relation curve; and taking the envelope curve of the second relation curve and the third relation curve as the relation curve of the metal structure defect tolerance stress and the cycle number. The invention can establish a full-range defect tolerance safety stress-cycle time curve adaptive to safety indexes based on the existing high-cycle defect tolerance stress-cycle time curve equation, and meets the requirements of defect tolerance design and verification.

Description

Method for determining relationship between metal structure defect tolerance stress and cycle number
Technical Field
The invention relates to the field of helicopter structure fatigue design, in particular to a method for determining the relation between the tolerance stress of a metal structure defect and the cycle number.
Background
The verification of the structural defect tolerance of the helicopter is a new requirement of airworthiness regulation clause 'fatigue assessment of 29.571 structure', and a defect tolerance stress-cycle number curve is one of important inputs for designing and verifying the structural defect tolerance of the helicopter. Due to different safety indexes, the traditional fatigue stress-cycle time curve is different from the defect tolerance stress-cycle time curve.
Disclosure of Invention
In order to solve the above problems, the present invention provides a method for determining relationship between metal structure defect tolerance stress and cycle number, comprising:
the method comprises the following steps of firstly, constructing a plane coordinate system, wherein the horizontal coordinate of the plane coordinate system is cycle times, the vertical coordinate of the plane coordinate system is tolerance stress, and generating a high-cycle defect tolerance average stress-cycle times curve under the plane coordinate system;
step two, making a first tangent line of the high cycle defect tolerance average stress-cycle number curve passing through the starting point, and intersecting the first tangent line with the tangent point of the curve, so that a first relation curve is formed by a straight line segment from the starting point to the tangent point and the high cycle defect tolerance average stress-cycle number curve from the tangent point to the cycle number upper limit, wherein the starting point is a point where a yield limit value is located under a coordinate system;
step three, multiplying the stress of the first relation curve by a stress reduction coefficient of the metal structure to obtain a second relation curve;
step four, dividing the cycle number of the first relation curve by the service life dispersion coefficient of the metal structure to obtain a third relation curve;
and fifthly, taking the envelope curve of the second relation curve and the third relation curve as a relation curve of the metal structure defect tolerance stress and the cycle number.
Preferably, the envelope includes:
making a second tangent of the third relation curve at the starting point of the third relation curve, wherein the starting point is the same as the abscissa of the starting point in the first step;
taking a first turning point on a second tangent line, making a third tangent line of the second relation curve, and intersecting the third tangent line with the second relation curve at the second turning point;
the envelope curve comprises a first line segment from the starting point to the first turning point, a second line segment from the first turning point to the second turning point, and a second relation curve from the second turning point to the upper limit of the cycle times.
Preferably, the first line segment is the same length as the second line segment.
The invention has the advantages that: the full-range defect tolerance safety stress-cycle number curve adaptive to the safety index can be established based on the existing high-cycle defect tolerance stress-cycle number curve equation, and the requirements of defect tolerance design and verification are met.
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FIG. 1 is a graph illustrating the relationship between the defect tolerance stress and the cycle number of the metal structure according to a preferred embodiment of the method for determining the relationship between the defect tolerance stress and the cycle number of the metal structure.
Detailed Description
In order to make the implementation objects, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be described in more detail below with reference to the accompanying drawings in the embodiments of the present invention. In the drawings, the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The described embodiments are only some, but not all embodiments of the invention. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention. Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
The invention relates to a method for determining the relationship between the defect tolerance stress and the cycle number of a metal structure, which comprises the following steps:
the method comprises the following steps of firstly, constructing a plane coordinate system, wherein the horizontal coordinate of the plane coordinate system is cycle times, the vertical coordinate of the plane coordinate system is tolerance stress, and generating a high-cycle defect tolerance average stress-cycle times curve under the plane coordinate system;
step two, making a first tangent line of the high cycle defect tolerance average stress-cycle number curve passing through the starting point, and intersecting the first tangent line with the tangent point of the curve, so that a first relation curve is formed by a straight line segment from the starting point to the tangent point and the high cycle defect tolerance average stress-cycle number curve from the tangent point to the cycle number upper limit, wherein the starting point is a point where a yield limit value is located under a coordinate system;
step three, multiplying the stress of the first relation curve by a stress reduction coefficient of the metal structure to obtain a second relation curve;
step four, dividing the cycle number of the first relation curve by the service life dispersion coefficient of the metal structure to obtain a third relation curve;
and fifthly, taking the envelope curve of the second relation curve and the third relation curve as a relation curve of the metal structure defect tolerance stress and the cycle number.
In this embodiment, the envelope includes:
making a second tangent of the third relation curve at the starting point of the third relation curve, wherein the starting point is the same as the abscissa of the starting point in the first step;
taking a first turning point on a second tangent line, making a third tangent line of the second relation curve, and intersecting the third tangent line with the second relation curve at the second turning point;
the envelope curve comprises a first line segment from the starting point to the first turning point, a second line segment from the first turning point to the second turning point, and a second relation curve from the second turning point to the upper limit of the cycle times.
In this embodiment, the lengths of the first line segment and the second line segment are the same.
Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
As shown in fig. 1, the main steps are as follows:
the first relation curve constructed in the second step is as the uppermost curve in fig. 1, and the specific method comprises the following steps:
determining a full-range defect tolerance mean stress-cycle number curve: over start point PmB(0.001,R02) High cycle defect tolerance mean stress-cycle number curves
Figure GDA0003192162390000031
Is intersected at the tangent point PmA(NmA,SmA) Straight line segment PmBPmA(SmA＜S≤R02) And curve
Figure GDA0003192162390000032
(S∞mf＜S≤SmA) Forming a first relation curve; wherein R is02For yield limit, S is the amplitude of the alternating load and N is the number of failure cycles (unit 10) corresponding to the value of S6Sub), S∞mfFor the average fatigue limit of the defect tolerance, Af and alphaf are respectively the curve shape parameters of the defect tolerance stress-cycle times.
In step three, a full-range defect tolerance reduction stress-cycle number curve is determined: multiplying the stress of the curve determined in the second step by a stress reduction coefficient kSThen, a full-range defect tolerance reduction stress-cycle number curve, i.e., a second relationship curve, is obtained, referring to the middle thicker dashed line in fig. 1;
in step four, a full range defect tolerance stress-reduction cycle number curve is determined: dividing the cycle number of the curve determined in the second step by the life dispersion coefficient kNThen, a full-range defect tolerance stress-reduction cycle number curve, i.e., a third relation curve, is obtained, referring to the middle thinner dotted line in fig. 1;
in the fifth step, determining a full-range defect tolerance safety stress-cycle number curve: determining an envelope curve-straight line segment P of the curve results of the third step and the fourth stepsCPsB(SsB＜S≤SsC)、PsBPsA(SsA＜S≤SsB) And curve
Figure GDA0003192162390000041
(S∞sf＜S≤SsA) I.e. to construct a full range defect tolerance safety stress-cycle number curve.
Note: the straight line segments in the steps all adopt a logarithmic coordinate system, wherein the abscissa is lgN.
A specific example of a metal structure will be described. The present embodiment is based on the existing high-cycle scratch defect tolerance stress-cycle number curve equation, and is characterized in that the scratch defect tolerance stress-cycle number curve (including the low-cycle scratch defect tolerance stress-cycle number curve) is determined by the following steps:
step two, confirmDetermining a full-range scratch defect tolerance average stress-cycle number curve: over start point PmB(0.001,R02) High cycle defect tolerance mean stress-cycle number curves
Figure GDA0003192162390000042
Is intersected at the tangent point PmA(0.00689,155.8), straight line segment PmBPmA(155.8＜S≤R02) And curve
Figure GDA0003192162390000043
(S is more than 68.7 and less than or equal to 155.8) forming a full-range defect tolerance average stress-cycle number curve; wherein R is02237.6 MPa;
in the third step, determining a full-range scratch defect tolerance shrinkage stress-cycle number curve: multiplying the stress of the curve determined in the second step by a stress reduction coefficient of 0.661 to obtain a full-range defect tolerance reduction stress-cycle number curve;
in the fourth step, determining a full-range scratch defect tolerance stress-shrinkage cycle number curve: dividing the cycle number of the curve determined in the step two by the life dispersion coefficient 7 to obtain a full-range defect tolerance stress-shrinkage cycle number curve;
in the fifth step, determining a full-range scratch defect tolerance safety stress-cycle number curve: determining an envelope curve-straight line segment P of the results of the three and four curvessCPsB(120.0＜S≤155.2)、PsBPsA(S is more than 83.9 and less than or equal to 120.0) and curve
Figure GDA0003192162390000044
(S is more than 45.4 and less than or equal to 83.9), namely a full-range scratch defect tolerance safety stress-cycle number curve is obtained; wherein P issC(0.001,155.2) is the intersection point of the curve determined in step four and the straight line N equal to 0.001, PsCPsBTangent to the curve determined in step four; per PsB(0.00230,120) making a curve
Figure GDA0003192162390000051
Tangent line of (1), tangent point is PsA(0.0158,83.9)。
The parameters of the tolerance stress-cycle times curve of the scratch defect of an aluminum alloy are shown in the table 1.
TABLE 1 parameters of stress-cycle times curve for scratch defect tolerance of certain aluminum alloy
Parameter name Parameter value
Yield limit R02，MPa 237.6
Mean fatigue limit of defect tolerance S∞mf，MPa 68.7
Defect tolerance stress-cycle number curve shape parameter Af 0.113
Defect tolerance stress-cycle number curve shape parameter alphaf 0.486
Stress reduction factor kS 0.661
Life spread coefficient kN 7
Defect margin safety fatigue limit S∞sf，MPa 45.4
The invention has the advantages that: the full-range defect tolerance safety stress-cycle number curve adaptive to the safety index can be established based on the existing high-cycle defect tolerance stress-cycle number curve equation, and the requirements of defect tolerance design and verification are met.
Finally, it should be pointed out that: the above examples are only for illustrating the technical solutions of the present invention, and are not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims (3)

1. A method for determining the relationship between the defect tolerance stress and the cycle number of a metal structure is characterized by comprising the following steps:
the method comprises the following steps of firstly, constructing a plane coordinate system, wherein the horizontal coordinate of the plane coordinate system is cycle times, the vertical coordinate of the plane coordinate system is tolerance stress, and generating a high-cycle defect tolerance average stress-cycle times curve under the plane coordinate system;
step two, a first tangent of the high-cycle defect tolerance average stress-cycle number curve is made to pass through a starting point, the first tangent and the high-cycle defect tolerance average stress-cycle number curve are intersected at a tangent point, a straight line segment from the starting point to the tangent point and the high-cycle defect tolerance average stress-cycle number curve from the tangent point to a cycle number upper limit jointly form a first relation curve, and the starting point is a point where a yield limit value is located under a coordinate system;
step three, multiplying the stress of the first relation curve by a stress reduction coefficient of the metal structure to obtain a second relation curve;
step four, dividing the cycle number of the first relation curve by the service life dispersion coefficient of the metal structure to obtain a third relation curve;
and fifthly, taking the envelope curve of the second relation curve and the third relation curve as a relation curve of the metal structure defect tolerance stress and the cycle number.
2. The method of determining metal structure defect tolerance stress versus cycle number of claim 1, wherein said envelope comprises:
making a second tangent of the third relation curve at the starting point of the third relation curve, wherein the starting point is the same as the abscissa of the starting point in the first step;
taking a first turning point on a second tangent line, making a third tangent line of the second relation curve, and intersecting the third tangent line with the second relation curve at the second turning point;
the envelope curve comprises a first line segment from the starting point to the first turning point, a second line segment from the first turning point to the second turning point, and a second relation curve from the second turning point to the upper limit of the cycle times.
3. The method of determining metal structure defect tolerance stress versus cycle number of claim 2, wherein the first line segment is the same length as the second line segment.
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CN110884685B (en) * 2019-12-04 2022-06-21 中国直升机设计研究所 Load monitoring method for helicopter blade
CN111046547B (en) * 2019-12-04 2022-09-16 中国直升机设计研究所 Method for rapidly calculating high-cycle life of rectangular composite flexible beam of helicopter

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CN1587967A (en) * 2004-08-10 2005-03-02 东华大学 Measuring device for flexural fatigue property of flexible material
CN102854252A (en) * 2012-09-10 2013-01-02 北京理工大学 Method and system used for detecting metal material fatigue state
CN102854245A (en) * 2011-06-27 2013-01-02 霍尼韦尔国际公司 Structural damage index mapping system and method
CN103380057A (en) * 2010-09-29 2013-10-30 航空机器股份有限公司 Novel systems and methods for non-destructive inspection of airplanes
CN106596301A (en) * 2016-11-30 2017-04-26 中国直升机设计研究所 Method for determining inspection period of metal structure defects of helicopter

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Publication number Priority date Publication date Assignee Title
CN1587967A (en) * 2004-08-10 2005-03-02 东华大学 Measuring device for flexural fatigue property of flexible material
CN103380057A (en) * 2010-09-29 2013-10-30 航空机器股份有限公司 Novel systems and methods for non-destructive inspection of airplanes
CN102854245A (en) * 2011-06-27 2013-01-02 霍尼韦尔国际公司 Structural damage index mapping system and method
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