CN112733398A - Method for determining repair-free limit of impact damage of pit-type hard object - Google Patents
Method for determining repair-free limit of impact damage of pit-type hard object Download PDFInfo
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Abstract
The invention discloses a method for determining the repair-free limit of impact damage of a pit-type hard object, which comprises the following steps: extracting steady-state stress of blades of a fan/compressor of the engine and modal vibration stress of at least the first six orders; determining the most dangerous shape of the pit type hard object caused by impact damage; adding pit type damage to a designated position of a blade basin or a blade back of the blade, calculating stress distribution of the damaged bottom of the pit, and calculating the stress intensity factor range of the damaged bottom of the pit under the steady-state stress and the vibration stress; establishing a stress ratio related pit type damage crack non-propagation model under high cycle fatigue load to obtain a repair-free limit of the pit type damage at the designated position of the blade; and (4) another position of the blade basin or the blade back is appointed, the third step and the fourth step are repeated, and the repair-free limit of the pit-type damage of the position is obtained until the repair-free limit of the pit-type damage of all the positions is obtained. The invention forms a standardized repair-free limit making process aiming at the damage of the blade basin/blade back pit type of the fan/compressor blade of the aircraft engine.
Description
Technical Field
The invention relates to a method for determining a repair-free limit of impact damage of a pit-type hard object, and belongs to the field of design and maintenance of damage tolerance of a hard object of an aircraft engine blade.
Background
Hard object impact damage is caused by the fact that an aircraft engine inevitably sucks hard objects such as metal, fragments, gravel, stones and the like to impact a blade rotating at a high speed during takeoff, landing or low-altitude flight of the aircraft. The impact damage of the hard object can accelerate the premature initiation of high-cycle fatigue cracks of the blade under the action of centrifugal force and vibration load, thereby causing the blade fracture accident and having serious influence on the working reliability of the engine.
In order to meet the continuing airworthiness requirements of aircraft engines, engine original equipment manufacturers must develop reasonable service manuals for users to guide engineers to maintain engine blades according to normative means. Among them, the "available limit" for judging whether the damaged blade of the hard object is available or not or is free of repair is one of the important contents for establishing a repair manual. At present, the main size for judging the damage severity of the hard object is the damage depth, and the maximum allowable damage depth is often adopted as the available limit of the blade after the hard object damage occurs in an engine service manual. The purpose of making available limit is to reduce the times of disassembly, repair or replacement of the engine blade after suffering from impact damage of hard objects to a certain extent, and improve the economy and readiness of the engine.
At present, engine companies do not make a standardized program of blade usable limits after hard object damage, the usable limits of engine blades newly designed in the past are usually based on the use and maintenance experience of old engines, however, with the continuous development of blade design technologies, a novel blade structure (such as a blisk, a hollow blade and the like) enables the empirical extrapolation mode to face a huge challenge.
Dimple-type damage is damage caused by hard objects impacting a thicker portion of the pressure (or bucket) or suction (or blade back) side of a blade, and such damage is one of the most common types of hard object impact damage to engine fan/compressor blades, and may even occur on turbine blades, for example, due to the internal falling objects of the engine impacting the turbine blades downstream with high-speed airflow. The invention provides a method for determining the usable limit of impact damage of a pit-type hard object, aiming at solving the problem of formulating the usable limit of reasonable and standard impact damage of the pit-type hard object, and considering the most common high-cycle fatigue failure of the current aeroengine blade.
Disclosure of Invention
The invention aims to stand at the most common high-cycle fatigue failure angle of the blade of the current aero-engine, and provides a method for determining the usable limit of impact damage of a pit-type hard object, so as to solve the problem that the usable limit of the impact damage of the pit-type hard object lacks reasonable specifications at present to make a flow.
In order to achieve the purpose, the invention adopts the technical scheme that:
a method for determining the repair-free limit of impact damage of a pit-type hard object comprises the following steps:
firstly, extracting steady-state stress and modal vibration stress of at least the first six orders of an engine fan/compressor blade by combining an aircraft engine rotating speed spectrum, a temperature spectrum and a Campbell diagram;
secondly, determining the most dangerous shape of the pit type hard object caused by impact damage through external field investigation, simulation test and stress concentration degree analysis;
thirdly, adding pit type damage to the designated position of a blade basin or a blade back of the blade, calculating stress distribution of the damaged bottom of the pit, and calculating the stress intensity factor range delta K of the damaged bottom of the pit under the steady-state stress and the vibration stress;
fourthly, establishing a pit type damage crack non-expansion model related to the stress ratio under the high cycle fatigue load, and judging the pit depth when the dangerous shape pit type damage does not generate crack expansion under the first six-order modal vibration load, namely the allowable pit depth under the order modal vibration load, so as to obtain the repair-free limit of the pit type damage at the appointed position of the blade;
and fifthly, another position of the blade basin or the blade back is designated, the third step and the fourth step are repeated, and the repair-free limit of the pit-shaped damage of the position is obtained until the repair-free limit of the pit-shaped damage of all the positions of the blade basin or the blade back is obtained.
In the first step, the steady state stress σstaFor superposition of centrifugal, aerodynamic and thermal stresses to which the blades of the fan/compressor of an engine are subjected, modal vibratory stress sigmadynThe characteristic frequency, the natural vibration mode and the modal vibration amplitude are calculated.
In the second step, determining the impact condition of the outfield typical hard object through the outfield investigation of the pit type damage; the pit-type damage simulation test is carried out under the laboratory condition, a light gas gun test system is adopted to carry out the pit-type hard object impact damage simulation test under the test conditions of outfield typical impact speed, impact angle, hard object type, hard object shape and hard object size, the hard object impact shape and the pit-type damage shape are observed to establish pit-type damage geometric models in different shapes, the stress concentration degree of the pit-type hard object impact damage in different shapes is calculated through a finite element numerical analysis method, and the shape with the largest stress concentration degree is selected as the most dangerous shape of the pit-type damage.
In the third step, the geometric model of the most dangerous shape of the pit-type damage established in the second step is added to the designated position of the blade basin or the blade back of the blade in a Boolean operation mode to form a blade geometric model with the pit-type damage, the stress distribution at the bottom of the pit-type damage is calculated by a finite element numerical analysis method, and a power function distribution stress expression is adopted for fitting, namely:
wherein, sigma (x) is stress distribution on the crack surface, a is the surface crack depth of the pit type damage, x is the coordinate along the crack propagation direction, and the origin of the coordinate is a unilateral crack and a front edge pointOf intersection point, σiIs a polynomial coefficient, i is a polynomial index, and n is less than or equal to 7; calculating stress intensity factor range delta K of pit damage bottom crack tip under steady-state stress and vibration stress by adopting a universal weight function methodmaxNamely:
ΔK＝Kmax-Kmin
ΔKmax＝max(ΔKA，ΔKB)
wherein, KmaxAnd KminRespectively the maximum stress intensity factor and the minimum stress intensity factor of the crack under the cyclic load, when the stress ratio R is>At 0, KminNot equal to 0, when the stress ratio R is less than or equal to 0, Kmin＝0；KAAnd mA(x, a) and KBAnd mB(x, a) stress intensity factor and universal weight function of the deepest point and surface point of the surface crack at the bottom of the pit-type damage respectively, M1A、M2A、M3AAnd M1B、M2B、M3BIs a general weight function coefficient; the maximum stress intensity factor range of the crack tip at the bottom of the pit type damage is the maximum value of the crack tip points A and B; Δ KmaxAs a function of the depth d of the pit-type damage with the dangerous shape and the crack size a.
And the surface cracks are I-shaped cracks and are positioned at the bottom of the pit-shaped damage, the depth of the surface cracks is the maximum distribution depth of the microscopic damage at the bottom of the pit-shaped damage, and the surface cracks are obtained by carrying out metallographic observation on the most dangerous shape of the pit-shaped hard object impact damage obtained in the second step.
In the fourth step, the stress ratio related pit type damage crack non-propagation model under the high cycle fatigue load is as follows:
ΔKmax(d,a)≤ΔKth(R)
wherein the crack propagation threshold value delta Kth(R) is a material constant related to the stress ratio R, which is constant, i.e., Δ K, for modal vibrations at a given order at a given position of the bladeth(R) is a constant value; the surface crack depth a is a fixed value, and the delta K is increased along with the increase of the pit type damage depth dmax(d, a) monotonically increasing, when Δ Kmax(d,a)＝ΔKthAnd (R), the damage depth d is the allowable pit depth under the vibration of the order mode.
And in the fourth step, the minimum allowable damage depth value under the first six-order modal vibration is taken as the repair-free limit of the pit type damage at the position of the blade.
Has the advantages that: the invention provides a reasonable and standard available limit determination method and a reasonable and standard available limit determination process for pit type hard object damage commonly suffered in the use process of the blade of the aero-engine. The invention considers the most common high cycle fatigue failure of the blade after being damaged by hard objects, determines the allowable pit depth by adopting a simple and efficient pit bottom surface crack stress intensity factor integral calculation method and a crack non-propagation principle, provides a standard step of establishing the repair-free limit of the tearing/crack type damage in the front edge and the rear edge of the blade, and the process is convenient to realize by a computer program so as to improve the engineering efficiency.
Drawings
FIG. 1 is a Campbell chart for determining the frequency and amplitude of the first six orders of modal vibration of an engine blade;
FIG. 2 is a simplified high cycle fatigue load steady state stress and vibrational stress amplitude example;
FIG. 3 is an example of a typical dimple-type damage macro-topography;
FIG. 4 is an example of a geometric modeling approach for impact damage to a spherical hard object;
FIG. 5 is an example of dimple type damage finite element meshing;
FIG. 6 is an example of stress concentration distribution at the bottom of a pit-type damaged section;
FIG. 7 is a dimple type damage bottom crack model;
FIG. 8 is a flow chart of a method of the present invention.
Detailed Description
The invention is further explained below with reference to the drawings.
The invention discloses a method for determining the repair-free limit of impact damage of a pit-type hard object, which comprises the following steps of:
firstly, extracting steady-state stress and modal vibration stress of at least the first six orders of an engine fan/compressor blade by combining an aircraft engine rotating speed spectrum, a temperature spectrum and a Campbell diagram; the method specifically comprises the following steps:
the high cycle fatigue failure is the most main failure mode after the aircraft engine blade is subjected to hard object impact damage, so that the repair-free limit of the pit type damage is determined by considering which pit type damage depth meets the requirement of high cycle fatigue performance. The crack of the damaged part does not expand under the high cycle fatigue load (namely infinite life) is the simplest and intuitive embodiment for meeting the high cycle fatigue performance requirement. Therefore, it is first necessary to determine the high cycle fatigue loads of the aeroengine blades. High cycle fatigue loads that lead to crater type damage fatigue failure of aircraft engine fan/compressor blades include steady state stresses and vibrational stresses. Stress σ in steady statestaThe rotational speed and the temperature of the blades are determined by a rotational speed spectrum and a temperature spectrum for the superposition of the centrifugal stress, the aerodynamic stress and the thermal stress to which the blades are subjected. The actual vibration stress process of the engine blade is difficult to measure and predict, and the vibration stress is the vibration stress under the dangerous condition of the blade, namely the resonance condition, and is a conservative estimation method. Wherein the modal vibratory stress σ of the bladedynAnd calculating through the natural frequency, the natural vibration mode and the modal vibration amplitude. The modal vibration amplitude is determined by the engine campbell diagram, as shown in fig. 1.
Steady state stress sigma of high cycle fatigue loadstaAnd the vibration stress sigmadynAn example of (2) is shown in figure 2.
Secondly, determining the most dangerous shape of the pit type hard object caused by impact damage through external field investigation, simulation test and stress concentration degree analysis; the method specifically comprises the following steps:
and determining the impact condition of the outfield typical hard object through the outfield investigation of the pit type damage. The pit-type damage simulation test is carried out under the laboratory condition, a light gas gun test system is adopted to carry out the pit-type hard object impact damage simulation test under the test conditions of outfield typical impact speed (for example, the rotating linear speed of a part of an engine blade which is frequently subjected to hard object impact damage is 300m/s), typical impact angle (30 degrees and 60 degrees), typical hard object type (steel and titanium alloy), typical hard object shape (spherical and square, generally, a square hard object can simulate the damage which is similar to the impact damage shape of the outfield hard object) and typical hard object size (3mm) and the like, the hard object impact shape and the pit-type damage shape are observed to establish pit-type damage geometric models with different shapes, and calculating the stress concentration degree of the impact damage of the pit-type hard objects in different shapes by a finite element numerical analysis method, and selecting the shape with the maximum stress concentration degree as the most dangerous shape of the pit-type damage. Figure 3 shows the damage patterns obtained by impacting a blade simulator with spherical and square steel hard objects. The invention takes the impact damage of a spherical hard object as an example, and shows the geometric modeling process of the pit-type damage, as shown in fig. 4, the finite element meshing of the pit-type damage is shown in fig. 5, and the stress concentration distribution of the pit-type damage is shown in fig. 6.
Thirdly, adding pit damage to the designated position of a blade basin or a blade back of the blade, calculating stress distribution of the damaged bottom of the pit, and calculating the stress intensity factor range delta K of the damaged bottom of the pit under the steady-state stress and the vibration stress; the method specifically comprises the following steps:
adding the geometric model of the most dangerous shape of the pit-type damage established in the second step to the designated position of the blade basin or the blade back of the blade in a Boolean operation mode to form a blade geometric model with the pit-type damage, calculating the stress distribution at the bottom of the pit-type damage by a finite element numerical analysis method, and adopting power function distribution stress expression fitting, namely:
wherein σ (x) is stress distribution on the crack surface, a is surface crack depth of the pit bottom, the crack size a is maximum depth of microscopic damage region at the pit type damage bottom (which is a dangerous hypothesis) or half of the limit value of nondestructive testing 0.38mm, the surface crack length is 2 times of the surface crack depth, as shown in fig. 7, x is coordinate along the crack propagation direction, the origin of the coordinate is the intersection point of the unilateral crack and the leading edge point, σ is the intersection point of the unilateral crack and the leading edge point, andiis a polynomial coefficient, i is a polynomial index, and n is less than or equal to 7. Calculating stress intensity factor range delta K of pit damage bottom crack tip under steady-state stress and vibration stress by adopting a universal weight function methodmaxNamely:
ΔK＝Kmax-Kmin
ΔKmax＝max(ΔKA，ΔKB)
wherein, KmaxAnd KminRespectively the maximum stress intensity factor and the minimum stress intensity factor of the crack under the cyclic load, when the stress ratio R is>At 0, KminNot equal to 0, when the stress ratio R is less than or equal to 0, Kmin＝0。KAAnd mA(x, a) and KBAnd mB(x, a) is a pit-type damage bottom surface crackStress intensity factor and universal weight function, M, for the deepest and surface points1A、M2A、M3AAnd M1B、M2B、M3BAre coefficients of a general weight function. The maximum stress intensity factor range of the crack tip at the bottom of the pit type damage is the maximum value of the point A and the point B of the crack tip. Δ KmaxAs a function of the depth d of the pit-type damage with the dangerous shape and the crack size a.
Fourthly, establishing a pit type damage crack non-expansion model related to the stress ratio under the high cycle fatigue load, judging the pit depth when the dangerous shape pit type damage does not generate crack expansion under the first six-order modal vibration load, namely the allowable pit depth under the first-order modal vibration load, and finally selecting the minimum allowable pit depth under the first six-order modal vibration load as the repair-free limit of the pit type damage at the specified position of the blade;
the pit type damage crack non-propagation model related to the stress ratio under the high cycle fatigue load is as follows:
ΔKmax(d,a)≤ΔKth(R)
wherein the crack propagation threshold value delta Kth(R) is the stress ratio R-related material constant:
when 0 ≦ R < 1:
when 1 ≦ R < 0:
wherein the content of the first and second substances,
the range of effective stress intensity factors when the stress ratio R is 0,
A0＝0.00729、A1＝1.0108、A2＝0.3959、A3the number of coefficients is 0.10356,
the stress intensity factor range when the stress ratio R is 0,
for modal vibrations at a specified order at a specified position of the blade, the stress ratio R is a constant value, i.e. Δ Kth(R) is a constant value. The crack size a is a constant value, and delta K is increased along with the increase of the pit type damage depth dmax(d, a) monotonically increasing, when Δ Kmax(d,a)＝ΔKthAnd (R), the damage depth d is the allowable pit depth under the vibration of the order mode.
And fifthly, another position of the blade basin or the blade back is designated, the third step and the fourth step are repeated to obtain the repair-free limit of the pit-type damage of the position until the repair-free limit of the pit-type damage of all the positions of the blade basin or the blade back is obtained.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (7)
1. A method for determining the repair-free limit of impact damage of a pit-type hard object is characterized by comprising the following steps: the method comprises the following steps:
firstly, extracting steady-state stress and modal vibration stress of at least the first six orders of an engine fan/compressor blade by combining an aircraft engine rotating speed spectrum, a temperature spectrum and a Campbell diagram;
secondly, determining the most dangerous shape of the pit type hard object caused by impact damage through external field investigation, simulation test and stress concentration degree analysis;
thirdly, adding pit type damage to the designated position of a blade basin or a blade back of the blade, calculating stress distribution of the damaged bottom of the pit, and calculating the stress intensity factor range delta K of the damaged bottom of the pit under the steady-state stress and the vibration stress;
fourthly, establishing a pit type damage crack non-expansion model related to the stress ratio under the high cycle fatigue load, and judging the pit depth when the dangerous shape pit type damage does not generate crack expansion under the first six-order modal vibration load, namely the allowable pit depth under the order modal vibration load, so as to obtain the repair-free limit of the pit type damage at the appointed position of the blade;
and fifthly, another position of the blade basin or the blade back is designated, the third step and the fourth step are repeated, and the repair-free limit of the pit-shaped damage of the position is obtained until the repair-free limit of the pit-shaped damage of all the positions of the blade basin or the blade back is obtained.
2. The method for determining the repair-free limit of impact damage of a dimple-type hard object according to claim 1, wherein: in the first step, the steady state stress σstaFor superposition of centrifugal, aerodynamic and thermal stresses to which the blades of the fan/compressor of an engine are subjected, modal vibratory stress sigmadynThe characteristic frequency, the natural vibration mode and the modal vibration amplitude are calculated.
3. The method for determining the repair-free limit of impact damage of a dimple-type hard object according to claim 1, wherein: in the second step, determining the impact condition of the outfield typical hard object through the outfield investigation of the pit type damage; the pit-type damage simulation test is carried out under the laboratory condition, a light gas gun test system is adopted to carry out the pit-type hard object impact damage simulation test under the test conditions of outfield typical impact speed, impact angle, hard object type, hard object shape and hard object size, the hard object impact shape and the pit-type damage shape are observed to establish pit-type damage geometric models in different shapes, the stress concentration degree of the pit-type hard object impact damage in different shapes is calculated through a finite element numerical analysis method, and the shape with the largest stress concentration degree is selected as the most dangerous shape of the pit-type damage.
4. The method for determining the repair-free limit of impact damage of a dimple-type hard object according to claim 1, wherein: in the third step, the geometric model of the most dangerous shape of the pit-type damage established in the second step is added to the designated position of the blade basin or the blade back of the blade in a Boolean operation mode to form a blade geometric model with the pit-type damage, the stress distribution at the bottom of the pit-type damage is calculated by a finite element numerical analysis method, and a power function distribution stress expression is adopted for fitting, namely:
wherein σ (x) is stress distribution on the crack surface, a is surface crack depth of the pit type damage, x is coordinate along crack propagation direction, origin of coordinate is intersection point of single-side crack and leading edge point, σiIs a polynomial coefficient, i is a polynomial index, and n is less than or equal to 7; calculating stress intensity factor range delta K of pit damage bottom crack tip under steady-state stress and vibration stress by adopting a universal weight function methodmaxNamely:
ΔK＝Kmax-Kmin
ΔKmax＝max(ΔKA，ΔKB)
wherein, KmaxAnd KminRespectively the maximum stress intensity factor and the minimum stress intensity factor of the crack under the cyclic load, when the stress ratio R is>At 0, KminNot equal to 0, when the stress ratio R is less than or equal to 0, Kmin＝0；KAAnd mA(x, a) and KBAnd mB(x, a) stress intensity factor and universal weight function of the deepest point and surface point of the surface crack at the bottom of the pit-type damage respectively, M1A、M2A、M3AAnd M1B、M2B、M3BIs a general weight function coefficient; the maximum stress intensity factor range of the crack tip at the bottom of the pit type damage is the maximum value of the crack tip points A and B; Δ KmaxAs a function of the depth d of the pit-type damage with the dangerous shape and the crack size a.
5. The method for determining the repair-free limit of impact damage of a dimple-type hard object according to claim 4, wherein: and the surface cracks are I-shaped cracks and are positioned at the bottom of the pit-shaped damage, the depth of the surface cracks is the maximum distribution depth of the microscopic damage at the bottom of the pit-shaped damage, and the surface cracks are obtained by carrying out metallographic observation on the most dangerous shape of the pit-shaped hard object impact damage obtained in the second step.
6. The method for determining the repair-free limit of impact damage of a dimple-type hard object according to claim 1, wherein: in the fourth step, the stress ratio related pit type damage crack non-propagation model under the high cycle fatigue load is as follows:
ΔKmax(d,a)≤ΔKth(R)
wherein the crack propagation threshold value delta Kth(R) is a material constant related to the stress ratio R, which is constant, i.e., Δ K, for modal vibrations at a given order at a given position of the bladeth(R) is a constant value; the surface crack depth a is a fixed value, and the delta K is increased along with the increase of the pit type damage depth dmax(d, a) monotonically increasing, when Δ Kmax(d,a)＝ΔKthAnd (R), the damage depth d is the allowable pit depth under the vibration of the order mode.
7. The method for determining the repair-free limit of impact damage of a dimple-type hard object according to claim 1, wherein: and in the fourth step, the minimum allowable damage depth value under the first six-order modal vibration is taken as the repair-free limit of the pit type damage at the position of the blade.
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