Method for calculating fatigue strength of cabin structure of wind turbine
Technical Field
The invention belongs to the technical field of calculation of fatigue strength of a wind turbine cabin structure, and particularly relates to a calculation method of fatigue strength of a wind turbine cabin structure.
Background
The cabin structure is one of the most critical and most complex bearing components in the permanent magnet direct drive wind generating set, and the design reliability directly influences the running safety of the set. In hundreds of wind power accidents at home and abroad in the past decade, most of the wind power accidents are caused by fatigue failure. The design life of a wind turbine is usually 20 years, and the wind turbine must bear alternating stress generated by alternating load and possibly fatigue damage in the design life, so that the safety and reliability of the wind turbine generator set must be checked by scientific and standard calculation.
When the fatigue strength analysis is carried out on the wind turbine cabin structure in the past, the consideration of factors influencing the structural fatigue strength performance is incomplete, the external load only considers the load under the fixed coordinate system of the hub center, the SN curve only adopts the SN curve of one standard sample, and the calculation result is inaccurate and unreliable.
Disclosure of Invention
The invention aims to enable fatigue check of a nacelle structure of a wind turbine to be more accurate and reliable, solve the problems of incomplete consideration of fatigue strength influence factors, inaccurate fatigue damage calculation and the like in the traditional calculation process, obtain stress results under unit load components and generator gravity load through a finite element analysis method, obtain stress time sequence through combining fatigue time sequence, obtain stress spectrum through a rain flow counting method, establish S-N curve families under different stress ratios of materials, and finally calculate final damage through a Miner linear cumulative damage theory.
The invention is realized in the following way:
a calculation method for fatigue strength of a wind turbine cabin structure specifically comprises the following steps:
step one: determination of stress spectrum
Firstly, obtaining a linear relation between load and part stress; the stress time course is calculated by adopting a finite element analysis method, namely, the linear product of the finite element calculation result of the structural unit load and the gravity load of the generator and the time load course is used for calculating the stress time course, and the stress formula of the wind turbine cabin structure is as follows:
σx(t)=SxFx*Fx(t)+SxFy*Fy(t)+SxFz*Fz(t)+SxMx*Mx(t)+SxMy*My(t)+SxMz*Mz(t)+SxG
σy(t)=SyFx*Fx(t)+SyFy*Fy(t)+SyFz*Fz(t)+SyMx*Mx(t)+SyMy*My(t)+SyMz*Mz(t)+SyG
σz(t)=SzFx*Fx(t)+SzFy*Fy(t)+SzFz*Fz(t)+SzMx*Mx(t)+SzMy*My(t)+SzMz*Mz(t)+SzG
τxy(t)=SxyFx*Fx(t)+SxyFy*Fy(t)+SxyFz*Fz(t)+SxyMx*Mx(t)+SxyMy*My(t)+SxyMz*Mz(t)+SxyG
τyz(t)=SyzFx*Fx(t)+SyzFy*Fy(t)+SyzFz*Fz(t)+SyzMx*Mx(t)+SyzMy*My(t)+SyzMz*Mz(t)+SyzG
τxz(t)=SxzFx*Fx(t)+SxzFy*Fy(t)+SxzFz*Fz(t)+SxzMx*Mx(t)+SxzMy*My(t)+SxzMz*Mz(t)+SxzG
wherein: σx, σy, σz, τxy, τyz, τxz are stress components; fx (t), fy (t), fz (t), mx (t), my (t), mz (t) are temporal load history components; others are finite element stress component results;
determining the average stress and the stress range of each stress cycle by using a rain flow counting method through Ncode software, and finally determining a stress spectrum, namely a matrix relation of one-to-one correspondence of the average stress, the stress amplitude and the cycle number;
step two: calculating S-N curve family of material under different stress ratios
According to GL2010 specifications, determining an S-N curve family of the corresponding material, wherein a specific formula refers to the content in a fifth chapter annex B in the specifications; all the formulas are programmed into excel for presentation; wherein the wall thickness of the structure is input according to the maximum thickness, and the tensile strength and the yield strength of the material are referred to accessory related standards; roughness, quality level and flaw detection method refer to relevant standards, and in order to obtain S-N curves under different stress ratios, the stress ratio is taken as a value: 0.5,0.25,0, -0.5, -1, -2, etc.; finally, an S-N curve family of the cabin structural material under different stress ratios is obtained, as shown in figure 1;
step three: cabin structural fatigue strength check
Converting a stress spectrum into full cycle times under each stress level by a rain flow counting method by combining a material S-N curve family synthesized according to specifications to obtain a rain flow matrix, comparing the rain flow matrix with the material S-N curve family, performing interpolation calculation on adjacent curves which are not on stress ratio curves, extracting different types of stresses in a cabin structure by adopting an absolute value maximum principal stress method, a signed mises stress method and a critical plane method, and finally calculating a damage value; calculating the fatigue life according to the fatigue accumulated damage theory, wherein the fatigue algorithm adopted is a Miner linear accumulated damage theory; the total fatigue damage is:
wherein the method comprises the steps of
d i -damage under i-th class load;
m-total number of load levels in the load spectrum;
n i -number of actions of the i-th stage load;
N i -number of permitted roles of the i-th stage load.
The beneficial effects of the invention are as follows:
when the fatigue strength analysis is carried out on the wind turbine cabin structure in the past, the consideration of factors influencing the structural fatigue strength performance is incomplete, the external load only considers the load under the fixed coordinate system of the hub center, the SN curve only adopts the SN curve of one standard sample, and the calculation result is inaccurate and unreliable. According to the invention, external wind load is considered, the gravity influence of the generator is also considered, an S-N curve family under the multi-stress ratio which can clearly represent the cabin structural material is used, and finally the fatigue damage of the cabin structure is calculated through an absolute value maximum main stress method, a signed mises stress method and a critical plane method. The fatigue damage of the wind turbine cabin structure can be accurately analyzed more accurately and more closely fit with the actual load working condition, and the running safety and reliability of wind turbine equipment are improved.
Drawings
FIG. 1 is a family of S-N curves for multiple stress ratios.
Detailed Description
The invention is further described below with reference to the drawings and examples.
A method for calculating fatigue strength of a nacelle structure of a wind turbine comprises the following steps:
step one: determination of stress spectrum
Linear fatigue refers to fatigue caused by a load time series under the condition that the stress magnitude of a part linearly changes with the external load magnitude. Wind turbine nacelle structural fatigue is a type of fatigue. First, a linear relationship between load and part stress is obtained. The stress time course is generally calculated by adopting a finite element analysis method, namely, the linear product of the finite element calculation result under the structural unit load and the gravity load of the generator and the time load course is utilized to calculate the stress time course, and the stress formula of the wind turbine cabin structure is as follows (the calculation formula is compiled by the inventor according to specific working conditions).
σx(t)=SxFx*Fx(t)+SxFy*Fy(t)+SxFz*Fz(t)+SxMx*Mx(t)+SxMy*My(t)+SxMz*Mz(t)+SxG
σy(t)=SyFx*Fx(t)+SyFy*Fy(t)+SyFz*Fz(t)+SyMx*Mx(t)+SyMy*My(t)+SyMz*Mz(t)+SyG
σz(t)=SzFx*Fx(t)+SzFy*Fy(t)+SzFz*Fz(t)+SzMx*Mx(t)+SzMy*My(t)+SzMz*Mz(t)+SzG
τxy(t)=SxyFx*Fx(t)+SxyFy*Fy(t)+SxyFz*Fz(t)+SxyMx*Mx(t)+SxyMy*My(t)+SxyMz*Mz(t)+SxyG
τyz(t)=SyzFx*Fx(t)+SyzFy*Fy(t)+SyzFz*Fz(t)+SyzMx*Mx(t)+SyzMy*My(t)+SyzMz*Mz(t)+SyzG
τxz(t)=SxzFx*Fx(t)+SxzFy*Fy(t)+SxzFz*Fz(t)+SxzMx*Mx(t)+SxzMy*My(t)+SxzMz*Mz(t)+SxzG
Wherein: σx, σy, σz, τxy, τyz, τxz are stress components; fx (t), fy (t), fz (t), mx (t), my (t), mz (t) are temporal load history components; others are finite element stress component results.
Fatigue loading spectra are typically determined using a cycle count method. There are tens of methods available for cycle counting, with the most widely used being the rain flow counting method. The method has sufficient mechanical basis and high accuracy, and the average stress and the stress range of each stress cycle are determined by using a rain flow counting method through Ncode software, so that the stress spectrum, namely the matrix relation of one-to-one correspondence of the average stress, the stress amplitude and the cycle number is finally determined.
Step two: calculating S-N curve family of material under different stress ratios
According to the GL2010 specification, an S-N curve family of the corresponding material is determined, a specific formula refers to the content in an annex B of a fifth chapter in the specification, and all the used formulas are programmed into excel for calculation convenience and are presented. Wherein the wall thickness of the structure is input according to the maximum thickness, and the tensile strength and the yield strength of the material are referred to accessory related standards; roughness, quality level and flaw detection method refer to relevant standards, and in order to obtain S-N curves under different stress ratios, the stress ratio is taken as a value: 0.5,0.25,0, -0.5, -1, -2, etc. Finally, an S-N curve family of the cabin structural material under different stress ratios is obtained, as shown in figure 1.
Step three: cabin structural fatigue strength check
And combining a material S-N curve family synthesized according to specifications, converting a stress spectrum into full cycle times under each stress level through a rain flow counting method to obtain a rain flow matrix, comparing the rain flow matrix with the material S-N curve family, performing interpolation calculation on adjacent curves which are not on stress ratio curves, respectively extracting different types of stresses in a cabin structure by adopting an absolute value maximum principal stress method, a signed mises stress method and a critical plane method, and finally calculating a damage value. And (3) calculating the fatigue life according to the fatigue cumulative damage theory, wherein the fatigue algorithm adopted is the Miner linear cumulative damage theory. The total fatigue damage is:
wherein the method comprises the steps of
d i -damage under i-th class load;
m-total number of load levels in the load spectrum;
n i -number of actions of the i-th stage load;
N i -number of permitted roles of the i-th stage load.
The embodiment of the present invention has been described in detail with reference to the embodiments, but the present invention is not limited to the embodiments, and various changes can be made within the knowledge of those skilled in the art without departing from the spirit of the present invention. The invention may be practiced otherwise than as specifically described in the specification.