CN116698993A - Method for evaluating durability of metal structural member - Google Patents
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Abstract
The invention discloses a durability evaluation method of a metal structural member, which comprises the following steps of: adopting an ultrasonic probe to detect cracks of the metal structural member; based on crack morphology information of the position, the size and the shape of cracks in the metal structural member, converting the metal structural member into a finite element model; applying unit displacement to the front end of the crack to introduce a non-zero crack tensile stress, so that stress concentration is generated at the front end of the crack, and further calculating a stress intensity factor K_I at the tip of the crack; acquiring material parameters of the metal structural member, and calculating fracture toughness of the metal structural member; calculating crack growth rate; according to the crack expansion rate and the crack morphology information, the fatigue cycle number is combined, and the durability of the metal structural member is evaluated; the durability evaluation method for the metal structural part can comprehensively and accurately evaluate the durability of the technical structural part and determine the failure condition of the metal structural part, thereby providing a basis for subsequent maintenance and maintenance of the metal structural part.
Description
Technical Field
The invention relates to a durability evaluation method for a metal structural member.
Background
The durability evaluation of a metal structural member refers to a method of evaluating fatigue and life that the metal structural member is subjected to during actual use. The evaluation can predict the possible failure mechanism and failure form of the metal structural part in the service life, and the problems of failure time, failure reason and the like.
In the design and manufacturing process of a metal structural member, durability evaluation is a very important link. Through durability evaluation, the problems of safety and reliability of the metal structural part in long-term use can be effectively solved, and an important reference basis is provided for design, manufacture and maintenance of the metal structural part. Meanwhile, according to different application scenes and use environments, the most suitable materials, design and processing technology can be selected through durability evaluation, so that the service life and performance of the metal structural member are improved.
The existing durability evaluation of the metal structural part is not comprehensive and accurate enough, and the possible failure mode and failure time of the metal structural part in the service life are difficult to judge.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide the metal structural member durability evaluation method which can comprehensively and accurately evaluate the durability of the technical structural member and determine the failure condition of the metal structural member, thereby providing basis for subsequent maintenance and maintenance of the metal structural member.
The technical scheme adopted for solving the technical problems is as follows:
the durability evaluation method of the metal structural part comprises the following steps:
performing crack detection on the metal structural member by adopting an ultrasonic probe to obtain crack morphology information of the position, the size and the shape of cracks in the metal structural member;
based on crack morphology information of the position, the size and the shape of cracks in the metal structural member, converting the metal structural member into a finite element model;
applying unit displacement to the front end of the crack to introduce a non-zero crack tensile stress, so that stress concentration is generated at the front end of the crack, and further calculating a stress intensity factor K_I at the tip of the crack;
acquiring material parameters of the metal structural member, and calculating fracture toughness of the metal structural member according to the crack form information and the stress intensity factor K_I;
calculating crack propagation rate according to the load, crack morphology and material parameters of the metal structural member;
and evaluating the durability of the metal structural member according to the crack expansion rate and the crack morphology information and the fatigue cycle times.
Preferably, the method for detecting the cracks of the metal structural part by adopting the ultrasonic probe comprises the following steps:
by adjusting the control parameters of the pulse echo instrument, ultrasonic signals can penetrate the inside of the metal structural member and return;
moving the ultrasonic probe along the surface of the position to be detected, and simultaneously recording the echo time and amplitude of the ultrasonic signal;
the recorded ultrasonic signal data is processed by a computer.
Preferably, the method for processing the recorded ultrasonic signal data by a computer comprises the following steps:
preprocessing the collected original data, removing background noise and filtering;
converting the ultrasonic signal into characteristic quantities of amplitude, echo time and energy;
according to the information obtained by the feature extraction, converting the signal into a frequency domain through FFT (fast Fourier transform), finding the frequency response of the crack, and analyzing the signal through a time domain method to obtain the position, size and shape information of the crack in the metal structural member.
Preferably, the method for converting the metal structural part into the finite element model comprises the following steps:
the geometric shape of the metal structural part is obtained through measurement, scanning and modeling, and modeling software is used for modeling the geometric shape of the metal structural part;
decomposing the geometric shape of the metal structural member into a series of small units, and dividing finite element units of different line elements and shell elements;
determining boundary conditions and loading conditions of constraint and load of the metal structural member;
determining material parameters of elastic modulus, poisson ratio and yield stress of the metal structural part;
performing finite element analysis, and simulating the response of the metal structural member under different load conditions to obtain the physical quantity of stress, strain and displacement of each point;
and (5) visualizing, plotting and processing data on the analysis result to determine stress distribution, deformation degree and damage degree under the applied load.
Preferably, the method for calculating the stress intensity factor k_i at the crack tip is:
defining crack angle and radius parameters of a crack tip;
calculating a Williams coefficient W:
W＝3-4sin(θ/2)+sin^3(θ/2)
wherein θ is a crack angle, the value range is 0 to 2pi, and when the crack is a straight crack (θ=pi), the Williams coefficient is the smallest, which is 0.395;
calculating stress intensity factors:
K_I＝F(W)σ√πa
where K_I is the stress intensity factor at the crack tip; f (W) is a function related to Williams coefficient W; sigma is the load size; a is the crack length.
Preferably, the material parameters of the metallic structural member include a limiting stress σf and an extension δ of the plastic region.
Preferably, the method for obtaining the material parameters of the metal structural part comprises the following steps:
selecting a metal sample with corresponding specification and shape, and preparing the sample;
mounting the sample on a tensile testing machine for tensile test;
and recording the stress-strain curve of the test sample during the test;
the ultimate stress σf of the sample material and the elongation δ of the plastic region are obtained from the stress-strain curve.
Preferably, the method for calculating the fracture toughness of the metal structural part comprises the following steps:
calculating a fracture toughness index K_IC value:
K_IC＝YK_I(ρ_c/a)^(3/2)
where ρ_c is the equivalent crack size, a is the crack length, ρ_c can be calculated by the following formula:
ρ_c＝0.5(a+t)
wherein t is the component thickness.
Preferably, the method for calculating the crack growth rate is as follows:
da/dt＝K_I^2/[YSigmaSqrt(pi*a)]
wherein da/dt represents the crack growth rate; K_I represents a stress intensity factor; y is a dimensionless number; sigma is the stress of the metal structure; a is the crack length;
the finishing formula can be obtained:
da/dt＝C(K_I/Sigma)^2
wherein c=1/[ YSqrt (pi×a) ] is the modulus of elasticity.
Preferably, the method for evaluating the durability of the metal structural member includes:
searching the fatigue cycle life of the metal structural part under the corresponding stress level according to the S-N curve;
the fatigue cycle life is compared with the expected number of cycles of use to determine the durability of the metal structural member.
The beneficial effects of the invention are as follows:
in the scheme, the ultrasonic signals are processed and analyzed by the computer, the related information of cracks in the metal structural member can be rapidly and accurately obtained, the analysis and calculation are carried out by combining the basic theory of application mechanics, the durability and the safety of the metal structural member are evaluated, important references are provided for maintenance of the structural member, meanwhile, the finite element model is used for analyzing the metal structural member by the computer so as to determine the parameter information of the metal structural member, and finally, the durability of the metal structural member is comprehensively evaluated through evaluation of parameters such as crack expansion rate, fatigue cycle number and the like.
Detailed Description
The principles and features of the present invention are described below in connection with the following examples which are provided for the purpose of illustrating the invention and are not intended to limit the scope of the invention. The invention is more specifically described by way of example in the following paragraphs. Advantages and features of the invention will become more apparent from the following description and from the claims.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and/or" as used herein includes any and all combinations of one or more of the associated listed items.
Examples
The durability evaluation method of the metal structural part comprises the following steps:
performing crack detection on the metal structural member by adopting an ultrasonic probe to obtain crack morphology information of the position, the size and the shape of cracks in the metal structural member;
based on crack morphology information of the position, the size and the shape of cracks in the metal structural member, converting the metal structural member into a finite element model;
applying unit displacement to the front end of the crack to introduce a non-zero crack tensile stress, so that stress concentration is generated at the front end of the crack, and further calculating a stress intensity factor K_I at the tip of the crack;
acquiring material parameters of the metal structural member, and calculating fracture toughness of the metal structural member according to the crack form information and the stress intensity factor K_I;
calculating crack propagation rate according to the load, crack morphology and material parameters of the metal structural member;
and evaluating the durability of the metal structural member according to the crack expansion rate and the crack morphology information and the fatigue cycle times.
The method for detecting the cracks of the metal structural part by adopting the ultrasonic probe comprises the following steps:
preparation device: ultrasonic probes, pulse echo meters, computers, etc.;
determining a detection position: determining a position to be detected according to the actual condition of the metal structural member;
debugging equipment: placing an ultrasonic probe on a position to be detected, and enabling an ultrasonic signal to penetrate through the inside of the metal structural member and return by adjusting control parameters of the pulse echo instrument;
detection is performed by using ultrasonic wave beams: the ultrasonic probe is slowly moved along the surface of the position to be detected, and simultaneously the echo time and amplitude of the ultrasonic signal are recorded. If the metal structural part has cracks, obvious echo is generated at the cracks;
analysis data: and processing and analyzing the recorded ultrasonic signal data by using a computer to obtain the information of the position, the size, the shape and the like of the crack in the metal structural member.
The method for processing the recorded ultrasonic signal data by using the computer comprises the following steps:
and (3) data acquisition: firstly, collecting ultrasonic signal data of a structural member to be detected, and storing the ultrasonic signal data into a computer;
data preprocessing: because the ultrasonic signals are affected by scattering, reflection and other factors after being transmitted inside the metal structural member, the collected original data need to be preprocessed, such as background noise removal, filtering and other operations;
feature extraction: feature extraction is a process of converting an ultrasonic signal into a feature quantity having physical meaning. The common characteristics comprise amplitude, echo time, energy and the like, whether cracks exist or not can be judged through extracting the characteristics, and relevant information of the cracks is obtained;
and (3) signal processing: and processing and analyzing the signals according to the information obtained by the feature extraction. For example, the signal can be converted into a frequency domain through FFT (fast Fourier transform), the frequency response of the crack is found, and the signal is analyzed through a time domain or frequency domain method, so that more accurate crack information and judgment result are obtained;
crack evaluation: using the processed data, the crack can be quantitatively evaluated, including crack length, depth, shape, etc.;
and (3) outputting results: and outputting the processed information to a screen for evaluating and repairing the safety of the structural member.
The method for converting the metal structural part into the finite element model comprises the following steps:
obtaining the geometric shape of the structural part: the geometric shape of the structural member is obtained through measuring, scanning, modeling and other modes, and CAD software is generally used for modeling;
dividing grids: the geometric shape of the structural member is decomposed into a series of small units (also called grids), and different types of finite element units, such as line elements, shell elements and the like, are divided according to actual conditions;
determining boundary conditions: in the finite element model, boundary conditions and loading conditions of the structural member, such as constraint, load and the like, need to be determined;
determining material parameters: in the finite element model, material parameters of the structural member, such as elastic modulus, poisson ratio, yield stress and the like, need to be determined;
finite element analysis was performed: finite element analysis is carried out by applying a finite element theory and a calculation method, and the response of the structural member under different load conditions is simulated to obtain key physical quantities such as stress, strain, displacement and the like of each point;
and (3) post-treatment: the analysis results are visualized, plotted, data processed and the like so as to further analyze and evaluate the mechanical properties of the structural member, such as stress distribution, deformation degree, damage degree and the like under the applied load.
The method for calculating the stress intensity factor K_I at the crack tip is as follows:
defining crack tip morphology: parameters such as crack angle, radius and the like of the crack tip are required to be defined firstly;
calculating Williams coefficients: the Williams coefficient W is calculated according to the following formula:
W＝3-4sin(θ/2)+sin^3(θ/2)
wherein θ is a crack angle and has a value ranging from 0 to 2pi. When the crack is a straight crack (θ=pi), the Williams coefficient is minimum, 0.395;
calculating stress intensity factors: and (3) calculating a stress intensity factor by using an elastic mechanics theory according to an expression of a crack hyperbola analytical solution through a Williams coefficient, a load and stress distribution at the crack tip. The specific formula is as follows:
K_I＝F(W)σ√πa
where K_I is the stress intensity factor at the crack tip; f (W) is a function related to Williams coefficient W; sigma is the load size; a is the crack length;
evaluation results: the calculated stress intensity factor may be used to evaluate the fracture toughness of the structural component. It is often necessary to compare the calculation results with experimental data to verify the accuracy and reliability of the calculation method.
The material parameters of the metal structural member comprise ultimate stress sigma_f and elongation delta of a plastic region, and the method for obtaining the material parameters of the metal structural member comprises the following steps:
selecting a metal sample with corresponding specification and shape, and preparing the sample;
mounting the sample on a tensile testing machine for tensile test;
and recording the stress-strain curve of the test sample during the test;
the ultimate stress σf of the sample material and the elongation δ of the plastic region are obtained from the stress-strain curve.
The method for calculating the fracture toughness of the metal structural part comprises the following steps:
determining material parameters: determining the definition of a fracture toughness index K_IC of the metal structural part, the limit stress sigma_f of the material and the extension delta of the plastic area;
calculating a correction factor Y: because the actual stress field and the theoretical stress field are different, the stress intensity factor needs to be corrected, the correction factor Y is related to the stress state and can be obtained through documents or experimental data, and if the actual stress field and the theoretical stress field cannot be obtained, Y=1 is generally adopted;
calculating a K_IC value: the k_ic value is calculated according to the following formula:
K_IC＝YK_I(ρ_c/a)^(3/2)
where ρ_c is the equivalent crack size and a is the crack length. ρ_c can be calculated by the following formula:
ρ_c＝0.5(a+t)
where t is the component thickness, here we assume that the crack is a semi-infinite long straight crack, i.e. t > > a;
evaluation results: the calculated k_ic value may be used to evaluate the fracture toughness of the metal structure, and it is generally necessary to compare the calculation result with experimental data for verifying the accuracy and reliability of the calculation method.
The ultimate stress σf of a material refers to the maximum value at which plastic deformation begins to occur when the stress of a metal sample reaches a certain value in a tensile test, and the stress no longer increases linearly with strain. Below this stress value, the metal material is elastically deformed, i.e., plastic deformation does not occur, and in general, a higher value of σ_f indicates a greater strength of the metal material.
The elongation δ of a plastic region refers to the maximum elongation from the point where plastic deformation of a metal sample begins to occur to before tensile fracture in a material tensile test, and is an important parameter for characterizing toughness of a metal material. In general, a larger extension indicates a better toughness of the material.
Accurate determination of the ultimate stress and elongation of the plastic region of a material requires the use of specialized tensile testing equipment, and for some specific materials, such as composites, its own characteristics can also lead to differences in the testing methods, which may need to be designed on a case-by-case basis.
The method for calculating the crack growth rate comprises the following steps:
da/dt＝K_I^2/[YSigmaSqrt(pi*a)]
wherein da/dt represents the crack growth rate; K_I represents a stress intensity factor and reflects the stress intensity of crack endpoints; y is a dimensionless coefficient, called a geometric coefficient, depending on the crack morphology; sigma is the stress of the metallic structure, indicating the degree of stretching or compression of the material when loaded; a is the crack length.
The finishing formula can be obtained:
da/dt＝C(K_I/Sigma)^2
where c=1/[ YSqrt (pi×a) ] is a parameter of a material, called elastic modulus, depending on the material properties and crack morphology and size.
The method for evaluating the durability of the metal structural member comprises the following steps:
searching the fatigue cycle life of the metal structural part under the corresponding stress level according to the S-N curve;
comparing the fatigue cycle life with the expected number of use cycles to determine the durability of the metal structural member;
the durability of the metal structural member is evaluated by considering the relation between the crack growth rate and the fatigue cycle number, and when the crack length reaches a certain degree, the crack growth rate is obviously increased, so that the metal structural member is invalid; therefore, after parameters such as crack length, crack expansion rate, fatigue cycle number and the like are determined, the parameters can be evaluated by using an S-N curve, wherein the S-N curve is a method for describing the fatigue life of the material and represents the relation between stress amplitude and fatigue life;
where S represents stress amplitude (stress amplitude) and N represents fatigue life (Numberof cyclestofailure). The S-N curve is usually represented by a double logarithmic scale, i.e. the stress amplitude is represented by logarithm on the abscissa and the fatigue life is represented by logarithm on the ordinate;
in the S-N curve, when the stress amplitude is smaller, the fatigue life of the material is long, that is, the material can bear more cyclic load, and when the stress amplitude is gradually larger, the fatigue life is gradually shortened, and the material is more prone to failure problems such as fatigue crack, fracture and the like.
The beneficial effects of the invention are as follows:
in the scheme, the ultrasonic signals are processed and analyzed by the computer, the related information of cracks in the metal structural member can be rapidly and accurately obtained, the analysis and calculation are carried out by combining the basic theory of application mechanics, the durability and the safety of the metal structural member are evaluated, important references are provided for maintenance of the structural member, meanwhile, the finite element model is used for analyzing the metal structural member by the computer so as to determine the parameter information of the metal structural member, and finally, the durability of the metal structural member is comprehensively evaluated through evaluation of parameters such as crack expansion rate, fatigue cycle number and the like.
The above-mentioned embodiments of the present invention are not intended to limit the scope of the present invention, and the embodiments of the present invention are not limited thereto, and all kinds of modifications, substitutions or alterations made to the above-mentioned structures of the present invention according to the above-mentioned general knowledge and conventional means of the art without departing from the basic technical ideas of the present invention shall fall within the scope of the present invention.
Claims (10)
1. The method for evaluating the durability of the metal structural member is characterized by comprising the following steps of:
performing crack detection on the metal structural member by adopting an ultrasonic probe to obtain crack morphology information of the position, the size and the shape of cracks in the metal structural member;
based on crack morphology information of the position, the size and the shape of cracks in the metal structural member, converting the metal structural member into a finite element model;
applying unit displacement to the front end of the crack to introduce a non-zero crack tensile stress, so that stress concentration is generated at the front end of the crack, and further calculating a stress intensity factor K_I at the tip of the crack;
acquiring material parameters of the metal structural member, and calculating fracture toughness of the metal structural member according to the crack form information and the stress intensity factor K_I;
calculating crack propagation rate according to the load, crack morphology and material parameters of the metal structural member;
and evaluating the durability of the metal structural member according to the crack expansion rate and the crack morphology information and the fatigue cycle times.
2. The method for evaluating the durability of a metal structural member according to claim 1, wherein the method for detecting cracks in the metal structural member by using an ultrasonic probe comprises:
by adjusting the control parameters of the pulse echo instrument, ultrasonic signals can penetrate the inside of the metal structural member and return;
moving the ultrasonic probe along the surface of the position to be detected, and simultaneously recording the echo time and amplitude of the ultrasonic signal;
the recorded ultrasonic signal data is processed by a computer.
3. The method for evaluating durability of a metal structural member according to claim 2, wherein the method for processing the recorded ultrasonic signal data by a computer is:
preprocessing the collected original data, removing background noise and filtering;
converting the ultrasonic signal into characteristic quantities of amplitude, echo time and energy;
according to the information obtained by the feature extraction, converting the signal into a frequency domain through FFT (fast Fourier transform), finding the frequency response of the crack, and analyzing the signal through a time domain method to obtain the position, size and shape information of the crack in the metal structural member.
4. The method for evaluating durability of a metal structural member according to claim 1, wherein the method for converting the metal structural member into a finite element model is:
the geometric shape of the metal structural part is obtained through measurement, scanning and modeling, and modeling software is used for modeling the geometric shape of the metal structural part;
decomposing the geometric shape of the metal structural member into a series of small units, and dividing finite element units of different line elements and shell elements;
determining boundary conditions and loading conditions of constraint and load of the metal structural member;
determining material parameters of elastic modulus, poisson ratio and yield stress of the metal structural part;
performing finite element analysis, and simulating the response of the metal structural member under different load conditions to obtain the physical quantity of stress, strain and displacement of each point;
and (5) visualizing, plotting and processing data on the analysis result to determine stress distribution, deformation degree and damage degree under the applied load.
5. The method for evaluating durability of a metal structural member according to claim 1, wherein the method for calculating the stress intensity factor k_i at the crack tip is:
defining crack angle and radius parameters of a crack tip;
calculating a Williams coefficient W:
W＝3-4sin(θ/2)+sin^3(θ/2)
wherein θ is a crack angle, the value range is 0 to 2pi, and when the crack is a straight crack (θ=pi), the Williams coefficient is the smallest, which is 0.395;
calculating stress intensity factors:
K_I＝F(W)σ√πa
where K_I is the stress intensity factor at the crack tip; f (W) is a function related to Williams coefficient W; sigma is the load size; a is the crack length.
6. The method for evaluating the durability of a metal structural member according to claim 5, wherein: the material parameters of the metallic structural member include the ultimate stress σf and the elongation δ of the plastic region.
7. The method for evaluating the durability of a metal structural member according to claim 6, wherein the method for obtaining the material parameters of the metal structural member comprises:
selecting a metal sample with corresponding specification and shape, and preparing the sample;
mounting the sample on a tensile testing machine for tensile test;
and recording the stress-strain curve of the test sample during the test;
the ultimate stress σf of the sample material and the elongation δ of the plastic region are obtained from the stress-strain curve.
8. The method for evaluating the durability of a metal structural member according to claim 7, wherein the method for calculating the fracture toughness of the metal structural member is:
calculating a fracture toughness index K_IC value:
K_IC＝YK_I(ρ_c/a)^(3/2)
where ρ_c is the equivalent crack size, a is the crack length, ρ_c can be calculated by the following formula:
ρ_c＝0.5(a+t)
wherein t is the component thickness.
9. The method for evaluating the durability of a metal structural member according to claim 8, wherein the method for calculating the crack growth rate is:
da/dt＝K_I^2/[YSigmaSqrt(pi*a)]
wherein da/dt represents the crack growth rate; K_I represents a stress intensity factor; y is a dimensionless number; sigma is the stress of the metal structure; a is the crack length;
the finishing formula can be obtained:
da/dt＝C(K_I/Sigma)^2
wherein c=1/[ YSqrt (pi×a) ] is the modulus of elasticity.
10. The method for evaluating the durability of a metal structural member according to any one of claims 1 to 9, wherein the method for evaluating the durability of a metal structural member is:
searching the fatigue cycle life of the metal structural part under the corresponding stress level according to the S-N curve;
the fatigue cycle life is compared with the expected number of cycles of use to determine the durability of the metal structural member.
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