This paper summarizes the commonly used mechanical fatigue analysis methods, including: nominal stress method, local stress-strain method, energy method, field strength method, fracture mechanics method, reliability design method, and probability fracture mechanics. sheet metal contractors near me
Nominal stress method
The nominal stress method is based on the nominal stress of the structure as the basis for the test and life estimation. The rain flow method is used to take out independent and unrelated stress cycles. In combination with the S-N curve of the material, the structural fatigue is estimated by the linear cumulative damage theory. A method of life. sheet metal contractors near me
The basic assumption is that for any component (or structural details or components), as long as the stress concentration factor KT is the same and the load spectrum is the same, their lifetime is the same. The nominal stress in this method is the control parameter. This method takes into account the influence of the load sequence and residual stresses and is simple and easy to implement.
However, there are two major deficiencies in this approach:
Firstly, due to the study of fatigue in the elastic range, the effect of local plastic deformation at the root of the notch is not taken into consideration. When calculating the fatigue life of the structure with stress concentration, the calculation error is large. sheet metal contractors near me
Second, the determination of the equivalent relationship between the standard sample and the structure is very difficult, this is due to the relationship with the structure of the geometry, the loading method and the size of the structure, materials and other factors.
It is precisely because of the above defects that the nominal stress method is not suitable for predicting the formation of fatigue cracks, and this method requires SN curves with different stress ratios R and different stress concentration factors KT, and it takes a lot of data to obtain these materials. Funding. Therefore, the nominal stress method is only suitable for calculating the fatigue life of high-cycle fatigue and unnotched structures with lower stress levels. In recent years, the nominal stress method has also been continuously developed, with the S.ST, the effective stress method, and the DRF. sheet metal contractors near me
Local stress-strain method
The basic idea of local stress-strain method is to analyze the local stress at the notch by means of local stress-strain method according to the nominal stress history of the structure. Then according to the local stress at the notch, the S-N curve of the member is combined with the cycle of the material. A curve, E-N curve and linear cumulative damage theory estimate the fatigue life of the structure.
The basic assumption is that if the stress-strain history of a dangerous part (point) of a component is the same as the stress-strain history of a smooth small sample, the lifetime is the same. In this method, local stress-strain is the control parameter.
The local stress-strain method is mainly used to solve the fatigue life problems of high-strain low-cycle fatigue and notched structures. The characteristic of this method is that through a certain analysis and calculation, the nominal stress on the structure can be transformed into the local stress and strain at the notch. It can analyze the non-linear relationship between local stress and strain at the notch carefully, and consider the influence of load sequence and residual stress on fatigue life. Therefore, so far, the local stress-strain method is a better method for estimating fatigue life. It overcomes two major drawbacks of nominal stress method, but it also has its own inherent defects: sheet metal contractors near me
First, the effects of stress gradient and multiaxial stress near the root of the notch are not considered;
The second is that the fatigue life calculation result is very sensitive to the fatigue notch factor K value.
In actual work, it is very difficult to accurately determine the K value of the structure, which affects the accuracy of the local stress-strain method to estimate the fatigue life. In addition, the local stress-strain method uses the CN curve of the material, and the EN curve is obtained under the condition of strain control fatigue test, the test data data is less, not as easy as SN curve, which also affects the method’s use. sheet metal contractors near me
Basic assumption: Components (elements or structural details) made of the same material, if subjected to the same local strain energy history in the fatigue danger zone, have the same fatigue crack formation life.
The material performance data of the energy method is mainly the cyclic stress-strain curve of the material and the cycle energy consumption-life curve. Although in the existing energy method it is assumed that the energy consumption of each cycle is linearly additive, and in fact due to the continuous expansion of the damage interface within the material during cyclic loading, the relationship between total energy consumption and cycle number is not. Linear. This key issue has caused the energy law to be difficult to apply to engineering practice. Therefore, the energy method may not be a very reasonable and promising method. sheet metal contractors near me
Field strength method
Basic assumption: Components (elements or structural details) made from the same material, if subjected to the same stress field strength history in the fatigue failure zone, have the same fatigue life. The control parameter of this method is the stress field strength. When the field-strength method is used to predict the formation fatigue life of a structure, a cyclic stress-strain curve and an S-Nf curve (or £-Nf curve) are required. The analysis and calculation are more complicated.
According to the characteristics of each of the four fatigue life prediction methods mentioned above, different prediction methods are required for different known conditions: For example, a nominal stress method can be used for a connection or a structural member made of a material having a large amount of fatigue performance data; Some structural members with complicated geometric shapes and under complex loads can use local stress-strain method, especially the transient cycle; a combination of curve and £-Nf curve; stress field strength method can be used for The local stress-strain method has the same material fatigue performance data, namely the cycle a-curve and the SN or £-Nf curve. sheet metal contractors near me
Fracture mechanics method
The theory of fracture mechanics is based on the fact that the material itself has flaws or cracks. Based on the mechanics of the deformed body, the expansion, instabilities, and crack arrests containing defects or cracks are studied. Through the quantitative analysis of fractures, the fatigue crack growth rate of the components in practical work (applicable to the Paris fatigue crack growth rate formula) is widely used, the fatigue life of the components is reasonably estimated, and the crack forming time of the components is determined and evaluated. Manufacturing quality is conducive to the correct analysis of the cause of the accident. In fact, this method solves many catastrophic low-stress brittle fracture problems in engineering and makes up for the deficiencies of conventional design methods. It has now become one of the important methods for failure analysis. sheet metal contractors near me
Fatigue fracture is the main mode of structural component failure. According to statistics, 85%-90% of major accidents caused by failure of structural components are related to fatigue fracture. According to the viewpoint of fracture mechanics, the fatigue failure of metal structural parts is caused by the extension of the main crack to critical dimensions. The life of the structure depends on the initiation and propagation of cracks in the dangerous parts of the structure.
This method divides the fatigue fracture process into three phases:
One is that the component generates an initial crack under the action of alternating force (the initial crack definition still has no uniform standard, and it is customarily 0.5-1mm);
Second, cracks begin to expand, resulting in larger macroscopic cracks; sheet metal contractors near me
The third is the rapid expansion of cracks, which quickly leads to damage. Its life is often very short. It is called the instantaneous fracture life and it is not examined in engineering.
According to the cracking time, the first stage can be defined as the initial cracking life, and the second stage is defined as the crack propagation life (commonly known as the remaining life). The measure of life is generally expressed as the number of cyclic loads experienced. The theory is that the fatigue limit is objectively present. That is to say, when the cyclic load amplitude of the component is less than the fatigue limit of the material of the component, the component can not be destroyed due to cracking. It is infinite. In addition, the fatigue life is not only related to the cyclic load amplitude and the physical and chemical properties of the material, but also related to the frequency of the load change. Therefore, the fatigue life has the characteristics of high-cycle fatigue and low-cycle fatigue.
The above-mentioned nominal stress method, local stress-strain method, etc. are all studies of initial cracking life. The study of remaining life is more complicated. At present, it is a hot issue. The engineering community has not yet proposed a generally accepted assessment method. sheet metal contractors near me
In recent years, the theory of fracture mechanics has made considerable progress. However, it is still imperfect. The mechanism of fracture failure is still not very clear. Therefore, it is still a long time to apply this theory to a simple, accurate and reliable fatigue life prediction formula.
Reliability design method
The reliability design method is the application of statistical data of reliability theory and design parameters, and the design of parts, equipment or systems under a given reliability index. Its purpose is to discover and determine the hidden dangers and weaknesses of products, and to improve the inherent reliability of products through prevention and improvement. However, the reliability research of mechanical systems is still immature, and the reliability design method cannot solve the problem of fatigue residual life assessment. sheet metal contractors near me
Probabilistic fracture mechanics
Fracture mechanics is an estimation method based on deterministic parameters. Probabilistic fracture mechanics takes the parameters such as crack size, fracture toughness, stress intensity factor, and crack propagation rate as the random variables in fracture mechanics for reliability analysis. This improves the reliability of fracture mechanics engineering analysis methods. However, this method has certain flaws:
First, it involves the use of normal distribution and three-parameter Weibull distribution to generate random variables and random numbers. It is obviously not enough to fully reflect the actual situation.
The second is insufficient test data. sheet metal contractors near me
Therefore, this method has been limited in practical applications.
At present, there are also people who use fuzzy mathematics and statistical simulation methods to comprehensively evaluate the technical state of a metal structure, and based on this, calculate its remaining life. The reliability of these methods depends not only on the mathematical method but also on the subjective factors of the person.
Theoretical basis for fatigue life assessment of metallic structures
The test focuses on the method of selecting the actual measurement of the metal structure that is suitable for the project, finds the basis for the application of the actual judgment, and thus correctly evaluates its life. The use of computer virtual technology to improve the processing of measured data, the establishment of metal structure of the expert system, the assessment of the metal structure of the remaining fatigue life and the remaining technical indicators, and then study the metal structure of the design, manufacture and technological innovation of artificial intelligence system . sheet metal contractors near me
In the future theory of metal structure fatigue life assessment, experts agree that the following aspects should be studied:
Theoretically, it focuses on the optimization of critical state and multi-critical state of the system, and studies the second-order moment method with multiple criteria.
Study and validate the effective method of critical failure model; sheet metal contractors near me
Improve fatigue strength theory and fracture mechanics methods;
The study is more suitable for the system’s probabilistic failure model, and improves the current method for calculating fracture probability.
Further study methods for calculating reliability;
Study the sensitive parameters that affect the system, and specifically study the method of parameter sensitivity analysis of the system, so that the system can effectively handle its sensitivity index. sheet metal contractors near me