1 Nominal stress method

The nominal stress method is based on the nominal stress of the structure as the basis for testing and life estimation. The rain flow method is used to extract independent and uncorrelated stress cycles. Combined with the S-N curve of the material, the structural fatigue is estimated according to the linear cumulative damage theory. A way of life.

 

Basic assumption: For any member (or structural detail or element), as long as the stress concentration factor KT is the same, the load spectrum is the same, and their life is the same. In this method, the nominal stress is the control parameter. This method takes into account the effect of load sequence and residual stress, and is simple and easy to implement.

 

However, this approach has two major drawbacks:

First, because the fatigue problem is studied in the elastic range, the influence of the local plastic deformation at the root of the notch is not considered, and the calculation error is large when calculating the fatigue life of the structure with stress concentration;

 

Second, it is very difficult to determine the equivalent relationship between the standard specimen and the structure, because this relationship is related to the geometry of the structure, the loading method, and the size and material of the structure.

 

It is precisely because of the above defects that the nominal stress method has a low ability to predict the formation of fatigue cracks, and this method requires S-N curves under different stress ratios R and different stress concentration factors KT, and obtaining these material data requires a lot of 's expenses. 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, and the stress severity factor method (S.ST), the effective stress method, and the rated factor method (DRF) have appeared one after another.

 

2 Local stress-strain method

The basic idea of the local stress-strain method is to analyze the local stress at the notch by means of the local stress-strain method according to the nominal stress history of the structure. Then according to the local stress at the notch, combine the S-N curve of the component and the cycle of the material. A curve, E-N curve and linear cumulative damage theory to estimate the fatigue life of the structure.

Basic assumption: If the stress-strain history of a dangerous part (point) of a component is the same as that of a smooth small specimen, the life is the same. The local stress-strain is the controlling parameter in this method.

The local stress-strain method is mainly used to solve high-strain low-cycle fatigue and fatigue life of notched structures. The characteristic of this method is that the nominal stress on the structure can be transformed into the local stress and strain at the notch through certain analysis and calculation. It can analyze the nonlinear relationship of local stress and strain at the notch in detail, and can consider the effect of load sequence and residual stress on fatigue life. Therefore, so far, the local stress-strain method is a better fatigue life estimation method. It overcomes two major drawbacks of the nominal stress method, but it also has its own inherent drawbacks:

 

First, the influence of stress gradient and multiaxial stress near the root of the notch is not considered;

Second, the calculation result of fatigue life is very sensitive to the fatigue notch coefficient K value.

In practical work, it is very difficult to accurately determine the K value of the structure, which affects the accuracy of the local stress-strain method for estimating fatigue life. In addition, the local stress-strain method uses the C-N curve of the material, while the E-N curve is obtained by performing fatigue tests under the condition of controlled strain. The test data is relatively small, and it is not as easy to obtain as the S-N curve, which also affects the method. use.

 

3 Energy method

Basic assumption: Components (elements or structural details) made of the same material have the same fatigue crack formation life if they are subjected to the same local strain energy history in the fatigue risk zone.

The material property data of the energy method are mainly the cyclic stress-strain curve and the cyclic energy consumption-life curve of the material. Although the existing energy methods assume that the energy consumption of each cycle is linearly additive, in fact, due to the continuous expansion of the damage interface inside the material during the cyclic loading process, the relationship between the total energy consumption and the number of cycles is not linear. This key problem makes the energy method difficult to apply to engineering practice. So the energy method may not be a very reasonable and promising method.

 

4 Field strength method

Basic assumption: Components (elements or structural details) made of the same material will have the same fatigue life if they are subjected to the same stress field strength history in the fatigue failure region. The control parameter for this method is the stress field strength. The cyclic stress-strain curve and the S-Nf curve (or ￡-Nf curve) are required when the field strength method is used to predict the formation life of the fatigue crack of the structure, and the analysis and calculation are complicated.

 

According to the characteristics of the above four fatigue life prediction methods, different prediction methods need to be used for different known conditions: for example, the nominal stress method can be used for connecting parts or structural parts made of materials with a large amount of fatigue performance data; Some structural parts with complex geometric shapes and under complex loads can use the local stress-strain method, especially the transient cycle; a method combining a curve and a ￡-Nf curve; the stress field strength method can be used with The same material fatigue performance data as the local stress-strain method, ie the cyclic a-curve and the S-N or £-Nf curve.

 

5 Fracture mechanics method

The theory of fracture mechanics is based on the fact that there are defects or cracks in the material itself. Based on the mechanics of deformed bodies, it studies the propagation, instability and crack arrest of defects or cracks. Through the quantitative analysis of the fracture, the fatigue crack growth rate of the component in actual work is obtained (the Paris fatigue crack growth rate formula is widely used), and the fatigue life of the component is estimated reasonably, the time for the component to form cracks is determined, and the evaluation of its The 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, makes up for the insufficiency of conventional design methods, and has now become one of the important methods of failure analysis.

 

Fatigue fracture is the main mode of failure of structural components. According to statistics, 85%-90% of major accidents due to failure of structural components are related to fatigue fracture. According to the point of view of fracture mechanics, the fatigue failure of metal structural parts is caused by the expansion of the main crack to the critical size, and the life of the structure depends on the initiation and propagation of cracks in the dangerous parts of the structure.

 

The method divides the fatigue fracture process into three stages:

One is the initial crack of the component under the action of the alternating force (the definition of the initial crack still has no unified standard, and it is customary to be 0.5-1mm);

The second is that the cracks begin to expand, resulting in larger macroscopic cracks;

The third is the rapid expansion of the crack, which quickly leads to damage, and its life is often very short, called the instantaneous fracture life, which is not considered in engineering.

 

According to the time of crack generation, the first stage can be defined as the crack initiation life, and the second stage can be defined as the crack propagation life (commonly called the remaining life). A measure of life is generally expressed in terms of the number of cyclic loads experienced. The theory believes that the fatigue limit exists objectively, that is, when the cyclic load amplitude of the component is less than the fatigue limit of the component material, the component cannot be damaged due to cracks, that is, the life of the component is examined from the perspective of fatigue life. 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 to the change frequency of the load, so the fatigue life is divided into high-cycle fatigue and low-cycle fatigue.

 

The aforementioned nominal stress method and local stress-strain method are all used to study the crack initiation life. The study of remaining life is more complicated. It is currently a hot issue, and the engineering community has not yet come up with a generally accepted assessment method.

 

In recent years, the theory of fracture mechanics has been developed by leaps and bounds, but it is still not perfect, and the mechanism of fracture failure is not very clear, so it will take time to apply this theory to obtain a simple, accurate and reliable fatigue life prediction formula.

 

6 reliability design method

Reliability design method is the design of components, equipment or systems under given reliability indexes by applying reliability theory and statistical data of design parameters. Its purpose is to find and determine the hidden dangers and weak links of the product, and improve the inherent reliability of the product through prevention and improvement. However, the reliability research of mechanical system is still immature, and the method of reliability design cannot solve the problem of fatigue residual life evaluation.

 

7 Probabilistic Fracture Mechanics

Fracture mechanics is an estimation method based on deterministic parameters. Probabilistic fracture mechanics is a reliability analysis that takes the crack size, fracture toughness, stress intensity factor, crack growth rate and other parameters in fracture mechanics as random variables. This improves the reliability of the fracture mechanics engineering analysis method. However, this method has certain drawbacks:

 

First, the random variables and random numbers involved are currently mainly generated by normal distribution and three-parameter Weibull distribution, which is obviously not enough to fully reflect the actual situation;

The second is the lack of experimental data.

Therefore, this method is limited in practical application.

 

At present, some people use fuzzy mathematics and statistical simulation methods to comprehensively evaluate the technical status of metal structures, and on this basis, calculate its remaining life. Whether these methods are reliable depends not only on mathematical methods, but also on human subjective factors.

 

8 Theoretical Basis for Fatigue Life Evaluation of Metal Structures

In the test, it focuses on the research and selection of the actual measurement method of the metal structure suitable for the project, and finds the judgment basis for practical application, so as to correctly evaluate its life. Using the virtual technology of the computer, improve the processing of the measured data, establish the expert system of the metal structure, evaluate the fatigue residual life and other technical indicators of the metal structure, and then study the artificial intelligence system for the design, manufacture and technical transformation of the metal structure. .

 

In the future fatigue life evaluation theory of metal structures, experts agree that the following research should be carried out:

Theoretically, it focuses on the optimization of the critical state and multi-critical state of the system, and studies the first-order second moment method in the case of multiple criteria;

Investigate effective methods for validating critical failure models;

Improve fatigue strength theory and fracture mechanics methods;

Research the probability failure model more suitable for the system, and improve the current method for calculating fracture probability;

Further research on methods for calculating reliability;

Sensitivity parameters affecting the system are studied, especially the parameter sensitivity analysis method for the system, so as to systematically deal with its sensitivity indicators effectively.