What is the stress amplitude of the fatigue limit of a material? What is the exact definition?

The material is fatigued by stress cycles that are high and low.

For some equipment conditions, the maximum and minimum stress values are fixed, and the sum of the highest stress value and the lowest stress value divided by 2 is called the average stress of cyclic stress.

The difference between the highest stress value and the average stress value or the difference between the average stress value and the lowest stress value is called the stress amplitude of the cyclic stress. In practice, the stress amplitude is equal to half of the difference between the highest and lowest stress values, and the average stress value is often one of the considerations for fatigue problems.

The fatigue life of the larger stress amplitude is less, the fatigue life of the smaller stress amplitude is long, and there are different fatigue life under different stress amplitudes, that is, the fatigue limit, and they are not the only ones for the same material.

Difference Between Fatigue and Static Failure

Mechanics of Materials. It is based on static experiments to determine the mechanical properties of materials.

For example, elastic limit, yield limit, strength limit), these mechanical properties do not fully reflect the characteristics of the material under alternating stress. Therefore, if the parts or structures working under the action of alternating loads are still designed according to the static load, sudden failures often occur during use.

There are many obvious essential differences between fatigue failure and traditional static failure:

1. Static failure is a failure under the action of a maximum load: fatigue failure is a failure under the action of multiple repeated loads, which does not occur in a short period of time, but takes a certain period of time or even a long time before the failure occurs.

2. When the static stress is less than the yield limit or strength limit, static failure will not occur; Fatigue failure may occur when the alternating stress is much less than the static strength limit, or even less than the yield limit.

3. Static failure usually has obvious plastic deformation: fatigue failure usually has no signs of external macroscopic significant plastic deformation, even if it is a metal with good plasticity, just like brittle failure, it is not easy to detect in advance, which indicates that fatigue failure has greater danger.

The material is fatigued by stress cycles that are high and low.

The stress amplitude of the standard specimen after an infinite number of stress cycles without fatigue failure is called the fatigue limit of the material.

The ultimate stress of the material is calculated as follows:

Let's start with the assumptions: normal and axes.

The positive and consistent surface is the front; Faces that coincide with the negative direction of the coordinate axis are negative.

Furthermore, it is stipulated that the stress pointing to the positive direction of the coordinate axis on the front side is positive, and the stress on the reverse side is negative. The stress that is negative on the negative side of the coordinate axis is positive, and vice versa. There are a total of nine ultimate stress components (including three normal stresses and six shear stresses) on the three fronts.

The first subscript of the ultimate stress component represents the normal direction of the plane of action; The second chain transport subscript indicates the direction of stress action. The two subscripts of the positive studio shooting stress are the same, so one subscript is abbreviated.

Due to the shear stress equality theorem, the shear stresses in the diagonal diagonal in the above matrix are equal, i.e.: xy= yx, yz= zy, zx= xz. Therefore, this matrix is symmetrical.

Six of the nine ultimate stress components are independent.

Properties: When an object is deformed due to external factors (force, humidity, temperature field changes, etc.), the internal force that interacts with each other in the object, and the internal force per unit area is called stress. Stress is a vector quantity, the component along the normal direction of the section is called the normal stress, and the component along the tangential direction is called the shear stress.

The stress of a point in an object in all possible directions is called the stress state of that point. However, after a point, an infinite number of planes can be made, and through analysis, it can be seen that the stress state of the point can be represented by any group of three planes perpendicular to each other at only one point, and the stresses on other cross-sections can be represented by this group of stresses and their azimuth relationship with the cross-section to be investigated.

For plastic materials, when they reach yield and significant plastic deformation occurs, they lose their normal working ability, so the yield limit is usually taken as the ultimate stress; For plastic materials without obvious yield phase, the stress corresponding to the plastic strain is taken as the ultimate stress. For brittle materials, since the material will not produce obvious plastic deformation before failure, and only when it is fractured, it will lose its normal working ability, so the strength limit should be taken as the ultimate stress.

Strictly speaking: for the steel workpiece, the greater the surface compressive stress, the higher the fatigue life. For example, in order to improve the fatigue life of some steel products, shot peening can be carried out at the end of the process, which is intended to increase the surface compressive stress.

It is not appropriate to say in general terms that "the greater the stress, the higher the fatigue life". General stress is divided into two types: tensile stress and compressive stress, if the tensile stress is large, it will only reduce the fatigue strength.

Accelerate workpiece fatigue failure. Compressive stress should be confirmed. Cong Sheng.

In addition, stress usually refers to internal stress.

The surface stress is mostly responsible for fatigue strength; It is beneficial to improve the fatigue life of the material, and the surface compressive stress should be used.

Answer]: c fatigue limit refers to the maximum stress when the stress alternating cycle is so large that there is no limit and the sample is still not damaged in the fatigue potato group test, and the ultimate stress is the fatigue limit of the material when the fatigue strength meter is calculated; The yield limit, also known as the flow limit, refers to the obvious plastic deformation of the material when it is subjected to an external force to a certain limit, even if the load is not increased. The strength limit refers to the maximum stress that occurs when an object fails under the action of external force, also known as failure strength or failure stress; The elastic limit refers to the ability of a metal material to resist an external force within this limit.

When calculating the fatigue strength, the ultimate stress should be ().

a.Shrink socks to the limit.

b.Fatigue limit.

c.Strength limit.

d.Elastic limit.

The only answer is the fatigue limit.

What is steel fatigue? How is the formula for calculating the allowable stress amplitude obtained?

What is steel fatigue? How to get the formula for calculating the allowable stress amplitude? Hello, happy to answer your <>

Under the action of alternating stress, although the stress is lower than the yield point of the material, the phenomenon of cracking or sudden complete fracture after a long period of work is called metal fatigue. (The stress of each point of the metal parts in the working process changes periodically with time, and the stress that changes periodically with time is called alternating stress (also known as cyclic stress). The fatigue strength max of the component is closely related to the stress cycle characteristics represented by the stress ratio r.

By introducing a safety factor to max, the allowable value of fatigue stress max =f(r) for design is obtained. Limiting the stress to max is known as the stress ratio criterion. Since welded structures are used to withstand fatigue loads, the engineering community has gradually realized from practice that it is not the stress ratio r that is closely related to the fatigue strength of this type of structure, but the stress amplitude δ.

The formula for calculating the stress amplitude criterion is δ is the allowable stress amplitude, which varies with the structural details and also with the number of cycles before failure. The fatigue calculation of welded structures should be based on the stress amplitude, because of the residual stress inside the structure. Non-welded members.

For stress cycles with r>=0, the stress amplitude criterion is fully applicable, because there is little difference in the fatigue strength of components with and without residual stress. For the stress cycle of r<0, the stress amplitude criterion family Gao Ying is more safe. Extended information, I hope mine can help you with the <> of Megaplex

Do you have any other questions?

What is the relationship between the allowable stress amplitude in the fatigue calculation of steel and the static strength of steel: dear, very happy with your question! The ultimate stress of the static strength design is a fixed value; The stress of the fatigue strength design is variable, with the change of cycle characteristics and the magnitude of the life.

Answer: I will make up for the fatigue of steel bars, which refers to the phenomenon of sudden brittle fracture of steel bars after a certain number of jujube imaginary numbers under the action of repeated and periodic dynamic loads. The fatigue strength of the steel bar refers to the maximum stress value of fatigue failure after being subjected to a certain number of cyclic loads within a certain specified stress range.

The cause of fatigue fracture of steel bars is generally believed to be due to internal and external defects of steel bars, which are prone to stress concentration. If the stress is too high, the steel grain will slip, resulting in fatigue cracks, the number of stress repetition will increase, and the cracks will propagate, thus building a stool to burn and fracture. There are two methods for fatigue testing of rebar:

One is to directly carry out the axial tensile test of a single undisturbed steel bar; The other is to bury the steel bar in the concrete to repeat the test of tension or bending. China adopts the method of directly doing the axle tensile test of a single steel bar. The Code for Design of Concrete Structures stipulates the limit of fatigue stress amplitude of different grades of steel bars, and stipulates that this value is related to the ratio of minimum stress to maximum stress of steel bars on the same fiber of the cross-section.

It is necessary to meet the different fatigue stress ratios, and the stress value is the fatigue strength of the reinforcement under the condition that the number of cycles is 2 million cycles. The fatigue strength of the steel bar is related to the amplitude of stress change, and other influencing factors are: the size of the minimum stress value, the geometric size and shape of the outer surface of the steel bar, the diameter of the steel bar, the strength of the steel bar, the processing section of the steel bar and the use environment, and the frequency of loading.

Due to the repetitive loads, the fatigue strength of the reinforcement is lower than its ultimate strength under static loads. Undisturbed steel bars have the lowest fatigue strength.

Summary. The larger the stress amplitude, the smaller the corresponding fatigue strength and the fewer the corresponding stress cycles.

The larger the stress amplitude, the smaller the corresponding fatigue strength and the fewer the corresponding stress cycles.

Fatigue strength refers to the maximum stress value that the material can withstand under the stress cycle and the accompanying load, and fatigue failure will occur when the stress borne by the material exceeds the fatigue strength. The stress amplitude is the difference between the maximum stress value and the minimum stress value of the material under stress cyclic loading. According to the fatigue life curve, when the stress amplitude increases, the fatigue strength decreases, and the number of stress cycles decreases.

This is because increased stress amplitude makes it easier for small defects or cracks in the material to propagate, resulting in a shorter fatigue life of the material. When designing and using materials, the influence of stress amplitude on fatigue strength and the number of stress cycles needs to be considered to ensure the service life and safety of the material.

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