Fatigue Fracture: Definition, Type, Characteristics, Improvement Methods

1. Concept of fatigue and fracture

1. Fatigue: The performance change of metal materials under the repeated action of stress or strain is called fatigue.

2. Fatigue fracture: when the material is subjected to alternating cyclic stress or strain, the local structural changes and the continuous development of internal defects will cause the mechanical properties of the material to decline, eventually leading to the complete fracture of the product or material.

This process is called fatigue fracture, which can also be referred to as metal fatigue.

The stress causing fatigue fracture is generally very low.

The occurrence of fatigue fracture is often characterized by sudden, highly local and sensitivity to various defects.

2. Classification of fatigue fracture

1. High cycle fatigue and low cycle fatigue

If the stress level acting on parts or components is low and the number of cycles of failure is higher than 100000, it is called high cycle fatigue.

For example, high cycle fatigue is commonly seen in spring, transmission shaft, fastener and other products.

Low cycle fatigue refers to fatigue with high stress level and low cycles of failure, generally less than 10000 times.

For example, the fatigue damage of pressure vessels and turbine parts belongs to low cycle fatigue.

2. Stress and strain analysis

Strain fatigue – low cycle fatigue with high stress and low cycle times;

Stress fatigue – low stress, high cycle times, called high cycle fatigue.

Composite fatigue, but in practice, it is often difficult to distinguish between stress and strain types.

Generally, there are both types, which is called composite fatigue.

3. Classification according to load type

Bending fatigue, torsion fatigue, tension and compression fatigue, contact fatigue, vibration fatigue, fretting fatigue.

3. Characteristics of fatigue fracture

Macro: crack source → propagation zone → transient fracture zone.

Crack source: the area with grooves, defects or stress concentration on the surface is the precondition for the generation of crack source.

Fatigue propagation zone: the section is relatively flat, and the fatigue propagation is perpendicular to the stress direction, producing obvious fatigue arc, also known as beach grain or shell grain.

Instantaneous fracture zone: It refers to the area where the fatigue crack rapidly extends to the instantaneous fracture.

The fracture surface has metal slip traces. Some products have radioactive stripes and shear lip areas in the instantaneous fracture zone.

Microscopy: The typical feature of fatigue fracture is the appearance of fatigue striation.

Cleavage and quasi cleavage phenomena (names in crystallography, small planes appearing in microscopic images), as well as micro area characteristics such as dimples will also appear in some microscopic samples.

4. Characteristics of fatigue fracture

(1) There is no obvious macroscopic plastic deformation during fracture, and there is no obvious omen before fracture.

It is often sudden, causing damage or fracture of mechanical parts, which is very harmful.

(2) The stress causing fatigue fracture is very low, often lower than the stress load of yield strength under static load.

(3) After fatigue failure, it is generally possible to clearly show the components of the three areas of crack initiation, propagation and final fracture at the fracture surface.

5. Case analysis

A motorcycle in a motorcycle factory had a mechanical failure after running for 2000km.

After disassembly and inspection, it was found that the engine crankshaft connecting rod was broken.

It is reported that the connecting rod is made of 20CrMnTi and its surface has been carburized.

The working principle of the connecting rod is shown in Fig. 1.

The reciprocating motion of the connecting rod drives the two drive crankshafts to rotate.

20CrMnTi is an alloy structural steel with a carbon content of about 0.2%, manganese content of about 1% and titanium content of about 1%.

This material is generally used for shaft parts, and carburizing is required.

Fatigue Fracture: Definition, Type, Characteristics, Improvement Methods 1

Fig. 1

1. Macro inspection

There are two fractures of the failed connecting rod.

Many cracks parallel to the fracture can be seen on the bearing camber at the fracture end of the connecting rod [Fig. 3 (a)];

One side of the fracture end has a strong friction trace [Fig. 3 (b)], and the wear depth is 0.5mm;

The blue-gray high-temperature oxidation trace can be seen at one end of the bearing arc near the friction side [Fig. 3 (c)].

Fracture 1 is relatively smooth and flat, the edge of the fracture has been worn, and the fatigue arc can be seen in the middle [Fig. 3 (d)];

No fatigue arc is found on fracture 2.

Fatigue Fracture: Definition, Type, Characteristics, Improvement Methods 2

Fig. 2

Fatigue Fracture: Definition, Type, Characteristics, Improvement Methods 3

Fig. 3

2. Scanning electron microscope analysis

Fracture 1 shows fatigue arc under scanning electron microscope [Fig. 4 (a)];

The fatigue source can be found according to the trend of the arc.

The fatigue source is located at the upper right corner of [Fig. 4 (d)].

The local magnification shows that most of the fine tissues in the source area have been worn, but the radial edge feature can be seen [Fig. 4 (b)];

Fatigue stripes and secondary cracks can be seen in the fatigue growth zone [Fig. 4 (c)];

Fracture 2 has no fatigue stripe, only dimples. It can be seen that fracture 1 is the first fracture, while fracture 2 is the secondary fracture.

Fatigue Fracture: Definition, Type, Characteristics, Improvement Methods 4

Fig. 4

3. Chemical composition

Take samples from the connecting rod body and analyze the chemical composition (mass fraction,%).

The results meet the requirements of GB/T3077-1999 20CrMnTi for chemical composition.

4. Result analysis

Based on the above inspection results, the chemical composition of the failed part material meets the technical requirements.

One side of the broken end of the connecting rod shows abnormal severe friction.

The blue gray oxide film on the end of the bearing arc near the friction surface is a mixture of black iron oxide (Fe3O4) and red iron oxide (Fe2O3), and its formation temperature is above 400 ℃.

It indicates that the friction between the connecting rod and an output shaft causes overheating in this area.

The SEM analysis of the fracture surface shows that the fatigue crack source of the fracture surface is at the corner near the oxide film, which is in the high temperature region.

Surface oxidation will increase the chance of crack generation, and high temperature will increase the possibility of creep damage.

On the other hand, friction leads to rough metal surface, which is easy to form surface stress concentration, and increases the possibility of fatigue source.

The origin of fracture often occurs at the level of maximum tensile stress.

According to the analysis of the force on the connecting rod movement, the tensile stress on the section of fracture 1 is the largest, and it is easy to form a crack source near the corner of the friction surface on this section.

At the same time, due to the existence of coarse carbides in this area, the continuity of the matrix structure is destroyed, the formation and propagation of cracks are accelerated, the fatigue strength is reduced, and finally the fatigue fracture is caused.

Excessive carbides on the carburized surface of connecting rod are related to the improper carburizing process.

Coarse blocky carbides are mainly caused by high carbon concentration, which is particularly easy to form at the sharp corners of the workpiece, leading to a significant decline in the life of the parts.

Therefore, the carbon potential of the carburizing atmosphere should be strictly controlled in the carburizing process to avoid the formation of coarse carbides on the workpiece surface due to the excessive carbon potential.

5. Conclusion

The fracture of the crankshaft connecting rod belongs to fatigue fracture.

The reason for fracture is that the connecting rod is subjected to severe friction during use, which leads to local stress concentration and high temperature, reducing the fatigue strength of materials.

The large blocky carbides on the surface of the connecting rod corners accelerated the germination and propagation of the cracks.

6. Improvement

Reducing the roughness of friction part during design can reduce the stress concentration and reduce the fatigue strength of parts.

At the same time, the high temperature caused by friction is reduced, and the possibility of creep damage is reduced.

To improve the carburizing process, the excessive carbide on the carburized surface of the connecting rod is related to the improper carburizing process.

Coarse blocky carbides are mainly caused by high carbon concentration, which is particularly easy to form at the sharp corners of the workpiece, leading to a significant decline in the life of the parts.

Therefore, the carbon potential of the carburizing atmosphere should be strictly controlled in the carburizing process to avoid the formation of coarse carbides on the workpiece surface due to the excessive carbon potential.

6. Methods to improve the fatigue limit or fatigue strength of materials

Generally, it is difficult to change the service conditions of parts, so it is necessary to improve the design of parts as much as possible, such as starting from the surface effect.

As long as the surface stress concentration of structural materials and mechanical parts is prevented, dislocation slip accumulation is hindered, and plastic deformation is restrained, fatigue cracks are not easy to nucleate and expand, which will increase the fatigue limit or fatigue strength.

1. Measures to reduce stress concentration

Square or sharp corner holes and grooves shall be avoided in the design.

Where the section size changes suddenly (such as the shoulder of stepped shaft), the transition fillet with sufficient radius shall be used to reduce the stress concentration.

If it is difficult to increase the radius of the transition fillet due to structural reasons, thinner grooves or undercuts can be cut on the shaft with larger diameter.

There is obvious stress concentration at the edge of the fitting surface of the tight fitting hub and shaft.

If the load relief groove is cut on the hub and the fitting part of the shaft is thickened to narrow the stiffness gap between the hub and the shaft, the stress concentration at the edge of the fitting surface can be improved.

At the fillet weld, the stress concentration degree of groove welding is much better than that of non-groove welding.

Related reading: Complete List of Welding Symbols

2. Enhance surface strength

Mechanical method is used to strengthen the surface layer (such as rolling, shot peening, etc.) to make the component surface form a pre compression stress layer, reduce the surface tensile stress that is easy to cause cracks, so as to improve the fatigue strength, or heat treatment and chemical treatment are used, such as high-frequency quenching, carburizing, nitriding, etc.

A small steel ball with a diameter of 0.1-1mm is used to impact the surface of the sample at a high speed to remove sharp corners, burrs and other stress concentrations on the surface, and the surface is compressed to the depth of 1/4-1/2 of the diameter of the steel ball, so that residual stress is generated on the surface of the part and fatigue crack growth is restrained.

Fatigue Fracture: Definition, Type, Characteristics, Improvement Methods 5

Shot peening

Leave a Comment

Your email address will not be published. Required fields are marked *