In the process of smelting and forming, metal materials will inevitably produce internal defects such as inclusions and segregation, resulting in discontinuity of internal structure.
The shape of groove steps, positioning holes, edges and corners in mechanical and equipment components will also affect the continuity of external surface structure of parts.
Structural discontinuity will lead to stress concentration at local positions of parts in use.
In engineering, such discontinuous structures are often considered as “notches”. These notches will not only cause stress concentration of materials, but also change the stress and deformation state of notch root.
For example, during the tensile process, the stress state at the root of the notch will change from unidirectional tension to bidirectional or three-way tension, and the plastic deformation near the notch tip will also be significantly constrained.
The results show that the influence of notch on fracture behavior of materials is different due to different plasticity of materials.
Few scholars directly compare the fracture behavior of notched specimens of different plastic metal materials.
For this reason, researchers conducted tensile tests on three different plastic metal materials respectively.
By comparing the tensile strength and fracture morphology of notched specimens, the impact of notches on the tensile strength and fracture behavior of different plastic materials was studied.
1. Test method
Three kinds of metal materials with different plasticity, 10CrNi3MoV steel, 5083 aluminum alloy and 500-7 ductile iron, were used in the test.
According to the technical requirements of Metallic Materials Tensile Testing Part 1: Room Temperature Test Method (GB/T 228.1-2010), the R4 cylindrical tensile test sample is processed. The V-shaped notch is processed in the middle of the parallel section of the test sample.
The notch angle is 60 °, the notch tip radius is 0.1mm, and the root diameter D of the notch is 6, 8, 10mm (the corresponding notch depth is 2, 1, 0mm, and the specimen with the notch depth of 0mm is smooth).
See Fig. 1 for the shape and size of the tensile specimen.
Fig. 1 Shape and Size of Tensile Specimen
The tensile test is carried out on an electronic universal testing machine, and the tensile speed is 1mm · min-1.
The tensile test results of smooth samples of the three materials are shown in Table 1.
It can be seen that the elongation after fracture A of the three materials is quite different.
The elongation after fracture of 10CrNi3MoV steel is greater than that of 5083 aluminum alloy and 500-7 ductile iron.
The tensile strength of smooth samples and notched samples of three materials is shown in Table 2.
It can be seen that the tensile strength of 10CrNi3MoV steel and 5083 aluminum alloy notched samples is higher than that of smooth samples, and the tensile strength of notched samples of 500-7 ductile iron is lower than that of smooth samples.
Table 1 Tensile Properties of Smooth Specimens of Three Materials
|Elongation after fracture |
|Reduction of area|
|5083 aluminum alloy||175||345||16.4||21.7|
|500-7nodular cast iron||400||604||10.2||7.4|
Table 2 Tensile Strength of Smooth Specimens and Notched Specimens of Three Materials
|Material||Smooth specimen||Specimen with notch depth of 1mm||Specimen with notch depth of 2mm|
|5083 aluminum alloy||345||398||453|
|500-7nodular cast iron||604||575||556|
2. Test results
2.1 Tensile strength
The tensile strength notch depth curves of the three materials are shown in Fig. 2.
It can be seen from Fig. 2 that with the increase of notch depth, the tensile strength of 10CrNi3MoV steel and 5083 aluminum alloy increases.
Among them, the increase of 10CrNi3MoV steel is higher than that of 5083 aluminum alloy.
For example, when the notch depth is 2mm, the tensile strength of the former is 1.74 times that of the smooth sample, and that of the latter is 1.31 times.
For 500-7 ductile iron, the relationship between tensile strength and notch depth is contrary to that of 10CrNi3MoV steel and 5083 aluminum alloy.
That is, with the increase of notch depth, the tensile strength decreases.
When the notch depth is 2mm, the tensile strength is 0.92 times that of smooth sample.
Fig. 2 Relation curve between tensile strength and notch depth of different plastic metal samples
2.2 Fracture morphology
Fig. 3 shows the macro morphology of the fracture surface of smooth 10CrNi3MoV steel sample and notch samples with different depths.
It can be seen that the fracture surface of the smooth sample is typical cup cone shape.
The elongation after fracture of this sample is 27.5%, the reduction of area is 78.1%, and the plasticity is good;
The notched specimen was necked, and the reduction of area of the specimen with the notch depth of 2mm was 33%.
Fig. 3 Macro morphology of smooth specimen and notch specimen of 10CrNi3MoV steel
With the increase of notch depth, the area of the shear lip at the edge of the fracture gradually decreases, and the area of the central fiber area gradually increases.
When the notch depth is 2mm, the area of central fiber area accounts for 90%, as shown in Fig. 4a).
Fig. 4b) shows the SEM morphology of the area shown by the arrow in Fig. 4a).
It can be seen from the figure that the fiber area in the core of the sample is a dimple fracture with the characteristics of normal tension fracture, indicating that the starting position of the tensile sample is located in the central area of the sample.
Fig. 4 SEM Morphology of Fracture Surface of Specimen with 2mm Notch Depth of 10CrNi3MoV Steel
Figures 5 and 6 show the macro and SEM morphology of the fracture surfaces of 5083 aluminum alloy smooth samples and notched samples.
It can be seen that the fracture surfaces of the smooth samples show typical 45 ° shear failure fracture characteristics, with certain axial deformation and necking.
The elongation after fracture is 16.4%, and the reduction of area is 21.7%, as shown in Fig.5a);
The fracture load of the sample with a notch depth of 1mm is 20.00 kN, which is 13.74 kN higher than the yield load of the smooth sample.
Therefore, there is obvious plastic deformation at the fracture surface.
The fracture surface is serrated and has certain directivity. The crack initiation position is at the edge notch.
The fiber fracture is the main part near the crack initiation position. The serrated area consists of fiber fracture and 45 °shear fracture, as shown in Fig. 5b) and Fig. 6a).
In addition, obvious shear failure zone and fiber zone with normal tensile failure characteristics can be seen, as shown in Fig. 6b) and Fig. 6c);
For the specimen with a notch depth of 2mm, its breaking load is 12.83kN, which is smaller than the yield load of the smooth specimen, the reduction of area is almost 0, and the fracture surface is mainly fibrous, as shown in Fig. 5c), Fig. 6d) and Fig. 6e).
Only the edges can see obvious shear failure areas, as shown in Fig. 6e).
Fig. 5 Macro morphology of 5083 aluminum alloy smooth sample and notch sample
Fig. 6 SEM Morphology of 5083 Aluminum Alloy Notch Specimen
Fig. 7 shows the macro morphology of smooth and notched samples of 500-7 ductile iron and the SEM morphology of the smooth sample fracture.
The smooth sample has no obvious necking, but has certain plastic deformation, and the reduction of area is 7.4%, as shown in Fig. 7a);
The reduction of area of notched specimen is almost 0, without plastic deformation, as shown in Fig. 7b) and Fig. 7c);
There is no obvious difference between the fracture surfaces of smooth samples and notched samples, both of which are in cleavage, belonging to brittle fracture.
The cleavage morphology of smooth samples is shown in Fig. 7d).
Fig. 7 Macro morphology and fracture SEM morphology of 500-7 ductile iron tensile sample
3. Analysis and discussion
The existence of notch makes the specimen change from uniform uniaxial stress state to non-uniform triaxial stress state in the tensile process, and there is obvious stress concentration at the root of notch.
The notch will also restrain the tip and limit the deformation of the notch tip.
Due to the different plasticity of materials, the stress concentration and binding caused by notches vary greatly in the evolution of the whole plastic deformation process of samples, resulting in different effects of notches on the tensile strength of different materials.
The plasticity of 10CrNi3MoV steel is good, and the smooth tensile specimen has good lateral and axial deformation capacity.
For notched specimens, although there is notch binding effect, there is still some plastic deformation in the tensile process to cushion the stress concentration caused by the notch.
The crack initiation position of tensile specimens with different notch depths is located in the center of the specimen, and the large area of fiber zone in the center is a dimple fracture with normal tensile fracture characteristics.
During tensile plastic deformation, when the axial stress in the center exceeds the normal tensile fracture resistance of the material itself, the specimen will crack.
And because the notch constrains the deformation, the tangential plastic deformation contributes less to the stress release, and the stress level of the entire fracture plane is very high when the fracture occurs.
During the process of crack initiation and outward expansion, the entire fracture is a dimple fracture that is damaged due to exceeding the normal tensile fracture resistance.
Only a small number of shear lips are at the edge, which is characterized by tangential fracture.
The fracture surface of 5083 aluminum alloy smooth specimen is a typical 45 ° shear fracture, with certain axial deformation and necking.
When the notch of the sample is 1mm, the crack initiation position is at the edge of the sample.
When the stress exceeds the yield stress during the tensile test, 45 ° shear deformation begins to occur near the notch of the sample, and the fracture continues to shrink during the tensile test. The shear strain will occur in the entire notch section along the 45 ° direction, and the stress at the location where the shear strain occurs will be released.
However, due to the stress concentration near the notch tip and the inability to produce a large amount of shear deformation, the axial stress gradually increases.
When the notch edge load exceeds the fracture resistance, local normal tensile failure occurs from the edge, and the axial stress is subsequently transmitted to the entire fracture.
During the fracture propagation process, the specimen will be damaged along the part that has undergone 45 ° shear deformation, forming a serrated fracture.
When the specimen notch is 2mm, the crack initiation position is located at the junction of plastic deformation and elastic deformation of the notch section.
Since the stress at the time of fracture of the notched specimen does not exceed the yield stress, the specimen does not have a large area of deformation in the 45 ° shear direction.
Due to the stress concentration at the root of the notch, when the stress is greater than the yield stress of the sample, a small amount of plastic deformation will occur.
Because of the binding effect of the notch and the movement characteristics of the aluminum alloy slip system, the sample cannot have a large amount of plastic deformation in the radial direction, and the plastic deformation zone cannot extend to the center of the sample.
Therefore, the maximum force is borne at the junction of the plastic deformation zone and the elastic deformation zone.
When the maximum force exceeds the fracture resistance of the material, the normal tension failure occurs at the maximum force, and then extends to the entire notch section.
The fracture surface is dominated by the dimple shape with normal tension fracture characteristics.
The fracture surface of 500-7 ductile iron smooth sample is a flat fracture perpendicular to the stress direction, with obvious brittleness characteristics.
The smooth specimen has certain axial and radial deformation in the tensile process, and the deformation is caused by the maximum shear stress.
For notched specimens, stress concentration is formed at the edge of the specimen, and the stress in the tensile process will reach the fracture resistance earlier and start to crack, and then rapidly expand to the entire section.
Due to the binding state of the notch and the brittleness tendency of the material, the ability of the specimen to relieve the stress concentration near the notch through plastic deformation is poor, and the normal stress of the specimen from the notch to the center will be very different.
Discontinuities in shape generally produce stress concentrations.
For brittle materials, stress concentration tends to cause premature fracture of the specimen, leading to strength decline;
The greater the depth of the notch, the higher the stress concentration at the root, the earlier the specimen will fracture, and the lower the tensile strength.
The plastic material at the notch tip can relieve the stress concentration through a certain degree of plastic deformation, and re distribute the stress on the notch section to reduce the stress concentration.
According to the third strength theory, the maximum shear stress is the main factor leading to plastic deformation and failure of materials, and the normal stress at this time is far less than the maximum normal stress that can cause material fracture and failure.
For the notched specimen, because the binding state limits the deformation of the material along the direction of the maximum shear stress, the fracture mode changes from cutting to pulling, and the tensile strength will be improved accordingly.
For materials with better plasticity, the stress distribution of the entire notch can be more uniform through plastic deformation.
The section where the entire notch is located is closer to the theoretical tensile strength of the material, and the tensile strength increases more significantly.
Therefore, the tensile strength of notched specimen of 10CrNi3MoV steel is most obviously improved than that of smooth specimen.
However, if the plasticity is not good enough or the notch binding is large, and the strain cannot extend to the center, the notch section will be destroyed at the junction of elastic deformation and plastic deformation.
Before the fracture, some of the interface forces are still in the elastic zone.
Therefore, the tensile strength of 5083 aluminum alloy notch specimen is higher than that of smooth specimen, but the increase degree is less than that of 10CrNi3MoV steel.
In addition, the deeper the notch is, the smaller the plastic deformation can make the core of the specimen reach the theoretical tensile strength, and the strength near the notch decreases less, which will also increase the tensile strength of the notched specimen.
(1) Notch will lead to stress concentration of materials under stress.
For materials with good plasticity, the stress distribution of the notch section can be conducted again through the plastic deformation of the notch tip to alleviate the stress concentration without reducing the strength of the material.
For brittle materials, the plastic deformation capacity at the notch tip is small, and the role of relieving stress concentration is not obvious.
Therefore, the stress concentration will lead to local failure of the material and expand to the entire section, reducing the overall strength of the material.
(2) Notch will change the stress state and fracture mode of plastic materials during deformation.
For plastic materials, the fracture stress changes from shear stress to normal stress, and the fracture mode changes from shear fracture to axial normal tensile failure.
Therefore, notches tend to increase the tensile strength of materials, and the better the plasticity is, the higher the proportion of normal tensile failure is, the more obvious the tensile strength is.
For brittle materials, due to the effect of notch stress concentration, there will be a great gradient in the normal stress from the notch root to the center of the sample when fracture occurs.
The microcrack will first form at the root, and then rapidly expand to the center, so the tensile strength of the material will be reduced, and the fracture mode will not change.