In the casting and forming process, metallic materials inevitably produce internal defects, such as inclusions and segregation, which can result in discontinuity of the internal structure.
In addition, the shape of groove steps, positioning of holes, edges and corners on mechanical and equipment components can also affect the continuity of the outer surface structure of parts.
Structural discontinuity can lead to stress concentration in the local positions of parts during use. Such structures are often considered “notches” in engineering.
These notches cause stress concentration in the materials and change the stress and deformation state of the notch root. For example, during the tensile process, the stress state at the root of the notch changes from one-way stress to two-way or three-way stress, and the plastic deformation near the notch tip is significantly restricted.
The influence of notches on the fracture behavior of materials differs due to the variation in plasticity of materials. However, few scholars directly compare the fracture behavior of notched samples of different metallic plastic materials.
Therefore, the researchers performed tensile tests on three different metallic plastic materials to compare the tensile strength and fracture morphology of notched samples. This study aims to understand the impact of notches on the tensile strength and fracture behavior of different plastic materials.
1. Test method
The test used three types of metallic materials, namely 10CrNi3MoV steel, 5083 aluminum alloy and 500-7 ductile iron, each with different levels of plasticity.
Following the technical requirements stipulated in the Ambient Temperature Test Method section of the Tensile Test of Metallic Materials Part 1 (GB/T 228.1-2010), R4 cylindrical tensile test specimens were processed and a V-shaped notch was made in the center of the parallel section of each sample.
The notch angle was 60°, the notch tip radius was 0.1 mm, and the notch root diameter D was 6 mm, 8 mm, or 10 mm (with corresponding notch depths of 2 mm, 1 mm and 0 mm, respectively, and a smooth sample without notches).
See Figure 1 for the shape and dimensions of the tensile sample.
Fig. 7 Macro morphology and SEM morphology of fracture of 500-7 ductile iron tensile sample
3. Analysis and discussion
The presence of a notch causes a uniform uniaxial stress state to transform into a non-uniform triaxial stress state during the tensile process, resulting in an obvious stress concentration at the root of the notch. The notch also restrains the tip and limits deformation of the notch tip.
Due to the different plasticity of materials, notches can cause varying degrees of stress concentration and bonding during the entire plastic deformation process, resulting in different effects on the tensile strength of different materials.
The 10CrNi3MoV steel has good plasticity, and the smooth tensile specimen has good lateral and axial deformation capacity. Although the notched samples experience the bonding effect of the notch, there is still some plastic deformation during the tensile process that can buffer the stress concentration caused by the notch.
The crack initiation position for tensile specimens with different notch depths is located at the center of the specimen, where there is a large fiber zone that exhibits normal tensile fracture characteristics with a wavy fracture. When the axial stress at the center exceeds the normal tensile fracture strength of the material itself, the specimen cracks. Due to the restriction of the notch, tangential plastic deformation contributes less to stress release, and the stress level of the entire fracture plane is very high when fracture occurs.
During the process of crack initiation and external expansion, the entire fracture has a wavy fracture caused by exceeding the normal tensile fracture strength, with only a small number of shear edges at the edge, which is characterized by tangential fracture.
The fracture surface of the smooth 5083 aluminum alloy sample is a typical 45° shear fracture with some axial deformation and necking. When the notch of the sample is 1 mm, the crack initiation position is at the edge of the sample.
During the tensile test, if the stress exceeds the yield stress, shear deformation at an angle of 45° begins to occur near the notch of the specimen, and the fracture continues to shrink during the test. Shear deformation will occur throughout the notch section in the 45° direction, causing stress to be released at the location where shear deformation occurs.
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 strength, local normal tensile failure occurs at the edge and 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.
For a sample with a 2 mm notch, the crack initiation position is located at the junction of the plastic and elastic deformation of the notch section.
Since the stress at the moment of fracture of the notched sample does not exceed the yield stress, the sample does not undergo a large area of deformation in the 45° shear direction.
When the stress at the root of the notch exceeds the yield stress of the sample, a small plastic deformation occurs due to stress concentration.
However, due to the notch binding effect and the movement characteristics of the aluminum alloy sliding system, the specimen cannot undergo significant plastic deformation in the radial direction, and the plastic deformation zone cannot extend to the center of the sample. As a result, the maximum force is supported at the junction of the plastic deformation zone and the elastic deformation zone.
If the maximum force exceeds the fracture strength of the material, normal stress failure occurs at the maximum force, which then extends to the entire notch section. The fracture surface has a ripple shape with normal stress fracture characteristics.
On the other hand, the fracture surface of the smooth 500-7 ductile iron sample appears flat, perpendicular to the stress direction, and with clear brittle characteristics. The smooth sample undergoes certain axial and radial deformation during the tensile process, which results from the maximum shear stress.
Notched specimens produce stress concentration at the edge, and the stress in the tensile process reaches fracture strength earlier, causing the specimen to begin to crack and expand rapidly to the entire section.
Due to the bonding state of the notch and the brittle tendency of the material, the ability of the sample to relieve the stress concentration near the notch through plastic deformation is poor. Therefore, the normal stress of the sample from the notch to the center will be significantly different.
In general, discontinuities in shape lead to stress concentrations.
In brittle materials, stress concentration can cause premature fracture of the sample, leading to a decline in strength.
As the depth of the notch increases, the stress concentration at the root becomes greater, resulting in earlier fracture of the sample and lower tensile strength.
However, the plastic material at the notch tip can alleviate the stress concentration by undergoing a certain degree of plastic deformation and redistributing the stress along the notch section.
Based on the third strength theory, maximum shear stress is the main factor that leads to plastic deformation and failure of materials, while normal stress is much lower than the maximum normal stress that can cause material fracture and failure.
In notched specimens, the bonded state limits the deformation of the material along the direction of maximum shear stress, causing the fracture mode to change from shear to tensile and subsequently increasing the tensile strength.
For materials with better plasticity, plastic deformation can result in a more uniform stress distribution across the notch, and the section where the notch is located is closer to the theoretical tensile strength of the material, leading to a more significant increase in strength. to traction.
The tensile strength of the 10CrNi3MoV steel notched sample is significantly higher than that of the smooth sample. However, if the plasticity is insufficient, or if the notch bond is significant and the deformation cannot extend to the center, the notch section may be destroyed at the junction of elastic and plastic deformation.
Some of the interface forces remain in the elastic zone before fracture. As a result, the tensile strength of the notched sample of 5083 aluminum alloy is higher than that of the smooth sample, but the increase is smaller than that of 10CrNi3MoV steel.
Furthermore, the deeper the notch, the smaller the plastic deformation that can allow the sample core to reach theoretical tensile strength. Consequently, the strength near the notch decreases less, leading to an increase in the tensile strength of the notched sample.
4. Conclusion
(1) The notch will lead to stress concentration of the materials under tension.
In materials that exhibit good plasticity, the stress concentration in the notch section can be relieved by conducting plastic deformation of the notch tip. This process helps distribute stress evenly without compromising the strength of the material.
However, for brittle materials, the notch tip has limited plastic deformation capacity, which makes it difficult to effectively relieve stress concentration. As a result, stress concentration can lead to local failure of the material, which can then spread throughout the section, ultimately reducing the overall strength of the material.
(2) The notch will change the stress state and fracture mode of plastic materials during deformation.
The fracture stress of plastic materials changes from shear stress to normal stress, causing the fracture mode to change from shear fracture to axial normal tensile failure. This means that notches typically increase the tensile strength of materials, and the greater the plasticity, the greater the proportion of normal tensile failure and the more noticeable the increase in tensile strength.
However, for brittle materials, the effect of notch stress concentration leads to a significant gradient in normal stress from the root of the notch to the center of the sample during fracture. As a result, a microcrack will initially form at the root and rapidly propagate towards the center, leading to a reduction in tensile strength without changing the fracture mode.
