Mechanical properties near fasteners in mechanical joints

In various systems and devices, there is inevitably a connection between components and components, components and components, and mechanical fastening connections are the most commonly used form of connection.

It is generally believed that although there is little interaction between the nail holes because they are far enough apart, conventional elastic mechanics cannot be directly applied. Therefore, although theoretically, a lot of researches have been done on the stress concentration and structural damage of the hole edge, and rich results have been obtained. However, when applied to engineering practice, it is still necessary to conduct in-depth research on multiple problems.

The first problem is the nail loading problem. Even if the nail hole connections are exactly the same, the load they bear can still be different, and the load ratio between them is the nail load distribution relationship; then the stress concentration around each nail hole The problem is that stress concentration has direct and close relationship with structural damage such as cracking and fatigue, and has a significant impact on structural safety. Therefore, it has received extensive attention in both theoretical and practical work, and research on related issues has emerged one after another.

In this paper, the ANSYS finite element method is used to study the nail-loading relationship of single-row multi-nail connection structure and the stress concentration problem at the edge of the hole. The effects of structural dimensions such as plate thickness, joining process and material properties on it are analyzed. The influence of the tolerance between the nail and the hole on the distribution relationship of the nail load is discussed, and the optimization design of the structure is studied, which provides a theoretical basis for engineering practice.

1. Modeling and analysis of multi-nail connection structure

Figure 1 shows the geometric parameters of the multi-nail connection structure model. Where W is the width of the upper and lower plates, L and L2 are the length of the two plates and L=L2, δi and 82 are the thickness of the upper and lower plates respectively, and M is the distance between the ends of the two plates, e is The distance from the nail hole to the end near the end of the plate, s is the spacing of adjacent nail holes, D is the diameter of the nail hole, and E and E2 are the elastic modulus of the two plates respectively.

The boundary conditions are set as follows: the Z-direction displacement of the upper surface of the plate 1 and the lower surface of the plate 2 is 0, the Z-direction displacement of the upper and lower surfaces of the pin is 0, the left end surface of the plate 2 is fully constrained, and the right end surface of the plate 1 is loaded with a displacement load. When meshing, the stress around the hole is more complicated. There are both the hole edge extrusion zone and the tension zone, which is the most prone to stress concentration and dangerous points. Therefore, the mesh division near the hole edge is dense.

The contact between the nail holes is set to friction contact, and the friction factor is 0.2. The influence of the tolerance of the nail holes on the load distribution is considered in the calculation. The pin is modeled according to the original size, and the interference is not considered. The gap interference is set in the Interface Treatment option.

2. Finite element analysis

2.1 Influence of plate thickness and material properties on nail load distribution

The ANSYS finite element method is used to analyze the nail load distribution. The influence of the thickness and material strength of the plate on the load distribution is shown in Fig. 2.

It can be seen from Fig. 2 that as the strength of the material of the plate 2 increases, the pin near the loaded end becomes larger and the yield fracture is more likely to occur. When the material strength of the plate 2 is smaller than the material strength of the plate 1, the pin near the load receiving end becomes smaller, and the nail load distribution tends to be uniform. When the thickness of the plate 2 is increased, the pin near the loaded end is less loaded, and the load near the fixed end pin is significantly increased, and the risk of breakage is likely to occur.

In the case of δ/82=1, as the material strength ratio changes, the nail 1 load distribution is reduced by 11%, and the nail 3 load distribution is increased by 11%. Table 1 shows the effect of the elastic modulus ratio on the nail load distribution when δ = δ 2 (the sum of the load distributions does not reach 1 because there is friction between the plates).

It can be seen from Table 1 that when the elastic modulus ratio is increased from 1 to 2, the load distribution change of the pin near the both ends is 15% to 19%, and the change in the pin load in the middle is small. The doubling of the elastic modulus of the plate 2 means that the rigidity of the plate 2 is doubled, and the rigidity ratio of the plate 2 to the plate 1 is reduced by half, which obviously makes the load distribution between the fasteners more unbalanced. The load transmitted by the fastener near the fixed end is further increased.

Table 2 shows the effect of thickness variation on each nail load distribution when E2/E1=1.0.

It can be seen from Table 2 that when the thickness ratio is increased from 1 to 2, the nail load changes nearly doubled. As the plate thickness increases, the load distribution of the pin near the fixed end becomes larger, and the load distribution of the remaining pins becomes smaller. An increase in the thickness of the plate 2 means an increase in the area of ​​the constraint, which will result in a significant increase in the coercive constraint of the deformation of the upper and lower plates at the fastener joints near the restraining end, and will still have a smaller effect on the connection of the distant fasteners. .

For fasteners near the loading end, a doubling of the thickness of the panel 2 means that the stiffness ratio of the panel 1 to the panel 2 is reduced by half, and the restraining effect on the deformation of the fastener is also significantly increased. Therefore, the load transmitted from the plate 1 to the plate 2 by this fastener will be significantly less without considering the influence of the restraining end.

2.2 Distribution of hole edge stress

According to the above analysis, the nail 3 is subjected to a large load, and the hole edge is easily damaged. The local coordinate system of the surface nail 3 on the upper surface of the plate 2 is as shown in Fig. 3, wherein the central axis of the hole 3 is the Z axis, the upper surface of the plate 2 is Z=0, and the radius of the hole edge is R from the central axis, and the X axis The angle is the day.

Figure 4 shows the dimensionless Mises stress distribution at different radii and different levels of the nail 3 hole.

It can be seen from Fig. 4 that the variation trend of each curve is basically the same, only the area near the two sides of the hole perpendicular to the direction of the external force intersects the circumference, which is mainly due to the fasteners in this part. The pressing action between the plate holes is relatively weak, starting from the position of θ=40°, the Mises stress monotonously decreases, and reaches a minimum value at θ=90°, which is apparently because the semi-cylindrical near the restraining end for the plate 2 The surface of the hole does not theoretically come into contact with the fastener and is squeezed.

However, in fact, due to the inclination of the fastener, this surface will also have a non-strong squeezing effect, and the squeezing is relatively weak at the θ=90° position. After this part, the Mises stress Increasingly monotonically, small fluctuations occur in the vicinity of θ = 180°, and are maintained at a maximum value in the region of θ = 240° to 300°, and then rapidly decrease. The effect of the thickness of the plate on the stress at the edge of the nail 3 is shown in Fig. 5. The effect of the material strength on the stress at the edge of the nail 3 is shown in Fig. 6.

It can be seen from Fig. 5 and Fig. 6 that the stress change in the far layer from the surface layer (Z-O) tends to be moderate.

3. Conclusion

In this paper, the finite element method is used to analyze the nail load distribution and stress concentration in the typical fastener connection structure. The analysis results show that the plate thickness and material properties have an impact on the pin load distribution. The calculation results are consistent with the prediction of stress severity coefficient method (SSF), which indicates that the results of this paper are correct and credible, help to solve the fatigue problem of related structures and guide the optimal design of the corresponding structure.

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