Bolts are important fasteners, especially in dynamic machines where the bolts are more pronounced. For example, the characteristics of large internal combustion engine power, large mass, and high vibration determine that some bolts need to bear huge loads. In order to ensure high speed, smooth, safe and long-term operation of the internal combustion engine, the pre-tightening force of the bolt needs careful consideration and implementation. Insufficient pre-tightening force, long-term operation will loosen and produce noise, accelerate wear; pre-tightening force over-head, exceeding bolt strength, easily cause fatigue damage. In order to study the influence of the bolt pre-tightening force on the bolt, a transparent patch-epoxy resin was attached to the surface of the bolt head. The epoxy resin grade 618, light yellow, thickness 5mm, material stripe value 12.5KN/m grade, with torsion The wrench tightening nut produces a certain pre-tightening force. After the bolt is stressed, the epoxy patch produces streaks in the polarized light field, and the stripe is the main stress difference stripe (isochromatic line) of the patch. By analyzing the stripe, the bolt force can be quickly evaluated.
1 Principle of reflected photoelastic measurement
A photo-elastic material [1] (such as epoxy resin) is attached to the surface of the member. When the member is stressed, the photo-elastic patch will be deformed according to the deformation of the surface of the member, and the photo-elastic patch can be observed by the reflective photoelastic device. A stress light pattern related to the surface deformation of the component is generated, which is a temporary birefringence phenomenon [2], that is, after the release force, the relevant stripe disappears. The photo-elastic experimental data is obtained by photographing the stress light map by a digital camera, and then the surface strain or stress of the member to be tested is obtained by calculation. The reflected light beam path is shown in Figure 1 below, and the direction of the arrow indicates the light path.
2 bolt preload test
Fig. 2 shows the state when the bolt is fixed, and the workpiece 4 to be fixed is the upper and lower parts, and is connected by the bolt 1, the nut 2 and the washer 3, and the photoelastic patch 5 is attached to the head of the bolt 1, and the connection is placed in Fig. 1 In the illustrated optical path, the photoelastic patch 5 faces the light source. Torque force is applied to the nut 2 by a torque wrench (shown in Figure 3) to generate the bolt pre-tightening force. The external force mainly received by the bolt head is the tensile force of the root of the bolt and the contact surface of the fixed workpiece, and the shearing force of the hole edge to the root of the bolt. And complex combinations of torsional forces generated by friction of the contact surfaces. When different torques are applied, the force on the head of the bolt is different. The corresponding stress light diagram [3] obtained by the reflected photoelastic instrument of Fig. 1 also changes (shown in Fig. 4).
A torque wrench is a tool that can apply a set torque to a bolt. Using the torque wrench of Figure 3 above, the author conducted a comparative experiment on the national standard bolts (M12, M14, M16) of different diameters. Because of the different bolt diameters, the stiffness will be different. If the same torque T is applied, different diameters The deformation of the bolts is also relatively different. The reflected stress maps differ in shape, and the number of stripes is different. From the point of view of the photoelastic fringe series analysis [4], the stress light map reflects the deformation of the bolt head under the complicated force such as pressing and shearing under the tightening condition, resulting in the force of the photoelastic patch. In the case, as long as the key data of the stripe level is correctly interpreted, the main stress difference of the patch can be obtained by substituting the parameters of the material stripe value and the patch thickness by the formula (1). Due to space limitations, only the experimental results of the photo-elastic patch stress light diagram of the head of the M14 hex head bolt are listed, as shown in Figure 4 below.
3 Several key issues in the experiment
The experiment found that the shape of the stress light diagram of different diameter bolts is not the same, it may be that the stress surface of the bolt is not flat, or the bolt processing has errors, so the direction of the force is not strictly perpendicular to the bolt head, that is, the direction of the force is also It may be different; in the experimental scheme, the thickness of the photoelastic patch, the bonding strength, and the uniformity of the material are all important components of the error; the process of information acquisition is oblique illumination of the light, and the digital camera is also inclined when receiving information. The error is difficult to overcome. The optical effect thus observed is the average over a certain width of the measured point. If the orthogonal type reflective photoelastic light path arrangement shown in the figure below is used, the above-mentioned oblique error can be significantly reduced, and the direction of the arrow indicates the optical path, as shown in FIG.
The orthogonal reflection type photoelastic light path is provided with a half anti-half lens, and the incident light and the reflected light are perpendicular to the patch. The added half-reverse half lens has the function of changing the light direction and equally dividing the light energy, thereby ensuring the positive path of the light path. The intersection and the incident light and the reflected light pass through the same point of the photoelastic patch, and no oblique error can be generated. However, the system is troublesome to adjust, and the half-reflex lens loses part of the light intensity. For current high-power solid-state lasers (such as 532nm solid-pump laser power up to 150mw) or high-energy LED lights can be easily obtained, it is still acceptable.
4 Stress analysis
The free surface of the member is completely bonded to the patch and has the same deformation [5]. The subscript b below indicates the bolt head and c indicates the photoelastic patch. According to the plane stress-optical law, and considering that the light passes through the patch twice, the principal stress difference of the patch is
Where n is the photoelastic stripe order, and f 0 is the stress fringe value of the patch material. This value is constant and can be found in Table [6], where t is the patch thickness. According to Hooke's law, the main strain difference of the patch is
It can be seen that the number of fringe lines n of the isotropic line of the photoelastic patch is measured by a reflective photoelastic instrument, and the surface main strain difference and principal stress of the bolt head can be obtained by the equations (3) and (4). difference. The fringe series n is the key data therein and can be obtained from the stress map shown in Fig. 4 by means of fringe compensation [7].
The photoelastic stripe can directly obtain the principal stress difference. For the principal stress of each point, supplementary conditions are needed, which are obtained by stress separation [8] (such as photoelastic stripe oblique method) or calculation (shear stress difference method). For example, the shear stress difference method uses the equidistant line and the other important data of the photoelastic—the isosceles line (ie, the principal stress direction line), by calculating the interpolation from the boundary, the principal stress can be obtained point by point. This article focuses on the relationship between the maximum number of stripes and the preload. Table 1 below shows the maximum number of fringe levels n (units) obtained by fringe compensation for the three bolts M12, M14, and M16 under different torque (T) preloads.
5 Conclusion
The bolt test piece is placed in the optical path. During the gradual loading process, the process of gradually changing the main stress contour line can be seen, and the relationship between the maximum number of fringes and the pre-tightening force is established, so that the appropriate pre-tightening force can be determined. Can meet enough pre-tightening requirements without loosening, and can ensure that the bolt pre-tightening force will not overshoot and cause damage. The method has many advantages such as high sensitivity, intuitiveness and simplicity, and is suitable for on-site measurement and detection. Even after the bolt is tightened, the photo-elastic patch can be pasted to monitor the residual stress (or its release) of the bolt head for a long time, similar to a full-field real-time optical strain gauge, thus giving a suitable working condition for the bolt. The evaluation is performed by finite element method, in order to obtain the direct relationship between the number of photoelastic stripes, the torque, the surface stress of the bolt, etc., so that the method has great popularization and application value.
references:
[1] Cai Lizhong. Optics (third edition) [M]. Beijing: Science Press, 2007.
[2] Chengdu Institute of Geology, Rock Teaching and Research Office. Crystal optics [M]. Beijing: Geological Publishing House, 1797.
[3] Ji Xinhua, Zhang Lina. Full field isosceles and isobaric phase in digital photoelastic phase shift method [J]. Journal of Optics, 2008, 28(2): 273-278.
[4] Han Yongsheng, Zhang Dongsheng, Luo Wei. Determination of photoelastic parameters by dual wavelength method and phase shift method [J]. Engineering Mechanics, 2008, 25(2): 62-65.
[5] Lu Peng, Zhang Wei, Wu Junyi, etc. Application of photoelastic method on flexible coupling of flexible coupling [J]. Diesel Engine, 2008, 30 (5): 48-50.
[6] A. Cosko, G. Robertson. Photoelastic stress analysis [M]. Shanghai: Shanghai Science and Technology Press, 1979.
[7] Zhang Wei. Application of Photoelastic Elasticity in Diesel Engine Design [R]. Marine diesel engine (internal reference), 1976 (1).
[8] Zhao Qingcheng. Photometric mechanics [M]. Shanghai: Shanghai Science and Technology Press, 1982.
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