Experimental Research on the Effect of Bolt Tightening Torque on Composite Material Connection Strength

Composite materials are a new class of materials that emerged in the 1960s. They have high strength, high rigidity, light weight, and have a series of advantages such as fatigue resistance, vibration reduction, high temperature resistance, and designability. vitality. At present, advanced composite materials have been widely used in aerospace, energy, transportation, construction, biology, medical and other fields.

Compared with metal materials, advanced composite materials have many excellent characteristics such as high specific strength and specific modulus, designable performance and easy integral forming. When used in aircraft structures, they can reduce weight by 25% to 30% compared to conventional metal structures. Therefore, more and more composite materials are used in the structural design of aerospace and various fields. Especially in the field of civil aviation, the amount of composite materials for Boeing 787 aircraft has reached 50%. The amount of composite materials for the newly developed Airbus A350 aircraft will be More than 50%.

Due to the limitation of the technical level of the composite material molding process and the composite material component is inevitably connected to the metal parts, there are still a lot of connection problems in the composite material structure, and due to the particularity of the composite material, the connection analysis is largely Rely on experimental research. The bolt tightening torque is one of the factors that affect the connection strength of the composite material. This article gives the connection failure load of the same layer of carbon fiber composite material under different tightening torques, which can provide a basis for the formulation of the composite tightening torque standard.

1 Test type and test conditions

2 Test results and analysis

(1) In the case of double-shear connection, as the tightening torque increases, the ultimate failure load gradually increases until the tightening torque reaches 8 N · m, and the joint strength decreases. The 4% deformation load showed a downward trend as the tightening torque increased.

(2) In the case of a single shear connection, as the tightening torque increases, the ultimate failure load also gradually increases, showing the same trend as the double shear connection.

(3) Comparing the deformation load of 4% of the same bolt tightening torque in the case of double shear and single shear connection, the load of double shear connection is smaller than that of single shear connection.

(4) Comparing the failure load of the same bolt tightening torque in the case of double shear and single shear connection, when the fastener is 2 N · m tightening torque, the load of the double shear connection is increased by 8.7% compared to the single shear connection; the fastener is When the tightening torque is 4 N · m, the load of the double shear connection is reduced by 7.8% compared to the single shear connection.

(5) The failure mode of the double-shear lap test piece is mainly the squeeze failure of the hole edge of the middle plate, the single-shear lap test piece has a squeeze failure at the hole edge of the lower laminate, and the bolt rod bends when the tightening torque is 2 N · m At 4 N · m, the bolt rod shear failure.

3 Conclusion

The connection form is the same as that of the fastener. Only when the tightening torque is different, the final damage load gradually increases with the increase of the tightening torque, but there is an extreme value. Instead, the load will decrease.

In the case of double shear and single shear connection, the deformation load of the same bolt is 4%, the double shear connection is smaller than the single shear connection load; in the case of double shear and single shear connection, the damage load of the same bolt tightening torque, the fastener is 2 N・ When the tightening torque is m, the double shear connection has a larger load than the single shear connection, and the fastener is 4 N. ・ When the tightening torque is m, the double shear connection has a smaller load than the single shear connection; the failure mode of the double shear lap test piece is mainly in the middle Plate hole edge extrusion failure, single shear lap test piece extrusion failure occurs at the lower laminate hole edge, and the bolt rod bends at 2 N · m tightening torque, and the bolt rod shear failure occurs at 4 N · m.

The above data only obtained the influence of the tightening torque on the connection strength under some connection conditions. More research on this aspect will be carried out in the future, and more data will be obtained for the study of bolt tightening torque.