Tension and compression
This kind of deformation is caused by a pair of forces of equal magnitude and opposite direction, and the line of action coincides with the axis of the rod, which is manifested as the length of the rod is elongated or shortened. For example, the deformation of the steel cable for lifting heavy objects, the rods of the truss, and the piston rod of the hydraulic cylinder are all tensile or compression deformations. In engineering, we often see rods under tension or compression. For example, when tightening the screw, when the nut is tightened, the compressed workpiece has a reaction force on the screw, and the screw is stretched; the screw of the jack is at the top When lifting heavy objects, they are subjected to compression. The former is elongated and deformed, and the latter is shortened and deformed. When the straight rod is subjected to external forces of equal magnitude and opposite direction along the axis, when the elongated or shortened deformation occurs, it is called the axial tension or compression of the straight rod. This chapter only discusses the axial tension and compression of straight rods. If the shape and force of the rod subjected to axial tension or compression are simplified, it can be simplified into the force diagram shown in. In the figure, the solid line represents the shape before force, and the dashed line represents the shape after deformation.
Cut
Members that are subjected to shearing are often seen in engineering. The common feature of this kind of rods is that they are subjected to the same magnitude and opposite directions on both sides of the member, and the action lines are very close and perpendicular to the rod axis. Under such external force, the main deformation of the rod is: taking the cross section m-m between the two forces as the interface, the two parts of the member move relative to each other along this surface. This form of deformation of the component is called shear, the section m-m is called the shear plane, and the shear plane is parallel to the direction of the external force. When the external force is large enough, the component will be sheared along the shear plane. There is only one shear surface, which is called simple shear. At the same time, the component is compressed, and the two sides are also squeezed by other components. This phenomenon of local surface compression is called extrusion. If the pressure is high, significant plastic deformation will occur in the local area of the contact surface, causing the structure to not be used normally. This phenomenon is called extrusion failure. In addition to being sheared and squeezed, the coupling is often accompanied by other forms of deformation. For example, bending or stretching deformation. However, since these deformations are secondary to shear and extrusion deformations, they are generally not considered. This type of deformation is caused by forces of equal magnitude, opposite directions, and lines of action parallel to each other, which manifests as the relative displacement of the two parts of the shear bar along the direction of the external force. Commonly used couplings in machinery, such as keys, pins, and bolts, all produce shear deformation.
Twist
This type of deformation is caused by two couples of equal size, opposite directions, and both acting surfaces perpendicular to the rod axis. It appears as the relative rotation of any two cross-sections of the rod around the axis. The drive shaft of the automobile, the main shaft of the motor and the turbine are all torsion rods. When there is a force coupling in a plane perpendicular to the rod axis, the rod will produce torsional deformation, that is, the cross sections of the rod will relatively rotate around the rod axis. The torsional deformation of the rod has the following characteristics: Force: Two couples are acting in a plane perpendicular to the axis of the rod at both ends of the rod, the torque of the couple is equal, and the direction of rotation is opposite. Deformation: Each cross section on the rod rotates relative to the axis of the rod. The relative rotation angle between any two cross sections is called the relative torsion angle. The torsional deformation of components is often encountered in engineering. For example, the two hands of the driver apply a force of equal magnitude, opposite directions, and parallel lines of action in the plane of the steering wheel. They form a force couple that acts on the end of the joystick, and the end of the joystick receives the force from the steering gear. The action of the reaction couple, so that the joystick is twisted. 4. Bending, this kind of deformation is caused by the transverse force perpendicular to the axis of the rod, or a pair of equal and opposite force couples acting on the longitudinal plane containing the axis of the rod, which manifests as the axis of the rod changes from a straight line. For the curve. In engineering, bending members are one of the most frequently encountered situations. The deformation of the girder, various mandrels and turning tools of the bridge crane are all bending deformations. After the force is applied, the axis of these straight rods will be bent from the original straight line into a curve. This deformation is called bending. Bars that are mainly bent and deformed are usually called beams. There are also several rods that undergo several basic deformations at the same time. For example, when the lathe spindle is working, three basic deformations such as bending, torsion and compression occur; two basic deformations of stretching and bending occur simultaneously on the column of a drill press. This situation is called combined deformation.
