This type of deformation is caused by a pair of forces which are equal in size, opposite in direction, and which coincide with the axis of the rod, showing that the length of the rod is elongated or shortened [fig 3-1 (A, b)]. such as lifting the weight of the cable, truss of the rod, hydraulic cylinder of the piston and other deformation, all belong to tensile or compression deformation. It is often seen in engineering to withstand tensile or compressive rod parts. For example, a fastening screw [fig $number (a)], when tightening the nut, the pressed workpiece has a reaction to the screw, the screw bears the tensile force, and the screw of the Jack [Fig $number (b)] is subjected to compression when the weight is lifted. The former occurs elongation deformation, the latter occurs in a shortened deformation, straight rod along the axis by the size of the opposite force, the direction of the reverse, when the elongation or shortening of the deformation, called the axial tensile or compression of the rod. This chapter deals only with axial stretching and compression of straight rods. If the shape and force of the rod subjected to axial stretching or compression is simplified, the Force diagram shown in Fig. 3-1 can be simplified. A solid line is used to indicate the shape of a force before it is deformed.