14 2.Fundamental Mechanical Properties of Materials FIGURE 2.3.Distortion of a cube caused by shear stresses △a Txy and Tyx. Tyx The force is measured in newtons(1 N=1 kg m s-2)and the stress is given in N m-2 or pascal(Pa).(Engineers in the United States occasionally use the pounds per square inch(psi)instead, where 1 psi=6.895 X 103 Pa and 1 pound =4.448 N.See Ap- pendix II.)The strain is unitless,as can be seen from Eq.(2.2) and is usually given in percent of the original length. The result of a tensile test is commonly displayed in a stress-strain diagram as schematically depicted in Figure 2.4. Several important characteristics are immediately evident.Dur- ing the initial stress period,the elongation of the material re- sponds to o in a linear fashion;the rod reverts back to its orig- inal length upon relief of the load.This region is called the elastic range.Once the stress exceeds,however,a critical value, called the yield strength,oy,some of the deformation of the material becomes permanent.In other words,the yield point separates the elastic region from the plastic range of materials. Stress Tensile strength U T Yield strength Breaking strength y Plastic part Necking 复 △U FIGURE 2.4.Schematic rep- resentation of a Tension △e stress-strain diagram for a ductile material.For ac- Strain s tual values of oy and or, see Table 2.1 and Figure Compression 2.5.The force is measured in newtons (1 N
1 kg m s2) and the stress is given in N m2 or pascal (Pa). (Engineers in the United States occasionally use the pounds per square inch (psi) instead, where 1 psi
6.895 103 Pa and 1 pound
4.448 N. See Ap￾pendix II.) The strain is unitless, as can be seen from Eq. (2.2) and is usually given in percent of the original length. The result of a tensile test is commonly displayed in a stress–strain diagram as schematically depicted in Figure 2.4. Several important characteristics are immediately evident. Dur￾ing the initial stress period, the elongation of the material re￾sponds to in a linear fashion; the rod reverts back to its orig￾inal length upon relief of the load. This region is called the elastic range. Once the stress exceeds, however, a critical value, called the yield strength, y, some of the deformation of the material becomes permanent. In other words, the yield point separates the elastic region from the plastic range of materials. 14 2 • Fundamental Mechanical Properties of Materials Plastic part Elastic part   Stress Yield strength y Tensile strength T Breaking strength B Tension Compression Strain Necking FIGURE 2.4. Schematic rep￾resentation of a stress–strain diagram for a ductile material. For ac￾tual values of y and T, see Table 2.1 and Figure 2.5. a  xy yx a FIGURE 2.3. Distortion of a cube caused by shear stresses xy and yx.