2.Fundamental Mechanical Properties of Materials 15 Nylon WoodL PVC Ice Polyurethane Concrete W Pressure- foam a-Fe Stainless vessel Al Epoxy steel steel Ultrapure Au Diamond fcc metals Cast iron Ni SiO2 SiC Pb Cu Alkali ALO3 halides 10 100 1.,000 10,000 o,[MNm-2] FiGURE 2.5.Yield Polymers strengths of materials (given in meganewtons Ceramics per square meter or Metals megapascals;see Ap- Composites- pendixΠ). This is always important if one wants to know how large an ap- plied stress needs to be in order for plastic deformation of a workpiece to occur.On the other hand,the yield strength pro- vides the limit for how much a structural component can be stressed before unwanted permanent deformation takes place As an example,a screwdriver has to have a high yield strength; otherwise,it will deform upon application of a large twisting force.Characteristic values for the yield strength of different materials are given in Table 2.1 and Figure 2.5. The highest force (or stress)that a material can sustain is called the tensile strength,or(Figure 2.4).At this point,a localized decrease in the cross-sectional area starts to occur.The material is said to undergo necking,as shown in Figure 2.6.Because the cross section is now reduced,a smaller force is needed to con- tinue deformation until eventually the breaking strength,oB,is reached (Figure 2.4). The slope in the elastic part of the stress-strain diagram(Fig- ure 2.4)is defined to be the modulus of elasticity,E,(or Young's modulus): △d=E. (2.3) △e Equation(2.3)is generally referred to as Hooke's Law.For shear stress,T[see above and Figure 2.3],Hooke's law is appropriately written as: △=G, (2.4) △y Sometimes called ultimate tensile strength or ultimate tensile stress,ours.This is always important if one wants to know how large an ap￾plied stress needs to be in order for plastic deformation of a workpiece to occur. On the other hand, the yield strength pro￾vides the limit for how much a structural component can be stressed before unwanted permanent deformation takes place. As an example, a screwdriver has to have a high yield strength; otherwise, it will deform upon application of a large twisting force. Characteristic values for the yield strength of different materials are given in Table 2.1 and Figure 2.5. The highest force (or stress) that a material can sustain is called the tensile strength, 1 T (Figure 2.4). At this point, a localized decrease in the cross-sectional area starts to occur. The material is said to undergo necking, as shown in Figure 2.6. Because the cross section is now reduced, a smaller force is needed to con￾tinue deformation until eventually the breaking strength, B, is reached (Figure 2.4). The slope in the elastic part of the stress–strain diagram (Fig￾ure 2.4) is defined to be the modulus of elasticity, E, (or Young’s modulus):   
E. (2.3) Equation (2.3) is generally referred to as Hooke’s Law. For shear stress,  [see above and Figure 2.3], Hooke’s law is appropriately written as:    
G, (2.4) 2 • Fundamental Mechanical Properties of Materials 15 FIGURE 2.5. Yield strengths of materials (given in meganewtons per square meter or megapascals; see Ap￾pendix II). 1Sometimes called ultimate tensile strength or ultimate tensile stress, UTS. Ultrapure fcc metals Polyurethane foam Wood
Nylon PVC Ice Concrete –Fe Al Au Pb Ni Cu Cast iron Epoxy Stainless steel Alkali halides W SiO2 Pressure￾vessel steel Al2O3 SiC Diamond 1 10 100 1,000 10,000 y[MNm–2] Polymers Ceramics Metals Composites