2.Fundamental Mechanical Properties of Materials 13 FIGURE 2.1.Schematic representation of a bend test.Note that the convex surface is under tension and the concave surface is under compression.Both stresses are essen- tially parallel to the surface.The bend test is particularly used for brittle materials. tial unit area,Ao,is mostly used;see below.If the force is ap- plied parallel to the axis of a rod-shaped material,as in the ten- sile tester (that is,perpendicular to the faces Ao),then o is called a tensile stress.If the stress is applied parallel to the faces (as in Figure 2.3),it is termed shear stress,7. Many materials respond to stress by changing their dimen- sions.Under tensile stress,the rod becomes longer in the direc- tion of the applied force (and eventually narrower perpendicular to that axis).The change in longitudinal dimension in response to stress is called strain,e,that is: e=1-b-4 (2.2) where lo is the initial length of the rod and l is its final length. The absolute value of the ratio between the lateral strain (shrinkage)and the longitudinal strain (elongation)is called the Poisson ratio,v.Its maximum value is 0.5 (no net volume change).In reality,the Poisson ratio for metals and alloys is gen- erally between 0.27 and 0.35;in plastics (e.g.,nylon)it may be as large as 0.4;and for rubbers it is even 0.49,which is near the maximum possible value. Sample FIGURE 2.2.Schematic repre- 0 sentation of a tensile test equipment.The lower cross-bar is made to move downward and thus ex- tends a force,F,on the test piece whose cross-sectional area is Ao.The specimen to be tested is either threaded into the specimen holders or held by a vice grip.tial unit area, A0, is mostly used; see below. If the force is ap￾plied parallel to the axis of a rod-shaped material, as in the ten￾sile tester (that is, perpendicular to the faces A0), then is called a tensile stress. If the stress is applied parallel to the faces (as in Figure 2.3), it is termed shear stress, . Many materials respond to stress by changing their dimen￾sions. Under tensile stress, the rod becomes longer in the direc￾tion of the applied force (and eventually narrower perpendicular to that axis). The change in longitudinal dimension in response to stress is called strain, , that is: 
l  l0 l0
 l0 l , (2.2) where l0 is the initial length of the rod and l is its final length. The absolute value of the ratio between the lateral strain (shrinkage) and the longitudinal strain (elongation) is called the Poisson ratio, . Its maximum value is 0.5 (no net volume change). In reality, the Poisson ratio for metals and alloys is gen￾erally between 0.27 and 0.35; in plastics (e.g., nylon) it may be as large as 0.4; and for rubbers it is even 0.49, which is near the maximum possible value. 2 • Fundamental Mechanical Properties of Materials 13 10kg FIGURE 2.1. Schematic representation of a bend test. Note that the convex surface is under tension and the concave surface is under compression. Both stresses are essen￾tially parallel to the surface. The bend test is particularly used for brittle materials. Sample F A0 FIGURE 2.2. Schematic repre￾sentation of a tensile test equipment. The lower cross-bar is made to move downward and thus ex￾tends a force, F, on the test piece whose cross-sectional area is A0. The specimen to be tested is either threaded into the specimen holders or held by a vice grip.