tailieunhanh - Material Science_ Vol 1 of 2 - US DOE (1993) Episode 7

Tham khảo tài liệu 'material science_ vol 1 of 2 - us doe (1993) episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Properties of Metals DOE-HDBK-1017 1-93 STRAIN Figure 2 Change of Shape of Cylinder Under Stress When metal experiences strain its volume remains constant. Therefore if volume remains constant as the dimension changes on one axis then the dimensions of at least one other axis must change also. If one dimension increases another must decrease. There are a few exceptions. For example s train hardening involves the absorption of strain energy in the material structure which results in an increase in one dimension without an offsetting decrease in other dimensions. This causes the density of the material to decrease and the volume to increase. If a tensile load is applied to a material the material will elongate on the axis of the load perpendicular to the tensile stress plane as illustrated in Figure 2 a . Conversely if the load is compressive the axial dimension will decrease as illustrated in Figure 2 b . If volume is constant a corresponding lateral contraction or expansion must occur. This lateral change will bear a fixed relationship to the axial strain. The relationship or ratio of lateral to axial strain is called Poisson s ratio after the name of its discoverer. It is usually symbolized by V. Deformation of Cubic Structures Whether or not a material can deform plastically at low applied stresses depends on its lattice structure. It is easier for planes of atoms to slide by each other if those planes are closely packed. Therefore lattice structures with closely packed planes allow more plastic deformation than those that are not closely packed. Also cubic lattice structures allow slippage to occur more easily than non-cubic lattices. This is because of their symmetry which provides closely packed planes in several directions. Most metals are made of the body-centered cubic BCC face-centered cubic FCC or hexagonal close-packed HCP crystals discussed in more detail in the Module 1 Structure of Metals. A face-centered cubic crystal structure will deform more .