Material is selected for various applications in a reactor facility based on its physical and chemical properties. This chapter discusses the physical properties of material. Appendix A contains a discussion on the compatibility of tritium with various materials.

EO 1.14           DEFINE the following terms:

a.         Strength                                    d. Ductility

b.         Ultimate tensile strengthe.         Malleability

c.         Yield strength               f. Toughness

g.         Hardness

EO 1.15           IDENTIFY how slip effects the strength of a metal.

EO 1.16           DESCRIBE the effects on ductility caused by:

a.         Temperature changes    b. Irradiation

c.         Cold working

EO 1.17           IDENTIFY the reactor plant application for which high ductility is desirable.

Strength

Strength is the ability of a material to resist deformation. The strength of a component is usually considered based on the maximum load that can be borne before failure is apparent. If under simple tension the permanent deformation (plastic strain) that takes place in a component before failure, the load-carrying capacity, at the instant of final rupture, will probably be less than the maximum load supported at a lower strain because the load is being applied over a significantly smaller cross-sectional area. Under simple compression, the load at fracture will be the maximum applicable over a significantly enlarged area compared with the cross-sectional area under no load.

This obscurity can be overcome by utilizing a nominal stress figure for tension and shear. This is found by dividing the relevant maximum load by the original area of cross section of the component. Thus, the strength of a material is the maximum nominal stress it can sustain. The nominal stress is referred to in quoting the "strength" of a material and is always qualified by the type of stress, such as tensile strength, compressive strength, or shear strength.

For most structural materials, the difficulty in finding compressive strength can be overcome by substituting the tensile strength value for compressive strength. This substitution is a safe assumption since the nominal compression strength is always greater than the nominal tensile strength because the effective cross section increases in compression and decreases in tension.

When a force is applied to a metal, layers of atoms within the crystal structure move in relation to adjacent layers of atoms. This process is referred to as slip. Grain boundaries tend to prevent slip. The smaller the grain size, the larger the grain boundary area. Decreasing the grain size through cold or hot working of the metal tends to retard slip and thus increases the strength of the metal. Cold and hot working are discussed in the next chapter.

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