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Tcnsllolue»'is^S!hT«ypo ot stre* in which thc two sectlons of motcriol on <H.her slde of a stress piane tend to puli apart or elongate as lllustratcd In Figuro l(a).

Compro^wsTis^rLerso ot tensile stross. Ad|accnt parts of thc materiał tend to press agoinst each other through a typlcal stress piane as lllustratcd in Figurę l(b).

Shcar stress exists when two parts of a materiał tend to slide across each other m any typ.cal piane of shear upon application of force parallel to that piane as lllustratcd in Figurę l(c).

Strain. Strain, which in materials science is also callod deformation, is a change in the shape or size of an object duo to an applied force (the deformation energy in this case is transferred through work) or a change in temperaturę (the deformation energy in this case is transferred through heat). The first case can be a result of tensile (pulling) forces, compressive (pushing) forces, shear, bending or torsion (twisting). In the second case, the most significant factor, which is determined by the temperaturę, is the mobility of thc structural defeets such as grain boundaries, point vacancies, linę and screw dislocations, stacking faults and twins in both crystalline and non-crystalline solids. The movement or displacement of such mobile defeets is thermally acttvated, and thus limited by the ratę of atomie diffusion. As deformation occurs, interna! inter-moleeular forces arise that oppose the applied force. If the applied force is not too large these forces may be sufficient to completely reslst the applied force, allowing the object to assume a new equlllbrium State and to return to its original State when the load is removed. A larger applied force may lead to a permanent deformation of the object or even to its structural failure.

Now, sińce both stress and strain werc briefly presented I will go on to the subject of reiations between them and Hooke's law.

The relationship between the stress and strain that a particular materiał displays is known as that materiał^ Stress-Strain curve. It is unique for each materiał and is found by recording the amount of deformation (strain) at distinct intervals of tensile or compre$sive loading (stress). These curves reveal many of the properties of a materiał (Including data to establish the Modulus of Elasticity, E). Stress-strain curves of various materials vary widely, and different tensile tests conducted on the same materiał yield different results, depending upon the temperaturę of the specimen and the speed of the loading. It is possible, however, to distinguish some common characteristics among the stress-strain curves of various groups of materials and, on this basis, to divide materials into two broad categories; namely, the ductile materials and the brittle materials.

Ductile materials, which comprise structural Steel, as well as many alloys of other metals, are characterized by their ability to yield at normal temperatures.

Brittle materials, which compromise cast iron, glass, and stone, are characterized by the fact that rupture occurs without any noticeable prior change in the ratę of ełongatłon.

The typical stress and strain reiations diagram with the yarious stages of deformation is presented on the next page.