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MECHANICAL PROPERTIES FOR THE ELASTIC RANGE

(I)

The tensile mechanical properties of materials for the elas­tic range are those which represent ability to resist loads and deformations and capacity to absorb energy in the elastic range.

Elastic strength. A material has a high elastic strength if it resists loads without yielding and without being permanently deformed. Elastic strength is measured by the stress which re­presents the transition from the elastic range (where the strains are small and recoverable) to the plastic range (where the strains become large aid are partly nonrecoverable). The initial part of the plastic range is designated as the region where yielding of the material takes place.

The proportional limit is the elastic strength and is defined as the stress value beyond which the stress is no longer propor­tional to the strain. An exact determination of the stress cor­responding to the elastic limit is difficult because the change from the curve to the straight line is very gradual and the exact location of the point of transition is not definite. Another elastic strength value is the elastic limit which is defined as the maximum stress that can be applied to a material without producing a permanent plastic deformation when the load is re­moved. The proportional elastic limits are called ideal strength values. These strength values are seldom found because of the difficulties involved in their evaluation and the inaccuracies in the values determined. Fîr these reasons, elastic strength is designated by one of the following so called plastical elas­tic strength values. In all cases some geometrical construction is used to define the transition from the straight-line elas­tic to the curved plastic part of the stress-strain diagram.

 

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MECHANICAL PROPERTIES FOR THE ELASTIC RANGE

(II)

A lower yield point exists for some materials because the movement of the end of the specimen, as produced by the testing machine, does not proceed as rapidly as the plastic deformation of the specimen. This condition results in a decrease in the load and hence a lowering of the stress from the upper to the lower yield point. The foregoing explanation is confirmed by the fact that in a rapidly applied impact test of the same material, a lower yield point does not exist. For materials which exhibit both an upper and lower yield point, the lower yield point is used to define the elastic strength since its magnitude is less influenced by test variables than is the upper yield point.

Stiffness. A material has a high stiffness value when its deformation in the elastic range is relatively small. The pro­perty of stiffness is very important in certain designs where the deformations must be kept small. For example, in some ma­chine tools a slight deflection resulting from lack of stiff­ness could result in inaccurate machining.

 

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MECHANICAL PROPERTIES FÎR THE PLASTIC RANGE (I)

The tensile plastic properties of a material are the proper­ties that define the ability to resist loads and deformations and the capacity to absorb energy in the plastic range. These mechanical properties are called plastic strength, ductility, and toughness, and correspond respectively to elastic strength, stiffness and resilience in the elastic range.



Plastic strength. A material has a high plastic strength if it resists loads without fracture. The plastic range may be de­signated by either the ultimate strength, corresponding to the ultimate stress, or the rupture, breaking, or fracture strength, corresponding to the rupture, breaking or fracture stress. The ultimate stress equals the maximum tensile load divided by the original cross-section area of the specimen, and the rupture, breaking or fracture stress equals the rupture, breaking, or fracture load divided by the original cross-sectional area of the specimen. Since the rupture load is difficult to determine accurately it is not always determined, and the ultimate stress is usually selected as the measure of plastic strength. For some ductile materials the ultimate stress and the fracture stress coincide. For brittle materials the maximum and fracture loads are identical, and the ultimate stress is used as the measurå of plastic strength.

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MECHANICAL PROPERTIES FOR THE PLASTIC RANGE (II)

Ductility. The property of a material that represents its ability to deform in the plastic range is called ductility. Duc­tility is an important property of materials since it repre­sents an insurance factor against excessive loads that may not have been considered in a design. In other words, a material with a high ductility will allow considerable deformation to occur before fracture takes place. This allowance is important in situations where unforeseen loads, exceeding the yield loads, are encountered. If ductility is small, an unforeseen overload may cause fracture. For example, in structural steel riveted joints of bridges and buildings, overloads may produce yielding of the structural members in the vicinity of the rivet holes. Since structural steel has a high ductility, fracture does not occur and the deformation of the material results in a redis­tribution and a reduction of the stress at the points of stress concentration.

Ductility is also an important property in various material-processing operations such as drawing, rolling, forging and die casting. If the ductility is not adequate, the large deforma­tions produced in these various materials-processing operations result in fracture of the material. For example, a sheet of me­tal can be bent to a desired curvature only if the material is ductile and will not crack during bending.

Toughness. A material has a high toughness if it can absorb high values of strain energy in the plastic range. Toughness is sometimes measured by the modulus of toughness or the amount of energy absorbed per unit volume in stressing to fracture.

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Date: 2015-01-02; view: 1167


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