Variables - переменные

stiffness - жесткость

machining - обработка резаньем

lack - недостаток, отсутствие

N 11

MECHANICAL PROPERTIES FОR 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 measurе of plastic strength.

Notes:

tensile mechanical properties - механические свойства материалов на растяжение

plastic range - область прочности

plastic strength - пластическая деформация

ductility (toughness) - вязкость, ковкость

elastic strength - предел упругости

stiffness - жесткость

resilience - податливость

elastic range - упругая область, область упругого поведения

fraсture - разрыв

ultimate strength - предельное сопротивление

rupture, breaking, or fracture strength - предел прочности

ductile materials - ковкие материалы

brittle materials - ломкие материалы

N 12

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.

Notes:

materials-processing operations - процессы обработки материала

yielding - текучесть

drawing - вытяжка

unforeseen - непредвиденный

die casting - чугунное литье

N 13

COMPARISON OF THEORIES WITH ЕXРЕRIMENTAL RESULTS

(I)

In general, there is little experimental information on com­bined stress behaviour of materials. Some experiments have been conducted for biaxial tension-tension and tension-compression stress combinations which give good support to the distortion energy theory for predicting the yield strength. A limited amount of experimental data is available on yielding under tri­axial stresses. These results support the distortion energy theory but are restricted to the case where two of the princi­pal stresses are compressive and equal. Few experiments have been conducted to determine ultimate and fracture strengths of ductile and brittle materials under combined states of stress. For biaxial tension-tension, tension-compression, and certain triaxial stress combinations, the internal friction theory ap­pears to be in as good agreement with the test results as any of the theories. It is on the basis of the foregoing observa­tions that the distortion energy theory is recommended for pre­dicting biaxial yield strength while the more conservative in­ternal friction theory is recommended for predicting triaxial yield strengths and biaxial and triaxial ultimate and fracture strength.

A limited number of investigations. Including biaxial ten­sion-tension and biaxial tension-compression, indicate that the simple deformation theory is a good approximation for predict­ing plastic stress-strain relations under combined stresses. There are no sufficient results on triaxial states of stress to conclude which theory agrees best with the experimental re­sults.

N о t e s :

biaxial tension-tension and tension-compression - двуосное растяжение и двуосное растяжение-сжатие

distortion energy theory - теория искажения энергии

yield strength - предел текучести

axial tension - осевое растяжение

ultimate strength - предельное сопротивление

fracture strength - предел прочности

N 14

COMPARISON OF THEORIES WITH EXPERIMENTAL RESULTS

(II)

Perhaps the main reason why there is little experimental information available on properties of materials subjected to combined stresses is that special, complicated testing equip­ment is needed for the most of these tests. Another reason is that for certain stress combinations a suitable method of stress­ing the specimens has not been devised. Some of the earlier in­vestigations were unsatisfactory since they were made on speci­mens in which the state of stress throughout the specimen was not uniform. One of these was the solid round specimen subject­ed to torsion and axial tension. Although this loading pro­duces a combined state of stress, the torsional shear stress varies from the inside to the outside of the specimen. This means that the lower stressed inner fibers introduce a streng­thening effect on the outer fibers, thereby increasing the re­sistance to yielding of these fibers. Other specimens in which a strengthening effect influences the results include solid round specimens subjected to torsion and bending, thick-walled cylinders subjected to axial tension and internal pressure, and notched specimens subjected to tension and bending.

Notes :

torsional shear stress - касательное напряжение при кручении

lower stressed inner fibers - наиболее напряженные окна

strengthening effect – эффект упрочнения

notched specimens - образцы с надрезом

N 15

FATIGUE ROPERTIES (I)

Machine and structural members are often subjected to loads and stresses that do not remain constant but vary with time. Fоr example, the aerodynamic loads on an aircraft do not remain fixed in value, and the stresses produced by these loads fluc­tuate in value. Stresses that vary with time are called fluc­tuating, alternating, or fatigue stresses. If the loads are suddenly applied at a high rate of speed, the loads are called impact loads.

The study of fatigue properties of materials was started about 100 years ago, primarily as a result of fatigue failures in railroad equipment. Today the development of modern high-­speed transportation and power equipment of various types has increased the importance of the fatigue properties of engineer­ing materials. The nature of fatigue failure is now reasonably well understood, but the complexity of the problem is such that rational methods of design for fatigue are difficult to develop. Some of the reasons for this difficulty are that fatigue strengths of parts are affected not only by the material but also by de­sign features, fabrication methods, and service conditions. In addition, fatigue strengths of materials are influenced by small cracks and other flaws in the material and, for this rea­son, considerable variation in test results occurs. The great variation in properties makes it necessary to apply statistical procedures in the evaluation of fatigue strength.

Notes:

fatigue - усталость