Материаловедение: свойства металлов. Матросова Т.А - 17 стр.

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соотнесения результата испытания с требованиями к конструкции? 8.
Существует ли общий подход к оценке результатов испытаний? 9. В чем
состоит особое значение испытания на ударную прочность? 10. Приведите
пример хрупкого разрушения в практике эксплуатации конструкций из
низкоуглеродистой стали. 11. Где локализуются концентраторы напряжений в
испытуемых образцах? 12. Дайте определение коэффициента концентрации
напряжений. 13. Укажите способы определения коэффициента
концентрации
напряжений. 14. Опишите соотношение коэффициентов концентрации
напряжений для кручения, растяжения и изгиба.
Topic 6. Fatigue Properties and Endurance Limit
Although yield strength is a suitable criterion for designing components that
are to be subjected to static loads, for cyclic loading the behavior of a material must
be evaluated under dynamic conditions. The fatigue strength or endurance limit of
a material should be used for the design of parts subjected to repeated alternating
stresses over an extended period of time. As would be expected, the strength of a
material under cyclic loading is considerably less than it would be under a static load
(Fig. 6.1). The plot of stress as a function of the number of cycles to failure is
commonly called an S-N curve. It is interesting to note that for specimens of SAE*
1047 steel there is a stress called the endurance or fatigue limit below which the
material has an infinite life (>10
8
cycles), i.e., the steel would not fail regardless of
exposure time (Fig. 6.2). In contrast, the 2014-T6 aluminum alloy has no limiting
stress value.
Fatigue data are inherently more variable than tensile test data. In part the
scatter is caused by variation in surface finish and environment. Polished
specimens of the same material give significantly better life than machined or scaly
surfaces. Since most fatigue failures initiate at surface notches, fatigue behavior and
notch sensitivity are closely related. Thus mechanical or other treatments that
improve the integrity of the surface, or add residual compressive stresses to it,
improve the endurance limit of the specimen.
Overstressing above the fatigue limit for cycles fewer than necessary to
produce failure at that stress reduces the fatigue limit. Also, understressing below
the fatigue limit may increase the fatigue limit. Thus, cold working and shot peening
usually improve fatigue properties.
Several standard types of fatigue-testing machines are commercially
available. The results of extensive fatigue-life testing programs are now available in
the form of S-N curves that are invaluable for comparing the performance of a
material that is expected to be subjected to dynamic loads.
___________________________
* - Society of American Engineers

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