Материаловедение: свойства металлов. Матросова Т.А - 14 стр.

UptoLike

ВУЗ: 

УлГТУ | Ульяновск

Составители: 

Рубрика: 

14
Questions:
1. С объектами какого размера имеет дело квантовая механика? 2. Что
помогает понять изучение поведения молекулярных частиц с точки зрения
квантовой механики? 3. Каковы два основных понятия лежат в основе
квантовой механики? 4. В чем суть квантовой гипотезы? 5. Могут ли
энергетические уровни электронов быть произвольными? 6. Справедлив ли для
материи принцип минимума энергии? 7. Каковы
способы накачки энергии? 8.
Как происходит спонтанное излучение? 9. Как соотносится количество
электронов и протонов в атоме. 10. Какие свойства материалов можно понять,
основываясь на поведении электронов?
Topic 5. Impact Properties
In some designs, dynamic forces are likely to cause failure. For example an
alloy may be hard and have high compressive strength and yet be unable to
withstand a sharp blow. In particular, low-carbon steels are susceptible to brittle
failure at certain temperatures. Most impact tests use a calibrated hammer to strike a
notched or unnotched test specimen. In the former, the test result is strongly
dependent on the base of the notch, where there is a large concentration of triaxial
stresses that produce a fracture with little plastic flow. The impact test is particularly
sensitive to internal stress producers such as inclusions, flake graphite, second
phases, and internal cracks.
The results from an impact test are not easily expressed in terms of design
requirements because it is not possible to determine the triaxial stress conditions at
the notch. There also seems to be no general agreement of the interpretation or
significance of the result. Nonetheless the impact test has proved especially useful in
defining the temperature at which steel changes from brittle to ductile behavior.
Low-carbon steels are particularly susceptible to brittle failure in a cold environment
such as the North Atlantic. There were cases of Liberty ships of World War II
vintage splitting in two as a result of brittle behavior when traveling in heavy seas
during the winter.
In a particular design having a notch or any abrupt change in cross section,
the maximum stress occurs at this location and may exceed the stress computed by
typical formulas based upon simplified assumptions in connection with stress
distribution. The ratio of this maximum stress to the nominal stress is known as a
stress concentration factor, usually denoted
by K. Stress concentration factors may
be determined experimentally or by calculations based on the theory of elasticity.
Figure 5.1 illustrates stress concentration factors K for fillets of various radius
divided by the thickness of castings subjected to torsion, tension, and bending
stresses.

Страницы