Khả
năng hấp thu năng lượng cơ học là một đặc tính quan trọng của vật liệu đàn hồi,
đặc biệt trong các ứng dụng giảm xóc. Tài liệu này giới thiệu các đặc trưng hấp
thu năng lượng cơ học của vật liệu đàn hồi nói chung và PU nói riêng.
…
As
a generalization urethane elastomers have greater energy absorption properties
than other equivalent rubbers and plastics. It is useful to examine the
mechanisms of energy absorption in elastomers. These are usually quantified by
measurement of resilience, hysteresis energy and damping properties, which are
defined in the usual BS and ASTM or ISO standards. When a stress is applied to
an elastomeric material there is a small but positive time lag before the material
takes up the corresponding strain. This time lag is caused by the need for the
intermolecular attractions to be overcome by the vibrational energy of the
atoms. The practical result of this time lag in applications involving cyclic
deformation (or dynamic applications) is that the stress-strain curve in
recovery does not follow the same path as when the stress was applied and there
is consequently a loss of energy, or hysteresis, which is converted into heat.
This loss of energy can be measured, for example, by rebound resilience. Since
the vibrational energy of the atoms increases as the temperature increases,
then this time lag decreases as the temperature increases. Thus the rebound
resilience for polyurethanes, and other elastomeric materials, increases with
increasing temperature. Under dynamic stress there will be heat build-up which
in turn will improve the resilience property and result in a lower rate of heat
build-up.
This
energy loss and consequent heat build-up occurs in all elastomers to some
degree or other, and since these are poor conductors of heat the temperature
can rise sharply. This factor can lead to limitations of use under rapid
cycling or in severe dynamic applications and it is necessary to calculate or
measure the heat produced under these conditions when designing a specific
product. One advantage in the use of polyurethanes is that, due to their high
modulus compared with other elastomers, thinner sections can be employed. This
helps to inhibit the heat build-up by assisting in heat dissipation. For
continuous dynamic conditions a within material temperature of 80oC
may be considered as a maximum working limit for most polyurethanes.
…
Trích
đăng từ sách Polyurethane Elastomers, C. Hepburn, Elsevier Science Publisher,
1992, trang 372 – 373
(vtp-vlab-caosuviet)
