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 ElastomersC. Hepburn, Elsevier Science Publisher, 1992, trang 372 – 373


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