Dynamic stress

Knowledge in such behavioral responses of plastics as those ranging from short time static (tensile, flexural, etc.) to long time dynamic (creep, fatigue, impact, etc.) mechanical load performances in different environments are important to product designs subjected to those loads. Dynamic loading in the present context is taken to include deformation rates above those achieved on the standard laboratory-testing machine (commonly designated as static or quasi-static). These slower tests may encounter minimal time-dependent effects, such as creep and stress-relaxation, and therefore are in a sense dynamic. Thus the terms static and dynamic can be overlapping.

Long time dynamic load involves behaviors such as creep, fatigue, and impact. Two of the most important types of long-term material behavior are more specifically viscoelastic creep and stress relaxation. Whereas stress-strain behavior usually occurs in less than one or two hours, creep and stress relaxation may continue over the entire life of the structure such as 100,000 hours or more.

In many applications, intermittent or dynamic loads arise over much shorter time scales. Examples of such products include chair seats, panels that vibrate and transmit noise, engine mounts and other antivibration products, and road surface-induced loads carried to wheels and suspension systems. Plastics' relevant properties in this regard are material stiffness and internal damping, the latter of which can often be used to advantage in design. Both properties depend on the frequency of the applied loads or vibrations, a dependence that must be allowed for in the design analysis. The possibility of fatigue damage and failure must also be considered.

Mechanical loads on a structure induce stresses within the material such as those shown in Fig. 2.10. The magnitudes of these stresses depends on many factors, including forces, angle of loads, rate and point of application of each load, geometry of the structure, manner in which that structure is supported, and time at temperature. The behavior of the material in response to these induced stresses determines the performance of the structure.

Static and dynamic loads (courtesy of Plastics FALLO)

Static and dynamic loads (courtesy of Plastics FALLO)

Table 2.2 on p. 90 provides examples of reinforced thermoplastics flexural data.

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