Time-Dependence of Mechanical Properties

All thermoplastics flow at room temperature under load stresses far below the yield point, cross-linked plastics at higher temperatures. This clearly demonstrates why it is not permissible to use tensile and breaking strength or the tensile, bending. 

The long-term behavior of plastics must, in other words, be investigated using static methods. The best-known method is the time-to-mpture or creep test, whereby a workpiece is subjected to stress CTq at time t = 0 and the stress parameter is maintained at a constant level for the entire duration of the test Time-dependent deformation is then measured. Fig. 26. If the elongation is constant, the stress parameter drops off in time with plastics. This is known as relaxation. This knowledge has applications related, for instance, to screw cmmections (plastic 

Creep curves or lines are determined in so-called time-tensile tests for extensive deformations, i.e., far beyond the range of linearity. Instead of maintaining a constant stress level, the simpler mode of load application is used, namely constant initial stress. Fig. 27. 

The many creep lines for the parameter stress can be recalculated for a specific test temperature to obtain the creep modulus EJJ), entered as a function of time. 

Theoretically permissible stress levels, for example, can be derived from such creep line diagrams for each material (assuming an elongation of 1 % as is frequently done for calculations). The actual stress levels that can be used for 

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On the other hand knowledge of these functions and of the spectra of relaxation (or retardation) times derived from them, is very helpful for obtaining insight into the molecular mechanisms by which they are originated. Analysis of the time dependency of mechanical properties thus provides a powerful tool to investigate the relations between structure and properties. 

Time Dependence of Mechanical Properties and Domain Formation of Linear and Crosslinked Segmented Polyurethanes... 

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