Amorphous compressive yield strain

Figures 13.16 and 13.17 are plots of the compressive stress-strain data for two amorphous and two crystalline polymers, respectively, while Figure 13.18 shows tensile and compressive stress-strain behavior of a normally brittle polymer (polystyrene). The stress-strain curves for the amorphous polymers are characteristic of the yield behavior of polymers. On the other hand, there are no clearly defined yield points for the crystalline polymers. In tension, polystyrene exhibited brittle failure, whereas in compression it behaved as a ductile polymer. The behavior of polystyrene typifies the general behavior of polymers. Tensile and compressive tests do not, as would normally be expected, give the same results. Strength and yield stress are generally higher in compression than in tension.

Richeton, J., Ahzi, S., Vecchio, K. S., Jiang, F. C., and Adharapurapu, R. R. (2006) Influence of temperature and strain rate on the mechanical behavior of three amorphous polymers characterization and modeling of the compressive yield stress, Int. J. Solids Structures, 43, 2318-2335. 

Kim (82) estimated PED from compressive experiments on molded disks of a number of materials, as shown in Fig. 5.17. High modulus, yielding, amorphous polymers such as PS dissipate a large amount of mechanical energy, compared to lower modulus, polycrystalline polymers, as shown again in Fig. 5.17. Iso-PED and corresponding iso-ATadiab contours can be obtained from a number of cylindrical specimens compressed to various strains at various initial temperatures, as shown in Fig. 5.18(a) and 5.18(b). From such plots, the expected ATadiab from one or more successive E deformations can be obtained, as shown in Fig. 5.19, for PS compressed to successive eo = 1 deformations. 

The structure of compression molded samples is more uniform than the injection molded bars, so the cavitation occurs across the gauge of the sample. The presence of cavitation in those samples depends on the strain rate. If the deformation is slow, then the crystalline elements are able to deform plastically before reaching the cavitation threshold of the amorphous phase. When the strain rate is higher, the yield stress, related to crystal strength, is higher (38-38.5 MPa) and cavitation occurs first. 

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