D4 Mechanical Tests (Plastic Creep Tensile)

Topic

The tensile test—the experimental method described by this model—subjects the polymer to an increasing load and records the resulting stress-strain curve. Unlike the curve of a brittle material lacking plastic yield, this curve does not fracture immediately after leaving the initial elastic region; instead, it first reaches a yield point—the point where the polymer ceases to deform elastically and begins to deform permanently, without the recorded stress continuing to rise at the same rate as it did prior to that point. The fact that the polymer withstands higher stress before reaching this yield point means that its elastic region extends further along the stress-strain curve.

Once the polymer passes this yield point, the permanent deformation is no longer distributed uniformly along the test specimen; instead, it concentrates in a single region known as necking—a localized thinning of the polymer's cross-section. The onset of this phenomenon is predicted by the Considère criterion, which marks the point where the polymer's capacity for strain hardening (strengthening under increasing deformation) can no longer compensate for the reduction in cross-sectional area caused by that same deformation. Because the polymer undergoes less strain hardening under increasing deformation, the Considère criterion is met sooner, and necking consequently begins at a lower level of deformation. Consequently, in this model—characterized by an initial yield point and necking—the necking phenomenon does not immediately lead to fracture (as it would in a brittle material); instead, as the polymer's molecular chains progressively align with the direction of stretching within the necked region, orientation hardening occurs. The more these molecular chains align with the stretching direction, the greater the polymer's resistance to further deformation in that region—to the point where necking ceases to concentrate there and instead propagates stably across the remainder of the specimen. The fact that this polymer's stress-strain curve—featuring a yield point, necking, and orientation hardening—contrasts so sharply with the brittle fracture curves of ceramic and glassy materials (which lack any comparable plastic yielding phase) reveals that this polymer exhibits mechanical behavior fundamentally different from that of those brittle materials.

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