NONPROPORTIONAL LOADING OF NYLON-66 AT ROOM-TEMPERATURE

Tubular specimens of commercial Nylon 66 were subjected to non-proport ional loading in strain control at room temperature. All tests were pe rformed in a servohydraulic, computer-controlled, MTS axial-torsion me chanical testing machine using a biaxial clip-on extensometer. A point in axial-torsion strain space was reached by a square path consisting of separate axial and torsion segments. One specimen was first loaded to a certain axial strain which was subsequently held constant while the tube was twisted. For the other specimen torsional strain was appl ied first followed by axial loading. The stresses at the same strain p oint were found to be path dependent. The stress corresponding to the strain that is kept constant drops as the other axis is being loaded. There is considerable interaction between axial and shear behavior. Th e stress drop can either be a ''plasticity effect'' or regular relaxat ion. Separate tests show their nonlinear interaction. Various constitu tive models are examined regarding their capabilities of modeling the observed behavior in a qualitative way. Viscoelastic ''clock'' models based only on the first invariant of strain are not capable of reprodu cing the observed effect of torsion on axial loading. Even if other in variants are included, the long term behavior predicted by ''clock'' m odels is shown to be independent of the loading of the other axis and linear in the strain reached during first loading. This prediction app ears to be too restrictive in light of the present experiments. The ex periments suggest an explicit dependence on the invariants in a consti tutive equation. (C) 1998 Elsevier Science Ltd. All rights reserved.