Macro-mechanical Creep Response of Polyethylene

In viscoelasticity, the constitutive relationships are time dependent. Creep, time-relaxation, and loading rate effects are typical time-dependent responses. Creep tests are convenient for material testing and characterization. During a creep test, strain grows with time under a constant stress. The material deformation history can be described by a compliance (strain divided by stress)-time curve.

Creep tests on two high-density polyethylene (HDPE) materials were done in a previous study, and nonlinear viscoelastic models were generated for them (Liu & Polak, 2005). The models perform well in simulating creep behaviour at stresses below 10 MPa, where strains do not show drastic changes within the test durations (24 hours).

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Figure 5. Material creep test specimen (units in mm).

In this study, further tests are presented for a medium-density polyethylene (MDPE) material. Tensile test specimens are cut off from a MDPE pipe, as shown in Figure 5. Creep test results for the MDPE samples are shown in Figure 6. During the test, load is applied by dead weights through a lever arm. Due to the small deformations, the (engineering) stress is assumed to be constant. A clip-on strain gauge with a data acquisition system is used to record the deformation history of the specimens. Displacements between the knife edges of the strain gauge are recorded, and the displacement-time history is converted to engineering strain-time history by dividing each displacement by the initial knife edges distance. The tests are carried out at room temperature (~22°C) for 24 hours. Figure 6(a) shows the creep strain-time curves from the tests on the MDPE pipe materials. The compliance curves at different stresses indicate strong viscoelastic nonlinearity, as shown in Figure 6(b).