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).
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).