Evaluation of Time Dependent Behavior of a Wood Flour/High Density Polyethylene Composite

Abstract

Short-term flexural creep and stress relaxation tests were conducted on a wood plastic composite containing 30% high density polyethylene (HDPE), 67% fir wood flour, 2% compatibilizer (MAPE), and 1% lubricant. In creep tests, applied stress levels ranged from 30 to 60% of measured flexural strength. The principle of time-stress superposition was applied to form a master curve extending for a maximum of 4 years. The horizontal shift factors conformed to an Arrhenius type equation. Stress relaxation tests were also carried out at strain levels ranging from 30 to 60% of the ultimate strain. The principle of time-strain superposition was applied to form a stress relaxation master curve that extended for 67 days. The horizontal shift factors also conformed to an Arrhenius type equation. The resulting master curves were compared with extrapolated creep and stress relaxation models. To determine whether time-stress superposition is valid for the studied composite material, creep shift factors were applied to stress relaxation data and vice versa. In both creep and stress relaxation tests, it was found that the application of superposition was verified. The results indicated that the studied composite material was rheologically simple, and a single horizontal shifting on time axis was adequate to predict the long term performance of the material.

Mechanical and time-dependent behavior of wood–plastic composites subjected to tension and compression

“…13–15 Creep testing has also been performed predominately in flexure for relatively short durations, usually for the purpose of comparing manufacturing parameters. 16–19 A few studies have conducted longer flexural creep tests up to 90 days. 9,10 Flexure response, however, is mechanically complex due to both the material’s bimodal behavior and its nonlinear constitutive response.…”

Stress-relaxation behavior of lignocellulosic high-density polyethylene composites

“…Therefore, principles such as Boltzmann and time-temperature superposition are invoked to evaluate the long-term viscoelastic behavior of materials by utilizing shortterm data and constructing master curves. 8 According to time-temperature superposition principle, timestrain superposition and time-stress superposition were also employed previously by Dastoorian et al, 9 Sreekala et al, 3 and George et al 6 The objective of this study is to assess flexural stressrelaxation behavior of polyethylene-based composites, consisting of different types and contents of natural fibers, at various strain levels of their ultimate strain. Additionally, by horizontal shifting method, curves at different strain levels were superposed through which a master curve was generated in order to describe the long-time decay of stress in terms of strain fractions.…”

Application of time–temperature superposition principle to Chinese fir orthotropic creep

“…The limit of linearity was determined using a stressstrain sweep, for which the L, R, and T specimens were deformed with increasing stress, respectively. Generally, wood behaved linearly up to 50% of the ultimate stress [2,19]. Dried specimens were heated to the maximum experimental temperature 150°C at 10°C/min and held there for 10 min before the stress-strain sweep.…”