Creep Of High Density Polyethylene Research Articles

Creep of high density polyethylene filled with multiwall carbon nanotubes

The creep-recovery behaviour of two types of high density polyethylene (HDPE) filled with multiwall carbon nanotubes (MWCNT) is investigated. Nanocomposites with various contents of MWCNT were produced by using a commercially available masterbatch aimed to transfer the gained knowledge to an assessment of properties of industrial-scale products. Nanocomposites are characterized by the improved creep resistance compared to the neat polymers. Incorporation of 10 wt.% of MWCNT into the polymers resulted in a decrease of creep and residual strains for more than 3 and 5 times, respectively. The reinforcing effect of the nanofiller appeared also in a great increase of the elastic modulus (up to 100%) and ultimate strength (up to 60%) as well as a decrease of the coefficient of linear thermal expansion (down to 17%) of HDPE. Carbon nanotubes, being also good heat conductors, greatly contributed to the improvement of polyethylene’s thermal conductivity (up to 60%). Electrical percolation is determined below 2 wt.% of MWCNT. The electrical resistance changes monitored during creep-recovery tests are well correlated with the overall strain changes and residual strains in nanocomposites, that approve their in situ strain sensing capability during inelastic and long-term deformation.

Predicting Compressive Creep Behavior of Virgin HDPE Using Thermal Acceleration

The subject of this paper is the compressive creep behavior of viscoelastic materials, such as high-density polyethylene (HDPE), commonly used to manufacture a multitude of civil engineering products, including polymeric piling, decking, and fender elements. Accelerated methods to predict the tensile creep of polymers are already available. The time-temperature superposition (TTS) model is the basis of several available methods, and one of its derivatives, the stepped isothermal method (SIM), is the basis for an ASTM standard for tensile creep. In this paper, both TTS and SIM have been adapted to study the time- and temperature-dependent compressive creep of HDPE. Experimental test results on virgin HDPE indicate that both TTS and SIM are applicable for predicting compressive creep with some limitations. Preliminary results indicate that the tested virgin HDPE loaded in compression is expected to creep by approximately 2% in 100 years when loaded to an ultimate stress of 2.8 MPa (400 psi) at room temperat...

Comparison of Accelerated Compressive Creep Behavior of Virgin HDPE Using Thermal and Energy Approaches

This article compares two available approaches for accelerating the creep response of viscoelastic materials, such as High Density Polyethylene (HDPE), which is increasingly gaining attention for use in construction. Thermal acceleration methods to predict the tensile creep of polymers are already available. The Time-Temperature Superposition (TTS) phenomenon is the basis of several available methods, and an ASTM standard for tensile creep of geosynthetics is based on one of its derivatives, the Stepped Isothermal Method (SIM). In this article, both TTS and SIM have been adapted to study the compressive creep of virgin HDPE. An alternate approach, based on the equivalence of strain energy density (SED) between conventional constant-stress creep tests and strain-controlled stress-strain tests, is also adapted for accelerated compressive creep of HDPE. There is remarkably a good agreement among the creep behaviors obtained from conventional tests, TTS, SIM, and SED predictions for virgin HDPE.

Effect of strain rate on stepwise fatigue and creep slow crack growth in high density polyethylene

The effects of frequency and R-ratio (the ratio of minimum to maximum stress in the fatigue loading cycle) on the kinetics of step-wise crack propagation in fatigue and creep of high density polyethylene (HDPE) was characterized. Stepwise crack growth was observed over the entire range of frequency and R-ratio examined. A model relating crack growth rate to stress intensity factor parameters and applied strain rate was proposed by considering the total crack growth rate to consist of contributions from creep and fatigue loading components. The creep contribution in a fatigue test was calculated from the sinusoidal loading curve and the known dependence of creep crack growth on stress intensity factor in polyethylene. At a very low frequency of 0.01 Hz, fatigue crack growth rate was found to be completely controlled by creep processes. Comparison of the frequency and R-ratio tests revealed that the fatigue loading component depended on strain rate. Therefore, crack growth rate could be modeled with a creep contribution that depended only on the stress intensity factor parameters and a fatigue contribution that depended on strain rate.

Correlation of stepwise fatigue and creep slow crack growth in high density polyethylene

The kinetics and mechanism of slow crack growth in fatigue and creep of high density polyethylene were studied. The relationship between fatigue and creep was examined by varying the R-ratio (the minimum/maximum loads in the fatigue loading cycle) in the tensile mode such that loading ranged from mainly dynamic (R = 0.1) to static (R = 1.0, creep test). The stepwise crack propagation mechanism characteristic of long-term failures in polyethylene was observed for all loading conditions studied. Fatigue fracture kinetics allowed for extrapolation to the case of creep failure, which suggested that short-term fatigue testing can be used to predict long-term creep fracture properties. The size of the craze damage zone ahead of the arrested crack tip was controlled only by the mean stress, however the lifetime of the zone was determined by both the maximum stress and the mean stress. Crack growth rate was related to the maximum stress and the mean stress by a power law relationship, which described crack growth over the entire range of loading conditions studied.

Creep of high-density polyethylene in active and passive loading regimes

Creep of high-density polyethylene in active and passive loading regimes

An instability leading to failure of polyethylene in uniaxial creep

AbstractBased on a continuum model, a point of instability is predicted for the uniaxial creep of high density polyethylene. From dead load experiments it has been found that the instability occurs for linear polyethylene at around 10–12 percent strain, depending upon molecular weight and molecular weight distribution. It is shown that in the range of applied stresses for which the specimens neck during uniaxial creep, the time required to reach the critical point is related by a constant factor to the time at which the neck appears. A synopsis of theoretical considerations, as well as experimental work in support of this idea, is given.

Creep of crystalline polymers under conditions of a plane stress state with constant loading

1. The creep of high-density polyethylene is studied under conditions of simple and biaxial tensioning with constant loadings. 2. Within the limits of the assigned time frame of the test, isochronic creep curves of HDPE in different stress states may be represented, in first approximation, by a single curve in σi~ei* coordinates. 3. Theoretical creep curves of HDPE in a plane stress state are described using relationships based on nonlinear differential equations that take into account the presence of a discrete spectrum of relaxation time.