Necmi Dusunceli

The Effects of Loading History and Manufacturing Methods on the Mechanical Behavior of High-Density Polyethylene

This article describes a series of experiments conducted to determine the effects of loading history and manufacturing techniques on mechanical behavior of high- density polyethylene (HDPE). The main reason for undertaking the research was to investigate multiple creep, multiple relaxation, and cyclic loading on uniaxial tension. The samples used for tensile tests were obtained from extruded pipe and compression-molded sheets. The stress—strain responses of both samples under uniaxial tensile were found to be independent of the loading history. It was observed that the compression-molded specimens exhibit greater deformation ratio than the extruded specimen. Understanding the deformation behavior under different loading can offer the designer of high-density polyethylene products reliable data relevant to practical applications.

The unusual creep and relaxation behaviour of polypropylene

Abstract This paper focuses on experimental investigation and modelling of the unusual creep and relaxation behaviour of polypropylene. A polypropylene specimen was loaded up to three different levels of maximum strain (ɛmax), unloaded to various minimum stresses (σmin), creep and relaxation tests were performed at seven different strain levels. In addition to investigate the effect of loading history on unloading paths, multiple creep experiments were conducted at various stress levels. It was observed that unusual creep and relaxation behaviour of polypropylene depended on a strain increment (Δɛ) which is the difference between the minimum (ɛmin) and maximum strain level (ɛmax). Increasing the strain increment affects the creep and relaxation behaviour on loading path. As a result, the creep and relaxation behaviours of polypropylene show non-monotonic characteristics. The viscoplasticity theory based on overstress for polymer (VBOP) model was used to model the unusual creep and relaxation behaviour of polypropylene. From the comparison between the VBOP model predictions and the experimental results, it can be seen that the VBOP model can model both the unusual creep and relaxation behaviours of a semicrystalline polymer.

Universal mechanical response of polypropylene under cyclic deformation

Abstract Experimental data are reported on isotactic polypropylene in uniaxial tensile (i) cyclic tests (oscillations between maximum strain 0.15 and the zero minimum stress) with various cross-head speeds and (ii) relaxation tests at temperatures ranging from room temperature to 100°C. Observations in cyclic tests show that maximum stresses decrease, while minimum strains grow with the number of cycles. A universal character is revealed of the dependence of minimum strain on the number of cycles: this function is independent of temperature and weakly affected by strain rate. To rationalize this observation, constitutive equations are proposed in cyclic viscoelasticity and viscoplasticity of semicrystalline polymers and a scenario is suggested for damage accumulation. Two types of damage are distinguished which are induced by (i) lamellar fragmentation in the crystalline phase and (ii) nucleation and growth of micro-voids in the amorphous matrix. The stress–strain relations involve five adjustable parameters that are found by fitting observations (50 cycles of loading–retraction) at room temperature and α-transition point. Numerical simulation demonstrates that the model correctly describes experimental data and can predict observations in tests with a large number of cycles.

Cyclic Behavior of High Density Polyethylene (HDPE)

This article presents the mechanical behavior of high density polyethylene (HDPE). Samples were prepared by extracting extruded HDPE pipe. Cyclic and strain rate jump behavior of HDPE were studied under uniaxial tensile loading conditions. The strain jump tests indicated that mechanical behavior of HDPE has deformation memory. Further, it was found that increasing cycle number on cyclic loading test increased strain accumulation amount and HDPE exhibited ratcheting behavior at on loading‐unloading‐reloading at constant stress level.

Modeling Finite Deformation Behavior of Semicrystalline Polymers under Uniaxial Loading—Unloading

The aim of this work is to investigate the finite deformation behavior of polymeric materials under monotonic loading—unloading. The strain rate sensitivity behaviors of polymeric materials were modeled using viscoplasticity theory based on an overstress (VBO) model. The modeling capability of the VBO model was improved to describe the nonlinear stress—strain behavior of the fully inelastic flow region in loading at small strain level. In this model, the tangent modulus (Et) is taken nonlinearly to simulate this polymeric material behavior. The numerical results were compared to the experimental data in the literature. These results were in good agreement with the experimental data.