Polyethylene Stresses Research Articles

Analysis of time‐dependent mechanical behavior of polyethylene

AbstractNew data analysis based on a spring‐dashpot model is developed to provide very close simulation of stress variation in polyethylene (PE) when subjected to a multi‐relaxation (MR) test. The model consists of a quasi‐static branch and two (long‐ and short‐term) viscous branches. The study shows that viscous stresses applied to the two dashpots and the model parameters can be quantified as functions of displacement (stroke) by simulating closely the stress variation in relaxation. Using these parameters, influences of stress triaxiality and material grade on PE's stress response at relaxation stages are examined, and along with experimental data at the loading stages, spring constants in the two viscous branches are determined. The results indicate that among three PE grades, spring constants and their trend of variation are similar in the long‐term viscous branch, but not in the short‐term counterpart. The results also show that by decreasing specimen gauge length 10 times to increase stress triaxiality, spring constants in the two viscous branches become closer to each other. The work concludes that the new data analysis can determine all parameters in the model, which can then be considered to quantify the role of the viscous stress response for PE's load‐carrying performance.Highlights Use a simple model to simulate complex, multiple relaxation behaviors of PE. Determine all model parameter values to characterize PE's relaxation behavior. Illustrate the effect of stress triaxiality on PE's viscous stress response. Show the similarity and difference of the viscous stress response among 3 PEs.

Effect of polyethylene particles on dibutyl phthalate toxicity in lettuce (Lactuca sativa L.)

The effects of dibutyl phthalate (DBP) on the toxicity and edible quality (e.g., soluble proteins, soluble sugars, and vitamin C) of green and purple lettuce in the presence of polyethylene (PE) fragments were evaluated. The results revealed that PE treatment for 28 days decreased DBP and monobutyl phthalate content in lettuce roots and leaves, but enhanced the inhibitory effects of DBP on root growth and activity, reduced soluble protein and sugar content in lettuce leaves, and increased vitamin C content in lettuce leaves. Scanning and transmission electron microscopies revealed that PE only adhered to the root surface and did not enter the lettuce roots. Moreover, separation of the cell wall was aggravated in lettuce roots treated with DBP+PE, but not in lettuce treated with individual DBP or PE, and even led to the expansion of endoplasmic reticulum vesicles and cell rupture. Gaussian analysis indicated that PE interacted with DBP molecules through van der Waals interactions, which decreased DBP transport from the culture solution into the lettuce roots. In addition, purple lettuce was more sensitive to exogenous pollutants than green lettuce. This study provides new insights for food safety related to DBP fate and toxicity under PE stress.

A Study on Polyethylene Stresses in Mobile-Bearing and Fixed-Bearing Total Knee Arthroplasty (TKA) using Explicit Finite Element Analysis

Mobile-bearing total knee arthroplasty designs were thought to be associated with lesser stresses on the polyethylene (PE) insert as compared to fixed-bearing designs. The wear rate of the PE liner was felt to be less in the mobile-bearing knees due to the decrease in the stresses. However, a reevaluation of the biomechanics of the mobile-bearing design is necessary due to the recent clinical reports on the long-term outcome of mobile-bearing knees which have not demonstrated any significant benefit in terms of implant survival and polyethylene wear rate. In this explicit finite element analysis of mobile-bearing and fixed-bearing knee designs using LS-DYNA, no significant differences have been found in the maximal stresses in the superior (articulating) surface of the PE insert in mobile- and fixed-bearing designs. On the inferior surface of the PE insert, the peak stresses were nearly 30% higher in the mobile-bearing compared with the fixed-bearing design. Thus, contrary to earlier expectations, mobile-bearing designs may be associated with higher overall PE stresses and wear than the fixed-bearing designs. Further research is necessary to minimize the wear rate of the PE insert in mobile-bearing total knee arthroplasty.