Long‐term hydrostatic strength characteristics of thermoplastics pipe

2004

ABSTRACT:The universal viscoelastic model was further validated in this study using two acrylonitrile-butadienestyrene (ABS) viscoelastic materials to better elucidate the direct relationship between the failure criterion characteristics involving creep, constant strain rate, stress relaxation, and pipe burst. Using the yield strain as the failure criterion for constant strain rate and stress relaxation measurements and the strain at critical creep, the failure condition for creep, it was found that the universal viscoelastic model allowed these failure criteria to yield remarkably good agreement on a projected time scale. The relationship of the failure criteria between these three different techniques for characterizing a viscoelastic material was successfully used to identify several complementary approaches to predict long-term pipe burst for two different ABS materials. The pipe burst data for ABS-A appeared to fit the Tresca failure criterion better than the von Mises failure criterion, as predicted from failure criteria using constant strain rate, creep, and stress relaxation measurements. For ABS-A the extrapolated creep and the constant strain rate failure criterion appeared to best predict the Tresca failure criterion pipe burst data. However, the hoop burst stress for ABS-N, adjusted with the von Mises failure criterion modification, was found to give the best agreement with the yield stress equivalent failure criterion for constant strain rate, creep, and stress relaxation. For ABS-N, the extrapolated creep failure criteria appeared to best predict the von Mises pipe burst failure criterion. In this study, the relationships between the failure criteria for the three different experimental techniques of constant strain rate, creep, and stress relaxation were shown to be reasonably interchangeable relative to a three-dimensional failure configuration such as pipe burst. Because this interchangeability approach was found to work so well in the laboratory, there is no reason to believe that this same technique, using the universal viscoelastic model, would not also work as well to predict failure criteria for design applications using finite-element analysis.

Section: Constant Strain Rate Failure Criteria For Materials Abs‐a Anmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Comparison of the failure conditions for creep, stress relaxation, and constant strain rate measurements to predict pipe burst for two ABS materials using the universal viscoelastic model

2004

“…The relationship described by eq. (5) between yield stress σ y and time to yield t y was addressed using the following simple relationship currently included in ASTM D2837‐98a (Standard Test Method for Obtaining Hydrostatic Design Basis for Thermoplastic Pipe Materials): this relationship has also been used by Reinhart21 to predict long‐term failure stress (which is normally close to the stress evaluated from the stress relaxation of the yield stress) as a function of time.…”

Section: Summary Of the New Universal Viscoelastic Modelmentioning

confidence: 99%

Comparison of the viscous and elastic components of two ABS materials with creep, stress relaxation and constant strain rate measurements using the universal viscoelastic model

2003

ABSTRACT:In general, the universal viscoelastic model evaluated in this study was found to adequately predict constant strain rate, creep, and/or stress relaxation measurements from the constants determined from constant strain rate measurements. The elastic and viscous components for two acrylonitrile-butadiene-styrene (ABS) viscoelastic materials were also easily isolated using this new universal viscoelastic model. The creep measurements for ABS-A (25383-A) and ABS-N (LL-4102-N) at three different stresses allowed elucidation of the common creep intercept strain of the calculated creep slopes that was designated as the "projected elastic limit." Once the values for n and ␤ were evaluated from creep measurements, then the creep variation of the universal viscoelastic model yielded a reasonably good fit of the measured creep data for both ABS-A and ABS-N. The extensional viscosity constant E was found to be 7.2% greater for ABS-A than for ABS-N. Consequently, ABS-N was found to have a lower extensional viscosity in secondary creep than that of ABS-A at any specific strain rate. The value of the efficiency of yield energy dissipation n for ABS-N as determined from creep measurements was also 37.6% larger than the value of n for ABS-A. In addition, the projected elastic limit ⑀ I for ABS-A was 2% greater than the projected elastic limit for ABS-N. These observations indicated that ABS-A should be slightly more solidlike than ABS-N. However, both ABS-A and ABS-N were significantly more solidlike than liquidlike because both of their values for the efficiency of yield energy dissipation n were very close to zero. In general, values of n range from 0 Ͻ n Ͻ 1 with a material characterized as being essentially pure elastic having a value of n ϭ 0. Using the yield strain as the failure condition for constant strain rate and stress relaxation measurements and the strain at critical creep, the failure condition for creep, it was found that the universal viscoelastic model allowed these failure criteria to yield remarkably good agreement on a projected time scale. This agreement resulted even though separate and independent data were used to evaluate these three different techniques for both ABS-A and ABS-N.

“…The relationship between yield stress, σ y , and time to yield, t y , can be addressed using the following simple relationship currently included in ASTM D2837‐98a (Standard Test Method for Obtaining Hydrostatic Design Basis for Thermoplastic Pipe Materials): where σ y is the engineering yield stress, t y is the time to yield, n is the efficiency of yield energy dissipation, and β is the constant. This relationship has also been used by Reinhart16 to predict long‐term failure stress (which is normally close to the stress evaluated from the stress relaxation of the yield stress) as a function of time.…”

Section: Brief Review Of the Universal Viscoelastic Model Relating Comentioning

confidence: 97%

Evaluation of the characteristics of a viscoelastic material from creep analysis using the universal viscoelastic model. I. Isolation of the elastic component designated as the “Projected Elastic Limit”

2003

ABSTRACT:In a preceding publication this author introduced a new universal viscoelastic model to describe a definitive relationship between constant strain rate, creep, and stress relaxation analysis for viscoelastic polymeric compounds. One extremely important characteristic of this new model is that it also characterizes secondary creep very well. Because secondary creep is the linear portion of creep after the completion of primary creep, then a straight line with a slope and an intercept can describe secondary creep. To effectively define a straight line in the secondary creep region it was found necessary to obtain averages of the instantaneous slope and the instantaneous intercept strain by averaging over a series of equally spaced data points in the secondary slope region. Most importantly, this average intercept strain was found to be independent of creep stress and creep time. This means that all the secondary creep straight lines must pass through the same intercept creep strain for all creep stresses. The results presented in this study strongly indicate that this secondary creep intercept strain is independent of creep stress and creep time, and appears to increase as the value of the efficiency of yield energy dissipation decreases. Because a decrease in the efficiency of yield energy dissipation, n, appears to correlate with an increase in the elastic solid like character of a material, then it appears that this secondary creep intercept strain should be a direct measure of the strain that the material can survive to retain its full elastic character. Therefore, this secondary creep intercept strain has been designated as the "Projected Elastic Limit" of a given viscoelastic material.