Creep rupture time

Figure 12.1 Creep-rupture (time to failure plotted horizontally against applied hoop stress) of polyethylene pipes under pressure at different temperatures...

Figure 12.26 Double logarithmic plot showing the creep rupture time, tf, as a function of the applied stress, a, for poly(methyl methacrylate) at 24°C. Data obtained for (A) freshly quenched samples and (O) samples aged at 24°C for 5 years. (From Ref. 30.)...

Since experimental creep rupture times rarely exceed 10" h, it is necessary to extrapolate the data, using a straight line extension of the ductile rupture line on the log-log graph. The British Gas Specification for polyethylene pipe required the 50 year creep rupture stress cr o > 10 MPa. The International Standard ISO 9080 classifies polyethylene as PE80 if the lower confidence limit of the 50 year creep rupture strength lies between 8.0 and 9.9 MPa, and as PEIOO if it lies between 10.0 and 11.9 MPa. 

Fig. 14.2 shows creep rupture data for a particular MDPE, for ductile failure at a range of temperatures. An Arrhenius plot, of the logarithm of the creep rupture time versus the reciprocal of the absolute test temperature, is usually a straight line graph. This can be used to estimate the creep rupture times at lower temperatures. If there is ductile failure in the higher temperature tests, it is unlikely that brittle failure will occur at long times at low temperatures. 

As experience with creep rupture testing of polyolefins has been gained, elevated temperature tests have been used for quality control purposes, and standards set using such tests, i.e. the creep rupture time for pipes for natural gas distribution must exceed 170 h at 80 °C and a hoop stress of 3 MPa. Care must, however, be exercised if a polyethylene made by a different process is introduced, because the use temperature is close to 10 °C when the pipe is buried in the ground the slope of the Arrhenius plot varies between different polyethylenes. 

Fiber Mean length (pm) Tensile strength (kpsi) Unnotched Izod (ft-lbF/in.) Tensile creep rupture time (hr)... 

By assuming that creep rupture occurs at a constant creep strain limit (2), Eq. (13.4) can be transformed into an expression of creep rupture time tcR as a function of stress and temperature ... 

Figures 13.15 and 13.16 depict the relationship between applied stress load and measured creep rupture times for a series of composite materials at 50 and 80°C,...

Figure 13.15 Tensile creep rupture time-load plots at 50°C. , P7-10FG-0600 , P7-20FG-0600 A, P7-30FG-0600 , P7-40FG-0600.

Chamber temperature T[°C) Axial load Lm Tensile stress S(psi) Creep rupture time f(hr) Independent variables Response function Y = x, x2) Time f(hr) Calculated load (kg) for creep rupture f = 1000 hr... 

To prevent unrealistically short creep rupture times, one needs to maintain stress loads below about 60-70% of the measured tensile strength at the same test chamber temperature. This is an important criterion for selecting appropriate weights for accelerated tensile strength determination. 

Equation (13.6) was used to develop a set of creep rupture envelopes to predict attainable creep rupture time at given load L for three mega-coupled composites at 80°C as shown in Fig. 13.18. With measured data only at creep rupture times <100 hr for accelerated tensile creep determinations, we can make estimates for anticipated service lifetime as a function of applied stress load that is indicated by the weight L. 

One of the relations used to express creep-rupture time, t, as a function of the activation energy of creep, Qc, is ... 

Fig. 36. Comparison of cr—N curve under a square-wave loading with creep-rupture data for a typical ABS polymer (175). Note that the creep-rupture time scale is equivalent to that of the fatigue cycle scale in terms of time under tensile load. To convert MPa to psi, multiply by 145.

Inorganic-Reinforced Styrene Poiymers. Glass reinforcement of PS and SAN markedly improves their mechanical properties. The strength, stiflfiiess, and fracture toughness are generally at least doubled. Creep and relaxation rates are significantly reduced and creep rupture times are increased. The coefficient of... 

Table 11.2. Creep rupture strength of several alloys (after [39,125,129]). The creep rupture strength iim/iooooo/T i- e., the stress needed to cause fracture in a specimen at temperature T after 10 hours (creep rupture time), is stated. The creep resistance of the ferritic steels with large amounts of vanadium and chromium is significantly larger than that of simpler steels because vanadium and chrome carbides have a better temperature stabihty. Due to their close-packed face-centred cubic structure, the creep resistance of austenitic steels is larger. The creep strength of the nickel-base superalloys IN 738 (polycrystaUine) and SC 16 (single crystalline) were estimated from Larson-Miller data...

If the activation energy, E, is equal to the quantity, yo, the creep rupture time is independent of temperature and implies the existence of a common pole as shown in Eig. 11.20. That is. 

Fig. 11.27 Comparison between Reiner-Weissenberg theory and experimentally determined creep rupture times.

Figure 10.6 Linear regression lines for creep-rupture times at 30°C, 45°C and 60°C (Pritchard and Speake).

Improvements of creep rupture time by more than one order of magnitude have been reported [134,135]. 

In extending the allowable stresses to 500,000 h, the most important point is to use a reliable database of creep mpture data. Fig. 18.1 shows an example of creep data on Mod.9Cr-lMo steel [10—12]. A number of data whose rupture time is over 100,000 h have been obtained at 550°C. This leads to a time-wise extrapolation of a factor of 4—5 to determine allowable stresses up to 500,000 h. There is no unique scientific way to determine how much extrapolation would be allowed. ASME Boiler and Pressure Vessel Code Section IB Division 5 Subsection HB Subpart B Appendix HBB-Y Guidelines for Design Data Needs for New Materials (ASME Appendix HBB-Y) [13], which was published as a nonmandatory appendix in 2015, provides some guidance on the extrapolation of creep rupture time as part of the requirements for adoption of new materials into the Code. It assumes use of time—temperature... 

Figure 18.1 Formulation of creep rupture time for Mod.9Cr-lMo steel [12].

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