Creep rupture limit

Example 2.21 A rod of plastic is subjected to a steady axial pull of 50 N and superimposed on this is an alternating axial load of 100 N. If the fatigue limit for the material is 13 MN/m and the creep rupture strength at the equivalent time is 40 MN/m, estimate a suitable diameter for the rod. Thermal effects may be ignored and a fatigue strength reduction factor of 1.5 with a safety factor of 2.5 should be used. 

The creep-strength limit, which is the initial stress that will just cause rupture or a specified strain in a specified time (under the stated environmental conditions). 

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. 

Most non-aerospace CMC applications require long service lives. For these applications CMC components must avoid creep rupture and must exhibit creep strains lower than 1 percent after 10,000 hours of service (e.g., at 1,200°C [2,192°F] and 100 MPa [14.5 ksi]) components must also be chemically and microstructurally stable. These stringent demands present major challenges to researchers and engineers, particularly for material development and accelerated testing. The performance objectives limit the material choices to polycrystalline oxides, SiC, or amorphous Si-C-N-B compositions (single-crystal fibers are not affordable). 

In the case of (a), since there can be substantial variations in both compliance and strength (creep rupture) with angle, this may result in creep in some directions involving extremely low strains, and therefore presenting severe measurement problems, whilst in other directions very rapid large creep or rupture may occur thus limiting the information available. It has therefore been found preferable to employ (b) and to choose stress levels in different directions so as to produce equal strains after a specified creep time in all directions. Furthermore if correlation of creep behaviour with deformation mechanisms is sought it may well be desirable to compare the polymer response when the different mechanisms produce similar strains. Selection of appropriate stress-levels is achieved by use of the isochronous stress-strain curves. 

Creep rupture tests are used to measure the long-term response of a material to a continuously applied stress at a given temperature. The ideal material should be able to support significant stresses for extended periods of time without accumulated permanent strain or breakage. The SiOC-Nextel 312 BN 2-D composites were tested in limited stress... 

At the ultimate limit state the effects of creep (e.g. creep rupture) shall be taken into account by the choice of the appropriate value of yn, 3 (see 2.3.3.2). 

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 ... 

Creep rupture from long term loading, creep-fatigue, and creep-buckling should also be addressed and procedures to do so within the Code have been around for some time now. In the past, at least some portions of the Code were limited to temperatures with the intent to avoid any time-dependent effects on the material, meaning that maximum permitted temperatures for materials were kept below the creep range. 

Creep rupture data minimum jreep rate as a function of the time to failure. Although the data set is limited, time-to-failure seems to be determined uniquely bye the rate of creep in this material, suggesting that improvement of creep rate is of paramount importance to improving lifetime. 

The tube materials are limited by design temperature and creep rupture strength. Recent progress in the formulation of construction materials (Jones, 90/91) and (Thuillier et al., 1978) has allowed safe designs with exit gas temperatures exceeding 950°C corresponding to tube wall temperatures up to 1050°C. The new materials which have superior creep rupture characteristics, allow a design with a considerable reduction in tube wall thickness. The use of a thin tube wall... 

Figure 11.21 Design stress domains recommended forNb-lZr, Ta-8W-2Hf, TZM [112], and V tCr-4Ti [113], The creep strain limit was made by two-thirds of creep rupture stress for 10 h for V-4Cr-4Ti and 1% creep strain in 7 years for the others.

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