Stress creep rupture

Figure 4.17 1000 hour creep rupture stress as a function of temperature for various steels (Waterman and Ashby, 1991)... 

Creep leads ultimately to rupture, referred to as creep-rupture, stress-rupture or static fatigue. Creep-rupture of thermoplastics can take three different forms brittle failure at low temperatures and high strain rates ductile failure at intermediate loads and temperatures and slow, low energy brittle failure at long lifetimes. It is this transition back to brittle failure that is critical in the prediction of lifetime, and it is always prudent to assume that such a transition will occur [1], A notch or stress concentration will help to initiate failure. 

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. 

Shear modulus Poisson s ratio Tfensile stress at yield Tbnsile creep rupture stress Shear strength intensity factor K c ThnsUe stress at yield ThnsUe creep rupture stress Poisson s ratio Compression strength Tfensile stress at yield thermal expansion... 

Maximum stress Tfensile stress at yield IfensUe creep rupture stress... 

Maximum stress Tfensile stress at 5deld Tfensile creep rupture stress Shear strength... 

Tensile strength at yield Tensile creep rupture stress Stress relaxation " Crystalline melting temperature... 

Several design standards may be used for the design of reactor tubes, for example, the guidelines AP530 of the American Petroleum Institute (51), which gives guidelines for creep rupture stress calculations. 

Goldfein, G. General Formula for Creep Rupture Stresses in Plastics. Modern Plastics, Apr. 1960. 

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.

Creep Rupture. The results from creep mpture tests on tubes under internal pressure at elevated temperatures (71,72) may be correlated by equation 16, in which is replaced by the tensile creep mpture stress after time t at temperature T. 

Mechanical Properties Mechanical properties of wide interest include creep, rupture, short-time strengths, and various forms of ductihty, as well as resistance to impact and fatigue stresses. Creep strength and stress rupture are usually of greatest interest to designers of stationary equipment such as vessels and furnaces. 

Metals Successful applications of metals in high-temperature process service depend on an appreciation of certain engineering factors. The important alloys for service up to I,I00°C (2,000°F) are shown in Table 28-35. Among the most important properties are creep, rupture, and short-time strengths (see Figs. 28-23 and 28-24). Creep relates initially applied stress to rate of plastic flow. Stress... 

Times-to-failure are normally presented as creep-rupture diagrams (Fig. 17.9). Their application is obvious if you know the stress and temperature you can read off the life if you wish to design for a certain life at a certain temperature, you can read off the design stress. 

The hoop stress in the tube under the working pressure of 50 bar (5 MPa) is 5 MPa X 50 mm/5 mm = 50 MPa. Creep data indicate that, at 900°C and 50 MPa, the steel should fail after only 15 minutes or so. In all probability, then, the failure occurred by creep rupture during a short temperature excursion to at least 870°C. 

Depending upon the stress load, time, and temperature, the extension of a metal associated with creep finally ends in failure. Creep-rupture or stress-rupture are the terms used to indicate the stress level to produce failure in a material at a given temperature for a particular period of time. For example, the stress to produce rupture for carbon steel in 10,000 hours (1.14 years) at a temperature of900°F is substantially less than the ultimate tensile strength of the steel at the corresponding temperature. The tensile strength of carbon steel at 900°F is 54,000 psi, whereas the stress to cause rupture in 10,000 hours is only 11,500psi. 

Above temperatures of 900°F, the austenitic stainless steel and other high alloy materials demonstrate inereas-ingly superior creep and stress-rupture properties over the chromium-molybdenum steels. For furnace hangers, tube supports, and other hardware exposed to firebox temperatures, cast alloys of 25 Cr-20 Ni and 25 Cr-12 Ni are frequently used. These materials are also generally needed because of their resistanee to oxidation and other high temperature corrodents. 

Furnace tubes, piping, and exchanger tubing with metal temperatures above 800°F now tend to be an austenitic stainless steel, e.g., Type 304, 321, and 347, although the chromium-molybdenum steels are still used extensively. The stainless steels are favored beeause not only are their creep and stress-rupture properties superior at temperatures over 900°F, but more importantly because of their vastly superior resistance to high-temperature sulfide corrosion and oxidation. Where corrosion is not a significant factor, e.g., steam generation, the low alloys, and in some applications, carbon steel may be used. 

Figure 9.6. (a) The temperature dependence of the flow stress for a Ni-Cr-AI superalloy containing different volume fractions of y (after Beardmore et al. 1969). (b) Influence of lattice parameter mismatch, in kX (eflectively equivalent to A) on creep rupture life (after Mirkin and Kancheev... 

Creep Rupture. When a plastic is subjected to a constant tensile stress its strain increases until a point is reached where the material fractures. This is called creep rupture or, occasionally, static fatigue. It is important for designers... 

Other factors which promote brittleness are geometrical discontinuities (stress concentrations) and aggressive environments which are likely to cause ESC (see Section 1.4.2). The absorption of fluids into plastics (e.g. water into nylon) can also affect their creep rupture characteristics, so advice should be sought where it is envisaged that this may occur. 

If the values for Uq and y for the material are not known then a series of creep rupture tests at a fixed temperature would permit these values to be determined from the above expression. The times to failure at other stresses and temperatures could then be predicted. 

For convenience, in the previous sections it has been arranged so that the mean stress is zero. However, in many cases of practical interest the fluctuating stresses may be always in tension (or at least biased towards tension) so that the mean stress is not zero. The result is that the stress system is effectively a constant mean stress, a superimposed on a fluctuating stress a a- Since the plastic will creep under the action of the steady mean stress, this adds to the complexity because if the mean stress is large then a creep rupture failure may occur before any fatigue failure. The interaction of mean stress and stress amplitude is usually presented as a graph of as shown in Fig. 2.76. 

This represents the locus of all the combinations of Ca and Om which cause fatigue failure in a particular number of cycles, N. For plastics the picture is slightly different from that observed in metals. Over the region WX the behaviour is similar in that as the mean stress increases, the stress amplitude must be decreased to cause failure in the same number of cycles. Over the region YZ, however, the mean stress is so large that creep rupture failures are dominant. Point Z may be obtained from creep rupture data at a time equal to that necessary to give (V cycles at the test frequency. It should be realised that, depending on the level of mean stress, different phenomena may be the cause of failure. 

Creep rupture tests on a particular grade of uPVC at 20°C gave the following results for applied stress, ct, and time to failure, /. 

A uPVC rod of diameter 12 mm is subjected to an eccentric axial force at a distance of 3 ttun from the centre of the cross-section. If the force varies sinusoidally from — F to f at a frequency of 10 Hz, calculate the value of F so that fatigue failure will not occur in 10 cycles. Assume a safety factor of 2.5 and use the creep rupture and fatigue characteristics described in the previous question. Thermal softening effects may be ignored at the stress levels involved. 

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