Rupture stress

Fig. 8. Stress—rupture curves for annealed H-grade austenitic stainless steels. AISI numbers are given (see Table 3). Rupture iu 10,000 h (35). To convert...

Fig. 10. 1000 h stress—rupture curves of wrought cobalt-base (Haynes 188 and L-605) and wrought iron-base superalloys (49). To convert MPa to psi,... 

Fig. 14. Stress—rupture properties of NiAl alloys and composites compared with superaHoy Renit 80 (77). The Heusler precipitates (ppts) = 50 Ni—25 Al—25X (at. %), where X = a Group 4 (IVB) or Group 5 (VB) element such as Hf. To convert MPa to psi, multiply by 145.

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. 

For statie design to be valid in praetiee, we must assume situations where there is no deterioration of the material strength within the time period being eonsidered for the loading history of the produet. With a large number of eyelie loads the material will eventually fatigue. With an assumed statie analysis, stress rupture is the meehanism of failure to be eonsidered, not fatigue. The number of stress eyeles in a problem eould... 

The formulations for the failure governing stress for most stress systems can be found in Young (1989). Using the variance equation and the parameters for the dimensional variation estimates and applied load, a statistical failure theory can be formulated for a probabilistic analysis of stress rupture. 

Stress rupture - duetile and brittle fraeture for simple and eomplex stresses... 

Figure 4.41 shows the Stress-Strength Interference (SSI) diagrams for the two assembly operation failure modes. The instantaneous stress on the relief section on first assembly is composed of two parts first the applied tensile stress,. v, due to the pre-load, F, and secondly, the torsional stress, t, due to the torque on assembly, M, and this is shown in Figure 4.41(a) (Edwards and McKee, 1991). This stress is at a maximum during the assembly operation. If the component survives this stress, it will not fail by stress rupture later in life. 

In the stress rupture ease, the interferenee of the stress, L, and strength, Sy, both following a Normal distribution ean be determined from the eoupling equation ... 

The eontribution of eaeh variable to the final stress distribution in the ease of stress rupture ean be examined using sensitivity analysis. From the varianee equation ... 

However, too great a separation, and overdesign may oeeur. The overload eondition is represented by a unique stress, whieh is very mueh greater than the working stress, applied suddenly whieh eauses only the weak link to failure due to stress rupture. 

Note, a eomprehensive list of failure modes and eauses of failure for meehanieal eomponents is provided by Dieter (1986). These tables are partieularly useful when assessing the likely stress rupture failure meehanism for reliability work. 

Stress-rupture data are often presented in a Larson-Miller eurve, whieh indieates the performanee of an alloy in a eomplete and eompaet graphieal style. While widely used to deseribe an alloy s stress-rupture eharaeteristies over a wide temperature, life, and stress range, it is also useful in eomparing the elevated temperature eapabilities of many alloys. The Larson-Miller parameter is... 

The stress rupture properties of this alloy are shown in Figure 11-6. 

Figure 11-6. Turbine Wheei Aiioys stress rupture eomparison.

It is not known to what extent each of the previous mechanisms contributes to turbine blade degradation during service. It is also probable that each alloy will respond differently to a particular temperature/stress combination. Figure 21-12 shows the typical variation in stress/rupture life determined at 1350°F (375 °C) with service time for forged Inconel X-750 blades. 

Figure 21-12. The variation of remaining stress rupture iife at 1350°F (735 °C) with serviee time in forged Ineonei aiioy X-750 turbine biades. (Courtesy of Westinghouse Eieotrio Corp., Gas Turbine Div.).

Figure 21-13. Comparison of stress rupture life at 50ksi/1350°F (345 MPa/735°C) in service exposed, commercially reheat-treated, laboratory reheat-treated, and HIP reheat-treated used Inconel X-750 turbine blades. (Courtesy of Westing-house Electric Corp., Gas Turbine Div.)...

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. 

Bismuth Niobium is resistant to bismuth at temperatures up to 560°C but is attacked at higher temperatures and is therefore not considered a suitable container for handling liquid bismuth even under oxygen-free conditions Furthermore, the stress-rupture properties of niobium are significantly lowered when the metal is tested in molten bismuth at 815°C . 

Lead Although subject to slight penetration at 980°C it shows no detrimental effects in stress rupture tests when tested in molten lead at this temperature or at 815°C . It is highly resistant to mass transfer in liquid lead as indicated by data obtained in tests at 800°C with a thermal gradient of 300°C . ... 

Bismuth Liquid bismuth has little action on tantalum at temperatures below 1000°C " , the rate of attack at 870°C being less than 0.13 mm/y, and exerts not detrimental effects on the stress rupture properties of tantalum at 815°C, but is causes some intergranular attack at 1000°C . 

Lead Tantalum is highly resistant to liquid lead at temperatures up to 1000°C with a rate of attack of less than 0.025 mm/y. It exhibits no detrimental effects when stress rupture tests are conducted in molten lead at 815°C . 

In the majority of cases, the tests are conducted using a dead-weight lever-arm stress-rupture rig with an electric timer to determine the moment of fracture, but a variety of test rigs similar to those shown in Fig. 8.89g are also used. The evaluation of embrittlement may be based on a delayed-failure diagram in which the applied nominal stress versus time to failure is plotted (Fig. 8.103) or the specimen may be stressed to a predetermined value (say 75% of the ultimate notched tensile strength) and is considered not to be embrittled if it shows no evidence of cracking within a predetermined time (say 500 h). Troiano considers that the nature of delayed fracture failure can be described by four parameters (see Fig. 8.103) ... 

Fig. 2-36 Stress-rupture data for rigid 2 in. diameter PVC pipe as a function of temperature.

Failure can be considered as an actual rupture (stress-rupture) or an excessive creep deformation. Correlation of stress relaxation and creep data has been covered as well as a brief treatment of the equivalent elastic problem. The method of the equivalent elastic problem is of major assistance to designers of plastic products since, by knowing the elastic solution to a problem, the viscoelastic solution can be readily deduced by simply replacing elastic physical constants with viscoelastic constants. 

Rate theory An alternate method available involves the manipulation of the rate theory based on the Arrhenius equation. This procedure requires considerable test data but the indications are that considerably more latitude is obtained and more materials obey the rate theory. The method can also be used to predict stress-rupture of plastics as well as the creep characteristics of a material, which is a strong plus for the method. 

Designing plastic Basically the general design criteria applicable to plastics are the same as those for metals at elevated temperature that is, design is based on (1) a deformation limit, and (2) a stress limit (for stress-rupture failure). There are, of course, cases where weight is a limiting factor and other cases where short-term properties are important. 

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