Creep allowable tensile stress

In the ASME Code, Section II, Part D, Table 2A, the allowable tensile stress values for the materials that are governed where creep and stress rupture govern are indicated by italics. However this is not the sole criteria... 

The design criteria of the ASME Code, Vni-1, and Section III, Division 1, Subsections NC except NC-3200, ND, and NE, are similar to those for Sections I and rv except that the ASME Code, VIII-1, and Section III, Division 1, Subsections NC, ND, and NE require cylindrical shell thickness calculations based on both the circumferential and the longitudinal directions. The minimum required thickness may be set by stresses in either direction. In addition, the ASME Code, VIII-1, permits the combination of primary membrane stress and primary bending stress to go as high as 1.5 5 at temperatures where tensile and yield strength control and 1.25 5 at temperatures where creep and rupture control, where 5 is the allowable tensile stress values. 

Therefore, substituting v = 0.4, the creep hoop strain will be smaller than that in a tensile creep test by a factor (1 — v ), if the hoop and tensile stresses are equal. Examining Fig. 7.6, the tensile stress to cause a creep strain of 3/0.84 = 3.6% after 50 years is approximately 4 MPa. For a 4 bar pressure to cause a hoop stress of 4 MPa, the pipe SDR = 21 by Eq. (14.2). This is the maximum SDR allowed. 

High UTS retention after interphase exposure at intermediate temperatures in wet oxygen (allows CMC toughness retention when exposed, uncracked or cracked, to combustion gases) High creep resistance at upper use temperature under high tensile stress (allows long Itfe, dimensional control, tow residual CMC stress)... 

Resistance to creep is dependent on the alignment of fibres to match the external loading and minimise stresses in the matrix. When subjected to tensile stresses carbon composites resist long term creep very well. In off axis situations creep rates will be higher and in compression the contribution by the matrix to local fibre stability is critical and lower allowable stresses are required. 

Example 4.1. A user is requesting code approval for a new material that has a minimum specified tensile stress of 120 ksi and a minimum specified yield stress of 60 ksi at room temperature. Tensile and yield values for various heats and temperatures are shown in Fig. 4.5. Creep and rupture data are given in Figs. 4.6 and 4.7, respectively. What are the allowable stress values at 300 and 1200°F based on criteria given in Section 2.4 ... 

Tensile Testing. The most widely used instmment for measuring the viscoelastic properties of soHds is the tensile tester or stress—strain instmment, which extends a sample at constant rate and records the stress. Creep and stress—relaxation can also be measured. Numerous commercial instmments of various sizes and capacities are available. They vary greatiy in terms of automation, from manually operated to completely computer controlled. Some have temperature chambers, which allow measurements over a range of temperatures. Manufacturers include Instron, MTS, Tinius Olsen, Apphed Test Systems, Thwing-Albert, Shimadzu, GRC Instmments, SATEC Systems, Inc., and Monsanto. 

A rod of polypropylene, 10 mm in diameter, is clamped between two rigid fixed supports so that there is no stress in the rod at 20°C. If the assembly is then heated quickly to 60°C estimate the initial force on the supports and the force after 1 year. The tensile creep curves should be used and the effect of temperature may be allowed for by making a 56% shift in the creep curves at short times and a 40% shift at long times. The coefficient of thermal expansion for polypropylene is 1.35 x 10 °C in this temperature range. 

This is because although 0 = (10), in general, cr(10) oQ (it will usually be less). In principle, the quantities we have defined, E(t), Dit), Gif), and J(i), provide a complete description of tensile and shear properties in creep and stress relaxation (and equivalent functions can be used to describe dynamic mechanical behavior). Obviously, we could fit individual sets of data to mathematical functions of various types, but what we would really like to do is develop a universal model that not only provides a good description of individual creep, stress relaxation and DMA experiments, but also allows us to relate modulus and compliance functions. It would also be nice to be able formulate this model in terms of parameters that could be related to molecular relaxation processes, to provide a link to molecular theories. 

E-3 spectrometer. A servo-hydraulic system was built which allowed loading of the samples in a wide variety of modes, and permitted tensile tests to be conducted at constant stress rate, at constant stress (creep), in cyclic fatigue, at constant strain rate, at constant strain or step strain. Provision was made for the simultaneous recording of stress-strain data and ESR spectra and a variable temperature control unit was employed. 

The creep of materials can also occur solely by diffusion, i.e., without the motion of dislocations. Consider a crystal under the action of a combination of tensile and compressive stresses, as shown in Fig. 7.4. The action of these stresses will be to respectively increase and decrease the equilibrium number of vacancies in the vicinity of the boundaries. (The boundaries are acting as sources or sinks for the vacancies.) Thus, if the temperature is high enough to allow significant vacancy diffusion, vacancies will move from boundaries under tension to those under compression. There will, of course, be a counter flow of atoms. As shown in Fig. 7.4, this mass flow gives rise to a permanent strain in the crystal. For lattice diffusion, this mechanism is known as Nabarro-Herring creep. The analysis showed that the creep rate e is given by... 

Thermomechanical analysis (TMA) measures the deformation of a material contacted hy a mechanical prohe, as a function of a controlled temperature program, or time at constant temperature. TMA experiments are generally conducted imder static loading with a variety of probe configurations in expansion, compression, penetration, tension, or flexime. In addition, various attachments are available to allow the instrument to operate in special modes, such as stress relaxation, creep, tensile loading of films and fibers, flexural loading, parallel-plate rheometry, and volume dilatometry. The type of probe used determines the mode of operation of the instrument, the manner in which stress is apphed to the sample, and the amount of that stress. 

At te mp)erature.s below the creep range, allowable stR s.s values were c.HtuhIislwd at the lowest value of stress obfaiiHid IVofn (a) 25% of the spM cifi( [Pg.24]

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