Stress values

Critical properties Critical stress values Critical value... 

A rotational viscometer connected to a recorder is used. After the sample is loaded and allowed to come to mechanical and thermal equiUbtium, the viscometer is turned on and the rotational speed is increased in steps, starting from the lowest speed. The resultant shear stress is recorded with time. On each speed change the shear stress reaches a maximum value and then decreases exponentially toward an equiUbrium level. The peak shear stress, which is obtained by extrapolating the curve to zero time, and the equiUbrium shear stress are indicative of the viscosity—shear behavior of unsheared and sheared material, respectively. The stress-decay curves are indicative of the time-dependent behavior. A rate constant for the relaxation process can be deterrnined at each shear rate. In addition, zero-time and equiUbrium shear stress values can be used to constmct a hysteresis loop that is similar to that shown in Figure 5, but unlike that plot, is independent of acceleration and time of shear. 

Pressure or Stress. Values with an uncertainty of more than 2% may be converted without rounding by using the approximate factor 1 Ibf/in. = 7 kPa. 

Physical Factors. Unsatuiated elastomers must be stretched for ozone cracking to occur. Elongations of 3—5% are generally sufficient. Crack growth studies (10—18) have shown that some minimum force, called the critical stress, rather than a minimum elongation is required for cracking to occur. Critical stress values are neady the same for most unsaturated mbbers. However, polychloroprene has a higher critical stress value than other diene mbbers, consistent with its better ozone resistance. It has been found that temperature, plasticization, and ozone concentration have httie effect on critical stress values. 

Both Watts and sulfamate baths are used for engineering appHcation. The principal difference in the deposits is in the much lower internal stress obtained, without additives, from the sulfamate solution. Tensile stress can be reduced through zero to a high compressive stress with the addition of proprietary sulfur-bearing organic chemicals which may also contain saccharin or the sodium salt of naphthalene-1,3,6-trisulfonic acid. These materials can be very effective in small amounts, and difficult to remove if overadded, eg, about 100 mg/L of saccharin reduced stress of a Watts bath from 240 MPa (34,800 psi) tensile to about 10 MPa (1450 psi) compressive. Internal stress value vary with many factors (22,71) and numbers should only be compared when derived under the same conditions. 

In shaded areas, allowable-stress values wliich are printed in italics exceed two- tliirds of the expected yield strength at temperature. All odier allowable-stress values in shaded areas are equal to 90 percent of expected yield strength at temperature. See ANSI B31.3. 

The higlier stress values at 566 C (1050 F) and above for tliis material shall be used only when the steel has an austenitic micrograin si2e No. 6 or less (coarser grain) as defined in ASTM E112. Odierwise the lower stress values shall be used. 

For temperatures above 53S C (lOOO F), diese stress values may be used only if the material has been heat-treated at a temperature of 1090 C (2000 F) minimum. 

For temperatures above 3S C (lOO F) these stress values apply only when the carbon content is 0.04 percent or liiglier. 

Stress values shown include the casting quality factor shown in this table. Higlier stress values can be used if specijJ inspection is accomplished. 

After use above the temperature indicated by a single bar (I), use at a lower temperahire shall be based on the stress values allowed for the annealed condition of the material. 

The SE values in Table 10-49 are equal to the basic allowable stresses in tension S multiplied by a quality factor E (see subsection Pressure Design of Metallic Components Wall Tliick-ness"). The design stress values for bolting materials are equal to die basic allowable stresses S. The stress values in shear shall be 0.80 times the allowable stresses in tension derived from tabulated values in Table 10-49 adjusted when applicable in accordance widi Note 13. 8tress values in bearing shall be twice those in shear. 

The stress values given for tliis material are not applicable when either welding or thermal cutting is employed. 

For stress-relieved tempers (T3.51, T3510, T3.511, T451, T4510, T4511, T651, T 510, T6511) stress values for material in the listed temper shall be used. 

Pipe produced to tliis specification is not intended for high-temperatnre service. The stress values apply to eitlier nonexpanded or cold-expanded material in the as-rolled, normalized, or normalized and tempered condition. 

Subsection C This subsection contains requirements pertaining to classes of materials. Carbon and low-alloy steels are governed by Part UCS, nonferrous materials by Part UNF, high-alloy steels by Part UHA, and steels with tensile properties enhanced by heat treatment by Part UHT. Each of these parts includes tables of maximum allowable stress values for all code materials for a range of metal temperatures. These stress values include appropriate safety fac tors. Rules governing the apphcation, fabrication, and heat treatment of the vessels are included in each part. 

Ultimate tensile strength the maximum stress value as obtained on a stress-strain curve (Figure 30.1). 

If we assume that the maximum stress applied is +3cr from the mean stress, where this loading stress value eovers 99.87% those applied in serviee ... 

The reliability, R/, as a funetion of the maximum stress value used is shown in... 

The usefulness of this formula is restricted by the difficulty of obtaining good values to substitute in it. They must apply to the alloy selected, and be derived from carefully controlled tests on it. The stress value, S, reflects an engineer s Judgment in the selection of elastic limit or some arbitrary yield strength. The modulus value must match this. The restraint coefficent, K, is seldom known with any precision. 

To be effective, the antiozonants should have two important functions decrease the rate of crack growth in the rubber, and increase the critical stress value (i.e. the stress at which crack growth occurs). Therefore, the following properties of an antiozonant are desirable. 

Residual stresses occur from welding and other fabrication techniques even at very low stress values. Unfortunately, stress relief of equipment is not usually a reliable or practical solution. Careful design of equipment can eliminate crevices or splash zones in which chlorides can concentrate. The use of high-nickel stainless steel alloy 825 (40% nickel, 21% chromium, 3% molybdenum and 2% copper) or the ferritic/austenitic steels would solve this problem. 

For most traditional materials, the objective of the design method is to determine stress values which will not cause fracture. However, for plastics it is more likely that excessive deformation will be the limiting factor in the selection of working stresses. Therefore this chapter looks specifically at the deformation behaviour of plastics and fracture will be treated separately in the next chapter. 

Dynamic tensile failure, called spall, is frequently encountered in shockloading events. Tension is created as compression waves reflect from stress-free surfaces and interact with other unloading waves or release-wave profiles. Spall has been widely studied by authors such as Curran, Ivanov, Dremin, and Davison and there is considerable data. As shown in Fig. 2.19, the wave profiles resulting from spall are characterized by an additional loading pulse after release of pressure. The late pulse is caused by wave reflection from the internal void of the tensile fracture. Analysis of such wave profiles yields appropriate spall stress values. 

The interlaminar shear stress, t, has a distribution through half the cross-section thickness shown as several profiles at various distances from the middle of the laminate in Figure 4-54. Stress values that have been extrapolated from the numerical data at material points are shown with dashed lines. The value of is zero at the upper surface of the laminate and at the middle surface. The maximum value for any profile always occurs at the interface between the top two layers. The largest value of occurs, of course, at the intersection of the free edge with the interface between layers and appears to be a singularity, although such a contention cannot be proved by use of a numerical technique. 

The maximum allowable stress values at normal temperature range for the steel plates most commonly used in the fabrication of pressure vessels are given in Table 12-3. For stress values at higher temperatures and for other materials, the latest edition of the ASME Code should be referenced. 

Maximum Allowable Stress Value for Common Steels... 

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