Sustained load

The sum of the longitudinal stress (in the corroded condition) caused by pressure weight or other sustained loadings is not allowed to exceed Where the sum of these stresses is less than the difference between and this sum may be added to the term 0.25 in equation 25. 

As contrasted with stress from sustained loads such as internal pressure or weight, displacement stresses may be permitted to cause hm-ited overstrain in various portions of a piping system. When the system is operated initially at its greatest displacement condition, any yielding reduces stress. When the system is returned to its origin condition, there occurs a redistribution of stresses which is referred to as self-springing. It is similar to cold springing in its effects. 

Fracture Mechanics Tests One problem of both sustained load and slow strain-rate tests is that they do not provide a means of predicting the behaviour of components containing defects (other than the inherent defect associated with the notch in a sustained load test). Fracture mechanics provides a basis for such tests (Section 8.9), and measurements of crack velocity as a function of stress intensity factor, K, are widely used. A typical graph of crack velocity as a function of K is shown in Fig. 8.48. Several regions may be seen on this curve. At low stress intensity factors no crack growth is... 

Wei, R. P. and Landes, J. D., Correlation between sustained-load and fatigue crack growth in high strength steels . Materials Research and Standards, 9, 25-27, 44, 46 (1969)... 

Fig. 8.105 Spring-loaded rig for sustained load testing of a steel specimen in gaseous hydrogen at high pressure (after Cavelt and van Ness )...

Delayed Failure see under Sustained Load Failure. 

The mechanical behavior of plastics on time-dependent applied loading can cause different important effects on materials viscoelasticity. Loads applied for short times and at normal rates (Chapter 2) causes material response that is essentially elastic in character. However, under sustained load plastics, particularly TPs, tend to creep, a factor that is included in the design analysis. 

Figure 8.12 General form of strain versus time under sustained load... 

Creep and stress-relaxation tests measure the dimensional stability of a material, and because the tests can be of long duration, such tests are of great practical importance. Creep measurements, especially, are of interest to engineers in any application where the polymer must sustain loads for long periods. Creep and stress relaxation are also of major importance to anyone interested in the theory of or molecular origins of Viscoelasticity. 

Esc Sustained axial force including the effects of weight, other sustained loads, and internal pressure N lb... 

B-3 LIMITS OF CALCULATED STRESSES DUE TO SUSTAINED LOADS AND DISPLACEMENT STRAINS... 

The standard methods for testing creep, the elongation and possible rupture of a plastic under sustained load, are ISO 899-1 [34] for tension and ISO 899-2 [35] for flexure. Tests last typically for 1,000 hours or six weeks. Tests at higher temperatures may be required either because of a higher service temperature or to provide a prediction of longer term behaviour by time-temperature shifting. 

Limits of Calculated Stresses due to Sustained Loads and Displacement Strains... 

Longitudinal stresses Sl- The sum of longitudinal stresses Sl in any component in a piping system, due to sustained loads such as pressure and weight, shall not exceed the product S W, where is the basic allowable stress at maximum metal temperature expected during the displacement cycle under analysis, and W is the weld joint strength reduction factor. 

D High-viscosity fuel for use in medium- and low-speed engines utilized in sustained load, constant-speed applications... 

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