Sustained load tests

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... 

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 )...

The slow strain-rate test method (10 s" to 10" s" ) is an accelerated test method when compared with a sustained-load test method. The slow strain-rate test provides an "accelerated test for measuring the threshold when compared with the 5000 hours required by ASTM E 1681, for steels at a hardness of 50-52 HRC. 

Condition/ plications requiring high toughness and moderate Microstructure strength. Ti-621/0.8, on the basis of fractime appearance, is considered resistant to seawater stress corrosion. However, sustained-load tests on... 

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. 

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. 

Methyl methacrylate impregnated concrete-polymer specimens subjected to creep tests exhibited expansion under sustained load (negative creep), in contrast to contraction in length (positive creep) shown by ordinary concrete. If confirmed, this property could influence significantly the design of concrete-polymer structural and prefabricated members. 

In the static crack growth experiments reported in this chapter, the time period for the application of the sustained load (before the interruption of the test for crack observations) was also typically smaller than the transition time, tT. This permits the stress intensity factor K to be used as a characterizing parameter for static crack growth. 

Creep tests are run by subjecting a grouted soil sample to a sustained load less than the short-term Unconfined Compression strength, until failure occurs. As the sustained load decreases in value, the time to failure increases. Data from such tests plot as shown in Fig. 11.22, and indicate an asymptote at what may be called a creep endurance limit. For acrylamides these values will range from as low as 20% Unconfined Compression for triaxial tests at low lateral pressure to as high as 40% for triaxial tests at at rest lateral pressures. (For silicates, values may be taken from Fig. 11.11.)... 

Because of the time dependence of polymer properties, it is important that the time scale used in our standard tests corresponds exactly with that of the moment loading. That is, if the moment is to be a sustained load, then the tensile test should also be at very long loading, for example, in the equilibrium modulus region. 

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