Creep rupture tests

The usually stationary use of plastics pipes places special requirements on their long-term strength, guaranteeing a service life of at least 50 years. The serviceability of a pressurized plastic pipeline depends on the particular operating conditions, i.e., on mechanical loading by Internal pressure and occasionally additional temperature and media loading [162]. 

The strength of plastic pipes Is usually characterized by their hydrostatic (internal 

For the internal pressure creep resistance, lifetime at constant internal pressure load is determined. Subsequently, internal pressure creep resistance is plotted in the internal pressure creep diagram as a function of lifetime (s. Section 2.5.4). If the test is performed in air as the medium, information about thermal oxidative resistance is obtained. 

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If the values for Uq and y for the material are not known then a series of creep rupture tests at a fixed temperature would permit these values to be determined from the above expression. The times to failure at other stresses and temperatures could then be predicted. 

Creep rupture tests on a particular grade of uPVC at 20°C gave the following results for applied stress, ct, and time to failure, /. 

Creep rupture. Creep-rupture data are obtained in the same way as creep data except that higher stresses are used and the time is measured to failure (Figs. 2-28 and 29). The strains are sometimes recorded, but this is not necessary for creep rupture. The results are generally plotted as the log stress versus log time to failure (110). In creep-rupture tests it is the material s behavior just prior to the rupture that is of primary interest. In these tests a number of samples are subjected to different levels of constant stress, with the time to failure being determined for each stress level. General technical literature and product data sheets seldom provide a complete description of a material s behavior prior to rupture. It should include the development of any crazing and stress whitening, its strain-time... 

Time to rupture can be predicted by using the accelerated times generated by the creep data, and the creep-rupture characteristic generated by performing twelve of these tests over a range of loads. Conventional long-term creep strain and creep-rupture tests have so far confirmed the validity of the predictions for polyester fibres. Comments on the method have been published by Greenwood and Voskamp [10]. 

Life is influenced by chemical and physical changes occurring in the insulation with time, e.g., oxidation. Under certain conditions, it is possible to initiate a thermal runaway failure mechanism. This power law is the electrical analogue of creep-rupture testing, except that the material fails under a sustained voltage instead of a sustained mechanical load. 

F. C. Monkman and N. J. Grant, An Empirical Relationship between Rupture Life and Minimum Creep Rate in Creep-Rupture Tests, Proc. ASTM, 56, 593-620 (1956). 

Creep of fibers also occurs much more rapidly than happens in bulk materials, in part because of the much smaller grain sizes present in the fibers. Creep and creep rupture test techniques for composites are described elsewhere in this volume (Chapter 4 Weiderhom and Fuller) but performing such tests on small diameter fibers requires somewhat different apparatus.57 To date, only a small number of fiber compositions have been characterized.58... 

The tensile standard uses the tensile dumbbells standardized in ISO 527-2 and requires that the load applied be maintained within I % of the desired load. The loading mt chan-ism must be designed both to apply the load smoothly and without overshoot and to do this consistently. In creep testing it is essential to avoid any shocks at any time during the test. The strain must be measured to an accuracy of 0.01mm, and the elapsed lime to 0.1 %. Contactless optical extensometers are recommended, e.spccially for creep rupture tests. The standard slates that strain gauges are only suitable w-here they can be attached by adhesive, and only when the adhesion quality is constant throughout the lest. 

A creep rupture test. specific to plastic pipes is given in ASTM D1598 [112], in which a length of pipe is pressurized by gas or liquid as appropriate and the time to failure is recorded. Pressure stability for up to a 100 hour test duration must be 0.5 o. Above this... 

Creep rupture tests of plastic pipe, described in Chapter 14, are typical of whole-lifetime testing. It is difficult to manufacture plastic products without incorporating foreign particles, of size about 0.1mm, such as undispersed pigment or stabiliser, or metal wear fragments from extruder screws. The... 

Failure modes in a pressurised MDPE pipe (a) Ductile parrot s beak fracture (b) brittle section through a welded joint that has failed in a creep rupture test in water at 80 °C. 

As experience with creep rupture testing of polyolefins has been gained, elevated temperature tests have been used for quality control purposes, and standards set using such tests, i.e. the creep rupture time for pipes for natural gas distribution must exceed 170 h at 80 °C and a hoop stress of 3 MPa. Care must, however, be exercised if a polyethylene made by a different process is introduced, because the use temperature is close to 10 °C when the pipe is buried in the ground the slope of the Arrhenius plot varies between different polyethylenes. 

The results of creep rupture tests on a drawn PMMA sheet with a lovyer level of molecular orientation (birefringence of only (HXX)6) are presented in Fig. 18. This shows a high level of anisotropy of creep rupture which contrasts with the relatively low level of anisotropy of stiffness found in the more highly oriented sample. The level of molecular orientation, even in the sample with lower birefringence, must therefore be regarded as significant, and the low anisotropy of stiffness must be... 

Lug] TEM, creep rupture tests 400°C, 10 mass% Ni, 1.5 mass% Fe... 

The most important goal for each NASA-developed CMC system was to be able to operate under potential component stress levels for long time at its selected upper use temperature (UUT). To evaluate this capability, tensile test specimens from the various CMC panels were subject to creep-rupture testing in ambient air at their goal UUT and at stresses of -60% of their room-temperature cracking stress. The primary performance objective was to demonstrate greater than 500-hour life without specimen rupture. Since high-temperature mpture of an initially uncracked CMC is typically controlled by CMC... 

Creep rupture tests are used to measure the long-term response of a material to a continuously applied stress at a given temperature. The ideal material should be able to support significant stresses for extended periods of time without accumulated permanent strain or breakage. The SiOC-Nextel 312 BN 2-D composites were tested in limited stress... 

TABLE 6. Tensile Creep Rupture Tests at 566°C/1050°F of SiOC Nextel 312 BN 2-D Composites... 

Long-term durability of adhesively bonded joints may require resistance to a number of individual or combined degradation modes, including environmental attack, fatigue and time-dependent failures. Time-dependent failure mechanisms are often characterized nsing either a strength approach, involving creep and creep-rupture tests, or a fracture approach, in which debond rate is determined. In creep-rupture tests, adhesive joints are subjected to... 

Creep rupture n. The rupture of a plastic under a continuously apphed stress that is less than the short-time strength. This phenomenon is caused by the viscoelastic nature of plastics. Creep-rupture tests are generally conducted over a series of loads ranging from those causing rupture within a few minutes to those requiring several years or more. 

The results of creep rupture tests on single-lap joints yielded the reduction factor, /l, for an adhesive bond subjected to constant static loading (Fig. 30). The strength of the adhesive decreases with increasing exposure. In constant-load tests of this kind, particularly at higher temperatures, creep strain is observed in the adhesive layer where a certain initial load is exceeded. 

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