Long-term creep behavior

Long-term tests were carried out [50] for 10000 h (14 months) at 20 in a room adapted specially for this purpose. No other experiments were performed in that room. The contents of PLC in the PP + PLC blends were 0, 5, 10, 15 and 20 wt%. In all tests the stress level was 5 MPa. 

In the present case it is possible to verify directly the applicability 

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The thermal behavior of tensile strength values in Fig. 13.17 tend to mirror the effects of temperature on long-term creep behavior. Obviously, load-bearing capability to resist creep is greatly reduced by elevation of ambient temperature. 

Time-Temperature Superposition In order to predict the long-term creep behavior based on short-term creep measurements, it is generally assumed that the polymer does not change its structure with time, and consequently the time-temperature superposition (TTS) principle can be adopted. ITS has been used to obtain the master curves for creep compliance against time. According to TTS, the creep at a given temperature (To) is related to the creep at another temperature... 

Long time dynamic load involves behaviors such as creep, fatigue, and impact. T vo of the most important types of long-term material behavior are more specifically viscoelastic creep and stress relaxation. Whereas stress-strain behavior usually occurs in less than one or two hours, creep and stress relaxation may continue over the entire life of the structure such as 100,000 hours or more. 

Viscoelastic and rate theory To aid the designer the viscoelastic and rate theories can be used to predict long-term mechanical behavior from short-term creep and relaxation data. Plastic properties are generally affected by relatively small temperature changes or changes in the rate of loading application. 

Binder, K., Non-Isothermal Long Term Creep (17 yrs.) Behavior of Thermoplastics Outdoors, Kunststoffe, No. 1988. 

Consider a specimen of length Ls containing a very large number of wholly intact fibers. A stress a is suddenly applied to the specimen parallel to the fibers. The temperature has already been raised to the creep level and is now held fixed. Upon first application of the load, some of the fibers will break. The sudden application of the load means that the initial response is elastic. This elastic behavior has been modeled by Curtin,16 among others, but details will not be given here. If the applied stress exceeds the ultimate strength of the composite in this elastic mode of response, then the composite will fail and long-term creep is obviously not an issue. However, it will be assumed that... 

Struik, L. C. E., Mechanical behavior and physical aging of semicrystalline polymers 3, Prediction of long-term creep from short-time tests. Polymer, 30, 799—814 (1989a). 

It is now well-recognized that pendant chains make a significant contribution to the long-term relaxation behavior of cross-linked rubbers as seen in stress relaxation and creep experiments. The molecular mechanism accountable for this long-term process is the diffusion of pendant chains in the presence of entanglements. 

Characterization of Long-Term Creep-Fatigue Behavior for Glass Fiber-Reinforced Polypropylene... 

The section that follows discusses two important types of long-term viscoelastic behavior creep and stress relaxation. These forms of creep may occur over the life of a part or structure on a time scale as long as 100,000 hours or more (see Figs. 3-28 to 3-31). 

Table 9.3. Parameters of the Burgers model fitting the creep behavior of SBS block copolymer and relative nanocomposites. Long-term creep tests were performed up to 225 h at a constant applied stress of 8.5 MPa [37]...

Recent progresses concerning long-term creep and fatigue behavior of austenitic stainless steels... 

One of the main challenges for some reactor components in austenitic stainless steels at high-temperature in-service conditions is the demonstration of their behavior up to 60 years. The evaluation of creep lifetime of these stainless steels requires on the one hand to carry out very long-term creep tests and on the other hand to understand and model the damage mechanisms in order to propose physically based predictions toward 60 years of service. 

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