Long-term creep

Creep. The creep characteristic of plastic foams must be considered when they are used in stmctural appHcations. Creep is the change in dimensions of a material when it is maintained under a constant stress. Data on the deformation of polystyrene foam under various static loads have been compiled (158). There are two types of creep in this material short-term and long-term. Short-term creep exists in foams at all stress levels however, a threshold stress level exists below which there is no detectable long-term creep. The minimum load required to cause long-term creep in molded polystyrene foam varies with density ranging from 50 kPa (7.3 psi) for foam density 16 kg/m (1 lb /ft ) to 455 kPa (66 psi) at foam density 160 kg/m (10... 

Resistance to common aircraft fluids such as water, salt water, hydraulic fluid and jet fuel is determined by additional shear testing after exposure to these fluids. Since adhesives are typically only exposed at bond edges, are protected by secondary primers and enamels and are not expected to be exposed to these fluids (save for water) for extended periods, exposure time prior to testing is relatively short. Lastly, the adhesive is tested for propensity to creep rupture under load in standard and aggressive environments. This testing indicates whether the polymer is crosslinked sufficiently to resist long-term creep under low load. 

Basics Creep data can be very useful to the designer. In the interest of sound design-procedure, the necessary long-term creep information should be obtained on the perspective specific plastic, under the conditions of product usage (Chapter 5, MECHANICAL PROPERTY, Long-Term Stress Relaxation/Creep). In addition to the creep data, a stress-strain diagram under similar conditions should be obtained. The combined information will provide the basis for calculating the predictability of the plastic performance. 

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

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]. 

The formula applies to results from both long-term creep and short-term forced vibration tests, but by itself provides a poor fit at longer durations. 

Polymers present a more or less plastic behaviour under stresses, leading to lower modulus and ultimate strength retentions, and higher long-term creep or relaxation when the temperatures rise. 

Even though the above work is providing a stable, non-sintering, creep-resistant anode, electrodes made with Ni are relatively high in cost. Work is in progress to determine whether a cheaper material, particularly Cu, can be substituted for Ni to lower the cost while retaining stability. A complete substitution of Cu for Ni is not feasible because Cu would exhibit more creep than Ni. It has been found that anodes made of a Cu - 50% Ni - 5% A1 alloy will provide long-term creep resistance (36). Another approach tested at IGT showed that an "IGT" stabilized Cu anode had a lower percent creep than a 10% Cr - Ni anode. Its performance was about 40 to 50 mV lower than the standard cell at 160 mA/cm. An analysis hypothesized that the polarization difference could be reduced to 32 mV at most by pore structure optimization (37). 

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... 

Fig. 9.2 Threshold for long-term creep of a uniaxially reinforced composite as a function of the Weibull modulus for the fiber strength distribution.

In selecting the materials ( W), major emphasis is given to neutron activation and radiation damage effects in materials close to the plasma and further considerations are given to other factors such as corrosion, long-term creep strength, fabrication technology, and cost. 

Table 10.4 Minimum load to cause long-term creep in molded polystyrene foam"...

The material chosen for the IHX and die steam reformer of the Japanese HTTR is HASTELLOY XR. Corrosion tests and long-term creep tests in helium atmosphere in the temperature range 800 - 1000 C have been conducted reaching 50,000 hours. No significant degradation in creep properties was observed even after carburization. Creep rupture was found to be caused by nucleation, growth, and link-up of grain boundary cavities [26]. 

KURATA, Y., et al.. Evaluation of Long-Term Creep Properties of Hastelloy XR in Simulated High-Temperature Gas-Cooled Reactor Helium, (3rd JAERI Symp., Oarai, 1996), Proc. JAERI-Conf 96-010, Japan Atomic Energy Research Institute (1996) 338-352. 

The long-term creep curve at 20 °C is just the main curve of creep compliance vs. time at the reference temperature of Og =20 °C. The parameters in shift factor can be detennined according to the main curve and creep curves at other temperatures. By parallel moving the curves at temperatures 60°C, 80°C, 100°C and 150°C in positive direction of abscissa t and finding out the corresponding points on the main curve where the J(t) values are the same as those before moving curves, we could find out the corresponding values of t y at the above reference temperatures and the four other temperatures. With eq. (25) and the four additional simultaneous equations the parameters C , C/, C, and Ca, were solved as listed in Table 1. 

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). 

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