Mechanical behavior creep

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. 

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. 

The creep of a viscoelastic body or the stress relaxation of an elasacoviscous one is employed in the evaluation of T] and G. In such studies, the long-time behavior of a material at low temperatures resembles the short-time response at high temperatures. A means of superimposing data over a wide range of temperatures has resulted which permits the mechanical behavior of viscoelastic materials to be expressed as a master curve over a reduced time scale covering as much as twenty decades (powers of ten). 

The mechanical properties of Shell Kraton 102 were determined in tensile creep and stress relaxation. Below 15°C the temperature dependence is described by a WLF equation. Here the polystyrene domains act as inert filler. Above 15°C the temperature dependence reflects added contributions from the polystyrene domains. The shift factors, after the WLF contribution, obeyed Arrhenius equations (AHa = 35 and 39 kcal/mole). From plots of the creep data shifted according to the WLF equation, the added compliance could be obtained and its temperature dependence determined independently. It obeyed an Arrhenius equation ( AHa = 37 kcal/mole). Plots of the compliances derived from the relaxation measurements after conversion to creep data gave the same activation energy. Thus, the compliances are additive in determining the mechanical behavior. 

These differences in the mechanical behavior are not reflected, within the experimental error, in the temperature dependence of the mechanical properties. As shown by the examples of Figures 1 and 3, the relaxation modulus and creep compliance data showed very little scatter and could be shifted smoothly into superposition along the logarithmic time axis. The amounts of shift, log Or, required to effect superposition are plotted against the temperature, T, in Figure 5 for the relaxation data, and in... 

From a practical point of view, the main consequence of physical ageing by structural relaxation is embrittlement (decrease in fracture resistance Chapter 12). For the other aspects of mechanical behavior, ageing has either no effect or a favourable effect (increase of relaxation times, leading to a decrease of creep or relaxation rates). This is the reason why, in most thermoset applications, the knowledge of short-term properties is considered to be sufficient for engineering design, as far as fracture and durability are not concerned. 

This is because although 0 = (10), in general, cr(10) oQ (it will usually be less). In principle, the quantities we have defined, E(t), Dit), Gif), and J(i), provide a complete description of tensile and shear properties in creep and stress relaxation (and equivalent functions can be used to describe dynamic mechanical behavior). Obviously, we could fit individual sets of data to mathematical functions of various types, but what we would really like to do is develop a universal model that not only provides a good description of individual creep, stress relaxation and DMA experiments, but also allows us to relate modulus and compliance functions. It would also be nice to be able formulate this model in terms of parameters that could be related to molecular relaxation processes, to provide a link to molecular theories. 

In 1935 Erwin SchrOdinger published an essay questioning whether strict adherence to the Copenhagen interpretation can cause the weirdness of the quantum world to creep into everyday reality. He speculated on how the principle of superposition, which is so fundamental for the quantum-mechanical behavior of microscopic systems, might possibly affect the behavior of a large-scale object. 

The analysis of full rheological curve illustrates how the complex mechanical behavior can be subdivided into several regions, and how within each of these regions it can be represented by a simple model that utilizes only one or two constant parameters. For this reason, such phenomena as Schwedov s creep and Bingham s viscoplastic flow, whose molecular mechanisms are so different, can be described by substantially different parameters within otherwise the same model. Such subdivision of complex behavior into a finite number of simpler constituents with particular quantitative characteristics illustrates the universal role of macrorheology. At the same time, detailed description of a mechanism involved in each of these elementary stages requires the use of molecular-kinetic concepts and may be characterized as a microrheological approach. 

While IPNs can be and have been made extremely tough and impact resistant, many of the proposed applications involve such diverse fields and sound and vibration damping, biomedical materials, and non-linear optics. This is because the presence of crosslinks in both polymers reduces creep and flow, allowing relatively stable materials with a wide range of moduli to be prepared. Thus, those materials with leathery mechanical behavior, combinations of elastomers and plastics, are especially interesting to scientists, inventors, and engineers. 

The mechanical behavior of NijAl and NijAl-based alloys, including creep, fa-... 

Alnico alloys are brittle and hard, they can only be machined by grinding, spark erosion, and electrochemical milling, and they resist atmospheric corrosion well up to 500 °C (Fiepke, 1990). The mechanical behavior, in particular creep, of Alnico-type, Fe-Ni- Al alloys has been studied in detail, and both Fe-rich alloys with precip-... 

The mechanical behavior of FeCo has been studied intensively since the order-disorder transition allows a direct study of the effects of ordering on the mechanical properties including creep, fatigue and... 

The restraining influence of the crystallite alters the mechanical behavior by raising the relaxation time T and changing the distribution of relaxation and retardation times in the sample. Consequently, there is an effective loss of short T, causing both the modulus and yield point to increase. The creep behavior is also curtailed and stress relaxation takes place over much longer periods. Semicrystalline polymers are also observed to maintain a relatively higher modulus over a wider temperamre range than an amorphous sample. 

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