Stress and temperature

Fig. 1. Stress and temperature ranges of apphcation for Zr02 (—), Si N (-), and SiC (--) advanced stmctural ceramics. To convert MPa to psi,...

Times-to-failure are normally presented as creep-rupture diagrams (Fig. 17.9). Their application is obvious if you know the stress and temperature you can read off the life if you wish to design for a certain life at a certain temperature, you can read off the design stress. 

This competition between mechanisms is conveniently summarised on Deformation Mechanism Diagrams (Figs. 19.5 and 19.6). They show the range of stress and temperature (Fig. 19.5) or of strain-rate and stress (Fig. 19.6) in which we expect to find each sort of creep (they also show where plastic yielding occurs, and where deformation is simply elastic). Diagrams like these are available for many metals and ceramics, and are a useful summary of creep behaviour, helpful in selecting a material for high-temperature applications. 

Over the present ranges of stress and temperature the alloy can be considered to creep according to the equation... 

The Larson-Miller parameters are plotted in Figure 11-4 for the speeified turbine blade alloys. A comparison of A-286 and Udimet 700 alloy curves reveals the difference in capabilities. The operational life (hrs) of the alloys can be compared for similar stress and temperature conditions. 

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 data for periods at 100 and 1000 hours (or more, if available) covering stress and temperature conditions closely comparable to those of product application. 

These recent tests were conducted at applied stress levels similar to those that might be experienced by ASME Section Vm, Division 2 vessels. Test exposure times exceeded 50,000 hours depending on applied stress and temperature. The test specimens were from weldments of thick section plates and represented base metal, weld metal, and heat-affected zone. Detrimental effects of hydrogen were found down to the Figure 1 limit of 850°F (454°C) at 2000 pounds per square inch absolute (14 megapascals) and 3000 pounds per square inch absolute (21 megapascals) hydrogen partial pressure. 

In pure metals at low stresses and temperatures, the gas-like mode is important, and the momentum carriers are electrons and phonons. For pure, simple metals there is essentially no shear bonding at the cores of dislocations, so the... 

Figure 5.44 Influence of applied stress and temperature on creep behavior. Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, 5th ed., p. 227. Copyright 2000 by John Wiley Sons, Inc.

Thus the effects of the rate of application of stress and the ambient temperature must be recognized when polymers are used as structural materials, and definite rates and temperatures must be specified for tests, such as those for tensile and flexural strengths cited in Chapter 3. A knowledge of the structure of polymers is essential for the understanding of these effects, which differ from the effects of stress and temperature on all other materials of construction. 

Creep rates of three glassy polymers are much greater during electron irradiation than before or after. Radiation heating is eliminated as a possible cause. Essentially the same concentration of unpaired electrons and ratio of cross-linking to scission were found in polystyrene samples in the presence or absence of stress. The effects of radiation intensity, stress, and temperature on creep during irradiation are examined. The accelerated creep under stress is directly related to a radiation-induced expansion in the absence of stress. This radiation expansion is decreased by increase in temperature or plasticizer content and decrease in sample thickness. It is concluded that gas accumulation within the sample during irradiation causes both the expansion under no stress and the acceleration of creep under stress. 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

In all these studies uniform stress and uniform temperature are assumed in the sample and the tests are of the Kelvin relation connecting stress and temperature. 

The physical factors include mechanical stresses and temperature. As discussed above, IFP is uniformly elevated in solid tumors. It is likely that solid stresses are also increased due to rapid proliferation of tumor cells (Griffon-Etienne et al., 1999 Helmlinger et al., 1997 Yuan, 1997). The increase in IFP reduces convective transport, which is critical for delivery of macromolecules. The temperature effects on the interstitial transport of therapeutic agents are mediated by the viscosity of interstitial fluid, which directly affects the diffusion coefficient of solutes and the hydraulic conductivity of tumor tissues. The temperature in tumor tissues is stable and close to the body temperature under normal conditions, but it can be manipulated through either hypo- or hyper-thermia treatments, which are routine procedures in the clinic for cancer treatment. 

Goetze C, Evans B (1979) Stress and temperature in the bending lithosphere as constrained by experimental rock mechanics. Geophys J Royal Astr Soc 59(3) 463-478... 

In Eyring s theory, yielding occurs by stress and temperature-activated jumps of molecular segments (McCrum et al., 1992). The applied stress reduces the activation barrier (AH) and segment motions define an activation volume, V. ... 

This new book focuses on the fundamental understanding of composite materials at the microscopic scale, from designing microstructural features, to the predictive equations of the functional behaviour of the stmcture for a specific end-application. The papers presented discuss stress and temperature-related behavioural phenomena based on knowledge of physics of microstructure and microstructural change over time. 

These equations are solved numerically under the assumptions of velocity, shear stress, and temperature continuity at all interfaces. They use the Sabia 4-parameter viscosity model (69), because of its ability to include the Newtonian plateau viscosity, which is important for multilayer extrusion, because of the existence of low shear-rate viscosities at the interfaces. 

For practical applications empirically determined creep data are being used, such as D(t) or, more often, E(t) curves at various levels of stress and temperature. The most often used way of representing creep data is, however, the bundle of creep isochrones, derived from actual creep curves by intersecting them with lines of constant (log) time (see Figure 7.7). These cr-e-curves should be carefully distinguished from the stress-strain diagram discussed before, as generated in a simple tensile test ... 

Thixotropy is the tendency of certain substances to flow under external stimuli (e.g., mild vibrations). A more general property is viscoelasticity, a time-dependent transition from elastic to viscous behavior, characterized by a relaxation time. When the transition is confined to small regions within the bulk of a solid, the substance is said to creep. A substance which creeps is one that stretches at a time-dependent rate when subjected to constant stress and temperature. The approximately constant stretching rates at intermediate times are used to characterize the creeping characteristics of the material. 

Resistance to process conditions, such as corrosion, erosion, stress and temperature. 

---