Minimum stress

Fracture Mechanics Methods These have proved very usebd for defining the minimum stress intensity K[scc. t which stress corrosion cracking of high-strength, low-ductihty alloys occurs. They have so far been less successful when apphed to high-ductility alloys, which are extensively used in the chemicm-process industries. 

In Fig. 8.18, we illustrate this just sufficient distribution in comparison to a hypothetical flaw distribution for an actual material. In this example, we envision a solid of finite extent, which will have a single critical flaw that activates at a minimum stress tensile stress, the population of flaws which activate should increase rapidly, perhaps as illustrated in Fig. 8.18. In contrast, a flaw distribution just sufficient to satisfy the energy balance criterion increases smoothly as JV [Pg.294]

The basic guidelines for preventing cracking would seem to be to operate at minimum stress levels, at as low an HjS concentration as possible and to make sure that welding procedures are adequately specified and followed. Furthermore, extensive periods of operation at temperatures that might cause sigma phase formation should be avoided. 

Quite large elastic strains are possible with minimal stress in TPEs these are the synthetic rubbers. TPEs have two specific characteristics their glass transition temperature (7 ) is below that at which they are commonly used, and their molecules are highly kinked as in natural TS rubber (isoprene). When a stress is applied, the molecular chain uncoils and the end-to-end length can be extended several hundred percent, with minimum stresses. Some TPEs have an initial modulus of elasticity of less than 10 MPa (1,500 psi) once the molecules are extended, the modulus increases. 

Figures 4 and 5 give a broad indication of the relevant biomechanical properties of a number of flow sensitive biomaterials. In the case of the data shown in Fig. 5, the surface mechanical properties are lumped into a single measure of the surface integrity. Admittedly, in view of what has been said in the introduction about the viscoelastic nature of the wall material, the information given in Figs. 4 and 5 are oversimplistic. The data in Fig. 5 are based on reported critical minimum stresses (often expressed in terms of the mean bulk fluid stresses) at which physical damage is first observed. Figure 6 gives an indication of the...

Fig. 5. Estimated characteristic strength of typical biological particles of interest to biotechnology data are based on in-situ measurements of the minimum stresses necessary to cause permanent breakage of particles. For comparison data are shown based on Van der Waals and pendular liquid bridges between two 10-pm particles, 0.01 pm apart...

Whole body Variety and number of animals Chronic studies possible Minimum restraint Large historical database Controllable environment Minimum stress Minimum labor Messy Multiple routes of exposure skin, eyes, oral Variability of dose Cannot pulse exposure easily Poor contact between animals and investigators Capital intensive Inefficient compound usage Difficult to monitor animals during exposure Cleaning effluent air Inert materials Losses of test material Even distribution in space Sampling Animal care Observation Noise, vibration, humidity Air temperature Safe exhaust Loading Reliability... 

The reformer tubes typically operate at maximum temperatures of 1,600°F to 1,700°F and are designed for a minimum stress-to-rupture life of 100,000 operating hours. A 35/25 Ni/Cr alloy is used that is modified with niobium and microalloyed with trace elements such as titanium and zirconium. Smaller tube diameters provide better heat transfer and cooler walls. This reduces tube and fuel costs and increases tube life. But more tubes increases the pressure drop. The optimum inside tube diameter is 4 to 5 in. The wall thickness may be as low as 0.25 inch with a length of 40 to 45 ft. The lane spacing between tube rows must be enough to avoid flame impingement from the burners. Typical spacing is 6 to 8 feet. 

The minimum stress necessary to introduce yielding occurs when a single crystal is oriented such that (p = X = 45°, for which... 

In rotational instruments, one member (e.g., the cup in a concentric cylinder viscometer) rotates while the other (e.g., the bob) remains stationary. The sample is held, and sheared, in the gap between the two. In a controlled shear rate measurement, the rotational speed is constant, and the torque on one member caused by the viscous resistance to flow exerted by the sample is measured. In a controlled stress measurement, a constant torque is applied to one member and its speed of rotation measured. Controlled stress instruments are particularly useful for measuring yield stress, the minimum stress causing flow of a plastic material. 

A minimum stress [Pg.866]

Stress ratio. Rates of corrosion fatigue crack propagation generally are enhanced by increased stress ratio R, which is the ratio of the minimum stress to the maximum stress. 

A critical minimum stress exists, below which delayed cracking will not take place. The critical stress decreases with increase in hydrogen concentration. These effects are shown in Figure 6.61 for SAE 4340 steel (0.4% C) charged with hydrogen by cathodic polarization in sulfuric acid, then cadmium plated to help retain hydrogen, and finally... 

Figure 6.61 Delayed fracture times and minimum stress for cracking of 0.4% C steel for various hydrogen concentrations obtained by different baking times at 150°C of cathodically charged specimens (Craig)5...

Figure 6.69 Venn diagram illustrating the interrelationship among stress corrosion, corrosion fatigue, and hydrogen embrittlement R — ratio of minimum stress to maximum stress (Phull)5...

At the end of me treatment period, groups of animals are killed under minimum stress conditions, preferably by rapid decapitation. The time point 24 hours after last dosing is not always suitable, e.g. when changes in pituitary hormone contents need to be assessed, the time point 2 hours after last dosing is preferable. Satellite groups need to be included if necessary for me exact time course of changes after dosing. 

When smooth specimens of PS are cycled at 26.7 Hz in a tensile mode at a maximum stress of 17.2 MPa discontinuous crack growth bands similar to those shown in Fig. 8 are seen in the mirror region near the fracture source. These bands are also similar in morphology to those observed in testing notched PS specimens at a minimum stress that is maintained at one-tenth the maximum stress However, DCG bands were not seen when tests are carried out under a fully reversed stress of 17.2 MPa at... 

Fig. 14a and b. Fatigue fracture surfaces of HIPS tested at 21 Hz in a tensile mode. The maximum stress was 17.2 MPa and the minimum stress was 3.4 MPa a Low magnification scan b High magnification scan... 

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