Safe stress

A centrifuge with a phosphor bronze basket, 380 mm in diameter, is to be run at 67 Hz with a 75 mm layer of liquid of density 1200 kg/m3 in the basket. What thickness of walls are required in the basket The density of phosphor bronze is 8900 kg/m3 and the maximum safe stress for phosphor bronze is 87.6 MN/m2. 

Safety Factors Technically, a safety factor (SF) refers to the ratio of a failure-producing load to the maximum safe stress a material may carry. Some call the maximum safe stress the allowable stress. Failure may not be by rupture or fracture. A failure could be a change in area or properties of the material that affect the load-carrying capacity and its safety. 

The most significant type of SCC data is determination of the safe stress level below which failme will not occur. Other data forms are time to failme, number of failmes, and percent failure (or survival). SCC failure times are not normally distributed, but the logarithm of failure times tend to have a normal distribution. This permits probability plots of cumulative percent failme (or survival) versus exposme time. Refer to Chapter 25. 

Plastics Type Unfilled Safe Stress Glass-filled ... 

Alternatively, a safe stress or working stress, cr, is used instead of design stress. This safe stress is based on the yield strength of the material and is defined as the yield strength divided by a factor of safety, N, or... 

As a result of uncertainties in both measured mechanical properties and in-service applied stresses, design or safe stresses are normally utilized for design purposes. For ductile materials, safe (or working) stress is dependent on yield strength and factor of safety as described in Equation 6.24. 

The majority of polymer flow processes are characterized as low Reynolds number Stokes (i.e. creeping) flow regimes. Therefore in the formulation of finite element models for polymeric flow systems the inertia terms in the equation of motion are usually neglected. In addition, highly viscous polymer flow systems are, in general, dominated by stress and pressure variations and in comparison the body forces acting upon them are small and can be safely ignored. 

After long periods of time at operating temperature, the britde—ductile transition temperature in autoclave steels increases (13). At temperatures much above 200°C for the solutions and fiHs used in ordinary hydrothermal processes, pressures and hence stresses in autoclaves can cause faHure of metal in the brittie state. OrdinarHy, the brittie region is weH below these temperatures but careful monitoring of the brittie—ductile transition of the steel is necessary for safe autoclave use over many years. 

First, the pressure vessel must be safe from plastic collapse that is, the stresses must everywhere be below general yield. Second, it must not fail by fast fracture if the largest cracks it could contain have length 2a (Fig. 16.4), then the stress intensity K CTV must everywhere be less than K. Finally, it must not fail by fatigue the slow growth of a crack to the critical size at which it runs. 

It is worrying that a vessel which is safe when it enters service may become unsafe by slow crack growth - either by fatigue or by stress corrosion. If the consequences of catastrophic failure are very serious, then additional safety can be gained by designing the vessel so that it will leak before it breaks (like the partly inflated balloon of Chapter 13). Leaks are easy to detect, and a leaking vessel can be taken out of service and repaired. How do we formulate this leak-before-break condition ... 

In some materials and environments, cracks grow steadily under a constant stress intensity K which is much less than (Fig. 23.8). This is obviously dangerous a structure which is safe when built can become unsafe with time. Examples are brass in ammonia, mild steel in caustic soda, and some A1 and Ti alloys in salt water. 

Fracture mechanics can be applied to this problem, much as it is to fatigue. We use only the final result, as follows. If the standard test which was used to measure (Tts takes a time f(test), then the stress which the sample will support safely for a time t is... 

A meehanieal eomponent is eonsidered safe and reliable when the strength of the eomponent, S, exeeeds the value of loading stress, L, on it (Rao, 1992). When the loading stress exeeeds the strength, failure oeeurs, the reliability of the part, R, being related to this failure probability, P, by equation 4.26 ... 

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