Working stress

Correlation between the body forces and the stress state in the head was investigated both by the strain gauge method and the optical coat work stress examination method, and the magnetic measurements were performed at the same time. 

During the optical coat work stress examination method the upper plate of the head of some of the bolts was covered with an optical coat work (Fig. 4). On the head of some other bolts strain gauges were stuck which measured the plain biaxial stress state in the middle of the top surface of the head of the bolt (3.5 x 3 mm). The magnetic probe detected average stresses up to 0.1 mm depth in an area of 14 mm diameter in the middle of the head of the bolt. 

The distribution of the main stresses during the optical coat work stress examination method is illustrated by a section of a stress state in Fig. 5. 

In practice a uniform distribution of the working stress cannot be achieved, but it may be approached by various methods of constmction such as compound shrinkage, tape winding, and autofrettage which have their origin in the design of ordnance. 

Find the maximum allowable working stress if this is to equal... 

However, too great a separation, and overdesign may oeeur. The overload eondition is represented by a unique stress, whieh is very mueh greater than the working stress, applied suddenly whieh eauses only the weak link to failure due to stress rupture. 

Table 3.6 gives commonly used maximum working stresses for various grades of cast irons up to 600°C. 

Tabic 3.6. Maximum Working Stresses for Various Grades of Cast Iron up to 600 C... 

For most traditional materials, the objective of the design method is to determine stress values which will not cause fracture. However, for plastics it is more likely that excessive deformation will be the limiting factor in the selection of working stresses. Therefore this chapter looks specifically at the deformation behaviour of plastics and fracture will be treated separately in the next chapter. 

S = allowable unit working stress in pounds per square inch T z= Net thickness in inches... 

The allowable working stress, based on successful performance at conditions of product usage. 

An important subject to introduce concerns the allowable working stress for a spe-... 

The viscoelastic nature of the material requires not merely the use of data sheet information for calculation purposes, but also the actual long-term performance experience gained that can be used as a guide. The allowable working stress is important for determining dimensions of the stressed area and... 

Selecting an allowable continuous working stress at the required temperature must be a procedure that allows for making an estimation of the elongation at the end of the product s life. For example, if a product will be stressed to 1,700 psi at a temperature of 66°C (150°F), and data are available for 2,000 psi stress at 71°C (160°F), this information plotted on log-log paper should allow to extrapolate the long-term behavior of the material. 

The next step is to determine an allowable working stress. This is done by using a safety factor usually of IV2 to 2V2 on the yield strength or tensile strength. If the type of stress is clearly defined, the IV2 factor is adequate otherwise, it should be higher (Chapter 2, Safety Factor). 

The final step is to calculate the elongation that the product would experience under the selected allowable working stress to see if such an elongation would permit the proper functioning of the product. The elongation could conceivably become the limiting component, and the working stress can be calculated from ... 

When materials are evaluated against each other, the flexural data of those that break in the test cannot be compared unless the conditions of the test and the specimen dimensions are identical. For those materials (most TPs) whose flexural properties are calculated at 5% strain, the test conditions and the specimen are standardized, and the data can be analyzed for relative preference. For design purposes, the flexural properties are used in the same way as the tensile properties. Thus, the allowable working stress, limits of elongation, etc. are treated in the same manner as are the tensile properties. 

The stress levels and the temperature of the test for a material is determined by the manufacturer. The guiding determinants are the continuous allowable working stress at room temperature and the continuous allowable working stress at temperatures of potential applications. 

Pipe schedule number = 1000P/S, approximately, where P is the internal pressure psig and S is the allowable working stress (about 10,000 psi for A120 carbon steel at 500°F). Schedule 40 is most common. 

Allowable working stresses are one-fourth of the uitimate strength of the materiai. 

For brittle materials the UTS is generally used as the basis for obtaining the working stress ow = cu/nu where ou is the UTS and nu is a factor of safety. The UTS may be used as a basis for arriving at the working stress even for ductile materials in situations where some permanent deformation is acceptable. 

Estimate the safe working pressure for a 4 in. (100 mm) dia., schedule 40 pipe, carbon steel, butt welded, working temperature 100°C. The safe working stress for butt welded steel pipe up to 120°C is 6000 lb/in2 (41.4 N/mm2). 

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