22 Budynas-Nisbett:Shigley's I.Basics 1.Introduction to T©The McGraw-Hill Mechanical Engineering Mechanical Engineering Companies,2008 Design,Eighth Edition Design 16 I Mechanical Engineering Desigr to zero.But the strength remains as one of the properties of the spring.Remember,then, that strength is an inherent property of a part,a property built into the part because of the use of a particular material and process. Various metalworking and heat-treating processes,such as forging,rolling,and cold forming,cause variations in the strength from point to point throughout a part.The spring cited above is quite likely to have a strength on the outside of the coils different from its strength on the inside because the spring has been formed by a cold winding process,and the two sides may not have been deformed by the same amount. Remember,too,therefore,that a strength value given for a part may apply to only a par- ticular point or set of points on the part. In this book we shall use the capital letter S to denote strength,with appropriate subscripts to denote the type of strength.Thus,S,is a shear strength,Sy a yield strength,and S an ultimate strength. In accordance with accepted engineering practice,we shall employ the Greek let- ters o(sigma)and r(tau)to designate normal and shear stresses,respectively.Again, various subscripts will indicate some special characteristic.For example,o is a princi- pal stress,oy a stress component in the y direction,and o,a stress component in the radial direction. Stress is a state property at a specific point within a body,which is a function of load,geometry,temperature,and manufacturing processing.In an elementary course in mechanics of materials,stress related to load and geometry is emphasized with some discussion of thermal stresses.However,stresses due to heat treatments,molding, assembly,etc.are also important and are sometimes neglected.A review of stress analy- sis for basic load states and geometry is given in Chap.3. 1-10 Uncertainty Uncertainties in machinery design abound.Examples of uncertainties concerning stress and strength include Composition of material and the effect of variation on properties. Variations in properties from place to place within a bar of stock. Effect of processing locally,or nearby,on properties. Effect of nearby assemblies such as weldments and shrink fits on stress conditions. Effect of thermomechanical treatment on properties. Intensity and distribution of loading. Validity of mathematical models used to represent reality. .Intensity of stress concentrations. Influence of time on strength and geometry ·Effect of corrosion. ·Effect of wear.. Uncertainty as to the length of any list of uncertainties. Engineers must accommodate uncertainty.Uncertainty always accompanies change. Material properties,load variability,fabrication fidelity,and validity of mathematical models are among concerns to designers. There are mathematical methods to address uncertainties.The primary techniques are the deterministic and stochastic methods.The deterministic method establishes aBudynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition I. Basics 1. Introduction to Mechanical Engineering Design 22 © The McGraw−Hill Companies, 2008 16 Mechanical Engineering Design to zero. But the strength remains as one of the properties of the spring. Remember, then, that strength is an inherent property of a part, a property built into the part because of the use of a particular material and process. Various metalworking and heat-treating processes, such as forging, rolling, and cold forming, cause variations in the strength from point to point throughout a part. The spring cited above is quite likely to have a strength on the outside of the coils different from its strength on the inside because the spring has been formed by a cold winding process, and the two sides may not have been deformed by the same amount. Remember, too, therefore, that a strength value given for a part may apply to only a par￾ticular point or set of points on the part. In this book we shall use the capital letter S to denote strength, with appropriate subscripts to denote the type of strength. Thus, Ss is a shear strength, Sy a yield strength, and Su an ultimate strength. In accordance with accepted engineering practice, we shall employ the Greek let￾ters σ (sigma) and τ (tau) to designate normal and shear stresses, respectively. Again, various subscripts will indicate some special characteristic. For example, σ1 is a princi￾pal stress, σy a stress component in the y direction, and σr a stress component in the radial direction. Stress is a state property at a specific point within a body, which is a function of load, geometry, temperature, and manufacturing processing. In an elementary course in mechanics of materials, stress related to load and geometry is emphasized with some discussion of thermal stresses. However, stresses due to heat treatments, molding, assembly, etc. are also important and are sometimes neglected. A review of stress analy￾sis for basic load states and geometry is given in Chap. 3. 1–10 Uncertainty Uncertainties in machinery design abound. Examples of uncertainties concerning stress and strength include • Composition of material and the effect of variation on properties. • Variations in properties from place to place within a bar of stock. • Effect of processing locally, or nearby, on properties. • Effect of nearby assemblies such as weldments and shrink fits on stress conditions. • Effect of thermomechanical treatment on properties. • Intensity and distribution of loading. • Validity of mathematical models used to represent reality. • Intensity of stress concentrations. • Influence of time on strength and geometry. • Effect of corrosion. • Effect of wear. • Uncertainty as to the length of any list of uncertainties. Engineers must accommodate uncertainty. Uncertainty always accompanies change. Material properties, load variability, fabrication fidelity, and validity of mathematical models are among concerns to designers. There are mathematical methods to address uncertainties. The primary techniques are the deterministic and stochastic methods. The deterministic method establishes a