【正文】 izontal as N approaches a very large number． Thus the endurance limit equals the stress level where the curve approaches a horizontal tangent． Owing to the large number of cycles involved， N is usually plotted on a logarithmic scale， as shown in Figure ． When this is done， the endurance limit value can be readily detected by the horizontal straight line． For steel， the endurance limit equals approximately 50% of the ultimate strength． However， if the surface finish is not of polished equality， the value of the endurance limit will be lower． For example， for steel parts with a machined surface finish of 63 microinches ， the percentage drops to about 40%． For rough surfaces， the percentage may be as low as 25%． The most mon type of fatigue is that due to bending． The next most frequent is torsion failure， whereas fatigue due to axial loads occurs very seldom． Spring materials are usually tested by applying variable shear stresses that alternate from zero to a maximum value， simulating the actual stress patterns． In the case of some nonferrous metals， the fatigue curve does not level off as the number of cycles bees very large． This continuing toward zero stress means that a large number of stress reversals will cause failure regardless of how small the value of stress is． Such a material is said to have no endurance limit． For most nonferrous metals having an endurance limit， the value is about 25% of the ultimate strength． EFFECTS OF TEMPERATURE ON YIELD STRENGTH AND MODULUS OF ELASTICITY Generally speaking， when stating that a material possesses specified values of properties such as modulus of elasticity and yield strength， it is implied that these values exist at room temperature． At low or elevated temperatures， the properties of materials may be drastically different． For example， many metals are more brittle at low temperatures． In addition， the modulus of elasticity and yield strength deteriorate as the temperature increases． Figure shows that the yield strength for mild steel is reduced by about 70% in going from room temperature to 1000oF． Figure shows the reduction in the modulus of elasticity E for mild steel as the temperature increases． As can be seen from the graph， a 30% reduction in modulus of elasticity occurs in going from room temperature to 1000oF． In this figure， we also can see that a part loaded below the proportional limit at room temperature can be permanently deformed under the same load at elevated temperatures． CREEP: A PLASTIC PHENOMENON Temperature effects bring us to a phenomenon called creep， which is the increasing plastic deformation of a part under constant load as a function of time． Creep also occurs at room temperature， but the process is so slow that it rarely bees significant during the expected life of the temperature is raised to 300oC or more， the increasing plastic deformation can bee significant within a relatively short period of time． The creep strength of a material is its ability to resist creep， and creep strength data can be obtained by conducting longtime creep tests simulating actual part operating conditions． During the test， the plastic strain is monitored for given material at specified temperatures． Since creep is a plastic deformation phenomenon， the dimensions of a part experiencing creep are permanently altered． Thus， if a part operates with tight clearances， the design engineer must accurately predict the amount of creep that will occur during the life of the machine． Otherwise， problems such binding or interference can occur． Creep also can be a problem in the case where bolts are used to clamp tow parts together at elevated temperatures． The bolts， under tension， will creep as a function of time． Since the deformation is plastic， loss of clamping force will result in an undesirable loosening of the bolted joint． The extent of this particular phenomenon， called relaxation， can be determined by running appropriate creep strength tests． Figure shows typical creep curves for three samples of a mild steel part under a constant tensile load． Notice that for the hightemperature case the creep tends to accelerate until the part fails． The time line in the graph (the xaxis) may represent a period of 10 years，the anticipated life of the product． SUMMARY The machine designer must understand the purpose of the static tensile strength test． This test determines a number of mechanical properties of metals that are used in design equations． Such terms as modulus of elasticity， proportional limit， yield strength， ultimate strength， resilience， and ductility define properties that can be determined from the tensile test． Dynamic loads are those which vary in magnitude and direction and may require an investigation of the machine part’s resistance to failure． Stress reversals may require that the allowable design stress be based on the endurance limit of the material rather than on the yield strength or ultimate strength． Stress con