机械毕业设计外文翻译--故障的分析、尺寸的决定以及凸轮的分析和应用(编辑修改稿)

机械毕业设计外文翻译--故障的分析、尺寸的决定以及凸轮的分析和应用(编辑修改稿)内容摘要：

materials． This brings out an interesting fact． When actual machine parts fail as a result of static loads， they normally deform appreciably because of the ductility of the material. Thus many static failures can be avoided by making frequent visual observations and replacing all deformed parts． However， fatigue failures give to warning． Fatigue fail mated that over 90% of broken automobile parts have failed through fatigue． The fatigue strength of a material is its ability to resist the propagation of cracks under stress reversals． Endurance limit is a parameter used to measure the fatigue strength of a material． By definition， the endurance limit is the stress value below which an infinite number of cycles will not cause failure． Let us return our attention to the fatigue testing machine in Figure ． The test is run as follows： A small weight is inserted and the motor is turned on． At failure of the test specimen， the counter registers the number of cycles N， and the corresponding maximum bending stress is calculated from Equation ． The broken specimen is then replaced by an identical one， and an additional weight is inserted to increase the load． A new value of stress is calculated， and the procedure is repeated until failure requires only one plete cycle． A plot is then made of stress versus number of cycles to failure． Figure shows the plot，which is called the endurance limit or SN curve． Since it would take forever to achieve an infinite number of cycles， 1 million cycles is used as a reference． Hence the endurance limit can be found from Figure by noting that it is the stress level below which the material can sustain 1 million cycles without failure． The relationship depicted in Figure is typical for steel， because the curve bees horizontal 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 temper。