【正文】 es． 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 concentration occurs at locations where a machine part changes size， such as a hole in a flat plate or a sudden change in width of a flat plate or a groove or fillet on a circular shaft． Note that for the case of a hole in a flat or bar， the value of the maximum stress bees much larger in relation to the average stress as the size of the hole decreases． Methods of reducing the effect of stress concentration usually involve making the shape change more gradual． Machine parts are designed to operate at some allowable stress below the yield strength or ultimate strength． This approach is used to take care of such unknown factors as material property variations and residual stresses produced during manufacture and the fact that the equations used may be approximate rather that exact． The factor of safety is applied to the yield strength or the ultimate strength to determine the allowable stress． Temperature can affect the mechanical properties of metals． Increases in temperature may cause a metal to expand and creep and may reduce its yield strength and its modulus of elasticity． If most metals are not allowed to expand or contract with a change in temperature，then stresses are set up that may be added to the stresses from the load． This phenomenon is useful in assembling parts by means of interference fits． A hub or ring has an inside diameter slightly smaller than the mating shaft or post． The hub is then heated so that it expands enough to slip over the shaft． When it cools， it exerts a pressure on the shaft resulting in a strong frictional force that prevents loosening． 故障的分析、尺寸的決定以及凸輪的分析和應(yīng)用 摘要 ： 作為一名設(shè)計(jì)工程師有必要知道零件如何發(fā)生和為什么會(huì)發(fā)生故障，以便通過進(jìn)行最低限度的維修以保證機(jī)器的可靠性 ； 凸輪是被應(yīng)用的最廣泛的機(jī)械結(jié)構(gòu)之一 ，是一種僅僅 有兩個(gè)組件構(gòu)成的設(shè)備。 能夠被零件進(jìn)行吸收的載荷是相當(dāng)重要的。因此，每當(dāng)它不能夠再履行它要求達(dá)到的性能的時(shí)候，一個(gè)零件就都算是被毀壞了（即使它的表面沒有被損毀）。主動(dòng)件本身就是凸輪，而輸出件被稱為從動(dòng)件。 剛度： 剛度是指材料抵抗變形的一種屬性。 脆性： 一種脆性的材料就是指在任何可以被看出來的塑性變形之前就發(fā)生破裂的材料。同樣，一種材料的極限強(qiáng)度粗略與它的硬度成正比。 剪應(yīng)力測(cè)試： 軸，螺釘，鉚釘和焊接件被用這樣一種方式定位以致 于生產(chǎn)了剪應(yīng)力。在拉伸實(shí)驗(yàn)中，被分次、逐漸的加載的作用力也被叫作靜載荷。當(dāng)汽車的一個(gè)輪胎碰撞到道路上的一個(gè)突起或者路上的一個(gè)洞時(shí)，相同的沖擊荷載的類型也會(huì)在汽車的減震器彈簧上發(fā)生。外部的一個(gè)環(huán)形部分相對(duì)光滑一些，因?yàn)樵瓉肀砻嫔舷嗷ソ诲e(cuò)的裂縫之間不斷地發(fā)生磨擦導(dǎo)致了這種現(xiàn)象的產(chǎn)生。持久極限是用來評(píng)價(jià)一種材料的疲勞強(qiáng)度的一個(gè)重要參數(shù)。正如圖表 所示圖形，該圖被稱為持久極限曲線或者 SN 曲線。無論如何，已經(jīng)加工完成的表面如果不是一樣的光滑，持久極限的值就會(huì)被降低。這樣的一種材料據(jù)說沒有持久 極限。 正如從圖上可以看見的那樣，彈性模數(shù)在從室溫升高到 1000oC 過程中大約降低了 30%。否則，與此伴隨的或者相關(guān)的問題就可能發(fā)生。 總結(jié) 機(jī)器設(shè)計(jì)者必須理解進(jìn)行抗拉的靜止強(qiáng)度的測(cè)試目的。 被設(shè)計(jì)出來的機(jī)械零件被用于在低于屈服強(qiáng)度或者極限強(qiáng)度的一些允許的環(huán)境下使用。先將轂加熱后，由于熱脹冷縮，此時(shí)可以輕松的將軸插入其中。這現(xiàn)象在依靠干涉配合來進(jìn)行零件裝配時(shí)是有益的。尤其是在一塊平板上或一塊條板上有一個(gè)孔的情況下