INTRODUCTION
It is absolutely essential that a design engineer know how and why parts fail so that reliable machines that require minimum maintenance can be designed．Sometimes a failure can be serious，such as when a tire blows out on an automobile traveling at high speed．On the other hand，a failure may be no more than a nuisance．An example is the loosening of the radiator hose in an automobile cooling system．The consequence of this latter failure is usually the loss of some radiator coolant，a condition that is readily detected and corrected．
The type of load a part absorbs is just as significant as the magnitude．Generally speaking，dynamic loads with direction reversals cause greater difficulty than static loads，and therefore，fatigue strength must be considered．Another concern is whether the material is ductile or brittle．For example，brittle materials are considered to be unacceptable where fatigue is involved．
Many people mistakingly interpret the word failure to mean the actual breakage of a part．However，a design engineer must consider a broader understanding of what appreciable deformation occurs．A ductile material，however will deform a large amount prior to rupture．Excessive deformation，without fracture，may cause a machine to fail because the deformed part interferes with a moving second part．Therefore，a part fails(even if it has not physically broken)whenever it no longer fulfills its required function．Sometimes failure may be due to abnormal friction or vibration between two mating parts．Failure also may be due to a phenomenon called creep，which is the plastic flow of a material under load at elevated temperatures．In addition，the actual shape of a part may be responsible for failure．For example，stress concentrations due to sudden changes in contour must be taken into account．eva luation of stress considerations is especially important when there are dynamic loads with direction reversals and the material is not very ductile．
In general，the design engineer must consider all possible modes of failure，which include the following．




前言介紹：
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(guò)進(jìn)行最低限度的維修以保證機(jī)器的可靠性。有時(shí)一次零件的故障或者失效可能是很嚴(yán)重的一件事情，比如，當(dāng)一輛汽車正在高速行駛的時(shí)候，突然汽車的輪胎發(fā)生爆炸等。另一方面，一個(gè)零件發(fā)生故障也可能只是一件微不足道的小事，只是給你造成了一點(diǎn)小麻煩。一個(gè)例子是在一個(gè)汽車?yán)鋮s系統(tǒng)里的暖氣裝置軟管的松動(dòng)。后者發(fā)生的這次故障造成的結(jié)果通常只不過(guò)是一些暖氣裝置里冷卻劑的損失，是一種很容易被發(fā)現(xiàn)并且被改正的情況。
能夠被零件進(jìn)行吸收的載荷是相當(dāng)重要的。一般說(shuō)來(lái)，與靜載重相比較，有兩個(gè)相反方向的動(dòng)載荷將會(huì)引起更大的問(wèn)題，因此，疲勞強(qiáng)度必須被考慮。另一個(gè)關(guān)鍵是材料是可延展性的還是脆性的。例如，脆的材料被認(rèn)為在存在疲勞的地方是不能夠被使用的。
很多人錯(cuò)誤的把一個(gè)零件發(fā)生故障或者失效理解成這樣就意味著一個(gè)零件遭到了實(shí)際的物理破損。無(wú)論如何，一名設(shè)計(jì)工程師必須從一個(gè)更廣泛的范圍來(lái)考慮和理解變形是究竟如何發(fā)生的。一種具有延展性的材料，在破裂之前必將發(fā)生很大程度的變形。發(fā)生了過(guò)度的變形，但并沒有產(chǎn)生裂縫，也可能會(huì)引起一臺(tái)機(jī)器出毛病，因?yàn)榘l(fā)生畸變的零件會(huì)干擾下一個(gè)零件的移動(dòng)。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）。有時(shí)故障可能是由于兩個(gè)兩個(gè)相互搭配的零件之間的不正常的磨擦或者異常的振動(dòng)引起的。故障也可能是由一種叫蠕變的現(xiàn)象引起的，這種現(xiàn)象是指金屬在高溫下時(shí)一種材料的塑性流動(dòng)。此外，一個(gè)零件的實(shí)際形狀可能會(huì)引起故障的發(fā)生。例如，應(yīng)力的集中可能就是由于輪廓的突然變化引起的，這一點(diǎn)也需要被考慮到。當(dāng)有用兩個(gè)相反方向的動(dòng)載荷，材料不具有很好的可延展性時(shí)，對(duì)應(yīng)力考慮的評(píng)估就特別重要。
一般說(shuō)來(lái)，設(shè)計(jì)工程師必須考慮故障可能發(fā)生的全部方式，包括如下一些方面：