《材料的疲勞與斷裂》研究生課程課件試卷

真誠(chéng)為您提供優(yōu)質(zhì)參考資料，若有不當(dāng)之處，請(qǐng)指正2013年春研究生《工程材料疲勞與斷裂》課程試卷一姓名出生日期 年 月 日性別學(xué)校住址民族X(qián)XX電話現(xiàn)學(xué)習(xí)院系專(zhuān)業(yè)/導(dǎo)師本科學(xué)校院系入學(xué)時(shí)間本科學(xué)習(xí)專(zhuān)業(yè)畢業(yè)時(shí)間是否學(xué)習(xí)過(guò)以下課程材料科學(xué)導(dǎo)論斷裂與疲勞其它斷裂力學(xué)基礎(chǔ)結(jié)構(gòu)失效計(jì)算機(jī)等級(jí)外語(yǔ)等級(jí)1為什么學(xué)習(xí)這門(mén)課程？和研究課題有什么關(guān)系？你同時(shí)或稍后還有其它的學(xué)習(xí)計(jì)劃嗎？2請(qǐng)解釋傳統(tǒng)的強(qiáng)度設(shè)計(jì)概念、一般方法及它的優(yōu)缺點(diǎn)3你聽(tīng)說(shuō)或見(jiàn)過(guò)有關(guān)工程斷裂失效的事情嗎？請(qǐng)舉出一例，并分析它們的力學(xué)特點(diǎn)是什么?4什么是金屬材料的脆性斷裂，它的核心本質(zhì)是什么？你能說(shuō)出與之相關(guān)的理論觀點(diǎn)、術(shù)語(yǔ)嗎?5什么事疲勞？疲勞有哪些特征？你能畫(huà)出一個(gè)簡(jiǎn)單的循環(huán)載荷示意圖嗎？6什么是斷口分析，在失效分析中斷口能提供哪些信息？7疲勞斷口和靜載破壞斷口有什么不同？8已知循環(huán)最大應(yīng)力smax=200MPa,最小應(yīng)力smin=50MPa,計(jì)算循環(huán)應(yīng)力變程Δs,應(yīng)力幅sa,平均應(yīng)力sm和應(yīng)力比R9The S-N curve of a material is described by the relationship ,where N is the number of cycles to failure, S is the amplitude of the applied cyclic stress, and is the monotonic fracture strength ,i.e.,S= at N=1. A rotating component made of this material is subjected to 104 cycles at S=0.5.If the cyclic load is now increased to S=0.75, how many more cycles will the material withstand?10Translation E2C Fatigue Crack NucleationFatigue cracks nucleate at singularities or discontinuities in most materials. Discontinuities may be on the surface or in the interior of the material. The singularities can be structural (such as inclusions or second-phase particles) or geometrical (such as scratches or steps). The explanation of preferential nucleation of fatigue cracks at surfaces perhaps resides in the fact that plastic deformation is easier there and that slip steps form on the surface. Slip steps alone can be responsible for initiating cracks, or they can interact with existing structural or geometric defects to produce cracks. Surface singularities may be present from the beginning or may develop during cyclic deformation, as, for example, the formation of intrusions and extrusions at what are called the persistent slip bands (PSBs) in metals. These bands were first observed in copper and nickel by Thompson et al.4 They appeared after cyclic deformation and persisted even after electropolishing. On retesting, slip bands appeared again in the same places. Later, the dislocation structure in the PSBs was investigated extensively. Figure 14.11(a) shows a TEM micrograph of a polycrystalline copper sample that was cycled to a total strain amplitude of 6.4 × 10?4 for 3 × 105 cycles. Fatigue cycling was carried out in reverse bending at room temperature and at a frequency of 17 Hz. The thin foil was taken 73 μm below the surface. Two parallel PSBs (diagonally across the micrograph) embedded in a veined structure in polycrystalline copper can be seen. The PSBs are clearly distinguished and consist of a series of parallel ‘‘hedges” (a ladder). These ladders are channels through which the dislocations move and produce intrusions andextrusions at the surface Figure 14.11(c). Stacking-fault energy and the concomitant ease or difficulty of cross-slip play an important role in the development of the dislocation structure in the PSBs. Kuhlmann-Wilsdorf and Laird have discussed models for the formation of PSBs in metals.5 They compared the deformation substructures produced by unidirectional and cyclic (fatigue) deformation and interpreted them in terms of the differences between the two modes of deformation. The principal differences are as follows:1. Due to the much larger time spans of deformation in fatigue, the dislocation structures formed are much closer to the configurations having minimum energy than the ones generated by monotonic straining. That is, more stable dislocation arrays are observed after fatigue.2. The oft-repeated to-and-fro motion in fatigue minimizes the buildup of surpluses of local Burgers vectors, which are fairly prevalent after unidirectional (monotonic) strain.3. Much higher local dislocation densities are found in fatigued specimens.4 / 4。