真誠為您提供優(yōu)質(zhì)參考資料，若有不當(dāng)之處，請指正。 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 XXX電話 現(xiàn)學(xué)習(xí)院系專業(yè)/導(dǎo)師 本科學(xué)校院系 入學(xué)時間 本科學(xué)習(xí)專業(yè) 畢業(yè)時間 是否學(xué)習(xí)過以下課程 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 計算機(jī)等級 外語等級 1 為什么學(xué)習(xí)這門課程？和研究課題有什么關(guān)系？你同時或稍后還有其它的學(xué)習(xí)計劃嗎？ 2 請解釋傳統(tǒng)的強(qiáng)度設(shè)計概念、一般方法及它的優(yōu)缺點(diǎn)。 3 你聽說或見過有關(guān)工程斷裂失效的事情嗎？請舉出一例，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？你能說出與之相關(guān)的理論觀點(diǎn)、術(shù)語嗎? 5 什么事疲勞？疲勞有哪些特征？你能畫出一個簡單的循環(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R 9 The 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? 10 Translation E2C Fatigue Crack Nucleation Fatigue 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 and extrusions 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