《材料的疲劳与断裂》研究生课程课件试卷

2013年春研究生《工程材料疲劳与断裂》课程试卷一姓名出生日期年月日
性别学校住址
民族联系电话
现学习院系专业
/导师
本科学校院系入学时间
本科学习专业毕业时间
是否学习过以下
课程材料科学导论断裂与疲劳其
它断裂力学基础结构失效
计算机等级外语等级
1 为什么学习这门课程？和研究课题有什么关系？你同时或稍后还有其它的学习计划吗？
2 请解释传统的强度设计概念、一般方法及它的优缺点。

3 你听说或见过有关工程断裂失效的事情吗？请举出一例，并分析它们的力学特点是什么?
4 什么是金属材料的脆性断裂，它的核心本质是什么？你能说出与之相关的理论观点、术语
吗?
5 什么事疲劳？疲劳有哪些特征？你能画出一个简单的循环载荷示意图吗？
6 什么是断口分析，在失效分析中断口能提供哪些信息？
7 疲劳断口和静载破坏断口有什么不同？
8 已知循环最大应力s max =200MPa,最小应力s min =50MPa,计算循环应力变程Δs,应力幅s a ,平均应力s m 和应力比R
9 The S-N curve of a material is described by the relationship
)/1(10log max σS N -=,where N is the number of cycles to failure, S is the
amplitude of the applied cyclic stress, and max σis the monotonic fracture strength ,i.e.,S=max σ at N=1. A rotating component made of this material is subjected to 104 cycles at S=0.5max σ.If the cyclic load is now increased to S=0.75max σ, 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.。