exam2006 - Question 1: This question is concerned with the...

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Question 1: This question is concerned with the effect of microstructure on the mechanical properties of materials. A pure metal with a body centered cubic (BCC) crystal structure is commercially available in grain sizes of 10 m and 100 m . (a) If the yield-stress of the sample with 100 m grain-size is 200 MPa, estimate the yield-stress of the sample with 10 m grain size assuming that the intrinsic strength of the above metal is 50 MPa. [2 Marks, 1 for k and 1 for answer] MPa m MPa k k d k i 524 10 10 5 . 1 50 5 . 1 10 100 50 200 6 6 (b) Discuss the effect (if any) of grain refinement on the following materials properties: I. Young’s modulus. [2 Marks, ½ for answer and 1 ½ for explanation] Grain refinement has no effect on Young’s modulus. The Young’s modulus is determined primarily by the nature of bonding between the atoms and this is not changed by grain refinement. ( If a student discusses the effect of grain size on isotropy, let me know ). II. Ductile to brittle transition temperature. [2 Marks, ½ for answer and 1 ½ for explanation]
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III. Creep rate. [2 Marks, ½ for answer and 1 ½ for explanation] Grain refinement will increase the creep rate. mechanisms all i ss . The diffusional creep rate increases as 1/d 2 . IV. Wear rate. [2 Marks, ½ for answer and 1 ½ for explanation] Grain refinement will increase the yield stress and this will generally increase the wear resistance. The area of contact between two surfaces is: a = P / y . The area can be lowered by increasing the yield stress of the material. This will lower adhesive wear. As the same time, having a harder surface will also lower abrasive wear because the particles will not be able to penetrate this hard surface. [ Please note, some students might point out that if yield stress becomes very large, the material might become brittle and therefore grain refinement is not good. This is not acceptable, since grain refinement is the one strengthening mechanism which does not reduce toughness ].
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Question 2: This question is concerned with the fatigue behavior of materials. (a) Describe the main types of fatigue behavior. Give an example for each.
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exam2006 - Question 1: This question is concerned with the...

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