MultipleChoice,True-False Ouestions(60% i Clearly identify your final answer for
ID: 1765184 • Letter: M
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MultipleChoice,True-False Ouestions(60% i Clearly identify your final answer for each question by circling the single, best answer. For example not oec answer Question #1 (496) You bend a paperclip back and forth at a given amplitude 100 times without it breaking. On the 101" bend (still at the same amplitude) the paperclip breaks. This reflects a failure. ar (A) fatigue (B) brittle (C) ductile (D) thermal (E) none of the above In its most basic form, steel is an alloy of iron and carbon. As the carbon content in steel increases, which of the following does not occur? (A) Ultimate strength increases (B) Modulus of elasticity increases (C) Ductility decreases (D) Both B and C (E) None of the above Which of the following values is unlikely for the Poisson's ratio of an engineering material? (A) 0.1 (B) 0.3 (C) 0.5 (D) 0.7 (E) All of the aboveExplanation / Answer
Q-1 - Ans. - A - Fatigue
{Ref: - http://www.epi-eng.com/mechanical_engineering_basics/fatigue_in_metals.htm}
Long ago, engineers discovered that if you repeatedly applied and then removed a nominal load to and from a metal part (known as a "cyclic load"), the part would break after a certain number of load-unload cycles, even when the maximum cyclic stress level applied was much lower than the UTS, and in fact, much lower than the Yield Stress (UTS and YS are explained in Stress and Strain). These relationships were first published by A. Z. Wöhler in 1858.
They discovered that as they reduced the magnitude of the cyclic stress, the part would survive more cycles before breaking. This behavior became known as "FATIGUE" because it was originally thought that the metal got "tired". When you bend a paper clip back and forth until it breaks, you are demonstrating fatigue behavior.
Q-2 - Ans. - C - Ductility decreases - As the Carbon content in steel increases, Young's modulus remains unaffected but brittleness increases..i.e., ductility decreases
Q-3 - Ans. - D - 0.7 --- K = E/[3(1-2*Poisson's ratio)]......if Poisson's ratio = 0.7 (or > 0.5), K (bulk modulus) becomes negative..for engg. materials, K cannot be negative. So, ans. is 0.7.
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