# hw2 - range of 135 MNm-2 It is desired to extend the part...

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Yeditepe University Department of Mechanical Engineering 1 Instructor: Asst.Prof. Fethi Okyar Room: A709 Phone: x1464 Email: Assignment #2 Due Date: Oct.12, 2009 Questions 1. An aluminum alloy for a structural part was tested in the laboratory under an applied stress which varied sinusoidally with time about a mean stress of zero. The alloy failed under a stress range, Δ σ , of 205 MNm - 2 after 3 . 1 × 10 5 cycles; under a range of 165 MNm - 2 , the alloy failed after 8 . 2 × 10 7 cycles. (Hint: Find material constants a and C 1 related with the Basquin’s Law) 2. A part, in service, has encountered an estimated 2 . 5 × 10 8 cycles at a stress
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Unformatted text preview: range of 135 MNm-2 . It is desired to extend the part life by another 2 . 5 × 10 8 cycles by reducing the performance requirements (and thus the stress). Find the decrease in stress range necessary to achieve the additional life using Miner’s cumulative-damage law. 3. In a more careful analysis, it has been noticed that a mean stress of about 10 MNm-2 is sustained by the part described in the previous question, under service conditions. How much would this aﬀect your ﬁnal answer? (Hint: Repeat the calculation by taking into account a mean stress of 10 MNm-2 ) (Note: σ TS = 395MNm-2 )....
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