(a) During a recent laboratory session, you were able to observe the rotary bending fatigue test where the fatigue test

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A During A Recent Laboratory Session You Were Able To Observe The Rotary Bending Fatigue Test Where The Fatigue Test 1 (287.01 KiB) Viewed 35 times

(a) During a recent laboratory session, you were able to observe the rotary bending fatigue test where the fatigue test specimen is subjected to a completely reversed bending stress cycle. The data obtained after a large number of tests is used to plot the S-N curve for the material. Write a brief account on: 'Laboratory tests for fatigue and the effect of mean stress on fatigue failure'. [3 Marks] (b) An alloy steel power transmitting shaft of 50 mm diameter, shown in Figure Q5(b), rotating at high speed, is subjected to a steady axial tensile force F of 150 kN and a bending moment M of 1500 Nm acting on a vertical plane through the shaft axis, simultaneously. F M D M F Figure Q5(b) (i) Sketch the general form of the stress cycle at a point where the vertical plane intersects the shaft surface and determine the mean stress and the stress amplitude. [6 Marks] The ultimate tensile strength of the alloy steel is 580 MNm?. Calculate the fatigue limit of the steel if the shaft has been designed for 'infinite fatigue life' based on the Goodman criterion, with a safety factor of 1.5. [4 Marks] (c) An aluminium alloy to be used in an aircraft was tested for fatigue and its behaviour conformed to the expression 4o. (Nf)a = b where a and b are constants, Nr is the number of cycles to failure and Ao is the alternating stress range (for a completely reversed cycle). The following table gives two observed results: 1 2 40 (MNm-2) 200 250 NE 2 x 106 1.5 x 105 (i) (ii) Estimate the number of cycles to failure for a component of this alloy subjected to an alternating stress range of 170 MNm2 [5 Marks] An aircraft component of this alloy undergoes 106 cycles per year when operated at an alternating stress range of 170 MNm-2 for 10 years. It is desired to use this mponent for a further 10 years in similar operations by reducing the alternating stress range. Using Miner's Rule, estimate the new stress range required to achieve this additional life. [7 Marks] Total [25 Marks]