Question

EXERCISE 5 A mechanical part is made of steel with the properties Su= 560 MPa, Sv=490 MPa and S=210 MPa. The part is subjected to a bending stress that alternates between 100 and 200 MPa. Determine the equivalent reversing stress and evaluate the factor of safety corresponding to a life of 500,000 cycles.

Answer 1

@ Ans - Mechanical part is made of steel Ultimate tensile strength (Su) = 560 MPa Yield strength (Sy) = 490 mpa Endurance Strength (SE) = 240 MPa Bending stress alternates between too MPa and 200 MPa. (max. = 200 MPa (60min. = loompa Equivalent never versing stress We determine amplitude stress (a) = Gmax - of min. 2 2 ба 200-100 sompa Mean stres com Of max t on min, 200+100 2 1 Sompa 1 Gasompa Bending stren medy reverses between 180 sompa about mean stren = 1 pompel We shes. can easily see that sompa is neversing So equivalent reversing stren - sompa

* Evaluate the factor of safety conecponding to a life of 500,000 cycles We know that at endurance limit (SE=210 MPa) life is too cycles N = We do the problem using s-N curve Where S = Stress amplitude Number of Cycles at fatigue on Number of cycles of cycles before gailure A los o$uxo logos Crenerally ferrous dag material shows this type of behaviour lg N When stress amplitude is (o.g. Su), the no. of Cycles ist can sustain is you cycle. For life of 5x los cycles let the stress amplitude will be on 3 y spot Nowe we can use straight line low to determine stress amplitude o. Eleg, slag, (Buxoig) logo-log (sux 8-3

Page Jos (Sxlo) – 3 210 log, (560x0.9) Toge ) - log (960x 0.9% say ove) - 3 Hogoro 9.58 2.3604 229.282 MPa 229.282 MPa tor In our So stress amplitude corresponding life of 5x10 cycles is 229. 282m Pa loading shers amplitud (a) = sompa So factor of safety conresponding to life of Sy los cycles of safety Maximum stress amplitud at Sxlos Cycle Actual stem amplitude at 5xlo Scycle Facton of 229.282 so 4.586 So factor of safety 4.586