Question

Compute the estimated actual endurance strength (Sn) for SAE 3140 OQT 1000 steel bar with a rectangular cross section of w-30 mm and h 60 mm. It is to be machined and subjected to rev bending stress. Reliability of 99.9 % is desired.

Answer 1

Answer:

Actaul endurance stress = 190.826 MPa

Explanation:

we know ultimate tensile strength S_{UT}  for AISI 3149 steel is 689.5 MPa

endurance of material Sn = 0.5 Sut

                                           $= 0.5 \times 689.5 = 344.75 MPa$

surface factor

$K_{surf} a Sut^b$

for mechanical surface a  = 4.51 and b = - 0.265

PUTTING ALL VALUE

$K_{surf} = 0.79$

size modification factor Ksize

equivalent diameter $de = 0.808\times (hb)^{1/2}$

where b =w = 30 mm and h = 60 mm

so de is  34.2805 mm

$K_{size} = [\frac{d}{7.62}]^{-0.107}$

$K_{size} = [\frac{34.28}{7.62}]^{-0.107} = 0.85$

Loading factor

for reversal bending load Kb = 1

Reliability factor

$K_{rel} = 1 - 0.08 Za$

we know for 99% reliablity Za = 2.326

$K_{rel} = 1 - 0.08\times 2.326 = 0.814$

so Actaul endurance stress of materail

$Se' = Se\times K_{surf} \times K_{size} Kb \K_{real}$

$= 344.75\times 0.8\times 0.85\times 1\times 0.814$

     = 190.826 MPa