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

Actaul endurance stress = 190.826 MPa

Step-by-step 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 * 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* (hb)^(1/2)`

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

so de is 34.2805 mm

`K_(size) = [(d)/(7.62)]^(-0.107)`

`K_(size) = [(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* 2.326 = 0.814`

so Actaul endurance stress of materail

`Se' = Se* K_(surf)  * K_(size) Kb \K_(real)`

`= 344.75* 0.8* 0.85* 1* 0.814`

= 190.826 MPa