A component in the shape of a large sheet is to be fabricated from 4340 steel, w

Question

A component in the shape of a large sheet is to be fabricated from 4340 steel, which has a fracture toughness K_1c = 98.9 MPa.m^0.5 and a tensile yield strength of 860MPa. The sheets are inspected for crack flaws after fabrication (beta = 1), but the inspection device cannot detect flaws smaller than 0.5mm. The part is too heavy as designed. An engineer has suggested that the thickness be reduced and the material be heat-treated to increase its tensile strength to 1515MPa, which would result in decreasing the fracture toughness to60.4MPa.m^0.5 Assuming that the stress level does not exceed one half the yield strength, is the suggestion feasible? If not, why?

Explanation / Answer

At 0.5 * 10^-3 of crack length we get

sigma   = 98.9 * 10^6 / (5 * 10^-4)^0.5

sigma = 4422.942 MPa

Stress level = 860 / 2 = 430 MPa

Material is safe against fracture

After heat treatment

sigma = 60.4* 10^6 / (5 * 10^-4)^0.5

sigma = 2701.17 MPa

Stress Level   = 1515 / 2 = 757.5 MPa

Therefore stress level does not beyong fracture stress

Hence material is safe

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