QUESTION 2: A large bar of alloy steel is to be loaded in tension. The design st

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QUESTION 2: A large bar of alloy steel is to be loaded in tension. The design stress is the tensile stress (ors) but must include a safety factor of 2. A non-destructive testing (NDT) method is used to analyse the part in service, but the minimum detectable crack size for that technique is 2mm. In order to increase the maximum possible load of the part in service, it is proposed that the heat treatment of the steel should be changed, so that the tensile strength of the material is increased from 1500MPa to 2000MPa. This increase in strength reduces the fracture toughness from 70MPa m2 to 35MPa m12. Can you recommend this proposed change in heat treatment? Give reasons for your answer The shape factor for this configuration is Y 1.12 and Kc YoTa

Explanation / Answer

Minimum detectable crack size = 2 mm = 0.002 m.

The factor of safety = 2.

Heat treatment increases strength from 1500 Mpa to 2000 Mpa.

Since factor of safety is 2, the applied load = 2000 / 2 = 1000 Mpa.

Toughness at this heat-treatment level = 35 Mpa.

35 = 1.12*1000*(3.14*a)0.5

Critical Crack length = a = 0.3110 mm.

The NDT technique is able to detect the crack length of 0.3110 mm. I would recommend the increase in strength.

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