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Fracture Mechanics
 
Problem 4
This question illustrates the potential effect of quenching and residual stresses on fast fracture. It should take about 15 minutes to complete. During water quenching of steel components with a section thickness of 30 mm, heat transfer calculations indicate that a peak stress of 130 MPa is generated in the section. Prior to heat treatment, the components were ultrasonically inspected to detect defects. The inspection technique has a minimum detection size of 0.5 mm. a) What type of defect will be most critical? b) Calculate the size of defect which would cause fracture of the component during the quenching operation, given that the aspect ratio of the crack is 2c/a = 10. c) Would this inspection procedure guarantee integrity of the component if the quenching stresses approached the proof stress of the steel? Note that the value of the plane strain fracture toughness K1C = 30 MPa m1/2 and the proof stress = 620 MPa. The stress intensity calibration for this component and crack geometry is given in the figure below. 
Where, for surface flaws:
and for embedded flaws: 
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Failure Analysis - Fracture Mechanics - Failure As A Design Criterion |