Solution (Problem 3)
This question should take about 10 minutes to complete. It is an introductory level question.
A thin sheet of maraging steel has a tensile strength of 1950 MPa. Calculate the percentage reduction in strength due to the presence of a crack in the sheet, which is 4 mm long and orientated perpendicular to the stressed direction.
For this steel, E can be taken as 200 GPa, the energy of fracture surface as 2 J/m2, and the work of plastic deformation of each crack tip is
2x104 J/m2.
Answer: 59%
The Griffith formula can be extended to cover fracture of ductile structural materials through the inclusion of a plastic work term for each crack tip. One has to make an ssumption here about whether the crack is a central through-thickness crack or an edge crack - let us take it to be a central crack. We also need to assume either plane stress or plane strain, but as we have been told that the steel sheet is 'thin', let us assume plane stress conditions.
We need to modify the critical strain energy release rate term GC to take account of the two contributions to fracture toughness - surface energy and plastic work, hence:
Close Window