help please
 

Hi all, any help or pointers on answering the question below would be much appreciated.
For a large, centrally cracked plate of a steel pressure vessel (sY =1300 MPa, KIc = 180 MPaÖm), construct a graph of stress level against crack length for crack lengths up to75mm.
Indicate the regions in which (a) the plate will fail by yield, (b) the plate will fail by rapid fracture and (c) the plate may be used safely.