4.4. PP homopolymer shows Kic value of 4.7 MPa m1/2 in plane-strain at all temperatures between -100C and 25C. Tensile yield stresses (y) at 25C intervals over this range are 90, 85, 75, 50, 35, and 30 MPa at (10- s-). a) Use these data to calculate for each temperature the minimum crack length required to initiate fracture before yielding in a tensile bar (take y= 1.12 for small a/W). b) Plot a graph of critical crack length against temperature, and hence estimate the ductile-brittle transition temperature for PP specimens containing 1 mm, 2mm, and 4 mm notches and tested at this strain rate.