$1.)$ A high strength steel has the following properties: $E=200$GPa,${}_{\text{U}\text{T}\text{S}}=1950$MPa, specific surface energy $=2\frac{J}{m^2},$ plastic work factor $=2{10}^{3}J{m}^{-2}.$ Calculate the percentage reduction on strength caused by the presence of a $4mm$ long crack perpendicular to the direction of stress in a large plate of the material. Assume plane stress conditions.