Fatigue,Creep and Fracture 445 2bEy or for plane stress a where 2a initial crack length(in an infinite sheet) b sheet thickness y surface energy of crack faces. Irwin's expressions for the cartesian components of stress at a crack tip are,in terms of polar coordinates; K 8.381 √2πr -2 1+sin sin2 K .301 Ox=- 2πr 1-sin 2 sin2 K 30 Oxy=- cos sin-Cos 2πr 22 2 where K is the stress intensiry factor=o√πa or,for an edge-crack in a semi-infinite sheet K=1.12o√πa For finite size components with cracks generally growing from a free surface the stress intensity factor is modified to K=oY√a where Y is a compliance function of the form 12 3/2 、7/2 9/2 Y=A a a W -B c() +E W) In terms of load P,thickness b and width W P K bW2·P For elastic-plastic conditions the plastic zone size is given by K2 rp=- o for plane stress K2 and rp= 3no for plane strain rp being the extent of the plastic zone along the crack axis measured from the crack tip. Mode II crack growth is described by the Paris-Erdogan Law da dN :=C(△K)m where C and m are material coefficients.Fatigue, Creep and Fracture 445 or for plane stress 2 2bEY Of = - na where 2a = initial crack length (in an infinite sheet) b = sheet thickness y = surface energy of crack faces. Irwin’s expressions for the Cartesian components of stress at a crack tip are, in terms of polar coordinates; ayy = - cos - 1 + sin sin ”1 -2 e[ 22 a,, = - cos - 1 - sin - sin - -2 e[ 22 e 37 K e e 3e G222 Oxy = - cos - sin - cos - where K is the stress intensity factor = 06 or, for an edge-crack in a semi-infinite sheet K = 1.12a& For finite size components with cracks generally growing from a free surface the stress intensity factor is modified to K=aY& where Y is a compliance function of the form In terms of load P, thickness b and width W P bW1/2 K=----.Y For elastic-plastic conditions the plastic zone size is given by K2 rp = - for plane stress nay2 and K2 rp = - for plane strain 3na; rp being the extent of the plastic zone along the crack axis measured from the crack tip. Mode I1 crack growth is described by the Paris-Erdogan Law da - = C(AK)* dN where C and m are material coefficients