Abstract: Using dimensional analysis of the equations and solutions of the deformation theory of plasticity and the extrapolation of F. E. results for pure power hardening materials given by Hutchiuson etal, simple crack opening displacement expression have been obtained for center cracked tension strip made of pure power hardening materials under fully plastic condition. By interpolating the small scale yielding solutions and the fully plastic solutions for pure power hardening materials, simple formulas for estimating COD in terms of nominal strain for elastic/plastic materials have been given. Emphasis has been given on distinquishing two typical yielding cases. I. E. the general yielding case and the ligament yielding case. Significantly different relationships between φ=δ/2παε0 and ε/ε0 exist for these two typical yielding cases: For general yielding case, φ=ε/ε0-[εel/ε0-4/π2·(σ/σ0)ln sec(σ0/σ·πεel/2ε0)] For ligament yielding case, φ=1/π(κ2/κ3) ·(y/a) ·ε/ε0+A Notched tension strip COD experiments on low carbon Mn-steel with broad yielding plateau show that when the relative crack size is not small enough to achieved pure general yielding condition, following four states of deformation in the φ～ε/ε0 curve are observed: I) the state of small scale yielding, ii) the stage of ligament yielding with slope satisfying basically the equation dφ/d(ε/ε0)=y/πa, iii) the state of the spreading of the initial yielding plateau in the remaining parts of the plate with COD frozen. Iv) the general yielding stage with power hardening. The scattering of the data of some wide plate experiments below and above the Wells or Burdekin’s design curve can now be explained as due to the effect of ligament yielding with different y/πa values. Finally, under general yielding condition of tension strips with different crack geometry, the relation between φ and ε/ε0 can be expressed as φ=Q2(ε/ε0-0.25) For low carbon Mn-steel, the shape factors Q2 for different crack geometry had been obtained by measuring the slope of the φ～ε/ε0curve in the stage of general yielding.