Effect of Notch Acuity on Critical COD with COD Bend Test Specimens

Abstract

The conventional COD bend tests are frequently used for the purpose of obtaining a critical COD (δc). But it is well known that δc shows various values depending on notch acuities. Accordingly, COD bend test specimens have usually a fatigue notch. However the industrial material tests are desired to be simple. In this sense, it is desirable that COD bend test specimens have only machined notch.In this paper, the transformation equations of a mouth COD (Vtotal) for a machined notch to δc for a fatigue notch are proposed by considering the machined notch root displacement. These transformation equations are expressed as follows :δ= O.45 (W-a) /0.45W+0.55a+Z [(Vtotal-Ve) -V'] … (Vtotal-Ve≥2V')O.45 (W-a) /0.45W+0.55a+Z [(Vtotal-Ve) 2/4V'] … (Vtotal-Ve<2V')Ve= 0.8d0.3 … (Vtotal≥4V'){0.8d0.3-0.74d0.45 (1-Vtotal/4V')} Vtotal… (Vtotal<4V')whereV'=γσYW (1-v2) /E : limit elastic clip gauge COD defined by Wells, W : specimen width, a : crack depth, Z : distance from free surface to positions setting a clip gauge, d : initial notch root width (d≤0.4mm)The above proposed equations are obtained from results of F. E. M. analysis, and it was confirmed that the relation between δ by the above equations and the plastic zone size is not different in the case of various notch acuities of the specimens. These equations are engineeringly satisfactory in the range of d≤0.4 mm for various materials.