COMMON FORMAT FOR DEVELOPING CALIBRATION CURVES IN ELASTIC-PLASTIC FRACTURE-MECHANICS

The application of a ductile fracture methodology to predict structura l behavior in components requires the use of two inputs. One of them i s the material fracture toughness, usually characterized by the J-R cu rve of the material. The second input is the calibration functions, wh ich represent the relation between load, P, displacement, upsilon, and crack length, a. These three variables uniquely determine the behavio r of a structure. When considering only the plastic component of this displacement, upsilon(pl), the calibration functions are usually state d in the form P = G(b/W)H(upsilon(pl)/W), in which b is the uncracked ligament, W -a, and W is a characteristic dimension (width) of the fra cture test specimen. In this relation, G is a geometry-dependent funct ion, whereas H is a material-dependent hardening function. The calibra tion functions may be obtained analytically, empirically, or numerical ly. Most often, the latter two forms are more readily available. Numer ical solutions for the most commonly employed fracture testing geometr ies are normally obtained from the GE-EPRI Handbook, in which the plas tic components of such quantities as J, CTOD, or load-line displacemen t, are related to the load by means of a power law. The behavior of th e test specimen is described by using the Ramberg-Osgood stress-strain parameters, geometry-dependent variables, a crack length-dependent fu nction, and a constraint factor. The GE-EPRI Handbook approach therefo re assumes that a component or a test specimen behaves in a predictabl e pattern, given by the material stress-strain behavior and by appropr iate geometry-dependent functions. A unique feature of the Handbook ap proach is that it infers that specimens or components ''inherit'' the basic stress-strain properties to account for their fracture behavior. In other words, it implicitly assumes that, for a given material, any geometry one chooses to test follows the same pattern of behavior. Th e purpose of this work is to show that, based on the EPRI approach, bu t not stated explicitly in the Handbook, there exists a common format for fracture test specimen calibration curves, hence for common two-di mensional structural components containing defects. The concept of loa d separation into components G and H is essential to the purposes of s tating the the common format, since the only differentiating feature b etween the various test geometries, for any given material, is the G f unction. Based on this, and on the experimental fracture behavior data of ductile steels with different test geometries, a few examples will be presented that confirm the existence of a common format.