The objective of this paper is to investigate the implications of the load separation criterion for evaluating ductile fracture mechanics parameters. This criterion allows the load to be represented as the multiplication of two separate functions: a material deformation function and a crack geometry function. Load separation implies a method for J-integral evaluating using only a single load-displacement record. The original method for evaluating J, proposed by Begley and Landes, used the energy rate interpretation of Rice which requires several load-displacement records for identical specimens with varying crack lengths. A method based on load separation introduced a new parameter r/, r/,,~ and tb~ which greatly simplified J calculation. This parameter which can be a function of geometrical factors is generally evaluated experimentally using the energy rate interpretation of J.In this paper the load separation criterion is used to imply a simple method for evaluating q experimentally. Using blunt notched specimen load versus load point displacement results from the literature, four different configurations with a wide range of stationary crack lengths are evaluated. Also included are several different materials varying from low work hardening to high work hardening. The data include thin and thick sections so that both plane stress and plane strain conditions are evaluated. A new method for ~l-estimation derived from the implication of the load separation is proposed. This method avoids most of the errors that accumulate in the classical methods of estimation. Both the separation method and the energy rate method are evaluated by comparing the techniques and the results. The results show some new trends in ~l~,~ results for the different configurations evaluated in this paper.
Load separation is the theoretical basis for the single specimen J form and the incremental calculation of J-R and J~rR curves. It is based on the assumption that the load can be represented as a multiplication of two separate functions; a crack geometry function and a material deformation function. Until recently, the main experimental basis for such an assumption was the approximate agreement between the experimental results of the single specimen J form and the energy rate interpretation of J in blunt notched bending geometries. The load separation assumption has been also implied in the growing crack records in order to develop the R-curve analysis. Both the crack geometry and material deformation functions were assumed to maintain their forms as the crack grows. Recently, an experimental study investigated the load separation in the test records of stationary crack specimens of different geometry, material, and constraint. The study showed that the load can be represented by a separable form for the entire plastic region except for a limited region at the early region of plastic behavior. Also, it was found that the load separation is not limited to a certain geometry, material, or constraint but it is a dominant property in the ductile fracture behavior of stationary crack specimens. The study also showed that the crack geometry function is a power law function. Hence r/pt is a constant equal to the power law exponent of the geometry function.The objective of this study is to investigate the extension of load separation to growing crack records. Sets of test records from three different materials are used in this study. For each material three or four precracked specimen test records and one blunt notched record are analyzed for the compact specimen geometry. The study will discuss the main condition to have a separable behavior in a growing crack test record. It will also construct the geometry and deformation functions for the materials studied, these functions are compared with those obtained from stationary crack records.
A method for experimentally determining the eta (η) factor based on separation constants has been recently proposed. This method has two important implications for elastic plastic fracture toughness testing. First, the method can be used to determine the η factors for any new test specimen geometry which might be added to existing test standards. Such specimens as disk compact, arc bend, and arc tension are used in the KIc test standard. They can be added to the J based standards if the specimen calibrations are known, one being the η factor calibration. In this paper a step by step procedure is given describing η factor calibration for an arbitrary specimen geometry based on a series of blunt notched specimens. The procedure proposed in this paper was then applied to existing blunt notch data for the traditional test specimen geometries, the compact, and single edge notched bend specimens. The results of the study show different values for η from these in the existing standards both in magnitude and trend with a/W. In addition they show a slight material sensitivity. The consequences of having incorrect η factors in the test standards are explored in a sensitivity study. These results are used to evaluate the importance of having correct η factors and recommendations are made.
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