The role of constraint on the calibration functions for the prediction of ductile fracture behavior in structural components

Abstract

The role of constraint in ductile fracture has been studied for its effect on fracture toughness. When toughness is characterized by the J−R curve, a lowering of constraint will usually increase the level of the J−R curve. In a ductile fracture methodology both the J−R curve and the calibration functions which relate the applied load to the plastic deflection of a structure, are required to predict structural behavior. Constraint also influences the calibration functions. In many cases the calibration functions play a more important role in the prediction of maximum load than does the J−R curve. Therefore in predicting the behavior of a structural component the effect of the constraint must be accounted for in determining the calibration functions to be used as input in the ductile fracture methodology.

The calibration functions have been traditionally taken from the EPRI-GE Handbook [6]. Using the format of the Handbook calibration functions, a constraint factor Ω is derived which can be used to evaluate the relative constraint in the calibration function. For prediction of the behavior of a structural component from a laboratory test specimen the constraint factor must be known for each component. This constraint factor for the test specimen can then be used to predict the calibration function that is appropriate for the geometry and constraint of the structural component.

A number of examples taken from test specimens of various thicknesses are presented to show that the constraint factor for the calibration function can be reasonably predicted based on information given in the Handbook. This method of prediction is applied to develop calibration functions that can be used in a ductile fracture methodology for a new structural constraint. The methodology is then used to predict load versus displacement behavior for cases of different constraints. Results from a test with essentially plane strain constraint are used to predict behavior of a plane stress structure. Finally a list of general rules is developed to account for the constraint of an arbitrary structural component. Along with this, some indication is given of where additional information is needed to make the prediction of constraint more complete.