Fracture mechanics validity limits
Abstract
Fracture behavior is characteristics of a dramatic loss of strength compared to elastic deformation behavior. Fracture parameters have been developed and exhibit a range within which each is valid for predicting growth. Each is limited by the assumptions made in its development: all are defined within a specific context. For example, the stress intensity parameters, K, and the crack driving force, G, are derived using an assumption of linear elasticity. To use K or G, the zone of plasticity must be small as compared to the physical dimensions of the object being loaded. This insures an elastic response, and in this context, K and G will work well. Rice's Jintegral has been used beyond the limits imposed on K and G. J requires an assumption of nonlinear elasticity, which is not characteristic of real material behavior, but is thought to be a reasonable approximation if unloading is kept to a minimum. As well, the constraint cannot change dramatically (typically, the crack extension is limited to tenpercent of the initial remaining ligament length). Rice, et al investigated the properties required of Jtype parameters, J(sub x), and showed that the time rate, dJ(sub x)/dt, must not be a function of the crack extension rate, da/dt. Ernst devised the modifiedJ parameter, J(sub M), that meets this criterion. J(sub M) correlates fracture data to much higher crack growth than does J. Ultimately, a limit of the validity of J(sub M) is anticipated, and this has been estimated to be at a crack extension of about 40percent of the initial remaining ligament length. None of the various parameters can be expected to describe fracture in an environment of gross plasticity, in which case the process is better described by deformation parameters, e.g., stress and strain. In the current study, various schemes to identify the onset of the plasticitydominated behavior, i.e., the end of fracture mechanics validity, are presented. Each validity limit parameter is developed in detail, and then data is presented and the various schemes for establishing a limit of the validity are compared. The selected limiting parameter is applied to a set of fracture data showing the improvement of correlation gained.
 Publication:

Advanced Structural Integrity Methods for Airframe Durability and Damage Tolerance, Part 2
 Pub Date:
 September 1994
 Bibcode:
 1994asim.nasa..393L
 Keywords:

 Aircraft Structures;
 Crack Geometry;
 Crack Propagation;
 Fracture Mechanics;
 Mathematical Models;
 Nonlinearity;
 Predictions;
 Stress Intensity Factors;
 Structural Analysis;
 Validity;
 Aluminum Alloys;
 Elastic Deformation;
 Nickel Alloys;
 Plastic Deformation;
 StressStrain Relationships;
 Structural Mechanics