The Elastic-Plastic Finite Element Alternating Method (EPFEAM) and the prediction of fracture under WFD conditions in aircraft structures

Abstract

This report provides a summary of the Elastic-Plastic Finite Element Alternating Method (EPFEAM), the T Ã -integral fracture mechanics parameter, and the use of both tools to predict the residual strength of aircraft panels with multiple-site damage. Because this report is meant to be self-contained and each of the three subjects is a considerable research topic in itself, the report is written in three parts. Part I, EPFEAM Theory provides a summary of the elastic-plastic ®nite element alternating method (EPFEAM) and the algorithms for fracture analysis and crack growth predictions. Part II, Fracture and the T Ã -Integral Parameter provides a complete description of the T Ã -integral fracture parameter including a detailed discussion of the theoretical basis of T Ã and the practical aspects of its use for fracture predictions. Finally, Part III, Computational Predictions of the NIST Multiple Site Damage Experimental Results provides a series of predictions of a number of fracture tests performed at the National Institute of Standards and Technology (NIST). These predictions are then compared with the experimental data, thus validating the present model for computing the residual strength under wide-spread-fatigue damage conditions. The reader that is interested in all the topics can study all the three self-contained parts, while the reader that is only interested in the practical aspects of fracture predictions using these methods can read only Part III. This is the Part I report, EPFEAM Theory.

Influence of MSD crack pattern on the residual strength of flat stiffened sheets

“…The tests by deWit et al (1995) have also been analysed by Harris et al (1996) using a full elastic-plastic FE-analyses with a constant angle opening criterion and by Wang et al (1997a;1997b and1997c) using the elasto-plastic ®nite element alternating method but where the T Ã -integral parameter was used as the crack growth criterion. The predicted loads were very similar in all studies.…”

“…Nilsson and Hutchinson (1993); Wang et al (1997c)] and plastic interaction effects have to be accounted for in the residual strength assessment. The yielding in thin plane cracked sheets can be successfully modelled by Dugdale/ Barenblatt yield-strip models [Dugdale (1960); Barenblatt (1962)], and a particular application of the yield-strip for MSD-damaged sheets has been presented in Nilsson and Hutchinson (1993); Deng and Hutchinson (1996); Nilsson (1996); Nilsson (1997a).…”

Elastic plastic analysis iterative solution

“…Ortiz and Popov 1985;Simo and Taylor 1985;Simo et al 1988;Chaboche and Cailletaud 1996;Dvorkin 1996;Kojik 1997), or globally in terms of generalized stresses utilizing the ®nite element plasticity conditions (see e.g. Maier 1968aMaier , 1968bMaier , 1969Capurso and Maier 1970;Sayegh and Rubinstein 1972;Martin 1975Martin , 1985Polizzotto 1986;Polizzotto 1989;Muscolino 1989;Pyo et al 1995;Wang et al 1997). …”

Error indicators and mesh refinements for finite-element-computations of elastoplastic deformations

“…A recently published paper by Wang, Brust and Atluri [28] describes a novel scheme for ®nite element computations of elastoplastic deformations, based on the Schwartz±Neumann alternating method. It was shown, that in case of multiple site damage (MSD) reliable numerical results can be obtained without mesh re®nements while retaining accuracy.…”