Page 357 - Biaxial Multiaxial Fatigue and Fracture

BiaxiaVMultiaxial Fatigue and Fracture
Andrea Carpinkri, Manuel de Freitas and Andrea Spagnoli (Eds.)
0 Elsevier Science Ltd. and ESIS. A11 rights reserved. 34 1

THREE-DIMENSIONAL CRACK GROWTH. NUMERICAL EVALUATIONS AND
EXPERIMENTAL TESTS

Calogero CALi, Roberto CITARELLA, Michele PERRELLA
Department of Mechanical Engineering, University of Salerno
via Ponte don Melillo I, 84084 Fisciano (SA), Italy

ABSTRACT

Experimental observations of three- and two-dimensional fatigue crack growth are compared to
numerical predictions from the computer code BEASY. The two dimensional propagation
occur in a Multiple Site Damage (MSD) scenario created on a pre-notched specimen,
undergoing a traction fatigue load as defined by a general load spectrum. Experimental
analyses on a fatigue machine were carried out in order to validate the numerical simulation
and to provide the necessary material fatigue data for the aluminium plates. The numerical
code adopted (BEASY) is based on Dual Boundary Element Method (DBEM). General
modelling capabilities are allowed by this approach, with the allowance for general crack front
shape and a fully automatic propagation process. By means of a non-linear regression analysis,
applied on in house obtained experimental data, the material parameters for the NASGRO 2.0
crack propagation law were defined, capable to effectively keep into account the threshold
effect and the unstable final propagation (the crack closure option was switched off). A
satisfactory agreement between numerical and experimental crack growth rates was obtained,
even starting from a complex MSD scenario, created by the presence of three holes in the plate.
Moreover the load introduction to the specimen was monitored by strain gauge equipment. The
numerical simulation include also the through the thickness propagation, corresponding to 3D
part-through cracks; in this case some specimen were pre-notched by a comer crack on one of
the holes and the 3D experimental crack propagation monitored.

KEYWORDS

MSD, Part-through crack, Load Spectrum, D~al BEM, NASGRO 2.0, BEASY

INTRODUCTION
Damage Tolerance is used in the design of many types of structures, such as bridges, military
ships, commercial aircraft, space vehicle and merchant ships. Damage tolerant design requires
accurate prediction of fatigue crack growth under service conditions and typicaIly this is
accomplished with the aid of a numerical code. Many aspects of fracture mechanics are more
complicated in practice than in two-dimensional laboratory tests, textbook examples, or overly