Self-Affine Analysis of Fractures Surfaces of an Aluminum Alloy Using Fractographic Techniques M. Hinojosa 1,a , E. I. Morales 1,2,b , N. Mohamed 1,c 1 Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León A. P. 076 Suc. F, Ciudad. Universitaria, San Nicolás de los Garza, N. L. 66450, México. 2 Kemet de México S.A. de C.V.,. P.O.Box 3126, Brownsville, Tx.78523-5836, USA a hinojosa@gama.fime.uanl.mx , b edgarmorales@kemet.com , c nmn_270382@hotmail.com Keywords: Fracture surfaces, Hurst exponent, correlation length, aluminum alloys. Abstract. We report the self-affine analysis on fracture surfaces of an A319-type aluminum alloy with different modification and refinement treatments, broken both in Charpy impact and cyclic impulse tests. The in-plane and out-of-plane Hurst exponent as well as the correlation length are obtained using quantitative fractographic techniques. It is found that the Hurst exponent, ζ, has a value of about 0.8 and is not influenced neither by the crack propagation modes nor by the microstructural condition. The self-affine correlation length is found to be related to the grain size resulting from the refining and modifying treatments applied to the alloy. Introduction The fractal character of fracture surfaces was first recognized by Mandelbrot et al. in 1984 [1]. They developed and applied the first method to evaluate the fractal dimension of fractal surfaces. At the present time, it is clearly established that fracture surfaces are self-affine objects that can be characterized by one or more roughness exponents [2], which can be measured by a variety of methods [3]. For 3D crack propagation in mode I, it is possible to define three different Hurst exponents (also called roughness exponents), as indicated in Fig 1. If the crack front is propagating in the y-direction, its projection on the x-y plane has a roughness exponent ζ f (which is also described as the in-plane roughness exponent), whereas profiles along directions perpendicular and parallel relative to the propagation direction have roughness exponents ζ yz and ζ xz , respectively (which are also referred to as the out-of-plane roughness exponents). The vast majority of the reported data in the literature correspond to the exponent ζ yz , that is, to the perpendicular direction. There are only a few reported values for ζ f . It has been suggested [4] that the out-of-plane exponents have a universal value of about 0.8 for rapid propagation conditions, such as impact and tension tests. This conjecture has been supported by the results reported for a variety of materials covering metals, ceramics, and polymers. Recently, however, significant deviations from the universal values have been reported for plastic materials [5, 6] and it has been proposed that the roughness exponent has attractor values instead of universal values. The upper limit of the self-affine regime is called the correlation length, ξ, above which the fracture surface can be described as an Euclidean object. There is evidence [7-12] supporting the hypothesis that the correlation length should be associated to the largest microstructural features. The aim of the present work is to provide more experimental evidence of the self-affine character of fracture surfaces in metals by determining the three Hurst exponents mentioned above for a given material, in this case, an aluminum alloy. We also seek to document the possible relationship between the microstructure and the correlation length. Experimental Procedure The material studied is a cast A319-type aluminum alloy used in the automotive industry. Two Materials Science Forum Vol. 560 (2007) pp. 91-96 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net . (ID: 148.234.30.160-30/05/07,15:09:43)