Fractal analysis of crack initiation in polycrystalline alloys using surface interferometry

Microstructural degradation is a predominant source of damage in polycrystalline alloys that are commonly used in diverse applications. For early diagnosis and prognosis of failures, it is essential to understand the mechanisms of damage growth specifically in the crack initiation phase, which is still an intriguing phenomenon for scientists due to sensing inaccuracies and modeling uncertainties. Measurements of gradually evolving deformations on the material surface during crack initiation provide early warnings of forthcoming widespread damage. In this paper, a surface interferometer is used to generate 3-D surface profiles of polycrystalline alloy specimens under oscillating load. The concepts of fractal geometry are used to quantify the changes in the 3-D surface profiles as early indicators of damage evolution in the crack initiation phase. Copyright c © EPLA, 2012 Introduction. – Microstructural degradation is a predominant source of damage in polycrystalline alloy structures that are used in diverse applications [1]. The life of these structures subjected to oscillating load patterns is broadly classified into two phases: i) the crack initiation and ii) the crack propagation. This classification implies that there is a phase transition when microstructural damage in the form of surface and subsurface deformities (e.g., voids, slip bands, inclusions, casting defects, machining marks, and dislocations) develop into multiple micro-cracks, that in turn coalesce together to develop into a single large crack that propagates under oscillating load [1]. Therefore, for early diagnosis and prognosis of failures, it is essential to understand the mechanisms of damage growth specifically in the crack initiation phase, which is still an intriguing phenomenon for scientists due to sensing inaccuracies and modeling uncertainties. In the current state-of-the-art, early diagnosis in the crack initiation phase is a critical challenge. Furthermore, fatigue damage evolution is critically dependent on the initial defects present in the materials, which may form random crack nucleation sites [2]. This random distribution of microstructural flaws may produce a wide uncertainty in the crack initiation phase under similar loading conditions [3], thereby making damage evolution a stochastic (a)E-mail: [email protected] phenomenon. Since accurate identification of exact initial conditions is infeasible, sole reliance on model-based analysis in the crack initiation phase is inadequate due to lack of requisite modeling accuracy [4,5]. Many model-based techniques have been reported in the literature. Qian et al. [6] did a X-ray computed microtomography study of the ductile fracture process of an aluminum alloy. Stochastic approaches have been developed for modeling 2D crack propagation in heterogeneous materials [7] and characterization of surface profiles [8]. Some studies attempted to model short cracks [9] while others correlated the mechanical properties of materials with the fractured surfaces [10,11]. Apparently, no existing model, solely based on the fundamental principles of physics, can adequately capture the dynamical behavior of damage evolution in the crack initiation phase. Alternatively, data-driven techniques have been proposed based on different sensing devices (e.g., acoustic emission [12], eddy currents [13] and ultrasonics [14,15]). While each of these methods has its own limitations, ultrasonic sensing has been proven to be one of the most successful methods for early detection of subsurface damage during crack initiation, when no surface damage is visible on a typical (100×) optical microscope. This paper complements the prior work on ultrasonic sensing [3,15] by studying the surface phenomenon that occurs during crack initiation using the surface interferometry.