SPIE LASE 2004, January 26-30, 2004, San Jose, CA. Proceedings Volume 5339A-43 LASER-MICROMACHINED DEFECT ARRAYS FOR DC POTENTIAL DROP FATIGUE STUDIES C. B. Arnold *1 , B. Pratap 2 , A. Piqué 2 , A. B. Geltmacher 3 , and J. P. Thomas 3 1 Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, USA 2 Materials and Sensors Branch, Naval Research Laboratory, Washington, DC, USA 3 Multifunctional Materials Branch, Naval Research Laboratory, Washington, DC, USA ABSTRACT The experimental characterization of fatigue crack initiation and growth of structural materials can be very expensive and time consuming. Fatigue specimens are typically controlled by a single dominant defect and several specimens are needed to examine the fatigue response for each loading condition of interest. Time and expense add up as millions of load cycles are sometimes required to initiate a crack, and replicate tests are necessary to characterize the inherent statistical nature of fatigue. In order to improve the efficiency of experimentation, we are developing laser-based techniques to produce fatigue test samples with arrays of defects. Controlled arrays of oval shaped micro-defects are laser-micromachined in titanium alloy (Ti-6Al-4V). Crack initiation from the individual defects in the arrays is monitored using a DC potential drop technique. Results indicate the utility of this approach in multiplying the amount of fatigue data generated per specimen-test. The new fatigue test approach is applicable to a wide range of material systems and initial defect structures. 1. INTRODUCTION Failure of a material via fatigue is an important consideration in structural design determining its applicability to a particular structural application [1,2]. Fatigue crack initiation under cyclic loading conditions is probabilistic in nature and requires multiple tests in order to develop a statistical sampling for a given material [3]. These tests can be quite time consuming and expensive, since a large number of load cycles may be required to initiate a crack from a single defect in each specimen. The acquisition of a single data point from each experimental test, which can take several days to perform, also increases the time needed for the required tests. In order to improve the efficiency of laboratory fatigue testing, a method that can provide multiple fatigue crack initiation defects on a single test specimen is necessary. There are a number of experimental techniques used to monitor the fatigue “damage” present in a test sample. Potential drop (PD) methods (both DC and AC) can provide an indirect measurement of crack initiation and growth from a geometric defect introduced in the specimen under fatigue loading [2,4]. In these methods, electrical current is passed through the specimen, and the change in electrical potential across the defect (typically a hole or notch in the specimen) is measured and correlated with the crack length [4]. As a crack initiates and grows, the electrical resistance between the potential drop leads increases leading to an increase in measured potential under constant current conditions. Obviously, one must know where the fatigue crack will initiate and grow in the specimen in order to use these methods. Typically, geometric defects such as holes and notches are used to produce a concentration of stress at their edges that provides a driving force for crack initiation. A statistical sampling of fatigue behavior can thus be accomplished by producing an array of defects that are individually monitored by potential drop probes at each defect. Arrays of defects can be generated relative to specific material microstructural features to combinatorially generate fatigue data relevant to micromechanical modeling and analysis. In this paper, laser-based techniques for producing fatigue test samples with an array of elliptical holes is examined. Laser micromachining is used to create the array of small elliptical holes in a titanium alloy specimen while laser direct- * Corresponding Author: cbarnold@princeton.edu; phone 609-258-0250; fax: 609-258-5877