13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Experimental Investigation of Mode I Fatigue Crack Growth Behavior of Titanium Foils Lee Chang-Woo1, Liu Liu2,*, Holmes W John3 1Department of Advanced Analysis and Characterization, Korea Institute of Materials Science, Chang won, Korea 2 School of Aerospace Engineering, Beijing Institute of Technology, Beijing, 100081, China International Research Center for Clean Energy Materials and Systems, Beihang University, Beijing, 100191,China * Corresponding author: liuliu@bit.edu.cn Abstract A low-cost experimental apparatus has been developed to investigate the Mode I fatigue crack growth behavior of thin metallic foils and sheets. The apparatus utilizes magnetic coupling between a ceramic magnet and a rotating steel disk to induce cyclic tensile loads in notched rectangular test specimens.To illustrate the testing apparatus, Mode I fatigue crack growth in 30 �m thick high purity titanium foils was studied. Experiments were performed at ambient temperature using a loading frequency of 2 Hz and a nominal stress ratio of 0.1. The cyclic crack growth data could be fit to a Paris relationship between crack growth rate and stress-intensity range. The stress intensity factor exponent, m, in the Paris relationship was between 4 and 5, which is comparable to the relatively high values found in the literature for the tension-tension fatigue of other metallic bulk materials. Self-similarity analysis was used to explain the observed higher m values for thin metallic foils. Keywords Fatigue testing, Mode I crack growth, foil 1. Introduction Foil thickness materials, with a thickness in the range of 10 �m to 250 �m are used in a variety of applications, including micro-electro-mechanical systems (MEMS), integrated circuits, fuel cells and printers. The progressive trend of miniaturization of structural components leads requires knowledge regarding the mechanical behavior and properties of thin metallic materials. As discussed below, the tensile, fracture and fatigue behavior of foil-thickness materials can be very different from that observed in bulk materials. In addition to influencing the tensile properties (ductility and strength) of thin foils, grain size and grain orientation can have a major effect on crack initiation, crack growth and fracture toughness. Compared to bulk materials, microstructural inclusions or precipitates in thin foils will also have a larger impact on fracture toughness and crack growth. Surface damage, in the form of roughness, oxidation or corrosion will also have a much larger effect on the mechanical behavior of thin metallic foils. Several experimental and theoretical investigations of the mechanical properties of foils have been carried out during last two decades. Klein et al [1] studied the stress-strain behavior of Cu and Al with thicknesses ranging from 10 and to 250 �m and observed a thickness effect on fracture strain, which decreased with decreasing specimen thickness. In experiments with thin Cu films, Zhang et al [2] observed an increase in yield strength and a decrease in fatigue life as thickness was decreased. In another study, Zhang et al [3] found that although the mode of fatigue damage in 25 �m thick 304 stainless steel followed that of the bulk material, the fatigue strength was higher for the 25 �m foils; the increase in fatigue strength was attributed in part to the smaller grain size of the foils Fracture-mechanics-based test results. In a review of fatigue test techniques and experimental results for materials used in MEMS, Sharp and Turner found that the majority of available fatigue data for foils was in the form of S-N curves obtained from bending fatigue. Limited fatigue data was found for uniaxial loading with a positive mean stress [4]. The cyclic crack growth behavior from a pre-existing notch in free-standing thin foils is still a relatively unexplored area of research.
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