Development of Nondestructive Method for Prediction of Crack Instability
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frequent and costly inspection [I]. This work provides a new method to relate surface deformation at a crack front to possible failure under static load. Such findings will allow for a more efficient approach to aircraft maintenance. This work is one of the first laboratory applications of white light interference microscopy. White light interferometry enables topographical measurements with 3 nm vertical resolution and at highest magnifications lateral surface resolutions of 0.2 jtm. Such precision provides detailed topographic images. In addition, imaging cycle time for medium resolution scans, up to a square millimeter in size, is less than 5 minutes. Short scanning time requirements enable in-situ measurements such as the time dependent measurements of the crack front deformation performed in this work. Ti-6A1-4V was chosen for this work because it is the most commonly used titanium alloy in airframe applications. It is used in high performance primary components because of its toughness and fatigue strength [2, 3]. Specific aerospace applications of Ti-6AI-4V include turbine engine blades, parts of the fuselage of some high performance jet aircraft and the lower skin of aircraft wings.
61 Mat. Res. Soc. Symp. Proc. Vol. 5910 2000 Materials Research Society
EXPERIMENT Material Ti-6AI-4V alloy was investigated in this project. The primary microstructure in this study was mill-annealed sheet with grain sizes of approximately 5 p}m in the transverse direction and 20 gxm in the longitudinal direction (Figure la). Both longitudinal and transverse orientation behavior of this material was investigated. A duplex microstructure from a+p forged and solution treated plate, with grain sizes of approximately 10-20 pRm, was also examined (Figure lb). All specimens were flat 79 mm dogbone specimen with a 0.4 mm notch initiated in the center of the reduced section of the specimen. Prior to testing all specimens were polished to a low surface roughness (RMS 250 nm) to enable detailed examination of changes in surface topography.
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Figure 1: Microstructure of Ti-6AI-4V specimens (a) mill annealed sheet and (b) duplex microstructure forged plate Mechanical Testine Fatigue cracks were initiated from the notch of each sample by high cycle fatigue loading. As soon as a crack was visually detected the stress was reduced by 10% and the specimen was subjected to an additional 1000 cycles. This stress reduction was repeated several times to sharpen the crack front and reduce the surface deformation area. The precracked specimen was then loaded into a portable static load frame. The load frame was placed under a white light interference microscope for in-situ topographical data acquisition. Profilometry White light interference microscopy was used to document topographical changes at the crack front of each specimen. The micropsope is a Michelson interferometer [4] with one of the mirrors replaced by the surface being examined (Figure 2) [5]. The white light
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