Three-dimensional crack tip deformation measurement using combined Moiré-Sagnac interferometry

Abstract

ABSTRACTmA combined Moir6-Sagnae interferometry method is developed for in-plane (u and v) and out-of-plane (w) surface deformation measurement. The combined optical setup is used to measure three-dimensional crack tip deformations of AI 2024-0 and AI 2024-T4 specimens at room temperature and an Inconel 909 specimen at 570~ Measured displacements near the crack tip region of the AI 2024-T4 specimen are used as input nodal displacements to determine stress intensity factors based on two-dimensional and three-dimensional Jacobian derivative method. The values compare favorably with theoretical calculations. The extent of the three-dimensional crack tip deformation zone is also discussed.KEY WORDS--3-D crack-tip deformation, Moir6-Sagnac Interferometry, high-temperature measurement, Jacobian Derivative method Three-dimensional crack tip displacement measurements are often needed in the field of fracture mechanics research. The crack tip region may be subjected to large deformation gradients, and typical experimental methods, which can only measure discrete surface deformations, may not provide sufficient information about the deformations in this region. Acquiring crack tip displacements becomes even more complicated when the measurements are made under harsh environmental conditions such as under high temperatures. In these instances, it may not be possible or desirable to affix a measuring device to the specimen. As an alternative, various optical techniques have been developed to measure surface deformations. These optical techniques provide fullfield measurements. The response time is essentially instantaneous, and the measurement system does not interfere with the process examined.Moir6 interferometry has been shown to be applicable for in-plane deformation measurement of specimens under harsh environmental conditions, i.e., at elevated temperatures 1-4 or in liquid medium. temperature. 6-I~ Measurements obtained by Michelson interferometry or similar interferometric methods are sensitive to variations in the medium (such as temperature field fluctuation and flatness of the furnace window) and thus may not be suitable for high-temperature testing. Moir6 interferometry can be used to measure both in-plane and out-of-plane displacements; 8,9 however, the approach requires the placement of a reference grating near the surface of the specimen to measure the out-of-plane displacements. This configuration is not practical for a specimen under elevated temperature conditions. The technique of holographic interferometry requires the superposition of a deformed image onto an original undeformed image 6 and thus is sensitive to the stability of the mechanical system and environmental conditions, l~ A simpler method for obtaining the out-of-plane displacements is desired.The purpose of this paper is to report the development of a combined optical method capable of full-field threedimensional surface displacement measurements of specimens under elevated temperature conditions. Moir6 interferometry is used to obtain the in-...

Recent Applications of Moiré Interferometry

“…Kang and Anderson [121] used a combined Moire-Sagnac interferometry method to measure three-dimensional crack tip deformations at room as well as high temperatures. Kang and Lu [122] investigated the displacement fields near the tip of a crack in a bimaterial joint under mixed-mode loading.…”

“…It is recognized as permanent deformation in the neighborhood of the crack path, and for nonmetals it is known as the damage wake. Moiré interferometry has been used extensively for experimental fracture mechanics, most notably, crack tip displacement measurement [120][121][122][123][124][125] and deformation analyses of fracture zone near a crack tip [126][127][128][129][130][131][132][133][134].…”

Real-Time Measurements on Crack-Tip Deformation Fields of LY12 Samples under Impact Tension

“…Now, some optical testing methods have been used in the investigation of dynamic performance of materials. Moiré interferometry method is a representative one, and it has been used in in-plane deformation measurements at elevated temperatures [1][2][3][4] .However, most of uses of moiré interferometry methofs are related to statistic and quasistaitc measurements because the testing precision will be severely affected by the rough environment when it's applied under high-speed impact loading.…”