High-magnification moiré interferometer for crack tip analysis of steels

Abstract

ABSTRACT--A high-magnification moir6 interferometer, particularly suitable for near-tip field analysis in cracked materials, is described. It has a submillimeter field of view, a high-resolution image sensor (1.4 million pixels), X-Y-Z translation stage and an optical fiber light delivery system. These features enable the microscope head to observe the crack tip while the specimen is loaded in a standard tensile test machine. Automated fringe pattern analysis, using temporal phase shifting and spatial phase unwrapping, enables the x or y displacement component to be measured and the corresponding in-plane strain component computed. The displacement accuracy is better than 40 nm, and the effective strain gage dimension is ~ 25 txm. Furthermore, the interferometer has a built-in white light microscope that allows the observation of the specimen granular microstructure in exact registration with the displacement field. The interferometer has hence been employed to investigate the near-tip fields of a precracked stainless steel specimen under load. The influence of the grain boundaries on the measured displacement fields was relatively minor. The near-tip strain field shows a significant asymmetrical behavior despite pure mode I loading conditions. KEY WORDS--Moir6 interferometry, microstructure, crack tip, strain field Fracture criteria and fatigue crack propagation mechanisms are traditionally analyzed using empirical models such as the Paris law: 1dN with a indicating the crack length, N the number of cycles, AK the stress intensity range and C and m empirical parameters that depend on the material. This approach is well tested and satisfied when the material is considered as a continuous medium. However, it is incapable of yielding a better understanding of the physical processes that take place in a small volume of material ahead October4, 1999. of the crack tip (covering a few grains). While it is recognized that this region at the crack tip plays a crucial role, the mechanism of crack propagation in fatigue and fracture is still not well understood. It seems reasonable to expect that the interactions between deformation fields and microscopic features at and near the crack tip will exert a strong influence on empirical parameters such as C and m. For further advances in the understanding of these relationships, it then becomes necessary to describe accurately the deformation over a few grains ahead of the crack tip so as to take into account the inhomogeneous and anisotropic nature of real materials.Difficulties in theoretical descriptions of these fields arise because of the number of parameters involved. A recent mathematical model has been proposed in terms of dislocation description: the influence of neighboring grain boundaries during fatigue crack growth in a polycrystal is described by the introduction of a shear modulus dependence on the grain's orientation, and its influence over a discrete dislocation distribution in the slip bands. 2 For elastic materials in which strain gradient effects predomi...

Recent Applications of Moiré Interferometry

“…Within a small field of view, the relative displacements are typically small unless the strains are extremely large. For those applications, displacement resolutions were increased by digital image processing techniques: the optical/digital fringe multiplication (O/DFM) method [80,82] and the phase shifting technique [83].…”

“…La Porta et al [83] used microscopic moiré interferometry with the phase shifting technique to investigate the near-tip fields of a precracked stainless steel specimen under load. The approach proposed by Han and Post was combined with the O/DFM method to document the heterogeneous deformation of titanium in elastic tension [85], the fiber/matrix interactions in a unidirectional Boron/Aluminum metal-matrix composite [86], micromechanical thermal deformations of semiconductor packages and subassemblies [30,41] and the changes in microstrain across bonded dentin interfaces [87].…”

Utilization of Moiré interferometry to study strain transfer across polymer interfaces within multilayer thermoplastic elastomers

“…Moiré interferometry is a whole field, real time, high sensitivity, and high spatial resolution technique for measuring deformation 24. It is an especially valuable tool for the measurement of nonuniform and in‐plane deformation 24–28. While this technique has been widely used to study the strain distributions in rigid materials and composites, application of this technique to soft viscoelastic polymers above their glass transition temperatures is relatively new 24, 29.…”

Development of a transparent indenter measurement system and indentation analysis for material mechanical property evaluation

“…In practice, three and four fringe pattern images are frequently used to extract the phase. More than four fringe patterns can also be used and the redundant information is used to reduce the noise and system error [80] .…”

“…Due to its high sensitivity and high spatial resolution, moiré interferometry has been used to measure small area deformation, such as crack tip and MEMS. High magnification moiré interferometer has been used for crack tip analyses by La Porta et al [80] . By applying fiber optics, the portable moiré interferometer is very compact and is able to investigate the deformation of sub-millimeter area.…”