Moire Fringe Effect

Automated Moire interferometry for strain analysis



Carlos Ruiz, Dennis Poon and others at the Rolls Royce University Technology Centre in Solid Mechanics, associated with OCAMAC, are making high sensitivity optical strain measurements based on Moire interferometry. The strain measurement system is computer controlled for performing automated Moire pattern analysis, data acquisition, image editing and processing. The automated Moire interferometry provides:

Whole field measurements of in-plane displacements

Wide measurement range (20 µm - 25 mm) with a sensitivity of 0.4 µm yielding strain measurements from 0.002-2%

Fast and accurate strain measurement through an automated fringe pattern analysis

Information for diverse applications, particularly for composites such as in the study of strain concentration, crack initiation, residual strain and the micro/macro mechanics of composite structures.

The strain measurement system has been successfully applied to measure the strain distribution of a laminated composite specimen with a dropped ply where crack initiation and a high strain concentration is expected. As shown in the Moire fringe pattern, the fringes clearly indicate strain concentrations occurring on the right where a crack was formed at the ply drop-off. The fringes were analyzed by computer and the strain was calculated within a few seconds. The corresponding distribution of shear strain is also shown, where the shear strain has the maximum value at the crack tip.





Displacement fringe pattern produced by Moire interferometry after a crack is formed at the ply drop-off in a laminated composite




Distribution of shear strain in laminated composite (peak-valley value = 1.35%)

http://www.ocamac.ox.ac.uk/newsletters/Newsletter%203/StrainAnalysis.html

The Oxford Centre for Advanced Materials and Composites (OCAMAC) is sponsored by the Departments of Materials, Engineering, Chemistry and Physics at Oxford University.

The objective of OCAMAC is :

to foster interdisciplinary research into the scientific and technological problem of processing, properties, design and fabrication associated with advanced materials.





Moire Fringe Effect




High precision linear displacement measurements can be made with Moire Fringes. The Moire Fringe Effect. Both of the strips are transparent (or reflective), with black lines at measured intervals. The spacing of the lines determines the accuracy of the position measurements. The stationary strip is offset at an angle so that the strips interfere to give irregular patterns. As the moving strip travels by a stationary strip the patterns will move up, or down, depending upon the speed and direction of motion.




A device to measure the motion of the moire fringes is shown in See Measuring Motion with Moire Fringes. A light source is collimated by passing it through a narrow slit to make it one slit width. This is then passed through the fringes to be detected by light sensors. At least two light sensors are needed to detect the bright and dark locations. Two sensors, close enough, can act as a quadrature pair, and the same method used for quadrature encoders can be used to determine direction and distance of motion.


Measuring Motion with Moire Fringes

These are used in high precision applications over long distances, often meters. They can be purchased from a number of suppliers, but the cost will be high. Typical applications include Coordinate Measuring Machines (CMMs).

http://claymore.engineer.gvsu.edu/~jackh/books/model/html/model-187.html




Spectroscopy

Magnetic resonance
A further branch of spectroscopy that has provided information on nanostructures is magnetic resonance, which involves the study of microwave (radar frequency) and radiofrequency transitions. Most magnetic resonance measurements are made in fairly strong magnetic fields, typically B~ 0.33 T (3300G) for electron spin resonance ESR, also called electron paramagnetic resonance EPR, and B ~ 10 T for nuclear magnetic resonance NMR. Several types of magnetic resonance are discussed here.
Nuclear magnetic resonance involves the interaction of a nucleus possessing a nonzero nuclear spin I with an applied magnetic field B app to give the energy level splitting into 2 I + I lines with the energies

Em = h γ Bapp m

Where γ is the gyromagnetic ratio, sometimes called the magnetogyric ratio, characteristic of the nucleas, and m assumes integer or half integer values in the range -1 ≤ m ≤ +1 depending on whether I is an integer or a half integer. The value of γ is sensitive to the local chemical environment of the nucleus, and it is customary to report the chemical shift ∂ of γ relative to a reference value γR:

∂ = γ – γR/ γR

Chemical shifts are very small and are usually reported in parts per million ppm.


Nanosolids, Wiley


Gyromagnetic ratio

In physics, the gyromagnetic ratio (also sometimes known as the magnetogyric ratio in other disciplines) of a particle or system is the ratio of its magnetic dipole moment to its angular momentum, and it is often denoted by the symbol γ, gamma. Its SI units are radian per second per tesla (s^-1·T^-1) or, equivalently, coulomb per kilogram (C·kg^-1).

Wikipedia