ABSTRACT
A high-magnification moiré interferometer has been constructed with a spatial resolution of the order of 1 microm to measure the local in-plane displacement field associated with a material's microstructure. Laser illumination passes through phase-stepping optics and is delivered to the microscope head by polarization-preserving single-mode optical fibres. The head itself is a compact unit consisting of collimating optics, an objective lens and a charge coupled device (CCD) camera. Thin-phase gratings are cast onto the sample surface with a compliant epoxy resin and coated with ca. 5 nm of gold to enhance the fringe contrast and reduce speckle noise. By switching between the laser illumination and white-light illumination, the underlying microstructure is viewed in exact registration with the measured displacement fields. The application of the instrument is illustrated here by visualization of displacement fields in polymer-bonded explosives (PBXs) during deformation to failure. PBXs are highly filled polymers consisting of up to 95% by weight crystalline explosive bound in a variety of polymeric binders. The mechanical properties of PBXs are highly dependent on the microstructure, and moiré interferometry is an ideal tool for investigating the relationship between the 1-100 microm sized crystals and the displacement fields. Methods such as this are required if computer models of inhomogeneous materials are to be accurately validated.
Subject(s)
Azocines/chemistry , Crystallography/methods , Heterocyclic Compounds, 1-Ring/chemistry , Materials Testing/instrumentation , Moire Topography/instrumentation , Motion , Elasticity , Equipment Design , Feasibility Studies , Interferometry/instrumentation , Interferometry/methods , Materials Testing/methods , Moire Topography/methods , Quality Control , Stress, Mechanical , Tensile StrengthABSTRACT
A technique to measure two-dimensional deformation fields of a layer inside materials during dynamic events such as impact experiments is presented. Even optically opaque materials like cement can be evaluated when flash x rays are used. Blocks of polyester and cement were prepared with a layer of x-ray-absorbing lead particles. The specimens were then hit by a 9-mm-diameter steel sphere (ball bearing) fired from a 9-mm-bore gas gun at a velocity of 373.5 +/- 3.0 ms(-1). A 30-ns-long x-ray pulse exposed one radiograph before impact; another radiograph was exposed a short time after the impact on the specimen. The two-dimensional displacement field was obtained when the x-ray radiographs were digitized by a conventional flatbed scanner, and a digital speckle photography algorithm was used to calculate the displacements. The flash x-ray technique allowed examination of the deformation at the layer inside the material during failure, thus giving interesting data about the material flow field around the impactor.
ABSTRACT
A metrology system is presented that measures internal three-dimensional (3-D) displacement fields. The system uses a stereo pair of flash x-ray heads and correlation analysis to measure the true deformation of a layer of x-ray-absorbent particles inside the specimen. The 3-D deformation field inside blocks of polyester was determined. The polyester blocks were impacted by a 9-mm steel ball bearing fired from a 9-mm-bore gas gun at a speed of 373.5 +/- 3.0 m s(-1). At a given time after impact, a short-duration (30 ns) flash x-ray pulse exposes the x-ray radiographs and freezes the events during impact. Thereafter, the x-ray radiographs are scanned into a personal computer and analyzed as in digital speckle photography.
ABSTRACT
An algorithm for unwrapping noisy phase maps has recently been proposed, based on the identification of discontinuity sources that mark the start or end of a 2π phase discontinuity. Branch cuts between sources act as barriers to unwrapping, resulting in a unique phase map that is independent of the unwrapping route. We investigate four methods for optimizing the placement of the cuts. A modified nearest neighbor approach is found to be the most successful and can reliably unwrap unfiltered speckle-interferometry phase maps with discontinuity source densities of 0.05 sources pixel(-1).
ABSTRACT
An automated system has been constructed to process double-exposure speckle-photography and particle-image-velocimetry images. A 3 × 3 array of laser beams probes the photograph, forming nine fringe patterns in parallel; these are then analyzed sequentially by digital computer and the use of a two-dimensional Fourier-transform method. Results are presented showing that the random errors in the measured displacements from such a system approach the expected speckle-noise-limited performance, with a total analysis time per displacement vector of 160 ms.
