Per Gren

New high-speed photography technique for observation of fluid flow in laser welding

Recent developments in digital high-speed pho- tography allow us to directly observe the surface topology and flow conditions of the melt surface inside a laser evap- orated capillary. Such capillaries (known as keyholes) are a central feature of deep penetration laser welding. For the first time, it can be confirmed that the liquid capillary surface has a rippled, complex topology, indicative of subsurface turbulent flow. Manipulation of the raw data also provides quantitative measurements of the vertical fluid flow from the top to the bottom of the keyhole. C 2010 Society of Photo-Optical Instrumentation

Shock wave generation in laser ablation studied using pulsed digital holographic interferometry

Pulsed digital holographic interferometry has been used to study the shock wave induced by a Q-switched Nd–YAG laser (λ = 1064 nm and pulse duration 12 ns) on a polycrystalline boron nitride (PCBN) ceramic target under atmospheric air pressure. A special setup based on using two synchronized wavelengths from the same laser for processing and measurement simultaneously has been introduced. Collimated laser light (λ = 532 nm) passed through the volume along the target and digital holograms were recorded for different time delays after processing starts. Numerical data of the integrated refractive index field were calculated and presented as phase maps showing the propagation of the shock wave generated by the process. The location of the induced shock wave front was observed for different focusing and time delays. The amount of released energy, i.e. the part of the incident energy of the laser pulse that is eventually converted to a shock wave has been estimated using the point explosion model. The released energy is normalized by the incident laser pulse energy and the energy conversion efficiency between the laser pulse and PCBN target has been calculated at different power densities. The results show that the energy conversion efficiency seems to be constant around 80% at high power densities.

PHASE EVALUATION AND SPECKLE AVERAGING IN PULSED TELEVISION HOLOGRAPHY

Double-pulsed (image-plane) TV holograms of transient bending waves in plates are recorded on separate frames in a CCD camera. A small angular offset between the reference and object beams, giving a spatial-frequency bias to the recorded pattern, permits quantitative evaluation of the phase of the interference. The Fourier spectrum of the image exhibits distinct parts that can be filtered out and inverse transformed to yield the phase information. Three different apertures of the imaging system are tested: a single slit, a double slit, and a three-hole aperture. Spatial speckle averaging is possible in the cases of the double-slit and three-hole apertures.

Pulsed TV holography combined with digital speckle photography restores lost interference phase

The measuring of situations with optical measuring methods is difficult when a deformation field must be determined while it is superposed to comparatively large rotating or translating object motion. Interferometric methods such as pulsed TV holography might be suitable to measure the small transient deformation, but the often-large bulk motion makes the phase information disappear. However, by a combination of digital speckle photography (DSP) (also called digital image correlation) with pulsed TV holography, such measuring problems can be mastered. A method to calculate the bulk in-plane motion by DSP from the usual pulsed TV holography recordings and then to use this information to restore the interference phase is proposed. This technique may be attractive in the study of transient vibrations overlaid on rotating or translating motions.

On structure-borne sound : experiments showing the initial transient acoustic wave field generated by an impacted plate

The initial propagating transient acoustic field in air generated by an impacted plate is visualized. The transient traveling flexural waves in the plate created by the impact are shown, simultaneously. The experiments are performed using double pulsed holographic interferometry. It is shown that flexural waves in a plate act as a series of traveling acoustic sources. Since the flexural waves in the plate are dispersive the trace matched acoustical waves further away from the impact source propagate at a smaller angle relative to the plate than those closer to the impact. An observer situated close to the plate and away from the impact point will first receive high‐frequency components of the sound. A quantitative evaluation of the acoustic field also shows that if there is a transient increase in pressure on one side of the plate there will be a similar decrease on the other side.

Four-pulse interferometric recordings of transient events by pulsed TV holography

With traditional double-pulsed holographic interferometry or pulsed TV holography, the experiment usually has to be repeated to allow the recording of a time sequence of interferograms of the event. With the proposed technique a sequence of four interferograms of a solitary transient event is measured. A twin oscillator, injection-seeded, pulsed Nd:YAG laser is incorporated into a pulsed TV holography set-up. With orthogonal polarisation and double pulsing of each of the two channels of the laser, four pulses are recorded on two separate CCD-frames. Four interferograms of a laser-impacted plate obtained from the same experiment, show how the bending waves develop and propagate in the plate.

Transient acoustic near field in air generated by impacted plates

The transient acoustic near field in air from impacted cantilever plates of two different geometrical shapes and materials is studied using two‐reference‐beam double‐pulsed holographic interferometry. The acoustic waves, observed in two directions simultaneously, are generated by traveling flexural waves in the plates and by the impact process itself. A subsequent phase‐stepping procedure is used for quantitative evaluation of the registered two‐dimensional projections of the acoustic fields. An interesting observation is the acoustic radiation at the free end of the plate, where sound waves propagate into the air in oblique directions from the plate. At the region right above the free end no acoustic waves are observed. Supersonic flexural waves in the plates generate trace‐matched acoustic waves in the surrounding air. Calculations of the corresponding two‐dimensional transient acoustic fields are performed. Those are based on an analytical solution of the Kirchhoff plate equation. Calculated results re...

Speckle photography combined with speckle interferometry

By combining speckle interferometry (SI) measurements with speckle photography, the fringe visibility can be kept high despite the presence of a large bulk or rotating motion of the object. This combined technique improves the usability and measuring range of both pulsed and phase-stepped SI-methods. This paper reviews the theory of fringe formation in SI and shows some recent applications of this combined technique.

Mechanical and Vibration Characteristics of Laminated Composite Plates Embedding Shape Memory Alloy Superelastic Wires

Currently, there is a great interest in the study of shape memory alloy (SMA) composites, since SMA wires with a small diameter have become commercially available. Many potential uses have been found for SMA composites in shape control, vibration control, and for the realization of structures with improved damage tolerance. In this work, two types of SMA-hybridized composites are presented for investigating the mechanical and vibration characteristics. The first one contains unidirectional superelastic SMA wires, while the other has been realized with embedded knitted SMA layers. The samples from these laminates have been tested according to “Charpy method” (ASTM D256) and static flexural test method (ASTM D790) to evaluate the influence of the integration of thin superelastic SMA wires on the impact behavior and the mechanical properties of the hybrid composites. Moreover, since the SMA wires are expected to give damping capacity, by measuring the vibration mode of a clamped cantilever using laser vibrometry, the influence of both SMA arrangements on the vibration characteristics has been investigated. Finally, further tests have been carried out on composite panels realized by embedding unidirectional steel wires to distinguish the influence of the martensitic transformation from the pure introduction of a metallic wire into the polymeric matrix.

Laser vibrometry measurements of vibration and sound fields of a bowed violin

Laser vibrometry measurements on a bowed violin are performed. A rotating disc apparatus, acting as a violin bow, is developed. It produces a continuous, long, repeatable, multi-frequency sound fro ...