Pulsed impact

Oxide layers are removed under the impact of laser beam 5-7 mm in diameter as distinct from cutting and welding procedures wherein concentrated beams 0.1-0.3 mm in diameter are applied oxide layers are removed under one-pulse impact allowing attaining acceptable decontamination rates (e g., at 50 Hz pulse-frequency the decontamination capacity can reach 3-5 m per hour). 

This paper deals with the control of weld depth penetration for cylinders in gold-nickel alloy and tantalum. After introducing the experimental set-up and the samples description, the study and the optimization of the testing are presented for single-sided measurements either in a pulse-echo configuration or when the pump and the probe laser beams are shifted (influence of a thermal phenomenon), and for different kind of laser impact (a line or a circular spot). First, the ultrasonic system is used to detect and to size a flat bottom hole in an aluminium plate. Indeed, when the width of the hole is reduced, its shape is nearly similar to the one of a slot. Then, the optimization is accomplished for... 

The common civil engineering seismic testing techniques work on the principles of ultrasonic through transmission (UPV), transient stress wave propagation and reflection (Impact Echo), Ultrasonic Pulse Echo (UPE) and Spectral Analysis of Surface Waves (SASW). 

Figure Bl.10.7. Electron impact ionization coincidence experiment. The experiment consists of a source of incident electrons, a target gas sample and two electron detectors, one for the scattered electron, the other for the ejected electron. The detectors are coimected tlirough preamplifiers to the inputs (start and stop) of a time-to-amplitiide converter (TAC). The output of the TAC goes to a pulse-height-analyser (PHA) and then to a nuiltichaimel analyser (MCA) or computer.

The air bag industry has become one of the principal users of pyrotechnic compositions in the world. Most of the current air bag systems are based on the thermal decomposition of sodium azide, NaN, to rapidly generate a large volume of nitrogen gas, N2. Air bag systems must function immediately (within 50 ms) upon impact, and must quickly deploy a pulse of reasonably cool, nontoxic, unreactive gas to inflate the protective cushion for the driver or passenger. These formulations incorporate an oxidizer such as iron oxide to convert the atomic sodium that initially forms into sodium oxide, Na20. Equation 1 represents the reaction. 

Shock-compression processes are encountered when material bodies are subjected to rapid impulsive loading, whose time of load application is short compared to the time for the body to respond inertially. The inertial responses are stress pulses propagating through the body to communicate the presence of loads to interior points. In our everyday experience, such loadings are the result of impact or explosion. To the untrained observer, such events evoke an image of utter chaos and confusion. Nevertheless, what is experienced by the human senses are the rigid-body effects the time and pressure resolution are not sufficient to sense the wave phenomena. 

Ang, J.A., B.D. Hansche, C.H. Konrad, and W.C. Sweatt (1991), Pulsed Holography For Hypervelocity Impact Diagnostics, Sandia National Laboratories, SAND91-2871C. 

G.T. Gray III and P.S. Follansbee, Influence of Peak Pressure and Pulse Duration on the Substructure Development and Threshold Stress Measurements in Shock-loaded Copper, in Impact Loading and Dynamic Behavior of Materials (edited by C.Y. Chiem, H.-D. Kunze, and L.W. Meyer), Deutsche Gesellschaft fuer Metall-kunde, Germany, 1988, 541 pp. 

MPI is especially valuable for elemental analyses with typical useful yield of 10 . Because SALI is laser-based, expected improvements over the next few years, in particular for vacuum-ultraviolet laser technology, should have a significant impact. High repetition rate Nd—YAG systems with sufficient pulse energy are already available to 50 Hz, and probably can be extended to a few hundred Hz. 

SNMS sensitivity depends on the efficiency of the ionization process. SNs are post-ionized (to SN" ) either hy electron impact (El) with electrons from a hroad electron (e-)heam or a high-frequency (HF-) plasma (i.e. an e-gas), or, most efficiently, hy photons from a laser. In particular, the photoionization process enables adjustment of the fragmentation rate of sputtered molecules by varying the laser intensity, pulse width, and/or wavelength. 

Typical current pulses observed for x-cut quartz, z-cut lithium niobate, and y-cut lithium niobate are shown in Fig. 4.3. Following a sharp rise in current to an initial value (the initial rise time is due to tilt, misalignment of the impacting surfaces), the wave shapes show either modest increases in current during the wave transit time for quartz and z-cut lithium niobate... 

Fig. 4.7. The dielectric permittivity of impact-loaded dielectrics can be determined from current pulse measurements on disks biased with a voltage V. The magnitudes of the normalized current pulse values shown for two crystallographic orientations of sapphire are linear change with applied strain (after Graham and Ingram [68G05]).

Fig. 4X When x-cut quartz is subjected to impact loading whose duration is less than wave transit time, an anomalous current pulse can be observed after the stress release. The diagram shows locations at which experiments were conducted and delineates the region of normal and anomalous response (after Graham and Ingram ([72G03]).

Fig. 5.2. Current-versus-time records for x-cut quartz impact loaded to stresses of 2.5, 3.9, 4.5, 5.9, 6.5, and 9.0 GPa are shown, illustrating the drastic changes occurring with mechanical yielding and conduction. Time increases from right to left. The current pulses are in the center of each record and are characterized by a brief horizontal trace (zero current before impact) followed by a rapid jump to a current value (after Graham [74G01]).

Fig. 5.5. The electrical response of piezoelectric polymers under shock loading is studied experimentally by placing the thin PVDF element on the impact surface of a standard target, either the polymer, Kel F, z-cut quartz, or z-cut sapphire. The im-pactor is typically of the same material. The current pulse is recorded on transient digitizers with frequency responses from 250 to 1000 MHz.

Fig. 5.6. Typical current-time responses from impact-loaded PVDF are shown for samples on the standard materials indicated. In each record the upper traces are the full record showing short duration negative and positive current pulses due to loading and release in the standard. Time increases from left to right. The detail of each pulse depends upon the shock impedance of the materials. In each record, enlarged views of loading and release pulses are shown.

Fig. 5.11. The study of shock compressibility of pressure-sensitive magnetic alloys was carried out with the quartz gauge impact technique. Loading was either with the specimen material or a quartz gauge. Resulting stress pulses were recorded with a quartz gauge (after Graham et al. [67G01]).

Fig, 5.15. A measured current-time pulse for shock-loaded Invar is shown. Time increases from left to right. The wave shape is closely predicted by the simple theory. Time from impact to peak current is about 1 fis. 

Fig. 5.21. The shock-induced polarization of polymers as studied under impact loading is shown. For the current pulse shown, time increases from left to right. The increase of current in time is due to finite strain and dielectric constant change. (See Graham [79G01]).

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