Doublers

By slightly altering the circuit in Fig. 10.1, we can attain the cascade doubler, which has the output characteristics of the conventional doubler. This circuit is shown in Fig. 10.6a. The SPICE equivalent circuit is shown in Fig. 10.6b. 

This circuit used the same 1N4002 diodes as the circuit in Fig. 10.1, the same 1/u.F capacitors, and the same 5 kHz 3 V input pulse. A 100K resistor acts as a load for the circuit. The breadboard results are shown in Fig. 10.7a, with the IsSpice results in Fig. 10.7b. The top waveform is the DC output voltage and the bottom waveform is the input pulse. 

The results of the Micro-Cap and PSpice simulators are shown in Figs. 10.8 and 10.9, respectively. The results of the three simulators are tabulated and compared in Table 10.2. 

TABLE 10.2 Output Voltage Comparison Between SPICE and Breadboard 

Disadvantages Current capability limited by source, ripple not as controlled as in other topologies, no AC/DC isolation, no regulation 

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Optical second-harmonic generation (SHG) has recently emerged as a powerful surface probe [95, 96]. Second harmonic generation has long been used to produce frequency doublers from noncentrosymmetric crystals. As a surface probe, SHG can be caused by the break in symmetry at the interface between two centrosymmetric media. A high-powered pulsed laser is focused at an angle of incidence from 30 to 70° onto the sample at a power density of 10 to 10 W/cm. The harmonic is observed in reflection or transmission at twice the incident frequency with a photomultiplier tube. 

The use of inverter-type power suppHes (63) in place of doubler-type 6O-H2 suppHes results in significant reduction in the weight of microwave ovens. Disadvantages are primarily cost for the consumer market in addition to somewhat less efficiency and increased noise. Their use in commercial or industrial equipment is more attractive. 

Commercial frequency doublers have rehed on inorganic materials. The commercial future of doublers depends on not only the improvement in second-order materials but also the development of diode lasers capable of operating in the visible frequency domain. 

Doubler. A doubler is a pot stiU used to redistill whiskey and low wines from a beer stiU. The low wines are fed into the doubler where they are redistilled byway of steam enclosed in a scroU at the bottom of the stiU. The bottoms, the organic components remaining at the bottom of the stiU, are returned to the beer stiU to extract the alcohol. 

Steam is introduced at the base of the whiskey column through a sparger. Where economy is an important factor, as in a fuel alcohol plant, a calandtia is employed as the source of indirect heat. The diameter of the stiU, number of perforated and bubble cap plates, capacity of the doubler, and proof of distiUation are the critical factors that largely determine the characteristics of a whiskey. 

Bourbon Distillation. The basic distiUation system for the production of bourbon and other straight whiskeys consists of a beer stiU and a beer heater, thumper, or doubler (Fig. 4). The whiskey stiU consists of between 14 and 21 stripping trays. The upper portion of the stiU is fitted with either a bubble cap section or a section packed with copper rings to enhance the removal of unwanted flavors and ethyl carbamate precursors. The reduction of carbamate precursors requites strict adherence to a cleaning protocol with a 5% caustic solution as often as twice a week. 

Older tank cars have a center sill or beam running the entire length of the car. Most modern cars have no continuous sill, only short stub sills at each end. Cars with full sills have tanks anchored longitudinally at the center of the sill. The anchor is designed to be weaker than either the tank shell or the doubler plate between anchor and shell. Cars with stub sills have similar safeguards. Anchors and other parts are designed to meet AAR requirements. 

The impact forces on car couplers put high stresses in sills, anchors, and doublers. This may start fatigue cracks in the shell, particularly at the corners of welded doubler plates. With brittle steel in cold weather, such cracks sometimes cause complete rupture of the tank. Large end radii on the doublers and tougher steels will reduce this hazard. Inspection of older cars can reveal cracks before failure. 

As mentioned above, aircraft structure is typically quite thin with numerous small fasteners to achieve efficient load transfer through joints. Mechanical fasteners are laborious to install. Adhesive bonding of large area doublers and joints can be accomplished at significant labor hour savings over equivalent mechanically fastened designs. 

Repair. Repairs for damaged bonded structure can be either mechanical or adhesively bonded. Mechanical repairs are metallic doublers on one or both sides of a damaged component, held on by fasteners. The fasteners transfer the load through the doubler around the damaged site and restore part functionality. Although common for metal-to-metal bonded structure, mechanical repair of sandwich structure is rare because of the risk of further delamination. Unless the doubler and fasteners are perfectly sealed, water can travel into the honeycomb core eventually causing freeze-thaw damage and delamination. 

Bonded repair sizes, shapes and configurations vary greatly depending on the type of structure and size and location of damage. Typically the damaged area is removed and replacement details of like material and size are prepared to fit. Larger doublers and possibly triplers are installed over the replacement details to hold them in place and transfer load to them. Fig. 23 shows a typical bonded repair for a small damaged area on honeycomb bonded structure. 

Occasionally a repair is needed that will maintain an aerodynamically smooth surface on exterior surfaces of sandwich structure. In this case an internal doubler is used to hold the filler detail (called a dime-dollar repair for obvious reasons Fig. 24). Damaged trailing edges can be the most difficult to repair because of the difficult geometry (Fig. 25). Repairs of larger areas follow the same general concepts but must be engineered individually to account for specific part shape and load requirements. 

Local repair of delamination originally caused by non-durable surface treatment is only temporarily successful at best. The surface treatment on the unrepaired portion of the assembly remains susceptible to attack and the area of delamination will likely continue to grow once the assembly is put back into service and exposed to moist conditions. Replacement or complete remanufacture of the component is the only way to permanently address this type of damage. However, time-limited repairs using bonded or mechanical methods can be used to extend the life of the component until a major overhaul is scheduled. In some cases such as widespread disbond of fuselage doublers, mechanical repairs (rivets and fastened doublers) and continued inspection are used to extend the life of the skin indefinitely because of the high cost of replacement. 

Fig. 26. Bonded fuselage doubler pattern. Reproduced by permis.sioii of the Boeing Company.

Skins with bonded doublers have been used successfully on a large number of civil aircraft and are still used on new designs. The only widespread in-service problem with bonded doubler assemblies has been delamination caused by unstable surface preparation. Early fuselage skins with bonded doublers and inadequate surface preparation experienced severe delamination and subsequent corrosion. A majority of these delaminated doublers were the fail-safe tear straps (Fig. 27). Although the tear strap bond does not cany structural load, the bond... 

The first type of bonded design for this application was the beaded doubler panel (Fig. 28). This design was fairly successful at addressing the problems with simple riveted structure but had two primary drawbacks. The area under the beads remained a single thickness sheet and was still prone to fatigue. Reducing the unbonded areas under the beads was not a solution because it reduced the overall stiffness of the panel. Secondly, tooling for these panels was complex and not very robust. Autoclave pressure applied to the beaded areas of the doubler would cause them to collapse, so thick frames were fabricated with cutouts for the beads to protect them. A rubber layer bonded to the surface of the frames... 

Applications. Boron fibers are used as unidirectional reinforcement for epoxy composites in the form of preimpregnated tape. The material is used extensively, mostly in fixed and rotary wing military aircrafts for horizontal and vertical stabilizers, mdders, longerons, wing doublers, and rotors. They are also used in sporting goods. Another application is as reinforcement for metal matrix composites, in the form of an array of fibers pressed between metal foils, the metal being aluminum in most applications. 

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