Mechanical shock

Initiators. Explosives such as mercury fulminate and certain metallic azides which are extremely sensitive to mechanical shock, and are accordingly used in small quantities in detonators to initiate the explosion of larger masses of less sensitive material. 

Aluminum bromide and chloride are equally active catalysts, whereas boron trifluoride is considerably less active probably because of its limited solubiUty in aromatic hydrocarbons. The perchloryl aromatics are interesting compounds but must be handled with care because of their explosive nature and sensitivity to mechanical shock and local overheating. 

Decomposition Hazards. The main causes of unintended decompositions of organic peroxides are heat energy from heating sources and mechanical shock, eg, impact or friction. In addition, certain contaminants, ie, metal salts, amines, acids, and bases, initiate or accelerate organic peroxide decompositions at temperatures at which the peroxide is normally stable. These reactions also Hberate heat, thus further accelerating the decomposition. Commercial products often contain diluents that desensitize neat peroxides to these hazards. Commercial organic peroxide decompositions are low order deflagrations rather than detonations (279). 

The addition of thoria to a Re—W ahoy produces a material having 74 wt % W, 24 wt % Re, and 2% Th02 that is used for heated cathodes in electron tubes. This material has good ducthity and high resistance to breaking by mechanical shock. 

Two alloys containing tungsten are commercially available. The first, containing about 3 wt % rhenium, is used for heating filaments. The rhenium contributes improved resistance to thermal and mechanical shock. The second alloy contains about 25 wt % rhenium. This latter alloy is sold as sheet, rod, and heavy wire and may be fabricated for various uses. An important use of these rhenium alloys is in the constmction of thermocouples. Various combinations, 3 wt % Re—97 wt % W, or 25 wt % Re—75 wt % W, are usehil for measurement of temperatures to 2500°C (see Temperaturemeasurement). 

The result is a hard, abrasion-resistant surface, important in many appHcations of cast kon. The depth of the chill may be controlled by regulating the amount of tellurium added. The casting shows a sharp demarcation line between the chilled and unchilled regions there is no intermediate or motded zone. Yet, the chilled portion shows excellent resistance to spalling from thermal or mechanical shock. Tellurium-treated kon is more resistant to sulfuric and hydrochloric acids than is untreated, unchilled gray kon. The amount added ranges from 0.005 to 0.1% ca 60% is lost by volatilization. Excessive addition causes porosity in the castings. 

The SPRTs are devices of superb accuracy and resolution, but they are fragile and can easily be broken. They can also be put out of caUbration by strain, iaduced by even slight mechanical shock or vibration. The principal use of SPRTs ia scieace and iadustry is to maintain the cahbrations of working thermometers. 

Impervious graphite heat exchangers machined from solid blocks are also available (15,16). The solid block constmction is less susceptible to damage by mechanical shock, such as steam and water hammer, than are shell and tube exchangers. Block exchangers are limited in size and cost from 50—100% more than shell and tube units on an equivalent area basis. 

Irons cast, malleable, and high silieon (14.5 pereent). Their lack of ductility and their sensitivity to thermal and mechanical shock. 

Boro.silicate gla.s.s and impregnated graphite. Their lack of ductihty and sensitivity to thermal and mechanical shock should be taken into account. 

Mechanical cleaning includes scrubbing, scraping, brushing, mechanical shocking, and ultrasonic procedures. Scrubbing with a bristle brush and a mild abrasive is the most widely used of these methods the others are used principally as supplements to remove heavily encrusted corrosion products before scrubbing. Care should be used to avoid the removal of sound metal. 

Mechanical sensitivity can be divided into two categories—mechanical friction and mechanical shock. Mechanical friction can be defined as mechanical energy imposed by materials being wedged between surfaces and mechanical shock can be defined as mechanical energy imposed by materials undergoing an impact. Several tests for measuring the sensitivity to friction and the impact of materials are detailed in CCPS G-13. 

The polymers are, however, more brittle than polystyrene and not suitable for applications which are to be subject to mechanical shock. 

Solid explosives which are classified as extremely sensitive to mechanical shock. 

Materials which (in themselves) are capable of detonation or of explosive decomposition or of explosive reaction but which require a strong initiating source or which must be heated under confinement before initiation. Includes materials which are sensitive to thermal or mechanical shock at elevated temperatures and pressures or which react explosively with water without requiring heat or confinement. Fire fighting should be done from an explosive resistant location. 

Nickel/silicon alloy (10% silicon, 3% copper, and 87% nickel) is fabricated only as castings and is rather brittle, although it is superior to the iron/silicon alloy with respect to strength and resistance to thermal and mechanical shock. It is comparable to the iron/silicon alloy in corrosion resistance to boiling sulfuric acid solutions at concentrations above 60%. Therefore, it is chosen for this and other arduous duties where its resistance to thermal shock justifies its much higher price compared with iron/silicon alloys. 

Hammetter [91H01] has carried out DTA analysis of a shock-modified but unreacted 2A1 -t- Fe203 mixture and found a preinitiation event, indicating the presence of shock-produced Hercynite (spinel phase FeAl204). These preinitiation events observed in thermal analysis provide direct evidence for the influence of mechanical, shock-induced mixing in controlling solid state chemistry. 

It is not subjected to hydrogen embrittlement as is tantalum, niobium and nickel alloys, and thus is able to sustain thermal and mechanical shock after exposure to gaseous hydrogen at high temperatures. 

These are iron alloys that contain 14-18% Si and are reported as first being developed in 1912 , although it was not until 1954 that they were first evaluated for use as impressed-current anode material in cathodic protection. Its major disadvantage is that it is a hard brittle material unable to sustain thermal or mechanical shock. 

Ceramic anodes may be cast or sintered around a central steel core which acts as the electrical conductor. However, anodes produced in this form are brittle and susceptible to mechanical shock. 

Laminae 4116 and Selectron 5003, were found to be satisfactory and interchangeable for use in these compns. These expls are thermally stable, relatively insensitive to mechanical shock, and have a brisance approaching that of Comp C-3. Weight-drop impact tests indicate them to be of the order of sensitivity of TNT... 

Safety. Since organic peroxides can be initiated by heat, mechanical shock, friction or contamination, an enormous problem in safety presents itself. Numerous examples of this problem have already been shown in this article. Additional examples include the foilowing methyl and ethyl hydroperoxides expld violently on heating or jarring, and their Ba salts also are extremely expl the alkylidene peroxides derived from low mw aldehydes and ketones are very sensitive and expld with considerable force polymeric peroxides of dimethyl ketene, -K>-0-C(CH3)2C(0)j-n, expld in the dry state by rubbing even at —80° peroxy acids, especially those of low mw, and diacetyl, dimethyl, dipropkmyl and methyl ethyl peroxides, when pure, must be handled only in small amts and... 

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