test equipment and technioues for evaluating composites propellants

8/2/2019 Test Equipment and Technioues for Evaluating Composites Propellants

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TEST EQUIPMENT AND TECHNIOUES FOR EVALUATINGCOMPOSITE PROPELLANT PROCESSING AND HANDLING HAZARDS

byV. T. Dinsdalel

THIOKOL CHEMICAL CORPORATIONWasatch Division

Brigham City, UtahABSTRACT

An essentia l asp ect of r ocke t propellant manufacturing i s the accura te evalu-ation of oroces sing and handling haza rd s. To determ ine the hazard s of prope lla ntconstituents, intermediate com positions, and final compositions present i n compositesolid arooe llan t. adequate te st equipment and techn:ques were required. The prog raminitiated by Thiokol Chemical Corporation's Wasatch Division to develop the specializedtest equipment and techniques necessary will be the subject of this paper.

Hazards were evaluated for norm al proc ess conditions and for hazard ousconditions which could result fr om equipment failure o r operator e r r o r . The primaryobjective of the prog ram w as to develop equipment and techniques with pro ced ure srequiring minimum manpower and equipment expenditures.

Equipment was developed to obtain hazard data for friction, impact, t he rma l ,and electrostatic sens itivity. Pre lim ina ry work was als o conducted to determine thedetonation properties of wo pel lan ts :ising s ma ll laboratory sample s.

INTRODUCTIONWith rapid adv ances being made in the development of higher energy propellants,

the demand for hazard test data fo r new propellant const ituents, inter media te, andfinal compositions has al so increased. These data are often required pr io r to makinglaboratory samples larger than 10 gr am s because of personnel exposure during somehandling and manufacturing opera tions . Lack of a means to rapidly determin e thisinformation with sm al l labor atory sampl es reduc es operating efficiency due to delaysincurr ed while waiting fo r the t es t data and the excessive time requir ed t o ma!;e thenumber of test samples.

J

. ISupervisor , Explosive Development Laboratory


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Most of the effort expended during re cent y ea rs on hazar d t es t equipmentimprovement p rogra ms was on the development of pre cis e hazard mea sur ing equip-ment. The maj or portion of this equipment was un sati sfac tory for production-typetesting, because the equipment is not capable of testing minimum size test samplesin a short period of time.

The purpose of the pr ogram , as summarized in this paper, wa s to improveexisting hazar d test equipment design s and test techniques to perm it mor e efficienthazar d determination using minimum sample si zes.

INVESTIGATIONA surv ey wa s made of Thiokol Chemica l Corporation Wasatch Division pro-

pellant processing facilities to determine the type and extent of hazardous environ-ments that exis t during nor mal operations or that could exis t as a result of minorequipment failure o r operator err or . This information was classified into types ofhazardous environments and the physical st ate of mat eri als that would be pre sen t inthese processing environments, i e , powders, liquids, o r solids. Hazardousenvironments given major consideration were friction, impact, therm al, electrostatic,and detonation. Mat eri als curr ent ly being processed were thoroughly evaluated ;andwith existing facility design and safety practic es, these m ate ria ls d o not pose un-neces sary haz ard s to processing equipment or to personnel. However, du ring norma loperations a cer tai n amount of hazar dous environments exis t. New composi tion swhich mav be mo re sensitive and cr ea te a hazardous condition during proc essi ng mustbe evaluated. A l l combustible m ate ria ls, including fuel and oxidizer powders, pre-mixes, intermediate propellant com positions, and uncured and cured prope llant s, areevaluated with this equipment to s imulate the various pro ces s conditions.

Estimated energy levels fo r fri.ction, impact, and heat wer e obtained primarilyby duplicating operating and minor incident conditions. A study of types of equipmentnecessary to duplicate these environments was then made to asce rtain the mostsevere operating hazards present during norma l operations and minor failur es andalso t o establish a hazard limit.

A sur vey wa s then made of s ev eral propellant and explosive manufacturingfaci liti es in the United State s to dete rmin e availability of equipment that could be usedto obtain the r equir ed hazar d data. This equipment als o was t o comply with theprogr am obj ectives of being a ble to obtain this information with pr oce dure s re quiringsmall test sampl es and minimum manpower and equipment expenditure s. A summaryof the information obtained fro m this surve y and the result ant equipment designedduring this program is as follows.

h a z a r d l im it is the environment that will barely cause the test material to react.


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Friction TestingA review of the friction equipment used throughout the propellant and explosive

industry showed that there was only one satisfactory friction tester design available.This piece of equipment wa s a s tr ip fricti on tes te r designed by Allegany Bal lis tic sLabora tory. However, t his te st er would meet only part of the te st requ iremen ts.The design was more than adequat e, but Thiokol ordnance des igne rs felt that the sa medata could be obtained with a te st er l es s expensive to fabrica te and operate. Availablefriction tes ter s of designs that we re used f or a number of yea rs w ere not consideredsatis facto ry because of un rea lis tic frict ion values and difficulty of op eratio n.

Three basic friction te st er s were subsequently designed. The fir st was asl ur ry friction tes ter, which with minor changes, was converted into thre e s eparatete st er s to duplicate all friction environments and permit testing of the various typesof materials.ment si mi la r t o that shown ;nFigure 1. The applied forces are shown in Figure 2 -C .The piston is lowered on top of t he sample and into the cup pri or to s tar tin g the motor.The force is varied by adding weights to the piston c ar ri ag e, and the rotating speedis adjusted by changing pulley ratios.accura te friction measuri ng device; however, the tes ter does provide an evaluationof the frictio n sensit ivity of ma te ria ls that might be subjected to confined fri ctionenvironments, such as combustible ma teri als in a mixe r packing gland.

One of the te s t e r s us es a one inch diameter piston and cup arrange-

This type of t es te r was not considered an

The second s lur ry friction te st er utilized a twelve inch bowl with a spring-The tester and theoaded friction head that impin ges agains t the side of the bowl.

fo rc es involved ar e shown in Fi gu re s 2- A and 3 . The applied force and rotation speedare se t by changing the spr ing tension on the friction-head ass emb ly and changing thepulley ratios .Teflon directional scoops.uncured propellant compositions a r e the only materials tested with this device becausethe tes ter scrapes on a vertica l plane. This teste r simulates friction environmentsexperi enced in mixing and cleanup operat ions. Vario us types of fric tion head and bowlmat eria ls can be used to achieve the desire d friction environment. The vertic altester was considered in addition to the horizon tal scr ap er because of the constantte st radius eliminating the changing force variable.

The test material is distributed in front of the friction head by twoMedium-viscosity materials such as intermediate and

The third type of s lu rr y fricti on te st er al so utilized a twelve inch bowl, butthe friction.head applies for ce against a bottom portion of the bowl that is grooved tocontain low viscos ity liquids. The fri ction te st er and the application of forc es areshown in Fi gur es 2-B and 4 . Th e forc e on the friction head i s controlled by adding orremoving weights from the top of the assembl y. Fricti on conditions a r e simulated formixing and cleanup proc esses involving low-viscosity prem ixes and intermediate anduncured propellant compositions. Fri cti on heads and bowl mat eria ls are changed toachieve the desired friction environment.


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A st ri p friction te st er was designed for testi ng fine powders and propellant,sam ple s. Two thin st ri ps of me tal simi lar to those shown in Figure 5 a r e used foreach test . The strip surfaces are machine cut to eliminate surfa ce variab les. Thete st er and the application of fo rc es ar e shown in Figures 6 and 7.desired force to the rol ler compressing the str ips together is accomplished byremova l or addition of weigh ts on a lever. A falling weight provides the energ y topull the s tr ip s apart. The falling weight impinges upon a lever attached to a wheel,which in turn pulls one of the strips. The other strip is secur ed to the stand. Theapplied for ce and the m ass of the falling weight are varied to obtain the desire dfriction environment. The dir ect impact tes ter wa s modified to incorporate thisdevice as an attachment.weight as the direct impact teste r.

Obtaining the

The st rip test er also used the same framework and drop

This par ticul ar t est not only provides a means of determining friction se nsi-tivity of powder ma te ri al s and propellant sa mples that i s difficult to obtain on othertypes of friction tes te rs , but a ls o duplicates friction environments occurr ing inprocesses such as scrap ing of a drum on a floor contaminated with o xidiz er and p r o -pellant scraps.

A third type of friction test er called, a rotar y friction tes te r, was designed(Figure 8) . A rotating wheel impinges upon a shoe of known su rf ace area to producethe friction environment. This te st er is primarily used to determine friction sensi-tivity of viscous ma te ri al s (pr emix and uncured propellants)., Te st s are conductedon cured propellants, but the setup time i s excessive. The applied for ce is variedby adding o r removing weig hts on the end of a lev er attached to the frictio n sh oe(Figure 9). The wheel speed is varied by a variable-speed gear box and pulley syste m.Shoe and wheel m ate ria ls are changed to obtain any desi red combination.Impact Testing

Pri or to initiation of thi s prog ram , Thiokol's Wasatch Division ordnanceengin eers designed an impact t es te r based upon test and design data obtained fromthe Naval Ordnance Lab ora tor ies at Silv er Spr ing , Maryland. Throughout themissile industry ther e are numero us impact-tester desig ns, very few of which a r edirec tly comparable. Even when designs a r e the sam e, sampl e preparation andte st techniques are different. The Thiokol te st er was designed to reduce a s many ofthe test variables as possible and improve on test efficiency. The design consi stedof a piston and cup arr ang eme nt, with the piston guided into the cup as shown inFigures 10 and 11. Propellant samples are prepared by cutting thin slices using amicrotome cutter (uncured samples are froze n with COz prior t o slicing). Powderedsampl es a r e weighed out or measured volumetrically. A dis c of sandpaper is placedface down into the sample t o inc rease sensitivity. To avoid excess ive gall ing of thestr i ker surface, a thin shim of st ee l, 0 . 0 0 5 inch thick, is used between the st rik ersurfac e and the sandpaper disc.


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The second impact tester i s called a direct impact tester (Figure 12).becausethe st ri ker impacts direc tly on the sample as shown i n Figure 1 3 . A sma ll sampleof controlled size is placed on the anvil (Figure 14), and th e anvil is inser ted into theanvi l holder and impacted by the fall ing weight. The dro p height and weights a r echanged to obtain the de sir ed i mpact e nvironments.Autoignition Testing

Autoignition tests are conducted in a convective-type oven because most auto-ignition data ar e required on sam ple s too la rge for a Woods'metal bath arrangement.Depending upon whether the sa mp les ar e liquid or solid the samp les are suspended inthe oven a s shown in Fi gu re s 15 and 16. A thermocouple is suspended adjacent tothe sample to record the sampl e tempe rature during test and to indicate when thesample fires. The oven is design ed to withstand the firing of up to ten gr am propellantsamples and contains a blowout plug and exhaust ducting to prevent contamination of thesurrounding areas .Electrostatic Testing

Most of the e lec tro sta tic te st de vices used by the various facilitie s were notpermanent and lacked unity of design. Se vera l of th e des ign s used common devicesfor energy application, such as he use of a phonograph needle for one electrode,butthe devices still varied in sample si ze , and sample preparation. It was decided todesign an electrostat ic t es te r that would incorporate many of the better featu res ofexisting designs to meet program objectives. The electrostatic te st er and the majorsubassemblies a r e shown in Figures 17, 18 , and 19. A sketch of the sam ple tes tfixture is shown in Figure 20 . Samples ar e inserted into a plas tic holder of a givenhole si ze and then placed on a st ee l blank attached to one electrode.elect rode i s attached to a st ee l ball placed on top of the hole in the plastic holder.The ball is four time s the diame ter of the s ample hole.

The other

The capacitors are charged with a NJE high voltage power supply capable ofgenerating 60,000 volts . Switching i s accomplished with a solenoid operated knifeswitch t o eliminate switch bounce experien cedwith vacuum switches. Arrangeme ntof t he capa cito rs in a circ le with the contacts in the center permits rapid selectionof the desired capacitance value.Detonation Testing

The most important detonation charac teris tic in propellant processing facili tiesis the c ritical diameter. If a critical diameter exists within the maximum diameterof mater ial being proc essed , then other detonation charac teris tics such as minimum'Critical diameter is the minimum cha rge diameter that will sustain a detonationwhen initiated by a booste r cha rge gr ea te r than the minimum booster o r equal indiameter to the te st charge .


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boo ste r, deflagration-to-detonation, and projecti le impact sensitivity will bedetermined.

After reviewing othe r faciliti es, it w a s found that very few improvementswere made in the method of determining criti cal dia meters , and that a l l attem ptsto determine critical diameter s of compositions with su bcri tical diame ter sampleswere unsuccessful.

Two approaches to determine criti cal d iamet ers with subcritical diamet ercharges a r e being investigated. The fi rs t i s the development of a relationshipbetween the cr iti cal diamete r of a mater ial and grain reaction determinationsextrapolated to a theoretica l detonation tempe ratur e and othe r chemica l and physicalparam eter s such a s density, temp era tur e, and confinement. The second method isa study of the detonation reactivity of subcri tic al diameter propellant samples whensubjected to large booster ch arges .

INTERPRETATIONThe following is a summ ary of the test techniques and data interpre tati on for

each of the testing devices descr ibed.Frict ion AnalvsisOne Inch Diameter Piston-to-Cup Slurr y Tester--The most uniform r es ul ts wereobtained using a sample size of approximately 0.4 gram s o the ma ter ial would notbe extruded out of the cup. A verti cal force and rotational speed were obtained fo rwhich a refe rence uncured propellant sample would fi re in approximately one minute.A f i re is evidenced by an audible o r vis ual re action. If the cup o r piston temperatu rerises above 10 degre es Fahrenhei t during a te st , cooling is provided t o maintain atempe ratu re within 10 degre es of room temperature pr ior to the next test. Ten testswere run on each sample to obtain the minimu m, maximum, and average fi re times.

A sensitivity index is established by dividing the average fi re ti me of the testmaterial by the average fire tim e of th e refe rence mater ial and multiplying by ten.Hazard limits are established based on the index values.Twelve Inch Diameter Ve rti cal Slurrv Friction Tester--A thin film of propellant isspr ead around the periph ery of the bowl the sa me width as the friction head. Thesample size s vary between 0.5 and 2 grams. On some te st s it is desir able to putthe tes t sample in front of the fr icti on head instead of spreading it around the sideof the bowl. The speed and for ce a r e varied to obtain a fi r e point for a referencematerial of approximately 15 seconds. A f i re is evidenced by an audible o r visu alreaction. The tem per atu re of the bowl and fricti on head i s maintained to within10 degrees of ambient tempe rature pri or t o each test. One test replaceable metals t r ips are used on the fri ctio n head when conducting standard te sts to dete rmine thefric tion index.


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166The number of t es t s conducted and the evaluation of the te st da ta fo r establi sh-

ment of a friction index are the same as on the piston-and-cup te st s.Twelve Inch Diameter Horizontal Slurry Fric tion Tester--The horizontal frictiontes te r uses a sample siz e of approximately 5 gra ms. The rotational speed and forcesettings, number of te st s, and data evaluation ar e si milar to the vertical teste r.Strip Friction Tester--The most uniform test res ult s we re achieved by placing thesampl e between the st ri ps in front of the rol le r pres sur e point. The preparation ofthe samp le and uniformity of the st ri ps must be carefully monitored to produceaccurate t est results.

A standard is tested with either beta-HMX (Cyclotetramethylenetetranitramine)explosives o r a propellant comp ositi on, depending upon the type of te st mat eri al. Themost usefu l information is obtained by determining a drop height and weight to providesatisfactory result s with the standard. Then all similar mate rial s ar e tested using th edrop weight and height, with the applied forc e being varied.

A Bruceton-type analysis is followed to obtain a 50 perce nt fi re point. The50 percent f ir e point is used t o establish a friction index, as compared t o a referencematerial. A fire is evidenced by a n audible or uisual reaction.Rotarv Friction Tester--The rotary friction tes ter i s used for two types of te st s. Onetes t is to establish a rot ary fricti on sensitivity index using a stainless steel shoe andwheel. The other is a compar ative t es t using varying shoe and wheel combinationssi mil ar to that shown in Figu re 21.

The average time-to-fire is used on the standard test t o compute a rotaryfriction index, which is the average time- to-fi re of the te st ma te ri al divided by theavera ge time-to-fire of the st andard mat eri al multiplied by ten.

On the te sts using varying wheel and shoe combinations, the data a r e evaluatedby plotting the average time -to-fi re in seconds versus the ra dia l wheel velocity infeet pe r second times the applied for ce in pounds per square inch (Figure 22). Bythi s me thod, varying wheel and shoe s izes are used and still compared with other testsusing different size wheels and shoes.Impact AnalysisIndirect Impact Teste r--Pre parat ion of propellant samples fo r the indirect impacttes te r is accomplished by sl ici ng the propellant into 0.021 inch thick s tr ip s using amicrotome cutter. Dried ma ter ial s a r e sampled volumetrically. The Brucetontest technique is used to obtain the 0 , 50, and 100 percent fi re points, with the50 percent f ir e point being used f or d etermining the impact index. Beta-HMX is

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used as a standard; and the referenc e mat eri al, an uncured propellant composition,is used t o compute the impact index. The impact index is computed simiIa rly to the r otaryfriction index, or the 50 perce nt f ir e point of the unknown mate ria l is divided by the50 perce nt fire point of the ref erence uncured propellant multiplied by ten. A typicaltest data sheet is shown in Figure 23.Direc t Impact Tester--Because of the difficulty o r ti me consuming effort s in raisin go r lowering the weights on the direct impact te st er , a Probst analysis is used ratherthan the Bruceton analysi s. The Pr obs t ana lys is involves conducting 20 tests at eachheight within the ran ge of the 0 and 100 percent fi re points. Small (0.009 gram)samples ar e used. Propellants are sliced with a microtome cutter and punched outwith a ci rc ul ar punch. An audible sound is used to deter mine whether o r not thematerial fired.Autoignition Asalv sis

Autoignitjon tests are conducted with the oven set a t a constant temperature.A thermocouple placed adjacent to the sample records the tempera ture and thetime-to-fire. The temperature i s increased o r decreased until the autoignitioncharac terist ics fo r approximately one hour are obtained, si mil ar to the data shownin Figure 24. If a material t o be tes ted ha s an unknown autoignition tem perat urerange, a dynatpic te mpe rat ure environment test is performed on the materi al withthe oven tempeyature incre ased o ver a given period of ti me until the f ir e point i sreached. These data are then evaluated to determine the temperat ure range to b eused.Electrostatic Analysis

Electrostatic tests are conducted between the ran ges of 100 pf to 0 . 1 I.lfcapacitance and 100 to 60,000 volts. A 50 percent f ir e point is estimated using aBruceton-type anal ysis by i ncrea sing o r decrea sing the voltage. A chart fo r whichvoltages, capacitance, and energy levels are displayed is shown in Figure 25. Th esens itiv ity of ma te ri al s is compared by taking a constant ene rgy line i n joules whichis tangent to the tes t data curve. This energy value is then used to compute anelectrostatic sensitivity index, the same as for the friction and impact te st data.

U s e of the one test replaceable p lastic holders enables rapid testing and uni-formi ty of sample prepara tion with a minimum material required.Detonation Analvsis- ,-

A t the pres ent t ime , sufficient te st s had not been conducted to prove thevalidity of determining critic al diameters eithe r by reaction ra te s or reactivitymeasurements with laboratory samples.

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Tests a r e still being conducted using a stand ard pipe detonation test setupsimila r t o that shown in Fi gure 26 . A booster having the same diamet er as the testcharge is used to en sure adequate boost ering, and a cha rge of sufficient length isused to evidence the reaction. Pin gages ar e used to record the shock velocitie s, andthe signal s from the gages are reco rded with a coded pin mixer system to en sur emore reliable d a t a pickup.

CONCLUSIONThe friction, impac t, autoignition, and elec trost atic te st equipment and

techniques developed during th is pro gra m enable rapid and inexpensive determinati onof the hazardous char act eri sti cs fo r propellant constituents and compositions presentin propellant manufacturing and storage facil ities.

This equipment and these techniques are providing bet ter utilization of pro -pellant development faciliti es and manpower because hazard tes t da ta are beingobtained with l ess t es t de lay s than was possible with previous designs and techniques.


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DIRECTION OFROTATIOX

dDIRECTION OFROTATION

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Figure 2Application of Forces in Slurry F r i c t i o n Tes ters


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Figure 4. Twelve Inch Bowl Horizontal Slurry Friction Tes ter


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- H E M I C A L C O R P O R A T I O NI . , . 7 C * 0 1 " 1 , 1 0 * . a " , r * . * C I , " ) " I . "Figure 6. Strip Friction Tester


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Figure 1Deta i l s of I n d i r e c t Impact Te s te r 9~ Assembly

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Figure 16Autoignitj.on Test for So l i d Samples


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D E T O N A B I L I T Y .E S OP NO. 63-7 R E V I TH E S A M P L E m f D I D N O T ) D E T O N A T E IN A 2 NCH CHA RGE D I A M E T E R.

N O T E : 1. A FIVE POUND DROP WEIG'fT IS USE0 IN IMPAC T TESTIN G.l H E H M X ST A N D AR D 50% EXPLOSION WAS% INCHES.

-2. SENSITIVITY C O M P A R E D WITH STANDARD PROP_EL_LANTIT H AN A SS IGNED I M PA C-T I N D E X O F io. .ANY S A M P L E W T H AN IMPAC T INDEX LOWER THAN 5 IS I N T H E C R I T I C A L R A N G E A N D E X T R E M EC A R E O lOULD B E E X E RCI S E D I N M A NUF A CT U RE AND HA NDL I NG.

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test equipment and technioues for evaluating composites propellants

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