AD-A285 240

* Abrasive Properties of Test andTraining Site SoilsRelative Hardness of Fine Particle FractionAustin W. Hogan June 1994

934

S0 94-31475

10 03 083

AbstractThe experiment reported here shows that fine soil particles contribute to abra-sion, wear and ultimate failure of Oarachute materials in a manner somewhatanalogous to 'three-body abrasion' in metals. The 'hardness' of the particles

collected at several test, training and maneuver areas is examined and scaledto known natural materials and commercial abrasives. The geometric diametersof the soil grains that enter and imbed in the fibers are a primary factor forunderstanding the abrasion mechanism. In the case of cordage abrasion, thefraction of soil grains less than 0.2 mm was dominant within the strands andamong the fibers. The particles were applied to designated surface grids onrelatively large (3 x 3 to 7 x 7 cm) Mohs hardness specimens, glass photo-graphic plates and steel cuffing tools. All of the fine particles abraded glass

photographic plates, with the exception of a soft, nonmagnetic, black fractionfound in Camp Blanding fines. None of the materials scratched corundum,although it was possible to make a few scratches in Topaz with almost allspecimens. The general upper limit of hardness was similar to that of quartz,which showed some detectable abrasion by five specimens. Fines from theRiyadh, Saudi Arabia, area easily scratched quartz, and this material is thehardest measured to date.

For conversion of SI metric units to U.S./Brltish customary units of measurementconsult Standard Practice for Use of the International System of Units (SI), ASTMStandard E380-89a, published by the American Society for Testing and Mater-lais, 1916 Race St., Philadelphia, Pa. 19103.

This report Is printed on paper that contains a minimum of 50% recycledmaterial.

Special Report 94-16 M

US Army Corpsof EngineersCold Regions Research &Engineering Laboratory

Abrasive Properties of Test andTraining Site SoilsRelative Hardness of Fine Particle FractionAustin W. Hogan June 1994

Prepared forU.S. ARMY NATICK RESEARCH, DEVELOPMENTAND ENGINEERING CENTER

Approved for public release; distribution Is unlimited.

PREFACE

This report was prepared by Dr. Austin W. Hogan, Research Physical Scientist,Geochemical Sciences Branch, Research Division, U.S. Army Cold Regions Researchand Engineering Laboratory. Funding was provided by the Parachute EngineeringBranch, Airdrop Systems Division, Aero-Mechanical Engineering Directorate, U.S.Army Natick Research, Development and Engineering Center.

Technical review was provided by E. Cortez and Dr. G.S. Brar, both of CRREL.The contents of this report are not to be used for advertising or promotional pur-

poses. Citation of brand names does not constitute an official endorsement orapproval of the use of such commercial products.

- ii

CONTENTS

Preface ................................................................................................................................... iiExecutive summ ary ............................................................................................................. ivIntroduction .......................................................................................................................... 1Technique for measuring the hardness of fine particles ............................................... 2Results of experiments ...................................................................................................... 6Discussion ............................................................................................................................. 8Conclusions .......................................................................................................................... 9Literature cited ................................................................................................................... 9Appendix A: Soil specimens ............................................................................................ 11Abstract ................................................................................................................................. 13

ILLUSTRATIONS

Figure1. Scratches made by fine soil particles on quartz hardnes specimen in oblique

illumination ........................................................................................................ 32. Comparison of standard abrasive grit sizes with sieve sizes ............................... 43. Size distribution by mass of the materials examined ............................................ 44. Hardness specimens used in experiments ............................................................. 55. Comparison of Vickers, Knoop and Mohs hardness scales .................................. 7

TABLES

Table1. Comparison of hardness values of various materials on Mohs and Knoop

scales ................................................................................................................. 62. Soil size parameters .................................................................................................... 8

9,' ItS GRA&IDTIC TABUnannounced EJ

Just ificatlon

DiEtr ibut icyr:Av.-di abillty.3:,i,

Dlest [ pou.1

I....'

EXECUTIVE SUMMARY the Natick drop test pit were examined for soilresidue. The fine fraction of soil penetrated core-

Soil specimens from seven paratroop training less braid, and the smallest particles present weredrop zones, two proving ground cargo chute test retained within the strands of the braid. Fine par-areas, and the Natick RDEC drop test pit were ticles were found between core strands subjectedanalyzed to examine the influence of physical to a single test drop, although the surface of theproperties of soil on abrasion of parachute materi- exterior braid looked clean.als. Additional analyses were performed on soil Individual soil particles caused abrasive dam-retained within the braid and core of parachute age to delrin test plates in a simple sliding frictioncord test specimens, and test plates exposed near test. A more specific experiment was consideredrigger's tables at troop training sites. necessary to examine the abrasion of textile strands

The size distribution and clay mineralogy of the by the smallest size fraction of the soil material. Ispecimens were quite different. Surface material proposed that a microscopic method of measuringfrom the Saudi and Israeli deserts approached soil particle hardness by relating size-classifiedspherical symmetry, while surface materials from soil particles to Mohs hardness standards could betest sites at China Lake and Yuma Proving Ground used to provide an index of soil abrasion.are quite angular. There was a great difference in This experiment shows that fine soil particlesthe mass fraction of fines among the several sam- contribute to abrasion, wear and ultimate failureples, but all contained material in the range below in textile materials in a manner somewhat analo-sieve size. The fraction below sieve size consisted gous to "three-body abrasion" in metals. The par-of angular, aspheric particles in all cases, includ- ticles of diameter comparable to the strands ofing those specimens that were dominated by near- yam enter the interior of the yam during flexingly spherical particles in the larger size classes, and stress. Particles harder than the yam fiber

Several parachute lines that had previously imbed in some fibers, and then abrade anotherbeen strained and exposed to local soil material in "third body" fiber as stress and flexing continue. I

104- 7S

"" Knoop Hardness (Foster 1967)

0 Vickers Hard.nes (Tabor 1954)SiliconCarbounndum Carbide

0

Garnet Few Partices Marked Topaz103 'Hard* Tool Steel 0 Riyadh

Engineering Mean for Sandi_0 _O China Lake, Yuma, NatickF:c Sot oo te Blending, Bragg, Nowra -

E *San* Tool SteelTypical 'Glass' Soda-Ume GlassBronze

C0C0 Copper

Aluminum OCp_

102

Silver --

E2D C0

Babbita in o~ . 2 E

lol I I I I I I - A _

1 2 3 4 5 6 7 8 9 10

Mohs Hardness Number

Comparison of Vickers, Knoop and Mohs hardness scales.

iv

have examined the "hardness" of the particles col- although it was possible to make a few scratches inlected at several test, training and maneuver areas, Topaz with almost all specimens. A few particlesand attempted to scale the hardness of these fine of greater hardness are apparently naturally oc-particles relative to known natural materials and curring contaminants in many soils. The generalcommercial abrasives, upper limit of hardness was similar to that of

It is important to note that the geometric diame- quartz, which showed some detectable abrasionters of the soil grains that enter and imbed in the by five specimens. Fines from the Riyadh, Saudifibers are a primary factor for understanding the Arabia, area easily scratched quartz, and this ma-abrasion mechanism. Classic wear and friction terial is the hardest measured to date. Fines fromexperiments in conventional machinery show di- China Lake, Natick and Yuma quickly producedminishing wear as the diameters of abrasive parti- visible abrasion on quartz, but some additionalcles drop to less than 0.2 mm. In the case of cordage effort was necessary to define scratches made byabrasion, the fraction of soil grains less than 0.2 mm fines from the Sicily drop zone at Fort Bragg. All ofwas dominant within the strands and among the these materials scratched a steel parting tool. Thisfibers. This made it necessary to extract the fine indicates that the hardness was equivalent to aboutfraction of the soil particle population for these 700-1000 kg/mm2 on the Vickers scale. A diagramexperiments. The fine fraction was further classi- coordinating hardness scales, and showing rela-fiedinto0.105<d<0.125mm,0.044<d<0.053mm, tive hardness of the specimens, is given in theand d < 0.044 mnm classes to provide uniform grit. preceding figure.

The grits were applied to designated surface The last material received was that from thegrids on relatively large (3 x 3 to 7 x 7 cm) Mohs AustralianArmyparachute school at Nowra, NSW.hardness specimens, glass photographic plates and This soil differed from the other specimeni. as itsteel cutting tools. The grits were imbedded in the contained visible plant material, and was of aflat face cut at 300 to the axis of hardwood dowels much darker shade, perhaps because of organicand rubbed over the surface, simulating three- content. It was also unusual in that a second massbody abrasion. In some cases, abrasion was appar- mode was found in the particle size range 0.105 >ent through sound and feel; in other cases, micro- d > 0.053 num, in addition to the primary massscopic examination was necessary to determine if mode in the 0.250 > d > 0.125 mm size class. Thethe particles had deformed the hard surface. fine fraction of this material was sufficiently hard

All of the fine particles abraded glass photo- to scratch the photographic plate and the steel tool,graphic plates, with the exception of a soft, non- but had a "lubricated" feel. This may be attribut-magnetic, black fraction found in Camp Blanding able to organic coating, and finer, harder materialfines. None of the materials scratched corundum, may be present.

v

Abrasive Properties of Test and Training Site SoilsRelative Hardness of Fine Particle Fraction

AUSTIN W. HOGAN

INTRODUCTION The size distribution and clay mineralogy of thespecimens were quite different. Surface material

Interest in the failure of individual parachutes from the Saudi and Israeli deserts approachedas a function of service life and exposure has a long spherical symmetry, while surface materials fromhistory (Coskren and Wuester 1989). Segars (1992) test sites at China Lake and Yuma Proving Groundreviewed international literature concerning the were quite angular. There was a great difference indegradation of stored parachutes and other items the mass fraction of fines among the several sam-made of Nylon 6,6 over time, finding that con- pies, but all contained material in the range belowclusive evaluation of age deterioration was limit- sieve size. The fraction below sieve size consisteded by the lack of initial tests of material properties of angular, aspheric particles in all cases, includingat the time the parachutes were made. Examina- those specimens that were dominated by nearlytion of a sampling of parachutes used by the U.S. spherical particles in the larger size classes.Army and the U.S. Forest Service "Smoke jump- Severalparachutelines thathad previouslybeeners" indicated that suspension lines began to de- strained and exposed to local soil materials h-, thegrade during the first 30 uses. Natick drop test pit were examined for soil residue.

Laboratory tests by Rodier et al. (1989) con- The fine fraction of soil penetrated coreless braid,firmed that suspension line degradation w is the and the smallest particles present were retainedmost common mode of failure, and that this was within the strands of the braid. Fine particles wereprimarily the result of accumulated contaminants found between core strands subjected to a singlewithin the suspension lines. Rodier et al. used soil test drop, although the surface of the exterior braidnative to the Natick Laboratory as an initial test looked clean.material, and they found that soils from the drop Individual soil particles caused abrasive dam-zones at Yuma Proving Ground, Arizona, and age to delrin test plates in a simple sliding frictionChina Lake Naval Weapons Center, California, test. A more specific experiment was considereddegraded suspension line strength faster than did necessary to examine the abrasion of textile strandsthe Natick soil. They concluded that inherent geo- by the smallest size fraction of the soil material. Ilogical differences in soil properties would alter proposed that a microscopic method of measuringthe service life of parachutes deployed in varying soil particle hardness by relating size classified soilgeographic locales, particles to Mohs hardness standards could be

My initial report (Hogan 1992) described the used as an input to provide an index of soil abra-physical properties of surface soil specimens from sion.seven paratroop training drop zones, two proving This report provides some additional analysis ofground cargo chute test areas, and the Natick the size distribution of the fine fraction of the soilRDEC drop test pit, all used in the Rodier et al. samples and the results of scratch hardness exper-(1989) experiments. Additional analyses were per- iments to determine the relative hardness of soilformed on soil retained within the braid and core grains from the several sites. The effective hardnessof parachute cord test specimens, and test plates of some soil specimens, relative to acetal resinexposed near rigger's tables at troop training sites. (nylon, delrin), was examined in my initial series of

experiments. Small quantities of sieve-classified several techniques used to measure the effectivesoil were placed on 50-mm-diameter disks ma- hardness of textiles, plastics, polymers and otherchined from delrin bar stock and another disk was nonmetallic materials. Nonmetals have a tenden-placed atop the specimen. Light pressure was cy to restore deformation as pressure is released,applied and the disks moved for about 5 mm in and larger uncertainties are inherent in nonmetalsliding friction contact. The disks were separated hardness measurements. Hardness scales and mea-and examined for striations by incident light micro- surement techniques are reviewed by Sarkar (1980),scopy. Rabinowicz (1965) Howell et a]. (1959) and Tabor

These techniques raised questions, rather than (1951, 1954).provided conclusions, because the largest parti- The relative hardness of materials can be esti-cles present dominated the visible results. Goode's mated by relatively simple scratch tests. Accord-(1989) SEM analysis of fai'ed suspension lines, the ing to Tabor (1954) the Mohs (1822) hardness scaleconclusion of kodier et al. (1989) that desert soils is the oldest known hardness measuring methodare more damaging to suspension lines, and the and is used to establish the relative hardness ofanalysis of particles contained within braid and natural materials through their ability to scratchcore in my initial report (Hogan 1992) all indicated Mohs' defined hardness standard specimens. Thethat the fine particle fraction of the soil did the Mohs hardness scale has been translated to themost damage. So, it is necessary to isolate smaller Knoop scale by Foster (1967) and to the Brinnell,classes within the fraction that is below sieve size Rockwell and Vickers scales by Tabor (1954).to determine the effective hardness of the fine The Mohs hardness scale is usually applied byparticles. I hypothesized that experiments with successively attempting to scratch the surface ofdiscrete size fractions could speed the establish- Mohs standards of ascending hardness with thement of hardness criteria or the creation of a size- specimen. The specimen's hardness is then somehardness factor that could be used to estimate the intermediate value greater than the hardest stan-potential for abrasive damage by soils. dard scratched. The Mohs method is generally a

Micro-scale hardness measurements usually two-body comparison as described. The abrasiveemploy a stylus of known hardness and area (Ta- wear of cordage more likely approaches three-bor 1954, Howell et al. 1959, Rabinowicz 1965, body wear, as described by Rabinowicz (1965).Sarkar 1980), which is used to scratch or indent the The three-body wear geometry considers a foreignsurface of the unknown specimen. The works cit- particle confined between two surfaces as the abra-ed above indicate that particles of less than 50 Pm sive agent.in diameter are responsible for the braid and cord I have applied the Mohs scale in a three-bodyabrasion observed. The physical properties of parti- hardness experiment. A small, size-classified quan-cles in this size range may vary, even though the tity of the of the material of unknown hardness ischemical composition of them nay seem quite placed on the Mohs standard specimen. A 5-mrm-similar. As mentioned earlier, I proposed that a diameter hardwood dowel, truncated at aboutmicro-scale technique, capable of simultaneously 30°, is placed in contact with the unknown mate-examining the hardness of a large number of sim- rial and pressure is applied by hand, imbeddingilarly sized particles, would be most desirable to the fines in the dowel. The dowel is then moved inexamine the range of hardnesses encountered. a push-pull motion through about 5 mm on theThis report describes experiments using this tech- Mohs standard surface; the imbedded unknownsnique. scratch the standard surface if they are sufficiently

hard. An additional resistance to motion is pro-vided by abrasive friction when the unknown

TECHNIESS OFO MEPASRTING HES does scratch the standard, but this "feel" does notuniquely identify scratching of the surface. Each

The hardnesses of metals, alloys and industrial area is examined microscopically for scratches inmaterials are measured by several standardized parallel lines along the push-pull axis. This allowstechniques (Brinnell, Knoop, Rockwell, Vickers), unique identification of multiple, parallel scratch-which deform the material surface. A standard es by the unknown, in the presence of randomlysphere, cone or pyramid is pressed against the oriented scratches that may be already on the stan-specimen surface, and the hardness is expressed dard. A photomicrograph of a scratched quartzas the ratio of the applied pressure to the area or standard is given in Figure 1.depth of surface deformed. Tabor (1954) describes It is evident that, if this three-body simulation is

2

A

/ p

A C C

Figure 1. Scratches made by fine soil particles on quartz hardness specimen in oblique illumination. Residue of thesoil specimen lies along A-A, and striations of the hardness standard lie along B-B. Scratches made by theparticles in the standard are nominally normal to the striations in the sector C-C (microphotograph by E. Cortes).

applied using abrasive particles with a large range experiments. The 0.044 mm > d fraction containsof diameters, the largest particles will contact the the particles approximating the diameters of thosesurfaces and prevent abrasion by the smallest removed from within the cordage in the initialparticles. This requires that a relatively narrow experiment. Figure 2 relates equivalent standardrange of particle diameters be extracted from the grits to the size fractions used. The 0.125>d>0.105whole sample for three-body hardness experi- mm separation corresponds approximately to 320ments. This was done through sieve separation of grit; 0.053 > d > 0.044 mm to 800 grit; and thefines, using sieves from both the DIN and USS smallest fraction to 1000grit. The relative fractionsseries to obtain cuts with relatively narrow diam- of the mass of submillimeter components of theeter ranges. several soils are shown in Figure 3.

Relatively large (30-50 g) quantities of each soil A series of Mohs hardness specimens of suffi-specimen were first conventionally separated us- cient size to permit multiple hardness experimentsing a 5-cm-diameter frame DIN Prufsieb sieve (1.0, was obtained from Wards Natural Science Estab-0.5, 0.25, 0.125 mm) series. The fractions retained lishment. The specimens were ruled as shown inby the 0.25- and 0.125-mm screens were resieved, Figure 4, after gentle surface cleaning. The surfac-and the total quantity passing 0.125 mm was used es were examined microscopically, and the gridsfor microsieve separation in a 10-cm frame USS coded for later abrasive identification. The hard-sieve (0.105, 0.053,0.044 mm) series. ness specimens are natural materials and do not

The 0.125 > d > 0.105 mm, 0.053 > d > 0.044 mm, always present smooth, flat faces for scratch test-and 0.044 mm > d separations provided sufficient- ing. The three-body abrasive experiment requireslynarrow diameter ranges for three-body abrasive a minimum area of about 3 x 5 mm to examine each

3

10� � I'll I I 'I 'I'j I i

Figure 2. Comparison of standard abrasive grit sizeswith sieve sizes. The particles associated with cordagedamage correspond in size to 1000-grit commercialabrasives. The letters A, B and C designate the size

102 fractions used in the hardness experiments.

i0� 10

I i�i 1i1I I I ii ilil I Iii

SIeve SIze (dia.) at Largest Particle __________________________________1.00

075 V Near RinyeG� Saud Araba

0.50 ii0.25 1- A C0.75 1 I I

050k Camp Blending FL

0.25 1-I A 1B C

0.75 I I0.50 V NWC Chine Lake, CA

0.25 A [B C

0.75 IIat

0.50 YwyiePGAZ(C.� 0.25C.) A Br-.------tC.� 01 0.75 _______________________________________

0� 0.50 Nowra, NSW. AustraliaIL O.25�- A 1 B C I

0.75 I0.50 V Normaidy, Ft. Bragg

0.25 1-A1 B C

0.75 I

0.50 SicIly, Ft. Bragg

Figure 3. Size distribution by mass of the materials ex- 0.25 A �

amined. The letters A, B, and C designate the size frac- 0tions used in hardness determination, as in Figure 2. I

0.50 r Drop Teat Pit. NRD�C. MA

O.25II.� A B C

cc cc o a -

Nominal Selve Size (mm)

4

a. Apatite.

b. Quartz

c. Topaz.

Figure 4. Hardness specimens used in experiments. The diversity of surface on natural specimensrequired systematic application of the abrasive and microscopic verification that abrasion nadoccurred (photographs by P. Keene).

5

Table 1. Comparison of hardness values of various lower limit of hardness for the soil specimens. Thematerials on Mobs and Knoop scales (after Foster 1967) photographic glass was also used to verify that the

Mols Knwp three-body abrasive model could be approximatedSubstance Formula value value using the screened soil and a hardwood dowel by

Talc 3MgO 4SiO2 H 20 1 examining the contact area of the dowel microscop-Gypsum CaSO4 2H20 2 32 ically after applying the grit to the glass surface.Cadmium Cd 37 Commercial abrasives are generally harder thanSilver Ag 60 8 (topaz) on the Mohs scale. The topaz hardnessZinc Zn 119 specimens used were relatively flat and parallel-Calcite CaCO3 3 135Fluorite CaF2 4 163 faced, facilitating use of three 50- x 50-mm topazCopper Cu 163 specimens as upper hardness limit.Magnesia MgO 370 Hardened tool steels have hardness in the 700- toApatite CaF2 3Ca3(PO4)2 5 430 1000-kg/mm2 range, corresponding to Mohs hard-Nickel Ni 557Glass (soda lime) 530 ness of 5 (apatite) to 7 (quartz). A hardened andFeldspar (orthoclase) K20 A120 3 6SiO2 6 560 ground 10- x 150-mm parting tool was used as aQuartz SiO2 7 820 hardness specimen to allow comparison of resultsChromium Cr 935 to Tabor's description of textile-to-tool wear andZirconia ZrO2 1160Beryllia BeO 1250 friction.Topaz (AIF)2SiO4 8 1340Garnet A120 3 3FeO 3SiO 2 1360Tungsten carbide alloy WC, Co 1400-1800 RESULTS OF EXPERIMENTSZirconium boride ZrB2 1550Titanium nitride TiN 9 1800 Size-classified soil fractions from the NatickRDECTungsten carbide WC 1880 drop test pit, Yuma Proving Ground, China Lake,Tantalum carbide TaC 2000 Camp Blanding, Riyadh, Fort Bragg NormandyZirconium carbide ZrC 2100Alumina A120 3 2100 Drop Zone, and Fort Bragg Sicily Drop Zone (seeBeryllium carbide Be2 C 2410 Appendix A), as identified earlier (Hogan i992),Titanium carbide TiC 2470 were examined for hardness using the methodsSilicon carbide SiC 2480 described above. An additional specimen receivedAluminum boride AMB 2500Boron carbide B4C 2750 late in the program from the Australian Army Para-Diamond C 10 7000 chute School, Nowra, NSW, was also examined in

the same manner. This soil differed from the othermaterial sample, and it is necessary to use less than specimens, as it contained visible plant material,ideal curved or striated areas of some specimens to and was of a much darker shade, perhaps because ofprovide sufficient test area. The shape, thickness organic content. It was also unusual in that a secondand varying surface aspect of the Mohs standards mass mode was found in the particle size rangealso posed certain problems for microscopic exam- 0.105>d >0.053 mm, in addition to the primaryination of the test area. Coaxial incident illumina- massmodeinthe0.250>d>0.125mmsize class. Thetion provided unique identification of surface fine fraction of this material was sufficiently hard toscratches. Relatively high magnification was neces- scratch the photographic plate and the steel tool, butsary to defime the scratches in thepresence of residual had a "lubricated" feel. This may be attributable tofines when evaluating the hardness of the < 0.044- organic coating, and finer, harder material may bemm (1000-grit) fraction of the soil specimens. A present.hand-held field microscope was sufficient to define About 1 mg of a sieve-classified soil fraction wasthe scratches made by the 0.125 >d>0.105 mm (320 transferred to the surface of the test specimen. Agrit) fraction of the soils. glass photographic plate was used as the initial

Review of Tabor's works on bearing friction and specimen in each case, as this quickly establishedpreliminary experiments with common materials the minimum hardness of the material. All of theindicated that most of these soil specimens lay in soils examined readily marked the soft glass, withthe Mohs 5 to 7 hardness range. The comparison of the exception of the black fraction of the materialthe Mobs. and Knoop hardness scales given by found in Camp Blanding fines. This black materialFoster (1967) (Table 1)indicates that thehardness of is nonmagnetic and is inertially separable fromsoda hme ("soft") glass liesjust above 5 (apatite) on other particles of the same sieve size. It may bethe Mobs scale. This allowed flat, smooth, 50- x 75- graphitic carbon residue from forest or brush fires.mm photographic plates to be used to establish a Slightly lesser quantities of size-classified soil

6

were transferred to the surface of a lathe parting few particles of harder substance were present intool. The tool had been polished along its long axis the examined samples.with a 220-grit wheel (determined microscopically), This indicates that the specimens examined hadso only the 0.125 > d > 0.105 mm size fraction could hardness comparable to Mohs 7 (Quartz, Fig. 4b).be used, making scratches normal to those residual The rock crystal obtained as a Mohs 7 standard hadfrom manufacture. All of the specimens examined several spalled or choncoidal fractured surfaces,scratched the parting tool, although some difficul- and numerous striations on its surface. It was nec-ties were evident in the trial with the Australian essary to select very small surface sectors for appli-specimen. This and other preliminary examina- cation of the soil specimens and to examine the sur-tions indicated that these specimens had hardness face with incident light microscopy. The surfacein the Mobs 6 (Feldspar) to 7 (Quartz) range. striations made application of the d < 0.044 fractions

A relatively flat side of a large (Fig. 4a) micro- quite difficult.cine (Feldspar) standard was laid out on an ap- Most "sand" is descended from the weatheringproximately 1-cm grid. Milligram quantities of 0.044 of quartz, and an engineering mean hardness valuemm > d and 0.125 > d > 0.105 mm fractions of each for sand of 800 kg/mm2 is given by the Vickerssample were applied to alternate sides of individu- scale. Sand is confined to the larger size classes ofal grid squares. Stereo microscopic examination the particles examined. The finer particles that areshowed that allof the specimens rapidly eroded the able to penetrate and damage cordage are moremicrocline surface. Two smaller (5 x 5 cm) Mohs 8 angular than sand.(Opal) standards were similarly gridded and ex- Tlhe relative hardnesses of the soils examined areamined to define the.upper limit of hardness. Very compared vs. hardness scales in Figure 5. The rank-few scratches were observed, indicating that only a ing is somewhat subjective; it was quite easy to

10 4 ' I " 104

"1o

o Vickers (Tabor, 1954). Knoop (Foster, 1967)

.- JŽ

4C Carborundum SiC

Garnet 0Few Particles Marked Topaz

103 -"Hard* Tool Steel Riyadh 10Engineering Mean China Lake, Yuma, Natick

E - "Soft" Tool Steel for 0Sand Blanding, Bragg, NowraS -t_,_ Soda/me

0 Glass" " - Typical 'Glass'- 7 2

2 Bronze Softest Black Fines, Blanding C.

C 8

"Aluminum Copper>

102 • 102

Silver

0

EBabbits E IV'_

2 E

101 I I I I 11010 1 2 3 4 5 6 7 8 9 10

Moahs Hardness Number

Figure 5. Comparison of Vickers, Knoop and Mohs hardness scales (after Tabor 1954). Hardnessesof common materials are noted on the left, relative to Vickers hardness, and on the right, relativeto Knoop hardness. The soils tested are noted corresponding to their relative hardness.

7

make scratches with the Riyadh fines and quite dif- particle size decreases to less than 0.2 mm. A scratchficult to scratch adjacent places on the same speci- to one-half depth by a 0.2-mm particle would leavemen with the Fort Bragg fines, establishing the a 0.1-mm clearance in which finer particles could"hard" and "soft" limits. The remainder are ranked circulate without abrading the surface.on a subjective scale based on the degree of dif- The initial work done on this problem showedficulty associated with producing visible scratches that the fine particle component of soils penetratedon the hardness substrates. cordage, causing internalabrasion. Larger particles

that could produce these clearances were absent,

DISCUSSION and the abrasion was caused by particles less than0.05 mm in diameter. The most frequent particle

The hardness of the fraction of soils d <0.125 mm size observed within damaged cordage was 0.03 <from several test and training areas examined to d < 0.04 mm, a size ignored in many past industrialdate approximate that of quartz, "sand" and tool wear and hardness investigations. The fine compo-steels. Someharder"contaminants,"sinilarinhard- nent was extracted from the whole for these hard-ness to garnet, can be present in any specimen, and ness experiments. Size classification allows numer-much softer material made up a significant fraction ous fine particle faces to be applied to a surface. Theof fines obtained from Camp Blanding and Nowra. small diameter of the individuals results in applica-Subjective judgment, feel during application and tion of very large contact pressure, many times thedifficulty in microscopic detection of scratches af- initial force.ter application indicate that the "hardest" fines The cordage applies a classification criterion tocame from Riyadh, and the "softest" from Fort the fines it admits. If larger particles were admitted,Bragg and Camp Blanding. A composite scale has the pressure applied at the abrasive face wouldbeen prepared in Figure 5 that relates the Vickers diminish, and no pressure would be applied to theand Knoop hardness scales to the Mohs scratch finer members of particle dispersion. This wouldhardness scale. The hardnesses of several materials reproduce the classical wear diminution with size.from the literature are also defined on the axis. The The very great contact site pressure that can besubjective array of hardness of the soil fines exam- applied through relativel, uniform-sized fine par-ined is plotted. ticles apparently reverse: this size--wear relation in

It should be noted that a certain anomaly, or the case of cordage.perhaps internal contradiction, is presenthere, and Although there is a notable difference in hard-some caveats should be considered in interpreting ness among the several soils, all of the soils containor applying these results. The Mohs scale compares a fine component significantly harder than nylonthe surfaces of materials on a relative scale that is 6,6. The size distributionofallthesoilsapproximatesindependent of the size of the specimen, or the a log-normal distribution (Hinds 1982), and thepressure applied to produce a surface scratch. The pertinent size parameters are tabulated in Table 2.hardness scale relations provided by Tabor are The observation of Rodier et al. (1989) that soilsmacroscopic, derived from penetration of a sphere from the China Lake and Yuma test sites were espe-or pyramid as a function of pressure applied. Clas- cially damaging to cordage is supported by thesical research in three-body abrasion shows abra- data, as both sites have a relatively large fraction ofsive wear to diminish with particle size when mean the soil smaller than 50 pm.

Table 2. Soil size parameters.

Median Geometricdiameter standard Fraction less

Source (mm) deviation than 50 am

Rlyadh, Saudi Arabia 0.165 1.3 0.008Camp Blanding, Florida 0.32 1.5 0.002China Lake NWC, California 0.165 2.2 0.043Yuma Proving Ground, Arizona 0.25 1.9 0.028Nowra, NSW, Australia 0.21 2.8 0.045Fort Bragg, North Carolina 0.27 1.9 0.045

(Normandy drop zone)Fort Bragg, North Carolina 0.335 2.1 0.008

(Sicily drop zone)Natick, Massachusetts 0.34 2.0 0.019

8

There may or may not be a general relation than 50 aim in diameter. Those from Yuma, Chinaamongsoilsize and hardness propertiesand textile Lake and Nowra contained the largest mass frac-damage. There may be some mechanical and elec- tions of these.trostatic factors that facilitate and enhance the ad-hesion and intrusion of soil particles within cord- LITERATURE CITEDage. My preliminary experiment did not detectlarge quantities of soil dust precipitating in a rig- Coskren, R.J. and E.A. Wuester (1989) Investigationgers' loft Films of mass maneuvers with multiple of the service and age lives of U.S. Army personnelcanopy openings show each opening to be accom- parachutes. American Institute of Aeronautics andpanied by a dust cloud.* The accelerations of the Astronautics. AIAA paper 89-0915-CP.partially filled canopy apparently generate suffi- Foster, S. (1967) In Handbook of Physics and Chemis-cient force to detach the soil particles from them, try. Chemical Rubber Pub. Co., 46th edition, p. F17.although vigorousshakesin the riggers' loft do not. Goode, P. (1989) Scanning eleco microscope eval-

Several specimens of cordage that had been uation on MIL C 5040 Type II nylon cord. USAexposed tosoiling during drop tests atNatickwere Natick Research, Development and Engineeringdisassembled and examined for internal soil resi- Center, Internal Memo (unpublished).due. Cords subjected to a single exposure con- Hinds, W.C. (1982) Aerosol Technology. New York:tained observable internal soil deposits. It is diffi- John Wiley.cult to account for movement of soil grains through Hogan, A.W. (1992) Effects of the abrasiveness ofseveral strata of strands in a single brief exposure, test and training site soils on parachute life. USAalthough electrostatic effects may accelerate the Cold Regions Research and Engineering Labora-process. Contact electrification among particles of tory, Special Report 92-11.similar size has recently been demonstrated by Horn, R.G. and D.T. Smith (1992) Contact electrifi-Horn and Smith (1992), which may account for cation and adhesion between dissimilar materials.particle adhesion within the cordage. Science, 26: 362-364

Howell, N.G., K.W. Mieszkis and D. Tabor (1959)CONCLUSIONS Friction in Textiles. New York: Textile Book Publish-

ers (Interscience).Soil specimens from several test and training Rabinowicz, E. (1965) Friction and Wear of Materials.

sites were examined for particle size distribution New York: John Wiley.and the relative hardness of the fraction smaller Rodier, R.W., E.A. Wuester and J.E. Hall (1989) Per-than 0.125 mm diameter. There were some rela- sonnel parachute age/service life criteria. Ameri-tively soft particles included in specimens from can Institute of Aeronautics and Astronautics.Camp Blanding, Florida, and Nowra, NSW, Aus- AIAA paper 89-0916.tralia. Almost all the specimens included a few Sarkar (1980) Friction and Wear. New York: Aca-particles as hard as garnet. The bulk of all speci- demic Press.mens had hardness in the range 700-1000 kg/mm Segars, R.A. (1992) The degradation of parachutes:(Vickers scale) corresponding to 7 on the Mohs age and mechanical wear. USA Natick Research,scale. Subjective considerations place specimens Development and Engineering Center, Technicalfrom Riyadh, Yuma and China Lake in the greater Report Natick/TR-92/035.hardness categories, andthosefromFortBraggand Tabor, D. (1951) The Hardness of Metals. Oxford:Camp Blanding in the lesser hardness category. Oxford University Press.

All of the soils examined contained particles in Tabor, D. (1954) Mohs hardness scale-A physicalthe potentially damaging size classes that are less interpretation. Proceedings, Physical Society (Lon-

don), B67: 249-257.

*PersonalcommutncationwithE.A.WuesterUS. Army NatickResearch, Development and Engineering Center, 1991.

9

APPENDIX A: SOIL SPECIMENS

These specimens were collected at active train- collected from the surface, sealed in plastic bags,ing drop zones, at parachute test sites, and in an and transported to the laboratory. All spearea typical of those traveled by U.S. forces on the were received in a relatively dry condition,Arabian peninsula. Representative specimens were alyzed without additional drying or conditiw, ,g.

Table Al. Identification of surface soil specimens.

ID code Collection location

01/14V191 Phillips drop zone Yuma Proving Ground, Arizona02/14VI91 G4 test area NWC, China Lake, California03/14VI91 Test pit NRDEC, Natick, Massachusetts04/17VI91 Sicily drop zone Fort Bragg, North Carolina05/17VI91 Holland drop zone Fort Bragg, North Carolina06/17VI91 Normandy drop zone Fort Bragg, North Carolina07/17VI91 Dhahran-Riyadh highway Saudi Arabia08/27VI91 Special forces drop zone Camp Blanding, Florida09/27VI91 Frazer drop zone Fort Benning, Georgia10/28VI91 From coreless braid NRDEC, Natick, Massachusetts11/25VWI91 Drop zone South of Tel Aviv, Israel

11

Form ApprovedREPORT DOCUMENTATION PAGE I OMB No. 0704-0188

Public reporing burden for this collection of information is estimated to average 1 hour per response, incuding the ttme for reviewing instructions, searching existing data sources, gathenng andmaintaining the data needed. and completing and reviewing the collection od information. Send comments regardng this burden estimate or any other aspect of this collection of informatiOn,including suggestion for reducing thi burden. to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington.VA 22202-4302. and to the Office of Management and Budget. Paperwork Reduction Project (0704-01B8). Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE REPORT TYPE AND DATES COVEREDJune 1994

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Abrasive Properties of Test and Training Site Soils:Relative Hardness of Fine Particle Fraction

6. AUTHORS

Austin W. Hogan

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATIONREPORT NUMBER

U.S. Army Cold Regions Research and Engineering Laboratory72 Lyme Road Special Report 94-16Hanover, New Hampshire 03755-1290

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

Aero-Mechanical Engineering DirectorateU.S. Army Natick Research, Development and Engineering CenterNatick, Massachusetts

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161.

13. ABSTRACT (Maximum 200 words)

The experiment reported here shows that fine soil particles contribute to abrasion, wear and ultimate failure ofparachute materials in a manner somewhat analogous to "three-body abrasion" in metals. The "hardness" of theparticles collected at several test, training and maneuver areas is examined and scaled to known natural materi-als and commercial abrasives. The geometric diameters of the soil grains that enter and imbed in the fibers are aprimary factor for understanding the abrasion mechanism. In the case of cordage abrasion, the fraction of soilgrains less than 0.2 mm was dominant within the strands and among the fibers. The particles were applied todesignated surface grids on relatively large (3 x 3 to 7 x 7 cm) Mohs hardness specimens, glass photographicplates and steel cutting tools. All of the fine particles abraded glass photographic plates, with the exception of asoft, nonmagnetic, black fraction found in Camp Blanding fines. None of the materials scratched corundum, al-though it was possible to make a few scratches in Topaz with almost all specimens. The general upper limit ofhardness was similar to that of quartz, which showed some detectable abrasion by five specimens. Fines fromthe Riyadh, Saudi Arabia, area easily scratched quartz, and this material is the hardest measured to date.

14. SUBJECT TERMS 15. NUMBEF.F PAGES

Abrasion Parachute wear Soil hardnessParachutes Soils 16. PRICE CODE

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACTOF REPORT OF THIS PAGE OF ABSTRACT

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)Prescribed by ANSI Std. Z39-18298-102