/ 

Ballistic Limit of 6061 T6 Aluminum 

and Threat to Surface Coatings for 

Use With Orbiting Space Station 

Space Suit Materials 

Dick Fish 

Eloret Ins tit 11 te 

for 

NASA - Ames Research Center 

Fioffett Field, CA 94035 

N8 7- 1 3 9 90 (NASA-CR-179884) 

ALUflINUfl AND TBREBl ?C SUREACE CCATINGS PO& 

CSE #ISH CRBXIIHG SFACE STBTZCN SPACE SUIT 

EALLISTIC LIMIT OF 6063 T6 

#ATERIAL$ (Elcret Cor&,) 29 6 CSCL 06K G3/54 Unclas 43563 
Background 

Since before man ventured into space, concern was expressed for the 

advent of an impact of the space vehicle by meteoretic particles. 

Then, the concern was for asteroidal particles or cometary debris. The 

average cubic 

centimeter (0.5 g/cc) and the average encounter velocity was on the 

order of twenty kilometers per second (20 km/sec). Early probes such 

as impact 

into a variety of materials with a wide range of projectiles and 

velocities. 

particle density was of the order of one-half gram per 

the Explorers and Pegasus led to laboratory experiments of 

In support of this early research, this experimenter proposed a pcnetration criteria for a variety of metal targets at the 7th Hypervelocity Impact Symposium at Tampa in 1965. (1) This penetration 

criteria became known as the Fish-Summers penetration equation for 

thin targets. Further experimentation by others (2,3) substantiated 

the findings and led to adoption of the penetration criteria for 

single sheets into the Meteoroid Hazards Flonograph (4). 

As our space effort grew, early distribution theories of meteoroid 

population were modified to reflect the expanding knowledge of our 

solar system. 

In recent years, however, the numbers of orbiting satellites, spent 

components, collisions and explosions has populated the near earth 

orbit with debris potentially more hazardous than the average meteoroid debris. This new debris has an average density of aluminum (2.78 

g/cc) and and and average encounter velocity of 10 kmjsec. 

The space station will require many hours of work in this environment 

and there is concern over hazard to the assembly personnel. The 

proposed 'hard suit' design utilizes 6061-T6 Aluminum for most of its 

exposed area. The aluminum surface will be treated for thermal and 

radiation control. The basic thickness of this suit will be on the 

order of 1.78 mm (0.070 inches). 

Since the earlier work in the sixties was performed generally with 

2024-T4 and 1100-0 aluminum it was felt that some ballistics testing 

on 6061-T6 aluminum to verify its ballistic limit was warranted. This 

would allow updating the penetration criteria for the newer material. 

Further, it was felt that some impact testing of coated specimens was 

necessary to determine what if any debonding occurred during the 

impact process. 

Test Procedures: 

Preliminary Test: 

Before testing the newer suit materials, a shoulder section of current 

suit material was to be impacted. This operational section was tested 

in a configuration similar to that tested at Johnson Space Center at 

least as far as particle size was concerned. An aluminum sphere of 

1 
0.79 mm was selected to impact at approximately 5.5 km/sec. 

The section was sealed, pressure tested and monitored for average leak 

rate. Since the section could not be subjected to extremes in pressure, all 

times from outside the evacuated range before shooting. This was 

accomplished by using an evacuated accumulator tank and double gauges. 

See figure (1). 

on arrangement was devised to allow monitoring pressure at 

As the range was pumped down the closed suit section internal pressure 

began to rise. A valve on the external evacuated tank was opened to 

lower the suit pressure below its limit. When 3 to 5 mmhg range 

pressure trap 

the suit pressure at 8 psig on the gauge located inside of the range. 

Under these conditions the round was fired. 

was reached the accumulator tank valve was shut off to 

A small block of wood was placed inside of the suit section to catch 

any debris resulting from the impact. Rapid pressure loss followed 

the impact. The projectile was observed to have broken up into 3 or 4 

pieces which imbedded in the wood. The impact hole was readily seen 

but was easily patched with tape from the inside of the suit. 

( Comments on this initial test ) 

This suit shot was an effort to try and reproduce fabric shots in an 

untitled report from JSC of tests performed on various components of a 

candidate space suit. 

Several questions arose when reading this hazard assessment of a 

candidate 8psi space suit configuration. Some of these questions were 

answered in a phone conversation with Bert Cour-Palais. One of the 

questions 6061-T6 

Aluminum were calculated using the hypervelocity equation cited in the 

report, namely: 

posed was why the energy levels for penetration of 

(1) tp = K x d**(1.06) x (pp/pt)*"(0.5) x (V/C)**(.667) x (BH)s%(-.25) 

Whcrc d = diameter, cm 

> 2.78 g/cc 

> 2.78 g/cc 

V 

> 5E05 " 

BII = Brinnel Hardness > 95 

K = material constant 1.75 x 5.24 -> 9.2 

tp = threshold perforation, in crn 

pp = projectile density ------ 

pt = Target density ------ 

C = Sonic Velocity " ------ = Impact Velocity, cm/sec ----- > 10E05 crn/sec 

------ 

This equation was developed by Rockwell and also JPL some years ago. 

The penetration equation used K=5.24. JSC in this report uses a modified K which is 1.75 times the K=5.24, or K=9.2. Some earlier reports 

on meteoroid bumpers uses a factor of 2.0 times for a K=10.48 for a 

bumper of zero penetration. 

Several other groups use a modified Fish-Summers equation, namely: 

(2) tp = K1 x (pp)**(.1667) x (mp)%%(.352) x (Vp)"*(.875) 

2 
Were pp = Projectile density g/cc -__---- > 2.78 glcc 

------- > 10 km/sec 

Alum ----> 0.54 to 0.57 

mp = Projectile mass g > 

Vp = Impact Velocity km/sec 

K1 = Target Flaterial Constant 

tp = threshold perforation in cm 

------- 

The 1969 Meteoroid Environment Model used K1 = 0.54 for Aluminum. A 

1984 MSFC report by Dallas Smith on the debris impact uses Kl=0.57. 

Plots of these equations are shown in figures (2) & (3). If we look at 

a JSC 

equation with K=9.2 this is shown in figure (3). These plots show 

Thickness of penetration versus projectile diameter for impacts at a 

constant of 10 km/sec. 

plot of the F-S equation with K1=.54 and overlay a plot of the 

The foregoing discussion on which formulas are being used and the true 

ballistic limit of the 6061 T6 suit material will be determined in the 

plan to impact 6061 T6 Aluminum with Aluminum projectiles to determine 

the ballistic limit. This B.L. can then be used to determine the 

probability of no penetration of the proposed suit for the space 

station. The ballistic limit shots will preceed the shots to determine 

the effect of impact upon the protective coatings being considered.) 

Ballistic Limit of 6061-T6 Aluminum: 

An initial series of impacts using various sizes of projectiles was 

conducted into target plates of 6061-T6 aluminum. The projectiles all 

of aluminum and spherical. of 

sheets impacted with 0.79 mm (0.03125inch) and 1.q mm (0.0625 inch) 

aluminum spheres at approximately 5.5 I<rn/sec. However , the 

determination of velocity is very poor with projectiles in this size 

rongc. The detectors are extinction type, i.e., they look for an 

interruption of a bright field with the passage of the particle. \hen 

the particle size is small the beam interruption goes unnoticed being 

lost in-the-noise, counters, 

one has no garrantee that the signal was due to a particle passage or 

not. 

Figures (4) & (5) show a cross-section 

and if a stop signal is given the range 

Therefore, it was decided to use 3.175 mm (0.125 inch) diameter 

aluminum projectiles since reliable velocity determination could be 

made. If we assume that the behavior of 6061-T6 aluminum to be similar 

to that of other aluminums we can use the Fish-Summers equation to 

predict the approximate perforation velocity limits and reduce the 

, number of shots required for this series of tests. 

If we take the modified Fish-Summers equation cited above, namely: 

Where pp = Projectile density g/cc ---I--- > 2.78 gicc 

mp = Projectile mass g > 

Vp = Impact Velocity km/sec 

K1 = Target Nnterial Constant 

tp = 'Thickness penetrated in cm 

------- 

------- > 10 km/sec 

Alum ----> 0.54 to 0.57 

3 
We may use either K1 = 0.54 or K1 = 0.57 initially to get an impact 

velocity near that assumed for penetration of the specimens chosen for 

the impact tests. Figure (6) is a plot of penetration thickness 

versus impact velocity for a one-eighth inch diameter aluminum projectile into an aluminum target with K1 = 0.54. We see that a 

projectile of this size with a velocity in the range of 5 to 6 km/sec 

should penetrate an aluminum target ranging from 0.9 cm to 1.0 cm. 

Using 0.953 cm (0.375 inch) 6061-T6 aluminum plate as a target 

material to match the 0.3175 cm (0.125 inch) aluminum projectile 

should allow determining threshold perforation within the capabilities 

of the Vertical Gun Range Facility. 

Several rounds were required to zero in on the threshold Derforation 

limit, which was found to be 5.02 km/sec. This suggested that the 

value for K1 target constant tor 6061-T6 aluminum should be 0.57. 

Figure (7) is a plot of thickness versus impact velocity for 1/8 inch 

diameter aluminum projectile and a target constant K1 = 0.57. The 

threshold perforation point is noted on the plot. 

Figure (8) is a plot of suit thickness versus debris diameter with an 

impact velocity of 10 kmjsec. This suggests that for normal impact the 

suit drbris 

diameters greater thnrl 0.35 mm at 10 km/sec. 

thickness of 1.78 mm (0.070 inch) wouia be pcrforaLed by 

If we consider the appearance of the target cross-section at 

threshold perforation, earlier work (a) pointed to the difference between highly ductile and brittle materials. Consider figure (9), (a) & 

(b), where (a) is a drawing of a brittle material and (b) is that of a 

ductile material of the same density. 

We see that the brittle material, while still retaining a roughly 

hemispherical crater at threshold perforation, fails through cracks to 

the rear surface where spall material has been ejected by the 

reflected shock wave interaction. 

For the highly ductile material, (gb), the crater shape has distended 

to a conical section, of 

the crater, and the spall products retained in petals below the 

crater. 

failure occuring through a hole at the base 

If we compare these figures with figure (10) of the threshold 

perforation of 6061-T6 aluminum, we see some distension and spall 

retension like a ductile material but with some cracking through like 

that of a brittle material. This is indicative of behavior of 6061-T6 

aluminum with a balonce between brittleness and ductility. 

What Certainly all impacts will 

not be normal to the surface. Figure (11) shows 6061-T6 aluminum 

impacted at 30 degrees to normal, The point is just below threshold 

perforation. Compare this figure with that of the previous dlown figure (10) for threshold perforation at normal impact. The crosssections are cilnlost identical in appearance. The velocity is about 0.5 

km/sec higher than the earlier impact. The cratering phenomena occurs 

due to the pressures developed at impact which sets off shock waves at 

right angles to the surface at thc impact point. For all but extremely 

happens at other than normal impact? 

4 
low angles of impact, the craters will not betray the impact angle. 

The pressures developed will be governed by the normal component of 

the particle velocity, that is, a function of the cosine of the impact 

angle. We could probably rewrite the modified Fish-Summers to include 

this angular dependence. 

(3) tp = K1 x (pp)**(.1667) x (mp)**(.352) x (V)**(.875) x (cos(phi)) 

Where tp = Thickness penetrated 

pp = Particle density 

mP = V= 

AL = 2.78 g/cc 

mass 

phi = angle of impact (from normal) 

?? 

11 velocity 

Kl = Target constant (0.57 for 6061-T6 AL) 

Impact of Coated Specimens: 

Determination of the best available thermal and radiation control 

coatings was arrived at by a separate study. Two candidate coatings in 

different configurations were presented for impact study. These were 

Rluminum and Gold in both polished and sctin finishes. Also satin 

finished specimens of each were subjected to low-speed, high-mass 

impacts from tumbling with other materials such a5 tools, etc. These 

latter specimens had a large number of burnished areas resulting from 

these impacts. The specimens were dimensioned about 2 inch by 4 inch 

by 114 inch thick. It was felt that maximum effect would occur if the 

high speed impacts were with particles of insufficient mass and velocity 

to penetrate the specimens but rather provide a reasonable crater and 

be as uniform as possible in the impact energies. 

With the 1/4 inch thick plates of 606l-Tb aluminum coated spcxiimens it 

was decided to use 1/16 inch (1.59 mm) aluminum projectiles at aroclnd 

5 km1sec velocity. This would keep the impacts below perletfatior, pcjint 

and not give rise to any large spa11 areas on their rear surfacer,. 

Figures (12) to (17) show the results of these shots. Figure (12) is 

an overall view of all the specimens, the balance of the figures show 

the craters close up. The areas of highest stress in the coatings, 

right adjacent to the craters shoved many stress lines in the coatings 

but no evidence of any debonding or cracking of the coatings. Whatever 

coatings are used it appears that all tested very well under high 

speed impact. 

Conclusions: 

The selection of GOG1-TG Aluminum for space suits for use on the space 

station would appear to be worthwhile. The relatively ductile behavior 

of 6061-T6 aluminum is better than a choice of a more brittle 

material. 

The coatings selected show no signs of failure occuring around the 

impact craters as other materials might well do. 

The suit for long term orbit use will likely include insulation mater5 
ials. These can only act beneficially in reducing ballistic limits. 

Bib1 iography 

(a) Fish, R. H. & Summers, J. L. Threshold Perforation of Various 

Materials - 7th HyperVelocity Impact Symposium, 1964 

Tampa, Florida 
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10 'short proyr-am to cal cul ate peiletr-a1.i oil tlli ckiiesz of 

!0 'aluminum space 5uit5 using thr-estiold per for-ation -Formula 

30 'author dick fish, iiasa ame5 research center 1985 

40 'Get the data if di Cfer ent from dEt+aillts: (FUR ALUM) 

50 V = 10 : RD = 2-78 : K1 = -57 :I<lt=="CiLUI.lINlJM" 

60 DIM X (101 ) ,Y (la1 1 

L5 FH1NT:PRINT:F'RINT 

70 F'RIbJT "7-hi5 program calculates tlie Thi-e~lholll perforation" 

00 PRINT "for- debris impact into str ur-tui e5 1I.i ilea- earth orbil" 

WZl PRINT "Input will be asl:ed far, a '11' ~I~wE?I-- will select" 

1130 PFcIt\IT "deiault valutns. .. . 'I : I-'t?It41 : I%lPU 

11D II4F'UT "Clhaiqe veloci \:y of pr-cijecti le'? (dit 1ID I;m/51=c) LY/N3 ":Hf 

1'70 IF e.$ ,lly It CJR 4.s =Ivy 11 THEN 14ld 

140 INPUl "Lnter- new veluci ty in decimal km/sec ( X. X 1 ":V 

150 INPUT "Clianye projerlti le dcnsi ty (df t 2. 70) CY/tJ3 ";E$ 

168 IF D$ ="Y" [IR El$ ="y" rtIEN 180 

170 IF l3-S ="N" C3R I35 ="I)" 1IIEtJ 190 ELSE 133 

180 INPUl "Enter new pr o-jectile rjerisity iri qnic-/c-c ( X-XX ";It0 

15'0 INFUT "Lhar.hge targc-?t constant Kl';' (dF t is AI-U;-11NUM=.57) CY/E43 ";E$ 

200 IF CB ="Y" OR C$ ="y" rtiEN 22Ql 

210 IF ='IN" (IR C9 ="n" THEN 230 ELSt 15'GI) 

220 INPUT "Enter new tar-get conrstant (emperical data) (. XX) '';l:;l 37r= l.L~ LINE INI'lJT "What is tlie new inaterial? ";K1$ 

3 &-.0 :I V1 = V'..(.El'75) : tl =L (-S236)*RU L)1 = I<O."(.1667) 

240 DEF FNA(X) = Kl*Dl*Vl* (X"'.3S2) 

245 F'RINT:FRIi\IT 

2'50 PR1NT"Fi 1 ling array wit11 tar yet thicl.ner,s vs projectile diain-ter- values" 

26D FOR I = tu 10U 

270 X(1) = I/280 

2-75 tl1 =; (X ( I )..'-3) 0l.l 

280 Y(I) = FNA(EI1) 

2Yv1 NEXI- I 

3CU Ct IA IN I' PLOT XY 'I , EJUDld 

939 END 

62 CoiirioiJ x ( ) , Y t 

3 38 IF ="Id" UR A$ =''I)'' THEN 150 ELSE 1 ID