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FRACTURE CONTROL OFSPACE FLIGHT STRUCTURES AND PRESSURE
VESSELS
S .%tl)arsharlaJet Propulsion Laboratory, California lnstihjte
of Techncdogy
Pasadena, California
J. El. ChangThe Aerospace Corporation
El Segundo, California
and
MC. LOUJet Propulsion Lak)oratory, California Instihlte of
Technology
Pasadena, California
ABSTRACT
The fracture corwol requirements and their implementa-tion for
the design and operation of NASA and Airforcespaceflight hardware
are reviewed in this paper. Thepaper till focus on two categories
of spaceflight hard-ware: ( 1 ) Space Shuttfe payloads which
include signifi-cant potions of the international space station
underdevelopment and (2) flight pressure vessels. The NASAand
Airforce have developed detailed fracture controlrequirements for
space shutde payloads and pressurevessels and these will be
discussed in this paper, Thepaper till also discuss the lessons
learned in incorporat-ing tfle fracture control requirements in
space flight hard-ware especially in the Space Shutde program.
INTRODUCTION
Fracture control is tile application of design philoso-phy,
analysis method, manufacturing technology, qualityassurance, and
operating procedures to prevent prema-ture failure of hardware due
to the propagation of unde-tected cracks or crack-like defects
(flaws) duringfabrication, testing, transportdbon and handling,
andservice. Fracture control has been formally implementedin tile
space programs beginning in 1970. NASA-SF-BO4O[11 was the first
document to specify tf~e fracture controlrequirements for space
flight hardwdre and it was forpressure vessels in space flight
systems. [n 1972, the AirForce completed its development of a
requirementsdocument, the MIL-ST-[) 1522, for the design of
pressur-ized space and missile systems. lhis document wasrevised in
19B4 to become MIL-STEI 1527A [21. At pre-sent MIL-STD 1522A is
accepted as the primary require-ments document for space flight
vessels.
Although fracture corltiol methodology was beingused in other
industries such as commercial and militaryaviation and the nuclear
povwr industsy, the Space Shut-Copyr,ght 63 1997 by tie Anmtic. r,
lkwt~tte of Acronat, bcs and Asuon.mnm Inc.
de was the first space program to incorporate a compre-hensive
fracture control plan 131 for its design, develop-ment, and
operation, There MS no precedent for suchusage on a space system
since it was a reusable tingedvehicle designed for 100 launches and
landings [41. Afterthe Challenger mishap, NASA went through an
extensivesafety reviewofthe Space Shutde and it$ operations.
Oneoutcome of this review WS that NASA developed acentral fracture
control document, NHB B071. I [5], forpayloads and associated
hardware to be flovwr on theSpace Shutde. At present NHB 8071.1 is
the acceptedrequirements document for all payloads ffovw in
SpaceShuttle vhich include some key elements of the SpaceStation
mhich are to be assembled in space around theturn of this
century.
PAYLOAD REQUIREMENTS
For a flight payload systerrl to be flovm in the SpaceShuttle,
d~e design and use of each of its hardwarecomponents must be
revievwd to determine vhetfler apre-existing crack in the component
may lead to a cata-strophic hazard. A catastrophic hazard is
defined as anevent which can disable or came fatal personnel
injuryor Ic)ss of the Space ShUttfe Examples of such eventsincl(lde
a failure and a sutJsequer~t release from a payloadany part or
fragment having nrdss and or energy that canpotentially pUnCh
through the wdll of tile cargo bay, arelease of a significant
anlollnt of hazardous substanceinto ttle cargo bay, or a fdilure
that prevents closure oftt~e cargo-bay door. The payloads
requirements docu-ment NHB B071.1 contains tile prc)cedures for
fracturecontrol classification of all payload components asshovm in
the flowchart in Figure 1.
Non-Fracture Critical Categories
Components classified asexeniptparts,lovweleased-mass parts,
contained parts and krit-safe parts are not
All r,gt, c, rcsencd,1
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flactufe critical. Itlcy can t)e processed uIIder cor)ventional
aerospace industry verification ancl qualltyassurar)ce require
n}ents. [xernpt components are thosetl~at are clearly
norvstructural and not susceptible tofdilure as a result of crack
propagation. Corrlponerlui ttlatmay be included irl exempt category
are insulation blankets, tire bundles, and elastomeric seals, The
totalreleased mass, tf]e fracture toughness of ttle part mate.rialj
and if tl~e part is preloaded in tension till determinemhetl~er a
component may be categorized as a /owre/eased-mass part. A
preloaded, Iowfracture tougtlrresscomponent wth a mass less than
13.6 grams (0.03 Ibs)vhose failure wII not result in the release of
a larger partmay be classified as a non-fracture critical part.
However,this mass may be as high as 113.5 grams (0.25 Ibs) if
tl~epart is not preloaded and is made of high fracture tough-ness
material. A payload component nlay be classifiedas a contained part
if its failure and subsequently thefailure of its container does
not result in ~le release ofelements with a combined mass exceeding
tfle lowre -Iease-mass Iirnit, The containment capability may
beassessed by test or analysis. An empirical approactl thatis often
used calculates tf~e required thickness of thecontainer by equating
the kinetic energy of the releasedfragment to an estimate of the
work required to punchout a hole from the container mall. The final
fa//-safiicategory in general applies only to redundant
structuraldesigns that are not classified as pressure vessels or
Itighenergy rotating equipment. In addition to being
deemedredundant a fail-safe design should ensure that frag-ments
from a failed structural element does not exceedthe Iovweleasemass
limit. An example of a fail safecomponent may be an electronic box
attached tithmultiple fasteners,
Fracture Critical (;olnponents
Afl ttre parts tl~at cannot be classified as
norl-fracture-critical are deenled fracture-critical parts.
Fracture criticalcomponents stlould have their damage tolerance
arid/orsafe-life verifred by either test or analysis. ltle safe
fifeanalysis should be performed based on tlie state of-tl~e-art
fracture mechanics principles. Apart satisfies safwfifecriteria
vdlen nondestructive evaluation (ND[-) is ~jer-forrned to screen
out crack above a particular size andttlen show by analysis or test
that an assumed crack ofthat size till not grow to failure mtlen
subjected to tilecyclic and sustained loads encountered during four
cornplete service Iifetirnes. lhe selection of N[)E
rnf!tllodsarlcjlevel of inspectic)n should be based on fracture
rrlecl~anics analysis and tile safelife acceptance requirements ofa
specific part. lt~e mirlimurn initial crack sizes for safe
Ilfeanalysis using different NDE methods are shovm InTable 1, Proof
testis sometimes msed to establish ~ cracksize for safe life
analysis.
One complete service lifetime inclucles all sigrlifk.antloading
events that occur after NDE-. Software coc]es vittlproven
metllc)dology and reliable material cfat,) base
suctl (IS NAWW1.AGF{O [61 SIlou[d be used to per furmtfle
required safe-fife arnalyses. Crack growt}l analysis ofconlposite
materials is beyond ttle current state -of-tile-art 1 f]e fracture
control requirement for composite struc-tures is satisfied by proof
tf!sting it to 1.2 times tllc IirrlitIoacl ar)d tf~rough NDE. Nll[l
B071. I also allows for anapproach to keep tf~e Iinlit load strains
belowa tl~resholdstrain level at mllich tfle composite is damage
tolerant.
PRESSURE VE. SSE[ REQUIF{FMENTS
Afl space-flight pressure vessels are designated asfracture
critical. MiL-STD-1522Aoffers d~ree approaches,namely A, B, and C
as shovw in Figure 2, for designanalysis and verification of
pressure vessels. ApproachB, the strength of materials approach, is
not acceptableby botf~ USAF/SD and NASA since it assumes that
noflaws are present in the w!ssel and Approach C, theapproach
complying wth tl~e ASME pressure vesselcode, is seldom used for
flight vessel cleveloprnent sirlceit results in an overweight
vessel.
Ttlus, Approach A is tf~e c]n]y one applicable for
ffightvessels. lW paths may be followed in this approachbased on
m.hed]er the failure mode is (1) LBB tith r~on-hazardous contents,
or (2) Rritde (Norl-LBB) or ILBB wthhazardous contents. The LBEI
failure mode maybe demo-nstrated by analysis ortestl]yshoting
thataninibal flawshape (given by ratio of crack deptf] a to half
length c) inthe range .05s a/c < 0.5 till propagate through
tt~ethickness of tile pressure vessel before becoming critical.An
appendix contained in MIL-STD 1522 provides anacceptable approach
for failure n!c~de determination. Apressure vessel is considere(f
to exhibit a ductile fracturemode (1-t3B) when
d~ere K,c is tflt? plane strain fracture toughness of
tdnknlaterialf ~(11) is the operating stress level, (x is tf~e
prooftest factor, t is the wall ttlickrless, and OYS is ttle
yieldstrengtf~ of tank material,
l-or Patf~ (1) in fi>[)roaclt A, no further fracture
mechan-ics analysis is required vdlen tile Miners rule is
satisfiedby a fdtigue analysis of ttle urrffawt!d vessel.
Qualificationtesting of tfle vessel requlr~!s arl 1 BB
demonstratic)rr(waived ifsflowl arlalytically), pressure testing,
and ramdom vibratiorl testing. Pressure testing requires
tf~atttlere be no yield afler pressure cyclirlg for either (i) 2
xnunlber of opera tirlg cycles at 1.5 x mean expectedoperating
pressure (ME OP), cjr (ii) 4 x number ofoperat-ing cycles at 1.0 x
MEOP, and tflere be no burst vdlenpressurized to burst factor (R
F.) x ME OP. Acceptancetests conducted on each pressure vessel
before commit-ment to flighl require an NDl and a proof pressure
test.The proof test is intended to detect any errors in tile
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rl.,arl~lf,i(,t(lrirl~] processor ~JlkrllzirlstlilJar~d
illtlles~]cc(fication of the materials. Mc)stflight pressure
vessels aredesigned to a B.F. of 1,5
[01 Path (7) in Approactl A, a safe-life analyses (ortests) of
tl~e vessel is required witl I a pre-existing irlltialflavwto
showthat life isgreater than four times ttle servicelife oftt~e
vessel. Safe-life of the vessel is the period duringwhich it is
predicted not to fail in tl~e expected operatingenvironment. The
requirements for clualifying the vesselsby pressure testing and
random vibration are the sameas for Patti (1) above. t{ovwver, ttle
acceptance tests inthis case vwxdd k)e required to establish the
initial flawsize used in the safe-life analysis in addition to
clet[!ctingany errors in the rnanufacturirrg.
Non-composite vessels anti composite vessels vittlmetallic
liners satisfy the same requirements with a fewexceptions. However,
composite vessels without a loadcarrying metallic Iinerrnayordybe
designed by the ASMEpressure vessel code, Section X. The 1 BB
failure modedetermination for the metal lined vessel is only based
onthe liner, and fracture mechanics methodology is notapplicable to
the composite overwap. Conventiorlal fd -tigue life analysis of the
composite overvwap must verifythatthe Iineristhe clitical safe-life
component by shovingthat the life of the overwap is at least ten
times longerthan that of the liner
The section on pressurized systems contains generalrequirements
for system design featores, including rout-ing of piping and
tubing, physical separation of con~po-nents, electrical grounding,
anti safety of groundhandling, tooling, and testing. It also
includes pressurerelief requirements related to locations of
pressure reliefdevices, flowcapacity, sizing, venting, filterinfg,
and nega-tive pressure protection. Design requirementsf
orcorltroldevices and detail requirements specific to hydraulic
anc~pneumatic systems are also contained in this section.
Some modifications and exceptions to ttle MIL-S11)1522A are
specified in NASA fracture control require-m(!nts docurnerlt f o r
Space Shuttle payioarfs, NIIR8271.1 [51. For analytical I BH
ciernonstration, a crackgrowth analysis is preferred to using ttle
ductile fractureec~uation given in tile 1577Aappendix. 10
accomrr~odatetl~e use of one-o f-a kind vessels, t}ie NASA
cfoctlrnerltsBC71.1 and 1700.7 allowa proc)f test at a minimum of
1.5x MDP and a fatig~le analysis showing a minimum of tendesign
lifetimes in Iwu of tflf: t)urst ancl fatigue life testsreq~jired
by 1522A. NASA !equires an additiorral NDl oftt~e welds in tl~e
pressore vessels stlell after proof testir]gto screen the initial
NDl ffawsize ass~rmed for analysis.
For a cc)mposite pfessure vessel with a m~!tal liner,
anadditional NASA requirement is to denronstrate damagetolerance of
the overvwap [)arrlage tolerance is ttl(!ability of the pressure
vessel to resist failure doe to tilepresence of flaws for
Srlecifled period of uorepai(ed us-
a g e . Unllke for ttle metalllc Iirlcr, Ilrlear elastic
fracturenlc!ct}anics is not strictfy valid for tf~e composite
over-vwap. ll)us, fracture control ofttle overvwap till bet]asedorl
ttle use of NDI and manufacturing process cc)ntrol.
Recently, tl~e MIL-STD-1522A l~as been updated byI he Aerospace
Corporation. A draft copy of tl~e updatedstandard which specifies
the safe-life demonstration re.quirement for pressurized structures
containing hazard-OLIS fluids such as launch vehicle main
propellant tankshas been reviewed by government agencies and
aero-space industries. This updated nlilitary standard in
beingconverted into an industry standard which consists
oftvwvolumes. Volume I [71 covers metallic pressure
ves-sels(spacecraft liquid propellant storage tanks and
highpressore gas bottles), pressurized structures (launchvehicle
main propellant tanks), special pressurizedequipment (batteries,
heat pipes, cryostats and sealedcontainer) and pressure components
(lines, fittings,valves and hose). The requirements for composite
hard-vmre including composite overvwapped pressure vesselswII be
covered in Volume II Composite F{ardvwrre,
LESSONS LEARNE-D
lhe most important aspect of cost effectively inlple-menting
fracture control for flight hardware is to preparea safety
verification plan which incorporates the relevarltfracture control
requirements early in the program. It isimportantto make the
designers and analysts also awareof the requirements and damage
tolerant design ap-proaches at the onset since most structural
parts can bedesigned tolerant of initial flaw wthout significantly
com-plicating tt}e design or adding much cost. Every effortsl)ould
be made to desigr) tfre components of spaceIlardware so tt~at ttley
fall irlto one of tt~e norl-fracturecritical categories described
above. Iiovwver the frac-ture critical category cannot be avoided
for componentsSLICt) aS P[L2SSUW WSSC?!S and t/10S6? haVlng
highf30ergyr{)tatillg parts, and parts which need to be made
wtllbrittle materials, vwlded joints, composite materials,
ori)orlded joints.
[)(! Splk? tlaVlng fOLIr aVi311FIblL? OOn-fraCtLlrE? Crlical
C.3k-gories, implementing ttle req[iirements for certain
SpaceStj(]ttle payloads result~!d in extensive frtICtLlre
criticalparts lists. tiovwver, nlost of ttlese parts eittler
carrieclvery small loads, or made of highly ductile material
andvx!re nlade witl~ processes irl wt~ictl initial cracks
are[!xtrf!rllely unlikely. h] outcome of ttlis wds ttlc! creationof
yet anotl}er oon-fracture critical category called lowrisk parts in
tl~e revised Nt IB EK71.1 clc)cument. Low riskparts must be made
horn metal highly resistant to fracturetittl VWII establistled
processes and demonstrate low~)robahilities of crack presencf! and
growttl.
As mentioned above ttw Si)ace Shuttle was the frrstspace program
to incorporate a conlprehensive ap-proach to prevent stroctlurdl
failures resulting from cracks
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w crack-like defects. Fracture control plans were devel -oped
for the orbiter, solid rocket booster, main engine,and external
tank. It was planned to produce five fhghtvehicles, and orbiter
test verification for safe life wouldhave been relatively costly
(in terms of total programcosts) compared to similar verification
tests forconlnler-cial and military aircrafts programs. t{ence
fracture con-trol requirements were met for the Shuttle program by
amixture of analysis and test.
ACKNOWLEDGEMENTS
lhe vwrk reported here was carried out by the JetPropulsion 1
aboratory (JP[ ), California Institute of Tech-nology, under a
contract wth the National Aeronauticsand Space Administration and
by The Aerospace Cor-poration under a contract tith the United
Sates Aiforce.
REFERENCES
1., Fracture Control of Metallic Pressure Vessels, NASASP- 8CM0,
1970.
2. Standard General Requirements for Safe Designand Operation of
Pressurized Missile and Space Sys-tems, MlL-STD-l 522A 1984.
3. Space Shuttfe Orbiter Fracture Contrc)l flan, SD-73S}{-0382A/
1974
4. Forman, R. G., and Hu,T., Application of FractureMechanics on
the Space Shutde , ASTM S-I-P 842,1984.
5. Fracture Control Requirements for Payloads UsingThe National
Space Transportation system (NSTS),NASA NtHB 8071,1, 1988.
6. Fatigue Crack Growth Computer ProgramNASA/FL AGRO. NASA
.ISC-2?267, 1988.
7. AIM Standard, Aerospace Pressurizecl Systems,Volume 1-
Metallic Hardware, AIAA-S-XXX+ Draft 1996.
4
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FRACI URE ctwnc~ PART
---1
1.
[
.- .-.
.vE:?LOBE=FYkET-4: -?A:5E;incLA?*?::.J
INo
.-
{ - - - - - - - -- -RZOCSIGN
.-. -. .. . ._. _ ..1
Figure 1 Fracture Control Classification Process
. - ___
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Table 1 Minimum Inihal Crack Sizes for Fracture Analysis Based
on NDE Method
Us. cusT()~y m (iIIChCS)
.
crack .
Pm Crack Crack c r a c k Lmcation ThiCklle% t Dimension a
Dimension c _
EWQnaNQEOpen Surfaa t 0.050
I
0.020
{
0100pTc 1
0.050 0.0s0
Edge or Hole [ s 0.075 -IllrOugil 0.100t >0.075 Gxocr
.0.:75 0.075 _
Open Surface t s 0.050 Througb t 0.100.050< t < .07s
-nNOugb t O.ls-t
c >0.075
{0.02s
[0.12s
PTc0.075 0.07s
Edge or Hole to.lm _ , . O.hx) Cr.loo .._.. -
e NDE
Open Surfacx I s 0.075 Through t 0.12s[ >0.075
[ 0.03$
[0.188
P T c0.075 0.12s
Edge or Hole [ 0.07s Comer 0.2s00.;75 _ .. 6
open Surfacx .025 s t x 0.107 o.7t 0.075I >0.107 o.7t o.7t _
.
.
UWiQllidQE!Open Surface t 20.100
. . c &___LL
t!QkXj : gp?,z?$~h% very tight flaws (e.g., forging flaws or
lack of full pmetratioo in burlwelds)3.- Ccmpanblc to Class A
quality level
-
--l1.5 x MEOP (HF. > 2.0)OR(1 + B.F_.) X MFOP (B.F. <
2.0)A
+q- BURS1 . . .
[
IMECHANICSSAFE-LIFEDEMONSTRATION(ANALYSISOR TEST 1_._r.. .
PROOF ATL) ETEF{MINED BYFRACTURE MECHANICSSAFE-LIFE ANALYSIS(MIN
1.25 X MEOP)
EV
L 1-------r-- H-FT~rlq!a
B5.1.3 &5.2.3~
ANA1 YSIS
SECTION 4.2.5-
T
A
r].-.. ANALYSISSECTION 4.2.5D!Ei5:l l!!%O I[
MODEDETERMINATION --
~Ellw=)_-
LBB BRlrTLE OR 113BNON HAZARDOUS HAZARDOUS5.1.1 & 5.2.1
5.1.2 & 5.2.2
&
OR
*
3
I IFmrrm
[ fi: I-F?iT
ANA1 YSIS ANALYSIS. ..
- .-
J,xmmmksr-- 1
.
ACCEPT} WE TEST
1
PRO~ r 1.5 MtOP. . .
AC~+=AT15MEOP.
T
T-m- BURST... -- __-~..__.r__L__ . .. .-A-_ . . .._ACCEPTAFKE
DESIGN
c5.1.4 & 5.2.4
-xsmmurmmDOT TITLE 49SATISFIED
CY- BURSTCLE 1
--T... L
---7l_- __-. _ .- . .. . - .- - . .. . .- - -- .. -.FIGURE 2
APPROACHES FOR DESIGN AND VER1l-lCATION OF PF{ESSURIZED
COMPONEN1S IN MIL-STD 1527A
