Advanced composite materials, which are increasingly being used to build aircraft, have different properties than the metals they replace. Fasteners intended for composite-material joints must be designed and selected to allow for these differences. For example, blind fasteners (one-sided access) used to assemble composite-to-composite joints have been redesigned to expand to larger diameters to resist pull-through and cocking failures. The fastener designs needed for composite materials are reviewed. Topics discussed are: galvanic corrosion, pull-through resistance, fastener rotation, installation damage, fastener galling and conductivity. A blind fastener recently developed by SPS Technologies is described to show how these requirements are incorporated

Landt, R. C.

English

NASA Technical Reports Server

MECHANICAL FASTENERS FOR ADVANCED COMPOSITE MATERIALS 
R ichard  C. Landt 
SPS Technologies,  Inc.  
Jenkintown, PA 19046 
ABSTRACT 
Advanced composi te m a t e r i a l s ,  which a re  i n c r e a s i n g l y  be ing  used t o  b u i l d  a i r -  
c r a f t ,  have d i f f e r e n t  p r o p e r t i e s  than t h e  metd ls  t hey  rep lace.  Fasteners 
in tended  f o r  composi te-mater ia l  j o i n t s  must be designed and se lec ted  t o  a l l o w  
f o r  these d i f f e r e n c e s ,  For example, b l  i n d  f as tene rs  (one-s ided access) used t o  
assemble composi te- to-composi  t e  j o i n t s  have been redesigned. t o  expand t o  l a r g e r  
d iameters  t o  r e s i s t  p u l l - t h r o u g h  and cock ing  f a i l u r e s .  
Th is  paper rev iews t h e  f as tene r  designs needed f o r  composite m a t e r i a l s .  Topics  
d iscussed a r e  ga l van i c  co r ros i on ,  p u l l - t h r o u g h  res i s t ance ,  f as tene r  r o t a t i o n ,  
i n s t a l  l a t i o n  damage, f as tene r  ga l  1  i n g  and c o n d u c t i v i t y .  A  b l  i n d  f a s t e n e r  
r e c e n t l y  developed by SPS Technologies i s  descr ibed  t o  show how these r e q u i r e -  
ments a re  incorpord ted .  
https://ntrs.nasa.gov/search.jsp?R=19860001765 2020-03-23T06:20:33+00:00Z
I n t r o d u c t i o n  
----------.#- 
Commercial and m i l  i t a r y  a i r c r a f t  a r e  now be ing  b u i l t  u s i n g  advanced composi tes 
such as graphi te-epoxy m a t e r i a l s .  A1 though these m a t e r i a l s  o f t e n  reduce t h e  
number o f  s t r u c t u r a l  components and o f f e r  a l t e r n a t i v e  j o i n i n g  methods, mechani- 
c a l  f as tene rs  p l a y  a  v i t a l  r o l e  i n  a i r c r a f t  assembly. I n  t h e  commercial sec to r ,  
composite m a t e r i a l s  a r e  used i n  secondary s t r u c t u r e s ;  t h e  NASA A i r c r a f t  Energy 
E f f i c i c e n y  (ACEE) Program i l l u s t r a t e s  some t y p i c a l  a p p l i c a t i o n s  ( F i g u r e  1). 
These ACEE s t r u c t u r e s  were f a b r i c a t e d  f rom separate  graphi te-epoxy spars,  r i b s  
and s k i n s  t h a t  were assembled w i t h  mechanical  fas teners .  
Composite m a t e r i a l s  d i f f e r  from t h e  me ta l s  t hey  r ep lace  because t h e y  a r e  n o t  as 
d u c t i l e  and t h e i r  p r o p e r t i e s  a r e  d i r e c t i o n a l .  Fasteners develop c lamping f o r c e s  
and r e s i s t  j o i n t  l oads  which a c t  i n  t h e  th rough- the- th ickness  d i r e c t i o n  i n  a  
j o i n t ;  t h i s  i s  a l s o  t h e  weakest d i r e c t i o n  i n  a  composi te laminate.  Exper ience 
w i t h  composite j o i n t s  has i d e n t i f i e d  some p o t e n t i a l  problems r e l a t e d  t o  
f as tene r  des ign dnd s e l e c t i o n ,  
F i gu re  1, NASA ACEE program s t r u c t u r e s .  
--- 
Galvanic  C o m ~ a t i b i l  i t v  
When meta l  fas teners  a re  coupled w i t h  g r a p h i t e - f i b e r  composites i n  a  c o r r o s i v e  
environment, g r a p h i t e ' s  low e l e c t r i c a l  p o t e n t i a l  causes t h e  f as tene r  t o  a c t  as 
an anode and corrode, The r a t e  a t  which c o r r o s i o n  proceeds i s  measured by  t h e  
c u r r e n t  dens i t y ,  F igures  2 and 3 rank t h e  e l e c t r i c a l  p o t e n t i a l s  and c u r r e n t  
d e n s i t i e s  f o r  va r i ous  meta ls  coupled w i t h  g raph i t e ,  Cur ren t  d e n s i t y  i s  t h e  be t -  
t e r  i n d i c a t o r  o f  c o m p a t i b i l i t y ;  6Al-4V t i t a n i u m  a l l o y ,  f o r  example, forms a  
Figure  2. P o t e n t i a l  o f  meta ls  coupled w i t h  g r a p h i t e  ( r e f .  1). 
F igure  3. Cur ren t  d e n s i t y  o f  meta ls  coup1,ed w i th .  g r a p h i t e  ( r e f .  1). 
t i g h t l y  adhe r i ng  p r o t e c t i v e  ox i de  f i l m  which r e s i s t s  f u r t h e r  co r ros ion ,  T i t a n i -  
um and i t s  d l  l o y s ,  MULTIPHASEa a1 l o y s  MP159a and MP35W; and Incone l  a1 l o y s  600 
and 718 a r e  compat ib le  w i t h  g r a p h i t e - f i b e r  composites. These m a t e r i a l s  show no 
evidence o f  c o r r o s i o n  a f t e r  500 hours o f  5% s a l t - s p r a y  t e s t i n g .  Cor r ros ion-  
r e s i s t a n t  s t e e l s  A-286, Ph13-8Mo, Ph17-7, 301, 304 and 316 a r e  accepted f o r  use 
w i t h  graphi  t e - f i  be r  composites. Occas iona l l y  these s t a i n  i n  sa l  t - s p r a y  t e s t s ,  
t h e r e f o r e  f as tene rs  o f  these  m a t e r i a l  a r e  u s u a l l y  i n s t a l l e d  w i t h  sea lant .  Monel 
400 and 405 m a t e r i a l s  w i l l  p i t  and r a p i d l y  co r rode  and a re  n o t  recommended f o r  
g r a p h i t e  composites. C o r r o s i o n - r e s i s t a n t  s t e e l  440, a1 l o y  s t e e l s  and a1 uminum 
a l l o y s  a r e  n o t  compat ib le  w i t h  g raph i t e .  Even when fas tener  m a t e r i a l s  a re  
compat ib le ,  sea lan t s  a re  g e n e r a l l y  a p p l i e d  t o  deny access by t h e  c o r r o s i v e  
env i  ronment. 
Pu l l -Throuah Fai l 'u res 
F i gu re  4 i l l u s t r a t e s  t h e  p u l l - t h r o u g h  f a i l u r e  mode. The j o i n t  f a i l s  when 
through-p lane shear f o r ces  p u l l  t h e  f a s t e n e r  th rough  t h e  laminate.  The f a i l u r e  
l o a d  i s  i n f l u e n c e d  by bending moments, i n -p l ane  s t r esses  and dynamic e f f e c t s  
t h a t  a c t  i n  combinat ion w i t h  t h e  through-p lane shear s t r e s s  t o  lower  t h e  f a i l u r e  
1  oad. 
S t r u c t u r e s  j o i n e d  w i t h  shear, f lush-head f as tene rs  and b l  i n d  fas teners  a re  par-  
t i c u l a r l y  s u s c e p t i b l e  t o  t h i s  t ype  o f  f a i l u r e .  Pu l l - t h rough  s t r e n g t h  can be 
improved by u s i n g  f as tene rs  w i t h  l a r g e r  bea r i ng  c i rcumferences because they  
develop h i g h e r  through-p ldne shear loads, Cur ren t  des ign  p r a c t i c e  i s  t o  use. 
t ens i on -  r a t h e r  than  shear-head f as tene rs  i f  p u l l - t h r o u g h  s t r e n g t h  i s  c r i t i c a l ,  
B l i n d  fas tener  a p p l i c a t i o n s  have r e q u i r e d  washers under t h e  b l i n d  head t o  pro-  
v i d e  d e s i r e d  s t reng ths .  
F i gu re  4. Pu l l - t h rough  f a i l u r e  i n  j o i n t  t e n s i l e  s t r e n g t h  
-------- 
t e s t .  
Fastener Rotation 
----m--------- 
Fastener rotation refers t o  a fastener cocking in the joint ,  not the turning 
action that  occurs when d fastener i s  torqued, Shear loaded joints develop non-  
uniform bearing contdct between the fastener shank and hole. A t  these hiyhly 
stressed contact points, the composite structure f a i l s  and the fastener rotates 
into the ldminate. Figure 5 i l l u s t r a t e s  a failed single-lap shear joint. 
Eventually, the fastener embeds or pulls through the laminate. 
Fasteners with large bearing diameters r e s i s t  cocking forces and retard rota- 
tion. Figure 6 sumliiarizes joint tens i le  strength data for experimental 3/16" 
diameter, 130' flush-head, titanium bolts with stdinless-steel nuts. By 
increasing the n u t  flange diameter from .250" t o  .600" (1.3 to 3.2 shank diam- 
e t e r s ) ,  the ultimate joint strength improved 36%. However, as Figure 7 shows, 
increasing flange diameter l imit lessly will add weight and not improve joint 
strength. Based on wei g h t  efficiency (ultimate load/fdstener weight) a flange 
didrneter of 2X shank diameter optimizes th i s  jo in t ' s  strength. 
Figure 5. Single-lap joint i l lus t ra t ing  fastener rotation 
fdilure.  
Threaded Fastener Gal 1 i  n g  
......................... 
Titdnium bolts are ~ i d e l y  used with .graphi te - f i  ber composites because they are 
galvanically cornpati ble and 1 i  ghtwei g h t .  Ai rframe manufacturers have reported 
that gal 1 ing (seizing)  may occur when using titanium or stainless-steel pre- 
vai 1 ing-torque nuts with these bolts. This effectively destroys the bolt and 
n u t .  
To p reven t  g a l l i n g ,  A-286 and 300-ser ies  s t a i n l e s s  nu t s  must be coated w i t h  a  
d r y - f i l m  l u b r i c a n t  dnd i n  some cases, f i r s t  p l a t e d  w i t h  cadmium o r  coated w i t h  
aluminum. These f i n i s h e s  a r e  no t  compat ib le  and would be consumed i n  s e r v i c e  
b u t  l u b r i c a t e  t h e  n u t  o r  b o l t  f o r  t h e  i n i t i a l  assembly opera t ion .  
An a l t e r n a t i v e  t o  t h e  threaded f as tene r  i s  a  nonthreaded l o c k b o l t  w i t h  a  swaged 
c o l l a r .  
I n s t a l l a t i o n  Dam* 
---- 
Some fasteners  developed f o r  use i n  m e t a l l i c  s t r u c t u r e s  w i l l  damage composi te 
s t r u c t u r e s .  Uarnage occurs  when t h e  c lamping s t r e s s  exceeds t h e  compressi ve 
s t r e n g t h  o f  t h e  lamina te ;  r a d i a l  expansion i n  t h e  f as tene r  ho le  de laminates o r  
buck les p l i e s ,  impact f o r ces  de laminate t h e  s t r u c t u r e ,  o r  t u r n i n g  f as tene rs  
abrade o r  s p l i n t e r  t h e  composi te sur face,  
Cur ren t  des ign p r a c t i c e  i s  t o  avo id  i n t e r f e r e n c e  f i t s  w i t h o u t  us i ng  a  p r o t e c -  
t i v e  s leeve, f as tene rs  w i t h  inadequate b e a r i n g  area, o r  r i v e t s  and b l i n d  
f as tene rs  t h a t  expand r a d i a l l y ,  and us i ng  r i v e t  guns t o  i n s t a l l  fas teners .  
C o n d u c t i v i t y  
---- 
An e l e c t r i c a l l y  conduc t i ve  pa th  between t h e  f as tene r  and t h e  s t r u c t u r e  i s  
needed t o  p r o t e c t  g raph i  te-epoxy s t r u c t u r e s  d u r i n g  1  i gh tn i ng  s t r i k e s .  Uamage 
can occur  a t  t h e  at tachment,  e x i t  and t r a n s i t i o n  p o i n t s  ( j o i n t s ) ,  w i t h  a r c i n g  
a  danger near f u e l  c e l l s .  
Measures t o  p reven t  ga l van i c  c o r r o s i o n  o f t e n  i s o l a t e  t h e  f as tene r  w i t h  noncon- 
d u c t i v e  sea lan ts ,  coa t i ngs  and l u b r i c a n t s .  Improved f as tene r - t o -ho le  c o n t a c t  
and conduc t i ve  coa t i ngs  must be developed t o  so l ve  t h i s  problem. 
B l  i n d  Fasteners 
B l  i n d  f as tene rs  w i t h  t h e i r  s p e c i a l  des ign requi rements  cha l  l enge  t h e  f a s t e n e r  
des igner  concerned w i t h  composites. Because b l i n d  f as tene rs  can be i n s t a l l e d  
f rom one s i d e  o f  t h e  s t r u c t u r e ,  l a r g e  bea r i ng  d iameters  d re  d i f f i c u l t  t o  
achieve. I n  fo rm ing  t h e  " b l i n d  head" these f as tene rs  must no t  damage t h e  com- 
pos i  t e  s t r u c t u r e .  Typ ica l  a p p l i c a t i o n s  f o r  b l  i n d  f as tene rs  i n c l u d e  cover - to -  
spar  j o i n t s  i n  c l o s i  ng-out s t a b i  1  i zers,  a i  1  erons and rudders.  These 
a p p l i c a t i o n s  a r e  t r u l y  " b l i n d "  s i n c e  t h e  f a s t e n e r  b e a r i n g  su r f ace  c rea ted  i n s i d e  
t h e  s t r u c t u r e  cannot be seen and inspected.  There fo re  a  b l i n d  fas tener  must be 
r e l i a b l e .  More recen t l y ,  a i r f r a m e  manufacturers  a r e  e v a l u a t i n g  honeycomb 
sandwich-to-composi t e  j o i n t s  c o n t a i n i n g  b l  i nd f as tene rs  where t h e  f a s t e n e r ' s  
b l i n d  head forms i n s i d e  t h e  co re  m a t e r i a l  a g a i n s t  t h e  sandwich face  sheet.  
The aerospace f a s t e n e r  i n d u s t r y  i s  o f f e r i n g  a  v a r i e t y  o f  new fas tene rs  f o r  com- 
p o s i t e  m a t e r i a l s .  One o f  these i s  t he  COMP-TITE'" b l i n d  fas tener .  Shown i n  
F i gu re  8, i t  i s  comprised of f o u r  elements:  a  nut,  c o r e b o l t ,  c o i l e d  washer and 
sleeve. Dur ing  i n s t a l l a t i o n  (F i gu re  9 )  t h e  c o i l e d  washer and s leeve  a re  d r i v e n  
over  t h e  tapered  end o f  t h e  nu t  dnd expanded t o  t h e i r  f i n a l  d iameters.  The 
formed washer i s  then  seated dga ins t  t he  j o i n t  su r face  as t he  c o r e b o l t  ddvdnces. 
F igu re  6. E f f e c t  o f  n u t  bea r i ng  d iameter  on j o i n t  s t r eng th .  
F igure  7. E f f e c t  o f  n u t  bea r i ng  d iameter  on j o i n t  s t r e n g t h  
versus j o i n t  e f f i c i e n c y .  
The c o r e b o l t  d r i v e  stem shears a t  a  t o r q u e - l i m i t i n g  groove a f t e r  t h e  j o i n t  i s  
clamped. Th is  des ign approach ach ieves a  l a r g e  b l  i nd - s i de  bea r i ng  d iameter  
w h i l e  m i n i m i z i n g  t h e  p o s s i b i l i t y  o f  j o i n t  damage. due t o  excess ive bea r i ng  
s t r ess .  A separate  washer element a l s o  p r o t e c t s  t h e  composite su r face .  shou ld  
t h e  s leeve  t u r n  d u r i n g  i n s t a l l a t i o n .  
Other des ign  advantages a r e  a  f l a t  washer bea r i ng  su r face ,  c o n s i s t e n t  b l ind -head  
diameters,  no f as tene r  h o l e  expansion, and improved i n s t a l l a t i o n  t o l e r a n c e  o f  
h o l e  c o n d i t i o n .  
The 6A1-4V t i t a n i u m  nu t ,  MULTIPHASE@ a l l o y  MP159@ c o r e b o l t ,  304 S/S s leeve  and 
316 S/S washer s a t i s f y  ga l van i c  c o m p a t i b i l i t y  requ i rements?  
F igu re  8. COMP-TITE" b l  i n d  f as tene r .  
---- 
F igure  9. COMP-TITP b l i n d  f a s t e n e r  i n s t a l  l a t i o n .  
C o n c l u d i s  Remarks 
------- --- 
Fastener s e l e c t i o n  fo r  b o l t e d  composite j o i n t s  should i nvo l ve  a  review o f  the  
cons idera t ions  addressed i n  t h i s  paper. The fas tener  i n d u s t r y  i s  p r o v i d i n g  new 
fas tener  designs which can improve j o i n t  s t reng th  and reduce co r ros ion  problems. 
One o f  these i s  t h e  COMP-TITEw b l i n d  fastener .  which was designed f o r  improved 
pu l l - th rough s t reng th  and cocking res is tance.  
Reference 
--------- 
1. B. A. M i l l e r  and S. G. Lee: "The E f f e c t  o f  Graphite-Epoxy Composites on t h e  
Galvanic Corrosion o f  Aerospace A1 1 oys ", Six  Force Mater ia ls  Laboratory 
(AFML) ; Wright Patterson A i  r Force Base, Ohio, TR-76-121, September 1976. 


Mechanical Fasteners for Advanced Composite Materials

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