Mechanism of strengthening and fracture in composite materials - tensile strength, impact deformation, brittleness

Tetelman, A. S.

English

NASA Technical Reports Server

. P -  
INTRODUCTION 
This r epor t  descr ibes  research a c t i v i t i e s  t h a t  w e r e  performed dur-  
i n g  the  t h i r d  phase of  a program designed t o  inves t iga t e  the  mechanism o f  
s t rengthening and f r a c t u r e  i n  composite materials. As mentioned i n  pre-  
vious reports, th ree  d i f f e r e n t  types of  composite a r e  cu r ren t ly  being 
inves t iga ted .  These are discussed sepa ra t e ly  i n  the  following paragraphs: 
I. The Mechanisms o f  Strengthening and Frac ture  i n  Composites Containing 
Hard P a r t i c l e s  DisDersed i n  a So f t e r  Matrix 
a)  Tens i le  and Impact Deformation of Hypo-Eutectoid S t e e l s  
(with Dare1 Hodgson, Graduate Student) 
Hypo-eutectoid s t e e l s  are  o f  i n t e r e s t  because the  f e r r i t e  matrix, 
and hence the  composite as a whole, can be made to  undergo a d u c t i l e -  
b r i t t l e  t r a n s i t i o n  by changing the  test temperature and the  morphology of 
t h e  hard carb ide  (Fe C) p a r t i c l e s  by hea t  t reatment .  3 
To study the  e f f e c t  o f  varying carbide morphology, two steels 
containing 0.4 and 0.6% carbon were t e s t e d  i n  uniax ia l  t ens ion  under 
plane stress condi t ions i n  both the  p e a r l i t i c  and spheroidized forms. The 
tests were conducted a t  temperatures from room temperature t o  - 196OC t o  
determine the  e f f e c t  of  carbide morphology on the  d u c t i l e  to  b r i t t l e  
t r a n s i t i o n .  
I n  the  p e a r l i t i c  steels,  composed o f  lamellar p e a r l i t e  and pro-  
eu tec to id  f e r r i t e ,  a d u c t i l e - b r i t t l e  t r a n s i t i o n  w a s  observed a t  about 
- 15OoC f o r  both the  0.4 and 0.6 percent carbon s t e e l s .  The s i m i l a r i t y  
of t h i s  t r a n s i t i o n  t o  published resul ts  fo r  a low carbon (0.2X)steel of 
t he  same f e r r i t e  g ra in  s i z e  leads t o  the  conclusion* t h a t  t he  t r a n s i t i o n  
* Hodgson, Darel, M. S. Thes is ,  Stanford Universi ty ,  September, 1965) 
- 1 -  
https://ntrs.nasa.gov/search.jsp?R=19660006326 2020-03-16T21:39:50+00:00Z
temperature i s  determined so le ly  by the  proper t ies  of the pro-eutectoid 
f e r r i t e .  Increasing the  carbon content  ( i . e . ,  the  amount of  lamellar  
p e a r l i t e )  serves  only t o  r e f i n e  the  pro-eutectoid f e r r i t e  gra ins  and con- 
s t r a i n  y i e ld ing  i n  the  f e r r i t e .  While both of these e f f e c t s  r a i s e  the  
t e n s i l e  y i e ld  stress, oy, the  cleavage f r a c t u r e  stress a i s  also r a i sed  f 
by - n l J  ---- 
L C A L L I C ~ U C ~ ~ .  Consequently, the d u c t i l i t y  t r a n s i t i o n  
temperature ( i . e . ,  t he  h ighes t  temperature a t  which a = o ) i s  e s s e n t i -  
a l l y  unchanged by increas ing  p e a r l i t e  addi t ions .  
Y f 
The spheroidized s t e e l s  w e r e  q u i t e  d u c t i l e  a t  a l l  temperatures 
down t o  - 196OC. 
a homogeneous d i s t r i b u t i o n  of spheroidal carb ides  of about 0 . 5  micron 
average diameter.  The high d u c t i l i t y  o f  t he  spheroidized steels may be 
ascr ibed  to  the  extremely f i n e  carbide p a r t i c l e s  and f i n e  f e r r i t e  gra in  
s i z e .  The l a t t e r  prevented the nucleat ion of microcracks i n  the  f e r r i t e .  
I t  was observed t h a t  a number of the  l a r g e r  carbide p a r t i c l e s  near the  
f r a c t u r e  sur face  %ere cracked in  a l l  o f  the  specimens. I n  some cases ,  
these  cracks had begun t o  open up and form voids .  
They had a f ine  f e r r i t e  g ra in  s i z e  (ASTM No. 13) and 
B r i t t l e  f r a c t u r e  i n  medium and high s t r eng th  s t e e l s  occurs by the  
formation of  voids ( a t  cracked carbides  o r  inc lus ions)  and the  coalescence 
o f  these  voids by i n t e r n a l  necking on a microscopic sca l e .  It is  known 
t h a t  t he  f r a c t u r e  toughness %c decreases  with increas ing  s t r e n g t h  l eve l  
(0 ) and increas ing  volume f r ac t ion  of  l a rge  p a r t i c l e s  (e.g. ,  inclusions)  
but  the  r e l a t i o n  between these  t w o  va r i ab le s  has never been inves t iga ted  
i n  d e t a i l .  Quenched and tempered hypo-eutectoid s t e e l s  se rve  a s  exce l -  
l e n t  model ma te r i a l s  on which to  perform t h i s  i nves t iga t ion  s ince  t h e i r  
y i e ld  s t r e n g t h  l e v e l  can be varied ( f o r  a constant  p a r t i c l e  s i ze )  by 
Y 
-2  - 
varying the test temperature while the particle (i.e., carbide) size can 
be varied by heat treatment. 
Six vacuum melted and rolled hypo-eutectoid steels containing 
various carbon and manganese contents have been studied to determine the 
optimum heat treatment for obtaining spheroidized carbides of various 
s i e s .  
lowed by oil quenching from 825 C and tempering at 700 C for times up to 
24 hours yields a homogeneous, spheroidized microstructure containing 
~t vas cieetemined that auateiiizing for m e  hour at ~IOOOC foi- 
0 0 
carbide particles whose diameter varies up to 10 microns. 
treatments were conducted in static vacuum and no observable decarburiza- 
tion was found. 
All heat 
S l o w  bend specimens have been machined from three of the steels and 
a number of these specimens have been heat treated as described above. 
A slow bend jig and temperature controlling device has been constructed 
and bend testing has begun on one of the steels. Once the fracture load 
and general yield load has been determined as a function of test temperature, 
specimens will be loaded to various fractions of the fracture load, un- 
loaded, sectioned, and examined by optical and replica electron microscopy. 
The number of cracked particles near the notch root will be determined, 
and the process of void coalescence during slow crack growth will be ob- 
served, as a function of nominal stress, yield strengthleve1,and carbide 
size. 
b) Tensile Micro-Strain Studies of Tic-Ni-Mo Cermets 
(with F. Darwish, Graduate Student) 
The purpose of this investigation is to determine the mechanism 
of fracture in a composite containing large volume fractions of Tic dis- 
persed in a soft matrix of Ni and Mo. Mo additions to Ni enhance the 
- 3 -  
wet t ing  of t h e  T i c  p a r t i c l e s  by the  l i q u i d  phase during f i n a l  s i n t e r i n g  
and t h i s  r e s u l t s  i n  a refinement of the  carb ide  g ra in  s i z e .  
During the l a s t  per iod the following progress has  been made. Com- 
p o s i t e  powders with d i f f e r e n t  p a r t i c l e  s ize ,  a s  descr ibed i n  PR2, have 
been prepared. Green compacts have been produced by press ing  the  com- 
posite pc?wder at a pressure  of 20,990 p.s . i .  in a dog-bone-shaped d i e .  
The compacts a r e  cu r ren t ly  being pre-s intered i n  a hydrogen atmosphere 
a t  1200'F; t he  binder  being burned o f f  by hea t ing  a t  400°F f o r  one hour. 
The high frequency induction generator  has a r r ived ,  power has  
been connected, and a new water p ipe l ine  i s  being extended t o  the  gen- 
e r a t o r .  The Mo conductor and the  hea t  sh i e lds  have been completed, A 
thermocouple, s a tu rab le  co re  reac tor  cont ro l  and a c o n t r o l l e r  c a l i b r a t e d  
f o r  t he  thermocouple w i l l  be used t o  con t ro l  t h e  f i n a l  s i n t e r i n g  tempera- 
t u re .  The pre-s in te red  compacts w i l l  be f i n a l  s i n t e r e d  a t  2500 F f o r  a 
per iod of one hour independent of t h e i r  composition. The f i n a l  s in t e red  
compacts w i l l  be ground, and tes ted  on the  micros t ra in  apparatus.  The 
dens i ty  of microcracks i n  the  Tic p a r t i c l e s  w i l l  be determined a s  func- 
t i o n  of t h e  micros t ra in  using r e p l i c a t i o n  techniques and o p t i c a l  and 
e l e c t r o n  microscopy. 
0 
11. Theore t ica l  Aspects of Fracture  i n  Composite Mater ia l s  
(with I>. Barnet t ,  Graduate Student) 
Dis loca t ion  i n t e r a c t i o n s  with e l a s t i c  inhomogeneities of var ious  
shapes a r e  cu r ren t ly  being s tudied .  The inves t iga t ion  should y i e l d  in-  
formation of p r a c t i c a l  importance i n  f i b e r  or  laminate  strengthened com- 
p o s i t e  ma te r i a l s  which conta in  l a rge  inc lus ions  ( inc lus ions  whose mean 
diameter i s  2 1000 2) and a r e  therefore  sub jec t  t o  d u c t i l e  f r a c t u r e  by 
- 4 -  
. 
, 
1 1  
By defining new variables x = ax , t = at, and by noting that from the 
- 5 -  
void formation around the inclusions and subsequent void growth and coal- 
esc enc e. 
The problem of void formation about inclusions is being approached 
in the following manner. 
to the ductile matrix containing them, plastic flow in the matrix may 
result in d i 5 1 c c a t i m  pt l s i ips  at the particle-matrix interface. The re- 
sulting stress concentration may lead to particle fracture or interface 
cracking, depending upon the relative magnitude of surface energies in- 
volved as well as upon particle shape, size, and curvature. 
Because the inclusions are quite rigid relative 
The stress concentration due to pileups at various shaped inclu- 
sions is best treated by the method of continuously distributed disloca- 
tions. Two of the problems being investigated are screw and edge dis- 
location pileups at cylindrical inclusions and at laminates or fibers. 
The screw pileup at a cylindrical inclusion generates the following 
singular integral equation for the dislocation distribution function 
f (t): 
There are N dislocations in the slip line of length L, T is the applied 
the matrix shear modulus, , % shear stress, "a" is the particle radius 
and x is the distance from the center of the particle to a point on the 
slip plane (Figure la). This equation may also be written 
2 asymmetry of the pileup and its image that f(') = t f(t), the equation 
may be recast as 
t 
The solution is 
1 Note: f(x) = 2 f 
X 
with the condition 
The last condition 
rect answer for: 
gives N as a function of a and L and yields the cor- 
(1) a d - (Chou's soh. for semi-infinite second phase) 
(2) a o (Stroh's soh. for homogeneous material) 
Knowing f(t), the stresses in the particle and at the interface 
can be calculated and this will be done shortly. 
edge pileup is considerably more complex due to additional terms involv- 
ing image dislocation multipoles, but presumably the solution for the 
screw pileup will yield insight for treating the edge problem. 
The laminate or fiber problem has been formulated and it does not 
The equation for the 
appear that the distribution function f(t) is much different from that 
of a semi-infinite second phase as treated by Chou. Chou's results have 
- 6 -  
. - .  
I -  
, ,  * 
been extended by a c t u a l l y  ca l cu la t ing  the  stresses present  i n  the  second 
phase and a t  the  i n t e r f a c e  i n  c losed form. A l l  stresses have the  form 
( in  polar  co-ordinates) 
= F. . (e) An (2L/r), where r is  the  d i s t ance  from t h e  *ij ij 
p i l eup  t i p  f o r  t h e  case  of an edge d i s l o c a t i o n  pi leup.  
Fij  
The values  of 
are shown i n  Figure lb .  
The problem of in t e r f ace  cracking i s  being t r e a t e d  by c l a s s i c a l  
e l a s t i c i t y  methods. 
tions f o r  
For the  two above cases  t h i s  means obtaining solu-  
(11 The T - Crack (2) The Ci rcu lar  Arc Crack 
Cri ter ia  f o r  void formation and crack b lunt ing  i n  composites can then be 
formulated and appl ied  t o  McClintock's e a r l i e r  work on f r a c t u r e  by void 
coalescence. 
111. The Mechanism of Fracture  i n  Composites of Mechanically Dr i l led  
Holes and a B.C.C. Metal Matrix 
( w i t h  Charles A. Rau, Jr., Graduate Student) 
Small d r i l l e d  ho le s  can markedly improve t h e  toughness of speci-  
mens containing sharp notches. It was previously reported (PR 2) t h a t  
two holes  d r i l l e d  i n  t h e  y i e l d  zones of a notched sample lowered the  
charpy impact t r a n s i t i o n  temperature of i r o n  - 2% nicke l  a l l o y  by a s  
- 7 -  
L 
I 
i 
0 much as 35 C depending on their position. During the past six months, 
we have : 
(1) Performed additional charpy impact tests and 
(a) Further definedthe important geometrical parameters 
locating the holes. 
(b) Shownthat similar increases in notch-toughness can be 
achieved by holes which are drilled only partially 
through the specimen thickness. 
Shownthat the impact energy required to produce l o w  
energy tear fracture in charpy samples of ultra-high 
strength steel (250 grade Maraging) is doubled when 
appropriately drilled holes are present. 
Performed slow-bend tests on charpy-type samples and 
(a) 
(c) 
(2) 
Shown that the load carrying capacity of notched samples 
can be increased significantly (up to 70% greater in 
Fe - 2% Nil at low temperatures where samples cleave 
prior to general yielding. 
Shown that drilled samples have a lower general yield 
load than standard notched samples and require much 
larger bend angles to produce fracture after general yield 
at a given temperature. 
(b) 
(3) Observed the redistribution of strain around a notch caused 
by drilled holes by dislocation etch-pitting of Fe-3% Si notch-bend samples 
which had been loaded to various fractions of their general yield load. 
The charpy (l), slw bend (21, and portions of the etch-pitting (3) 
Metallographic examination of the results have been reported in detail*. 
* Tetelman, A . S .  and Rau, C.A., Jr., Technical Report 81 (NASA-NSG-622) 
August, 1965 - 8 -  
I 
surface of etch-pitted specimens revealed that the holes cause a pro- 
nounced redistribution of strain around the notch (see Tech. Rept. #l). 
Subsequent etch-pitting studies have revealed the variation in the 
strain patterns from the specimen surface (plane stress) to its center 
(plane strain). Figures 2a and 2b show the effect of the holes on the 
strain distributior: =i&ay between the surface and the center of s p e c i -  
mens loaded to 50% of general yield. Similar patterns were observed in 
the specimen center demonstrating that in the critical plane strain 
regions the holes redistribute the plastic away from the notch tip. 
Thus the plastic stress concentration factor is lower than for the un- 
drilled sample at any given bend angle. Attainment of the cleavage 
stress belaw the notch tip requires much larger displacements (bend 
angles) while fracture is encouraged to initiate between the notch sides 
and each hole by the high strains present there. 
to both holes the mechanism described in Technical Report #l operates 
producing the observed improvements in notch toughness. 
Once fracture occurs 
Preliminary results have indicated that the magnitude of the im- 
provement with holes is a function of the ease of fibrous or shear fail- 
ure in the material itself. Future work will employ a series of differ- 
ent hypo-eutectoid steels to study this effect in more detail. Notched 
sheet tensile specimens of Fe-3% Si alloy will be used to (1) define 
the nature of the reduction in plastic constraint produced by the holes, 
and (2) determine the changes in optimum geometrical parameters result- 
ing from tensile loading and plane stress conditions. A determination of 
the effect of more complex arrays of holes on notch toughness in tension 
and bending shall also be made. 
- 9 -  
T 
RIGID CYUNDRICAL INCLUSION, 
INFINITE IN 2 OlRECTlON 
SOFT iy PHASE RIGID PHASE 
(infinite in 
d i rec t ion s 
b) 
Y +  
' 1  1111 1 1 1 1 
*x 
\ 
PLANE OF MAXIMUM NORMAL STRESS 
FOR L - r  
2L *r { 8 + 112 sin 28 In 7
{ e  + 112 sin 28  } In 2L 
s2 1 rxx -= - 
3r2 
fry = - 
2 2L - 8r cos 8 In 7 r'wY - 
'MAXIMUM NORMAL STRESS IN SECOND PHASE OCCURS ALONG 
A PLANE AT 8 = 66.15O 
F i g u r e  I 
F i g u r e  2.  Comparison o f  t h e  y i e l d e d  zones of  a S tandard  (a) and d r i l l e d  
(b) Fe-3% S i  specimens rnidway between t h e  specimen s u r f a c e  and 
m i d s e c t i o n  a f t e r  loading  t o  50% of  t h e i r  r e s p e c t i v e  g e n e r a l  
y i e l d  l o a d s  i n  pure  bending. M i g n i f i c a t i o n :  30X 


Mechanisms of strengthening and fracture in composite materials Third progress report, 1 Jun. - 30 Nov. 1965

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server