Mechanisms of strengthening and fracture in composite material

Tetelman, A. S.

English

NASA Technical Reports Server

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MECHANISMS OF 
STRENGTHENING AND FRACTURE 
IN COMPOSITE MATERIALS 
A. S. Tetelman 
Associate Professor 
Stanford University 
i Department of Materials Science 
! Stanford, California 
Second Progress Report 
to NASA 
December 1, 1964 - May 31, 1965 
Contract No. NSG-622 
GPO PRICE $ 
CFSTI PRICE(S) $ 
ff653 July65 
https://ntrs.nasa.gov/search.jsp?R=19650020148 2020-03-17T01:57:22+00:00Z
c c* 
f ' -  
MECHANISMS OF 
STRENGTHENING AND FRACTURE 
IN COMPOSITE MATERIALS 
Second Progress Report 
to NASA 
December 1, 1964 - May 31, 1965 
Contract NO. NSG-622 
A. S. Tetelman 
Associate Professor 
Department of Materials Science 
Stanford University 
Stanford, California 
L. 
INTRODUCTION 
This report describes research activities that were performed during 
the second phase of a program designed to investigate the mechanism of 
strengthening and fracture of composite materials. A s  mentioned in 
Progress Report #1 (subsequently referred to as PR 1) three different 
types of composites are currently being 
separately in the following paragraphs. 
I) The Mechanisms of Strengthening and - Hard Particles Dispersed 
Hodason , 
Hypo-eutectoid steels are 
investigated. These are discussed 
Fracture & Composites Containing 
- in 5 Softer Matrix 
Hypo-Eutectoid Steels (with Dare1 
Graduate Student) 
composite materials containing varying 
volume fractions of hard particles of cementite (Fe C) dispersed in a 
ferrite matrix. 
view because they are used extensively in structural application. They 
are also interesting for use as model systems in studies of composites 
because 1) the shape and distribution of the hard particles can be varied 
by heat treatment and 2) because the ferrite matrix itself can behave in 
a ductile or brittle manner, depending on test temperature. 
3 
These systems are interesting from a practical point of 
The purpose of this phase of the program was to determine the effect 
3 of Fe C morphology and dispersion on the tensile properties of these 
steels, over a range of temperature in which the ferrite matrix is both 
ductile and brittle (susceptible to cleavage fracture). Flat tensile 
specimens, 0.040" thick and .250" wide, were machined out of stock con- 
taining 1.0 per centmanganeseand 0.4  per cent carbon (5.9 + 8.6 vol 
-2 - 
per  cen t  Fe C).  Each s tee l  was given two hea t  t r ea tmen t s .  The f i r s t  
(Q. T ) cons i s t ed  of  austeni l iz ing a t  1150 C f o r  one hour ,  quenching i n t o  
o i l  and then  tempering a t  695OC f o r  24 hours .  
uniform d i s p e r s i o n  o f  sphero ida l  Fe C i n  t h e  f e r r i t e  matrix. 3 
p a r t i c l e  s i z e  was about 3-5 microns d iameter .  The second hea t  t rea tment  
(S.C) cons i s t ed  of  austeni t iz ing a t  1150 C f o r  one hour ,  s lowly cool ing  
(approximately 1 / 4 O  C/min.) through t h e  e u t e c t o i d  t ransformat ion  a t  
705OC, and hold ing  a t  695OC for  24  hours .  
i s l a n d s  (53 & 80%) o f  coa r se ,  p a r t i a l l y  degenera te  p e a r l i t e  nodules  
( a l t e r n a t e  p l a t e s  o f  f e r r i t e  & Fe C) and i s l a n d s  o f  f r e e  f e r r i t e  (47 & 3 
3 
0 
This  t rea tment  gave a 
The average 
0 
This  t rea tment  produced 
20%). 
T e n s i l e  tes ts  were c a r r i e d  o u t  between -196OC & -t85OC i n  a s p e c i a l l y  
cons t ruc ted  c r y o s t a t .  F igure  1 shows t h a t  t h e  y i e l d  stress o f  a l l  com- 
p o s i t e s  increased  wi th  decreas ing  temperature .  Furthermore,  f o r  e i t h e r  
h e a t  t r ea tmen t ,  t h e  y i e l d  s t r e s s  o f  t h e  0.6% carbon s t ee l  i s  s l i g h t l y  
h ighe r  than  t h a t  o f  t h e  0.4%C s t ee l ,  a t  any t e s t  temperature .  F i n a l l y ,  
a t  any tes t  temperature ,  t h e  (Q.T) steelsare s t r o n g e r  than  t h e  (S.C) 
s tee ls .  This  may r e s u l t  from the  f i n e r  f e r r i t e  g r a i n  s i z e  i n  t h e  Q.T 
s t ee l s ,  ca rb ide  morphology, o r  both e f f e c t s .  The p o i n t  i s  c u r r e n t l y  
under i n v e s t i g a t i o n .  
The e f f e c t  o f  tes t  temperature on t h e  d u c t i l i t y  of  t h e  two types 
o f  composite i s  shown i n  Figure 2 .  The (Q.T) s t e e l s  were d u c t i l e  a t  
-196 C y  f r a c t u r e  i n i t i a t i n g  by f ib rous  cracking.  The (S.C) steels  
showed a sha rp  d u c t i l e  t o  b r i t t l e  t r a n s i t i o n  a t  -16OOC. 
g r a i n  s i z e  and ca rb ide  morphology are  probably r e s p o n s i b l e  f o r  t hese  
0 
Again, f e r r i t e  
-3- 
differences. The ductile-brittle transition also is evident from the 
fracture stress curves shown in Figure 3. Of immediate interest is 
the fact that particle shape plays a significant role in determining 
ductility, even when the composites fracture by fibrous propagation 
only. 
greater tensile ductility than steels of the same composition containing 
plate-like particles (S.C) . 
At 85OC the (Q.T) steels containing spheroidal particles show 
During the next six months extensive metallographic investigations 
will be carried out to determine the reasons for this behavior, as well 
as additional tensile testing on manganese free steels to determine 
the effect of manganese. Microstrain studies will also be carried out 
to determine the effect of particle shape on the tensile yield stress. 
b) Tensile Micro-Strain Studies of Tic-Ni-Mo Cermets 
(- with E. Darwish, Graduate Student) 
The purpose of this investigation is to determine the mechanism 
of fracture in a composite containing large volume fractions of hard 
phase dispersed in a soft, FCC matrix. The initial investigations 
were described in PR 1 .  During the past six months the following 
progress has been made. 
The Tuckerman micro-strain gauge and collimator have been set up 
and micro-strain measurements were made on an Fe-3% Si alloy to check 
out the apparatus. Our results were found to be in excellent agreement 
with those obtained elsewhere, indicating that our rig is in good 
working order. 
Composites containing 60% by weight of the hard phase (Tic), and 
40% Ni-Mo matrix are currently being prepared from powders of Tic, 
-4 - 
Ni, and Mo2C. The powders are being milled for varying period of time 
(12 hours up to 3 days) to obtain a range of Tic par.ticle sizes, dried 
in a vacuum furnace, and stored. 
The die, in the shape of a dog-bone tensile specimen, has now arrived 
and compacts will be pressed as soon as the sintering furnace is completed. 
Pre-sintering will be carried out in a hydrogen train furnace described 
in PR 1. The induction generator is expected to arrive shortly, and as 
soon as the heat shields are completed, specimen preparation will begin. 
During the next six months the tensile specimens, containing vary- 
ing volume fractions of Tic and varying Tic particle size, will be prepared 
by sintering, ground, and tested on the microstrain apparatus. Replication 
studies will be carried out at various microstrains to determine the density 
of microcraclsin the Tic particles as a function of stress and microstrain. 
11) The Initiation of Fracture in a Composite Material Containing 5 
Pre-Induced Crack (with 2.  Barnett, Graduate Student). 
During the last six months Mr. Barnett was completing a theoretical 
problem on the effect of boundaries on crack propagation, supported by 
an ARO-D contract, and has done no additional work on this problem. It 
is expected that he shall return to this problem within one month, and 
take up the program described in PR 1. 
111) The Mechanism of Fracture & Composites of Mechanically Drilled 
Holes and g B.C.C. Matrix -- 
(with Charles A_. e, Jr., Graduate Student) 
Small, second phase particles have been shown to increase the 
resistance to cleavage crack propagation in some brittle materials. This 
project is designed to investigate whether mechanically drilled holes can 
- 5 -  
s i m i l a r l y  improve p r o p e r t i e s  i n  t h e  presence of  a notch  without  sub- 
s t a n t i a l l y  lowering r e s i s t a n c e  t o  c rack  nuc lea t ion .  
months w e  have 
During t h e  p a s t  6 
(1) def ined  t h e  s t r a i n  concent ra t ion  around a s i n g l e  h o l e  i n  0.040” 
t h i c k  s h e e t  
( 2 )  r e l a t e d  t h e  measured, nominal, f r a c t u r e  s t r a i n  t o  t h e  l o c a l  s t r a i n  
around t h e  hole  a t  f r a c t u r e  
(3) determined t h e  e f f e c t  of  ho les  on nuc lea t ion  and propagat ion of  
c leavage from a sha rp  notch. 
Above t h e  d u c t i l e - b r i t t l e  t r a n s i t i o n  temperature  (TD),  a c r i t i c a l  
f r a c t u r e  s t r a i n  ( e  ) i s  required f o r  c leavage i n i t i a t i o n .  This  s t r a i n  
w a s  measured i n  s h e e t  t e n s i l e  samples o f  Fe-3% S i  f o r  d i f f e r e n t  s tandard  
. f 
condi t ions  of  g r a i n  s i z e  and t e s t  temperature .  When s i n g l e  holes  of  
va r ious  s i z e s  a r e  introduced i n t o  t h e  s h e e t  and t h e  t e n s i l e  tes ts  a r e  
r epea ted ,  t h e  nominal f r a c t u r e  s t r a i n  i s  reduced (Figure 4 ) .  This  
i n d i c a t e s  t h a t  t h e  holes  a c t  a s  s t r a i n  concen t r a to r s ,  i . e . ,  t h e  
c r i t i c a l  f r a c t u r e  s t r a i n  e 
c rack  i s  nuc lea t ed ,  a t  a lower nominal s t r a i n  than  i f  t h e  ho le  were no t  
i s  produced l o c a l l y ,  and an uns t ab le  c leavage f 
p re sen t  e 
A s t r a i n  concen t r a t ion  f ac to r  K (%) may be def ined  as the  f r a c t u r e  
s t r a i n  wi thout  t h e  ho le  d iv ided  by t h e  nominal s t r a i n  i n  t h e  presence o f  
a hole .  
e 
A t  - 1 4 8 O C ,  and wi th  t h i s  s p e c i f i c  specimen s i z e  (w = 0 .435” ,  
t = 0 .040” ,  gauge l eng th  1 .25 “ )  an  e x p l i c i t  r e l a t i o n s h i p  was obta ined  
from t h e  d a t a  i n  F igure  4 €or  both g r a i n  s i z e s  s tud ied :  
-6- 
where % i s  t h e  ho le  diameter  i n  inches .  The measured K (I-#,) i s  a r e s u l t  
o f  two e f f e c t s .  F i r s t ,  t h e r e  is  an e l a s t i c  and p l a s t i c  s t r e s s  concent ra -  
t i o n  f a c t o r  a s soc ia t ed  wi th  the ho le  which w i l l  concen t r a t e  s t r a i n  i n  
i t s  v i c i n i t y .  
width tends  t o  concen t r a t e  s t r a i n  i n  t h e  reduced s e c t i o n  u n t i l  l o c a l  
work hardening raises t h e  appl ied  load t o  t h e  y i e l d  load o f  t he  u n d r i l l e d  
gauge length .  For holes  0.070'' and l a r g e r  i n  d iameter ,  t h e  c r i t i c a l  
e f  i s  reached l o c a l l y  before  the y i e l d  stress o f  t h e  u n d r i l l e d  
reached. Consequently,  a l l  s t r a i n  i s  r e s t r i c t e d  t o  t h e  reduced c r o s s  
sect  ion .  
G 
Second, t h e  decreased s e c t i o n  a r e a  i n  t h e  sample of  f i n i t e  
a r e a  i s  
Although d r i l l e d  holes  reduce t h e  d u c t i l i t y  o f  unnotched t e n s i l e  
specimens, they can markedly improve t h e  toughness of  specimens con- 
t a i n i n g  sha rp  notches .  This  r e s u l t s  from t h e i r  a b i l i t y  t o  concen t r a t e  
s t r a i n  and cause t h e  notch  t o  b lun t  o u t ,  by t e a r i n g ,  before  a cleavage 
f r a c t u r e  i s  i n i t i a t e d  below the notch r o o t .  
0 Figure  5 shows t h a t  a r educ t ion  o f  about  35 C i n  charpy impact 
t r a n s i t i o n  temperature  (T ) can be achieved when small  ho les  are dhilled 
below t h e  notch r o o t .  The e f f e c t  o f  varying t h e  angular  coordinant  (e) 
f o r  a cons t an t  R = 0.0448" i s  shown i n  F igures  5 and 6 .  Figure 6 shows 
t h a t  0 = 75O produces t h e  greatest lowering o f  T wi th  good improvements 
a t  a l l  angles  between 45 and 75 . A t  l a r g e r  o r  smaller angles  t h e  ho le s  
f a l l  p a r t l y  o r  t o t a l l y  o u t s i d e  t h e  y i e l d  zones o f  t h e  notch (p red ic t ed  
by t h e o r e t i c a l  s l i p  l i n e  f i e l d )  and improvement i n  toughness i s  not  
achieved.  
D 
D 
0 
The r a d i a l  coord inant  (R) was a l s o  increased  t o  R = 0.074" a t  
0 
€I = 75 and 8 = 30°, and no change from s t anda rd  u n d r i l l e d  samples i s  
-7 -  
produced. I n  a l l  c a s e s ,  it i s  necessary  t h a t  t h e  two ho le s  l i e  w i t h i n  
t h e  y i e l d  zone o f  t h e  notch and t h a t  f r a c t u r e  i n i t i a l l y  spreads  from t h e  
r o o t  t o  both ho le s .  These experiments i n d i c a t e  t h a t  c leavage i n i t i a t i o n  
i s  prevented u n t i l  lower temperatures because t h e  ho le s  i n  t h e  y i e l d  
zones o f  t h e  notch are  a b l e  t o  concen t r a t e  s t r a i n .  They produce f r a c t u r e  
t o  both  holes  be fo re  s u f f i c i e n t  s t r a i n  i s  produced f o r  c leavage i n i t i a -  
t i o n  a t  t h e  notch  t i p .  Once f r a c t u r e  has occurred t o  both ho le s  t h e  
" e f f e c t i v e  rad ius"  o f  t h e  notch i s  very  large (Figure 7) and cleavage 
cannot be i n i t i a t e d .  
Future  work w i l l  employ slow-bend t e s t s  on Fe-3% S i  and subsequent 
e t c h - p i t t i n g  s t u d i e s  t o  more s p e c i f i c a l l y  d e f i n e  t h i s  f r a c t u r e  mechanism 
by determining t h e  e f f e c t  o f  holes  on the  f r a c t u r e  load and on t h e  
p l a s t i c  s t r a i n  d i s t r i b u t i o n  a t  t h e  notch r o o t .  
I 8 0  
160 
6 120 
a 
v) 
W 
c 
v) 
0 
W 
> 
-I 100 
a 
3 
W 
2 80 
60 
40 
20 
0, 
- 1  
I I I 1 I I 
- 0 0.4 C S.C. 
I3 0 . 6 C .  S,C. 
A 0 . 4 c ,  Q,T 
0 0 . 6  C. Q.T. 
I 1 I 1 1 I 
TEMP., O C 
K) -160 -120 -80 - 40 0 40 
Figure I 
70 
TEMR , OC 
1 I I I I I I 
0 0 . 4 C  S.C. 
El 0.6C S.C. 
A 0.4C Q.T. 
0 0,6C Q.T. - - 
h 
Figure 2 
220 
2 00 
I80 
I60 
W 
3 
k 
U 
a 
2 100 
LL 
80 
60 
40 
- 80 20 - 200 - I60 - 120 - 40 0 40 80 
TEMP., C 
Figure 3 
c 
0 
N 
0 
h 0 
Figure 4 
240 
220 
200 
cn 180 e 
f 160 
w" 140 
c 
Y 
m a 
m a 
>- 100 
a 
z 8 0  
6 0  
4 0  
2 0  
2 120 
c3 
W 
W 
-"----- 
L N O  HOLES, STD. CHARPY 
X NOHOLES 
A 30" TWOHOLES 
0 
TEST TEMP. ( O C  1 
The e f f e c t  of two 0.0292" holes  d r i l l e d  a t  various angles i n  
the y i e l d  zones of a notch on the charpy impact t r a n s i t i o n  
curve of a l l o y  V- 906. 
Figure 5 
n 
0 
W 
3 
a 
o, -50 
a 
2 
-60 
W 
I- 
z 
0 
F 
a 
# -80 
5 
a a 
6 -90 
-70 
2 
U c 
I- 
> a 
CB ONE DRILL HOLE, e =oo 
- 
I 
\ I I I I 
Figure 6 
' \\\\\\' 
F i g u r e  7 


Mechanisms of strengthening and fracture in composite materials second progress report, dec. 1, 1964 - may 31, 1965

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