The objective of this investigation was to characterize the pure mode 1 delamination growth in metal to metal adhesively bonded joints under static and fatigue loading conditions, using FM 73 adhesive. Double cantilever beam specimens (DCB) were used for pure mode 1 tests. Aluminum 2024-T3 adherends were bonded with FM 73 adhesive. Delamination was introduced during fabrication by inserting a Teflon film between the two adherends. The mode 1 strain energy release rate G sub I sub c was obtained directly from static DCB tests conducted in accordance with the ASTM designation D1876-12. Constant amplitude fatigue tests on DCB specimens were conducted to determine the relationship between delamination growth rate da/dN and strain energy release rate G sub I sub c for a pure mode 1 delamination growth. It is found that the debond propagation rate in adhesive joints using FM 73 is more sensitive to errors in design load than is typical crack growth rate in metallic structures

Biswas, S. C.

Jeelani, S.

English

NASA Technical Reports Server

I r 
I ' \  
MODE I DELAMINATION GROWTH IN ADHESIVELY BONDED 
JOINTS UNDER STATIC AND FATIGUE LOADS 
by 
Subodh C. Biswas 
Shaik Jeelani 
School of Engineering and Architecture 
Tuskegee University 
Tuskegee, AL 36088 
Prepared for 
NASA LANGLEY RESEARCH CENTER 
GRANT NO. NAG-1-447 
S EM I -ANNUAL RE PORT 
(NASA-Ch-179812) HODE 1 DELAklIPATION GROWTH N87-lOUC9 
IN ADLIESZVELY BONDED J O f N l S  UKDEB STATIC A N D  
E A T I G U E  L O A D S  SeHiannual  Efport ( T u s k e g e e  
I E S t . )  17 p CSCL 20K Unclas 
63/39 44331, 
https://ntrs.nasa.gov/search.jsp?R=19870000976 2020-03-20T13:55:29+00:00Z
I 
Abstract 
The objective of this investigation was to characterize the pure mode I 
delamination growth in metal to metal adhesively bonded joints under static 
and fatigue loading conditions, using FM 73 adhesive. 
Double cantilever beam specimens (DCB) were used for pure mode I tests. 
Aluminum 2024-T3 adherend were bonded with FM 73 adhesive. Delamination was 
introduced during fabrication by inserting a Teflon film between the two 
adherends. 
The mode I strain energy release rate G was obtained directly from 
IC 
static DCB tests conducted in accordance with the ASTM designation D1876-12. 
Constant amplitude fatigue tests on DCB specimens were conducted to 
determine the relationship between delaminat'ion growth rate da/dN and strain 
energy release rate G for a pure mode I delamination growth. I '  
C 
It is found that the debond propagation rate in adhesive joints using 
FM 73 is more sensitive to errors in design load than is typical crack 
growth rate in metallic structures. 
1. Introduction 
The advantages of'adhesively bonded joints over mechanically fastened 
joints has encouraged the aerospace industry to use the former in structural 
components and systems. Such joints produce lower stress concentration than 
the mechanically fastened joints and have high strength to weight ratio. 
Even with these potential advantages and encouraging experience with 
adhesive bonding, manufacturers still hesitate to use the technology in 
primary structural components. This is partly due to lack of understanding 
of failure mechanisms and durability. Several investigators (1-6) used the 
fracture mechanics concepts of strain energy release rate to model the 
failure of adhesively bonded joints. 
GT, of an adhesive joint can be resolved into three components GI, GII, GIII 
associated with three debonding modes: I (opening), I1 (sliding), and I11 
(tearing), respectively. However, in most practical applications, only GI 
and GII, due to peel and shear stresses, respectively, exist near the debond 
front. 
with peel stresses had a significant effect on cyclic debonding. 
The total strain energy release rate, 
Everett (5) showed that the strain energy release rate associated 
The objective of this study was to characterize a pure mode I 
delamination growth in metal to metal adhesively bonded joints under static 
and fatigue loading conditions using FM 7 3  adhesive. 
cantilever beam (DCB) specimens were used to characterize debond growth 
under opening mode I loading conditions. 
adherend. 
The widely used double 
Aluminum 2024-T3 was used as 
2. Fabrication of Test Specimens 
The adherends were made from commercially available aluminum 2024-T3 
sheets. 
inch in length. 
primer supplied by American Cyanamide. 
adherends and cure of the adhesive were performed as per the manufacturer's 
recommendations ( 7 ) .  A film of teflon was introduced to cause the desired 
initial delamination in the test specimens as shown in fig. 1. 
attached at the end of the DCB specimens to make certain that there was no 
bending moment introduced in the adherend during loading. 
Each adherend measured 1/8 inch x 1 inch in cross section and 8 . 5  
The adherends were bonded with FM 73 adhesive and BR 127 
The surface preparation of the 
Hinges were 
3. Description of DCB Test 
Static tests were conducted in a displacement-controlled mode. The 
test set up is shown in fig. 2 .  
controlling the tip of the displacement. 
shown in fig.2, the side surface was coated with typewriter correction fluid 
so that visible marks could be made at the extent of delamination. Prior to 
Stable delamination growth was achieved by 
Prior to mounting the specimen as 
recording the test data, the DCB specimens we-re loaded such that the length 
of the joint holding the teflon tape opened and introduced a sharply defined 
visible delamination. The initial delamination length was recorded with the 
help of a microscope held adjacent to the specimen. 
Static tests were conducted according to the ASTM standard (8) at 
The load (P) constant slow speeds to induce slow delamination growth. 
corresponding to the applied displacement (6) was also monitored. 
increased linearly with 6 when the delamination length (a) remained 
constant. 
When the tip displacement exceeded 6cr, a delamination growth (Aa) was 
observed, accompanied by a reduction in the load from the Pcr value. 
P 
This was continued until critical values (Per, 6cr) were reached. 
The 
-2 - 
. 
applied displacement was then decreased until a zero load reading was 
observed. It may be noted that a zero load reading did not correspond to a 
zero displacement reading. The loading/unloading procedure was repeated for 
the growth of delamination. The slope of the load displacement plots 
indicated an increase in compliance with an increase in delamination size. 
The critical load and deflection values and the compliance measured 
corresponding to the delamination sizes were recorded during each static 
test. Plots of critical loads and compliance as a function of the 
delamination size were obtained and slopes of the curves were used to 
compute G the mode I critical strain energy release rate. The 
computation procedure is explained in section 4 .  
I ’  
C 
Constant amplitude fatigue tests were conducted at a frequency of w = 
10 cycles and a stress ratio of R = 0.1. The maximum cyclic load was 
selected based on the static test data. 
their side surface coated with typewriter correction fluid to aid the 
Fatigue test specimens also had 
measurements. A microscope was used to monitor the delamination growth. 
The delamination size and the corresponding cycles were recorded at selected 
intervals. Delamination growth rate da/dN and its variation with 
delamination size (a) were obtained. Using available static test data these 
results were converted to da/dN versus GI 
delamination growth rate records. 
plots which are conventional 
C 
4 .  Results and Discussion 
Static double cantilever beam (DCB) tests were conducted to compute the 
strain energy release rate (GI) for a pure mode I delamination growth. 
These displacement controlled tests produced load-displacement (P versus 6 )  
curves similar to those shown in fig. 3 .  The compliances (C = 6/P) of the 
DCB specimen were obtained from the slope of the curves in fig. 3 .  
-3- 
Figure 4 shows the variation in the compliance with the delamination 
size. Figure 5 shows the variation in the critical load with delamination 
size. 
energy release rate for the DCB specimens. 
(GI) for the pure mode I is calculated using the following equations. 
The slope of the fig. 4 (dc/da) was used to calculate the strain 
The strain energy release rate 
C 
GI 
Table 1 shows 
GI /P. 
C 
Constant 
2 
= - -  'cr dc 
2w da ' where w = 1 inch 
the experimental data including the values of dc/da, GI , and 
C 
amplitude fatigue tests were conducted on DCB specimens at a 
stress ratio of 0.1 and a frequency of 10 HZ. 
based on the static test data. 
number of cycles were monitored. 
Delamination growth rate was calculated form the slope of this curve. 
Figure 7 shows dc/da versus a data obtained from the static tests on DCB 
specimens. 
mode I were calculated for the fatigue tests for the delaminationsize 'la." 
Figure 8 shows the variation of delamination growth rate with the strain 
energy release rate for DCB specimens. 
The maximum load was selected 
The delamination size and the corresponding 
The test data are shown in fig. 6. 
Using fig. 7 dc/da and the strain energy release rate for pure 
The straight line graph in fig. 8 may be represented by the following 
equation : 
da/dN = K(GI)I1 (2) 
where n is the slope of the line. 
quite high when compared with typical values of n derived from applying 
equation 2 to fatigue crack growth in aluminum and steel alloys, where n 
The value of n is 4 . 4 .  This value is 
-4- 
ranges from 1.5 to 3 (9). Steep slopes means that small changes in applied 
load causes a large change in debond growth rate. 
propagation rate in adhesive joints is more sensitive to design loads than 
is the case in metallic structures. The value of n found in this 
investigation compares well with that found in other investigations (1-6). 
Thus the debond 
5. Conclusions 
1. The value of n in adhesive joints is higher than the typical values 
found for aluminum and steel. 
In adhesively bonded joints a small change in the design load causes a 
large change in debond growth rate. 
The values of n found in this investigation compared with the results of 
previous investigations. 
2. 
3. 
References 
1. W. S. Johnson and S. Mall, "A fracture mechanics approach for designing 
adhesively bonded joints." NASA TM 85694, September 1983. 
2. S. Mall, W. S. Johnson and R. A. Everett, "Cyclic debonding of 
adhesively bonded composites.'' NASA TM 84577, November 1982. 
3. W. S. Johnson and P. D. Mangalgiri, "Investigation of fiber bridging in 
double cantilever beam specimens." NASA TM 87716, April 1086. 
4. S. Mall and W. S. Johnson, "Characterization of mode I and mixed mode 
failure of adhesive bonds between composites adherends." 
Materials Testing and Design, ASTM STP 893, J. M. Whitney edition, ASTM, 
PA, 1986, pp. 322-334. 
Composites 
5. R. A. Everett, Jr. "The role of peel stress in cyclic debonding." NASA 
TM 84504, June 1982. 
-5- 
6. R. L. Ramkumar, "Performance of a quantitative study of instability 
related delamination growth." 
Data product handbook, American Cyanamide Company, Polymer Division, 
Wayne, NJ 05470. 
NASA TM 166046. 
7. 
8. Standard test method for "Peel resistance of adhesives." ASTM 
designation D 1876-72. 
9. D. Broek, "Elementary engineering fracture mechanics.'' 3rd edition, 
Martinus Nijhoff Publishers, Boston, 1982. 
-6 - 
P 
65 
56 
50 
46 
45 
42 
38 
37 
36 
33 
33 
32 
32 
a 
1.623 
2.185 
2.674 
2.95 
3.216 
3.55 
3.872 
4.133 
4.317 
4.574 
4.844 
5.042 
5.184 
C 
0.00134 
0.00334 
0.00875 
0.0132 
0.0156 
0.0208 
0.025 
0.0291 
0.0219 
0.0396 
0.047 
0.052 
0: 00625 
dc lda  
0.004 
0.005 
0.0065 
0.0083 
0.01 
0.011 
0.0135 
0.016 
0.018 
0.0225 
0.027 
0.041 
0.052 
-7- 
8.45 
7.84 
8.125 
8.78 
10.125 
9.702 
9.747 
10.952 
11.664 
12.25 
14.7015 
20.992 
25.6 
G I C P  
0.13 
0.14 
0.1625 
0.19 
0.225 
0.231 
0.2565 
0.296 
0.324 
0.371 
0.4455 
0.656 
0.7875 
-8- 
L 
4 
t: 
\ 
0 
Q) 
Q 
m 
c, 
v) 
W 
c, 
h 
C3 u 
n 
5 
W 
N 
L 
W > aJ 
r fu 
0 
aJ 
3 
0 
D 
7 
n 
I 
.. 
4 
, 
F ig .  2: Test Set-up 

IO 
6 
4 
2 
IO 
6 
4 
2 
I 
I 
i 
2 4 6  IO 2 4 6 IO 
DELAMINATION LENGTH (a) IN 
Fig.  4: Variation o f  Compliance w i t h  Delamination length. 
-11- 
IO 
6 
4 
L a e 
TP 
I I I I I I I I 
3 4 6  IO 2 4 6  IO 
DELAMINATION LENGTH 4 a 
Fig  5: V a r i a t i o n  o f  C r i t i c a l  l o a d  w i t h  Delaminat ion  l e n g t h .  
. 
-12- 
"0 -
X 
6 
W 
J 
0 > 
0 
LL 
0 
80 
70 
60 
50 
40 
30 
I 
P 
2 3 4 5 6 
DEBOND LENGTH, a IN INCHES 
7 
F ig .  6: V a r i a t i o n  o f  Debond l e n g t h  "a"  w i t h  Number o f  Cycles. 
-13- 
0.06 
0.05 
- 0.04 
1 
a 
da-03 
- 
t 002  
0.01 
I 2 3 
6 a in 
4 
Fig .  7: V a r i a t i o n  o f  Delamination l e n g t h  "a"  w i t h  dc /da .  
5 
- 14- 
8 '  C 
n = 4.4 
K = 0.65 X IO-* 
I 10 IO' 
Gic 
STRAIN ENERGY RELEASE RATE (LBAN) 
F ig .  8: V a r i a t i o n  of S t r a i n  energy r e l e a s e  r a t e  w i t h  
Delaminat ion growth r a t e .  
-15- 


Mode 1 delamination growth in adhesively bonded joints under static and fatigue loads

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