Efforts to characterize and differentiate between two major failure processes in graphite/epoxy composites - transverse cracking and Mode I delamination are described. Representative laminates were tested in uniaxial tension and flexure. The failure processes were monitored and identified by acoustic emission (AE). The effect of moisture on AE was also investigated. Each damage process was found to have a distinctive AE output that is significantly affected by moisture conditions. It is concluded that AE can serve as a useful tool for detecting and identifying failure modes in composite structures in laboratory and in service environments

Garg, A.

Ishaei, O.

English

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NASA Technical Memorandum 84370
|NASA-TH-Oqj70) C||A_'CTERX_&TXON OF M6J-_B09?
D_LAHXNATXUN AND _hANSV_B_ CkACKXNG £a
_A(A_AIZT_/_U_X L&_LN&7_S BY ACOUStiC
_MZS_XON (NASA) 15 p HC A02/M_ A0| Cb_ I1_ Uoc_as
G3/2_ _8039
CHARACTERIZATION OF
DELAMINATION AND
TRANSVERSE CRACKING IN "_:
GRAPHITE/EPOXY LAMINATES
BY ACOUSTIC EMISSION _
t
Amar Garg andOri Ishai i
May 1983
NASA
Nal_onalAeronauticsand
Spa(;(.Aclm_r_tstrat,on
1983019826
https://ntrs.nasa.gov/search.jsp?R=19830019826 2020-03-21T02:06:11+00:00Z
NASA TechnicalMemorandum 84370
CHARACTERIZATION OF
DELAMINATION AND
TRANSVERSE CRACKING IN
GRAPHITE/EPOXY LAMINATES
BY ACOUSTIC EMISSION
Amar Garg and Ori Ishai, Ames ResearchCenter, Moffett Field, California
CHARACTERIZATIOH OF D_._MINATIONA_DTi_N_RSH CRACEING
IN GRAPHZTE/EPOXY I,AMINAT_ BY ACOUSTIC EMISSION
Amar Gats _ and Ot£ lsha£ *_
NASA Ames Research Center, Moggett Field, Calig. 94035
ABSTRACT
A study has been made to characterize and differentiate between two
major failure processes in graphite/epoxF composites -- transverse crack.inK
and Hode I delam£nation. Representative laminates were tested in un£aT£al
tension and flexure. The failure processes were monitored and identified by
acoustic emission (AB). The effect of moisture on AE was also investigated.
Each damage process was found to have a distinctive AE output that is signifi-
cantly affected by moisture conditions. St is ccacluded that AE can serve as
a useful tool for detecting and identifying failure modes in composite struc-
tures in laboratory and in service environments.
INTRODUCTION
One of the major complications in the study of the mechanics of composite
materials is the multiphase failure behavior. Generally, four failure modes
can act s_ arately or simultaneously, namely, fiber failure, matrix failure,
fiber-matrix interracial debond£ng, and lnterply delamination. The main
problems from an analytical and experimental point of view are: (1) how to
distinguish between the different failure modes_ and (2) how to assess the
*NRC Associate on leave from the Indian Institute of Technology in
Bombay, India.
**NRC Associate on leave from Technfon, Israel Institute of Technology,
Haifa, Israel.
l
ORIGINAL Pf,GE I_
OF POOR QUALITY
1983019826-TSA04
ORIGINAL PAG£ IS
OF POOR QU&LI'i'Y
£nd£vidu81 and combined contribu_ions of the f_Llure nodes to che overall
failure process.
In most laminates, £nitia¢iou and nit/maCe failure will occur aC s load
level significantly below the fiber strength. On the other hand. matrix fail-
ure is _requently ansociaced with sheer stresses that control yisXding and
vtscoelaac£city of the ductile polyuer. In general, most failures start as
£ntraply cracking (transverse cracking) and/or lnCerply delaaination [1-8].
The main objective of this investigation i8 to develop the methodology
for on-line characterization, detection, and follow-up of transverse cracking
and delaainet£on by the acouetic-e_Lselon (AE) method. In addition, the
effect of a hyBrothermal environment on these failure modes and AE response
was also studied.
TEST PROCEDURE
Transverse Cracking
The Lest specimens were designed to have a 90 ° ply at the external
surfaces so that transverse cracks can be easily identified. Tri-directional
L iminatc 'i')0 /_45 /qN ). made of AS/3501 material was used and specimen_ were
initially tested under uniaxial tensile and flcxural Loafing. Based on these
preliminary tests, it was determined that uniaxial tensile loading led to
complex AE response because of the simultaneous cracking of all 90 ° layers.
l[_nce, £t was decided to use 4-point flexural loading, where only a single
I_lyer (at the external surface) along a relatively small gage length would be
_ubjectod to the maximmn tensile stress. In this way the cracking process
could be easily isolated and monitored by AE, at least during initial loadin S.
Specimens were vacuum dried at 700C for several weeks. Then, some were
Immersed in 70°C water for a few weeks to develop a moisture content of about
[.OZ. Tests were conducted _tt room temperature. During Ioadlng0 the AE
2
1983019826-TSA05
response was recormed usinB a 17$-kHz rason_t sensor att_lmd to the mpaci-
men. In addition, load verauo 8tratn or crosshaad diaplacauunt was recorded.
A constant displacement rate of 0.025 mm/sac wan kept durin8 the test. Zn
several cases, the load was held constant at various erases of crick&n8 to
enable duplication of the cracl_.n8 pattern using a replicating tape
technique [2].
Mode I Delaminatton
To study Mode I dela_l.naCion behavior, double-cantilever-beam (DCB)
specimens were prepared £rom specially £abricated laminates 0£ T300/934. A
mylar tape was inserted into one end at the desired interface to help initiate
delaminat&on. Follovins cure, the specimens were vacuum dried at 70°C for
several weeks. Tests were conducted on dry specimens, on specimens exposed
to 700C/95Z relat&ve huad.d&ty, and on specimens &mersed in 700C water for
varying times. All tests were performed under displacement-controlled cond&-
tlons. The crosshead speeds were in£tially 0.08 mlsec for short cracks and
increased to 0.25 _/sec for Ion K cracks (beyond 90 un). The load was applled
untll some predeter_Lned delamlnatlon growth had occurred and the specimen
was unloaded. This process was repeated. Dur£n S delamlnatlon srowth, AE
response was recorded uslns a resonant sensor attached to the specimen.
RESULTS AND DISCUSSION
Transverse Cracking
Typical AE outputs for dry and wet specimens obtained durlns continuous-
loadin S tests ire shown in Fig. 1. A 8isnificant di£ference in A£ response
between dry and wet specimens is apparent. The typical crack density observed
in the external 90 ° ply at various levels of flexural strain is shown in
Fig. 2 for wet and dry specimens. The correspondin S AE counts as a function
OF POOR QUALII_
1983019826-TSA06
ORIGINAL _C_ IS
OF,,POORQLrry
of flcxuraZ strain Is shown In Fig. 3. As can be earn, crack initiation
occurs earlier in d:y epactmens. Crack density /neraancc almost ZincarZy_Lth
strain in dry and wet spcclmenso However, at high crack densities, crackin8
appears :o slow down and approach saturazion. At the lover strain levels,
coral /_ counts also increase linearly with strain, suageetin8 a direct rela-
tionship between ,_E and transverse cracktna. Generally, lower m peai_
i (Fig. 1), lower crack density (F18. 2), and fewer AE counts (Fig. 3) are
observed in the wet specimens, for a $1vcn strain level, as compared wlth
their dry counterparts.
Using an analysis sJ_Llar to the one presented by Harris at al. [9], the
total nLmber of AE counts, N. can be related to applied strain. ¢, a8 follows:
s - k _n(¢/¢ o) (1)
where ¢0 is the threshold value of strain below which no counts arc observed
and B is a constant of proportionality. For dry and wet specimens, c o was
found to be 0.0045 end 0.0059, respectively, and B was found to be
5 × 106 counts and 2.56 x 106 counts, respectively. In Fig. 3, the number of
counts as predicted analytically by Eq. I shows good agreement with experi-
mental results, especially in the case of wet specimens.
Delamination
Typical AE response of dry and wet specimens during delamination growth
is presented in Fig. 4. As can be seen, the rms peaks from the dry specimens
were again higher than for wet specimens. Horeover, in the dry specimens the
rise of the rms signal appears to be sudden near the maximum load, whereas in
the wet specimens, the t'ms signal rises slowly as maximum load is approached.
It is interesting ro note that the number of _£ counts varies linearly with
delamlnatlon growth as seen in Fig. 5 (shown only for a few dry specimens).
1983019826-TSA07
ORIGINALPAG£ IS
OF POORQU,%lry -o--
_ The _umber of layers has no eppreclable effect on AI_ reew ,,,e. The mote,,,,
Cure was found to reduce the AS counts, as ehtwu In Fig. 6.
CONCLUSIONS
From the foreso£n $ sCud£es lC may be concluded chat:
1. AB Qharaccerie_ice are different for trauevatse crack£nS and dnlM_L-
net£on. In delaminaC£on AB response is continuous with damase 8rowch, whereas
transverse crack£ng is characterized by a discontinuous AS response.
2. Total _ counts vat7 alaoet linearly with dela_l.nation 8to_ch and
v_th the density of r.ransveree cracks at low strain levels.
3. In both delam_Lnation and transverse craci_.ng, mo£eture was found to
reduce AS response per unit of dmuase Broth (both total AE counts and raw
peaks).
4. Durin8 delam£nac£on 8rowch, the CeCal n_ber of A_ counts is
unaffected by specimen thickness.
REFERENCES
1. O.Isha._ and R. Levensood, Israel J. Tech. 8, 101 (1970).
2. A. L. H18hsmtth and K. L° Reifmueider, ASTMSTP 775, 103 (1980).
3. F.W. Crossman -d A. S. D. Wa_8, ASTH STP 775, 118 (1980).
4. T. K. O'8r:Len, J. T. Ryder, and R. Canyon, AFWAL-TR-82-6007, 79 (1982).
5. A. S. O. Wang aud F. W. Crossmau, AF_AL-TR-82-4007, 91 (1982).
__ 6. T. K. O'Brte_o NASA TH-84592 (1983).
i 7. R. Y. Ktm, A,FWAL-TR-82-4007. 218 (1982).
8. D. i. W:Llkine et al., ASTM STP 775, 168 (1980,.
9. D. O. Harris et at., ASTM STP 505, 238 (1971).
1983019826-TSA08
Fi6ure Tltla_
F_8. i. Typ£eal aco_usttc s_ssion outpu_ for dry and wet 8raphite/apoxy
specl_ns (AS/3501, 90/±45 ) loaded in flexure (_ndJce¢£on of transverse
cracking only).
F£S. 2. Transverse crack density versus flaxural stra£1_ for dry and wet
graphite/epoxy specimens (AS/3501, 90/±45 ) loaded in flexure.
Fig. 3. Total AE counts versus flex_rel strain for dry end wet sraphite/
epoxy specimens (AS/3501, 90/±45 ) loaded in flexure.
F£8. 4. Typical AE response for dry and wet specimens during dela_natton
growth (T300/934, U.D.).
Fig. 5. AE counts versus delamiuatiou growth (T3OO/934, U.D.).
F£g. 6. AE counts versus moisture Content of specimen during delamtnatlon
(T300/934, U.D.).
ORIGINAL PAGE |9
OF POOR QLJ.%L.ITY
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1983019826-TSA09
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50 100 150 200 250
TIME, se¢
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7
1983019826-TSA10
ORIGINA'.. t_
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MAX, FLEXURAL STRAIN, %
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1983019826-TSA11
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8
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9
1983019826-TSA12
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10
1983019826-TSA13
ORIGIN;,:,
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OELAMINATiON GROWTH, mm
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1983019826-TSA14
OF P_J_ _L,,_,_,_,,
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e_E_ -1_ • (0] 16 (t _ 2.06 mm)
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1000
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1983019826-TSBO1


Characterization of delamination and transverse cracking in graphite/epoxy laminates by acoustic emission

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