'HFHPEHU

1$6$73±

2EVHUYDWLRQRI,QWUDODPLQDU&UDFNLQJLQWKH

(GJH&UDFN7RUVLRQ6SHFLPHQ

0LFKDHO:&]DEDM

/DQJOH\5HVHDUFK&HQWHU+DPSWRQ9LUJLQLD

-DPHV*5DWFOLIIH

1DWLRQDO,QVWLWXWHRI$HURVSDFH+DPSWRQ9LUJLQLD

%DUU\''DYLGVRQ

6\UDFXVH8QLYHUVLW\6\UDFXVH1HZ<RUN
NASA STI Program . . . in Profile

6LQFHLWVIRXQGLQJ1$6$KDVEHHQGHGLFDWHGWRWKH

DGYDQFHPHQWRIDHURQDXWLFVDQGVSDFHVFLHQFH7KH

1$6$VFLHQWLILFDQGWHFKQLFDOLQIRUPDWLRQ67,

SURJUDPSOD\VDNH\SDUWLQKHOSLQJ1$6$PDLQWDLQ

WKLVLPSRUWDQWUROH

7KH1$6$67,SURJUDPRSHUDWHVXQGHUWKH

DXVSLFHVRIWKH$JHQF\&KLHI,QIRUPDWLRQ2IILFHU

,WFROOHFWVRUJDQL]HVSURYLGHVIRUDUFKLYLQJDQG

GLVVHPLQDWHV1$6$¶V67,7KH1$6$67,

SURJUDPSURYLGHVDFFHVVWRWKH1$6$$HURQDXWLFV

DQG6SDFH'DWDEDVHDQGLWVSXEOLFLQWHUIDFHWKH

1$6$7HFKQLFDO5HSRUW6HUYHUWKXVSURYLGLQJRQH

RIWKHODUJHVWFROOHFWLRQVRIDHURQDXWLFDODQGVSDFH

VFLHQFH67,LQWKHZRUOG5HVXOWVDUHSXEOLVKHGLQ

ERWKQRQ1$6$FKDQQHOVDQGE\1$6$LQWKH

1$6$67,5HSRUW6HULHVZKLFKLQFOXGHVWKH

IROORZLQJUHSRUWW\SHV

x 7(&+1,&$/38%/,&$7,215HSRUWVRI

FRPSOHWHGUHVHDUFKRUDPDMRUVLJQLILFDQWSKDVH

RIUHVHDUFKWKDWSUHVHQWWKHUHVXOWVRI1$6$

3URJUDPVDQGLQFOXGHH[WHQVLYHGDWDRU

WKHRUHWLFDODQDO\VLV,QFOXGHVFRPSLODWLRQVRI

VLJQLILFDQWVFLHQWLILFDQGWHFKQLFDOGDWDDQG

LQIRUPDWLRQGHHPHGWREHRIFRQWLQXLQJ

UHIHUHQFHYDOXH1$6$FRXQWHUSDUWRISHHU

UHYLHZHGIRUPDOSURIHVVLRQDOSDSHUVEXW

KDYLQJOHVVVWULQJHQWOLPLWDWLRQVRQPDQXVFULSW

OHQJWKDQGH[WHQWRIJUDSKLFSUHVHQWDWLRQV

x 7(&+1,&$/0(025$1'806FLHQWLILF

DQGWHFKQLFDOILQGLQJVWKDWDUHSUHOLPLQDU\RURI

VSHFLDOL]HGLQWHUHVWHJTXLFNUHOHDVHUHSRUWV

ZRUNLQJSDSHUVDQGELEOLRJUDSKLHVWKDWFRQWDLQ

PLQLPDODQQRWDWLRQ'RHVQRWFRQWDLQH[WHQVLYH

DQDO\VLV

x &2175$&7255(32576FLHQWLILFDQG

WHFKQLFDOILQGLQJVE\1$6$VSRQVRUHG

FRQWUDFWRUVDQGJUDQWHHV

x &21)(5(1&(38%/,&$7,21

&ROOHFWHGSDSHUVIURPVFLHQWLILFDQG

WHFKQLFDOFRQIHUHQFHVV\PSRVLDVHPLQDUV

RURWKHUPHHWLQJVVSRQVRUHGRUFR

VSRQVRUHGE\1$6$

x 63(&,$/38%/,&$7,216FLHQWLILF

WHFKQLFDORUKLVWRULFDOLQIRUPDWLRQIURP

1$6$SURJUDPVSURMHFWVDQGPLVVLRQV

RIWHQFRQFHUQHGZLWKVXEMHFWVKDYLQJ

VXEVWDQWLDOSXEOLFLQWHUHVW

x 7(&+1,&$/75$16/$7,21

(QJOLVKODQJXDJHWUDQVODWLRQVRIIRUHLJQ

VFLHQWLILFDQGWHFKQLFDOPDWHULDOSHUWLQHQWWR

1$6$¶VPLVVLRQ

6SHFLDOL]HGVHUYLFHVDOVRLQFOXGHRUJDQL]LQJ

DQGSXEOLVKLQJUHVHDUFKUHVXOWVGLVWULEXWLQJ

VSHFLDOL]HGUHVHDUFKDQQRXQFHPHQWVDQGIHHGV

SURYLGLQJLQIRUPDWLRQGHVNDQGSHUVRQDOVHDUFK

VXSSRUWDQGHQDEOLQJGDWDH[FKDQJHVHUYLFHV

)RUPRUHLQIRUPDWLRQDERXWWKH1$6$67,

SURJUDPVHHWKHIROORZLQJ

x $FFHVVWKH1$6$67,SURJUDPKRPHSDJH

DWhttp://www.sti.nasa.gov

x (PDLO\RXUTXHVWLRQWRKHOS#VWLQDVDJRY

x )D[\RXUTXHVWLRQWRWKH1$6$67,

,QIRUPDWLRQ'HVNDW

x 3KRQHWKH1$6$67,,QIRUPDWLRQ'HVNDW



x :ULWHWR

67,,QIRUPDWLRQ'HVN

1$6$&HQWHUIRU$HUR6SDFH,QIRUPDWLRQ

6WDQGDUG'ULYH

+DQRYHU0'
1DWLRQDO$HURQDXWLFVDQG

6SDFH$GPLQLVWUDWLRQ

/DQJOH\5HVHDUFK&HQWHU 

+DPSWRQ9LUJLQLD 



'HFHPEHU

1$6$73±

2EVHUYDWLRQRI,QWUDODPLQDU&UDFNLQJLQWKH

(GJH&UDFN7RUVLRQ6SHFLPHQ

0LFKDHO:&]DEDM

/DQJOH\5HVHDUFK&HQWHU+DPSWRQ9LUJLQLD

-DPHV*5DWFOLIIH

1DWLRQDO,QVWLWXWHRI$HURVSDFH+DPSWRQ9LUJLQLD

%DUU\''DYLGVRQ

6\UDFXVH8QLYHUVLW\6\UDFXVH1HZ<RUN
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not 

constitute an official endorsement, either expressed or implied, of such products or manufacturers by the 

National Aeronautics and Space Administration. 
 1

OBSERVATION OF INTRALAMINAR CRACKING IN THE EDGE CRACK TORSION 

SPECIMEN 

M.W. Czabaj1*, J.G. Ratcliffe2

, and B.D. Davidson3

1 NASA Langley Research Center, Hampton, VA 23681 

2 National Institute of Aerospace, Hampton, VA 23666 

3 Syracuse University, Syracuse NY 13244 

*Corresponding author (michael.w.czabaj@nasa.gov) 

Abstract 

The edge crack torsion (ECT) test is evaluated to determine its suitability for measuring fracture 

toughness associated with mode III delamination growth onset. A series of ECT specimens with 

preimplanted inserts with different lengths is tested and examined using nondestructive and 

destructive techniques. Ultrasonic inspection of all tested specimens reveals that delamination 

growth occurs at one interface ply beneath the intended midplane interface. Sectioning and 

optical microscopy suggest that the observed delamination growth results from coalescence of 

angled intralaminar matrix cracks that form and extend across the midplane plies. The relative 

orientation of these cracks is approximately 45° with respect to the midplane, suggesting their 

formation is caused by resolved principal tensile stresses arising due to the global mode-III shear 

loading. Examination of ECT specimens tested to loads below the level corresponding to 

delamination growth onset reveals that initiation of intralaminar cracking approximately 

coincides with the onset of nonlinearity in the specimen’s force-displacement response. The 

existence of intralaminar cracking prior to delamination growth onset and the resulting 

delamination extension at an unintended interface render the ECT test, in its current form, 

unsuitable for characterization of mode III delamination growth onset. The broader implications 

of the mechanisms observed in this study are also discussed with respect to the current 

understanding of shear-driven delamination in tape-laminate composites. 
 2

1 Introduction 

Delamination has long been recognized as a key failure mode for laminated fiber-reinforced 

polymeric (FRP) composites. In addition to growth under predominantly mode I (opening) 

loading conditions, delamination may also occur due to loading in mode II (in-plane shear), 

mode III (anti-plane shear), or some mixture of the three. Common procedures for characterizing 

delamination growth onset are generally based on the critical strain energy release rate, Gc, 

which is associated with a particular delamination mode or mode-mix. Methods for measuring 

Gc associated with mode I, mode II, and mixed mode I/II delamination at unidirectional ply 

interfaces are well established and are available as standardized testing methods [1-3]. Although 

many test methods have been proposed for measuring the value of Gc associated with mode III 

delamination (GIIIc), identifying a candidate most suitable for standardization has proven 

difficult. Although early efforts considered various split beam geometries [4-6], the majority of 

work in this area has focused on the edge crack torsion (ECT) test [7]. 

The ECT test geometry is depicted in Fig. 1. The specimen consists of a rectangular laminate 

of tape composite material that contains a preimplanted insert at its midplane, which spans the 

specimen length and creates an edge delamination. The specimen is twisted via equal-andopposite couples resulting in mode III dominated crack tip stresses acting along the delamination 

front. The two plies bounding the insert are oriented such that their fiber direction is parallel to 

the expected direction of delamination advance (90o orientation in Fig. 1) [7]. This ensures that 

the delamination growth is in the direction of the adjacent plies consistent with the mode I, mode 

II and mixed-mode I/II delamination tests [1-3]. 

Because delamination advance cannot be visually documented during an ECT test, Lee 

proposed two data reduction methods for calculating GIIIc. The first method uses a laminated 

plate theory solution that requires experimental determination of the compliance of the cracked 

regions of a test specimen. Thus, this method requires that, subsequent to fracture, test specimens 

are split into two halves about their midplane and the compliance of each half is measured by 

loading it in the ECT fixture [7,8]. The second data reduction method uses an experimental, 

multi-specimen compliance calibration procedure [7]. Here, multiple ECT specimens with 

different insert lengths, a, (as defined in Fig. 1) are tested. This yields a direct measurement of 

specimen compliance versus delamination length and avoids many of the practical difficulties 

associated with the former method. One disadvantage of this compliance calibration procedure, 

however, is that the compliance versus delamination length expression for any single specimen is 

based on test results from several different specimens. Regardless of the data reduction method 

adopted, the inability to monitor delamination propagation in an ECT specimen means that GIIIc

corresponding only to delamination growth initiation from the insert can be measured. 

A number of previous investigations into the ECT test have highlighted other issues. 

Ratcliffe [8] reported that the specimen’s force versus deflection response deviated from linearity 

prior to delamination growth onset, with growth onset typically being indicated by a sudden drop 

in force. Also, when using the compliance calibration data reduction method, measured values of 

GIIIc were found to increase with increasing insert length. This is unexpected, given that all of 

these values correspond to delamination initiation from the insert, i.e., no R-curve effects should 

be present to cause such a dependence. X-ray inspection of ECT specimens conducted during 

this study suggested that the deviation from linearity was partly due to the onset of localized 

delamination growth and splitting of the plies bounding the insert front [8]. Analyses conducted 

by Ratcliffe also suggested that delamination growth onset can be stable over certain 
 3

delamination length ranges. This implies that small amounts of delamination growth may 

actually occur prior to the sudden force drop. 

Other researchers have reported similar observations during ECT testing. Specifically, the 

two most common observations are the deviation from nonlinearity in the ECT loading response 

and dependence of GIIIc on insert length. In one such study, significant nonlinearity prior to 

macroscopic delamination advance was observed, yet post-test X-ray radiograph examination 

revealed no ply splitting [9]. Through a combined experimental and numerical analysis, this 

nonlinearity was attributed to an R-curve effect [9]. However, R-curve behavior is intrinsically 

associated with delamination advance, and thus it is unclear how this mechanism affects the 

initiation toughness in specimens with different insert lengths. Others have reported evidence of 

ply splitting [10,11], fiber bridging [7,10], and migration of the delamination to other ply 

interfaces [7,11,12]; however, these mechanisms were not directly linked to the observed 

nonlinearities in the force-displacement response. In summary, there appears to be little 

consensus regarding the source of nonlinear behavior commonly exhibited by ECT specimens. 

Collectively, the uncertainties related to data reduction methods, the nonlinearities in the forcedisplacement response prior to the onset of macroscopic delamination advance, and the potential 

presence of additional energy dissipating mechanisms may provide an explanation for the 

dependency of GIIIc on insert length [7-10, 12-14]. 

Based on the above discussion, the current study re-examines the ECT test by performing a 

set of post-test nondestructive and destructive evaluations of the ECT specimens. Specifically, 

these evaluations are used to focus on the nature and evolution of damage mechanisms that occur 

between the onset of nonlinearity in the force-displacement response and macroscopic 

delamination advance. The resulting observations are used to evaluate the suitability of the ECT 

test for measuring mode III fracture toughness associated with delamination growth onset. 

In what follows, fabrication of the ECT specimens and the test and data reduction procedures 

are described, including the procedure used to evaluate specimen behavior between the onset of 

nonlinearity and macroscopic delamination advance. Calculation of apparent GIIIc (labeled 

“apparent” GIIIc here due to the uncertainties of the ECT test) from the tests is presented and is 

followed by a description of results from ultrasonic inspections and from microscopy of 

sectioned ECT specimens. The paper is concluded by discussing the results and other possible 

implications of this study. 

2 Specimens and Fabrication 

Two 305 mm × 305 mm plates from which ECT specimens were cut were manufactured as 

shown in Fig. 2. Here, the heavy square represents the plate’s outer boundary, and the lighter, 

internal lines represent the cuts made to divide each plate into individual specimens, each of 

width b = 38.1 mm. As indicated in the figure, each plate produced three groups of six ECT 

specimens with normalized insert lengths, a/b, ranging from 0.10 to 0.35 in increments of 0.05. 

Note that after specimen machining the actual crack lengths varied slightly from the intended 

dimensions. This variation will be reflected in all subsequent plots. The range of insert lengths 

was chosen based on data from [8], where it was observed that shorter inserts encouraged more 

uniform delamination advance along the insert front over the length l shown in Fig. 1, i.e., inbetween the loading pins. In such cases, delamination onset is expected to be more evident from 

the force-displacement response. The inserts, depicted as shaded regions in Fig. 2, were 

manufactured using a 13 μm thick Teflon film. 
 4

The ECT specimens consisted of 32 plies of IM7/8552 carbon/epoxy pre-preg. During 

fabrication each plate was debulked at room temperature and subsequently cured in a hot press 

per the recommended cure cycle from the pre-preg manufacturer [15]. The average thickness of 

plates 1 and 2 was 3.85 mm and 3.87 mm, respectively. The stacking sequence of plate 1 

specimens was modified from the original design [7] to eliminate the bend-twist couplings (D16 

and D26) in the cracked and uncracked portions of the specimen. The new, 32-ply, specially 

orthotropic stacking sequence was [(90/45/(-45)2/90/(45)2/-45)s ||]s, where the “||” symbol 

designates the location of the Teflon insert, and the ply orientations are as defined in Fig. 1. The 

stacking sequence of plate 2 was identical to plate 1 with one exception. In order to lessen the 

effect of fiber nesting on crack face sliding during mode III loading, the midplane plies were 

rotated by approximately 2o in the opposite directions to each other. To balance this asymmetry, 

the outermost plies were also rotated by an equal and opposite amount. The resulting stacking 

sequence of plate 2 was [-88/(45/(-45)2/90/(45)2/-45)s/88 || -88/(45/(-45)2/90/(45)2/-45)s/88]. Note 

that in both the cracked and uncracked regions of plate 2, the bend-twist couplings D16 and D26

are zero. Due to slight asymmetry, the extension-twist couplings B16 and B26 are nonzero; 

however, these terms are quite small, and hence, their influence on the laminate's curvature is 

negligible. Correspondingly, no warpage of these plates was observed on cool-down or during 

cutting. 

3 Experimental Procedures 

3.1 ECT test procedure 

The ECT test fixture used in this study is pictured in Fig. 3. The diameter of the loading and 

supporting hemispherical tipped pins is 6.4 mm. The distance between the pins in the length and 

the width directions is l = 76.2 mm and w = 31.8 mm, respectively. The test fixture compliance 

was measured and found to be less than 5 percent of the least compliant specimen. All specimens 

were tested quasi-statically in displacement control at a loading rate of 0.25 mm/min. The 

applied force and fixture displacement were recorded throughout each test once every second. 

The various tests conducted are described in Section 3.4. 

3.2 Data reduction 

A typical force-displacement response of an ECT specimen with a/b ≅ 0.2 is illustrated in 

Fig. 4. In this figure, PNL designates the onset of nonlinearity in the force-displacement response 

determined from the procedure outlined in [8], and Pmax is the maximum force achieved, which is 

typically assumed to correspond to the onset of delamination growth. The dashed line represents 

extrapolation of the linear portion of the force-displacement curve and is intended to illustrate the 

extent of the nonlinearity. 

The reduction of data to obtain toughness was performed using the multi-specimen 

compliance calibration procedure. Here, the relationship between specimen compliance and 

crack length was assumed to take the form [7]: 

A[ ] m( ) a b

C = 1− 1 (1) 
 5

In Eqn. (1), C is specimen compliance, and A and m are parameters calculated by performing a 

linear regression analysis on the experimentally determined values of 1/C versus a/b. In this 

study, the compliances of all specimens were determined over the load range 0.44 to 1.33 kN, as 

illustrated in Fig. 4. Apparent GIIIc was calculated based on the following expression [7]: 

GIIIc = mC P( ) max

2

2lb 1− mab ( ) ⎡

⎣ ⎤⎦

(2) 

3.3 Post-test specimen inspection

Following all tests, specimens were inspected nondestructively with an ultrasonic C-scan unit 

using a 10 MHz pulse-echo transducer and 0.13 mm scan increment. A selected number of 

specimens were also sectioned and examined using an optical microscope. The specimen 

sectioning was performed using two distinct cuts. First, each specimen was cut along plane A-A 

depicted in Fig. 1. The newly exposed surface was polished in preparation for microscopy. Next, 

one of the two resulting specimen halves was cut lengthwise along plane B-B. The location of 

the plane B-B cut was slightly behind the insert front on the insert side of the specimen. After 

removal of the region that contained the insert, the newly created surface was ground and 

polished until the resin rich region that forms ahead of the insert front was fully exposed. As will 

be shown subsequently, this apparently new approach to assessing near-tip damage in ECT test 

specimens yields new and significant insights into the fracture process. 

3.4 Test matrix 

As shown in Fig. 2, specimens from each plate were divided into three groups of six 

specimens, where each group contained specimens with normalized insert lengths, a/b, from 0.10 

to 0.35. Table 1 summarizes the entire test matrix. 

The first two groups of specimens from each plate were used for “apparent GIIIc testing.” 

That is, these specimens were tested until there was a sudden drop in the applied force, similar to 

the results shown in Fig. 4, where macroscopic delamination growth was assumed to have 

occurred. The data from these specimens were used to compute the apparent GIIIc following the 

procedure described in Sec. 3.2. These specimens were subsequently ultrasonically inspected and 

sectioned as described in Sec. 3.3. 

The remaining group of specimens from each plate was used for “damage progression 

testing.” The purpose of these tests was to investigate the initiation and development of any 

damage that occurred between the point at which a specimen's response deviated from linearity 

(PNL) and the anticipated onset of delamination growth (Pmax). To this end, two specimens from 

plate 1 were loaded until the specimens’ response deviated from linearity (PNL), and the 

remaining four specimens were loaded to 95% of Pmax, where Pmax was the average result from 

the group 1 and 2 specimens from plate 1 at the appropriate insert length. This approach was 

chosen in order to compare cross-sectional views at the two different force levels. 

For plate 2, one specimen (a/b = 0.185) was tested six times. In the first five loading 

increments, the specimen was tested to the following force levels: PNL, 82% Pmax, 87% Pmax, 91% 

Pmax, and 95% Pmax. During the sixth and final force increment, the specimen was tested until 

macroscopic delamination growth was assumed to have occurred (as indicated by the sudden 

force drop). At each force level, the specimen was unloaded, removed from the test fixture, and 
 6

ultrasonically inspected as described in Sec. 3.2. The remaining 5 specimens from the third 

group of plate 2 were tested to 95% of Pmax, where Pmax is as defined above but using plate 2 

specimens. Following the completion of these tests, all specimens were ultrasonically inspected, 

sectioned, and then optically inspected using the methods detailed in the section 3.3. 

4 Results and discussion 

4.1 Specimen response 

The force-displacement data of specimens from group 1 of plates 1 and 2 are presented in 

Figures 5a and 5b, respectively. In each case, all specimens exhibited nonlinearity prior to a 

sudden force drop (corresponding to the assumed onset of delamination growth). For specimens 

with longer insert lengths, the extent of nonlinearity prior to the force drop was more pronounced 

and the force drop was typically more gradual, especially in the case of the plate 2 specimens. 

The nonlinear response of ECT specimens is consistent with previous observations 

[7,9,10,14,16]. 

4.2 Apparent GIIIc testing and observed delamination growth 

Data from the group 1 and 2 specimens from each plate were used to compute apparent GIIIc

following the procedure described in Sec. 3.2. A summary of all apparent GIIIc values versus 

normalized crack length, a/b, is presented in Fig. 6. For comparison, data from the previous 

study using the same material [8] are also included. The data from these two studies are 

consistent, indicating that, over the range of a/b considered in this study, the apparent value of 

GIIIc is relatively constant in comparison to data obtained over a larger range of a/b [8,10,14]. 

The only exception is specimens from plate 1, group 2, where apparent GIIIc was the lowest for 

the intermediate insert lengths. Interestingly, if one was to restrict measurement of apparent GIIIc

to essentially the same range used in this study, (i.e., a/b = 0.1 to 0.4) then the data presented in 

[8,10,14] also appears somewhat independent of the insert length. However, while it is possible

that the apparent toughness obtained from this shorter range of insert lengths could be a material 

property, this is not at all clear. It is equally possible that whatever mechanism is responsible for 

the geometry-dependence of the apparent toughness at longer insert lengths simply occurs to the 

same degree in all specimens over the range a/b = 0.1 to 0.4. A finding of this type was made in 

a parallel study using mode III split beam-type tests [17]. Thus, the question remains as to why 

apparent GIIIc tends to increase with values of a/b beyond 0.4. The answer to this question should 

provide insight as to whether the average value of apparent GIIIc obtained from specimens within 

the current range of a/b represents a true material property. 

To further explore the above observations, the ECT specimens were examined by performing 

nondestructive and destructive evaluations. Specimens used to measure apparent GIIIc (see Table 

1) were C-scanned using the procedures described in Sec. 3.3. Figure 7 depicts an example of 

ultrasonic time-of-flight images for specimens with the shortest, intermediate, and longest insert 

lengths (plate 1, group 1, a/b = 0.118, 0.268, 0.358). In Fig. 7, a darker shade of gray 

corresponds to a through-thickness location of the detected feature that is closer to the back 

surface. The light gray region on the bottom of each image corresponds to the location of the 

insert, and the dark gray region at the top corresponds to the signal reflected from the back 

surface. Examination of the time-of-flight data from the specimens in Fig. 7 shows that 

macroscopic delamination advance did not occur at the midplane as intended. Rather, for all 

specimens, delamination advance occurred one ply interface below the midplane at the 90/45 
 7

interface. For a few specimens (e.g., specimens with a/b = 0.268 or 0.358 in Fig. 7), some 

delamination advance was observed at the midplane; however, the extent of this advance was 

small relative to the extension along the 90/45 interface. A few specimens exhibited limited 

advance at the interface above the midplane at the 45/90 interface. Overall, the delamination 

advance observed in this study is different from what has typically been assumed to occur in 

ECT tests. 

To both corroborate and further understand the nature of delamination growth observed in 

the C-scan images, specimens were sectioned and examined using an optical microscope. Fig. 8 

shows an image of a sectioned specimen with a/b = 0.358 (corresponding to the bottom 

specimen in Fig. 7) taken at the intersection of planes A-A and B-B defined in Fig. 1. The 

various gray bands correspond to individual plies as noted on the left side of the figure. As 

evidenced by this image and being consistent with the C-scan data in Fig. 7, macroscopic 

delamination advance took place at the 90/45 interface, with only very limited advance occurring 

at the midplane. In addition to delamination, a large number of angled intralaminar cracks were 

visible through the midplane 90° plies and terminating at the adjacent 45° plies. 

The observed intralaminar cracks provide a potential explanation for the presence of vertical 

lines (aligned with the 90° direction) inside the delaminated regions seen in C-scan data in Fig. 

7. Interestingly, without performing destructive inspection, the presence of these vertical lines 

gives the C-scan data an appearance of delamination growth occurring at the midplane, 90/90 

interface. Overall, the above observations were consistent between specimens from plate 1 

(90/90 interface) and plate 2 (+88/-88 interface). Thus, the attempt to reduce fiber nesting by 

slightly misaligning the midplane plies (see Sec. 2) had little or no effect on the observed 

damage state. 

4.3 Damage progression testing 

The results in the preceding section illustrate that the current ECT specimens exhibit 

extensive intralaminar matrix cracking in the midplane plies and macroscopic delamination at an 

unintended ply interface (90/45) away from the midplane. Given these observations, the 

remainder of this study was conducted to address the following two questions: 

1) Does matrix cracking initiate prior to or during macroscopic delamination advance? 

2) What mechanisms cause (or contribute to) the observed intralaminar matrix cracking? 

To answer question 1, one specimen (a/b = 0.185) from the third group of plate 2 was 

incrementally tested to different force levels between PNL and macroscopic delamination advance 

and ultrasonically inspected after each loading cycle (see description in Section 3.4). The 

resulting C-scan images are presented in Fig. 9. At PNL no damage was observed, and the C-scan 

image was essentially identical to the image obtained prior to testing. As the force level was 

increased beyond PNL to 82%, 87%, and 91% of Pmax (Figs. 9b-d, respectively), changes were 

detected in the reflected ultrasonic signal in the region just ahead of the Teflon insert. This 

appears as the thin black band at the delamination front in Figs 9b-d, within which no reflected 

signal was obtained. Subsequently, it will be shown that this ultrasonic attenuation was caused 

by the presence of intralaminar matrix cracks bisecting the midplane plies. 

At 95% Pmax (Fig. 9e) the C-scan image indicates that macroscopic delamination advance in 

this specimen is at the 90/45 and 90/90 interfaces. Fig. 9f is a post-test scan, i.e., after the final 

increment of loading and the associated sudden force drop. Here, the delamination has extended 
 8

primarily at the 90/45 interface, with a limited amount of delamination extension observable at 

the 90/90 interface. Sectioning and optical microscopy of this specimen along planes A-A and BB further confirmed that the damage was essentially identical to the post-test damage that was 

observed in specimens used to measure apparent GIIIc (Fig. 8). 

To answer question 2, several remaining specimens (see Table 1) were tested either to PNL or 

95% Pmax, ultrasonically inspected, and then sectioned for optical microscopy. Typical results 

obtained from this evaluation are presented in Fig. 10. The left column in Fig. 10 shows C-scan 

images obtained after testing and prior to sectioning, while the right column depicts a 

representative micrograph taken along plane B-B from each respective specimen. The 

micrographs in Fig. 10 are termed “representative” as the damage seen in each image was 

repeated with relatively constant frequency across the length of each specimen. In each image on 

the right, the gray band in the middle corresponds to the resin rich region that typically forms 

ahead of the insert during fabrication. Optical inspection along plane B-B of the specimens tested 

to PNL (Fig. 10a) revealed a large number of angled microcracks in the resin rich region. In the 

specimen tested to 95% Pmax (Fig. 10b), the total number of microcracks across the length l (the 

frequency of the resin microcracks) increased. In addition, a large number of the microcracks 

appeared to have extended intralaminarly through the two midplane 90° plies, terminating at the 

surrounding 45° plies. Generally, the microcracks in the resin rich region and the intralaminar 

cracks through the midplane plies were oriented at approximately 45° to the midplane. Any 

significant deviations from this orientation appeared to have been caused by local variations in 

microstructure, such as voids, resin rich regions or nested fibers. 

As may be observed in Fig. 10b C-scan image, in some cases a small amount of midplane 

delamination growth occurred as the angled intralaminar cracks in the resin rich region extended 

into intralaminar cracks through the 90° plies. More significant, however, is that upon reaching 

the 45° plies, some of these intralaminar cracks turned and extended interlaminarly as 

delaminations along the 90/45 interface. Fig. 10c shows that advance and interconnection of 

these delaminations along the 90/45 interface likely resulted in the primary mode of macroscopic 

advance and confirms that this is the event corresponding to the observed force drop at Pmax. 

To gain further insight into the processes discussed above, it is useful to consider results in 

the literature for cracked geologic materials, homogeneous materials, and for delaminated 

unidirectional composite laminates subjected to anti-plane shear loadings. In the first two cases, 

it has been well-established that crack advance does not occur as an essentially planar advance of 

the original crack front, but rather as a series (or echelon) of cracks that initiate and grow at a 45° 

angle to the original crack plane, where the angle of inclination is oriented perpendicularly to the 

direction of maximum tensile stress (e.g., [18-23]). This discontinuity in crack surface evolution 

is often described as a combination of crack front segmentation and rotation [24-26]. Consistent 

with all of the experimental results in these references, a recent model [26] suggests that there is 

a linear instability that will cause an initially planar mode III crack in a homogenous material to 

always advance in this manner. Similar results were postulated to occur in unidirectional 

laminated composites when there was a correspondence between the fiber direction and the 

intended direction of mode III advance [27]. This was recently found to be the case in 

companion and collaborative works [17,28]. Here, it is shown that the evolution of damage, 

beginning with 45° microcrack initiation in the resin rich region and progressing to 45° 

intralaminar crack growth through the midplane plies prior to any macroscopic delamination 

advance, occurs similarly to the ECT specimens studied herein when unidirectional coupons are 
 9

tested in anti-plane shear. Thus, the current findings and the results of [17,28] for unidirectional 

composites are all locally consistent with those for homogeneous and geologic materials. 

4.4 Fracture mechanisms in the ECT specimens and assessment of the test method 

The C-scan and fractographic data presented in the previous two sections suggest that the 

overall sequence of events that occur on plane B-B at the end of the Teflon insert in ECT 

specimens progresses in three general stages. Figure 11 illustrates the overall process. In Stage 1,

microcracking initiates in the resin rich region ahead of the insert. In the current study, these 

microcracks were present when the specimen’s response deviated from linearity (i.e., at P ≅ PNL). 

However, it is likely that microcracking must be present in a critical amount in order to manifest 

itself in the specimen’s force versus deflection response. Thus, although not confirmed in the 

current investigation, the exact instance of microcrack initiation likely occurs prior to PNL. As 

previously described, these microcracks are oriented at approximately 45° to the specimen’s 

midplane and are orthogonal to the direction of the of the maximum tensile stress associated with 

the KIII field, where KIII is the mode III stress intensity factor at the insert. This stress 

decomposition is shown in Fig. 12. During Stage 2, frequency of microcracking increases, and 

some microcracks extend through the midplane plies and into the adjacent interface, as the force 

is increased from PNL to a value approaching Pmax. This process proceeds analogously to that 

originally described by [20] for homogeneous materials. Initial microcracks occur due to 

irregularities along the insert front. This relieves the stress locally but increases the likelihood of 

another crack developing in the vicinity. The overall spacing of the cracks should become 

essentially uniform and can be approximated using a shear lag analysis, similar to that which has 

been considered for microcracking associated with mode II loading [29]. Because the two 

midplane plies have their fiber orientations aligned with the direction of macroscopic advance, 

microcrack initiation and extension during this stage is essentially unconstrained by the fibers 

(see Fig. 12). Stage 2 concludes when the microcracks reach the more constraining fiber 

orientation associated with the 45° plies and interlaminar delamination initiates at the 90/45 

interface, as shown in Fig. 10b. Stage 3 then commences as the force approaches Pmax. Here, 

delaminations at the 90/45 interface and, to a lesser extent at the 90/90 interface, link together. 

Macroscopic advance, as shown in Figs. 9e and 10c, occurs when the force reaches Pmax. The 

final outcome is a fully three-dimensional, relatively rough surface with a “factory roof” [25] 

profile that the geologic fracture literature refers to as echelon cracking [18,21,23] and the 

literature on fracture of homogeneous materials refers to as being comprised of “lances” [19,30], 

“river lines” [24] and/or “facets” [26]. 

In view of the above discussion, the presence of extensive intralaminar cracking in the 

midplane plies prior to onset of delamination clearly invalidates the assumptions associated with 

computation of GIIIc using Eq. 2. As a consequence, the data presented in Fig. 6 does not

correspond to the true measure of mode III initiation fracture toughness, but instead represents a 

“structural” response of the ECT specimen to the applied mode III loading. Given that all ECT 

tests reported in the literature use a similar stacking sequence (i.e., having 90° plies bounding the 

insert), it is possible that the commonly observed dependence of apparent GIIIc on edge 

delamination length is, in fact, due to the occurrence of mechanisms similar to those observed 

herein. Further, as the microcracks initiate prior to macroscopic delamination advance, it follows 

that the relatively constant value of apparent GIIIc observed earlier for the range a/b = 0.1 to 0.4 

cannot be considered a material property. Thus, it is concluded from these findings that the ECT 
 10 

test with its current stacking sequence is unsuitable for measuring mode III fracture toughness in 

laminated FRP composites. 

4.5 Broader implications for shear-driven delamination in tape laminates 

In many respects, the fracture mechanisms observed in ECT specimens are similar to those 

observed in unidirectional mode II fracture toughness test specimens. In mode II, 45° 

microcracks also initiate ahead of the insert, within the resin rich region at the specimen’s 

midplane. These microcracks arise due to the resolved principal tensile stresses that are 

associated with the applied in-plane shear loads, similar to microcrack development in the ECT 

specimens. However, in mode II, the microcracks are inclined in the direction of intended 

macroscopic advance, and their through-thickness growth (i.e., into the bounding plies) is 

constrained by the fibers of the bounding plies. Thus, it is energetically more favorable for these 

microcracks to remain within the resin rich region at the midplane; with increasing load, these 

microcracks link and coalesce and create what is macroscopically observed to be mode II 

delamination [29]. Thus, strictly speaking, “mode II delamination” in laminated FRP composites 

is a series of mode I events, but the fact that the fibers constrain mode I intralaminar crack 

advance means that a useful engineering quantity is obtained from these tests [31]. In the case of 

the ECT specimens however, the plies bounding the insert are oriented such that the intended 

delamination growth direction would follow the direction of the fibers of these plies. This is 

intended to make the test consistent with the unidirectional tape specimens used for measuring 

mode I, mode II and mixed-mode I/II fracture toughness. However, by applying such a ply 

orientation to the ECT specimen, the microcracks resulting from the resultant tensile stresses 

(from the mode III shear loading) are not contained by the bounding plies. Thus, rather than 

coalescing into a macroscopic delamination, as is the case in mode II loading, the microcracks 

simply grow transversely through these bounding midplane plies. This intralaminar crack growth 

continues until a ply is reached that is able to terminate (constrain) this process. In the ECT 

specimens this happens at the 90/45 ply interfaces above and below the midplane plies. 

The preceding discussion raises the question of whether it is possible that a different choice 

of stacking sequence for the ECT specimen could be used such that the plies bounding the 

midplane would constrain the microcracks to the midplane interlaminar region. Such an 

approach could potentially produce a value of GIIIc that is independent of delamination length for 

some range of bounding ply angles. That is, this would provide a mode III test that would 

produce a useful engineering quantity, similar to the approach that has been adopted in mode II. 

At present, the determination of appropriate bounding ply angles would need to be established 

experimentally. In addition, this may allow for a correspondence to be made between mode II 

and mode III toughness tests: specifically, to determine whether there is a certain range of 

bounding ply angles, measured with respect to the direction of delamination advance, that will 

constrain delamination growth to occur along the intended interface in both modes II and III. 

While this would surely be an important development in an overall engineering approach to 

predict delamination growth, note that it also brings to light many of the deficiencies in our 

understanding of the true details of delamination advance in both modes II and III, and highlights 

the need for further study in order to develop a mechanistically accurate approach to both 

delamination growth characterization and its prediction. 
 11 

5 Concluding Remarks 

Edge crack torsion specimens made with preimplanted inserts between two slightly different 

ply interfaces (+88/-88 and 90/90 ply interfaces) were manufactured from IM7/8552 prepreg 

tape. Specimens of each layup contained normalized insert lengths ranging from 0.10 to 0.35. A 

generally accepted procedure for measuring GIIIc using these specimens was followed, and both 

specimen types produced essentially identical results. Apparent GIIIc was found to be largely 

independent of the insert length over the range considered, and measured values were 

comparable with previously reported values for the same material system. However, ultrasonic 

and fractographic inspections revealed that delamination advance in all specimens 

unintentionally took place primarily at the 90/45 ply interface, one ply below the midplane. It 

was found that a series of microcracks developed prior to macroscopic delamination advance 

within the resin rich region ahead of the insert. These microcracks were observed to develop into 

intralaminar cracks that bisected the two plies bounding the midplane. Their growth was arrested 

at the adjacent 90/45 interface. A portion of these cracks then changed direction and continued 

advancing as interlaminar delaminations. Linkage of these delaminations resulted in the observed 

macroscopic delamination growth. 

It is clear from these results that delamination advance in ECT specimens occurs quite 

differently than that which is assumed in any of the commonly employed methods of data 

reduction. Thus, the apparent value of mode III toughness that is extracted from the ECT test 

does not reflect the mechanics or physics of the processes that occur, and there is no reason to 

expect that such results will be valid. This explains the commonly observed dependence of 

measured GIIIc on insert length. Moreover, the above findings confirm that ECT specimens in 

their current form, with the plies that bound the insert oriented at or near to 90°, are unsuitable 

for measuring GIIIc associated with mode III delamination growth onset. To address this 

deficiency, it has been suggested that alternative ECT stacking sequences may be possible that 

constrain the interlaminar microcracks at the midplane from growing intralaminarly. Tests of 

these specimens may provide a mode III toughness that is independent of insert length, and 

which could therefore be interpreted in a manner that is analogous to what is currently done for 

mode II. More broadly, this approach may be extended to an assessment of whether there is a 

certain range of bounding ply angles, measured with respect to the direction of delamination 

advance, that will constrain delamination growth to occur along the intended interface in both 

modes II and III. This would provide a consistent approach for characterizing both modes of 

shear-driven growth. 

6 References

1. American Society for Testing and Materials. Standard Test Method for Mode I Interlaminar 

Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites, ASTM 

Standard D5528-01; 2008. 

2. Davidson, BD. Determination of the mode II interlaminar toughness of unidirectional fiber 

reinforced polymer matrix composites using the end-notch flexure (ENF) test, ASTM Work 

Item WK-37680; 2012. 

3. American Society for Testing and Materials. Standard Test Method for Mixed Mode 

I-Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer 

Matrix Composites, ASTM Standard D6671-06; 2006. 
 12 

4. Donaldson SL. Mode III interlaminar fracture characterization of composite materials. 

Compos Sci Technol 1988; 32(3):225–49. 

5. Robinson P, Song DQ. A new mode III delamination test for composites. Adv Compos Lett 

1992; 1:160-164. 

6. Sharif F, Kortschot MT, Martin RH. Mode III delamination using a split cantilever beam. In: 

Martin RH, editor. Composite materials: fatigue and fracture - Fifth Volume, ASTM STP 

1230. Philadelphia: American Society for Testing and Materials; 1995, p. 85-99. 

7. Lee SM. An edge crack torsion method for mode III delamination fracture testing. J Compos 

Technol Res 1993; 15(3):193-201. 

8. Ratcliffe JG. Characterization of the edge crack torsion (ECT) test for mode III fracture 

toughness measurement of laminated composites. NASA-TM-2004-213269; 2004. 

9. de Morais AB, Pereira AB, de Moura MFSF, Magalhães AG. Mode III interlaminar fracture 

of carbon/epoxy laminates using the edge crack torsion (ECT) test. Compos Sci Technol 

2009; 69(5):670-676. 

10. Li X, Carlsson LA, Davies P. Influence of fiber volume fraction on mode III interlaminar 

fracture toughness of glass/epoxy composites. Sci Technol 2004; 65(2):295-300. 

11. Greenhalgh, ES, Rogers, C and Robinson, P. Fractographic Observations on Delamination 

Growth and the Subsequent Migration Through the Laminate. Compos Sci Technol 2009; 

69(14):2345-2351. 

12. de Morais AB, Pereira AB, de Moura AFSF. Mode III interlaminar fracture of carbon/epoxy 

laminates using the Six-Point Edge Crack Torsion (6ECT). Compos Part A 2011; 

42(11):1793-1799. 

13. Pennas D, Cantwell WJ, Compston P. The influence of strain rate on the mode III 

interlaminar fracture of composite materials. J Compos Mater 2007; 41(21):2595–614. 

14. Browning G, Carlsson LA, Ratcliffe JG. Modification of the edge crack torsion specimen for 

mode III delamination testing. Part II – experimental study. J Compos Mater 2011; 

45(25):2633-2640. 

15. Hexcel Corporation. HexPly® 8552 Product Data Sheet - http://hexcel.com; 2007. 

16. Marat-Mendes R, de Freitas M. Characterization of the edge crack torsion (ECT) test for the 

measurement of the mode III interlaminar fracture toughness. Eng Fract Mech 2009; 

76(18):2799–2809. 

17. Johnston AL, Davidson BD, Simon KK. Assessment of split-beam-type tests for mode III 

delamination toughness determination. Int J Fract 2013, DOI: 10.1007/s10704-013-9897-1. 

18. Roering C. The geometrical significance of natural en-echelon crack-arrays. Tectonophysics 

1968; 5(2):107-123. 

19. Knauss WG. An observation of crack propagation in anti-plane shear. Int J Fract Mech 1970; 

6(2):183-187. 

20. Palaniswamy K, Knauss WG. On the problem of crack extension in brittle solids under 

general loading. Mech Today 1978; 4(3):87-148. 
 13 

21. Pollard DD, Segall P, Delaney PT. Formation and interpretation of dilatant echelon cracks. 

Geol Soc Am Bull 1982; 93(12):1291-1303. 

22. Li S, He T, Teng C, Zheng X, Viktor K. Theoretical and experimental study of 3-D initial 

fracture and its significance to faulting. Earthq Sci 2011; 24(3):283-298. 

23. Goldstein RV, Osipenko NM. Structures of fracture near a longitudinal shear main crack. 

Proceedings of the 19th European conference on fracture, Kazan, Russia; Aug 26-31, 2012. 

24. Hull D. Tilting cracks: the evolution of fracture surface topology in brittle solids. Int J Fract 

1993; 62(2):119-138. 

25. Lin B, Mear ME, Ravi-Chandar K. Criterion for initiation of cracks under mixed-mode I + III 

loading. Int J Fract 2010; 165(2):175-188. 

26. Leblond J-B, Karma A, Lazarus V. Theoretical analysis of crack front instability in mode I + 

III. J Mech Phys Solids 2011; 59(9):1872-1887. 

27. Trakas K, Kortschot MT. The relationship between critical strain energy release rate and 

fracture mode in multidirectional carbon-fiber/epoxy laminates. In: Armanios EA, editor. 

Composite materials: fatigue and fracture- Sixth Volume, ASTM STP 1285. Philadelphia: 

American Society for Testing and Materials; 1997, p. 283-304. 

28. Johnston AL, Davidson BD, Intrinsic coupling of near-tip matrix crack formation to mode III 

delamination advance. Int J Solids and Structures 2013. In Review. 

29. Lee SM. Mode II delamination failure mechanisms of polymer matrix composites. J Mater 

Sci 1997; 32(5):1287-1295. 

30. Sommer E. Formation of fracture 'lances' in glass. Eng Fract Mech 1969; 1(3):539-46. 

31. O’Brien TK. Composite Interlaminar Shear Fracture Toughness, GIIc: Shear Measurement of 

Sheer Myth? Composite Materials: Fatigue and Fracture, Seventh Volume, ASTM STP 1330, 

R.B. Bucinell, Ed., American Society for Testing and Materials; 1998, p. 3-18. 
 14 

Table 1. Summary of the test matrix. 

Group 1 Group 2 Group 3 

Plate 1 GIIIc tests GIIIc tests - 2 specimens tested to PNL

- 4 specimens tested to P = 95% Pmax

Plate 2 GIIIc tests GIIIc tests - 1 specimen tested 6 times between PNL and Pmax

- 5 specimens tested to P = 95% Pmax

Figure 1. The ECT test geometry. Planes A-A and B-B define cuts made for optical 

examinations. 

Preimplanted

Teflon insert

0°

90°

Preimplanted insert 

crack length, a

45°

b L

w l

L = 88.9 

l = 76.2

b = 38.1 

w = 31.8

(dimensions in mm)

P/2

P/2

P/2

P/2
 15 

Figure 2. Typical ECT specimen plate layout. The shaded regions correspond to the 

preimplanted Teflon inserts at the midplane. 

Figure 3. ECT test setup (note: alignment blocks are removed prior to testing). 

304.8

304.8

0°

90°

36.3

119.4

203.2

+45°

(all dimensions in mm, not to scale)

a/b = 0.35

19.8

19.8

19.8

b = 38.1

L = 88.9

a/b = 0.10

a/b = 0.30

a/b = 0.15

a/b = 0.25

a/b = 0.20

a/b = 0.00

group 1 group 2 group 3

Alignment blocks 

20 kN load-cell 

ECT specimen 
 16 

Figure 4. Typical force-displacement response of an ECT specimen. 

Figure 5. Force-displacement data from (a) plate 1 and (b) plate 2 specimens. 

0.0

0.5

1.0

1.5

2.0

2.5

0.0 1.0 2.0 3.0

Force (

kN)

Displacement (mm)

Compliance calibration

range: 0.44 -1.33 kN

PNL = 1.68 kN

Pmax = 2.08 kN

0.0

0.5

1.0

1.5

2.0

2.5

0.0 1.0 2.0 3.0

Force (

kN)

Displacement (mm)

Plate 1

a/b = 0.36

a/b = 0.39

a/b = 0.27

a/b = 0.20

a/b = 0.18

a/b = 0.12

0.0

0.5

1.0

1.5

2.0

2.5

0.0 1.0 2.0 3.0

Force (

kN)

Displacement (mm)

Plate 2

a/b = 0.29

a/b = 0.39

a/b = 0.27

a/b = 0.19

a/b = 0.18

a/b = 0.11

Plate layup:

[(90/45/(-45)2/90/(45)2/-45)s ||]s

Plate layup:

[-88/(45/(-45)2/90/(45)2/-45)s/88|| 

-88/(45/(-45)2/90/(45)2/-45)s/88]

(a) (b)
 17 

Figure 6. Apparent GIIIc versus normalized crack length. 

Figure 7. Typical C-scans of specimens used to compute apparent GIIIc (plate 1, group 1).

0.0

0.5

1.0

1.5

2.0

0.1 0.3 0.5 0.7

Apparent 

GIIIc (kJ/m2)

a/b

plate 1, group 1

plate 1, group 2

plate 2, group 1

plate 2, group 2

Ref. [8]

a/b = 0.118 

a/b = 0.268 

a/b = 0.358 

A-A 

B-B 

Teflon insert 

10 mm 

90/45 delamination 
 18 

Figure 8. Optical image of specimens used to compute apparent GIIIc at the intersection of planes 

A-A and B-B (plate 1, group 1, a/b = 0.358 ). 

Figure 9. C-scans from progressive loading of specimen with a/b=0.185 (plate 2, group 3).

 90 

preimplanted Teflon insert front 

delamination 90 

 45 

 45 

intralaminar cracks 

midplane midplane 

P = 95% Pmax (2.11 kN) after load drop

P = 91% Pmax P = 87% Pmax (1.93 kN) (2.03 kN )

P = 82% Pmax P (1.83 kN) NL = 78% Pmax (1.74 kN)

(a) (b)

(c) (d)

(e) (f)

90/90 delamination

90/45 delamination

attenuated signal

attenuated signal
 19 

Figure 10. C-scan and micrograph (plane B-B) of specimen loaded to (a) PNL, (b) 95% Pmax, and 

(c) delamination growth (a/b = 0.10).

50 m 

200 m 

PNL (plate 1) 

95% Pmax 

(plate 2) 

after load drop 

(plate 1) 

midplane 

midplane 

midplane 

10 m 

(a)

(b)

(c)

intralaminar crack

delamination

intralaminar cracks

delamination

microcracks

C-scan Section B-B 
 20 

Figure 11. Stages of crack formation in the ECT specimen. 

Figure 12. Principal stress resultant on plane B-B from mode III loading. 

0.0

1.0

2.0

3.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Force (

kN)

Displacement (mm)

P ≅ PNL

PNL ≤ P < Pmax

P approaching Pmax

Stage 1

Stage 2

Stage 3

midplane

90°

45°

45°

90°

90°

45°

45°

90°

90°

45°

45°

90°

zx 

xz 

zx 

xz 

1 

1 2 

2 
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Publication

 4. TITLE AND SUBTITLE

Observation of Intralaminar Cracking in the Edge Crack Torsion Specimen 

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Czabaj, Michael W.; Ratcliffe, James G.; Davidson, Barry D.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

L-20343

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 24

Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

The edge crack torsion (ECT) test is evaluated to determine its suitability for measuring fracture toughness associated with mode III delamination growth 

onset. A series of ECT specimens with preimplanted inserts with different lengths is tested and examined using nondestructive and destructive techniques. 

Ultrasonic inspection of all tested specimens reveals that delaminationgrowth occurs at one interface ply beneath the intended midplane interface. Sectioning 

and optical microscopy suggest that the observed delamination growth results from coalescence of angled intralaminar matrix cracks that form and extend 

across the midplane plies. The relative orientation of these cracks is approximately 45° with respect to the midplane, suggesting their formation is caused by 

resolved principal tensile stresses arising due to the global mode-III shear loading. Examination of ECT specimens tested to loads below the level 

corresponding to delamination growth onset reveals that initiation of intralaminar cracking approximately coincides with the onset of nonlinearity in the 

specimen’s force-displacement response. The existence of intralaminar cracking prior to delamination growth onset and the resulting delamination extension at 

an unintended interface render the ECT test, in its current form, unsuitable for characterization of mode III delamination growth onset. The broader 

implications of the mechanisms observed in this study are also discussed with respect to the current understanding of shear-driven delamination in 

tape-laminate composites.

15. SUBJECT TERMS

crack; delamination; fracture strength; microscopy; torsion

18. NUMBER

 OF 

 PAGES

25

19b. TELEPHONE NUMBER (Include area code)

(443) 757-5802

a. REPORT

U

c. THIS PAGE

U

b. ABSTRACT

U

17. LIMITATION OF 

 ABSTRACT

UU

Prescribed by ANSI Std. Z39.18

Standard Form 298 (Rev. 8-98)

3. DATES COVERED (From - To)

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

794072.02.07.03.03.03

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

NASA/TP-2013-218143

16. SECURITY CLASSIFICATION OF:

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and 

Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person 

shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 12 - 2013