August 2010 

NASA/TM–2010-216838 

Mode II Interlaminar Fracture Toughness and 

Fatigue Characterization of a Graphite Epoxy 

Composite Material 

T. Kevin O’Brien 

NASA Langley Research Center, Hampton, Virginia 

William M. Johnston 

Lockheed Martin, Hampton, Virginia 

Gregory J. Toland 

University of California, Santa Barbara, California 
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August 2010 

NASA/TM–2010-216838 

Mode II Interlaminar Fracture Toughness and 

Fatigue Characterization of a Graphite Epoxy 

Composite Material 

T. Kevin O’Brien 

NASA Langley Research Center, Hampton, Virginia 

William M. Johnston 

Lockheed Martin, Hampton, Virginia 

Gregory J. Toland 

University of California, Santa Barbara, California 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

Trade names and trademarks are used in this report for identification only. Their usage does not constitute 

an official endorsement, either expressed or implied, by the National Aeronautics and Space 

Administration. 
2 

Abstract 

Mode II interlaminar fracture toughness and delamination onset and growth 

characterization data were generated for IM7/8552 graphite epoxy composite materials 

from two suppliers for use in fracture mechanics analyses. Both the fracture toughness 

testing and the fatigue testing were conducted using the End-notched Flexure (ENF) test. 

The ENF test for mode II fracture toughness is currently under review by ASTM as a 

potential standard test method. This current draft ASTM protocol was used as a guide to 

conduct the tests on the IM7/8552 material. The results for material produced by the two 

suppliers were similar for all material property values measured. The insight gained is 

valuable for current and future material property standards development. The draft 

ASTM procedure for GIIc was followed, and produced consistently repeatable 

interlaminar fracture toughness values. Furthermore, the testing procedures developed in 

this study, and the associated test results, will be useful for developing mode II fatigue 

standards. Methods were developed to parse and analyze fatigue data. Development of a 

Mode II fatigue delamination onset curve for precracked specimens and specimens with 

inserts was discussed. The effects of performing material property tests from either 

precracks or inserts were discussed and compared. Testing from a precracked specimen 

gave more conservative properties than testing from an insert, for both interlaminar 

fracture toughness and fatigue onset data. 
3 

Introduction 

As part of the Space Act Agreement (SAA) between the Center for Rotorcraft 

Innovation (CRI) and NASA Langley Research Center, fracture mechanics based 

analyses are being performed to predict delamination onset and growth in structural 

subcomponents made of IM7/8552 carbon-epoxy composite materials. Delamination 

characterization data for opening (mode I) and shear (mode II) fracture modes are needed 

as input to these models. The mode II characterization of this material includes the testing 

of fracture toughness during static loading and the determination of delamination onset 

and growth during fatigue loading. Both the fracture toughness testing and the fatigue 

testing were conducted using the End-notched Flexure (ENF) test. The ENF test for mode 

II fracture toughness is currently under review by ASTM as a potential standard test 

method [1]. This current draft ASTM protocol was used as a guide to conduct the tests on 

the IM7/8552 material. There is no current ASTM standard for conducting Mode II 

fatigue delamination characterization. This report summarizes the test approach, methods, 

procedures and results of this characterization effort. 

Experimental Scope and Approach 

Characterization tests were conducted on IM7/8552 ENF coupons produced by two 

different rotorcraft manufacturers and made to their own specifications. The Mode II 

characterization included both the determination of the interlaminar fracture toughness, 

GIIC, as well as the fatigue delamination onset and growth. 

Tests were conducted using the End-notched Flexure specimen. The ENF test 

involves loading a beam with a mid-plane starter crack at one end, in three-point bending, 

as shown in figure 1. Crack length, a, is measured from the support roller, and the half-

 



 The terms “delamination” and “crack” will be used interchangeably throughout this 

report. 
4 

span length, L, is the distance from the support rollers to the center loading roller. A 

compliance calibration (CC) was performed for each specimen, before testing, to 

determine the relationship between specimen compliance and crack length. The 

compliance calibration relationship was used to estimate the crack length and to 

determine strain energy release rate (G) levels during the fracture and fatigue tests. 

Fracture tests were conducted according to the ASTM Draft Standard. Non-precracked 

(NPC) and precracked (PC) fracture toughnesses were determined for each specimen. A 

thin polytetrafluoroethylene (PTFE, or Teflon®) film was inserted on the mid-plane to act 

as a crack starter for the NPC tests. The specimens were pre-cracked from this Teflon 

insert during the NPC fracture test to create a naturally occurring crack, from which to 

propagate the crack in the subsequent PC tests. Fracture testing was conducted on a total 

of 5 specimens from each manufacturer to determine GIIC. The average mode II fracturetoughness was calculated for each material group. 

Fatigue characterization tests were run in load control at four different maximum 

cyclic loads, corresponding to initial GIImax values of 50%, 40%, 30%, and 20% of the 

average value of the PC GIIC from the fracture toughness tests for each material 

manufacturer. Five specimens were tested at each applied load level. Specimens were 

precracked using the same procedure as the static specimens. The compliance calibration 

method was used to determine the crack length and G levels during the test. After 

testing, specimens were pulled apart and the initial and final crack length and crack front 

shapes were measured. These revised crack lengths were used in the final data reduction. 

Crack growth rate was determined as a function of the maximum applied energy release 

rate, GIImax. The number of cycles before fatigue delamination growth onset occurred 

was also determined. 

Loading Roller

L L

a

Cracked 

End Support Roller

Specimen

Base

Insert or 

Pre-crack

P-

 

Fig.1. End-notched Flexure (ENF) 3-point bend test 
5 

Test set up and procedure 

Twenty-four ply, zero-degree unidirectional test coupons, measuring 7.0 inches long 

by 1.0 inch wide, were manufactured for this study by each of the two manufacturers. 

Specimens were nominally 0.175 inches thick. Actual specimen thicknesses and widths, 

measured at three locations along the length and averaged, are shown in Appendix 1 for 

both manufacturers (referred to as Source 1 and Source 2). A 3.0-inch long, 0.0005-inch 

thick, polytetrafluoroethylene (PTFE or Teflon®) film was embedded in one end at the 

mid-plane of the specimen thickness, across the 1.0-inch width of the test specimen, to 

act as a crack starter. The specimen edges were coated with a white spray paint to easily 

view the onset of delamination from the insert 

Non-precracked Fracture Toughness Tests from the Teflon Insert 

All specimens, including those tested in fatigue, were first statically precracked from 

the insert (NPC testing). Figure 2a shows a schematic of the specimen and nonprecracked testing dimensions that were used for the NPC fracture test. Figure 2b shows 

a test specimen in the three-point bend fixture. This configuration was used to generate 

the NPC fracture toughness and a shear pre-crack of approximately 0.5 inches from the 

end of the insert. The end of the PTFE film was located at 3.0 inches from the cracked 

end and placed 1.5 inches from the left roller for fracture testing. Tick marks were made 

on the specimen at 3.5 inches from the cracked end, and at +0.4 and -0.4 inches away 

from this mark, to use as guides for positioning the center load nose during compliance 

calibration loadings. The compliance calibration and NPC fracture test consisted of three 

loadings. The first two loadings used a maximum load below the failure load, so as not 

to cause delamination growth. First, the specimen was positioned in the three-point-bend 

fixture with the center load nose above the +0.4 mark and loaded to produce a plot of 

load versus stroke to obtain the specimen compliance. Next, the specimen was shifted to 

align the center load nose with the -0.4 inch mark, and the specimen was reloaded to 

determine the compliance again. Finally, the specimen was repositioned in the threepoint-bend fixture to the testing position (center tick mark under the center load nose) as 

shown in figure 2, loaded until the crack extended, and then unloaded. Load and 

displacement were recorded through the complete load-unload cycle. The initial loading 

data prior to crack growth provided a third compliance value for the compliance 

calibration. The unloading compliance after fracture occurred was used with the 

compliance calibration to calculate the precrack length. During the NPC fracture test, the 

delamination grew about 0.5 inches from the end of the insert to the vicinity of the central 

load nose. Photos taken after unloading (figure 3) showed the two specimen edges and 

provided a visual reference of the crack length (insert length from first roller plus precrack). The tick marks used for aligning the specimens for the CC loadings are also 

shown in the photo. The crack length, a, after NPC testing was approximately 2.0 inches. 
6 

2h

7.0

3.0

1.5

2.0 2.0

(a) (b) 

Fig.2 Static ENF pre-cracking test configuration (dimensions are in inches) 

Fig.3 Specimen edge showing crack growth past the central loading nose. 

Precracked Fracture Toughness Tests 

After precracking the specimen during the NPC fracture test, the specimen was ready 

for PC fracture testing. The specimen is shown in the PC fracture testing configuration in 

the three-point-bend fixture in figure 4a. The location of the center load nose was 

marked on the specimen at approximately 4.5 inches from the cracked end. The end of 

the shear pre-crack was placed 1.0 inch from the left roller. Figure 4b shows the 

specimen prior to testing in the three-point bend fixture. A compliance calibration was 

completed with the pre-cracked specimen prior to testing. Tick marks were made on the 

specimen edges at +0.4 and -0.4 inches from the center load nose mark for positioning 

the specimen for the precracked compliance calibration loadings. The specimen was 

shifted so that the center load nose was positioned first above the +0.4 mark, and then 

above the -0.4 inch mark, and loaded in each case to yield a plot of load versus stroke to 

obtain the specimen compliance, keeping the load below a level that would cause further 

delamination growth. In order to measure a pre-cracked value of GIIC, the specimen was 

shifted to the testing position as shown in figure 4, and the specimen was loaded until the 

crack extended, and then unloaded. The initial loading portion of the test was used for 

the third compliance calibration run. The delamination grew about 1.0 inch, from the 

initial 1.0-inch crack length at the end of the shear precrack to near the central load nose. 
7 

2.0

1.0

2h

2.0

7.0

Insert 

Pre-crack

 (a) (b) 

Fig.4 ENF test configuration with a pre-cracked specimen (dimensions are in inches) 

Fatigue Testing 

To conduct fatigue tests to measure delamination onset and growth, precracked 

specimens were placed in the fixture as shown in figure 4. The specimens were then 

subjected to a sinusoidal cyclic loading at a frequency of 5 Hz, and a stress ratio 

(Pmin/Pmax) of R = 0.1. Tests were conducted in load control using the Fracture 

Technology Associates (FTA) software for data accumulation and retrieval [2]. Tests 

were run at maximum cyclic loads, Pmax, corresponding to GIImax values equal to 50%, 

40%, 30%, and 20% of the average value of GIIC from the pre-cracked fracture tests. 

Prior to fatigue testing, a compliance calibration was conducted to determine the 

relationship between compliance and crack length and to calculate the appropriate Pmax, 

for each of five specimens being tested at the same GIImax value. For all three crack 

lengths used to obtain this compliance calibration, the load was kept below a level that 

would cause further delamination growth. During fatigue, the compliance was measure at 

a constant interval of cycles until the test was stopped. This interval was either every 2 or 

10 cycles. The FTA system monitored compliance and calculated a corresponding crack 

length. The system stopped cycling when the calculated crack length reached 2 inches. 

During fatigue the delamination grew about 1.0 inch from the shear precrack to the 

vicinity of the central load nose. After unloading, the specimens were pried open to 

allow measurement of the initial and final crack front shapes. Photos were taken of the 

two specimen surfaces to obtain a visual reference of the crack lengths and shapes. 

Data reduction 

Data reduction was conducted to produce fracture toughness and fatigue properties. 

Compliance calibration was used before and after precracking for all of the specimens to 

determine G levels. The compliance calibration was also used in the fatigue tests to 

determine crack length. Fracture toughness was calculated from both the PC and NPC 

tests. Fatigue data was reduced to produce delamination onset and growth curves. 
8 

Compliance Calibration 

The CC method was first performed on the NPC fracture tests as follows. Three 

loadings were used to obtain three plots of compliances versus crack length. The three 

compliances are those from the two CC tests (at ao–0.4 =1.1 inches, and ao+0.4 = 1.9 

inches) and from the initial portion of the fracture test (ao = 1.5 inches). Figure 5 shows 

typical compliance calibration load-displacement plots from the NPC fracture test. Figure 

6 shows a typical ENF load-displacement plot from the NPC fracture test. 

Fig. 5 Typical Compliance Calibration load displacement plots 
9 

Fig. 6 Typical fracture test load-displacement plot 

At each crack length, the compliance was determined by a least-squares linear 

regression analysis of the digital data to obtain the slope of the displacement versus load 

( vs P) data. The resulting compliances were plotted against the corresponding crack 

lengths raised to the third power and fit a line given by equation 1. 

3 C  A  ma (1)

The CC coefficients, m and A, were determined using a least-squares linear regression 

analysis of the compliance, C, versus crack length cubed (a

3

) where A is the intercept and 

m is the slope obtained from the regression analysis. Rearranging equation 1, yields a 

relationship between crack length and compliance, as shown in Equation 2. 

1/ 3





 



   m

C A

a

(2)

Figure 7 shows a typical NPC fracture test fit of compliance as a function of crack length. 

During the NPC fracture test from the PTFE insert, the crack did not always grow to a 

length of exactly 2.0 inches under the load nose. Therefore, the unloading modulus, CUL,

was measured to estimate the actual crack length from equation 2. 
10 

This calculated crack length was used to determine the initial conditions for the PC 

fracture tests and the fatigue tests. For each specimen, the difference between this 

estimated crack length and the nominal final crack length of 2.0 inches was added to the 

assumed 1.0 inch initial crack length for the pre-crack configuration, shown in figure 4, 

to perform the compliance calibration on the precracked specimens. The pre-cracked 

compliance calibration was performed using the same procedure as for the NPC 

specimens, starting from the configuration shown in figure 4. The new values of A and m

from equation 1 from this pre-cracked compliance calibration were then used to 

determine the PC GIIc, and to estimate the maximum cyclic load levels for the fatigue 

tests. All of the compliance calibration coefficients from the NPC and PC tests are 

presented in Appendix 1. 

The energy release rate, GII, was determined using the compliance calibration relation 

specified in the ASTM draft standard [1]. Equation 3 shows the equation for GII for a 

specimen with a constant width, B. Substituting the compliance relation from Equation 1 

into Equation 3 and taking the derivative, yields equation 4, where GII is a function of the 

compliance calibration constant, m. 

a

C

B

P

GII



  2

2 (3)

B

mP a

GII

2

3 2 2



(4) 

The compliance calibration data and equation 4 were used to calculate GIIc and maximum 

cyclic GII levels. 

1.5 10-4

2.0 10-4

2.5 10-4

3.0 10-4

3.5 10-4

4.0 10-4

1234567

/P,

in/lb

a3

, inches 3

m

C = A + ma3

Fig.7 Typical fit of compliance as a function of crack length for NPC fracture test. 
11 

Fracture Toughness Calculations 

The mode II NPC and PC fracture toughness was determined using equation 4, where 

m is the CC coefficient, P is the maximum load from the fracture test, a is the initial crack 

length, and B is the specimen width. As a further check on the estimated crack lengths, 

test specimens were pried open following final failure. Using the end of the PTFE film 

and the tick marks as guides, the initial delamination lengths were corrected, providing 

accurately measured values. 

Fatigue Property Calculations 

Fatigue tests were conducted at maximum cyclic GII levels corresponding to 50%, 

40%, 30%, and 20% of the average pre-cracked GIIc for each source. These maximum 

cyclic load levels were obtained by solving equation 4 for P (equation 5) and then 

substituting the appropriate GIImax value. 





 



 

m

BG

a

P

3

1 2 IImax

1/ 2

0

max

(5)

As a method to validate and correct the compliance calibration estimated crack 

lengths, all test specimens were pried open following fatigue loading. Figure 8 shows the 

fracture surface of a typical ENF fatigue specimen. It was possible to clearly delineate the 

end of the PTFE film, the end of the static pre-crack, the cyclic growth region, and the 

final crack length where the specimen was pried open. The precrack does not always 

coincide exactly with the tick marks on the edges of the specimen where the loading nose 

was aligned. Therefore, the initial pre-crack length was corrected based on these 

measurements, and the corrected value was used to perform the compliance calibration 

before finally reducing the data. 
12 

Fig.8 Fracture surfaces of typical ENF fatigue specimen 

The FTA data acquisition system used during the fatigue tests provided compliance as 

a function of cycle count. To determine crack growth rate as a function of GIImax, the 

compliance data were initially converted to crack length using Equation 2 and the CC 

data for the specimens (Figure 9). The derivative was taken numerically to provide the 

crack growth rate, da/dN. 

Because the data sets produced were large and contained noise, compliance was 

recorded at 2-cycle intervals for the 50% GIIc tests and at 10-cycle intervals for the 20-

40% GIIc tests to ensure that no data were lost. These intervals were not optimized to 

ensure a significant (above the noise level) increment of crack growth. To remedy this, 

the data were parsed to remove any crack growth interval of less than 0.002 inches, 

which was about twice the noise level in the data. The raw data and the resultant parsed 

data for one specimen are shown in Figure 10a. These parsed data were then analyzed to 

determine the crack growth rate. 

The fatigue crack growth rate, da/dN, was calculated from the parsed data using the 

secant method and the incremental polynomial method, described in ASTM E647 [4]. 

Using the secant method, fatigue crack growth rate was calculated as, 

i 1 i

i 1 i

N N

d /dN 

 



 a a

a (6) 

where ai is a crack length corresponding to cycle count Ni, and ai+1 and Ni+1 are crack 

length and cycle count values corresponding to the next available data point. The crack 
13 

length used to calculate G from this two point linear fit method is taken as the midpoint 

(average a) of the crack growth increment. Using the incremental polynomial method 

helps reduce error induced by noisy experimental data. The incremental polynomial 

method fits a parabola to successive sets of seven points of data. The fatigue growth rate 

is calculated as the derivative of this fit evaluated at the center point. The crack length 

used to calculate G is also taken from the center of the fit. The maximum cyclic G level 

was determined by substituting the estimated crack length and cyclic load for each 

growth rate measurement into equation 4. The data were plotted in the typical log-log 

form used for Paris law crack growth [5]. Typical results for fatigue crack growth are 

shown for the secant (2-point) and the sliding polynomial (7-point) data. The difference 

between the two reduction methods is illustrated in figure 10b. The sliding polynomial 

had significantly lower noise in the reduced data. For this paper the sliding polynomial 

method was used to reduce all of the crack growth vs. GIImax data. 

The section of the curve that represented stable fatigue crack growth was isolated and 

analyzed. The lower limit of this curve was taken as the delamination onset point. The 

upper end of the linear region was determined visually and extracted to synthesize the 

delamination growth law. For each test specimen, the linear region data were plotted in 

the typical log-log form of a Paris law and the data were fit to a relationship in the form 

of Equation 7. 

n DG

dN

da  IImax

(7)

The data beyond the linear growth region of the curve were truncated. The data from 

each source were combined and fit to a single growth law equation. The data were also 

fit separately for each of the five load levels for each manufacturer. 

The reduced crack growth rate data are shown in Figure 11 for an entire fatigue test. 

The onset and growth regions are determined from these data. The onset of delamination 

growth was defined as the point where da/dN reaches its absolute minimum. The crack 

length corresponding to the minimum da/dN value was then used to linearly interpolate 

the corresponding compliance and cycle count from the parsed data. The cycle count was 

recorded and used to formulate GIImax vs. N onset curves and the associated change in 

compliance was also recorded. This curve represents the threshold when stable fatigue 

crack growth begins. Onset curves were developed for each manufacturer and a 

combined curve for the material was also generated using all of the data. 
14 

15.0

20.0

25.0

30.0

35.0

40.0

0 10000 20000 30000 40000

C,

in-5/lb

N, cycles 

0.50

1.00

1.50

2.00

2.50

0 10000 20000 30000 40000

a,

in

N, cycles

 (a) (b) 

Fig.9 Typical plot of compliance (a) and crack length (b) as a function of fatigue cycles. 

0.910

0.915

0.920

0.925

0 500 1000

a,

in

N, cycles

Raw Data

Parsed Data

 

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

1.00E‐01

1.0 10.0

da/dN,

in/cycle

GIImax,

in-lb/in2

2 point Fit

7 point Fit

10 ‐1

10 ‐2

10 ‐3

10 ‐4

10 ‐5

10 ‐6

 (a) (b) 

Fig.10 Typical plot of (a) raw and parsed crack length data as a function of fatigue cycles 

and (b) reduced crack growth data using both ASTM E647 data reduction methods. 
15 

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

1.00E‐01

0.4 4.0

da/dN

in/cycle

GIImax, in-lb/in2

Onset Region Linear Paris Fit 

Growth Region

Truncated

Region

Fig.11 Reduced fatigue crack growth data with the onset and growth regions shown. 

Fracture and Fatigue Test Results 

Fracture toughness results for the NPC and PC fracture tests are shown in figure 12 for 

both sources of the material. The individual data points appear alongside an average 

value with corresponding error bars. The fracture test from the Teflon inserts (NPC tests) 

had significantly higher toughness than the shear precracked specimens (PC tests). The 

coefficient of variation for the precracked data is less than 10% and the data is reasonably 

consistent between the two manufacturers. The non pre-crack and the pre-crack mode II 

interlaminar fracture toughness data measurements for both sources are given in table 2. 

0.0

2.0

4.0

6.0

8.0

10.0

0 2 4 6

GIIC,

in‐lb/in2

Source 1 Source 2

Teflon

Insert

Shear

Precrack

Fig.12 Fracture toughness values for both sources 
16 

Specimen ID PC GIIC (in-lb/in2) NPC GIIC (in-lb/in2) 

Source1 

187 4.32 6.55 

186 3.69 7.06 

213 4.55 6.03 

206 4.20 6.14 

220 4.34 6.73 

Average 4.22 6.50 

Source2 

1S 4.92 8.48 

16TF 4.46 7.49 

09CF 4.36 7.22 

12CFHW 4.36 6.89 

12TFHW 4.95 7.85

Average 4.61 7.58 

Table 2. Non pre-crack and pre-crack mode II interlaminar fracture toughness. 

The fatigue tests were run at loads corresponding to the different percentages of GIIC

as shown in table 3. The tabulated pre-crack mode II delamination onset data 

measurements for both sources are given in Appendix 1. Table 3 shows the GIImax values 

used to calculate Pmax for the fatigue tests. The actual GIImax values shown in figures 13 

and 14, and tables A1-3, are adjusted by using the measured precrack lengths in equation 

4. 

Figure 13 shows the delamination onset data and curve fits for both manufacturers’ 

specimens. The combined delamination onset threshold data from both suppliers are 

shown in figure 14 along with a fit to the data. The PC fracture toughness data appears 

on the left side of the plots. The data presented in Figure 13 demonstrated very consistent 

results from both manufacturers as shown in Figure 14. 

Table 3. GIImax levels used to calculate Pmax for each fatigue specimen. 

Source 1 Source 2 

GIIcAVG = 4.22 in-lb/in2 GIIcAVG = 4.61 in-lb/in2

50% Gmax 2.11 50% Gmax 2.31 

40% Gmax 1.69 40% Gmax 1.84 

30% Gmax 1.27 30% Gmax 1.38 

20% Gmax 0.84 20% Gmax 0.92 
17 

0.00

1.00

2.00

3.00

4.00

5.00

6.00

1 100 10000 1000000

GIImax

in‐lb/in2

N, cycles

Source 1

Fit Data

GIImax=A×Nm

A = 4.24

m = ‐0.153

0.00

1.00

2.00

3.00

4.00

5.00

6.00

1 100 10000 1000000

GIImax

in‐lb/in2

N, cycles

Source 2

Fit Data

GIImax=A×Nm

A = 4.65

m = ‐0.158

 (a) Source 1 (b) Source 2 

Fig.13 Power law fits applied to the delamination onset threshold data. 

0.00

1.00

2.00

3.00

4.00

5.00

6.00

1 100 10000 1000000

GIImax

in‐lb/in2

N, cycles

Source 1

Source 2

Fit Data

GIImax=A×Nm

A = 4.47

m = ‐0.158

Fig.14 Power law fit applied to the delamination onset threshold data for all specimens. 

The percent change in initial compliance at the onset of delamination growth was also 

calculated and plotted in Figure 15. The individual data points appear alongside an 

average value with corresponding error bars. The change in stiffness is similar to the one 

percent increase in compliance, which the Mode I fatigue standard prescribes as the onset 

value [6]. 
18 

0.0

0.5

1.0

1.5

2.0

% Increase in Compliance

50% Gmax

40% Gmax

30% Gmax

20% Gmax

0.0

0.5

1.0

1.5

2.0

50% Gmax 40% Gmax 30% Gmax

% Increase in Compliance

20% Gmax

(a) Source 1 (b) Source 2 

Fig.15 Percent increase in initial compliance at the onset of stable delamination growth. 

The delamination crack growth results are summarized in Figure 16. Figure 16 shows 

the delamination growth laws derived by combining results for all five cyclic load levels 

into one log-log plot for the Source 1 (fig. 16a) and Source 2 (fig.16b) specimens. 

 (a) Source 1 (b) Source 2 

Fig.16 Power law fits applied to the delamination growth data. 
19 

Discussion 

The difference between the response of precracked and non-precracked specimens 

demonstrated the importance of using the proper data and testing method when using 

material data for delamination onset predictions. Both the interlaminar fracture 

toughness and the fatigue delamination onset are influenced greatly by the presence of a 

Teflon insert or a precrack. The importance of this difference is illustrated in figure 17, 

which compares the fatigue delamination onset behavior between precracked and nonprecracked specimens [7]. The static fracture toughness values are shown along the yaxis of this linear-log plot, corresponding to N = 1/4 cycle. Not only is there a significant 

difference in the interlaminar fracture toughness values, but there are order of magnitude 

differences in fatigue delamination onset lives. The difference in cycles to onset becomes 

larger as the load decreases. Hence, the fatigue delamination onset behavior is strongly 

affected by the condition of the delamination front, and this information must be 

considered before using these data for analysis. 

The fatigue delamination behavior of precracked and non-precracked fatigue 

specimens is shown schematically in Figure 18. For the precracked specimen tests, the 

initial instability in the delamination growth rate is denoted by a high growth rate that 

rapidly decays to a minimum after which stable increasing growth rates are observed. In 

contrast, for the test from a Teflon insert, the growth rate starts low and increases at a 

relatively constant Gmax level, until it reaches a point where the Paris law crack growth 

rate begins. This difference in behavior can be explained by considering how mode II 

shear delaminations initiate and propagate. Shear delaminations propagate through the 

coalescence of micro-tension cracks formed by the resolved shear stress ahead of the 

delamination crack front [8]. In addition to creating the precracked zone, the precracking 

process creates an area of uncoalesced microcracks ahead of the delamination front. The 

rapid initial growth of the PC specimens corresponds to the coalescence of these voids. 

When the formation of new voids and the coalescence of old voids have reached a steady 

state, the growth rate reaches a minimum. Conversely, cycling from the insert requires 

the initial formation of voids before the crack can grow, thus slowing the initial growth 

rate. When these voids begin to coalesce, stable growth begins. 
20 

0.00

2.00

4.00

6.00

8.00

10.00

1 100 10000 1000000

GIImax

in‐lb/in2

N, Cycles

Pre‐cracked Data

Insert Data [6]

Static GIIC

Fig.17 A comparison of fatigue onset behavior between precracked and nonprecracked 

specimens.. 

Fig.18 Fatigue onset behavior compared for precracked a non-precracked specimens. 
21 

Summary 

Mode II interlaminar fracture toughness and delamination onset and growth 

characterization data were generated for IM7/8552 graphite epoxy composite materials 

from two suppliers for use in fracture mechanics analyses. The results for material 

produced by the two suppliers were similar for all material property values measured. 

Insight was gained into testing that should be valuable for current and future material 

property standards. The draft ASTM procedure for GIIc was followed and produced 

consistently repeatable interlaminar fracture toughness values. Furthermore, the testing 

procedures developed in this study, and the associated test results, will be useful for 

developing mode II fatigue standards. Methods were developed to parse and analyze 

fatigue data. Development of a Mode II fatigue delamination onset curve for precracked 

specimens and specimens with inserts were discussed. The effects of performing material 

property tests from either precracks or inserts was discussed and compared. Testing from 

a precracked specimen gave more conservative properties than testing from an insert, for 

both interlaminar fracture toughness and fatigue onset data. 
22 

References 

[1] ASTM Draft Standard Test Method for Determination of the Mode II Interlaminar 

Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites 

using the End-Notched Flexure (ENF) Test. 

[2] Fracture Technology Associates, Bethlehem, PA, 

http://www.fracturetech.com/index.html. 

[3] D. Broek, Elementary Engineering Fracture Mechanics, 1991, p.124. 

[4] ASTM International, E 647-00, “Standard Test Method for Measurement of Fatigue 

Crack Growth Rates”. 

[5] P. C. Paris, M. P. Gomez and W. E. Anderson, “A Rational Analytic Theory of 

Fatigue,” The Trend in Engineering (1961). 13, pp. 9-14. 

[6] Standard Test Method for Mode I Fatigue Delamination Growth Onset of 

Unidirectional Fiber- Reinforced Polymer Matrix Composites, ASTM standard D6115-

97, American Society for Testing and Materials Annual Book of standards, Vol. 15.03, 

1997, pp. 338-343. 

[7] R. Martin, U.S. Army European Research Office Report - Report# N68171-98-M5177, 1998. 

[8] O'Brien, T.K., “Composite Interlaminar Shear Fracture Toughness: GIIc: Shear 

Measurement or Sheer Myth?”, Composite Materials: Fatigue and Fracture, Seventh 

Volume, ASTM STP 1330, 1998, pp. 3-18. 
23 

APPENDIX 1 – Tabulated Data 

This appendix tabulates the data used in this report for future reference. Specimen 

thicknesses and widths, measured at three locations along the length and averaged, are 

shown in Table A1 for both manufacturers (referred to as Source 1 and Source 2). The 

non-precracked and precracked compliance calibration coefficients are summarized in 

tables A2 and A3 below for Source 1 and Source 2 specimens, respectively. The fatigue 

onset data is summarized in table A4 for both sources 

Source 1 Source 2 

Specimen ID Width, B (in) Thickness (in) Specimen ID Width, B (in) Thickness (in) 

181 0.992 0.1770 03CF 0.999 0.1723 

182 0.991 0.1773 09CF 0.997 0.1760 

183 0.991 0.1767 10CF 0.998 0.1752 

185 0.989 0.1777 12CF 0.998 0.1687 

186 0.993 0.1778 06CFHW 0.997 0.1793 

187 0.991 0.1765 09CFHW 0.997 0.1775 

199 0.991 0.1768 12CFHW 0.997 0.1748 

201 0.992 0.1772 13CFHW 0.998 0.1755 

202 0.991 0.1767 1S 0.994 0.1727 

203 0.991 0.1765 2S 0.991 0.1753 

206 0.990 0.1787 4S 0.993 0.1728 

207 0.990 0.1793 01TF 0.998 0.1742 

208 0.988 0.1792 03TF 0.998 0.1713 

209 0.991 0.1783 15TF 0.997 0.1677 

210 0.989 0.1782 16TF 0.997 0.1715 

213 0.986 0.1823 20TF 0.998 0.1720 

214 0.990 0.1788 25TF 0.998 0.1725 

216 0.990 0.1783 29TF 0.998 0.1698 

218 0.991 0.1783 02TFHW 0.990 0.1748 

219 0.990 0.1792 10TFHW 0.990 0.1760 

220 0.986 0.1807 12TFHW 0.989 0.1763 

223 0.990 0.1828 15TFHW 0.990 0.1723 

224 0.987 0.1813 17TFHW 0.990 0.1720 

225 0.988 0.1803 21TFHW 0.990 0.1765 

226 0.989 0.1788 25TFHW 0.992 0.1767 

Table A1. ENF specimen dimensions. 
24 

 Insert Configuration Pre-Cracked Configuration 

GIImax Level Specimen ID m x10-5 (in2

-

lb-1)

A x10-5 (in/lb) m x10-5 (in2-

lb-1)

A x10-5 (in/lb) 

Static 

(100%) 

187 2.6415 14.446 2.6843 14.564 

186 2.7341 14.394 2.6737 14.541 

213 2.4445 13.710 2.5445 13.636 

206 2.7227 14.463 3.0323 14.309 

220 2.5541 13.814 2.8471 13.836 

50% 

181 2.5861 14.721 2.5861 14.733 

199 2.5731 14.456 2.6077 14.380 

209 2.5086 14.694 2.7412 14.280 

214 2.4770 14.090 2.5395 13.990 

223 2.4366 13.843 2.3167 13.641 

40% 

182 2.5812 14.327 2.3375 14.604 

201 2.5652 14.457 2.6434 14.335 

207 2.5837 14.604 2.6963 14.145 

216 2.5558 14.294 2.5305 14.274 

224 2.4843 14.389 2.4869 14.004 

30% 

183 2.5122 14.232 2.4849 14.259 

202 2.5978 14.494 2.5067 14.369 

208 2.6412 14.595 2.5936 14.202 

218 2.5643 14.178 2.4898 14.015 

225 2.5974 14.476 2.5368 14.057 

20% 

185 2.4866 14.556 2.4325 14.424 

203 2.5760 14.604 2.5157 14.352 

210 2.5885 14.614 2.5925 14.263 

219 2.6300 13.888 2.5180 13.872 

226 2.5991 14.591 2.5596 14.246 

Table A2. Source 1 compliance calibration curve coefficients. 
25 

Insert Configuration Pre-Cracked Configuration 

GIImax Level Specimen ID m x10-5 (in2

-

lb-1)

A x10-5 (in/lb) m x10-5 (in2-

lb-1)

A x10-5 (in/lb) 

Static 

(100%) 

1S 3.1556 13.625 3.3283 15.064 

16TF 2.5786 14.231 2.5454 15.316 

09CF 2.8193 15.033 3.1732 14.987 

12CFHW 2.5424 14.783 2.8841 14.902 

12TFHW 2.7839 14.313 3.0763 14.506 

50% 

2S 2.8559 15.413 2.9685 15.264 

3CF 2.8934 15.305 2.9167 15.703 

10CF 2.9075 15.126 2.9479 15.129 

10TFHW 2.8235 15.359 3.0579 14.928 

29TF 3.0559 16.469 3.1425 16.513 

40% 

4S 2.9464 15.233 2.8777 15.141 

13CFHW 2.8328 15.013 2.7837 14.864 

15TFHW 2.9856 15.644 2.9545 15.493 

20TF 3.0174 16.035 3.0360 15.771 

21TFHW 2.7465 14.837 2.7322 14.842 

30% 

01TF 2.8563 15.642 2.7739 15.458 

02TFHW 2.8688 15.154 2.8589 15.006 

09CFHW 2.8417 14.643 2.6388 14.758 

15TF 3.1789 16.662 3.1954 16.742 

17TFHW 3.0134 15.494 3.0389 15.209 

20% 

03TF 3.1408 16.167 3.4396 16.548 

06CFHW 2.7612 14.538 2.6826 14.446 

12CF 3.2024 16.954 3.2070 16.450 

25TF 2.9606 15.339 2.8069 15.263 

25TFHW 2.8229 14.864 2.7799 14.690 

Table A3. Source 2 compliance calibration curve coefficients. 
26 

GIImax Level Specimen ID GIImax (in-lb/in2) Cycles to Onset 

Source 1-50% 

181 2.1133 156 

199 2.1197 133 

209 2.1039 170 

214 2.1079 116 

223 2.1174 199 

Source 1-40% 

182 1.6939 257 

201 1.6895 449 

207 1.6924 746 

216 1.6861 545 

224 1.6872 436 

Source 1-30% 

183 1.2681 1597 

202 1.2699 987 

208 1.2734 1275 

218 1.2696 2246 

225 1.2718 2420 

Source 1-20% 

185 0.8445 41642 

203 0.8446 11080 

210 0.8482 34114 

219 0.8487 41643 

226 0.8490 38271 

Source 2-50% 

3CF 2.3190 83 

10CF 2.3592 126 

10TFHW 2.3146 128 

29TF 2.3188 93 

Source 2-40% 

4S 1.8559 366 

13CFHW 1.8414 557 

15TFHW 1.8328 416 

20TF 1.8161 243 

Source 2-30% 

01TF 1.5271 1242 

02TFHW 1.4049 1654 

09CFHW 1.3827 1302 

15TF 1.3589 1678 

17TFHW 1.3681 1706 

 Source 2-20% 

06CFHW 0.91766 28653 

12CF 0.94497 22733 

25TF 0.92442 44969 

25TFHW 0.91098 20725 

Table A4. Pre-crack mode II cycles to delamination onset. 
27 

APPENDIX 2 –Delamination Growth Data 

Delamination growth data are shown along with fits for each load level in figures A1 

and A2 below for Source 1 and Source 2 specimens, respectively.

 (a) (b) 

 (c) (d) 

Fig.A1. Experimental data and power law fits applied to the delamination growth data 

from Source 1 for the 50% (a), 40%(b), 30%(c) and 20%(d) load level tests. 
28 

 

 (a) (b) 

 (c) (d) 

Fig.A2. Experimental data and power law fits applied to the delamination growth data 

from Source 2 for the 50% (a), 40%(b), 30%(c) and 20%(d) load level tests. 
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Mode II Interlaminar Fracture Toughness and Fatigue Characterizarion of a 

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Mode II interlaminar fracture toughness and delamination onset and growth characterization data were generated for IM7/8552 

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testing and the fatigue testing were conducted using the End-notched Flexure (ENF) test. The ENF test for mode II fracture 

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Graphite-epoxy composites; Fracture mechanics; Fatigue; Delamination

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