*. e. 

PROGRESS REPORT 

FRACTOGRAPHY OF COMPOSITE DELAMINATION 

NAG- 1-705 

1 OCTOBER 1967 - 15 APRIL 1986 

ABSTRACT 

Studies have been made of the fractography of mode I1 delamination 

and the impact damage of carbon fiber reinforced polymer comptes. 

Laminates fractured under mode I I loading were potted in a clear 

epoxy polymer, sectioned and polished and examined using transmission and 

reflection light microscopy. There were only occasional fibers bridging the 

mode I1 cracks. These cracks were not always visible probably because the 

crack opening displacement was to small as to be resolved using light 

AS4/3501-6 and IM6/350 1-6. Plots of cumulative impact energy rs 

cumulative absorbed energy exhibited a sharp change in slope which 

corresponded to the damage area reaching the edges of the specimen. The 

initial slope was highly reproducible for both compte materials. On the 

other hand, the intersection point between the two slopes, where the damage 

area reaches the speclmen edges, was highly variable between specimens. 

This variability is tentatively ascribed to differences in laminate quality. 

microscopy . 

A study was made of the effect of repetitive impacts on a laminates of 

RESULTS 

The work under this Grant during the reporting period has been on 

two topics; (a) mode I1 delamination and (b) impact damage. 

Mode I1 Delamination: 

The load frame shown in Figure 1 was constructed to load endnotched carbon fiber laminates to induce a pure mode I1 crack and then pot 

the cracked speclmen in a clear epoxy for sectioning and microscopy. As 

shown in Figure 2, the speamen is clamped at one end and deflected 

downward at the other end by turning a bolt with a short wooden dowel 

contacting the speamen. Crack growth at the edge of the speclmen was 

1 
observed using a telescope and when the crack reached about half the length 

of the specimen a mold was built around the free end of the laminate 

(including the dowel), filled with the potting resin and heat cured. 

The test laminate was a unidirectional 16ply, AS4/350 1-6 composite 

lin. wide and 4in. in length. Excessive bending of the specimen near the 

clamped end resulted in failure before a delamination could be produced. 

This problem was solved by supporting the speclmen with two laminates 

O.5in wide and 2.25in and 3.25in long respectively as shown in Figure 2. 

Using this configuration, a mid-plane crack could be initiated without any 

apparent breakage at the clamped end. 

Two loading configurations are shown in Figure 2. In one case (Figwe 

2A), the laminate was precracked using a knife blade, the upper section cut 

away and the dowel positioned against the protruding lower section. In 

order to minimize Mode I loading, a wire was wrapped around the specimen 

as shown in Figure 2A. Judging from visual observations the mode I opening 

displacements are minimal at the crack front that had propagated one-half 

the length of the speclmen so that mode I loading is minimal near the crack 

f r ont. 

The loading configurations shown in Figure 2B should produce a pure 

mode I I delamination. However, we have not yet induced a crack in this 

fasbon. The fractography results reported here are for cracks produced 

using the “mixed mode” technique (Figure 2A). 

The general appearance of the cut sections is shown in Figure 3. Two 

major cracks developed in the speamen ; the major mixed-mMe crack along 

the central plane and a pure mode I1 crack in the upper half of the 

specimen. For the most part, these cracks progressed through the matrix 

with relatively little fiber bridging. Occasionally, there was evidence of 

fibers spanning the crack (Figure 4). The mode I1 crack was not continuous. 

As shown in Figure 5, the crack is distinct on the left and right hand sides of 

the photomicrograph but disappears in the center. 

2 
1. 

3 c 

The general appearance of the cut sections is shown in Figure 3. Two 

major cracks developed in the specimen ; the major mixed-mode crack along 

the central plane and a pure mode 11 crack in the upper half of the 

specimen. For the most part, these cracks progressed through the matrix 

with relatively little fiber bridging. Occasionally, there was evidence of 

fibers spanning the crack (Figure 4). The mode I1 crack was not continuous. 

As shown in Figure 5, the crack is distinct on the left and right hand sides of 

the photomicrograph but disappears in the center. 

mode I1 crack 

__---- 

micro cracks 

mixed mode crack 

Figure 3 - Schematic of cracking in a mode 11 specimen 

Figure 4 - Central mode 11 crack (Figure 3) showing occasional fiber crouve 

2 
Figure 5 - Discontinuity in mode I1 crack 

As shown schematically in Figure 3, there were microcracks near the 

major crack and especially ahead of the crack front. It was sometimes 

difficult to distinguish microcracks from laminate imperfections but there is 

the distinct possibility that "satellite" microcracks develop ahead of the mode 

I1 cracks but then close-up once the crack front has passed. The formation 

of these satellite cracks constitute part of the energy of mode 11 crack 

propagation. 

The stressed laminate revealed extensive fiber breakage throughout 

the specimen. In Figure 6, photomicrographs are shown of a section cut 

through an unstressed laminate and a section through the stressed laminate. 

In the latter, many of the fibers appear to have broken into short segments. 

ons and Future Work, The mode 11 cracks appear to propagate 

through the matrix with relatively little fiber bridging. This is in distinct 

contrast to Mode I delamination where there was extensive fiber bridging. 

The pure mode 11 crack seemed to be discontinuous in that the crack 

"disappears" €or a short section and then reappears again. Quite possibly the 

the crack opening becomes too small to be observed at the magnifications 

used here. This explanation is supported by the fact that in the presence of 

a small mode 1 component, the crack was continuous. In principle, a mode 11 

crack should not involve any opening displacement. The fact that it can be 

seen at all indicates that at the local level there is some mode I component. 

Future work will include modifications of the load frame to prevent 

specimen cracking at the fixed end without having to resort to stiffening 

members. One possible solution is to use thinner ($ply) laminates so that the 

bending stress for delamination is less than the stress for transverse 

3 
cracking. The excessive fiber breakage (Figure 6) may have been the result 

of the high stresses needed to delaminate the stiffened specimen . 

Further tests are planned for the AS413501-6 laminate to confirm and 

possible extend the observations reported here. Subsequently, a 

unidirectional laminate with a higher fracture energy will be tested. 

Cross-ply (0/90) laminates will be tested. Based on impact damage 

fractography. delamination is expected to occur in the resin layer between 

plys rather than intra-ply. The experiments reported here on mode 11 

delamination and in previous reports on mode I are actually intra-ply 

failures which are rarely encountered in impact damage. 

4 
A 

B - 

0.01 mm 

Figure 6 - Fiber fracture in tested specimen (A) not observed in untested laminate 

5 
An investigation has been undertaken to determine the effect of 

matrix properties (and other factors) on laminate impact damage. The test 

method, which was originated at the DFVLR (1 1 in West Germany. involves 

1. Stellbrink. K.. The Repetitive Dropweight Test with Increasing Energy,' Wissenschaftiich-brhnhhe Tagung :12, Verstarkete Paste '88, Rrlin, DDR ,1988 p.21 

repetitively impacting a specimen and plotting the cumulative impact energy 

vsthe cumulative adsorbed energy. These plots typically exhibit a change in 

slope which has been attributed (1) to a change in the damage mode, Le., 

from predominantly delamination to the onset of fiber fracture. 

The test lends itself to materials development since the test laminate 

is relatively small and one specimen yields damage information over a range 

of impact loads. In this study the extent and type of damage at various 

stages of impact history are being determined using acoustic backscattering, 

x-ray corn p u te r ked tomography (CAT scanning and metallographic 

sectioning for microscopy. 

Exnew . entat The drop weight impact test equipment is shown in 

Figure 7. A round nose (lcm diameter) impacter is fitted with an 

accelerometer and an electro-optical detector records the drop and rebound 

heights. The computer controls calculate the incident and adsorbed energy 

as well as the force and displacement. The specimen was a 16ply, [0/90] 

1aminateIOcm in length and 5cm wide. The specimens were fabricated from 

AS4 and 1M6-3501-6 prepreg (Hercules Inc.) in the composite fabrication 

facility in the Mechanical Engineering Department of the University of Utah 

using conventional vacuum bag/autoclave procedures. The test laminates 

were clamped to steel blocks so that the impact area of 5cm X Scm was 

unsupported. 

Results, Typical repetitive impact test (RIT) results are presented in 

Figure 8 for AS41350 1-6 and 1M6/35O 1-6 specimens. There is a distinct 

change in slope in all cases. The force-displacement traces are shown in 

Figure 9 for impacts in the initial slope (stage 1) and the secondary slope 

(stage 11). These traces correspond to the Sth, 121h, 15th and 23rd impact 

(Figure 8A). In stage I ( impact 5 and 12) the impacter response is 

essentially linear (Figure 9A) but is distinctly nonlinear in the stage I1 region 

(Figure 9B). 

G 
Figure 7 - Drop-weight impact test equipment including devoted computer hardware 

(courtesy of Prof. John Nairn) 

7 
3 

i 

f 

H I 

AS41350 1-6 

50 

40- 

30- 

20 

10 

0 

3 

m 

f 

t 

B i 

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0 20 40 60 80 100 

ctrmulalive impact energy, 3 

.. 

AS413501-6 

80 1 

Figure 8 A - Typical RIT data; cumulative impact energy vs cumulative absorbed energy 

(AS41350 1-61 

8 
i 

5 I 

0 m 40 60 80 

cumulative impact energy, J 

IM6/350 1-6 

0 20 40 60 80 

cumulatlve impact energy, J 

Figure 8 B - Typical RIT data; cumulative impact energy vs cumulative absorbed energy 

(IM6/35O 1-61 

9 
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Figure 9A- Force displacement traces for the 5th (A) and 12th (B) impacts of an 

AS4/3501-6 laminate (Figure SA) 

10 
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Figure 9B- Force displacement traces for the 15th (A) and 23rd (B) impacts of an 

AS4/3501-6 Lamina (Figure SA) 

11 
Acoustic C-scans of the damage area for a specimen impacted in the 

stage 1 region and for a specimen impacted in the stage I1 region are shown 

in FigurelOA andlOB respectively. It would appear that for these specimens 

the transition from stage I to stage 11 corresponds to the extension of the 

damage area to the edges of the specimen. 

Three parameters of the RIT curves are of interest; the slope of the 

plots in stages I and I1 and the intersection point of the two stages. These 

parameters are listed in Table I. 

TABLE I 

Specimen Slope Intercept (3) 

IM613601-6 

1A 0.225 0.526 17 

1B 0.250 0.500 17 

1c 0.225 0.309 21 

28 0.300 0.357 51 

2c 0.274 0.507 19 

3A 0.274 0.510 23 

3c 0.238 0.5 18 29 

4c 0.250 0.36 45 

AS41350 1-6 

1A 0.287 0.485 24 

1c 0.273 0.230 62 

2B 0.225 0.535 10 

2c 0.26 1 0.383 49 

Average (SD) 0.260 +0.026 0.43 120.099 

12 
A 

B 

Figure 10 - Acoustic scans of impact damage to an AS4/3501-6 laminate; A - after 5 

impacts. B- after 13 impacts 

The slope of the data in stage I is relatively constant with a low 

standard deviation. The slope for stage 11 has a somewhat bher variance. 

The intercept of the two regions was quite variable and appears to be 

related to the position the specimen was cut from the panel. The alphabetic 

designation of the specimens listed in Table I - A,B,C - refers to the panel 

layout shown in Figure 11. 

13 
FiBure 11 - Three test laminates were cut from each panel and designated as indicated 

With the notable exception of the specimens cut from panel 2. the "C' 

specimens had higher intersections in the RIT data. Quite possibly, the 

differences in the intersection points in Table 1 reflect differences in 

specimen quality. 

CONCLUSIONS AND FUTURE WORK The repetitive impact test offers 

some interesting possibilities for characterizing impact damage. Clearly, the 

materials requirements are minimal which can be very useful in materials 

development studies. 

The results presented here suggest that the "rate" of cumulative 

damage is essentially constant so long as the damage area is within the 

bounds of the specimen but that there is an abrupt change in rate once the 

damage has reached the edges of the specimen. This result is intuitively 

reasonable in that the mechanical response of the plate must change once 

the damage has extended to the boundaries. 

However, in work in progress, we have found an abrupt change in the 

slope of the KIT data before the damage has reached the specimen edges. 

This was observed for 32ply specimens of 1M6/3SO 1-6 and for 32ply 

specimens of AS4/PEEK. The initial slope, stage 1, was the same for these 

materials as reported in Table I but the intercept was extended to much 

higher energies. The details of these current experiments will be reported 

later. 

Our current thinking is that the transition from stage I to stage 11 

represents a change in the type of mechanical response of the laminate; 

possibly from quasistatic behavior to dynamic behavior. This hypothesis is 

supported by the fact that the change in slope in the RIT corresponds to 

14 
distinct reductions in the laminate modulus determined from the force 

displacement curves (Fig 8).