P-c6 GRADUATE AERONAUTICAL LABORATORI

CALIFORNIA INSTITUTE OF TECHNOLOGY 

Firestone Flight Sciences Laboratory 

Guggenheim Aeronautical Laboratory 

Karman Laboratory of Fluid Mechanics and Jet Propulsion 

Pasadena 
SM Report 86-34 

1 

OBSERVATION OF THE INITIATION AND PROGRESSION OF 

DAMAGE IN COMPRESSIVELY LOADED COMPOSITE PLATES 

CONTAINING A CUTOUT 

Anthony Waas * 

Charles Babcock. jr ** 

Progress report 

Grant: NSG 1483 

November 1986 

Graduate Aeronautical Laboratories 

California Institute of Technology 

Pasadena, California 91125 

* Graduate Research Assistant ! 

** Professor of Aeronautics and Applied Mechanics I 
2 

Abstract 

I A series of experiments were carried out to determine the mechanism of failure 

in compressively loaded laminated plates with a circular cutout. Real time holographic interferoixietry and photomicrography are used to observe the progression 

of failure. These observations together with post experiment plate sectioning and 

deplying for iritcrior damage observation provide useful information for modelling 

the failure process. 

It is revealed that the failure is initiated as a localised instability in the zero 

layers, at the hole surface. With increasing load extensive delamination cracking is 

observed. The progression of failure is by growth of these delaminations induced 

by delamination buckling. Upon reaching a critical state, catastrophic failure of 

the plate is observed. The levels of applied load and the rate at which these events 

occur depend on the plate stacking sequence. 
3 

1.1 Introduction 

I 

The use of Graphite/Epoxy laminates in various fields of engineering has posed 

a new challenge to engineering design. In many instances, it is necessary to look for 

new techniques to characterise these materials which differ from isotropic , homogeneous materials in their response to mechanical load. Extensive research programs 

are being carried out throughout the country aimed at improving the understanding 

of the mechanics of Composite Materials. 

One situation where these laminates exhibit a different type of behaviour is 

when they are subjected lo compressive loads. One or several competing modes of 

failure are present and the researcher is left with the task of identifying the most 

important aspect of the failure process from a design standpoint. When structural 

components are made out of these laminates it is seldom that they will persist in 

an environment free of stress concentrations. Stress raisers can be lethal and are 

potential sites where damage can initiate in a structure. 

The open hole strengths of these laminates has been the subject of numerous investigations (1-41. Figure 1 depicts ultimate load carrying capacity of these 

laminates as a function of the normalised hole diameter. Mikulas (51, has shown 

that the results could be bounded by simple criteria based on notch sensitivity. If 

the material is notch insensitive, the failure strains are directly proportional to the 

reduction in cross-sectional area, while if the material is notch sensitive then it is 

postulated that the material fails when the strain concentration at the hole edge 

equals the failure strain of the material. It is seen that the latter assumption is nonconservative in that the failure strain is underestimated for (a/w) ratios less than 

0.4 . This situation indicates the possibility that the strain concentration effect 

alone cannot possibly account for the failure process and that a localised failure 
4 

mechanism may be present. Based on this premise Starnes et a1 [l] carried out 

open hole strength testing of these laminates and reported a shear crippling type of 

failure to be prevalent in the vicinity of the hole prior to catastrophic failure. This 

conclusion was based on post-mortem examination of damaged specimen. 

In these and other similar investigations no attempt has been made to follow the 

progression of failure. The progressive nature of failure, in many cases, by various 

stages is a characteristic property of laminated composites. Several different failure 

processes are prevalent when these laminates are stressed and what is required is 

to identify the key aspects of failure in a particular situation. 

Motivated from such an understanding, the present work is a detailed experimental investigation to understand the initiation and propagation of failure in 

laminates in the presence of a stress raiser. The stress concentration effect is produced by choosing a flat plate with a centered circular hole and subjecting it to an 

inplane compressive load. The hole is carefully cut in order to insure that excessive 

local damage does not occur during the machining process. The damage initiation and propagation throughout the entire load history is studied via real time 

Holographic Interferometry and Photomicrography of the hole surface. The results 

thus obtained are substantiated by post experiment examination of the damaged 

specimen Ultrasonically and under an Optical Microscope. 

In the sections to follow, the Experiments and associated methodology is presented first. Pertinent details are given where necessary but no attempt is made to 

go into extensive details. Thus, the reader is refered to relevant texts on Holography, Ultrasonic scanning etc. for a detailed treatment of the subject. We emphasise 

here that our main goal is to capture the phenomenological aspects of the failure 

process as opposed to conducting a parameter study. 
5 

I 

1.2 The Experiment 

1.2.1 Assembly 

Figure 2 is a photograph showing the arrangement of the various devices used 

in the experiment. On a pneumatically vibration isolated table, optical components 

are arranged to make a hologram and generate interferograms of the test specimen 

which is loaded by a table top compression device. An optical microscope is mounted 

on a tripod which is situated adjacent to the optical table by the testing machine. 

A 35 rnrn motor driven SLR camera attached to the microscope is used to take photographs of the test specimen hole surface, during the testing sequence. Recording 

and developing of the hologram is done on a thermoplastic plate which is housed in 

the body of the instant recording device(1RD) as marked in Figure 2. This device 

which is electronically controlled replaces the conventional wet chemical processes 

which are used had the recording being done on a photographic plate. A video 

camera placed behind the thermoplastic plate views the test specimen through the 

hologram. A VCR records the interferometric pattern generated during the loading 

sequence. A TV monitor is used to view the interferograms during a typical test, 

which enables testing to be done interactively. In the following sections the main 

components of the experiment are described. 

1.2.2 Specimen 

I 

The multi-layered laminated plates used in the tests were supplied by NASALangley Research Center. The composition of the laminae in the specimen consisted 

of Thornel-300 graphite fibers preimpregnated with American Cyanamide BP 907 

epoxy resin. This laminate is typical of those used in the Aerospace Industry. Two 

types of lamina stacking sequence were used in the tests. The first designated Type 

A is 48 ply [+45/-45/0/0/+45/-45/0/0/$45/-45/0/90]~~ while the other, Type B is 
6 

i 

obtained by rotating the first sequence by ninety degrees, the angles being measured 

with respect to the loading direction counterclockwise. Diamond impregnated core 

drills and saws are used for machining the specimen. Two sizes of specimen are 

used. The first designated "S" is 7.62 crn x 10.2 cm and the other "L" is 12.7 crn 

x 15.3 cm. 

1.2.3 Specimen Supports 

The laminated plates need to be supported in a manner that allows a smooth 

load transfer from the testing machine on to the plates. To do this specimen holders 

made of mild steel are used in these tests. The diameters of the holders are chosen 

to be the widths of the specimen. The holders contain grooves of 1.27 cm depth 

and widths large enough to accomodate the specimen and the thin layers of bonding 

agent. The test specimen are inserted into the grooves and bonded to the support 

holders with DEVCON bonding agent. Edge stiffeners are attached to the sides of 

the specimen to avoid plate buckling. These stiffeners increase the flexural stiffness 

of the plate but carry no load. A typical set of edge stiffeners is shown in Figure 3. 

1.2.3 Testing Machine 

The use of real time holographic interferometry requires vibration isolation of 

test specimeri and tesling device. As such, a special purpose testing device was 

designed and constructed for this purpose. Both tension and compression load can 

be applied using this device. At the commencement of a test one end of the specimen 

support holders is inserted into an adapter which threads onto the actuator arm of 

the loading device. A ball joint is attached to the other end. The mating part of 

the ball joint is positioned at the center of a 100,000 Zbf (445 kN) load cell. The 

load cell is mounted on one of the flanged box beams of the testing machine. A 
7 

I 

photograph of the testing machine and accessories positioned on the optical table 

is shown in Figure 4a. A photograph of a specimen mounted as described earlier 

with the balljoint arrangement is shown in Figure 5. 

The load-time history is regulated through a servo controlled loading mechanism. The servo valve and associated accessories are mounted on a plate which fits 

on the cylinder of the loading device. A function generator is used to program the 

loading path. A picture of the servo valve and accessories is shown in Figure 4b. 

1.2.4 Holographic Interferometry 

Real time holographic interferometry is used in this study to record the out of 

plane displacement changes Aw of the specimen during the loading sequence. The 

revelant details of the technique as applicable to the present experiment is given in 

section 1.2 . One novelty in the present experiment is the use of an electronically 

controlled process in the recording and development of a hologram. In conventional 

holography a wet chemical process is used in the development of the photographic 

glass plates. The time consumed during this process which is of the order of 7-9 

minutes, is a significant portion of the total duration of a test which is of the order 2 

hours. During a continuous loading sequence, the information on the displacement 

history is lost during this development time. The electronic process refered to above 

and explained in the next section overcomes this drawback. 

1.2.5 The Instant Recording Device 

In the reconstruction process of a hologram the conventional Silver Halide photographic plates cause diffraction via an amplitude grating which is representative 

of the spatial distribution of light( amplitude and phase ) that is emitted from the 

diffusely reflective surface of the object. However, the same task could be accomplished with a Phase grating i.e. , a surface contour variation which is representative 
r 

of the light off of the object. This is the basic idea behind the success of the thermoplastic recording device. The device has several stages of operation as shown 

in Figure 6. Steps (a) through (d) require less than one minute - a considerable 

saving of time. Furthermore, with real time holographic interferometry, a complete 

deformation history of the object up to failure can be recorded. This is important 

in the study of composites which exhibit several stages of failure. There are four 

basic steps in tlie creation of a hologram with this device. 

Step 1. 

A uniform charge is deposited on the thermoplastic plate as shown in Figure 

6a. A movable coronatron deposits this charge. A corresponding negative charge 

forms on the transparent electrical conductor which is connected to ground. 

Step 2. 

The plate is exposed. Charge passes through the photoconductor only in the 

illuminated areas (Figure 6b.). 

Step 3. 

Another charge is provided to the plate. Because the plate is now not exposed 

to the light the charge does not move across the photoconductor. Thus, the areas 

of the thermoplastic that were illuminated experience a greater electric field; the 

charge density is greater in the illuminated areas. 

Step 4. 

The development of the hologram is carried out by heating the thermoplastic 

plate. In areas where the electric field is greater, the heat softened thermoplastic 

becomes thinner(Figure 6d). This creates the surface contour variation which causes 

the diffraction of light in the reconstruction stage. 
9 

The coronatron is a device that consists of a wire anode running down the 

center of a slotted cylinder. The slot is oriented to face the thermoplastic plate. 

The slotted cylinder and plate are grounded and the anode is raised to a potential of 

several kilovolts. The resulting electric fields ionize the surrounding air, producing 

electrons and positive ions. The electrons are collected on the wire which results in 

a positive charge on the slotted cylinder and the thermoplastic plate (Figure 7). 

The detailed structure of the thermoplastic plate is shown in Figure 8. On a 

quartz substrate, an optically transparent electrode of Indium-Tin oxide is deposited 

on top of which a 1 pm thick layer of trinitro-fluorenone doped photoconductive 

organic polymer is placed and finally a 0.7 pm thermoplastic layer is added. Metal 

electrodes are attached to each side of the transparent electrode which provides a 

ground plane for the coronatron. A current passed through the transparent electrode produces resistive heating sufficient to soften the thermoplastic. 

The cycle of events explained above is sequenced by a control unit supplied with 

the recording device. Dry Nitrogen is used as the coolant for the thermoplastic plate 

in the erase process. This consists of reheating the thermoplastic to releive it of the 

surface contour variation and to produce a ripple free surface. Upon cooling, the 

plate is ready to be used again in the formation of a hologram. 

1.2.6 Exp erimerit a1 Procedure 

A schematic of the holographic interferometer used in this study is shown in 

Figure 9. It is a standard off axis holographic system. Proper choice of reference to 

object beam intensity ratio is important for producing the best holograms. With 

the use of the instant recording device, the quality of the hologram is insensitive to 

the exposure time. In the real time mode, the test specimen is illuminated in a state 
10 

S1 (say) with coherent light transmitted from the laser via the object wave path 

(this consists of the beam splitter, a beam exaprider-spatial filter, collimating lens). 

The corresponding load is P1 (refer to Figure 10). A hologram is then recorded 

by exposing the thermoplastic plate positioned at the junction of the reflected light 

and reference bcam. Upon completion of the first exposure the specimen which is 

continuosly loaded is viewed through a camera attached to a VHS recording system 

(a VCR and a monitor). The resulting image consists of fringes which are dynamical 

in nature representing contour maps of the changes, with respect to the state SI, of 

the out of plane displacement component UI of the stressed specimen. These fringes, 

together with the load cell reading are recorded. The load cell reading is obtained 

by focussing the display of the voltmeter, exhibiting the load cell reading, at the 

position of the circular hole in the plane of the test specimen. With increasing 

load on the specimen the fringe density becomes high, so that the resolution of the 

interferogram becomes poor. At this stage a new hologram of the test specimen 

is made and the above cycle of events is repeated. Thus, finally at the end of 

each experiment we obtain an interferometric recording of the complete load - out 

of plane displacement history of the specimen. This sequence of events is shown 

graphically in Figure 10. 

t 

Each of the holographic interference fringe field is related to the change in 

surface displacement of the specimen under load, by the vector expression 

nX = IC.2 

where, 

n fringe number 

X wavelength of the coherent laser light 
I 

- - - 

k sensitivity vector (iz - il) 

2 surface displacement vector at point of observation 

;1 and 12 are the unit vectors in the illumination and observation directions 

respectively. In the present experimental setup the diffusely reflective surface of 

the specimen is oriented normal to the bisector of the angle Za, between 

(Figure 11). Thus, since the sensitivity vector i, also points along this bisector, the 

interferometer senses only the out of plane displacement component w. Equation 

(1) then becomcs 

and 

nX = 2wcosa 

In the present experimental setup, the surface normal is 10' off the observation 

direction. With X = 0.633 pm, this shows that each fringe represents an out of plane 

displacement change of 0.321 pm. 

A schematic of the present experimental set up is shown in Figure 12. During 

the entire loading program, the surface of the hole is viewed through a microscope 

and photographs are taken at various times. A fibre optic cable attached to a high 

intensity lamp is used to illuminate the hole surface. The interferograms displayed 

on the TV monitor helps taking these photomicrographs interactively in an efficient 

manner. Table 2. summarises the test conditions and the results obtained. In this 

table the last two columns represent normalised "loads" at initiation and failure. 

Here, "A" is the cross sectional area of the specimen on which the loading is applied 

and ECLT is the Youiigs modulus in the load carrying direction, calculated using 

classical [aminated plate theory. In the second column, "AS" for example refers to 

type "A", small. The ply properties used are given in Table 1. 
12 

I 

The interferometric data reveal the changes in the w displacement component. 

These changes may be the manifestations of internal damage. Thus, some specimen 

were loaded to a post initiation level but were unloaded prior to catastrophic failure. 

The unloaded specimen were sectioned in the region of stress concentration and were 

examined under an optical microscope. In the following section the results obtained 

from Interferometry, Photomicrography and Interior damage studies are presented. 

1.3 Results and Discussion 

In this section the results obtained in the experimental program are presented 

and discussed. The interferometric data are presented only in the immediate neighbourhood of the hole. The photomicrographs are only captured on one side of the 

hole surface as indicated in Figure 13. The horizontal shiny stripes appearing on 

the photomicrographs are 0' layers while vertical lines are scribe marks for purposes 

of reference. The top edge of the hole surface appearing in the photomicrographs 

correspond to the side on which the interferometry is carried out. Only a part of 

the hole surface appearing in the photomicrographs is in focus, as this surface is 

viewed at an oblique angle through the microscope. 

We now consider the different stages of failure as exhibited by the interferometric results and substantiated by the photomicrographs. In the pre-damaged 

situation an increase in the load on the specimen results in a uniform pattern of 

fringes. These fringes are a few in number and correspond to the thickness changes 

due to the Poisson ratio effect, but coupled with some rigid body motion and/or 

slight bending. The rigid body motion and the bending cannot be totally avoided 

in a compression test. The slight bending may be the result of any initial imperfection in the specimen. Added to this we realise the high sensitivity of the technique 

which is a function of the wave length of the laser light. (displacement changes as 
13 

I 

small as 0.316 prn can be detected). In the discussion to follow we first consider the 

responses of Type A and Type B specimen separately and summarise the common 

features of the failure process at the end. 

Type B specimen 

This type of specimen is one in which there are a fewer number of Oo plies 

(25%). Figure 14. shows a series of interferometric patterns, which correspond to 

the first instance at which internal damage was detected. The numbers below each 

picture represent the load in Ibf (x 4.448 N) corresponding to each frame, while Ph 

indicates the load at which the hologram was made (Reference load). Notice the 

highly localised fringe cluster adjacent to the hole edge at approximately 90' to the 

loading direction. When any form of internal damage occurs that manifests itself 

as a localised perturbation in the normal displacement component w the resulting 

interferometric patterns display this event as shown in the figure. A view of the 

hole surface corresponding to this fringe pattern is shown in frame 2 of Figure 

15. Notice the damage to the midplane 0' plies which appear as a black spot. 

Adjacent to this spot is a delamination crack. Inspection of the photomicrographs 

prior to this event revealed no damage to the hole surface. In all tests it was 

revealed that always a perturbation in the Interferometric data corresponded to 

0' ply damage at the hole surface. We interpret this interferometric pattern as a 

localised bulging of the surface just above the damage site. This localised damage 

to the 0' plies is found to be the triggering of extensive delamination cracking and 

buckling. The above sequence of events is consistently observed in all of the Type 

B specimen. At this stage we point out that when we refer to damage we mean the 

destruction to the specimen caused by the application of mechanical load as picked 

up on the interferograms. The interferograms corresponding to the initiation event 

reveal the displacements within the damage zone to be increasing with increasing 
14 

I 

I 

load. This is clear by noting a sharp increase in the fringe density within the 

damage zone as loading proceeds. Subsequent to the initiation, the remaining Oo 

layers undergo failure while delaminations are seen to appear between various ply 

interfaces. An inspection of the sequence of photomicrographs in Figure 15 shows 

this. The delaminations undergo a stage of slow growth as shown in the next 

set of interferometric patterns (Figure 16). Here the reference state corresponds 

to a load Ph = 35,760 Ibf (159 kN) while the three frames.span a load increase, 

AP of 390 lbf(1.73 kN). The displacement patterns corresponding to this figure 

are shown in Figure 17. Here the displacements are measured with respect to 

displacements in areas remote from the hole edge. The origin of the axes is placed 

at the edge of the hole as indicated. The X and Y axes are scaled while the vertical 

axes representing Aw is l:l. The fringe patterns reveal the displacements to be 

growing faster within the damage zone than the rest of the specimen. Further 

the damage area is also growing. The Corresponding photomicrographs show the 

extent to which delamination cracking occur. Notice the zero fiber breaks as well 

as the large (on the order of a ply thickness) mode I type displacements. These 

observations lead us to beleive. that Delamination Buckling is the mechanism chiefly 

responsible for spreading of the damage to the undamaged areas of the plate. Once 

the delaminated portions reach a sizeable extent an increase in the growth rate is 

noticed. This is shown in the last series of interferograms (Figure 18). In these the 

fringe density within the damage zone is very high,so much so, that the resolution 

of the interferogram within this area is poor. However the manner in which these 

delaminations grow and respond can be deduced by follwing the fringe development 

within the damage area. This will be discussed later in the context of Type A 

specimen. It must be pointed out that playback of the recorded video information 

displays a continuos transition in the fringe data which enables following the damage 
15 

I 

progression un-interrupted as opposed to the limited number of frames displayed 

here. Our discussion is influenced from such a knowledge. The culmination of the 

last growth event is the complete loss of flexural stiffness of each of the delaminated 

portions, leading to catastrophic failure. These findings and conclusions are drawn 

from the consistency with which these failure events occur in all specimen. Figures 

19a through d depicts the same scenario of events leading to failure as described 

(for another specimen with a different geometry). 

Next, we turn to a discussion of post experiment microscopic studies. It was 

felt that the 0' ply damage observed in the preceeding section warranted further 

examination. As such, some specimen were loaded into the post initiation level but 

unloaded prior to catastrophic failure. In particular the specimen whose interferometric results were discussed earlier, was unloaded near its ultimate failure load 

and sectioned for further study. An enlarged view of the black spot appearing in the 

photomicrographs is shown in "A" of Figure 20. Notice that a part of the picture is 

out of focus. This is because the fibers in that region have jutted out of the plane of 

the hole surface (plane of the picture). Examination of similar damage to other Oo 

layers revealed the same type.of jutout failure. It was also of interest to determine 

the extent to which the hole surface damage propagated into the interior of the 

specimen. The laminate was sectioned at different locations as indicated in Figure 

21. Magnified views of the 0" layers at sections "E" and "D" are shown in Figure 20 

& 22. Notice the dark regions showing broken fiber segments scattered at different 

angles. In "D" an undamaged portion of the midplane 0' layer is shown contrasted 

against a damaged portion which clearly indicates a characteristic pattern. Here we 

see a V shaped matrix rich area, within which the fibers have undergone rotations. 

The maximum width of the V shaped zone measures to be approximately 0.3 mm. 

The matrix rich portion in the bottom right hand picture is seen inclined at an 
16 

angle of 65' to the 0' fiber direction. These characteristic patterns of failure have 

collectively been refered to as Fiber kinking in the literature. 

This type of kinking failure has been observed in other studies dealing with 

compressive strengths of laminates (see for eaxample [9]-[ 11)). What still remains 

as a nagging question is the origin of fiber kinking. 

Sectioning beyond cross-section "D" and inspection under the microscope revealed no further damage. Similar 'inspection to other specimen loaded to levels just 

prior to catastrophic failure revealed that the damage zone was contained within 

one hole radius. 

Planform views of the damaged midplane 0' layer as obtained by mechanical 

polishing is shown in (a) of Figure 23. Here we note the matrix rich damage zone 

which measures in width to approximately 70 fiber diameters (0.5 mm). Closer 

examination of the edge of the damage zone revealed that the fibers had undergone 

rotation in the plane of the picture. Thus, in this zone the fibers have not only 

undergone rotations in the XY plane, but also in the XZ plane as iiidicated by the 

sectioning studies earlier. Similar results were obtained from the dcplying study 

which consists of heating the 'damaged samples to about 600' F and removing ply 

by ply to observe internal damage. Shown in Figure 24a is a C-scan of a specimen 

loaded upto about 90% of its failure load. This scan is a planform view of the 

integrated through thickness damage. Here we note the clear resemblence between 

the shape of the damaged area and the portion of the interferometric patterns that 

contain a very high fringe density. 

Type A specimen 

These specimen have a higher number of 0' layers (42%) than Type B. Figure 

25 shows a series of interferograms corresponding to the first instance at which a 
17 

perturbation in the fringe pattern occured. A sharp localised cluster of fringes is 

seen formed at the edge of the hole which spreads very rapidly with a small increase 

in load. It is seen that the area in which the fringes are unresolvable extends out 

to about one hole radius from the hole edge. The corresponding photomicrograph 

is shown in frame 2 of Figure 26 (corresponds to P = 28990 lbf[129 kN]). Here a 

single Oo ply failure remote from the front surface of the specimen is visible. The 

interferometer detects this internal damage. A fringe count of frame 5 in Figure 

25 reveals a w displacement change of approximately 6-10 pm, at the hole edge, 

perpendicular to the direction of applied load. This displacement is on the order of 

a fiber diameter (7 pm). In the last frame of this sequence, the fringe density at the 

hole edge is too high, precluding any possibility of a fringe count. With increasing 

load surface buckle formation on the other edge of the hole approximately at 90' 

to the loading direction is found to occur. This event is shown in the series of 

interferograms that are displayed in Figure 27. The buckle formation occurs very 

rapidly and as indicated in the figure the duration between the last two frames 

is 0.1 seconds. Inspection of the photomicrographs shown in Figure 26 reveal the 

manner in which progressively the remaining zero layers fail with increasing load. A 

picture of the damaged, unloaded specimen is shown in the last frame of Figure 26. 

A similar pattern of failure is displayed by several other specimen and a sequence of 

such photomicrographs is shown in Figure 28. Here the first 0' fiber failure occurs 

at 27890 16j (124 kN) close to the front edge of the specimen and is shown in the 

third frame of the montage of pictures. The fourth frame corresponding to several 

zero ply failure and the formation of a surface buckle is designated the time T=O 

seconds. The subsequent progression of failure leading to catastrophic failure is 

shown in the next series of frames, with the final disintegration of the specimen 

occuring within about one minute subsequent to frame 4. Notice the damage to all 

0' fiber layers and the formation of surface buckles on both surfaces of the specimen. 
18 

I 

In many of the interferometric sequences discussed so far it was found that 

fringe resolvability was poor close to the hole surface. This is because the displayed 

pictures which are shot off of a TV monitor screen, are recorded using a standard 

video camera which is incapable of resolving all of the information stored in the 

thermoplastic plate. Thus, some picture quality is lost in this process. To check the 

regions of high fringe density we modified the recording procedure by placing a beam 

splitter. behind the thermoplastic plate and taking pictures of the interferograms 

using a 35 mrn SLR camera, while still recording on the VCR. A sequence of such 

interferograms showing the final stages of failure of a Type A specimen is shown in 

Figure 29. The fringes on the top half of the pictures display a continuous variation 

in w. Further, the fringe gradient is seen to rapidly decrease away from the hole 

edge. A fringe count of frame 3 reveals a w displacement change at the hole edge of 

10 pm. These pictures display a superior resolution. However,to adopt this method 

for the entire duration of a test would be cost ineffective. What is important here, 

is to compare the fringe patterns, to those taken off of the TV monitor. It is seen 

that the areas in which the fringes were unresolvable earlier could be smoothly 

extrapolated from the areas in which the fringes are resolvable. We also see that a 

surface buckle is already formed on the bottom half of the specimen. This damage is 

contained within a distance of approximately half a radius of the hole. The sequence 

1-4 depicts the formation of the bucklc on the top half of the specimen which is 

found to be the last event prior to catastrophic failure. The pictures were taken 

with an exposure time of approximately 1 second witli the duration between frames 

approximately 2-3 seconds. 

Bearing in mind the information obtained from the surfaces of the specimen, 

we now turn to a discussion of post experiment sectioning studies. Shown in Figure 

24b is a C-scan of the lower half of a typical damaged specimen. Notice the extent 
19 

three orthogonal directions were set in a Graphite matrix and subjected to localised 

impact. Post-experimental examination revealed typical kink orientation angles a 

t of 45O - 60' and kink boundary orientation /3 obeying /3 M 5 for broad(: 2 0.1) 

of the damage area which is contained within about a hole radius. In Figure 30a 

I 

which fall into the category $ 2 0.1 indicate p = 25' - 30' and CY NN 15" - 30". I 

I However, no definite trend between CY and p can be established. One difference in , 

the present case is that the specimen are loaded well into a post damaged state as 

opposed to being unloaded just after initial damage. I 

1 

It was seen earlier that the outermost layers of the laminate that underwent , 

large displacements in the buckling process suffered fiber breaks. A magnified view I 

of such a fiber break is shown in Figure 3O(c). It was of interest to examine the 

propagation of the initial fiber kinking failure into the interior of the specimen. 

This was done by sectioning at several positions and following the damage of a 

typical Oo layer. The results thus obtained are shown in Figure 32. Notice that the 

appearance of the kink is unaltered at several different cross sections. Further, the 
20 

same type of geometry as found before is seen to prevail with an area of undamaged 

fibers between two inclined bands of damaged fibers which have undergone rotation 

in the XZ plane. The lengths of the fibers in the damage zone are comparable to 

those seen before. A planform view of such a damaged 0' layer is shown in Figure 

23b and c. The damage zone width is approximately 0.27 mm (38 fiber diameters). 

Here we note the fiber jutout failure at the edge of the hole. The damage zone 

appears dark as the fibers in this area which are broken and kinked out of plane 

were polished off during preparation of the specimen for microscopic examination. 

Finally in Figure 33 we show the outer layer delamination crack at two different 

cross-sections. 

Summary of common featurcs of failure 

In this section we summarise the results presented so far and try to identify 

features that are common to both types of laminates. 

In both types the failure is initiated by a 0' fiber failure at the hole edge 

approximately at right angles to the loading direction. In type A, with increasing 

load more Oo fiber failure is seen to occur with simultaneous surface delamination 

buckling. These events occur very close to the catastrophic failure load. In type B, 

subsequent to O'? fiber failure which occurs at around 75% of the ultimate failure 

load, extensive delamination cracking is observed, with the outermost delaminated 

sections undergoing buckling. Photomicrographs typical of these two types are 

contrasted in Figure 34. In both typcs the delaminations are seen to grow with 

increasing load. Delamination Buckling is found to be the mechanism responsible for 

the spreading of the damage. When the delaminated portions grow to a critical size, 

approximately on the order of one hole radius the specimen fails catastrophically. 
21 

I 

The mechanics of the 0' fiber failure, which shows up at the hole edge and captured by the interferometer is found to be a localised microbuckling/fiber kinking. 

The question to be addressed is the process by which such kinks form. Information 

regarding fiber kinking published elsewhere and at present have been obtained once 

the damage has taken place and the structure loaded into a post-damaged state. 

Qualitative information regarding the pre damaged state, from the interior of the 

structure, is dilficult to obtain as these systems are opaque. However, the present 

experimental results lead us to beleive that a possible mechanism culminating in 

fiber kinking is elastic buckling of the fillers. 

Damage patterns typical of those observed so far are shown in Figure 35. A 

3-D view of the damage to a Oo ply is summarised in Figure 36. This 0' ply 

microbuckling/kinking which occurs both, in plane (XU) as well as out of plane 

(XZ), is found to persist well into the interior of the specimen. It is conceivable 

then, that this type of failure originates at the edge of the hole and propagates 

into the interior of the specimen, causing a narrow zone within the 0' plies to loose 

structural integrity. Continuous viewing of the recorded videotapes, shows the 

surface manifestation of this event as a well defined cluster of fringes originating 

at the hole edge and propagating rapidly away from the hole. The origination of 

fiber kinking at a free edge and its subsequent propagation into the interior of the 

structure has been reported before. We cite here the of work of Chaplin 191 and 

Hahn et a1 [ll]. 
22 

1.4 Conclusions 

In an attempt to understand the mechanisms responsible for the initiation and 

spreading of damage in composite laminates containing a cutout we have carried 

out a detailed experimental investigation and presented qualitative and quantitative 

results. The damage is found to initiate by a combination of Fiber microbuckling/ 

kinking. Delamination Buckling is found to be the mechanism by which the damage 

spreads away from the hole at 90' to the loading direction. When the delaminations reach a critical state the plate fails catastrophically. These events which are 

consistently observed, can form the basis for the formulation of simple mechanics 

models to highlight the various stages of failure. 
23 

I 

Acknowledgements 

The authors appreciate the support and interest of Dr. James Starnes. jr of 

NASA Langley research center. 

I 
24 

1.5 References 

1). Starnes J. and Williams J.G. - Failure characteristics of Gr/Epoxy struc 

tural components loaded in compression . NASA TM 84552, Sept. '82. 

2). Rhodes M., Mikulas M. and McGowan P. - Effect of orthotropic properties 

and panel width on the compression strength of Gr/Epoxy laminates with holes, 

AIAA paper No. 82-0749. 

3). Shuart M.J and Williams J.G. - Compression behaviour of [+45/-451 dominated laminates with a circular hole or impact damage . AIAA journal, vol. 24, 

No. 1, p 115, Jan '86. 

4). Knauss J., Starnes J. and Henneke E. - The compressive failure of Gr/Epoxy 

plates with circular holes . NASA CR 157115, N78-24295, 1978. 

5). Mikulas M. - Failure prediction techniques for compression loaded composite laminates . NASA CY 2142, 1980. 

6). Babcock C.D. and Waa. A.M. - Effect of stress concentrations in composite structures . GALCIT SM Report 85-12. 

7). Rosen B.W. - Mechanics of composite strengthening ., Fiber composite 

materials, Am.soc. for Metals seminar, 1965. 

8). Schuerch €1. - Prediction of compressive strength in uni-axial Boron fiber 

-metal matrix composite materials , AIAA journal, Jan. 1966. 

9). Chaplin C.R. - Compressive fracture in uni-directional glass reinforced 

plastics. , J. of Mat.Science, 12, p347-352, 1977. 

10). Evans A. and Adler A. - Kinking as a mode of structural degradation in 

Carbon fiber composites ,Acta Metallurgica, vol. 26, pp725-738, 1977. 

11). Hahn H. and Williams J.G. - Compression failure mechanisms in unidirectional composites , NASA TM 85834, Aug 1984. 
25 

12.) Budiansky B. - Micromechanics , Computers @ Structures, vol. 16, No.1-4, 

p3-12, 1983. 

13.) Chai H. - Growth of impact damage in compressively loaded laminates, 

P.hD thesis, Caltech, 1982 

14.) Vest C.M. - Holographic Interferometry , Wiley, 1979. 
2 

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Figure 1. Surnmary of test results for T3OO/BP 907, T300/5208 

composite plates. w = 13 cm. (from Ref. 1) 

ORIGINAL EAGZ i2 

OF POOR QUALITY 

Figure 3. Typical set of edge stiffeners 
a 

b 

Figure 4. (a) Testing machine on optical table 

(b) Servo-valve and accessories 
ORIGINAL PAGE 1s 

OF POOR QUALIM 

a 

b 

Figure 5. (a) Specimen inserted in holders 

(b) Ball joint arrangement 
b. 

*CHARGING 

++++++++++++ - THE- 

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Auniform -8 a-m the th.nnoglostr and @uXe 

ductor. 

d. 

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++++++++++++ 

---- 

.- 

Hasting of thermoo(8shc ' caw rmnnanent de(om\ahon. The 

imqulanties mH &tfract light to remeate the ongrnd ~maga. 

ttposure to laser fringe pattam redistnbutsr the chrpes through 

the photaconductor. 

ERASURE 

0. 

SECOND CHARGING 

+++++4++++++ 

++++ +.I.++ 

C. 

------------ 

%charge increases the dectnc field across the exposed area d 

the thermmastr. 

Ensure IS done by controtted healing. 

Figure 6. Process of creating o surface- relief hologram 

on a thermoplastic plate 
a node 

at 4Kv 

1 

corona 

cy1 i nder 

1 

movement 

of corona 

cy1 i nder 

Figure 7. The coronatron deposits a uniform charge on the 

thermoplastic plate 

Thermopl a s t i c 

Photoconductor 

Indium-Tin-Oxide 

transparent 

el ec trode 

Quartz substrate 

Figure 8. Detailed diagram of thermoplastic plate, showing 

quartz substrate and transparent electrode 
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TF - DURATION OF TEST 

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in terferog ro ms 
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Direction 

Direction 

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5 Sensitivity Vector 

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w Component Of Displacement 

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n Fringe Number 

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Figurell. Details of interferometer 
----- TI 

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TV 

YQN I TOR. 
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/ 

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Portion of hole surface in photomicrographs 

Figure 13. Photomicrography 
P = 27,705 Ibf 27,815 

Ph = 27,700 

Figure 14. Initiation of damage. 

(Spec. TB 1 1 ) 

27,960 
P = 1 5,0 0 0 Ibf 

36,98 0 

27,700 

35,770 

37100 

Figure 15. Hole surface photomicrographs 

(spec. TB Ill 
P = 35,770 Ibf 35,980 

P, = 35,760 

Figure 16. Buckling and spreading 

of delaminated portions, (Spec. TB 1 1) 

36,150 
t 

SCALE 1:0.37 

txav axes) 

I 

Figurel7. Summary of displacement 

corresponding to Fig.16 
ORIGINAL PAGE ?!3 

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Figure 21. Post-experiment microscopic examination (TB 11) 
I 

.E 

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cv 

0Rffi;ONAE PRGE 1s 

OF: P08R QUALITY 

m 

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0 

aa 

a 

cn 
Figure23 Planform view of internal damage to 0' layers. 

almid plane 0' layer (TB11). b) second from bottom 0' layer (TB17). 

c) inset of (b). 

b 

C 
TB16 

TB15 

I load direction-' x 

Figure 24. C-scan of damaged specimens 
0 

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(ref. 
1 

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Figure 33. Sectional study indicating surface 

delamination (spec. TB 17) 
13765 Ibf 27050 

27800 15710 

16155 

28250 

18265 

28350 

19160 

28400 

20000 

28450 

20250 

.125crn 

TB13 

Figure 34. Sequential photomicrographs of hole surface 

exhibiting damage development. 
a 

b 

Figure 35. Forms of internal damage. 

33mm 

17mm 
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