N O T I C E 

THIS DOCUMENT HAS BEEN REPRODUCED FROM 

MICROFICHE. ALTHOUGH IT IS RECOGNIZED THAT 

CERTAIN PORTIONS ARE ILLEGIBLE, IT IS BEING RELEASED 

IN THE INTEREST OF MAKING AVAILABLE AS MUCH 

INFORMATION AS POSSIBLE 
GRADuATEAERONAUTICAL LABORATOPJ-ES

CALIFORNIA INSTITUTE OF TECHNOLOGY

N

( NA SA-CB- 1764 10) EFFECT CF CG HCENTB ITIONS IN C,CAFCSITE

Progress Report (California

47 p PC A03/Ht 101

DEC 10

RECEIVED.

*3A Sn FAQ+

. 0% -

SIRiss

STRUCTURES

Icst. of Test► ,)

CSCL llu

N66-1`_348

Uucla as

G3/24 04988

Firestone Flight Sciences Laboratory

Guggenheim Aeronautical Laboratory

Karman Laborator,.- of Fluid Mechanics and Jet Propulsion

Pasadena
SM REPORT 85-12

EFFECT OF STRESS CONCENTRATIONS

IN CONPOSITE STRUCTURES

by

C.D Babcock' and A . M. Wass**

Graduate Aeronautical Laboratories

California Institute of Technology

Pasadena, CA 91125

Progress Report

NASA Grant: NSG 1483

November 1985

C -

1. INTRODUCTION

^. Composite structures have found wide use in many engineering fields

and a sound understanding of their response under load is important to

their utilization. An experimental program is being carried out to gain a

fundamental understanding of the failure mechanics of multilayered composite structures at GALCIT. As a part of this continuing study, we are

interested in assessing the performance of laminated composite plates in

the presence of a stress gradient and the failure of composite structures

at points of thickness discontinuity. In particular we address the questions of initiation of failure and its subsequent growth to complete

' Professor of Aeronautics and Applied Mechanics I

'• Graduate Research Assistant

9

WAPd
1 ^

2

failure of the structure.

2. FAILURE INITIATION AND PROPAGATION IN THE PRESENCE OF A

STRESS GRADIENT AND A FREE EDGE

1

, t For the experimental program studying the mechanisms of failure initiation and propagation it was desirable ' to know where the failure is most

likely to initiate. For this reason a flat plate with a hole was chosen

as a test structure. The hole is carefully cut in order to insure that

excessive local damage does not occur during the machining process (Ref.

1). Compression loading is applied using a hydraulic Jack.

The loading device and the optical components tilat are used in the

experiments are mounted on a 5 ' x7' optical table with pneumatic vibration

Isolation. As reported earlier, Holographic Interferometry is used to

capture the deformation occv. rring around the hole. One difficulty we

encountered was the choice of a suitable recording medium for the inter- 1

i

ferograms. Real time studies were difficult due to the weL chemical

processes involved in the developing of the photographic glass plates.

Moreover, the time consumed between the exposures was excessive. We have

succesfully circumvented this difficulty by purchasing a newly developed

thermoplastic instant recording device ( manufactured by Honeywell Corp.

with NRC as the marketing agent). A brief description of this follows.

2.1. The Instant Recording Device.

In the reconstruction process of a hologram the conventional silver

halide photographic plates cause diffraction via an amplitude gating

Which is representative of the spatial distribution of the light ( amplitude and phase) that is emitted from the diffusely reflective surface of

the object. However, the same task could be accomplished with a hp aseg ting i.e.. ra a surface contour variation which is representative of the

reflected light off the object. This is the basic principle behind the
71, * %yrs -

- 3 -

success of the thermoplastic recording device. The device has several

F stages of oper-tion as shown in Fig.i. Steps a through d require less

than 1 minute - a considerable saving of time. Furthermore. with real

E 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 the creation of a hologram with this device.

Step 1 '

i

A uniform charge is deposited on the plate as shown in Fig. la. A

} movable coronation deposits this charge. A corresponding negative charge

forms in the transparent electrical conductor which is connected to

- ground.

a

_.

r

Step 2.

The plate is exposed. Charge passes through the photo conductor only

In the illuminated areas (Fig. lb). j

Stec 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 (Fig. ld). This creates the surface contour variation which causes the diffraction of light in the
- 4 -

reconstruction stage.

The coronatron is a device that consists of a :.,re anode running down

the center of a slotted cylinder. The slot is oriented to face the thermoplastic plate. Ve slotted cylinder and the plate are grounded and the

&-I 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 (Fig. 2).

The detailed structure of the thermoplastic plate is shown in Fig. 3.

On a quartz substrate, an optically transparent electrode of indium-tin

oxide is deposited on top of which a 1 ga thick layer of trinitrofluorenone doped photoconductive organic polymer is placed and finally a

0.7 an 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 th,!trmoplastic.

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

supplied with the recording device Dry Nitrogen is used as the coolant

lor the thermoplastic plate in the erase process. This consists of re- s;}

heating the thermoplastic to relieve it of the surface contour variation

to produce a ripple-free surface. Upon cooling, the plate is ready to be

used again in the formation of a hologram.

2.2. Specimens.

Test specimen are cut from the panels remaining from the impact

delamination study. Diasond impregnated core drills and saws are used for

cutting purposes. The geometry of the specimen is shown in Figure 4. The

dimensions of the test specimens are chosen to induce failure due to the

stress concentration effect prior to a global buckling failure of the

plate. A number of specimen have been =ed to failure. The complete
- 5 -

history of 2 of the?- has been recorded in the form of interferograms and

will be discussed in detail. The plate fiber/matrix system is T300/5208 .A I

and are 48-ply with a stacking sequence (+ 45/90 2 /+ 45/902 /+ 45/90/0)2s

(angles are measured from the load axis). They are fabricated from unidirectional tape of Thornel 300 graphite fibers pre-impregnated with

either matrix. The resin volume fraction and total laminate thickness are

28% and 6.1 an for T300/5208 and 38% and 6.7 mm for T300/BP907 respectively.

Several of the T3CO/5208 specimens have been tested. Of the 2 specimen for which complete histories were recorded, one was tested with the

stacking sequence given above. The other panel was cut with the load axis

900 to the Indicated lay up.

2.3. Experimental Procedure.

A schematic of the holographic interferometer used in this study is

shown in Figure 5. It is a standard off axis holographic system. Apart

from a few double exposure holograms it is used for real time observations. 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 S 1 (say) with coherent light transmitted from the laser via the

object wave path (this consists of the beam splitter, a beam expanderspatial filter, collimating lens). A hologram is then recorded by exposing the thermoplastic plate positioned at the junction of the reflected

light and reference beam. Upon completion of the 1st exposure the specimen is loaded and viewed through a camera attached to a VHS recording system. (a VCR and a monitor). The load is slowly increased, which, results

in fringes which are dynamical in nature, since the load is continuously

varying. 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

....._ _. ,. 1 

:eti.

ti
r:

nA = k.d (1)

- 6 -

hole in the plane of the test specimen. Thus a typical frame would look

like that displayed in Figure 6. Wen the fringe density becomes high,

the resolution of the interferogram becomes poorer. 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.

Each of the hologre is interference fringe field is related to the

change in s^irface displacement of the specimen under load, by the vector

expressica

A

where

n fringe number

A wavelength of the coherent

laser light

k sensitivity vector (12-11)

i 1 and 1 2 are the unit vectors in the illumination and observation direction respectively and d is the surface displacement vector at the point of

observation of the specimen. In the present experimental setup the diffusely reflective surface of the specimen is oriented normal to tae bisector of the angle 2a, between 1 1 and 1 2 (Figure 7). Thus, since the sensitivity vector k, also points along this bisector, the interferometer

senses only the out of plane displacement component, w. Equation (1) then

becomes

nX - w2cosa

w nX

2cosa (2)

6W

0
LV Pr'. r wr'= rr7- _

- 7 -

In the present experimental setup, the surface normal is loo deg.

off the observation direction. With A - 0.633 An. this shows that each

fringe represents an out of plane displs-3ement change of 0.321 us.

A schematic of the present expeA ntitbl setup is shown in Pig. 8.

r

Using the newly incorporatea recording devi ,-r we have studied the failure

r

of 2 specimens so far. A considerable amount of experience has been

t gained with a view toward improving further the experimental setup. One

outstanding prublem we are presently faced with is the application of

load, which we accomplish with the aid of a hand pump to drive the actuator of the loading device. Incorporating a servohydraulic loading system

is currently being pursued. Once a test is completed, the recorded interferometric information is replayed and photographs are made from the

screen of the TV monitor using a standard 35 mm camera.

2.4. Results and Discussion.

In order to interpret the holographic fringe data presented in this

section it is important to understand the procedure by which the photographs are generated. The test specimen is loaded to the reference load Po

and the valves of the hydraulic system are closed. The hologram is

exposed and developed. The valves of the system are opened and the load

is increased until the fringe density becomes too great for proper viewing. These steps are then repeated. The difficulty with this procedure

is that while the hologram is being produced and the valves of the

hydraulic system are closed, the load tends to drop off. This is due to

either leakage in the hydraulic system or creep of the test specimen.

This is the reason we have decided to replace the hand pump by a servocontrolled loading mechanism which would ensure a more controlled loading.

Since there may be a load drop between making a hologram and viewing it in

f 

some of the interferometric data presented the reference load state is

higher than the load state at which the interferogram was made. One of

our main objectives 's to understand the spreading of the damage that initiates at the edge of the hole. Thus, this minor drasback in the loading

J
WIT-77 777'.

- 8 -

device should not deter us f:o n extracting some important features that we

have captured so far.

For the first sequence of events ( i.e. specimen with dimensions 5"x5"

see Figure 9). Figure 9b shows the first instance at which a delamination

was detected. A delamination appears as a cluster of fringes on the hole

sI!

edge. Notice that there are some extraneous fringes approximately 3 in

number representing a displacement change of 0.963 μm. These are due to a

rigid body motion of the specimen coupled with some slight bending. However, these are not be mistaken with the fringes resulting from an initiated delamination.

The rigid body motion or/and slight 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 must realize that the

sensitivity of the technique, which is a function of the wavelength of the

laser light, is extremely high (displacement changes as small as 0.316 gm

can be detected).

Figure 9b-p shows sequentially the growth and subsequent spreading of

the initiated delamination. The initiation occurs at the hole edge at

approximately 90 to the load direction and subsequently spreads, with

increasing load. Notice the distortion of the fringes around the hole

edge. Figure 9p contains a high density of extraneous fringes thus making

it difficult to extract any qualitative features regarding the delaminated

region. However, this interferogram was included for comparison with Figure 9q which shows the change in fringe pattern with delamination spreading just prior to final failure. Figures 9p and 9q have the same reference load while "q", which is subsequent to "p", demonstrates an area with

an extremely high fringe density, so mtech so that the fringes cannot be

.t .

resolved. This area then has an out-of - plane displacement change which is

much higher than the rest of the plate. Note that in Figure 9p,

P s =1.014 P f , indicating a higher load than the failure load PF . This is

because PF is the catastrophic failure load. The maximum load recorded

f

.A

^'r
kv 1

V

• t

- 9 -

during the experiment occurred prior to catastrophic failure.

..

In the second sequence of interferograms ( specimen dimension 3"x3",

see Figure 10) the first sign of initiation is seen in Figure 10a. With

Increasing load the delamination does not show signs of spreading as in

the previous case. Instead, we see clearly a symmetric pattern of " delamination spots" around the hole contour (Figures 10a - p). We must point out

that, although the interferograms presented in sequence show very little

evidence of any difference in the disp .laceant field around the hole edge, {

viewing the fringes in real time makes it such easier to capture the

points at which delamination occur. Thus our discussion is motivated from

such a knowledge. In the first test, we recorded an initiation load P I of

38,916 lbf while the failure load P = 44,476 lbf. In the second test we

noticed P I = 24,254 lbf and P F = 31,724 lbf. In both cases initiation

occurs at 75 -87% of the ultimate failure load of the specimen. Notice the

difference in the initiation points around the hole contour for the different specimens. While we are not able to offer any explanation for this

difference we note the difference in specimen dimensions and more importantly the directional properties.

Summarizing we see that the behavior of the plate can be broken down

into a sequence of events. Firstly, when the applied load reaches 75-87%

of the catastrophic failure load P F, a delamination initiates. Then, with

:icreasing load the delamination spreads around the hole edge, away from

the direction of load application. Upon reaching a critical delaminated

size the delaminated portion buckles. This is followed by catastrophic

failure of the specimen. Notice that we have only addressed the question

of failure by delamination. As reported elsewhere (2) other failure

mechanism, such as matrix shearing, is not addressed.

r

S

^- 3

r ,^

^* -ids"---^-.'
(9

w

- 10 -

2.5. Relation of Analytical Developments to Experimental.

Observations

The problem of initiation of delamination in laminated composites has

received a lot of attention in th 4t literature. Recently, O'Brien-Ra,)u [3]

addressed the question of delamination initiation around a circular hole.

However, the model, originally developed for straight-edge delamination

growth, predicted onset strains about 30% higher than, those experimer.-

tally observed. Wang and Choi [4) further extended the earlier work of

Pipes and Pagano [S] and found analytically that stress singularities

occur along straight free edges. Using Wang's results, Ericsson et al.

I [6] attempted to predict onset strains for [0/90] s laminates with a circular hole. The analysis used total strain energy release rate (G) as

opposed to separating G into its components G I ,G II , and GIII' Many other

researchers have attempted to obtain the complete 3D stress field using

the finite element method (see ref. 7 for example).

The difficulty in obtaining a satisfactory solution to the complete

r

3D stress field in composites lies in the inherent singularity that is f

introduced in modelling the laminate as consisting of discrete layers of

different material. The usefulness of the singular stress field in

predicting delamination initiation has not been established. The initiation problem is also complex, since many parameters enter into the problem. A satisfactory solution to the intiation problem arou nd circular i

cutouts must display the observed experimental findings. In particular,

the position around the hole edge at which initiation occurs, the interface associated with the delamination, and the load level at initiation

must be satisfactorily predicted.

.^ Once the delamination initiates it spreads around the hole edge away

k,.

fronthe directior of load appl ! cation. It is felt that the principal

F

mechanism of spreading is by delamination buckling, a phenomenon observed

i earier [8] in the spreading of impact damage. However, more experimental

s

evidence is needed to substantiate this claim. A simple analysis based on
4

- 11 -

W!

an energy method is being developed to calculate the bucking load of an

arbitrarily located delamination at the hole edge. Details of this

analysis will be available in the future.

2.6. Conclusions.

Holographic interferograms showing the behavior of laminated composites containing a circular cutout under inplane compression have been

presented. From these observations the behavior of the composite is seen

to consist of delamination initiation at the edge of the hole, followed by

spreadYng of the delaminated area leadirI to catastrophic failure. The

applied load at which initiation occurs is about 80% of the catastrophic

failure load P 

3. DELAMINATION AT A THICKNESS DISCONTINUITY

Experimental and analytical work have continued on the problem of

determining the conditions that govern failure at a thickness discontinuity. The experiments are carried out using d simulated

stiffener/panel joint shown in Figure 11. A photograph of the test specimen is shown in Figure 12. Previous work on this problem concentrated on

the so called "macro failure" conditions. Teats were carried out to

determine the axial force, Nx , failure condition and the bending moment

transverse shear, Mx /Vx , failure condition. It was found that the axial

force required to cause failure was so high that its role in the failure

of the joint could be assumed to be negligible. The bending/transverse

shear tests showed that the moment was the dominate force in the failure

condition. Additional failure tests have been carried out as described in

the next section. An analysis of the test configuration has been

developed and is described in Section 3.2.

A
- 12 -

3.1. Experiments.

A new test was developed that more closely simulates 'lie ,onditions

that occur in a post buckled stiffened panel (ref. 1). This test consists

of using the previously described specimen in a column buckling test. In

the test, the column is buckled and loaded into the postbuckled range.

The loads in the test specimen, shear, axial, and bending moment, are

found from the applied load and the configuration of the buckled column.

3.1.1. Description of Tests. The specimen ends are ground to form a Vshape and are supported on V-notched blocks. This simulates simply supported end conditions. A back plate with a tracing paper is kept adjacent

to the specimen (Fig. 13) and at various load levels the specimen configuration is traced with a pencil. This curve is used to obtain the

deflection and slope of the specimen as a function of load which enables

calculati,ig the bending moment and shear force. This procedure is deemed

satisfactory by comparing the results obtained with the analv4is presented

In Section 3.2 which shows good agreement.

3.1.2. Results. In all of the tests initiation of failure in the form of

edge cracks appeared at the point of maximum moment. Post experiment

examination of the specimen revealed edge cracks between layer 2 and 3

(Figure 14). These delaminations are due to the interlamirar stresses

that develop at the free edge. These stresses have been shown to be

singular for the idealized problem at the free edge (Ref. 4). It is

worthwhile noting that experiments on laminates under tensile loading

(Ref. 9) have exhibited a similar failure initiation mechanise. In all of

the specimens, these edge cracks grew in the axial direction, but in some

specimens failure occurred by rupture at the point of maximum moment. In

two of the specimen, the failure occurred by stiffener separation. Figures 15 and 16 show the two types cf failure observed in the experiments.

The results obtained in the stiffener separation case are plotted along

with results from previous experimens (Ref. 1) in Figure 17.

(^).
- 13 -

3.2. Analysis.

An analysis of the buckling and postbuckling of the simulated joint

test specimen was carried out in order to help in evaluation of the experimental data. The usual assumptions of beam theory were made with the

properties of the teat specimen determined from independent tests. The

following sections describe the two analyses.

Buckling

The coordinate systems and the nomenclature used for the buckling

- analysis is shown in Figure 18. It is assumed that at the ends of the

column there is no moment. This is simulated in the experiment by the

knife edge supports. However, due to the eccentricity of the neutrzl axes

of the two parts of the column, there is a shear force at the e.ids o. the

column. This is easily found from overall equilibrium. With the end

forces and moments known, the column buckling equations can be written in

the second derivative form. These equations are given as follows:

2

(EI)1d 2= - lexl - Pv1

dx1

2

(E I) 

2 dx

2 = + 1 x2 - Pv2

2

The solution of these equations is straight forward, with the results as

giver, below.

v1 - AsinA l x i + BcosA l x l - 1e 1 . Ai = P(EI)1

.r

v2 = CsinA 2x2 + DcosA 2 x2 - 1e 2 , A2 = P( EI)2

f

I

10

- ^_ ____ter
i4 -

The four constants introduced in the solutions are found by the displacement conditions at the ends of the column and the matching of slope and

displacement at the thickness step. These conditions and the resulting

equations are given below.

v1 (o) = 0

v 1 (o) = 0

v1 (al) = v2 (1 - al)

dvldv2

dx (al) _ - dx (1 - al)

1 2

( 1

sinX 1 la -sinX21(1 - a) i (A1 Iea21

t f

li^ l cosX l la 1A 2cosX 2 1 ( 1 - a) ^Bj = Iae I

I 1

The buckling load is found from the equation developed by setting the

determinate of the coefficients equal to zero. This leads to the following transcendental equation which was solved numerically for the particular experimental conditions.

(1 ,\ 2 )sin(X l la)cos(X 2 1)(1-a) + (1,\ 1 )cos(,\ l la)sin(X 2 1)(1-a) = 0

The results for the buckling experiments and the analysis are shown

in Table 1. The comparison is satisfactory and lends confidence to both

the experiment and the analysis.

Post Suckling

The postbuckling analysis is carried out in a manner similar to

Euler ' s Elastic& problem ( Ref. 10). As in the previous analysis the

column is analyzed for the two sections where ET is constant and matching

111
15 -

enforced at the step. The coordinate systems and the nomenclature are

shown in Figure 19.

Starting with Section 2, the moment at any point can be written as

Pe ( _ dB M _ -x2 1 + Pv2 -(EI) EI)2

- R 2 ds2

where the expression for curvature, R, is no longer approximated as in the

previous analysis. f

A

F

Differentiating this expression with respect to s 2 and using the

relations between v2 , 3 2 , and 0 we obtain:

2

ds2 + ^

2 sin9 = X2 (1 cos8

2

dv 

2 sing , ds2 = cos6, X

2 P

ds 2 (EI) . 2 2 2

i

This is the nonlinear differential equation for the right section of the

column. Multiplying by 2d9 and integrating, the following equation is

obtained.

2

I 

ds2

2 ( -A-8 ) ds 2 + 2X2 I sinOd9 - 2X2 ii) sOdO I co

2

or

(_! ) 2 - 2X2cose - 2A 2 ( 1 )sine - C ds

a 2

The constant C is deter~=ned by using the condition at 0-#, dO30 since the

moment is equal to zero Rt the end.
-0 -V/ 0

sin9d9

v2 (1 2 ) I dv2 = I sin9ds 2 = I

0 4 -0\12X2 \Icos9-coso

These expressions may be integrated to give

F -.

- 16

( _00 ) 2 - 2X 2 (cos9 - coso) + 2X 2 ( e )(sin9 - sino)

ds2

2 1

or

ds _ _ d8

2

\I2X '(cost-cosO)+e (sin9-sinO)

'. 2\1 1

where the negative sign has been chosen for the square root since 9

decreases as s 2 increases. Assuming that e/1 is a small quantity, an

integration over the length 1 2 =1-a1 can be performed. In this integration

theta goes from to -W where the angle definitions are positive as given

s

_ in Figure 19.

1 = 1

1

d9

2

\12X 2 -y+ \Icos9-cosh (1j

A similar expression can be obtained for 11=al

11 1

I

d9

\12X1 ;P \Icos9-cosy (2)

The displacements at the step can be found using some of the previously

derived expressions. The compatibility condition produces the following

result.

0 w

y sin9d9

v1 (1 1 ) = Idv1Isin9dsl

I _

'Y y O\I2X1\Icos9-cosy
- 17 -

L 1 2 _ (EI) 2 _ cosy-costi

( X 2 ) (EI)1 cosp-coso (3)

Equations (1),(2), and (3) constitute a set of three equations for the

four unknowns p,y,W,p. In order to solve these equations one unknown must

be choosen. In using this analysis for comparison it was decided to use $

as the control variable. This was matched to the experiment at the time

of failure. The other quantities could then be calculated from the equations developed in the analysis. Some additional rearrangements of these

equations were carried out to eliminate the load P from the equations and

to put the elliptic integrals in proper form. Once this was carried out

the equations were solved numerically for the unknown angles and the load

then determined from equation (1) or (2).

The results of this calculation are given in Table II as well as the

comparative experimental quantities. In addition, the displacement at the

step is calculated and compared to the experiment. i

The agreement between the experiment and the analysis is quite good

for this type of problem. This lends confidence to the experimental procedure and to the results for shear and bending moment at failure that are

calculated from three experimentally measured quantities.

3.3. Conclusions.

The results of the column buckling tests were tainted by the edge

delaminations that occurred prior to stiffener separation. Two of the

test points were considered reliable and extended the macro failure

envelope somewhat (Fig. 17)-. Continuing to decrease the length of the

column would extend the envelope only slightly. Therefore these tests

have been discontinued. Other simple tests to extend that failure

envelope have not been devised. Future wor L,

this problem will concentrate on plate buckling tests.

F ^ ^
r 

\^ ­^ -.-t.. _ __ _

18 -

4. REFERENCES

1. Babcock, C.D. and Knauss, W.G., "Effect of Stress Concentrations in

Composite Structures," Progress Report, SM 84-17. October 1984.

2. Shuart, M.J. and Williams, J.G., "Compression Failure Characteristics

of t 450 dominated Laminates with a Circular Hole or Impact Damage,"

25th SDSM Conference Proceedings, Pals Springs, California, May 14-

16, 1984.

3. O'Brien, T.R. and Raju, S . S., "Strain Energy Release Rate Analysis of

Delamination Around an Open Hole in composite Laminates," 25th SDSM

Conference Proceedings, Pals Springs, California, May 14 - 16, 1984.

4. Wang, S.S. and Choi, I., "Boundary Layer Effects in Composite Laminates," Parts I 8 II, Journal of Applied Mechanics, Vol 49, Sept. }

1982.

5. Pipes, R.B. and Pagano. N.J., "Interlaminar Stresses in Composite

Laminates Under Uniforr Axial Extension," Journal of Composite

Materials, Vol. 4, 1971. p. 538-548.

6. Ericson, K., Persso.:. M., Carlsson, L., Gustaysson, A., "On the Prediction of the Initiation of Delamination in a [0 / 90] 8 and [ 90/0]8,

Laminate with a Central Circular Hole," Composites Technology Review,

Vol. 4, No. 4, Winter 1982, pp. 116-124.

7. Carlsson, L., "Interlaminar Stresses at a Hole in a Composite Member

Subjected to in Plane Loading," Journal of Composite Materials, Vol.

17, May 1983.

,i

i{

1^ r
- 19 -

8. Chai, H., Babcock, C.D., "Two-Dimensional Modelling of Composite

Failure in Delaminated Laminates," J. of Composite Materials, Vol.

19, Jan. 1985.

9. O'Brien, T.K. "Characterization of Delamination Onset and Growth in a

i r Composite Laminate," ASTM STP 775, 1982, pp. 140-167.

10. Timoshentto, S.P., Gere, J.M., "Theory of Elastic Stability," McGraw

Hill, 1961.

t

I i...il

o^j'l
1

1

- 20 -

Table 1.

Buckling Loads of Test Specimens

(EI) 1 ) 559 lbin2 . (EI) 2 = 232 lbin2

Specimen 1(in) a Buckling Load (lb)

s Experiment Analysis

1 8.00 0.38 38 42

2 9.00 0.33 33 32

3 7.50 0.40 46 49

4 7.25 0.41 47 53

5 7.00 0.43 59 58

6 6.75 0.44 65 63

IL .

Y

o.1
\e./

- 21 -

Table 2.

Condition at Failure of the Test Specimens

Specimen ¢ 0 y P v2(al)

E E A A E

1 54 29 28 40 44 46 1.81 1.73

2 66 42 39 51 37 37 2.25 2.15

3 59 29 27 43 55 55 1.81 1.79

4 60 27 26 43 51 60 1.94 1.77

5 50 21 20 35 66 63 1.44 1.48

6 41 16 15 29 71 67 1.25 1.21

Angles in Degrees, E - Experiment, A - Analysis

It 
k,.

1

(MIGINAL PAG_

OF POOR QUALITY

10 M

r;

•C*wUMNG

++++

a.

TH&MOPLASTTC -

^— Mg0T00DwXCrM

A u. Won. charge is I poritb on the thennopiestic and photoconductor. DEVELOPMENT

d.

EXPOSURE 

LOW

Flestng of ^10Q^ Tc cOueeearrn panev dskxff lorL TAe at

LOW 4Tegtrlaht ws wW diffract light to recreate the oripinN wnaa. g

II

.

 I 

t++++t+t++t+

b.

Exposure to laser hinge pattern rediatributes the charges tt eugh

the photoconductor.

ERASURE 1

SECOND CHARGI NG Erasure is done by conwAw heetwg.

C.

+ttt^++++f t+

Recharge increwws the oNK:vx field across the exposed area of

the therrnopiastorFigure 1. Process of creating a surface-relief hologram on a

thermoplastic plate.
Photoconductor

Indium-Tin-Oxide

transparent

electrode

quartz substrate

iA

U lIl

.

n

corona

cylinder

movement

of corona

anode

cylinder

t 4 Kv

thermoplastic

plate

Figure 2. The coronatron deposits a uniform charge on the thermoplastic

plate

Thermoplastic

i

1=

Figure 3. Detailed diagram of thermoplastic plate, showing quartz

substrate and transparent electrode.

o1.1
%dfte

t^

-2-

i

Nf

1

Is

6

Material; T300/5208 ' 1..1l

b.

units are in

inches.

Hole diameter = 1.0

Specimen # 111" "b" "t"

1 5 5 0.265

2 3 3 0.265

Figure 4. Perspective of specimen used in compression testing

V

any ^_ ri
^4

BS — BEAM

SPLITTER

M1

------------^M2 — MIRRORS

M3

L1 _ LENSES

L2

15 mw He/Ne BEl_ BEAM 40 -, )

LASER IBE2^ EXPANDERS

ID — INSTANT

RECORDING

DEVICE

BS

M2

BEi I

M Ll

VIDEO

CAME A I

PECI'IFil

ID

L2

BE2 LOADING

TO VIDEO I DEVICE

RECORDER i M3

I

I

III ^— ---- -- -- ----J EDGE OF

OPTICAL

TABLE

Figure 5. Schematic of Holographic Interferometer.

.E

i

.I
ILLUMINATION

DIRECTION

FRONT SURFACE

OF SPECIMEN

Figure 7. Nomenclature for equations associates with Interferometer.

w

iv1

O^.

. (01

Figure 6. Hologram showing load cell reading

ORIGINAL

()F POOR QUALITY

it UNIT VECTOR IN ILLUMINATION

DIRECTION

i 2

UNIT VECTOR IN OBSERVATION

DIRECTION

' i. A

i2 lac K SENSITIVITY VECTOR

OBSERVATION

DIRECTION
I

I OPTICAL TABLE

HAND PUMP I I

i TEST FRAME

I OR

WITH SPECIMEN

I ^

I

SERVO CONTROL l I

LOADING

' OPTICAL COMPONENTS

DEVICE I I

TO MAKE 1

IHOLOGRAPHIC

NTERFEROGRAM L

L--Z

INSTANT

RECORDING

DEVICE

i

VIDEO CASETTE

RECORDER

Figure 8. Flow chart of experiment Drocedure.

All
E ?

,f

^'•\^ •-- u^ ^,-.^ -..ter.- + 

^ r„ ,

W ,

A

vnotograpnea

section

(Figs. 9 a - q)

Specimen ; 1

Lay up ; (+ 45/902/+45/902/+45/90/0)2s

Failure load (P f ) ; 44,476 lbf

Fiyure 9. Data interpretation guidE for photos 9 a - q.

I
Figure 9 a

Po = 0.6?5

Ps = 0.65E

AP = 0.03T

Figure 9 b

Po = 0.859

P s = 0.875

AP = 0.016

V,1

ORIG"'' .

Nomenclature OF POOR QUALITY

P 

= failure load

Po = load carried by the specimen at the time the hologram was

41

made(reference load) normalised by P 

P s = load in the specimen corresponding to the interferogram

7 displayed in photograph, normalised by Pf

' ^. _ 11

I

_r ^ r Q •f
1

N

ORIVI IAL' - FP.G'z— 1

OF POOR QUAI rry

Figure 9 c

Po = 0.859

ps = 0.867

dp = 0.008

.61

Figure 9 d

Po = 0.930

Ps = 0.937

pp = 0.007

i

s

Fi g ure 9 e

po = 0.953

ps = 0.953-0.883

pp = 0-0.07

i
ORIGNAL

OF FLOOR QUALITY

Figure 9 f

po = 0.953

ps = 0.953-0.883

LP = 0-0.07

.01

Figure 9 9

po = 0.883

ps = 0.891

t Ap = 0.008

V^/ '

Ohl

Fi g ure 9 h

pa = 0.883

ps = > 0.891

Ap = > 0.008

I

_,.d" _^
It

. W",

Figure 9 i Figure 9 j

Figures_12i

pa = 0.984

ps = 0.969 - 1.0

ap = ; 0.016 ORiGiNA i-r' :'i:

OF POOR QUALITY

9 k Figure 9 1
ORIGINAL PA .^. ". '

OF POOR QUALI-i Y

Figure 9 m

po = 0.984

ps = 0.969 - 1 .0

pp=±0.016

161

Figure 9 n

po = 0.984

ps = 0.969 - 1.0

Ap=±0.016

' i. A

Iry

Figure 9 p

po = 1 .0

ps = 1 .023

pp = 0.023

I

^n

`v 1
Figure 9 q

Po = 1 .0

p

s

 = ?

A 

!'l

m 

F-.. .

^^A
zr

+I

Photographed

section

(Figs.10 a - p)

V

Specimen ; 2

Lay l ip ; (-45/02/-45/02/-45/0/90)2s

Failure load (p f ) ; 31,724 lbf

I^

I

fi

Figure 10. Data interpretatior guide for photos 10 a - p.

QD
PWGi1AL r ; .

06 POOR QUAU, i Y

Fi qur• e 10 a

po = 0.778

ps = 0.765

A p = -0.073

.e n

F

1-`

Figure 10 b

po = 0.786

Ps = 0.773

Qp = -0.013

Is

Figure 1'l c

Po = 0.786

P, = 0.774

Qp = -0.012

"I

IF I

1,41

1

^Q
0RIGiNA!- PAS

OE POOR QUAD f Y

Figure 10 d

po = 0.786

ps = 0.777

pp = -0.009

Al

I

Fi g ure 10 e

po = 0.786

ps = 0.781

dp = -0.005

Figure 10 f

po = 0.777

ps = 0.778

dp = 0.001

i

I
ORIGINAL FAt^E ,3

OF. POOR QUALITY

Fi oure 10 g

po = 0.777

P = 0.778 - 0.776

Ap =-0.001

. n

i n

Fi qure 10 h

po = 0.786

ps = 0.785

ti p = -0.001

Fi g ure 10 i

po = 0.806

pg = 0.809

Ap = -0.003

I
Figure 10 1 pa = 0.849 ps = 0.848

Qp = -0. )01 fl

IN

ORIGINAL

OF POOR

Fi aure 10 j

Po = 0.825 ps = 0.818

Ap = - 0.007

t

Fi nure 10 k po = 0.832 pj = 0.830

Ap = -0.002 I1

,t

E:
Fi nure 1 0 m

p = 0.849

p = 0.848 - 0.850

AP = - 0.001

Fi vure 10 n

o = 0.875

ps = 0.831

pp = -0.044

^r

I^

I

Fiqure 10 p

po = 0.953

p = 1 .014

Ap 0. G61

(frinae density too high)

% DO
4=

E

f

E

l

E

s:

^I

8 ply, t 45°

x--16 ply

Quasi - lsatd-apia

l

12.0' — - - -

Figure 11. Stiffener delamination test specimen.

; FAF

:M

It •L

Figure 12. Simulated stiffener joint.

l
f

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

' fM

i

OR1GINA:- t.._..:

OF POOR QUAD TY

Figure 13. Backplate used to trace specimen configuration.

Figure 14. Photo showing edge cracks visible between layers 2 & 3.

^rra^r

o.
IN 1 I ^,

I^I ^

I

•' I

It{ I \

r ` I

I`^t

V

II ^

\I

IIIf

II

I I I I

{ ^ I I

"I

`III

`II

l{I

^I

fl

f

i I 1 ^ cI

^. I

l ,

^•i I^ I. I NYViViO

evE

a7 t

C1L

ro w0

4-1

R3

Sea

CL

auNi

CJ ad C

O M

4- N • r L NwO

L N

U ^J

cu ^- YN

I.C; L7

LLI^II

s r

O

bda

C71 mE

/a

a: rN

G1L7

roac

CL t

.3CU

(JGN

4- a..)

O ^

t O

V1..1

G1 u:

Y ^

V) +-

1

Ln

cui

LL7

0;.JGINA`'_ 'r

OF POOR QUALi dY
C

i J

M X

CY

O

i O

t

&/

6 i

t _

Bonding rvioment,M X , In-Lb/!n

H

>0

.01

i

r•

Figurel7. Bending Moment-Shear Failure Envelope
a V

w

a

'N

f A

n

X

v

M

r\

rrW

a

a^

^^ M

xl

ci

euc

i

O

O

v

c

ry

S

i

v

O

cu

c

E

U

W

v

i

Y

I
u

CL

a

a

`X

v

E

a^

U ^

LO

. I v^

All

Y

V

Im ^

to

O

a

I v

I C^•r

1

C

O

C

GJ s

„^ L

IX

L

7

b

U

G

CJ

E

^^ Z

''. _ 11

IN

v

L

1

ar

^. L

a i