The failure of optical fibres embedded in composite materials

The failure of optical f!bres . embedded tn compostte matertals S. R. WAITE and G. N. SAGE (The City University, UK) This paper shows the use of an available 'fibre in matrix' model to represent optical fibre embedded in a composite three-point bend specimen. The model was used to predict the optical fibre stress level whilst embedded in the loaded structure, and experimental failure of such embedded fibre was compared with the failure stress of similarly treated non-embedded fibre. Indications are that knowledge of optical fibre strength combined with use of the model will permit prediction of embedded optical fibre failure. Key words: optical fibre; composite material; optical fibre stress; "fibre in matrix" model; embedded optical fibre failure prediction

The growth in optical fibre sensor technology I has resulted in many interesting applications for optical fibre within engineering environments. One such application is that of embedding optical fibres into composite materials for the purpose of monitoring the integrity of such materials. Work in this field has ranged from that considering the use of simple optical fibre failure resulting from severe structural damage 2' 3 to that utilizing the sensitivity of the light path in the optical fibre to geometric and photoelastic changes developed by the application of stress. 4-6 Despite the importance of optical fibre surface treatment to optical fibre strength levels (and scatter) and to the efficiency of the bonding of the fibre to the composite, many publications give little detail of fibre pretreatment. The surface treatment is of importance to both the meaningful interpretation of optical fibre failure prior to composite failure and to reliable and efficient stress transfer for the more sensitive techniques. This work, part of a more extensive study, aims to consider the use of a specified optical fibre pretreatment, ie silane coating, to provide optical fibre which fails prior to the composite. The calculated embedded optical fibre failure stress is then compared with that of similarly treated non-embedded fibre. The results should establish whether such a model is valid for the representation of embedded optical fibre in composite material. It will also indicate whether or not the embedding process etc., produces any major deviation from the

non-embedded fibre results. Such knowledge will increase confidence in the use of the optical fibre and provide meaning to the interpretation of results. Consquently a strain threshold indicator may be constructed using optical fibre of known pretreatment. This study considers the embedding of silane treated stripped optical fibre in three-point bend specimens constructed from woven glass reinforced plastic (eRe), see Fig. 1. GRP was selected because of its importance to the maritime industry. 7 Acrylate buffered fibre was also tested to provide comparison. Non-embedded fibre was tested 'dynamically'.

THEORY An existing model of a 'fibre in matrix '8 was used to represent the optical fibre, buffer layer and matrix in a

Embedded fibre /

Load

/

t-~O 7~

Mirror

Light source

Specimen

Fig. 1 Three-point bend test

0010-4361/88/070288-07 $3.00@1988 Butterworth & Co (Publishers) Ltd 288

COMPOSITES. VOLUME 19. NUMBER 4. JULY 1988

Table 1. Input data

z re/

Matrix EL (GPa) ET (GPa) v12 v13 O~L (10-6/°C) OCT(10-6/°C)

Sur rour'~'d Coati ng/g /

ii---~

Buffer

72.2 72.2 0.2 0.2 0.6 0.6

0.089 0.089 0.4 0.4 9 9

Matrix 3.96 3.96 0.39 0.39 50 50

Surround 23.5 4 0.042 0.042 10 50

~

r- 1

Fig. 2 Cylindrical model

surround of mean composite properties. This model was constructed from four concentric cylinders defined relative to polar axes (r, 0, z), see Appendix A and Fig. 2. Composite stresses, curvature etc., were determined using laminate analysis,9 see Appendix B. An alternative might be to consider Shear lag theory,~° but this is not shown here. Both theories assume the longitudinal strain (~z) is constant across the section of the model when considering a continuous fibre. Consequently both show that the fibre stress is approximately equal to the optical fibre Young's modulus, typically 72 GPa, multiplied by the strain arising from the application of load, plus thermal load. The model calculates longitudinal, radial and circumferential stresses. The longitudinal model axis was considered to be parallel to the axis of the three-point specimen length for the purposes of this work. The longitudinal component of the stress output was of primary importance, although the other components would be of interest to those working with sensitive systems or when there is a poor fibre-matrix chemical bond. In this case a tensile radial stress would not permit friction load transfer to the fibre. This model was originally developed for use with silica carbide (SIC) coated graphite fibre in an aluminium matrix. Consequently transverse isotropy was assumed. This is similar to the case of a composite laminate of a unidirectional lay-up. However, in the work reported here, woven composite was used. Therefore transverse isotropy is not strictly applicable and may provide a source of error. Curing stresses, produced upon composite cooling from cure, were accounted for via application of a ( - 120°C) temperature decrement to the model. The resulting stress change was presented as a fraction of EL 0eLA T, where EL = longitudinal Young's modulus of the surrounding material, 0¢i~= thermal expansion coefficient and A T = temperature change. The longitudinal stress was presented as a fraction of the mean composite applied stress. The surrounding cylinder properties were originally calculated from the Rule of Mixtures applied to the fibre, coating and matrix components. This was altered for the purposes of this work because the composite properties are defined by the properties of the composite fibre and matrix, not the properties of the

COMPOSITES. JULY 1988

Fibre

optical fibre and buffer. The alteration made was simply to replace the relevant section of the Fortran program, from Reference 8, with direct input of the composite properties. This should permit the consideration of optical fibres embedded in carbon systems etc. It was assumed that the effect of the optical fibre upon the structure was negligible. 11 In this study a single 140 p.m diameter fibre (Coming 140 ~tm Multimode Coreguide Optical Fibre) only contributes approximately 0.05% to the cross sectional area of a typical 120 x 10 x 3 mm specimen. The input data for the model was as shown in Table 1. Constants were used for the evaluation of stress using the model. The data assumes that the optical fibre can be treated as a fibre in a unidirectional material ie, the surround being considered transversely isotropic. The fibre properties were typical of those of silica fibre, 12 whilst the properties of the buffer were those of urethane acrylate. 13 Matrix properties were those of 914C, 14 because 916G properties were not available at the time. The surrounding material properties (the composite) were those derived by the author in the laboratory. These are referred to in terms of longitudinal (L) and transverse (73 components. EXPERIMENTAL WORK The experimental work was conducted in two parts. Firstly non-embedded optical fibre was dynamically tested, and secondly embedded optical fibre was tested in three-point bending. The 'dynamic' testing of the non-embedded optical fibre was made using an Instron tensile testing machine. The optical fibre was gripped by winding the ends of the test lengths onto 75 mm diameter drums. The optical fibre gauge length (152 mm) was mechanically stripped of the buffer layer and treated with 5% silane solution (Union Carbide A-187). Previous work by the author showed that this treatment reduces the scatter of strength results for dynamically tested non-embedded optical fibre, when compared to untreated stripped optical fibre. However, a further loss of mean strength was also observed to result from the increased handling during treatment. It should be noted that the removal of the buffer considerably reduces the optical fibre strength eg, buffered fibre failure strength 3.4 GPa, compared to 1.14 GPa for stripped silane treated optical fibre. The test was then conducted and recorded using the tensile test machine load-time chart.

289

The embedded fibre testing was made using mechanically stripped silane treated optical fibre embedded between plies 11 and 12 of a 12 ply 916G (Ciba Geigy) woven glass epoxy prepreg composite specimen. The specimens (120 × 10 x 2.7 mm) contained no angled plies. These had been cut from a single panel cured at 125°C in a heated press. Individual specimens were trimmed and the end sections polished, using 30 ~tm alumina paper, to ensure maximum light transmission through the longitudinally located fibre. The specimens were located in a three-point bending rig (Fig. 1) of 80 mm span. This was placed into the test machine and a simple light source was arranged to transmit light through the fibre. The output was viewed using a mirror. The testing was made in a darkened laboratory, and the light source had been shrouded with a dark cloth to reduce any error in visual judgement of optical fibre failure. Some specimens containing 250 p.m buffered 140 Ixm optical fibre were similarly tested for the purpose of comparison with the stripped optical fibre performance.

-

Applied longitudinal load

-

------

cO

Applied

Fibre

thermal load (cure)

Buffer

r/R

.D

Matrix

1.0 Fibre

radius

~J

o

The embedded optical fibre was tested at a crosshead speed of 12.5 mm min -1, equivalent to a centre span strain rate of 2.64% min -1 or integrated strain rate of 1.32% min -1. The centre span rate was calculated from bending theory: 12ziS ~x L2 where ~x = strain rate (strain min-1), z = location with respect to neutral axis (m). L = span (m) and ~ = cross-head speed (mm min-1), whilst strain is proportional to the distance from the support. Dynamic testing of the non-embedded optical fibre was at 5 mm min -1, equivalent to 1.25% min -1 for the chosen gauge length. Selection of similar strain rates was considered important because both glass reinforced plastic 15 and optical fibre 16 show strength to be strain rate dependent.

-3 Fig. 3

Optical fibre longitudinal stress fraction versus radial

location (longitudinal applied load and cure)

Combining the applied longitudinal load and thermal load, the longitudinal stress component of fibre, buffer and matrix may be represented in the form: Oi = Cli oc - C2i EL 0~LAT

RESULTS

where i = fibre, buffer, or matrix, oc = applied composite stress, Cli = longitudinal fraction due to applied longitudinal load and C2i = longitudinal fraction due to thermal load. Table 2 shows the values of these fractions for this work.

The following is a summary of the use made of the 'fibre in matrix' model and of the experimental results.

It may be noted that this model also produces radial and circumferential stress values, not shown here.

Fig. 3 shows the longitudinal stress calculated to exist in the buffered fibre, buffer, and matrix, as a fraction of the applied longitudinal stress, versus radial location, given with reference to the matrix outer radius. This was based upon a fibre volume fraction of 0.49 established for this specimen type by sectioning specimens and making a point count. It is clear that the fibre stress is approximately the applied stress multiplied by the Young's moduli ratio of the fibre/composite.

Figs. 4 and 5 show the use of the calculated stress fractions to produce stress values for the fibre, buffer and matrix respectively. The stress fractions were those of Table 2.

Fig. 3 includes the longitudinal stress calculated to exist in fibre, buffer and matrix due to applied thermal load ie, cooling from cure. The stress EL 0eLATbased upon input data (Table 1) is -29.375 MPa. This negative value accounts for cooling from the cure temperature of 125°C showing that the optical fibre would be subjected to thermally induced longitudinal compression, whilst the matrix may experience tension. 290

The calculated stresses are referenced to the case of 589 MPa maximum composite stress ie, not in initial audible damage, stress (420 MPa). Fig. 4 shows the longitudinal fibre stress for a buffered fibre. The optical fibre would not be expected to fail prior to the specimen because the optical fibre stress obtained upon failure is less than half that necessary to fail the buffered optical fibre, according to dynamic testing of the buffered fibre. This result was supported qualitatively in experiment when embedded 250 pm acrylate buffered 140 ~tm optical fibre was embedded in 916G. No buffered optical fibre failure was observed prior to specimen failure because the protection COMPOSITES. JULY 1988

Table 2.

Stress fractions Buffered fibre lO0

i

Cli

C2i

Fibre Buffer Matrix

3.07 0.008 0.2

-2.91 -0.058 0.808

Matrix stress

Matrix strength (51 MPa)

Stripped fibre i

Cli

C2i

Fibre Matrix

3.09 0.2

-3.14 0,76

Buffer strength (19,3 MPa) Buffer stress

J 1.0

tress/strength ratio

J

1.5

L

Buffered fibre strength(3.4 G P a )

Fig. 5 Buffer and matrix stress versus composite stress/ strength

I

provided by the buffer helps to maintain a high optical fibre strength. Similar testing conducted with optical fibre buffered with the older harder acrylate resulted in examples of total specimen failure when specimen separation into halves was only prevented by the optical fibre. This implies that some slippage of the fibre relative to the composite must have occurred. Although testing such fibre does show that the correct qualitative result is indicated by the model it does not validate the use of the model to accurately predict stress levels. Indeed a similar qualitative result could be produced by embedding a fibre with very poor fibre-matrix bonding.

589 MPa ] re ference_~

C3.

L~

L

L -Q

L~ % u O_

©

However, the model could be used to provide a prediction for the maximum stress that the fibre might experience, assuming strain continuity and ignoring local stress concentrations eg, weave/weft pattern, fibres touching, or damage. Work is under way to assess the validity of the strain continuity assumption.

/

I 1.0

0

Composite s t r e s s / s t r e n g t h ratio Fig. 4

Optical fibre stress versus composite stress/strength

COMPOSITES. JULY 1988

Fig. 5 similarly shows the calculated stress levels in the buffer. The very low values relative to the failure stress would indicate that buffer failure would not be expected prior to the composite specimen failure. However, published data presenting comprehensive property values for such buffer material is scarce and the properties may be altered greatly by adjusting the UV dose 17 upon cure. Therefore care is necessary when using buffer strength data. 291

~v

1.0

z

Embedded fibre -Least squares Fit V Experimental point

c~ 2

Non-embedded fibre -- Least squares fit

t.~

c~

x

Upperx ~ / "X /~/

Experimental point

~I~/~'/V

f

XV~ "

/ V /

V V

V

k.

0)

> 0~

E uD

/

Lover

-%

/V

I

//"

I

0.5

1.0

.5

Fibre stress (GPa) Fig. 6 Cumulative failure probability (F) versusfibre stress (GPa)

The performance of the buffer would prove difficult to check experimentally. Either the fibre fails upon total specimen failure, or it slips in which case a poor fibre-matrix bond is dominant. Fig. 5 also shows that the matrix strength was calculated to be exceeded prior to specimen failure. This comparison was made with strength data for 914 (Ciba Geigy) resin. 14 Qualitatively this result is correct because matrix failure is recognized as an early indication of damage development in both static and fatigue testing. The observed cracking of the matrix at high load (greater than 80% of the ultimate tensile stress) in this study would support this, although this was only assessed to have occurred because of surface crazing. The observed damage was at a considerably greater strain than indicated by using the 914 matrix properties with the model. This was believed to be the result of the 914 matrix being considerably more brittle than the 916 system used. The state of the matrix local to the embedded optical fibre was unknown. Matrix failure is not considered to be of major significance to the overall strength of composites with high volume fractions,12 although the effect upon an optical fibre would need to be clarified. Fig. 6 shows the least squares fit of the experimental data obtained from embedded and non-embedded stripped silane treated fibre, to the Weibull Cumulative probability expression: F=

1 - e [- (L-~)ml

where F = cumulative failure probability, Ix, [3, m = constants and x = stress. The non-embedded fibre failure strength was adjusted for length, from the gauge length of 0.152 m to the embedded length of 0.08 m using: 18

292

of2

\ L1]

where of -- fibre strength and L -- length. The embedded fibre stress was calculated with respect to specimen failure (589 MPa reference). Table 3 shows the values of the constants obtained from the least squares fit, and also mean strength. The non-embedded fibre data was represented as upper and lower (intrinsic and extrinsic failure) curves. The embedded fibre data was shown as one curve. Intrinsic failures are those governed by internal optical fibre defects, whilst extrinsic failures are those governed by external damage eg, cracks. Comparison of results shows that the non-embedded fibre data, adjusted for length, produces similar fibre strength to that of the embedded fibre, when assessed with respect to specimen stress at the integrated strain Table 3. Least squares fit of fibre data Embedded fibre

Non-embedded fibre Lower

m -1 IX(MPa) [3 (MPa) of mean (MPa) Coefficient of variation No. of fibres tested

0.13 -1.23 1153.82 1199.31 0.15 20

0.4 -1 1245

Upper

0.06 14 1171.61 1152.40 0.19 10

COMPOSITES. JULY 1988

(2)

Q.

(3) (4)

r~

E

©

A

Non-embedded f i b r e failure stress [mean + 1 S D ] ( 1 . 1 5 GPa)

B

Embedded f i b r e failure stress [mean + 1SD] (1.19 GPa)

neutral axis, because of its finite dimension in the specimen depth. The longitudinal strain gradient. Calculation was made under the assumption that the centre span strain level existed for the full embedded fibre length. This is clearly a simplification which should provide an early prediction for optical fibre failure. Stress concentrations. Stress concentrations due to weave/weft pattern, fibre touching and damage were ignored. Data. The material constants were largely based upon published data, and not data exact to the material tested. There is a need for more extensive test data concerning these material types ie, buffer, matrix. Also, the test data for the optical fibre was based upon a very small sample, because of time limitation. There is a need to extend this work to a much larger data base, and across a number of strain rates.

CONCLUSlON /

I 1.0

0 Composite stress/strength

Fig. 7

(1)

ratio

Optical f i b r e stress versus c o m p o s i t e stress/strength

(stripped silane treated fibre)

rate. The slightly higher mean strength of the embedded fibre may be attributed to the centre span strain rate being higher, and of more relevance, than the integrated rate assumed here. This needs further study. Fig. 7 shows the model predicted stress for stripped silane treated optical fibre (Table 2) vs composite stress/strength defined for specimen ultimate strength. Also marked are A, the non-embedded stripped silane treated fibre mean strength (1.15 GPa) + one standard deviation and B, the embedded stripped silane treated optical fibre mean strength (1.19 GPa) (according to the model) + one standard deviation. This shows that the scatter is large, but that both spreads coincide. This suggests that knowledge of the non-embedded fibre strength data and application of the 'fibre in matrix' model should permit the prediction of embedded optical fibre failure. Therefore characterization of differing surface treatment effects upon stripped fibre, and the reduction of the spread of data, could permit the development of a strain multilevel threshold detector. The similarity of results would also indicate that the fibre failure mechanism is not greatly affected by being encased in the matrix material. Factors which may have influenced the results, but were ignored in this work include: (1)

The through the thickness strain gradient. An optical fibre of 140 ~m diameter located in a specimen, as tested here, would experience a strain difference of _ 6.5% of the value calculated at the mean fibre location with respect to the

COMPOSITES. JULY 1988

(2)

(3)

(4) (5) (6)

The results indicate that, at similar strain rates, knowledge of non-embedded optical fibre strength and use of the available 'fibre in matrix' model may permit the failure prediction of stripped silane treated optical fibre embedded in composite material. This is in spite of the many simplifications and assumptions made. The similarity of results suggests that the optical fibre failure mechanism is not greatly altered by the surrounding matrix. Embedded stripped silane treated optical fibre fails prior to the composite specimen. This would permit the development of a strain threshold detector. Characterization of such fibre and manipulation of the pretreatment would enable refinement of such a system into a multilevel threshold detector. Embedded buffered optical fibre did not fail prior to the composite specimen, as calculated. However, the validity of assuming efficient stress transfer remains untested. There is a need for greater availability of matrix and buffer material property data. The effect of strain rate needs further study, because this influences both fibre and composite strength. Practical application of such a system would require knowledge of the cyclic fatigue performance of such embedded fibre. This is currently under study by the author.

REFERENCES 1 Jones, B. E. 'Optical fibre sensors and systems for industry' (Invited keynote paper) J Phys E Sci Int 18 (1985) pp 770-782 2 Crane, R. M., Marander, A. B. and Gagorik, J. 'Fiber optics for a damage assessment system for fiber reinforced plastic composite structures' Proc Review of Progress in Quantitative NDE, San Diego, CA, USA, 1982 (Plenum, New York, USA, 1983) pp 1419-1431 3 Hofer, B. 'Fibre optic damage detection in composite structures'

Proc 15th Congress ICAS Conference, London, September 1986, ICAS86-4.1.2 pp 135-143

293

4

Culshaw, B., Davies, D. E. N. and Kingsley, S. A. 'Fibre optic strain, pressure, and temperature sensors' Proc 4th European Conference on Optical Communications, Genoa, Italy, September 1987 pp 115-126 Reddy, M., Bennett, K. D. and Claus, R. O. 'Imbedded optical fibre sensor of differential strain in composites' Preprint of Review of Progress in Quantitative NDE Conference, San Diego, CA, USA August 1986 (Plenum, New York, USA) Butter, C. D. and Hocker, G. B. 'Fiber optics strain gauge' Applied Optics 17 No 18 (September 1978) pp 2867-2869 Roberts, J. 'Fiber Glass Boats, Construction Repair and Maintenance' (W. Norton & Co., New York, 1984) Yozo, M. 'Mechanics of coated fibre composites' PhD Thesis (Ann Arbor University, Mi, USA, 1985) 85-11217 Jones, R. M. 'Mechanics of Composites' (McGraw Hill, New York, 1975) Chamis, C. C. 'Mechanics of load transfer at the interface' in Composite Materials Vol 6 edited by E. P. Plueddermann (Academic Press, New York, 1974) Claus, R. O., Jackson, B. S. and Bennett, K. D. 'Non destructive testing of composite materials by OTDR in imbedded optical fibres' Preprint of Proc S. P. L E. Int Symp, San Diego, August 1985 Hull, D. 'An Introduction to Composite Materials' (Cambridge University Press, England, 1981) Schlef, C. L., Narasimham, P. L. and Oh, S. M. 'UV cured resin coating for optical fiber/cable' J Radiation Curing 9 No 2 (April 1982) pp 11-13 Berry, D. B., Buck, B., Cornwell, A. and Phillips, L. 'Resin Properties Part A--Cast Resins' (Yarsley Technical Research Centre, The Street, Ashtead, Surrey, UK, 1975) Chiao, T. T. and Moore, R. L. 'Strain rate effects on the ultimate tensile stress of the fiber epoxy strands. J Composite Mater 5 (1971) pp 124-127 Katsuyama, Y., Mitsunaga, Y., Hirokazu, K. and Ishida, Y. 'Dynamic fatigue of optical fibre under repeated stress' Appl Phys 53 No 1 (1982) pp 318-321 'De Solite Data--Optical Fibre Coating, Sheet No. 95131, 1986' (De Solite, October, 1986) Love, R. E. 'The strength of optical waveguide fibers in Proc Fibers and Integrated Optics SPIE, Vol 77, 1976, pp 69-77

5

6 7 8 9 10 11

12 13 14 15 16 17 18

where n denotes the phase of interest, 13is an expression in terms of thermal expansion coefficients, C n is an elastic constant, U = displacement (radial), W = displacement (longitudinal) and T = temperature. Application of boundary conditions which satisfy continuity of displacement at radii r2, r3, r4 enable the solution to Equations (1) in the general cylindrical equation form of: - + 2 f gn(r) dr Un(r) = Znr + -Bn r

1 2r

fr2gn(r)dr

W n ( Z ) = E n Z + Fn

(2a) (2a) (2b)

When An, B., E., Fn are constants determined by boundary conditions and gn(r) = Ln dVn dr Note that IU4(0)] < =

therefore B4 = 0,

and Fn [n = 1 to 4] = 0

yielding

Wn(z) = E Z

(3)

Substitution of (2) and (3) into the given stress-strain relationships of Reference 8 enables the evaluation of the radial, circumferential and longitudinal stress distributions. Reference 8 presents this solution in matrix form and provides a F O R T R A N program which permits this evaluation.

A U THORS The authors are with the Department of Aeronautics, The City University, Northampton Square, London EC1V 0HB, UK. Enquiries should be directed to Mr Waite.

Model mathematics The following is a brief outline of the mathematics used in Reference 8 for the calculation of the cylindrical model stresses. The same notation as that of Reference 8 has been used. Fig. 2 illustrates the model. Equilibrium was considered in terms of polar axes, (r,

0, z). Substitution of polar stress-strain relationships into the polar equilibrium equations yields equations of the form: -

dr 2 + -

1 dUn dr r

Un

dTn

r 2 = L . - dr

(la)

294

Ln -

131n Cll n

[Ox] = [Q] [Sx] where [o.] = stresses with respect to specimen axis, Q = stiffness matrix and [s.] = strains with respect to specimen axis. The strain comprises strain leo], plus strain due to bending, [zK]: [e] = [Co] + z[K] where z = Location with respect to neutral axis and [K] = curvatures

[El = [D - l ] [ M ]

(lb)

for a balanced laminate, and D -l = inverse 'D' matrix linking moments to curvature and [M] = moments applied to specimen.

(lc)

The restrictions of the testing rig imposed zero transverse curvature and twisting upon the specimen.

dZWn

dz----7- = 0 when

Laminate analysis The stress in a three-point bend specimen may be obtained using laminate analysis~:

APPENDIX A

d2Un

APPENDIX B

C O M P O S I T E S . JULY 1988