Fractoemission from Epoxy and Epoxy Composites - ACS Publications

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Fractoemission from Epoxy and Epoxy Composites J. T. Dickinson and A. S. Crasto Department of Physics, Washington State University, Pullman, WA 99164-2814 Fracto-emission (FE) is the emission of particles (electrons, positive ions, and neutral species) and photons, when a material is stressed to failure. In this paper, we examine various FE signals accompanying the deformation and fracture of fiber-reinforced and alumina-filled epoxy, and relate them to the locus and mode of fracture. The intensities are orders of magnitude greater than those observed from the fracture of neat fibers and resins. This difference is attributed to the intense charge separation that accompanies the separation of dissimilar materials (interfacial failure) when a composite fractures. When stressed, a material releases various types of emission prior to, during, and subsequent to ultimate failure. This emission includes electrons, positive ions, neutral molecules, and photons - including long wavelength electromagnetic radiation (radio waves), which we have collectively termed fracto-emission (FE). FE data have been collected from the fracture of a wide variety of single and multi-component solids, ranging from single crystals of molecular solids to fiberreinforced composites, and also from the peeling of adhesives (1-16). In this paper, we will restrict our attention to FE arising from the failure of polymer composites (fibrous and particulate), and the individual components thereof (fibers and matrix resins). Composites have a complex microstructure, and characterizing and monitoring the various fracture events when a sample is stressed to failure is not straightforward. In contrast to homogeneous materials where failure usually proceeds via the propagation of a single well-defined crack, composite materials exhibit several damage mechanisms, which may progress independently or in interaction with each other. These mechanisms include fiber breakage, interfacial debonding, and matrix cracking, well before ultimate failure. To aid our understanding of the performance of composite structures and the design of these products, it is essential to detect these pre-failure events and their sequence, which lead to crack growth. Acoustic emission (AE) is a technique that has been successfully employed to study fracture events in composites, where potentially, each failure mechanism has a unique acoustic signature (17-19). FE is another technique, which can be used in parallel with AE, and offers better sensitivity to the various microfracture processes. We have shown that interfacial failure between fiber and matrix in a composite produces significantly more intense emission and longer lasting decay 0097-6156/88/0367-0145S06.75/0 © 1988 American Chemical Society

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compared to cohesive failure in the matrix (2.7.9). Analysis of the emission may prove useful in determining the mechanisms involved in delamination, fatigue, and/or environmentally induced failure. This technique could aid the materials scientist in understanding how, when, and where cracks originate and grow in a wide variety of materials. Other workers employing fracto-emission to investigate composite failure include Udris et al. (20), Kurov et al. (21), and Nakahara et al. (22). Experimental Procedure Equipment, sample preparation, and data acquisition have been described in detail elsewhere (1-16). and will only be briefly reviewed here. Detection of charged particles and gaseous species accompanying fracture events is necessarily carried out in vacuum. While pressures below 10 torr are sufficient for electron emission (EE) and positive ion emission (PIE), mass spectroscopy studies require pressures below 10 torr. Charged particles are detected with continuous dynode electron multipliers, usually positioned about 1 cm from the sample. The front surface is biased typically +300V (for electrons) and -2500V (for positive ions), and typical background noise is 1 and 10 counts per second, respectively. Photon emission (phE) is usually detected with various photomultiplier tubes, with single photon sensitivity. For experiments performed in air, cooled housings have been used, which reduce the background counts to less than 10 per second. When intensity comparisons are made, care is taken to acquire data at the same detector gains. Acoustic emission (AE) accompanying failure is frequently used to correlate the emission signals with various microfracture events, or for triggering a multichannel scaler for pulse counting. The signal is detected by attaching a standard A E transducer directly to the specimen, or the grip. A sketch of the experimental setup is shown in Figure 1. To correlate the occurrence of macroscopic crack growth in a specimen with the observed emission, we have employed a technique involving resistive changes. A grid of thin, parallel gold strips (0.5 mm thick and 0.5 mm apart) is deposited onto one face of the specimen, perpendicular to the crack path. As the crack grows these strips break, and the resulting resistance change is converted to a voltage and digitized. Thus, the approximate crack tip position is determined as a function of time, and periods of crack growth compared in time with the corresponding emission curve. Standard techniques were used for sample preparation. Epoxy matrices were employed, including MY720, cured with diaminodiphenyl sulfone (TGDDM/DDS), and Epon 828, cured with Z-hardener (an aromatic amine eutectic from Shell Chemical Co.), m-phenylene diamine or Jeffamine polyoxyalkyleneamines. Cure schedules are summarized in Table I . Neat resins were cast in silicone rubber molds and cured to give dog-bone shaped specimens, as were model composites containing small volume fractions of aligned, continuous fibers. Unidirectional composite bars were also molded from prepregs. Glass, graphite, aramid and boron filaments, and alumina particles, were used to reinforce the above epoxy compositions. Notched and unnotched samples were fractured at various loading rates, and unless otherwise mentioned, in tension. Results and Discussion Neat EPOXV Resin. Neat epoxy resin (TGDDM/DDS) was found to be a relatively weak emitter of photons, electrons and positive ions. The general shape of all the emission curves consists of a relatively rapid burst, followed by a very low intensity decay which lasts approximately 100 ps. We frequently observed that during

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Table I. Stoichiometry and Curing Schedules for Matrix Resins Resin

Hardener

R/H Ratio

Cure Schedule

M Y 720

Diaminodiphenyl sulfone

100/27

lh @ 135C, 2h @ 177C

Epon 828

Z-hardener

100/20

4h 55C, 16h @ 95C

m-phenylenediamine

100/14.5

lh @ 80C, 0.5h @ 150C and 150 psi, 4h 155C

Polyoxyalkyleneamines D400:D2000 :: 53:10

100/63

2h @ 75C, 3h @ 135C

fracture, our detectors were saturating, ie. the instantaneous count rates for time periods of a few microseconds were greater than 10 counts/s. The time distributions of EE and PIE show nearly identical behavior, and this has been explained with an ion emission mechanism based on electron stimulated desorption (15). The total counts detected for these various emission components were 100150. It was frequently observed that notched specimens, and those broken under high strain rates, showed smoother, glassy fracture surfaces, compared to the rough surfaces produced with either unnotched specimens or low strain rates. Figure 2 compares the phE and surface characteristics of two notched specimens strained to fracture at 2%/s (data acquired at 100 ns/ch.). One specimen displays a smooth fracture surface and relatively low phE, while the other, with a considerably rougher fracture surface, yields much more intense phE. This correlation of emission intensity with surface roughness indicates that regions where significant plastic deformation and crack branching occur yield the highest emission rates. The molecular motion and bond alterations associated with this deformation may be important parts of the emission mechanisms. To correlate (in time) the crack motion (from initiation to specimen failure) with the simultaneous emission curve, the voltage drop is continuously monitored across a grid of gold strips deposited on the sample surface. The crack breaks strips as it progresses, and the corresponding voltage drops map the crack motion. In general, both PIE and phE are most intense near the start or middle of fracture, with a decrease in emission prior to final surface separation. After separation, the emission, though weak, continues, decaying within 50-100 yas. This after-emission is attributed to thermal relaxation processes stimulated by fracture. The fracture surfaces are "glowing" after formation, similar to phosphorescence. Occasionally, small clusters of counts are seen (usually in charged particle emission which has a better signal/noise ratio) several milliseconds prior to the onset of crack growth. Since these experiments are conducted in a vacuum at room temperature, we do not expect to see the type of photon signals seen by Fanter and Levy (23) in an oxygen atmosphere at elevated temperature, where stress-induced chemiluminescence was observed well before failure. Thus, the occasional discrete bursts of emission we see are more likely due to microcracking and crack initiation, occurring prior to catastrophic crack propagation. Typical results with such crack growth measurements are shown in Figure 3. In this example a notched specimen was used, with the first gold strip located immediately below the notch, and the phE recorded. The voltage is steady between

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1RS- C H A N N E L T R O N PHOTOMULTIPLIER MASS SPECTROMETER ENERGY

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Figure 1. Schematic design of the experimental arrangement f o r fractoemission i n v e s t i g a t i o n . (Reproduced with permission from Ref. 7. Copyright 1984 Plenum Press.) P H O T O N E M I S S I O N F R O M MY720 E P O X Y

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Figure 2b. Photon detector current as a function of time and corresponding fracture surface micrograph f o r rough fracture surfaces of TGDDM/DDS.

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Figure 8. EE accompanying the fracture of both unfilled and alumina-filled Epon 828, cured with Z-hardener.

12. DICKINSON AND CRASTO

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Figure 9. Peak EE from the f r a c t u r e of a l u m i n a - f i l l e d Epon 828 (Z-hardener) as a f u n c t i o n of the alumina/epoxy r a t i o (jpt). (Reproduced with permission from Ref. 7. Copyright 1984 Plenum Press.)

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DEFLECTION (mm) Figure 10. The EE, AE, and load accompanying the flexural straining of (a) ( 0 ) and (b)(±45) graphite/epoxy laminates (Thornel 300/NARMCO 5208). 16

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The results of these experiments indicate that E E may be used to detect microfracture events on the composite surface, and signal the early stages of composite failure. It also clarifies the source of A E as a function of strain by the presence or absence of A E - E E correlations, and allows more details of failure mechanisms and composite characterization to be obtained. To detect EE from interply failure, the newly-formed fracture surfaces have to be exposed to the detector, requiring a change of sample and testing configuration. Such a study is currently underway, with unidirectional composites made from Hercules A U (untreated) and AS4 (surface treated) graphite fibers. Fiber tows are impregnated with Epon 828 (cured with m-phenylene diamine) and the prepreg cut into plies and compression molded, with 16 plies to a composite. Thin Teflon strips are inserted between plies 8 and 9 prior to molding, which initiate an interlaminar crack when the bar is loaded as a double cantilever beam. A sketch of the arrangement is shown in Figure 11. Preliminary results indicate at least two orders of magnitude difference in the E E from the two specimen types, the untreated (and consequently poorly bonded) fibers giving the more intense emission. A comparison is shown in Figure 12. SEM micrographs of the fracture surfaces reveal interfacial failure with an abundance of resin-free fibers in the A U sample, while good fiber-matrix adhesion in the AS4 specimen is evidenced from the large extent of cohesive matrix failure. The intensity of emission therefore correlates directly with the extent of interfacial failure. Efforts are underway to use specimens of similar geometry to evaluate composite fracture toughness, Gjp, by a hinged double cantilever beam technique (32). and relate it to the observed EE. G j ç may in turn be varied with appropriate fiber surface treatments. Model Glass and Boron Fiber Composites. To detect charged particles, newly created fracture surfaces must be in communication with the surroundings, and further, a vacuum is necessary to enable the particles to reach the appropriate detectors. Photon detection is not as restricted, in that fracture events internal to relatively clear specimens may be detected prior to ultimate failure, and the experiments may be performed in air. Model unidirectional composites were therefore fabricated containing a single boron filament (100 pm) or 50-300 aligned Ε-glass filaments, embedded in a transparent matrix of Epon 828 (cured with Jeffamines D2000 and D400). These amine hardeners have different molecular weights, and by varying the ratio used, the modulus and failure strain of the matrix can be altered. The matrix was formulated to achieve a high failure strain (>30%) so that the filament(s) underwent multiple fracture prior to ultimate composite failure. These specimens were strained in tension at a rate of 15% /min, and phE and A E recorded (Crasto, A. S.; et al., Compos. Sci. Technol., in press). In general, the A E bursts correlated with breakages of the fiber bundle (or single filament) at various intervals along the length, and ultimate specimen failure. The phE bursts did not coincide with those of A E , but occurred a fraction of a second later. Results for specimens made from untreated and commercially sized (AR-120-AA) glass fibers are displayed in Figure 13. The total counts from the former are an order of magnitude greater than those from the latter. When the counts for ultimate specimen failure were isolated (for the untreated fibers), they were found to be a small fraction of the total count, indicating that the major emission occurred before ultimate failure. SEM examination of the tensile fracture surfaces revealed the untreated glass fibers protruding from the fracture plane to be clean and devoid of resin (Figure 13c), attesting to interfacial failure as a result of poor adhesion. On the other hand, the sized fibers, also exhibiting pull-out (Figure 13d), were coated with resin, indicating the occurrence of cohesive failure in the matrix. Degradation of the glass-epoxy bond in an aqueous environment was also easily discerned, from the sizable increase in the resulting FE. Consequently, this

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Applied Force

Applied Force

EE Detector

AE Transducer

Figure 11. Sketch of the specimen and test configuration to detect E E and A E from the interlaminar fracture of graphite/epoxy composites. 176.500 TOTAL COUNTS: 9.6 M

2

Channel Number (10~ s/channel)

2200

(b) TOTAL COUNTS: 26 Κ

2

Channel Number (10~ s/channel)

Figure 12. E E from the interlaminar fracture of graphite/epoxy composites made with (a) untreated, and (b) surface treated fibers (Epon 828/mpda).


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Untreated Ε glass Strand (approximately 300) fibers) in E p o n 828/Jeffamine Matrix 200 Κ I

( ) a

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Strain (%)

6.5

Commercially Sized Glass Strand (approximately 300 fibers) in E p o n 828/Jeffamine Matrix ι 20 Κ

.

5

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Strain (%)

5

·

5

Figure 13· Photon emission from the t e n s i l e loading of model u n i d i r e c t i o n a l epoxy composites made with (a) unsized and (b) sized Ε-glass f i b e r s i n Epon 828/jeffamine. (Reproduced with permission from Ref. 33· Copyright 1988 E l s e v i e r Applied Science.) (Continued on next page.)

164


Figure 13 Continued. (c) and (d) Fracture surfaces that correspond to Figures 13a and 13"b. (Reproduced with permission from Ref. 33· Copyright 1988 E l s e v i e r Applied Science.)

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technique can be used to assess bond deterioration as a result of environmental attack. When these specimens are stressed in tension, the fibers fracture first. If the fiber-matrix bond is weak, the resulting stress is relieved by debonding on either side of the break. These debonded fibers slide back into their holes in the resin, against considerable friction, and the load is built up again in the fibers. This motion is most likely accompanied by considerable "making and breaking" of surface contact at the interface. This activity, as well as surface separation in debonding, with accompanying charge separation, contribute to the observed phE. A significant difference between the interfacial events occurring here and those in previous studies, is that the earlier work always involved macroscopic separation of surfaces. Here, the fiber and matrix remain in relatively intimate contact after debonding, which very likely influences the intensity and time dependence of the resulting emission. For example, we expect possible quenching mechanisms involving the nearby surfaces and gases in the narrow void created by the broken interface, which would tend to reduce the intensity and duration (decay) after a separation event. With a strong interfacial bond, when a fiber fractures, the high stresses in the matrix near the broken ends are relieved by the formation of a short radial crack in the resin. There is no interfacial debonding and corresponding friction at a sheared interface, but rather, the load is transferred to the fiber by elastic deformation of the resin. The lack of adhesive failure in this case is responsible for the relatively low emission observed. The large diameter single filaments of boron yield intense emission when they fracture. Easily detectable A E was used in the interpretation of the phE arising from the straining of single-filament composites made with untreated and lubricated boron filaments. The A E data are similar, with sharp bursts occurring at each filament break (typically 4-8 breaks/filament). On the average, the untreated (and better bonded) filament fractured more than a lubricated (poorly bonded) filament, yet the latter displayed two orders of magnitude greater phE (see Figure 14). Almost all of this emission occurred prior to catastrophic failure of the specimen. When these specimens were stressed under an optical microscope, large debonded lengths were clearly visible in the lubricated-filament samples. This reinforces the contention that the major emission from composite fracture arises from interfacial failure. Conclusions In general, the use of FE signals accompanying the deformation and fracture of composites offer elucidation of failure mechanisms and details of the sequence of events leading upto catastrophic failure. The extent of interfacial failure and fiber pull-out are also potential parameters that can be determined. F E can assist in the interpretation of A E and also provide an independent probe of the micro-events occurring prior to failure. F E has been shown to be sensitive to the locus of fracture and efforts are underway to relate emission intensity to fracture mechanics


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30 Κ

a) phE

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Strain (%)

100 Κ

it

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Figure 1 4 . Photon emission from the t e n s i l e loading of s i n g l e filament boron/epoxy composites made with (a) untreated and ("b) l u b r i c a t e d filaments i n Epon 828/Jeffamine. (Reproduced with permission from Ref. 3 3 . Copyright 1988 E l s e v i e r Applied Science.)

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Acknowledgments The authors wish to thank Les Jensen, Russ Corey, and R. V. Subramanian of Washington State University, and Clarence Wolf of McDonnell Douglas, for their contributions to this work. These studies were supported by McDonnell Douglas Independent Development Fund, the Office of Naval Research (Contracts No. N00014-80-C-0213, N00014-87-K-0514), the Washington Technology Center, and the Graduate School at Washington State University.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

Dickinson, J. T.; Park, M. K.; Donaldson, E. E. J. Mater. Sci. 1981, 16, 2897. Dickinson, J. T.; Park, M. K.; Donaldson, Ε. E.; Jensen, L. C. J. Vac. Sci. Technol. 1982, 20, 436. Dickinson, J. T.; Jensen, L. C. J. Polym. Sci.: Polym. Phys. Ed. 1982, 20, 1925. Dickinson, J. T.; Jensen, L. C.; Park, M. K.Appl.Phys. Letts. 1982, 41, 443, 827. Dickinson, J. T.; Jensen, L. C.; Park, M. K. J. Mater. Sci. 1982, 17, 3173. Dickinson, J. T.; Jensen, L. C.; Jahan-Latibari, A. J. Mater. Sci. 1984, 19, 1510. Dickinson, J. T. In Adhesive Chemistry: Developments andTrends;Lee, L. H., Ed.; Plenum: New York, 1984; pp 193-244. Dickinson, J. T.; Jensen, L. C.; Jahan-Latibari, A. Rubber Chem. Technol. 1984, 56, 927. Dickinson, J. T.; Jensen, L. C.; Jahan-Latibari, A. J. Vac. Sci. Technol. 1984, 17, 1112. Dickinson, J. T.; Jensen, L. C.; Jahan-Latibari, A. J. Mater. Sci. 1985, 20, 1835. Dickinson, J. T.; Jensen, L. C. J. Polym. Sci.: Polym. Phys. Ed. 1985, 23, 873. Dickinson, J. T.; Jahan-Latibari, Α.; Jensen, L. C. In Polymer Composites and Interfaces; Kumar, N. G.; Ishida, H., Eds.; Plenum: New York, 1985; pp 111-131. Dickinson, J. T.; Jensen, L. C.; Bhattacharya, S. K. J. Vac. Sci. Technol. 1985, A3, 1398. Dickinson, J. T.; Jensen, L. C.; Williams, W. D. J. Am. Ceram. Soc. 1985, 68, 235. Dickinson, J. T.; Dresser, M.J.;Jensen, L. C. In Desorption Induced by Electronic Transitions DIET II: Brenig, W.; Menzel, D., Eds.; Springer -Verlag: New York, 1985; pp 281-289 Dickinson, J. T.; Jensen, L. C.; McKay, M. R. J. Vac. Sci. Technol. 1986, A4, 1648. Rotem, Α.; Altus, E. J. Testing Eval. 1979, 7, 33. Rooum,J.;Rawlings, R. D. J. Mater. Sci. 1982, 17, 1745. Jeffery, M. R.; Sourour, J. Α.; Schultz, J. M. Polym. Compos. 1982, 3, 18. Udris, A. O.; Upitus, Z. T.; Teters, G. A. Mekhanika Kompozitnykh Materialov 1984,5,805. Kurov, I. E.; Muravin, G. B.; Movshovich, Α. V. Mekhanika Kompozitnykh Materialov 1984, 5, 918. Nakahara, S.; Fujita, T.; Sugihara, K. Proc. 8th Exoelectron Emission Symp., 1985.

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Fanter, D. L.; Levy, R. L. In Durability of Macromolecular Materials; Eby, R. K., Ed.; ACS Symposium Series No. 95; American Chemical Society: Washington, DC, 1979; pp 211-217. Zakresvskii, V. Α.; Pakhotin, V. A. Sov. Phys. Solid State 1978, 20, 214. Bondareva, Ν. K.; Krylova, I. V.; Golubev, V. B. Phys. Stat. Sol. 1984, 83, 589. Wada, Y. In Electronic Properties of Polymers; Mort,J.;Pfister, G., Eds.; John Wiley: New York, 1982; pp 109-160. Grayson, Μ. Α.; Wolf, C. J. J. Polym. Sci.: Polym. Phys. Ed. 1985, 23, 1087. Edland, J. H. D. Photoelectron Spectroscopy; Butterworths: London, 1974; pp 34-37. Maxwell, D.; Young, R.J.;Kinloch, A. J. J. Mater. Sci. Lett. 1984, 3, 9. Becht,J.;Schwalbe, H.J.;Eisenblaetter, J. Composites Oct. 1976, p 245. Barnby, J. T.; Parry, T. J. Phys. D: Appl. Phys. 1976, 9, 1919. Standard Tests for Toughened Resin Composites. NASA Reference Publication 1092, 1983. Grasto, A. S.; Corey, R.; Dickinson, J. T.; Subramanian, R. V.; Eckstein, Y. Composites Sci. Technol. 1988, in press.

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Fractoemission from Epoxy and Epoxy Composites - ACS Publications

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