Fractography of CFRP composites damaged by impact and subsequently loaded statically to failure

Composites 26 (1995) 154-160 © 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0010-4361/95/$10.00

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Fractography of CFRP composites damaged by impact and subsequently loaded statically to failure L. Shikhmanter*, B. Cina and I. Eldror Metallurgy Group, Materials Engineering and Technology Development, Tashan Engineering Centre, Israel Aircraft Industries Ltd, Lod 70100, Israel (Received 13 May 1994) The fractography has been studied of quasi-isotropic CFRP tape composites first damaged by impact of low levels of energy and subsequently loaded to failure in static tension and compression. It was found that a distinction could be made between the damage caused by the impact and that due to both modes of static loading. Any features new to the fractography of CFRP are described and discussed. Visual examination and optical and scanning electron microscopy were employed for analysis of the fracture surfaces. (Keywords: CFRP composites;fractography;impact damage)

In previous work ~ a systematic study was made of the fractography of multidirectional CFRP composites tested statically. This work has now been extended to fractures induced by dynamic modes of loading. Of these perhaps the most important is impact since its immediate effect sometimes is seemingly innocuous, while in reality serious damage may have been caused within the component involved2'3. The present work will be confined to the fractography of damage caused by impact and of the damage caused by subsequent static loading in tension and in compression, in a simulation of what may occur in service. The effect of fatigue loading will be reported separately. EXPERIMENTAL PROCEDURE The work on impact was carried out on a quasi-isotropic plate made of 48 plies of unidirectional tape. The plate was of dimensions 430 x 180 × 6.2 mm. The commercial graphite-epoxy system Magnamite AS4/3502 (Hercules Corp., Magna, Utah, USA), having - 65 vol% of fibres, was employed as in the previous work 1. The diameter of the fibres was 7 /zm. The lay-up of the plate was [45°, 4 5 °, 90 °, 0°]s6. The impact on the plate was obtained by use of a steel impactor of cylindrical shape having a ball-shaped end, 0.5 in (12.6 mm) in diameter, and weighing 10 lb (4.54 kg). Two levels of impact energy were employed, 20 and 30 ft lb (27 and 40.7 N m). Both impact levels caused just barely visible damage to the rear side of the plate, but none to the impacted side, as described

* To whom correspondence should be addressed

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later. Two impacts were performed at the lower level of energy and one at the higher. The distance between the centres of the impacts was 125 mm. The extent of internal damage to the plate was determined by ultrasonic C-scan inspection. For determination of the effect of prior impact damage on the fractography of a specimen tested in compression, one of the regions in the above plate impacted with an energy of 20 ft lb was sectioned. Sectioning was through the centre of the impact such that a specimen 10 mm wide and 40 mm long was obtained. This specimen was loaded to failure in compression applied at the ends of the 40 mm long section. The 0° plies were parallel to the length of the specimen. For determination of the effect of prior impact damage on the fractography of a specimen tested in tension, the material used was also a quasi-isotropic plate made of 48 ply tape. The plate, however, had dimensions of 300 x 180 x 6.2 mm. The lay-up of the plate, [45°, ~t5 °, 0 °, 90°]s6, was slightly different from that tested in compression. Only one level of impact energy, 30 ft lb (40.7 N m), was investigated. The impact was made at the centre of the plate. The extent of internal damage was determined ultrasonically. For determining the effect of tension on the impacted material, a specimen of 100 mm width was sectioned from the length of the plate, such that the impacted zone was at its centre. The actual gauge length employed was 180 mm. The impacting, compressive and tensile testing were carried out at ambient conditions of temperature and humidity in the laboratory. The 0° direction served as the axis for all static loading. The fractured samples were examined visually, optically and by scanning electron microscopy (SEM).

Fractography of CFRP composites: L. Shikhmanter et al. RESULTS

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Damage due to impact There was no visible sign of damage to the plate on the side facing the impactor after applying impact energies of 20 and 30 ft lb (27 and 40.7 N m). However, in both cases, a few cracks up to -- 20 mm in length and parallel to the fibres in the outermost 45 ° ply, were visible on the rear side of the plate. Figure 1 shows the extent of internal damage in the plate, as determined by ultrasonic examination, the chain dotted boundaries having been drawn in to facilitate identification. The zones marked 1 and 2 refer to impacts of 20 ft lb (27 N m) while that marked 3 is from the 30 ft lb (40.7 N m) impact. It will be seen that the zones are roughly circular with little significant difference between them. The × in the centre of each zone represents the point of impact. Figure 2 shows the extent of damage in zone no. 1, as shown by a metallographic section taken through its centre. This section was taken through the width of the plate, henceforth termed the 0 ° direction. The dark bands in this figure are the double 0 ° plies. The white arrow shows the point of impact. The letters D and T in the figure refer to delaminations and transverse cracks, respectively. D1 will be referred to later. As a result of examination of this metallographic section at a magnification of × 25 it was possible to obtain a more detailed picture of the extent of cracking. The results are shown schematically in Figure 3. All the horizontal marks indicated in this figure are delaminations, while the others are transverse cracks. There is considerable symmetry in the location of the cracks with respect to the lay-up of the plies. It will be seen that a delamination formed between every 90 and 0 ° ply on progressing from the side of impact. There was, however, no delamination

Figure 1

Extent of internal damage due to impact as determined by ultrasonic examination. 1, 2 and 3 result from impacts of 20, 20 and 30 ft lb, respectively, x marks the point of impact

Damage of zone 1 of Figure 1 as revealed by a metallographic section. D, Delamination; T, transverse crack; white arrow, point of impact

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Schematic representation of extent of internal damage due to impact of energy of 20 ft lb (27 N m)

between 0 and 90 ° plies on progressing from the side of impact. With the exception of the plies immediately adjacent to the side of impact, a delamination was found between every -45 and +45 ° ply and also between every +45 and -45 ° ply on progressing from the side of impact. All the delaminations tended to increase somewhat in area on progressing from the side of impact. It is to be noted that the maximum extent of delamination, as measured metallographically on the above specific section and as measured ultrasonically over the whole volume of the plate, was 40 and 60 mm, respectively. This would imply that a metallographic section taken in a different direction would have given a different extent of delamination. A further symmetrical feature in Figure 3 is that of the transverse cracks. It can be seen that these occur predominantly in the double +45 ° plies and are fairly uniformly spaced around the axis of impact. In fact these transverse cracks seemingly serve to connect the +45°/ -45 ° delaminations, with the formation of the latter possibly preceding that of the former. These transverse cracks are in fact between the fibres. A further metallographic section, taken this time at 45 ° to the previous section, showed features of delamination very similar to those in the first section. There were, however, no transverse cracks in this section. Likewise a metallographic section taken through zone no. 2 as per Figure 1, albeit on a chord 5 mm removed from the centre of impact, showed features of cracking similar to those in zone no. 1. However, it was also observed that in addition to delaminations on 900/0 ° interfaces, there were delaminations on 00/90 ° interfaces.

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Fractography of CFRPcomposites: L. Shikhmanter e t a I. This implies that the latter delaminations were asymmetrical with respect to the axis of impact. Similar levels of energy of impact gave similar types and extent of damage. A section through the centre of zone no. 3, as per Figure 1, showed features of cracking similar to those in zone no. 1 except that the lateral extent of damage was significantly greater with increasing distance below the point of impact. Likewise the section through zone no. 3 showed an increased number of delaminations, these now appearing also between plies with a relative difference in orientation of 45 °, with transverse cracks between them.

Compression after impact Figure 4 shows the specimen tested to failure by compression after impact. The white arrow shows the point of impact. The black arrows show the direction of the applied compressive stress. The brightest bands are the double 0° plies. The specimen clearly failed by buckling of individual sublaminates concentrated close to the point of impact. The precise forms of failure were the formation of major and minor delaminations, separation into sublaminates and transverse cracking. Figure 5 shows the central section of this specimen at higher magnification. The black arrow shows the point of impact. It can now be seen that the delaminations

were formed between every two adjacent plies of different orientation. Furthermore, separation was observed both between adjacent 0° plies and within many 0° plies themselves. Many of the 0 ° plies were also fractured transversely. The +45 ° plies and 90 ° plies also showed transverse cracking but, in fact, these cracks were between the fibres. By comparison of Figures 4 and 5 with Figures 2 and 3, it can be deduced that the major delaminations observed after failure in compression developed from delaminations formed initially on impact. Not infrequently transverse cracks, observed at the ends of delaminations after impact, played no role on subsequent compression. The other delaminations observed after compression formed due to the compression itself, as did most of the transverse cracks. For comparison, Figure 6 shows a specimen taken from the same plate as above and tested in compression, but without prior damage from impact. Failure was due to shear kinking induced by buckling with attendant delaminations and transverse cracks, but the latter two modes of failure were of considerably smaller extent than in the specimen subjected previously to impact. The seeming distortion of the plies in Figure 6 was in fact made up of a series of broken straight sections. We now turn to the characterization of the surface of the delaminations in the specimen damaged by impact and subsequently tested in compression. Figure 7 shows a sublaminate which separated from this specimen. The surface observed is composed of two separate delaminations designated by bracketed areas 1 and 2 in the figure. These two areas in fact correspond with the delaminations indicated by the two white arrows in Figure 5, area 2 being that of the left white arrow. The arrow in Figure 7 shows the point of impact. Area no. 1 is a 00/90° delamination, while area no. 2 is a +45°/~5 ° delamination. Area no. 2 is itself divided into two zones of different reflectance, marked I and C in the figure, separated by a serrated boundary. The upper edge of zone I corresponds to the delamination identified as D1 in Figure 2. In fact the length of zone I, and of similar zones observed on other delamination surfaces, corresponded with the lengths of the several delamina-

Figure 4 Specimen tested in compression after impact

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Figure 6 Specimen tested in compression without prior damage from

Specimen shown in Figure 4 but at higher magnification

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impact

Fractography of CFRPcomposites: L. Shikhmanter et al.

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Figure 7 Surfaces of delaminations shown by white arrows in

Figure 5 tions observed in Figure 2. In Figure 7, therefore, the serrated boundary between zones I and C represents that between a delaminated surface formed by impact and its extension by compression. The nature of this boundary, as observed in the SEM, will be described later. It is also to be noted that banded structures, characteristic of delaminated surfaces formed due to static loading 1, can also be observed in both areas 1 and 2 in Figure 7. In area 1 these bands lie in the 0 and 90 ° directions, while in area 2 they lie in the +45 and -45 ° directions. In area 2, the bands in zones C and I are bright and dark, respectively. It should be noted that from examination of other delamination surfaces, those due to impact could show banding in up to three directions whilst those due to subsequent compression could show banding in up to four directions. The fractographic features specific to all these bands will be given later. The fractographic features of the delamination surfaces, as observed by SEM, will now be presented. The delamination due to impact showed alternate zones of shear and peel fractures, with the former predominating. The delamination due to compression likewise showed alternate zones of shear and peel fractures, but in this case the latter fractures predominated. The peel fractures were characterized by the presence of valleys and ridges, respectively, on mating surfaces, as shown in Figure 8 and 9, and by asymmetrical cusps, as shown in Figure 10. Cusps are a form of failure of the matrix due to the operation of shear stresses. They are generally of two types, symmetrical and asymmetrical. The former originate between the fibres, whereas the latter originate on the fibres TM.

Figure 8 Valleys, numbered 1-4, on a delamination due to impact and formed by peeling in the direction indicated

Figure 9 Ridges (r) formed by peeling on the delamination surface mating that in Figure 8. f denotes fibres; arrows indicate river lines

Figure 10 Asymmetrical cusps, some of which are an'owed, on a delamination due to impact and formed by peeling

The reason for the magnitude of the spacing between the valleys, as seen in Figure 8, is as yet not understood. The presence o f fine river lines on the ridges in Figure 9, indicated by arrows, hints at localized quasi-cleavage. We define this as quasi-cleavage because of the curved smooth nature of the surface between the river lines. In Figure 10 the small zone between the two furthest left arrows would also seem to have been formed by quasicleavage. Since the asymmetrical cusps themselves seem to have a brittle character, there would seem to be variations as to how the epoxy matrix can fail in a nonductile manner. Figure 10 also shows many fine pores in the imprints of the fibres. These pores probably resulted from volatiles, absorbed by the sizing on the fibre during the process of manufacture of the fibre and the prepreg, and which were evolved during the cure process of the laminate at elevated temperatures. The extent of this porosity varies widely from batch to batch of material, but no effect has been observed on mechanical properties or on macroscopic or microscopic modes of failure. The zones of shear fracture both in the delamination caused by impact, and in that caused by compression, were characterized by the presence of symmetrical cusps 1'4. Attention will now be turned to the nature of the boundary between the delamination formed by impact and its extension due to subsequent compression. This is shown at a relatively low magnification in Figure 11 as observed in the SEM. I and C represent delaminations due to impact and compression, respectively. Both these delaminations are between +45 and -45 ° plies. In

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Fractography of CFRPcomposites: L. Shikhmanter et al. fibres in quasi-isotropic specimens subjected to static compression ~. The bands observed on the delamination surfaces due to impact were associated with the alternate presence and absence of ridges or valleys. Such features were also observed in previous work ~ after static testing. It is to be noted that whereas in static loading, whether in compression or in tension, bands were additionally associated with alternate resin-rich and resin-poor zones, no such feature was observed with the banding formed due to impact.

Figure 11 Boundary between a delamination formed by impact and that due to subsequent compression

the I zone the fracture will be seen to lie on the -45 ° fibre-matrix interface, while in the C zone the fracture is on the +45 ° fibre-matrix interface. In fact it is the imprint of the +45 ° fibres which is visible in zone C, i.e. the fracture has jumped from one interface to another, through the epoxy layer separating them. Examination of the mating surfaces to zones I and C of course showed features opposite to the above. The reason why the delamination fracture jumped from one interface to another on the application of a compressive stress will be discussed later. Because of the difference in morphology between zones I and C, light is reflected in a different manner from each, thus allowing the appearance of a boundary between them, as observed both visually and by optical microscopy. The morphology of the surface of the transverse cracks which resulted both from impact and from subsequent compression was similar to that of the fracture obtained in unidirectional specimens tested in transverse compression5; the path of the crack was partly through the fibre-matrix interface and partly through the matrix itself, as can be seen in Figure 12 for a crack formed by impact. In addition to fibres and imprints of fibres, there can be seen two types of seeming cleavage in the matrix, as indicated by arrows. The reasons for the river line and tongue-like types of cleavage, as indicated by the arrows to the left and right, respectively, in this figure, are not fully understood but may well be associated with the complexity of the localized stress system operating. Very few cusps were observed on the fracture surfaces of all the transverse cracks. The morphology of fractured individual 0° fibres was very similar to that observed in

Figure 12 Surface of a transverse crack formed by impact

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Tension after impact Figure 13 shows the specimen fractured in tension after impact. White arrows in the figure delineate an approximately circular path of fracture through the fibres. This circular zone, in its size and form, corresponds with the impact-damaged zone detected by ultrasonic examination prior to the tensile testing. To the left in the figure, and marked by a black arrow, can be seen a sublaminate pulled out during the tensile test. Parts of the boundary of this sublaminate correspond with the circular zone observed in the right half of the figure. From all these geometrical correlations it can be deduced that, on tensile testing, the specimen failed at the boundary of the previously impact-damaged zone. On visual examination of the fracture it was observed that, on impact, delaminations had formed between plies with a difference in orientation of 90 °, such as 00/90 ° and +450/-45 °, and that these delaminations had extended due to subsequent tensile testing. Figure 14 shows a +450/-45 ° delamination on the reverse side of the righthand fracture in Figure 13. White arrows indicate the boundary between the delamination formed on impact and its extension by subsequent tensile testing, similar to that observed previously on compressive testing after impact (Figure 7). It should also be noted in Figure 14 that the boundary of the prior impact damage is not perfectly circular, in keeping with the observations made by ultrasonic C-scan examination. Figure 15 shows an example of delaminations between 0 and 90 ° plies caused by prior impact and extended by subsequent tensile testing. This photograph is, in fact, also the reverse side of the sublaminate indicated by a black arrow in Figure 13. The white arrows show the

Figure 13 Specimen tested to fracture in tension after impact

Fractography of CFRPcomposites: L. Shikhmanter et al.

Figure 14 Delamination formed between +45° and 4 5 ° plies in tension after impact

Figure 16 Serratedboundary betweendelaminationscaused by impact (I) and subsequent tensile testing (T)

Figure 15 Delaminationsformed between 0° and 90° plies in tension after impact

Figure 17 Serratedboundary shown in Figure 16 as seen by SEM

boundary of the delamination due to impact. Again this is not quite circular. The two zones marked A are parts of the same delamination between 0 and 90 ° plies, while zone B is part of a delamination between 90 and 0 ° plies. It should be noted that, in fact, B represents two 90 ° plies, so that the B surface visible is separated from the two A surfaces by these two 90 ° plies. Figure 16, taken by a stereoscopic microscope, shows the boundary between the delamination indicated by B in Figure 15 and the propagation of this delamination by tensile testing. This boundary is seen to be serrated. Such a feature was also observed in compression after impact. A view of this boundary region, as obtained by SEM, is given in Figure 17. The letters I and T represent delaminations due to impact and tensile testing, respectively. In the I region, the delamination lies on a 90 ° ply/matrix interface, so that 90 ° fibres and cusps formed in the matrix can be observed. In the T region, the delamination lies on a 0 ° ply/matrix interface, so that imprints of 0 ° fibres and cusps formed in the matrix can now be observed. The initial delamination due to impact propagated, on subsequent tensile testing, through the rupturing of the epoxy layer between the two plies, with the creation of delamination in the 0°/matrix interface. A kindred effect was observed for +45 and 4 5 ° delaminations formed due to impact and subsequent tensile testing, respectively. The epoxy layer between the

two interfaces ruptured, thus facilitating progression of the delamination. Two general observations can be made regarding the delamination surfaces formed by impact and by subsequent tensile testing. The first is that, on examination by SEM, alternate zones of shear and peel fracture could be observed, with the peel predominating in the delamination formed by tension. Likewise for impact followed by compression, peel predominated in the delamination formed due to compression. The second observation is that of bands on all the delamination surfaces as observed on compression after impact. The final morphological feature to be described is that of the cracks between the fibres which were obtained both after impact and after subsequent tension. These cracks were in the fibre-matrix interface and also in the matrix itself, as had been observed in specimens tested in compression after impact. Some of these cracks were located at the ends of delaminations formed on impact. No evidence was found for them playing any significant role under subsequent tensile loading. DISCUSSION Perhaps two of the most important and interesting features of the present work are that each delamination formed by impact was propagated by the subsequent application of a compressive or tensile load, and that the

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Fractography of CFRPcomposites: L. Shikhmanter et al. boundary between the two stages of the delamination could be deafly distinguished. The reason for the latter observation was that the delamination due to impact subsequently propagated through the matrix separating two adjacent plies by jumping from one fibre-matrix interface to another. The reason why the delamination due to impact did not continue to propagate on the same fibre-matrix interface can be understood in terms of the nature of the stresses operating on the specimen in impact and those operating in subsequent compression or tension. The delaminations form on impact due to shear stresses resulting from localized bending of the plies. On subsequent application of a tensile or compressive stress along the length o f the specimen, again shear stresses are developed because of the difference in rigidity of the adjacent plies. These stresses, however, will be concentrated at the ends of the existing delaminations. For specimens in compression, the propagation of the delamination through the matrix separating two plies could have been due to conditions of mixed modes I, II and III. For specimens in tension, similar propagation could have been due to mixed mode II and III conditions. It has also to be explained why the crack continued along the new interface it encountered in both tension and compression, instead of zig-zagging through the matrix between the plies. This could possibly be related to a reduced stress concentration maintained at the new interface because of the change in orientation of the crack from 0 ° to --45 ° with respect to the axis of loading. For both tension and compression, mode II will be the dominant condition. In each case the component of shear stress will be at a maximum in the 45 ° direction, in which the delamination first propagates on application of a tensile or compressive load. On reaching the new interface however, the crack is now inclined at --45 ° to its previous direction. If the crack were to zig-zag back through the epoxy layer, this would imply a 90 ° change of direction. There would be no shear stress concentration acting in such a direction at the tip of the crack where it meets the new interface, although there would still be an operative shear stress at such a location. Accordingly, there is now a greater combined effect forcing the crack to continue along the new interface, which it does. The reason that, on visual examination, a distinction could be made between a delamination due to impact and that due to subsequent static stressing, may be due to the fact that alternate resin-rich and resin-poor zones were not found in the former but were found in the latter, i.e. there was a topographical difference leading to a variation in reflectance. A further point for discussion is that of transverse cracks. Since these were often encountered at the ends of, close to, and far from the ends of delaminations due

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to impact, as shown schematically in Figure 3, they are not considered to have played any part in determining how the delaminations propagated subsequently due to the application of compressive or tensile stress. In fact these transverse cracks may well have preceded the delaminations formed by impact because o f localized tensile stresses. Insufficient work was done to explain the absence of transverse cracks in a metallographic section taken in the 45 ° direction of the plate. Finally, it is to be noted that banding, observed previously on delaminations formed by static stressing 1 and in all four principal orientations o f the fibres, has now also been observed, albeit only in three orientations of the fibres, on delaminations in specimens damaged in impact. The bands observed previously wei'e attributed to microcracks preceding final failure in static stressing. Their observation after impact, together with evidence for the formation of transverse cracks, would suggest that discrete mechanistic stages can be identified even in rapid failure of C F R P specimens. It is to be noted that banding was observed on delaminations formed by static tensile or compressive loading applied subsequent to impact. This is in keeping with previous work 1. CONCLUSIONS (1)

(2) (3)

Fractography can be successfully employed to distinguish between damage caused by impact and that by subsequent static stressing. Banding is common to delaminations formed statically and by impact. Mechanistic stages can be detected even in the rapid failure of C F R P specimens.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the assistance provided by Dr H. Rosenthal and Mrs E. Drucker who supplied the fractured specimens, and offer their thanks to D r A.K. Green for kindly reviewii]g the text of this article. REFERENCES 1 Shikhmanter, L., Cina, B. and Eldror, I., Fractography of multidirectional CFRP composites tested statically Composites 1991, 22(6), 437444 2 Rosenfeld,M.S. and Gause, L.W., Compressionfatigue behaviour of graphite/epoxyin the presence of stress raisers in 'Fatigue of Fibrous CompositeMaterials', ASTM STP 723, American Society for Testing and Materials, 1981, pp. 174-196 3 Clark, G. and Van Blaricum, T.J., Load spectrum modification effects on fatigue of impact-damaged carbon fibre composite coupons Composites 1987, 18(3), 243-251 4 Purslow, D., Matrix fractography of fibre-reinforced epoxy composites Composites 1986, 17(4), 289-303 5 Shikhmanter, L., Eldror, I. and Cina, B., Fractography of unidirectional CFRP composites J. Mater. Sci. 1989, 24, 167-172