Mechanisms of fracture in filled thermosetting resins

$Page 1: Mechanisms of fracture in filled thermosetting resins$
Mechanisms of fracture in filled thermosetting resins Stephen K . Brown*

Fracture properties of a wide range of filled unsaturated polyester resin composites have been investigated with respect to filler size, shape, loading and adhesion to the matrix resin. Mechanisms are proposed for the fracture behaviors found, based on geometrical considerations of interacting filler particles and stress concentrations which result from them.

1. INTRODUCTION

Fillers are added to polymers to impart certain useful mechanical properties. Some can predictably be improved; examples are stiffness, melt viscosity and heat distortion temperature. However, strength properties remain elusive and often unpredictable when filers are incorporated. Also strength measurements may be complicated by high levels of data scatter and susceptibility to surface imperfection, particularly with brittle materials. A technique which over- comes these problems is that where the energies for fractur- es to move through materials are measured. Together with microscopic examination of fracture surfaces, this technique provides a highly discerning tool for investigation of the fracture of materials with a view to determining the mechanisms irivolved. Reinforcement of glassy thermoplastics with rigid and rubbery inclusions is understood to result from widespread craze formation in the matrix due to stress concentrations by the inclusions. i2 Reinforcement of brittle thermosetting resins is less understood. Thermosets are not able to dissipate energy by craze formation, yet typically exhibit fracture energies many orders of magnitude above their theoretical surface energies. Uskov et a1 forcement of an epoxy thermoset by fillers depended on how they influenced a 'supramolecular structure' of the resin, but did not relate this effect to the fracture energy approach. A number of workers have investigated the effect of fillers on the fracture energies of thermosets. Lange and Radford drate composites, Broutman and Sahus glass-sphere-filled epoxy and unsaturated polyester (UP) resins and Hammond and Quayle glass-sphere-filled UP resins. All found that filler incorporation increased fracture propagation energies (Fp) which passed through distinct maxima as filler loadings increased. Such behaviours are not uncommon in two phase materials and have been found for materials as diverse as glass-ceramics,' sodium chloride crystal^,^ ceramic^,^ metal alloys l o and rubbers.' fracture energies of some materials are decreased by filler incorporation, e.g. polyphenylene oxide and polyethylene.' The filled thermosets investigated above exhibited such optimal loadings for fracture energies even when filler- matrix adhesion was very low, and various mechanisms were proposed for such behaviours. It is generally accepted that a crack front travelling through a material will be

found that rein-

investigated epoxy-alumina trihy-

On the other hand, the

Department of Industrial Science, Melbourne University, Parkville, Australia. *Present address CSIRO, Division of Building Research, Hkhett, Victoria, Australia. [Paper received 9 October 1979)

slowed when it meets an inhomogeneity and become tem- porarily pinned at that point. The pinned crack front is believed to bow forward between pinning points, as shown in Figure 1, until it forms semi-circular segments of dia- meters equal to the spacing between the points; then the segments will overlap behind the pinning points and the crack front will break away. This behaviour leads to the increase in Fp at low fdler loadings. Lange and Radford," following microscopic examination of fracture surfaces, proposed that filer particles in composites exceeding the optimal loading became too closely spaced for effective crack front pinning as above to occur. Broutman and Sahu proposed that the force to move a crack front was proportional to the inverse of its radius of curvature, such that past the optimum filler loading it became easier to break interfacial bonds than to bend the crack front to a breakaway position. This theory agreed with their data where Fp maxima occurred at lower filler loadings when filler-matrix adhesion was lower. They also found that at lower levels of adhesion, Fp values were greater at equival-

= filler particle

Fig.1 material under load.

Diagrammatic model of a pinned crack front in a fded

24 THE BRITISH POLYMER JOURNAL, MARCH 1980

ent filler loadings, an effect also noted by Wambach et at I * However, Hammond and Quayle,6 with an almost identical composite system, found quite the opposite effect . The present paper further investigates the fracture of resin composites and proposes mechanisms for observed behaviour. A further paper will consider the relationship between fracture energies and other mechanical properties, including fatigue. All data were prviously presented in a research thesis.

2. EXPERIMENTAL

2.1 Matrix resin

The matrix resin was Monsanto's Crystic 19 1 MV, a general purpose UP resin, catalysed with 1% w/w MEKP (methyl ethyl ketone peroxide, 50% w/w solution) and 0.2% w/w cobalt octoate (6% w/w solution). Specimens were cured at room temperature for 4 hours, postcured at 100°C for 4 hours and then cooled slowly back to room temperature.

2.2 Fillers

Fillers were incorporated in the resin using a high-speed stirrer under vacuum to prevent air entrapment. A number of spherical filers were incorporated: UP spheres, porous UP spheres and glass spheres. The porous UP spheres con- tained a closed cellular structure and were manufactured by a patented commercial process.' Rhombohedra1 calcite filers were also incorporated. Filler properties were varied to permit measurement of the influence of filler size and adhesion to the matrix. These properties are summarised in Table 1. Filler-matrix adhesion was varied by application of surface treatments. No adhesion resulted when fillers were released and treated with a resinous silicone (Dow-Corning DC-20). Strong adhesion to solid UP spheres resulted from treatment with a cationic silane coupling agent (Dow-Corning XZ-85069). Glass spheres were commercially supplied with a proprietary polyester coupling treatment (CW1, Potters Bros, Inc). Calcite fillers were supplied with surface treatments specific for UP resin (Omya BLR2) and polyethylene (Omya BLR3), providing high and low adhesion to the matrix resin, respectively.

2.3 Test specimens

Plane strain fracture energies were determined using a tapered, double cantilever beam cleavage specimen of a design developed by Mostovoy et a1 to provide a constant ratio of compliance (reciprocal stiffness) t o crack length, and shown in Figure 2. These were cast directly in a precision- made, three-piece aluminium mould, employing mould rotation to prevent settling of higher-density fillers. Speci- men ends were separated at O.Smm/min and crack front length ( L ) was measured with a travelling microscope. Fracture energy per unit area was calculated from the equation:

P2 dC

4 w dL (1) F = - -

where P i s the load across the specimen for a crack of length L to grow or arrest, w is the fracture surface width and dC/'dL is the constant ratio of compliance to crack length for the particular specimen.

Table 1 Filler properties

Size Median Filler Range Diam. Density Surface No Description (pml (pm) (g/cm3) Treatment

1

2

3

4

5

6

7

8

9

10

11

12

13

14

porous UP 1-1 1 spheres

porous UP 1-17 spheres





solid UP 1-75 spheres



glass spheres 5-90

glass spheres 5-90

rhombohedral 1-1 9 calcite

rhombohedra1 1-19 calcite

rhombohedral 1-1 9 calcite

3.8

5.1

25

48

65

90

18

18

18

36

36

4.6

4.6

4.6

0.33

0.38

0.59

0.59

0.59

0.59

1.23

1.23

1.23

2.45

2.45

2.71

2.71

2.71

nil

nil

nil

nil

nil

nil

nil

coupled

released

coupled

released

UP specific

PE specific

released

- + - = 4.6 mm-' h3 h F6.3 mrn

+A SECTION AA

: : . : < 1 SECTION BB

Fig.2 Design of tapered DCB cleavage specimen.

2.4 Fracture surface examinations

Fracture surfaces of a number of composites were exaniin- ed at high magnification by either transmission electron microscopy of carbon replicas or scanning electron microscopy of gold or graphite coated sections.

3. RESULTS AND DISCUSSION

Most of the composites studied exhibited discontinuous crack growth, similar to that outlined by Broutman and

THE BRITISH POLYMER JOURNAL, MARCH 1980 25

S r l 1 1 1 , ~ whcrcby a t a critical loading the crack rapidly jumps throiigh thc spcciinen to a diffcrencc length at which its travel is arrcstcd. A rcpctitive sequence of such events was usually found for each specimcn, pcrrnitting the calculation of energies for both fracture propagation ( F p ) and arrest (/=) to accuracies o f t 5-1 0% in most cases (error bars will be prcscntcd in figures for data failing to meet this accuracy). Somc composites, notably those of higher filler contents. exhibited stablc, continuous crack growth through tlic cpccinicn at a constant load, and for these only one fracture energy could be calculated.

3.1 Effect of filler size on fracture energies

This was investigatcd with composites of the porous UP spheres, which varied in particle size as shown in Table 1. Fillers 1 and 2 were significantly smaller than and less dense than others. Their effects on fracture energies are shown in Figures 3 and 4, respectively. Values of F p of both composite series exhibit similar and distinct maxima as filer contents are increascd, that of the larger filler composites occurring at greater filler loadings. Hing and McMillan found that the increases in values of Fp of glass-ceramics, as the crystalline ceramic content increascd, could be represented by the equation:

F,,(gc) = /+',,(g) + constant/Y (2)

where gc and g rcfer to the glass-ceramic and glass, respectively, and Y i s the mean free path between ceramic particles, detcrmined from:

100 I (3)

0.1 0.2 0.3 0.4 0 20 - 0.4 0 0.1 0.2 0.3

Volume Fraction Filler (u f )

0 0 matrix resin 25%w/w styrene 0 matrix resin 40%w/w styrene

Fip.3

26

Fracture energies of composites with filler 1.

100

80 Fracture Energy (Jtm )

60

FP n

F a A

201 0 0.1 0.2 0.3 0.4

Volume Fracture Filler ( v f )

Pig.4 Fracture energies of composites with filler 2.

where D,, is the average particle diameter and vf the volume fraction of the ceramic (or second) phase. A similar relationship was derived by Lange following observations of crack front interactions with inhomogeneities in brittle inorganic crystals. Lange introduced the pinning and bowing mechanism of crack front interaction shown in Figure 1 which was said to require an increase in total crack front length, for which strain energy must increase and thereby fracture energy. However, Hing and McMillan point out that such a mechanism alone is insufficient to explain the six-fold increase in Fp which they encountered. Fracture energies of composites of the present study have also been analysed in terms of r'. Results of analyses of filler 1 and 2 composites are presented in Figure 5. The increase in values of Fp of both composites are linear with

filler 1 0

filler 2 A

0

0

4 8 12 16

Y J (10-2 pm-ll

Fig.5 composites.

Relationship between Fp and Y* for filler 1 and filler 2

THE BRITISH POLYMER JOURNAL, MARCH 1980

Y-1 and are now virtually identical indicating a predomin- ating influence of filer spacing rather than filler size on the fracture energy increases. A further aspect of equation 2 was explored by examining a series of filer 1 composites where the styrene content of the matrix resin was 40% instead of the normal 25% w/w. This resulted in fracture energies Fp and Fa of the matrix resin of 39.2 and 28.7 J/m*, respectively. Fracture energies of these composites are also presented in Figure 3, and while all are greater than the corresponding normal matrix composite values, the increases appear variable and fail to follow the behaviour predicted by equation 2 . The other porous UP spheres (fillers 3 to 6) were considerably larger and FP-Y-' plots for their composites indicate different fracture behaviours to result from their greater sizes, as shown in Figure 6. Initial increases in Fp values are small, but linear with Y-' . However the two largest filler composites exhibit large and distinct increases in Fp values, deviating abruptly from this linearity and passing through maxima at considerably larger filler spac- ings than found with the smaller fillers. In comparison, fracture arrest energies exhibited little change with filer content or size, and for this reason are not presented. It is believed that the sharp rise in Fp may be the result of crack tip blunting when the large fillers are met. Due to the large size of these filers, a crack front interaction (by whatever mechanism) should be expected when the front first meets a filler boundary; consequently, Fp would be better interpreted against the mean particle centre spacing (MPCS, the sum of Y and fde r diameter) than Y. Data are replotted against MPCS-' in Figure 7, where it is seen that the abrupt rise in Fp values occurs at similar values of MPCS for both composite series, indicating that the mechanism responsible for the rises is associated with the crack fronts' contact with filerboundaries, which is consistent with a mechanism of crack tip blunting.

f i l ler 3

f i l ler 4

f i l ler 5

The interaction of porous W spheres with crack fronts was complicated by fracture of the fillers themselves in some cases, as shown by the electron micrograph of the fracture surface of a filler-1 composite in Figure 8. Consequently, only solid fillers were employed in further investigations.

f i l ler 5 x

f i l ler 6 0

01 I 0 0.2 0.4 0.6 0.8

MPCS' (1 0-2 prn.' )

Fig.7 filter 6 composites.

Relationship between Fp and MPCS' for filler 5 and

Fig.8 Fracture surface appearance of filler 1 composite (l0,OOOX).

3.2 Effect of filer-matrix adhesion

The other filer types investigated all provided different levels of filler-matrix adhesion, as presented in Table 1. Fracture energy results for the three filler composite systems are presented against filer loading in Figures 9 , 10 and 1 1. Note for all composite systems that Fp values increase and

I I pass through distinct maxima as filer loadings increase. 0.5 1 .o 1.5 Significant influences of filler-matrix adhesion are exhibited

in all cases, lower adhesion leading to composites of greater

Previous workers also found this maximum Fp behaviour, and the mechanisms they proposed were discussed earlier.

0- I 0

Y-' (10-2 prn-? Fp values.

Fig.6 composites.

Relationship between Fp and Y1 for filler 3 to 6


""i 120.

100.

80 Fracture Energly (J lm 1

60

20r 0 0.1 0.2 0.3 0.4 0.5

Volume fraction filler ( v f )

-

-

FP F a filler treatment

0 9 released A A 7 nil n 8 coupled

Fig.9 sphere composites.

Effect of filler-matrix adhesion on fracture energies of UP

1 160

0 0 0.1 0.2 0.3 0.4 0.5

Volume fraction filler (ufl

FP Fa filler treatment

0 0 11 released 0 10 coupled

Fig.10 sphere composites.

Effect of filler-matrix adhesion on fracture energies of glass

01 0 0.1 0.2 0.3 0.4

Volume Fraction Filler ( u f )

FP Fa filler treatment

0 0 14 released A A 13 PE specific 0 12 UP specific

Fig.11 calcite filled composites.

Effect of filler-matrix adhesion on fracture energies of

Evidence from the present study will be shown to support the mechanism of Lange and R a d f ~ r d . ~ Microscopic examination of the fracture surfaces of filler-7 composites of filler loadings before, at and after the Fp maximum reveal- ed steps or 'tail' features associated with filler particles in the first two composites but not in the last. These steps are believed to result from the overlapping of bowed crack front segments on different planes behind filer particles at the crack front breakaway position. Lange and Radford observed the same effect on fracture surfaces of epoxy- alumina trihydrate composites and concluded that the filer particles became too close for effective crack front interaction by bowing to occur. This conclusion can be checked with the prsent results by considering a model where filler particles are equal in size and spacing and arranged in a triangular configuration, as shown on Figure 12. It is seen that at a critical filler loading the bowing crack front segments will just contact leading filler particles at the breakaway position. At higher filler loadings progressively less bowing will occur before the leading filler particles are contacted, and if such contact is assumed to present a breakaway situation crack front pinning will become less effective and fracture energies should drop. Observations in the present and past studies are consistent with this model. The occurrence of continuous rather than


'stick-slip' crack propagation at high filler contents is predicted, as is the absence of steps or 'tails' at f i ler particles on fracture surfaces of composites past the F,, maxima. Maximum values of Fp are expected t o occur a t fdler loadings where the situation modelled in Figure 12 is reached. It can be derived from geometrical consideration of this model that: (a) occur when Y = 1.73 D,,, and (b) a volume fraction of filler of 0.37, irrespective of fdler size.

departure from a linear FP-Y-' relationship should

from equation (3), Fp maxima should occur around x crack f ron t

direct ion o f crack growth I

Fig.12 composite.

Model breakaway posit ion of crack f ron t pinned in f i l e d

Table 2 composite properties at peak F,s

Comparison of derived and experimental

Cornp osite Properties

Filler Ypeak Ypeak Vf,peak Vf,peak NO (derived) (exper) (derived) (exper)

1

2

5

6

7

8

9

10

11

12

13

14

6.6

8.8

113

156

31

31

31

62

62

8 .O 8.0

8 .O

4.8

7.6

126

177

38

nd

72

54

> 70

28 21

24

0.37

0.37

0.37

0.37

0.37

0.37

0.37

0.37

0.37

0.37

0.37

0.37

0.22

0.40

0.34

0.47

0.32

0.40

0.20

0.40

0.49

0.14

0.18

0.16

nd - not determinable; fil ler coupling to matrix led t o fracture o f a high prcpor t ion o f f i l ler particles in continuum w i th the matrix resin.

Comparison of thus derived values with those found in the present study is made in Table 2. Fillers 9, 11 and the calcite filler (12, 13, 14) composites show poor agreement with the derived values of Y , possibly due t o the lack of matrix adhesion by the former and the non spherical shape of the latter (note: examination of fracture surfaces of calcite-filled composites indicated a distinctly different

crack front interaction occurred which appeared t o involve the mechanical locking of the irregular filler particles by the matrix resin). The other composite systems show excellent agreement with the derived values in view of the assumptions made, and strongly support the model proposed for the occurrence of Fp maxima at critical filler contents. Note, however, that this model only considers why maxima values of Fp are observed and not how fracture energy is dissipated in the composite. Crack front pinning and bowing appears t o play an important role in slowing the crack front, but as Hing and McMillan out it alone is insufficient t o explain the large increases in Fp observed. The remaining discussion will consider this area further.

Filler-matrix adhesion is seen to exert a significant influence on Fp values for the spherical filler composites, with less influence for the calcite filler composites, possibly for the reasons outlined above. In all cases lower adhesion results in greater values of I; , in agreement with the find- ings of Broutman and Sahu.{ Fracture arrest energies exhibit considerably less influence by either filler content of adhesion. Rroutnian and Sahu found that the relative water absorptions of fracture surfaces t o bulk composites was greater for composites of lower filler-matrix adhesion. This was believed t o indicate greater sub-surface debonding and cracking in these cases which led t o the greater values of Fp. However, similar water absorption measurements on 1.5 mm thick sections of fracture surfaces of UP and glass sphere composites of the present study failed to find such a result.

A possible explanation for the effect of adhesion comes from the stress analyses of Goodier who showed that a void in a stressed material would cause greater stress concentration over greater volume of the material than a rigid inclusion. These stress concentrations may influence energy dissipating processes at a pinned crack tip in a UP resin when filler particles are present. While UP resins have crosslinked structures, these are far from infinite, their tensile moduli typically being 20 times less than that of an infinitely crosslinked structure, and their fracture energjes about 40 times greatei than the theoretical fracture energy (0.7 J / m 2 ) of organic resins.' sipation appears t o occur under the high stresses operative at a crack tip in a UP resin, possibly by localised plastic flow.

When filers are incorporated in the matrix resin, they may lead t o greater stress concentrations in the plastic zone around the pinned crack front. with consequent greater energy dissipation leading t o greater fracture propagation energies as filler loadings increase. If rigid fillers of n o adhesion t o the matrix resin are assumed t o act analogous t o the voids of Goodier's analysis, then their composites should experience greatest energy dissipation in the plastic zone and greatest fracture propagation energies, as is found. Fracture arrest, in which a fast-moving crack front stops abruptly, should involve less energy dissipation by plastic deformation due t o the greater rate of deformation involved, and should thus exhibit less influence by either filler incorporation or adhesion, as observed in this and past studies.

Plastic flow of the matrix resin can be restricted by lower- ing the test temperature during fracture. When filled composites are tested at -100°C a very significant change in fracture behaviour is found, as shown in Figure 13 for UP sphere composites. At -100°C: fracture propagation

point

Considerable energy dis-


32

30

Fracture 28. Ener ies (J/m 1 26- 9

22 24F

I

- -

I I

0.1 0.2 0.3 0.4

Volume Fraction Filler (uf)

FP F a filler treatment

0 0 9 released n A 7 nil

Fig. 13 UP sphere composites at -100 C.

energies exhibit only minor increases with filler incorporation and pass through shallow maxima. Also any effect of filler-matrix adhesion is virtually insignificant and fracture propagation and arrest energies for each composite are nearly identical. It is apparent that the energy dissipating processes leading to significant increases in Fp values with filler addition and lowered filler-matrix adhesion at room temperature are the same and become restricted at - 100°C. Microscopic examination of fracture surfaces formed at -100°C showed identical features to those found with room temperature fracture, indicating crack front pinning and bowing to be still operative. These observations suggest that energy dissipation from filler incorporation in room temperature fracture results from increased plastic deformation of the matrix at the pinned crack tip. At -1 00°C plastic deformation becomes restricted and crack front pinning is probably the major energy dissipating process.

Effect of fiier-matrixoadhesion on fracture energies of

4. CONCLUSIONS

Incorporation of fillers in an unsaturated polyester resin leads tq significant increases in fracture propagation energies which pass through distinct maxima at room temperature as filler loadings are increased. This enhancement of

Fp values is believed to result from a combination of factors, involving pinning of the crack front followed by stress-concentrating influences by filler particles. Rigid fillers of low interfacial adhesion are proposed to cause greater stress concentration, resulting in greater energy dissipation in the plastic zone at the crack tip by plastic deformation of the matrix resin. The occurrence of -

maximum values of Fp at critical filler contents is proposed to result from the filler particles becoming too close so that slowing of the crack fronts by pinning and bowing becomes less significant.

5. ACKNOWLEDGEMENT

The author is indebted to his Supervisor, Mr 0. Delatycki and the Engineering Department Workshop, Melbourne University, to Dulux (Aust.) Pty. Ltd. who provided a scholarship under which this research was carried out and to Mr A. Albrick (ICI), Mr H. Jaeger (CSIRO), Mr P. Paterson (RMIT) and Mr and Mrs Silver (MRL) who carried out electron microscope studies of fracture surfaces.

References 1 2

3

4 5

6

7 8

9

10

11 12

13

14

15

16 17 18

Hagerman, E. M., J. Appl. Polym. Sci., 1973,17,2203. Lavengood, R. E., Nicolais, L. G. Narkis, M., ibid, 1973,17, 1173. Uskov, I. A., Tarasenko, Yu G. & Nizhnik, V. V., Mekhanika Polimerov, 1967,3(6), 1060. Lange, F. F. & Radford,K. C., J. Mater. Sci., 1971,6, 1197. Boutman, L. J. & Sahu, S., Proceedings o f 26th Annual Tech- nical Conference on Reinforced Plastics Division, SPl, 1971, 14C. Hammond, J. C. & Quayle, D. V.. Second International Conference on Yield, Deformation and Fracture of Polymers, Gmbridge, 1973. Hing, P. & McMillan, P. W.,J. Mater. Sci., 1973,8, 1041. Gross, G. E. & Gutshall, P. L., International Journal of FractureMechanics, 1965, 1, 131. Passmore, E. M., Sprigs, R. M. & Vasilos, T., J. Am. Ceram. SOC., 1965,48, 1. Gurland, J. & Parikh, N. M., in Fracture VZZ, 1972, p 842, ed. H. Liebowitz, London: Academic Press. Kendall, K., Brit. Polyrn., 1978, 10, 35. Wambach, A., Trachte, K. & Di Benedetto, A., J. Compos. Mater., 1968,2(3), 266. Brown, S. K., Crack Propagation in a Filled Thermosetting Polymer. M . Appl. Sc. Thesis, Melbourne University, 1975. GiUan, J. & Kershaw, R. J., ‘Porous Crosslinked Polyester Resin Granules’. Balm Paints Ltd. Ger. Offen. 2,063,239 (Cl. C 08& 15 July, 1971, Australian App. 22 Dec. 1969,

Mostovoy, S. & Ripling, E. J., J. Appl. Polyrn. Sci., 1966, 10, 1351. Lange, F. F., Phil. Mag., 1970,22,983. Goodier, J. N., Trans. ASME, 1933,55, A39. Averbach, B. L., in Fracture: A n Advanced Treatise, Vol. 1, 1968, p 441, ed. H. Liebowitz, London: Academic Press.

48 PP.


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Mechanisms of fracture in filled thermosetting resins