13
Fracture, Strength and Fatigue of Filled Thermoset Composites Stephen K. Brown* reasons for this are discussed. 1. INTRODUCTION A previous paper l reported on the fracture properties of filled unsaturated polyester (UP) composites. It showed that fracture propagation energies increased and passed through distinct maxima as filler loadings increased, greater maxima resulting as filler-matrix adhesion was decreased. Mechanisms were proposed for these effects based on the geometrical interaction of filler particles with a crack front and the role of interfacial adhesion on stress concentrations around fillers. Broutman and Sahu * found similar fracture properties for glass bead-filled polyester and epoxy resin composites. However, when they determined impact energies, markedly different behaviour was evident. Impact energy decreased as filler loading increased and interfacial adhesion decreas- ed. The greater strain rate in impact and the expenditure of energy for crack initiation were believed responsible for this poor correlation. However, some studies have found strength properties of thermosets to increase with filler addition, similar to frac- ture energy behaviour. Uskov et a13 found some fillers to be reinforcing and others non-reinforcing in an epoxy resin. Strength characteristics (impact, compression, hardness and bend) of composites with reinforcing fillers exhibited maxima as filler loading increased. This effect was associated with deep-seated changes in the 'super- molecular structure' of the resin when it was cured in the presence of certain fillers. The present study compares the fracture properties of composites reported previously 1 as being reinforced by filler addition, with other mechanical properties of the same com- posites. Flexural strength and impact energy provide measures of the strengths of composites. Fatigue proper- ties provide information on the abilities of the composites to resist the development and growth of cracks (which eventually bring about failure) as a result of a large number of deformations. Also, little appears to have been publish- ed on this latter area for particulate filled composites, but it may be more greatly influenced by fracture propagation energies than by strength properties. For example, Hertz- berg et a1 found that the resistance to fatigue crack ad- vance in various polymers was enhanced by the presence of crystalline regions or rubbery regions. Department of Industrial Science, Melbourne University, Parkville, Australia. 'Present address CSIRO, Division of Building Research, Highett. Victoria, Australia. (Manuscript received 25 September 1981) 2. EXPERIMENTAL 2.1 Specimen preparation Composites consisted of an unsaturated polyester resin with a wide range of fillers (see Table l), as described previously. Filler-matrix adhesion was varied by application of surface treatments to the fillers. Fillers were incorporated into the resin using a high speed stirrer operated under vacuum to prevent air entrapment. Rod-shaped specimens were pre- pared by sucking these mixtures into release-treated Pyrex glass tubes. These were allowed to gel at room temperature before being postcured at 100°C for four hours. Specimens prepared in this way had very smooth and scratch free surfaces. Table 1. Filler properties Filler Size Median Density Surface description range diameter treatmen ts (run) (run) (s/cm31 Flexural strength, impact energy and fatigue resistance are reported for a wide range of filled unsaturated polyester resin composites. Behaviour of these properties is compared with fracture energies reported earlier. Little correlation with the latter is found and THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982 Vesiculated UP beads 1-11 3.8 0.33 Nil Solid UP beads 1-75 18 1.23 Nil, coupled or released Glass beads 5-90 36 2.45 Coupled or released R horn bohe- Coupled, poor dral calcite 1-19 4.6 2.71 bonding or released 2.2 Flexture testing Rod-shaped specimens 125mm long and 6mm in diameter were tested in three-point bend at a span of 104mm and crosshead speed of 2mm/min. Testing was carried out at room temperature (18 to 25OC) and humidity. Results represent averages of 8-10 replicates. Flexural properties of specimens of diameter D and test span L were determin- ed from the following equations : 8PL s = -- lrD3 4L3m TME = - 3 7rD4 (3) 1

Fracture, strength and fatigue of filled thermoset composites

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Page 1: Fracture, strength and fatigue of filled thermoset composites

Fracture, Strength and Fatigue of Filled Thermoset Composites Stephen K. Brown*

reasons for this are discussed.

1. INTRODUCTION

A previous paper l reported on the fracture properties of filled unsaturated polyester (UP) composites. It showed that fracture propagation energies increased and passed through distinct maxima as filler loadings increased, greater maxima resulting as filler-matrix adhesion was decreased. Mechanisms were proposed for these effects based on the geometrical interaction of filler particles with a crack front and the role of interfacial adhesion on stress concentrations around fillers. Broutman and Sahu * found similar fracture properties for glass bead-filled polyester and epoxy resin composites. However, when they determined impact energies, markedly different behaviour was evident. Impact energy decreased as filler loading increased and interfacial adhesion decreas- ed. The greater strain rate in impact and the expenditure of energy for crack initiation were believed responsible for this poor correlation. However, some studies have found strength properties of thermosets to increase with filler addition, similar to frac- ture energy behaviour. Uskov et a13 found some fillers to be reinforcing and others non-reinforcing in an epoxy resin. Strength characteristics (impact, compression, hardness and bend) of composites with reinforcing fillers exhibited maxima as filler loading increased. This effect was associated with deep-seated changes in the 'super- molecular structure' of the resin when it was cured in the presence of certain fillers. The present study compares the fracture properties of composites reported previously 1 as being reinforced by filler addition, with other mechanical properties of the same com- posites. Flexural strength and impact energy provide measures of the strengths of composites. Fatigue proper- ties provide information on the abilities of the composites to resist the development and growth of cracks (which eventually bring about failure) as a result of a large number of deformations. Also, little appears to have been publish- ed on this latter area for particulate filled composites, but it may be more greatly influenced by fracture propagation energies than by strength properties. For example, Hertz- berg et a1 found that the resistance to fatigue crack ad- vance in various polymers was enhanced by the presence of crystalline regions or rubbery regions.

Department of Industrial Science, Melbourne University, Parkville, Australia. 'Present address CSIRO, Division of Building Research, Highett. Victoria, Australia. (Manuscript received 25 September 1981)

2. EXPERIMENTAL

2.1 Specimen preparation Composites consisted of an unsaturated polyester resin with a wide range of fillers (see Table l), as described previously. Filler-matrix adhesion was varied by application of surface treatments to the fillers. Fillers were incorporated into the resin using a high speed stirrer operated under vacuum to prevent air entrapment. Rod-shaped specimens were pre- pared by sucking these mixtures into release-treated Pyrex glass tubes. These were allowed to gel at room temperature before being postcured at 100°C for four hours. Specimens prepared in this way had very smooth and scratch free surfaces.

Table 1. Filler properties

Filler Size Median Density Surface description range diameter trea tmen ts

(run) (run) (s/cm31

Flexural strength, impact energy and fatigue resistance are reported for a wide range of filled unsaturated polyester resin composites. Behaviour of these properties is compared with fracture energies reported earlier. Little correlation with the latter is found and

THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982

Vesiculated UP beads 1-11 3.8 0.33 Nil

Solid UP beads 1-75 18 1.23 Nil, coupled

or released Glass beads 5-90 36 2.45 Coupled or

released R horn bohe- Coupled, poor dral calcite 1-19 4.6 2.71 bonding or

released

2.2 Flexture testing Rod-shaped specimens 125mm long and 6mm in diameter were tested in three-point bend at a span of 104mm and crosshead speed of 2mm/min. Testing was carried out at room temperature (18 to 25OC) and humidity. Results represent averages of 8-10 replicates. Flexural properties of specimens of diameter D and test span L were determin- ed from the following equations :

8PL s = -- lrD3

4L3m TME = -

3 7rD4 (3)

1

Page 2: Fracture, strength and fatigue of filled thermoset composites

where S is flexural stress, E is flexural strain, TME is tangent modulus of elasticity, P is applied load, y is specimen deflec- tion and m is the slope of the tangent to the initial straight line portion of the load-deflection curve.

2.3 Impact testing

Impact energies were determined by an unnotched Charpy impact test performed with a Hounsefield Impact Tester (falling pendulum device). Rod-shaped specimens 6mm diameter and 45mm long were tested at a span of 32mm. Testing was done at room temperature and humidity and results were averaged from eight replicates. Notched spec- imens were tested initially but it was found that the impact energy measured was of similar magnitude to the kinetic energy of the broken pieces (determined from their dis- tance of flight). Kinetic energy constituted only about I0 per cent of the impact energy of unnotched specimens and so this method of test was adopted.

2.4 Fatigue failure

Constant strain (sinusoidal) flexural fatigue was carried out on rod-shaped specimens 6 to lOmm diameter at a test span of 203mm. As the thermoset composites fail in flexure at only a few per cent strain, a fatigue machine was designed and constructed to provide the low strain levels (0.2 to 1.2 pcr ccnt) involved. Full details of the design of the mac- hine are described elsewhere.6 Its main features were a motor-driven eccentric mounted along a moving arm pivot- ed at one end. Push rods from this arm deflected the middle of clamped specimens by distances dependent on their location along the arm. Testing was conducted at

O FP

Fa

600+20 cyles/minute and at room temperature (1 ~ 3 0 ° C ) and humidity.

3. RESULTS AND DISCUSSION

3.1

While the mechanical properties of composites are the main areas of interest covered by this report, it was considered that the fracture and strength behaviour of the unfilled UP resin should also be understood and this area is discussed first. Features investigated were the influences of test tem- perature and styrene content of the resin. Fracture energies for the UP resin were determined between -100°C and 70°C and are presented in Fig. 1. Note that fracture propagation energy, F,, increases slowly up to 40°C but thereafter increases dramatically. In contrast, fracture arrest energy, Fa, increases slowly over the whole temperature range and exhibits no marked increase. This behaviour is different from that reported for other thermo- sets ' $8 and was investigated further. Tangent moduli of elasticity (TME) in flexure were determined over this tem- perature range and were,found to exhibit a marked discon- tinuity around 35 to 40 C. suggesting that the glass transition occurred in this region. The glass transition is defined as the onset of extensive molecular motion under load and this transition may be responsible for the sharp rise in F,. However, Fa would also be expected to exhibit the sharp rise because the glass transition varies little with rate of testing. The lack of a sharp rise in Fa at 40°C suggests that a brittle-ductile transition (the onset of plastic flow)

Fracture and strength of UP resin

I

n n

h 1 W I 1 - I I

v -100 -40 -20 0 20 40 60

Test Temperature (OC)

I i g l Fracture energies of UP resin at different test temperatures.

2 THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982

Page 3: Fracture, strength and fatigue of filled thermoset composites

might occur in this region and be responsible for the frac- ture behaviour observed. By varying the styrene content of the unsaturated polyester resin, the crosslink structure of the cured resin can be varied. As supplied by the manufacturer, the resin contain- ed 25 per cent by weight of styrene which corresponded to a 1 :1 ratio of monomer to unsaturation. This offers the most efficient copolymerisation forming the maximum number of short crosslinks per unit of crosslinker em- p10yed.~ Complete crossfinking is never attained, even with a very great excess of styrene, as longer crosslinks can form.l0 All of these effects will influence plastic deform- ation in the resin and hence its fracture properties. The effedt of different styrene contents on the fracture proper- ties of UP resin is shown in Fig. 2. F, increases at high styrene contents, probably due to the formation of longer crosslinks. Increase in F, at low styrene contents may be due to fewer crosslinks being formed. Fracture energies vary little for styrene contents between 20 and 35 per cent by weight. This may reflect some balance between the number of crosslinks formed and their lengths and how these influence plastic deformationin the resin. Other strength properties showed similar variations with styrene content. Energy to break in flexure varied little between 25 and 40 per cent styrene, but then increased by more than half at higher contents. Unnotched impact

60 ..

50 -

O F P Fa

energies exhibited identical behaviour (Fig. 3) but did not increase at very low styrene levels. This latter aspect corre- lates closely to fracture arrest behaviour, due possibly to the high rate of deformation involved in both cases.

3.2 In a previous report tested at room temperature that:

Strength and impact properties of filled resin it was shown for all filed composites

F, increases with filler loading and passes through a distinct maximum; composites of fillers with lower interfacial adhesion exhibit greater F, values; F, either changes little with filer addition or mirrors the effects found for F, but t o a less pronounced extent.

Such trends will now be compared with the strength and impact properties of the same composites. These properties will be compared also with behaviours pre- dicted from other studies. Nielsen used theoretical models to predict the mechanical properties of sphere filled polymers relative to the volume fraction of filler and filler adhesion. In broad terms he predicted a sharp drop in mechanical properties with low filler loadings (around 5 volume per cent) followed by a slower decline. Composites

Fig.2 Styrene Content (% wlw)

Fracture energies of UP resin at different styrene contents.

THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982 3

Page 4: Fracture, strength and fatigue of filled thermoset composites

Styrene Content (% wlw)

l.ig.3 impact energies of UP resin at different styrene contents.

with greater filler adhesion were predicted to exhibit great- er strength. Data will be presented in this study relative to volume fraction of filler to allow comparison with these predicted behaviours. Also, Hing and McMillan l 2 found that the strength of glass ceramics increased in proportion to the inverse square root of thc mean free path between crystal particles (Y). As Griffith and the flaw size of a material at failure, they considered that the microcracks important to strength existed only in the glass phase, being terminated by crystal boundaries. In other words, rather than act as failure-generating flaws the crystals served to limit the lengths of microcracks causing failure. The relationship between strength and Y-” will be further explored in the present study.

predicted such proportionality between strength

3.2.1 Vesiculated UP bead composites Composites with this filler exhibited a very distinctive max- imum in F, as filler content increased, pealung around 35 per cent by volume filler. Flexural strengths exhibit pro- foundly different behaviour, decreasing throughout filler addition. When plotted against Yyz this decrease appears linear and extrapolates back to the strength of the unfilled resin (Fig. 4). Using these strengths and the values of F, reported previously, the inherent flaw sizes at failure ( ~ 0 ’ ) were determined from Griffiths equations and plotted against filler content (Fig. 5). It appears that rather than limiting the flaw size at failure, the presence of fillers in- creases it profoundly: As these fillers have a vesiculated internal structure which is weak, it is suggested that failure results initially from fdler collapse (as observed pre- viously I ) . This causes stress concentration on the matrix

resin resulting in the generation of flaws which lead to ul- timate failure.

3.2.2 Fillers with different interfacial adhesion Strength properties of composites with fdlers possessing different levels of interfacial bonding were investigated. Fillers used were solid UP beads and rhombohedra1 calcite as described in the Table. Composites with all of these were reported previously to exhibit pronounced maxima in fracture propagation energies as filler contents increased, greater maxima resulting from lower interfacial adhesion. Flexural strengths of all UP bead composites dropped sharply at low filer loadings but then showed little change as fdler was added (Fig. 6) . Filler adhesion influenced the level to which strength dropped, greater adhesion leading to greater strength. This behaviour is very similar to that pre- dicted by Nielsen.’ However, when the data were plotted against Y- K , no linearity was observed. Impact energies for these composites showed a sharp drop with filer incorpor- ation for the case of no adhesion only. Other composites exhibited a notably dower decrease with filler addition (Fig. 7), and little resemblance to behaviour predicted by Nielsen. When plotted against Y-yz, impact energies exhib- it reasonably linear decreases. This difference in behaviour from flexural strength data may reflect the susceptibility of the matrix resin to induction of flaws by an impact load. Flexural strengths and impact energies of composites with calcite fillers all dropped sharply with increasing filler load- ing, and showed no tendency to flatten out at higher load- ings (Fig. 8a and 8b). This behaviour is quite different from Nielsens predictions for sphere-filled composites and

4 THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982

Page 5: Fracture, strength and fatigue of filled thermoset composites

1.0 (

0.9 c H E f% 2 - - E 0.8

P

U u 4- .- -

0.7

0.6

0.5

0.4 1 I L 1 1 I I

0.05 0.10 0.15 0.20 0.25 0.30 0.35

y-% (/lrn.")

Fig.4 Relative flexural strength of vesiculated UP bead composites versus (mean free path)Jh.

may reflect some influence of filler shape on strength pro- perties. However, the decrease in these properties was found to be linear with Yya and, again, fillers of greater in- terfacial adhesion produced composites of markedly greater strength. Examination of cleavage fracture surface topographies indicates significant differences between the fracture of sphere-filled and rhombohedron-filled compos- ites. In the former case the plane of fracture is well defined and filler particles separate cleanly from surrounding matrix resin (Fig. 9). However, in the latter case, the asymmetrical orientation of filler particles relative to the fracture plane results in their being mechanically keyed to the matrix. Separation from such particles during fracture results in multiple failures of the resin on a microscopic scale and a smooth, coherent fracture plane is obliterated (Fig. 10). In none of the composites tested does flexural strength, energy to break in flexure, or impact energy correlate with fracture energy behaviour reported earlier.' Broutman and Sahu attributed this lack of correlation to factors such as differences in strain rate, or the added energy needed for crack initiation in strength testing. However, data present- ed earlier for unfilled resins showed good correlation be- tween fracture, strength and impact testing. This suggests that incorporation of fillers has a pronounced effect on crack initiation processes so that they overshadow the in- creased fracture propagation energies. Crack initiation

may occur more easily in composites of lower filler adhe- sion because these always exhibit lowest strength. Coodier 2 7 showed that fdlers of no adhesion exerted greatest stress concentration on their surroundings. Such stress concentrations may result in easier crack initiation in the matrix resin between filler particles and thus be associa- ted with the linear strength - Y-% relationships reported above.

3.3 Fatigue failure is believed to involve the incremental growth of cracks during cyclic testing at stresses lower than those that lead to catastrophic failure. However, it may also in- volve a fracture initiation step,14 and although there is a general lack of evidence for such a step it has been detected in some cases.' 5-1 8

Andrew l 4 derived the following equations for the number of cyles to fatigue failure (nf) , assuming that incremental crack growth commences at a preexisting intrinsic flaw of length co .

Fatigue properties of filled composites

nf = (p K2 '& cO)-l rate independent (4)

nf = (n - l)-l (q Kn Wn t con- l)-l rate dependent (5)

where p, q and K are constants, W the maximum strain

THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982 5

Page 6: Fracture, strength and fatigue of filled thermoset composites

6

o a o

i I i

t

a! 0 9

c

0

t 0

0 Lo

0 P

0 N

z

0

THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982

Page 7: Fracture, strength and fatigue of filled thermoset composites

11 strong-adhesion filler A untreated filler o no-adhesion filler

0.4 1 1 I I 1

0 10 20 30 40 50

Volume Fraction Filler (%I

Fig.7 Relative impact energy of solid UP bead composites.

energy for each cycle, t the time for a single cycle and n an empirical power function. For bother cases a plot of log nf versus log W should yield a straight line. If cyclic loading itself introduces flaws that then grow incrementally, then crack initiation will complicate fatigue failure and the above plot may not yield a straight line. The fatigue tester constructed for this study operated under cyclic strains ( E ) up to 0.012. In order to determine maxi- mum strain energies for each fatigue cycle, flexural moduli of specimens were determined over a range of testing speeds and then extrapolated to determine moduli (M) in Pascals at the rates involved in fatigue. Strain energies were then computed from %MeZ, and were used in analysis of fatigue data.

exhibited a similar fatigue behaviour on a strain basis, but as filers profoundly reduced specimen moduli, behaviours on a strain energy basis were markedly different (Fig. 1 I(b)). Note the linearity of the log W/log n f relationships and that fatigue resistance decreases with increased filler loadings. While some role of incremental crack growth is indicated, no correlation is found with fracture energy behaviour .

3.3.2 Solid UP bead composites Fatigue results for these composites at 5 and 20 per cent by volume filler and with different levels of interfacial adhesion are presented in Figs 12 and 13. Results are pre- sented for maximum strain energies only because these show the same trends as strain data, with filers exerting little influence on specimen moduli. Fatigue resistance of 5 per cent filled composites was greatly influenced by filer adhesion. The composite with no filer adhesion exhibited least fatigue resistance at high strain energies, and greatest resistance at low strain energies. In fracture testing, this composite exhibited greater fracture propagation energy

3.3.1 Vesiculated UP bead composites Fatigue results for composites with this filler at loadings 15 and 27 per cent by volume are presented compared to the unfilled resin in Fig. 1 I ( Q ) . The unfilled resin exhibited a well defined fatigue limit of 0.65 per cent strain, approxi- mately one-quarter of its static breaking strain. Composites

THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982 7

Page 8: Fracture, strength and fatigue of filled thermoset composites

0 strong-adhesion filler A weak adhesion filler 0 no-adhesion filler

8

1 .o

$ 0.8 al

w

H E

c 0.6

- 0

- a"

0.4

0.2 0 10 20 30 40 50 60

Volume Fraction Filler

Fig3 Relative flexural strength and impact energy of calcite filled composites.

THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982

Page 9: Fracture, strength and fatigue of filled thermoset composites

V

Fig.9 Fracture surface of 33 per cent - coupled glass bead composite (400~). V

Fig.10 Fracture surface of 19 per cent coupled calcite compbsite (2000~). V

unfilled x 15% v l u filler j 27% vtv f i l ler

THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982 9

Page 10: Fracture, strength and fatigue of filled thermoset composites

5 8

5 6

5 4

5 2

3 5 0

4 8

4 6

4 4

4 2

unfilled

x 15% vlv filler

f , 27% vlv filler

(bl

X-+

A+

lbip 1 1 l,atigue properties of vesiculated UP bead composites.

5 8

5.6

5 4

5 5.2 3 -

5 0

4 8

4 6

4 4

4 2

l; ig.l2

10

unfilled ;i strong-adhesion filler A untreated filler ( 1 no-adhesion filler

.--, n-+ A - 0-

1 2 ~ ~

5 6 7

Fatigue properties of 5 per cent solid UP bead composites. V

THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982

Page 11: Fracture, strength and fatigue of filled thermoset composites

0 unfilled s strong adhesion filler

'-\ no-adhesion filler

a- n--*

V

V Fig.13 Fatigue properties of 27 per cent - solid UP bead composites.

1.2

1 .o

c ? 6 - 0.8 d 0"

0.6

0.4

unfilled A 5% v/v filler. good adhesion A 5% vlv filler, no adhesion ( 1 25% vlv filler. good adhesion

25% vlv filler, no adlesion

1 I

5 6 7 1 2 3 4

log "f

Fig.14

THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982

Fatigue properties of glass bead composites.

11

Page 12: Fracture, strength and fatigue of filled thermoset composites

5 n t 5 unfilled + 5% vlv filler, good adhesion

A 5% vlv filler, no adhesion 1 25% v/v filler. good adhesron

25% vlv filler, 00 adhesion

1,'ip. 15 1;atigue properties of glass bead composites.

and such fatigue behaviour may suggest greater importance of crack propagation at low strain levels. Such a change in mode of failure is consistent with the non-linear log "/log nf plot found for this composite.

arising around filler particles. Such stress concentrations may lead to induction of flaws early in fatigue, larger flaws being induced by fillers of lower interfacial adhesion.

Fatigue resistance of 20 per cent filled composites shows little influence of fdler adhesion, although all exhibit great- er fatigue resistance than 5 per cent filled composites. This latter effect is consistent with fracture energy behaviour, al- though all composites exhibit less fatigue resistance than unfilled resin which is an inconsistency.

3.3.3 Glass bead composites Fatigue results for glass bead composites at 5 and 25 per cent filler are presented in Fig. 14 in terms of cyclic strain. Results based on maximum strain energies are presented in Fig. 15 and are markedly different from stain-based results due to the high modulus of the glass filer. On the latter basis, fatigue resistance is influenced little by interfacial adhesion in 5 per cent filled composites, but at 25 per cent loading high-adhesion composites possess much greater fatigue resistance. Also, increased filler loading results in greater fatigue resistance for composites of the highadhe- sion filler only, little effect being found with the no-adhe- sion composite. While some trends in fatigue behaviour are consistent with fracture energies reported previously, fdler incorporation appears to complicate fatigue beyond crack propagation effects. The linearity of log W/log nf plots for unfilled resin and most composites of this study suggest that incre- mental crack growth commences at preexisting flaws. However, the transposition of the plots with filler incor- poration indicates that these flaws vary in size with filler loading and filler adhesion. Incorporation even of low levels of fiuers leads to a sharp drop in fatigue resistance, a trend found also in some of the strength properties dis- cussed earlier and associated with the stress concentrations

4. CONCLUSIONS

Strength and impact properties of an unfilled UP resin with different styrene contents correlate well with fracture pro- perties. However, when fillers are incorporated into the resin, poor correlation is evident. Generally, strength and impact energy decrease greatly with fdler addition and are lower for composites of lower ffier-matrix adhesion - exactly opposite to fracture properties. Fatigue propoerties of composites exhibit limited trends consistent with frac- ture properties but are generally profoundly different. Addition of small amounts of filler causes a sharp drop in fatigue resistance below that of unfilled resin. Further filler addition results in an increase in fatigue resistance but not to that of unfiled resin. In general, composites of greater filler-matrix adhesion exhibit greater fatigue resistance. Strength, impact and fatigue properties of composites are believed to be significantly influenced by stress concentra- tions around fillers and their role in crack initiation pro- cesses. This influence is believed to overshadow any effect of fracture propagation in the failure of composites under such tests.

5. ACKNOWLELXEMENT

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. Jaegar (CSIRO), Mr P. Paterson (RMIT) and Mr and Mrs Silver (MRL) who carried out electron microscope studies of fracture surfaces.

12 THE BRITISH POLYMER JOURNAL, VOLUME 14, MARCH 1982

Page 13: Fracture, strength and fatigue of filled thermoset composites

References

1 2

3

4

5

6

7

8 '

Brown, S.K.,Brit. PoZym. J., 1980, 12 (I), 24. Broutman, L.J., & Sahu, S., Proceedings of 26th Annual Technical Conference on Reinforced Plastics Division, SPl, 1971,14C. Uskov, LA., Tarasenko, Yu. G., & Nizhnik, V. V., Mekhanika Polimerov, 1967,3 (6), 1060. Hertzberg, R.W., Manson, J.A., Lk Wu, W . C., ASTM STP 536, 1973,391. Schmitz, J.V., (Ed.), Testing of Polymers Volume 2. Inter- science, 1966, 329. Brown, S.K., Csack Propagation in a Filled Thermosetting Polymer, M. Appl. Sc. Thesis, Melbourne University, 1975. Broutman, L.J., & McGarry, F.J., J. Appl. Polym. Sci., 1965, 9,589. Nelson, B.E., &Turner, D.T., J. Polym. Sci. A2, 1973, 11, 1949.

9 10

11 12 13

14

15

16

17 18

Hayes,B.T., etal, Chem. andInd. (London), 1957, 1162. Hamman, D.K., Funke, W., & Gilch, H., Angew. Chem., 1959, 19, 71. Nielsen, L.E., J. Appl. Polym. Sci., 1966, 10,97. Hing, P. & McMillan, P.W.,J. Mat. Sci., 1973,8, 1041. Griffith, A.A., Proc. Int. Congr. Appl. Mech. (Delft), 1924, 55. Andrews, E.H., 'Fracture in Polymers: Oliver and Boyd Ltd, 1968,182. Bunsell, A.R., & Hearle, J.W.S., J. Appl. Polym. Sci., 1974, 18 (I), 267. Broutman, L.J., & Sahu, %,Proceedings of 24th Annual Technical Conference of Rein forced Plastics Division, SPI, 1969. Laird, C., & Smith, G.C., Phil. Mag., 1963,8,1945. Nakano, Y., & Sandor, B.I., J. Test. Eval., 1974, 2 (3), 196.

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