Creep in Glass Fiber Reinforced Plastics 1. R. BOW The rapidly increasing use of reinforced plastics as structural materials in the chemical industy has not been accompanied by a general appreciation of their mechanical characteristics. Designers, builders, and operators have not had the benefit of re-
views on the structural properties of reinforced p1a.stics. The authors treat one of the most important mechanical properties of this new class of materials with emphasis on load bearing applications interest in the use of reinforced plastics, Increasing notably glass fiber resin systems, for long term load bearing structures makes an assessment of their creep properties of particular i m p k n c e . Although a knowledge of short term mechanical properties gives same indication of the potential of these materials, the mechanisms responsible for degradation are far from fully understood. Creep behavior of such materials when in service is of considerable interest to the design engineer.
CHARACTERISTICS OF CREEP BEHAVIOR The creep behavior of a typical thermosetting glass reinforced material is shown in Figure 1. The curve is characterized by three distinct regions. The primary stage includes a period of elastic strain followed by a period of decreasing rate of creep. The secondary stage is characterized by a constant minimum creep rate dependent upon applied stress and temperature. The increase in creep rate heralds the tertiary stage of creep and the onset of failure of the composite. CREEP MECHANISMS IN RESIN MATRIX AND REINFORCEMENT A property of every amorphous polymer is the narrow temperature range in which it changes from a viscous rubbery condition, at temperatures above this region, to a hard and quite brittle condition below it. A temperature is defined within this narrow band and called the glass transition temperature. The Characteristics of the polymer above and below this glass transition temperature are very different. Thermoset resins differ from thermoplastic resins in that they have a cross-linked structure-Le., they have a three-dimensional random network structure imposing 46
INDUSTRIAL A N D ENGINEERING CHEMISTRY
A. J. BARKER
some degree of restraint on the movement of the internal structure. As a result of these internal structural differences, the glass transition temperature of the thermosetting plastic is, in general, higher than for a thermoplastic, and the transition from the glass to the rubbery state occurs over a greater temperature range. A thermoset resin is therefore less sensitive to temperature change which means that its creep characteristics are in marked contrast to those of a thermoplastic material. Creep in thermoplastics involves the rupture or change of molecular bonds resulting from stress. The action permits molecules, or segments of long molecules, to move relative to their neighbors. It is unlikely that all the bonds between one segment and its neighbors are broken or interchanged simultaneously. It is more likely that such breaks will occur in sequence as a result of the thermal vibrations of constituent atoms. The application of thermal energy induces thermal oscillations which will be of similar magnitude to some of the weaker bond energies. Temperature is therefore influential in producing weep, and a relatively small stress may produce strain energy sufficient to permit place change between certain atoms. Thus continuous deformation may result from stress as the thermal and local energies are redistributed with time. Resistance to creep in linear polymen, such as polyethylene, polypropylene, and methylmethacrylate, may be provided by such measures as increasing the chain length, by adding bulky side atoms to the backbone chain,
4w
15%
0
180 x13 220 240
TlMf h i n J
F i p e 1. Cracp
CUIUCS fm dtyertnt stress levels for polyester resin, chqopad mol lmiwtes in water at 43' C. (77)
and by crystallization, cross-linking, and branching. The effect of each of these conditions is to reduce the mobility of the molecular segments. Space polymers, such as phenolformaldehyde and polyester and epoxy resins, are much more rigid and resistant to creep than the linear polymers because of their three-dimensional network structure. The relatively small amount of creep that does occur in these polymers is probably the result of movement between those segments of the network which are not held by primary bonds but rather by weak secondary bonds. In tension, this mobi1ity is exhausted by relatively small creep strains, and fracture follows as movement begins to affect those segments of the network held by primary bonds. In compression, at elevated temperatures, creep may occur by a flow mechanism involving secondary bonds until the space network is distorted to such an extent that most of the load has been transferred to the primary bonds. At this point the rate of creep is markedly reduced. A difference has been observed in polymers between creep in tension and creep in compression when small stresses are applied at ambient temperatures. The difference can be explained in terms of the normal stress effect (7). Glass is considered to be a supercooled melt of inorganic oxides held together in an essentially amorphous manner by rigid primary bonds. Observation has shown that glass in a flawless fibrous form behaves like a perfect elastic material up to the point of failure. Creep in glass fibers appears to be nonexistent. In a composite the reinforcement can only assist the process of creep by crimped fibers tending to straighten out, groups of fibers physically slipping in the resin, and, particularly toward the onset of failure, the progressive fracture of groups of fibers.
CREEP MECHANISMS I N T H E COMPOSITE MATERIAL At or near the glass transition temperature of the resin, the creep of the composite probably involves the following sequence of events. Initially, ductile flow of the resin occurs. Fibers under tension straighten and those under compression buckle. High stress in the resin due to the transference of load from fiber to fiber causes the resin to creep. In the region of the interface, large shear forces parallel to the reinforcement will develop. Continued stress gradually ruptures the bond Filament-wound pipe, internally lighted, shows the angles of winding VOL. 5 9
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JULY 1 9 6 7
47
0 1.o TIM Ihn.1
F i p e 2. Effect of hfwature OR m‘rmmndal prfonnanu in distilled watm at ambient tanpatwe (75)
at the interface and so delamination begins. As the condition progresses, rupture of fibers occura randomly but at an increasing rate, until eventually complete failure under load occurs. At temperatures below the resin transition temperature (the usual condition for composition seMce), the method of onset of failure is modified. Creep is governed largely by crazing of the resin because of its lower ductility at these temperatures. Failure of the composite usually begins with the failure of the glass resin bond and delamination (3,4, 75), followed by budding of the composite when in compression and by massive rupture of the glasa fibers at points of greatest stress when the composite is under tensile loading. The creep of composites takes place in discrete steplike movements which occur throughout the creep period (75). This phenomenon for a woven glass cloth polyester system is dearly demonstrated by Figure 2. Steel (77) has made a similar observation for a similar reinforcement system using both epoxy and polyester resins. During initial stages of the creep deformation, the average magnitude of the step movements is small but increases as the creep progresses. The movements, which occur at all magnitudes of stress level, are more noticeable at ambient temperature than at elevated temperatures. The mechanism of the stepped movement is not at present completely understood, but theories suggest that it is a “stick and slip” mechanism associated with the physical adhesion of the glass resin interface, together with the progressive rupture of groups of fibers.
CREEP I N =ME SPECIFIC TYPES OF LAMINATE AND INFLUENCE O f CERTAIN PARAMETERS Creep is greater in laminates reinforced with chopped strand mat than in woven fabric reinforced composites when tested under identical conditions (75, 77, 78). The reason for the difference may be attributed to the fact that the glass to resin ratio is generally lower for chopped strand mat by virtue of its irregular fiber distribution and discontinuity, which aLw gives no particular directional strength within the laminate. Few 48
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
10
ICQ
1CJ
lW
1W
106
l l Y I 10 FhILURf lminl
F i p e 3. Flemal creep stress againrt failure tim fol tba laminate 20’ C. (77)
systems in air at
creep data are available for a comparison of the merits of one resin system over another. Data obtained by Steel (77) demonstrate the superiority of the epoxy laminate system compared to the polyester one when both are tested under dry conditions. However, cornparisan of the data obtained for the behavior of epoxy and polyester fabric systems in water at 20° C. shows that the epoxy system is more severely attacked (Figures 3 and 4). I t is common practice in testing laminates to use a flexural test since this provides a greater change in properties for a given exposure time. Tensile or compressive creep tests might, however, provide more useful structural design data. Smr.
The creep rate does not increase significantly with stress level for laminates stresed up to approximately 20% of their ultimate seength. Rawe (79, using an epoxy resin/woven glass cloth reinforcement, makes this obsyvation, and examination of the results obtained by Findley (8) for a silicone resin/woven glass cloth system appears to confirm this fact. However, when the stress level exceeds 30% of the ultimate strength, the tertiary stage Bf creep is reached very rapidly (Figure 1) for the rather extreme case of a polyester/mat laminate immersed in water at 48’ C. The effect of stress level on creep characteristics in any particular laminate system is, however, complicated by the presence of residual stresses. These internal stresses, which become of particular significance in the interfacial region because they influence the failure of the glass resin bond, arise from resin shrinkage following cure. Radial, tangential, and axial forces are induced as the resin cools to ambient temperature following the exothermic reaction of cure. At fiber spacings of the order of magnitude of those found in laminates, the axial force is small (TO), and the tangential and radial forces are of similar magnitude. The failure of the adhesive bond between the glass and the resin and the fracture of glass filaments and resin are not only functions of the externally applied loads but also of the internal system of residual strews. Baev and Malinin (7) have investigated the influence
&r
nMt m FNLWinin.)
Figure 4. F l e m d neep stress agaimt Iogm'thm of failure tim fm three laminate system in water at 2i.F C. (17)
of stres direction on creep characteristi@ of a woven glass cloth/polyester resin system. They suggested that if the stress is less than half the short term strength and the directions of the principal normal tensile stresses coincide with the direction of reinforcement, then creep is governed by the redistribution of stress between the glass fibers and the resin. This occurs during the stress relaxation process of the resin which, when complete, reduces to zero. If the stress level is in excess of about half the short term stress for similar conditions, the creep is governed mainly by the development of damage in the fibers and the redistribution of stress between damaged and undamaged regions, and no reduction in creep rate is obtained. If the directions of the principal normal stresses do not coincide with the directions of the glass reinforcement, then the creep is dependent on distortion of .the resin between the fibers under the action of shearing stresses. Tempwalum
An increase in temperature gradually increases the creep rate and reduces the l i e of the stressed laminate as shown by Figure 5. Steel (77) reports that the stress rupture results for both chopped strand mat and glass fabric/polyester systems can be represented by a series
of parallel straight lines, the intercepts of which are ned by the temperature. At 48' C. the scatter of results is small for both systems, whereas at lower temperatures a wider scatter band is apparent. The explanation is linked to the mode of failure. At temperatures near the glass transition temperature of the resin (56" C.), the failure is largely governed by the ductile flow of the resin, whereas below this temperature, the resin becomes less ductile, and failure occurs, through brittle fracture, in both the resin and the glass fibers. At low loadings it has been shown that about 95% of the creep deformation is recoverable after a rest period of about four times the loading time (2). As the applied stress is increased, the degree of irreversibility of the deformation increases, and the temperature to which the specimen has to be heated to relieve the residual deformation also increases. Deformation, reversible only with rise in temperature, appears to occur in overstressed microvolumes of the resin. As the bond at the interface is broken and the glass begins to rupture, a significant proportion of the creep remains irreversible even at temperatures above the glass transition temperature of the resin. Environnwnt
Except where grm chemical attack of the resin occu~s, aqueous environments degrade the mechanical properties of a composite more rapidly than do nonaqueous environments. Figure 6 shows the relative creep performance of laminates in air, water, and paraffin at 20' C. The mean humidity of the air sample exposed to the air environment was 80%. The effect of immersion in paraffin is to exclude moisture, thus giving improved performance for this sample. Steel (77) further reported that seawater was less severe than demineralized and tap water in degrading laminate properties. Water absorption figures showed that the ingress of water was inhibited, probably through preferential absorption of sodium chloride ions in the polymer lattice. Creep data on a polyester resin/glass fiber material immersed in water have been obtained by Romanenkov (76). He found the initial deformation of fabric glass fiber lami-
"I
. '\
. 0 I .o
. I 10 1 8 . 10s lW 'IP .: ;1w '
'
IIME TO FNLW Imin.1
Figure 5. Stress ruphue r d t s fm polyester resin chopped strand mat in water at 20° C.and 48' C. (17)
Figure 6. Stress rupture bchndor of polyester fabric in air, water, and p m a m at 20' C. (17) V O L 5 9 NO. 7
JULY 1967
49
nates under flexural stress in water, at temperatures from 18' to 24' C., increased by 180 to 200% in comparison with that in the dry state, and reached 300% in composites based on glass mat reinforcement. If severe chemical attack of the resin occurs, then the creep rate increases rapidly and the service life is reduced as shown by the performance of the polyester/glass fiber laminate, immersed in toluene (Table I). Rawe (15) noted that the pH of the aqueous solution was not significant within the range of pH covered by creep tests carried out on various polyester/glass reinforcement systems. However, there was a slight tendency for alkali environments to degrade the laminates more rapidly than acid environments. The environment most commonly encountered by laminates is water, in the form of both liquid and vapor. The presence of water in the resin matrix is considered to have a plasticizing effect on the resin and hence leads to a reduction in the degree of stiffness of the composite. Desorption of gross water from the composite leads to a recovery of a large part of the original stiffness so that it is the presence of this gross water in the laminate which represents a large proportion of the reversible degradation in the composite. The open random network structure of both thermoplastic and thermosetting resins enables individual water molecules to pass through the structure between chain segments. Only in regions of high molecular cohesion, such as crystallites, is the movement of water molecules restricted. If the resin is above its glass transition temperature, then the statistical fluctuations in density and alignment of small segments of the polymer chains allow much larger quantities of water to pass through the resin matrix. The presence of microvoids in the resin may represent critical flaws especially with respect to the shear performance of the laminate; their existence certainly assists the easy passage of bulk quantities of water through the resin matrix. Desai and McGarry (6) preloaded laminates to some known percentage of their ultimate strength before immersing them in distilled water. I t was found that the weight gained from the imbibed water was a direct function of the preload level after the latter had passed a certain critical value. The critical value appeared to be the stress level at which internal crazing of the laminate became significant. Rates of molecular diffusion of water can be much greater along the glass/resin interface than through the resin. The surface of glass fibers can act as capillaries to "wick" the water into the interior of the composite. The phenomenon of water wicking is a process of pressure drawing due to the absence of adequate bonding between the glass and the resin. The collection of water at the interface is regarded as one of the main reasons for the deterioration in composite properties. I t is believed that the water solvates the chemical and physical bonds at the interface. The existence of such compounds as polyvinylalcohol in many finishes with which glass fibers are treated tends to increase the water absorption in the interface region because of the hydrophilic properties of these compounds. 50
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
TABLE I. TYPICAL STRESSED ENVIRONMENTAL PERFORMANCE DATA
Stress level, UFS
Environment
0 '/z
a/
/ 3
Water
I
... 0.9 0.7
...
1 12
2o
0.9 0.8
...
100
Dilute acid
Creep Rate
35 800
... 0.4 0.7 0.8 0.8
...
Toluene
0.5
0.9 1.4 Kerosene
I
2o
0
0.9
2/ 3
0.9 0.85 0.7
Strength Retention ut 7000 Hr., "/o
1000 15 0.7
67
1000 36 2.4 1000 2 0.1
68
...
... 31
...
1000 1000 1OOOb 185 1.1
71 83.6 62.6
1000 1000 600 1.4
94 73
1000 1000 158
99 57
61
...
0
...
1000 250 1000
0
...
1000
110
0 0
...
1000 1000
95 97
'/z 1 13
I s 0 propyl-
..
I/ 2
Time to Failure, Hr.
... 110
alcohol Di-octyl a Average initial creep rate (in./ b Specimens observed to be very cl 15).
x to
...
703)measured oue failure a t conclurio
hr. 1000 hr. (from Rawe, Ref.
st 20
Recent work by Norman (73)suggests that water at the interface displaces resin from the finish rather than finish from the glass surface. However, it is probable that neither an absorbed monolayer nor a thin polymolecular layer of coupling agent and size is an effective barrier to the eventual diffusion of water through the loosely polymerized silane coupling agent layer to the glass surface. Upon reaching the glass surface some form of chemical interaction between the glass and water will almost certainly occur at the many hydroAUTHORS T . R. Bott and A . J . Barker are members of the Departmmt of Chemical Engineering, The University, Birmingham, England. They acknowledge assistance by the chairmen and members of the British Plastics Federation Reinforced Plastics Group Committees, and sponsorship of research by the Ministry of Defence. They also acknowledge J . N . Ratcliffe of the Plastics Institute f o r permission to reproduce data.
philic points on the surface of the glass causing it to become pitted with minute flaws. Hollinger and Plant ( 7 7) have proposed a mechanism for the stress corrosion of glass. The increase of p H within the corrosive layer due to the leaching of alkali components from the glass surface tends to make the glass dissolution autocatalytic, and acceleration of the reaction occurs. The foregoing discussion emphasizes how the presence of water within the composite is likely to influence its properties. While no data are available on the creep properties of laminates made using an untreated glass reinforcement, it is more than probable that the laminate’s creep performance in aqueous environments would be inferior to that of laminates using a silane treated reinforcement. Comparison of short term performance data, obtained for laminates made using both surface finished and unfinished reinforcements, suggests that untreated glass reinforcement leads to inferior mechanical performance.
PREDICTION OF CREEP BEHAVIOR IN COMPOSITE STRUCTU RES I t has been possible to analyze the behavior of linear polymeric materials in terms of microelasticity and related mechanical models (72). However, no satisfactory model has yet been derived that accurately represents the behavior of reinforced plastics. Findley (7) has developed a theory that accurately predicts the creep properties of a number of reinforced plastics. The analysis concerns the rate of activation of movement of one portion of the molecular structure of the resin past resistances to movement. The mechanisms involved include the sliding of segments of molecular chains by a process of place change of atoms bound by secondary forces, and the disruption of secondary and primary bonds. Each of these contributions may be precipitated if the energy available at any instant is in excess of the energy required for such action. According to the rate process theory, the frequency with which the action will be activated is given by: k T -(%)
-.e h In the absence of a shearing force the frequency of jumps past the barrier in two opposite directions will be equal and, therefore, no net deformation will occur. If a shear stress is applied, on the other hand, the barrier will be reduced in one direction by the work done by the stress, whereas the energy level for the jump to occur in the opposite sense is increased. The result will be to produce a net movement in one direction. Using this analysis, Findley developed.the equation y
=
(g)
(”)
dr kT - = 2 a - . esinh (2) dt h 2kT from which the creep may be determined with reasonable accuracy. A different approach to the problem of the prediction of creep behavior in reinforced plastic materials has been
adopted by Goldfein (9). Using the Arrhenius rate equation as a basis, he derived the expression :
T T o (20 - In t> = K -T
To
(3)
Master curves of stress rupture against K are prepared from data obtained in short term tests. If the temperature and stress conditions are specified, the time to failure can be calculated. An empirical approach to the prediction of creep behavior in the long term is the method of curve fitting to experimental data. I n this way it is possible to obtain empirical equations which have limited application.
CONCLUSION This brief discussion of the creep of glass reinforced laminates has shown the complex nature of the phenomenon. The creep mechanisms in the polymers alone involve many factors, added to which are the additional factors due to the presence of glass reinforcement. The complexity of the system is such that no satisfactory theory may be developed to predict creep in all applications and conditions. O n the other hand, specified laminates subject to specified conditions may lend themselves to reliable theoretical treatment. Adequate engineering design methods, whether based on experimental results alone or on theory substantiated by practical work, demand reliable data. Many more practical data are required, and it is to this end that workers are urged wherever possible to follow accepted standard procedures using standardized materials. NOMENCLATURE a
= constant
Af = change, i n free energy across the barrier h = Plank’s constant
k
= Boltzman’s constant
K = constant t = time to failure T = absolute temperature (” K.)
TO= absolute temperature of zero laminate strength-Le., y
=
X Y
= =
r
=
the
melting point of the glass (” K . ) shear strain average distance between equilibrium position of jumps “jump” frequency shearing stress
REF ERENC ES (1) Baev, L. V., Malinin, N. I., Soviet Plastics (7), 42 (1964).
( 2 ) Bershtein, V. A., Glickman, L. A,, Ibzd. ( l l ) , 36 (1963). (3) 16%. (l), 50 (1965). (4) Broutman, L. I.,Mod. Plastics 42 (8), 143 (1765). (5) Daniel, I. M., Durelli, A. J., S.P.I. 16th Annual Technical and Management Conference, Reinforced Plastics Division 8-D, February 1961. (6) Desai, M. B., McGarry, F. J., A.S.T.M. Pub. No. 76 (7) (1957). (7) Findley, W. N., Mach. Des. 32 (lo), 205 (1960). (8) Findley, W. N., Paper No. 16, 2nd International Reinforced Plastics Conference, London, November 1960. ( 9 ) Goldfein, S.,Mod. Plastics 37 (8), 127 (1760). (10) Haslett W. H McGarry F J S.P.I. 17th Annual Technical and Management Conierence,‘heinforced Pias& Division 14-D, February 1962. (11) Hollinger, D. L Plant H. T S.P.I. 17th Annual Technical and Management Conference, R h f o r c i d Plas&s Division 11-A, February 1964. (12) McKelvey, J. M., “Polymer Processing,” p. 6 , Wiley, New York, 1962. (13) Norman, R. H., James,D. I., Gale, E. M., Chm. Eng. 71, 182 (1964). (14) Outwater, J. O., West,D. C . , Mod. Plastics39 (l), 154 (1761). (15) Rawe, A. W., Tmns. PlasticsInst. 30, 27 (1962). (16) Romanenkov, I. G., Soviet Plastics (7), 43 (1963). (17) Steel, D. J., Trans. Plastics Inst. 33 (107), 161 (1965). (18) Thompson, A. W., Ibid., 30 (85), 39 (1962).
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