Factors affecting the stress corrosion of GRP in acid environments

Factors affecting the stress corrosion of GRP in acid environments R~ HOGG GRP fails by stress corrosion in acid environments which may result in catastrophic fracture. As design confidence in GRP improves and the value of arbitrary 'safety factors' is reduced, the need for reliable data on the long term behaviour of GRP in aggressive environments becomes increasingly important. Three factors which control stress corrosion, ie resin, fibre and gel-coats are considered. It is demonstrated that useful improvements may be achieved by varying the resin type and that the optimum resin system must be determined on the basis of mechanical as well as chemical properties. In addition, significant improvements are possible using acid-resistant glass fibre types. The use of resin gel-coats is shown to provide an excellent barrier to acid attack but gel-coat integrity is vital and damaged gel-coats are deleterious.

Key words: composite materials; acid conditions; compression testing; stress corrosion; hoop wound pipes; glass fibres; plastics; gel-coats

The suitability of a particular glass reinforced plastic (GRP) laminate for use in an aggressive environment is usually based on data obtained from simple immersion tests. ~ These tests are frequently performed at high temperature for accelerated testing. The success of GRP in a range of anti-corrosion applications is, in part, due to the successful interpretation of such data and the development of a reservoir of practical experience by fabricators and endusers. Another contributory factor has been the large safety factors that are frequently specified in GRP design, z which can mask errors made in predicting the long term strength of a particular laminate. It is generally recommended that to minimize the effects of corrosion on a GRP laminate an appropriate, chemically resistant, resin should be used. The resin should be post-cured and a non-structural barrier layer should be provided. 3 Typical barrier layers consist of a resin gel-coat reinforced with C-glass or organic fibre veil backed by a number of low volume fraction chopped strand mat (CSM) plies.4 Requirements for the glass reinforcement are usually concerned with the nature of the size and binder to minimize the possibilities of debonding and blister formation, s If corrosion does occur, it usually involves degradation of the resin or the fibre resin interface to produce effects such as debonding, wicking, delamination, disc-cracking and blistering. 6 In recent years it has become recognized that in certain environments, notably mineral acids, GRP will exhibit stress corrosion cracking. 7-12 Failure under such conditions is often catastrophic and involves the propagation of sharp

254

cracks in a brittle manner. 13'14 Under such conditions, the lifetime of a GRP structure cannot be predicted on the basis of immersion test results. Stress corrosion is likely to become an increasingly important consideration in design as confidence in GRP increases and operating safety factors are reduced, is It is therefore important to be able to assess the relative influence of certain laminate variables, eg resin type, on the stress corrosion behaviour of GRP. In this work, the effect of four laminate parameters, namely, resin type, resin post-cure, a resin gel-coat and using alternative glass fibres, were investigated to determine their influence on time to failure in stress corrosion. The test method used throughout was a modified form of the ASTM 3681 ring compression test 9 using cylindrical specimens cut from unidirectional Filament wound hoop pipe.

MA TERIALS AND TESTMETHODS Test specimens were cut in 70 mm lengths from hoop wound pipe of 76 mm internal diameter, produced by f'flament winding. The basic winding process is described in detail in Reference 16. The pipe sections were sealed at each end with transparent sheets of polyvinylidene chloride, bonded with epoxy resin. Test environments were introduced into the sealed pipe using a hypodermic syringe. Pipes were produced with a number of different glass and resin types and the various combinations are listed in Table l, together with the relevant cut'hag schedules.

0010-4361/83/030254-08$03.00© 1983 Butterworth & Co (Publishers) Ltd. COMPOSITES. VOL 14. NO 3. JULY 1983

Table 1.

Resins and fibres used to produce hoop wound pipe

Glass

Resin

Cure system (Amount added for every 100 cm 3 of resin)

Cure schedule

Suprewind 20/70

Impolex T500

1 cm 3 SD2+ 1 cm 3 NL49ST

24 h 20°C; 3 h 80°C; 3 h 120°C

Equerove 23/47

Crystic 272

0.5 cm 3 E + 1.5 cm 3 M

24 h 20°C; 4 h 80°C

Equerove 23/47

Crystic 272 + Crystic 586

0.5 cm 3 E + 1.5 cm 3 M

24 h 20°C; 4 h 80°C

Equerove 23/47

Derakane 411-45

6 cm 3 E + 2 cm 3 M

24 h 20°C; 4 h 80°C

Equerove 23/47

Beetle 870

0.5 c m 3 E + 0.5 cm 3 M

24 h 20°C; 4 h 80°C

Equerove 23/47

Atlac 382-05A

5 cm 3 E + 1 cm 3 M

24 h 20°C; 4 h 80°C

Equerove 23/47

Crystic 272 with a 0.25 mm gel-coat of Crystic 69PA

0.5 cm 3 E + 1.5 cm 3 M

24 h 20°C; 4 h 80°C

Equerove 23/47

Crystic 272 (machined pipe)

0.5 cm 3 E + 1.5 cm 3 M

24 h 20°C; 4 h 80°C

Equerove 23/47

Crystic 272 (machined pipe)

0.5 cm 3 E + 1.5 cm 3 M

24 h 20°C

Equerove 23/47

Crystic 600 PA

0.5 cm 3 M

24 h 20°C; 4 h 80°C

ECR-glass

Crystic 272

0.5 cm 3 E + 1.5 cm 3 M

24 h 20°C; 4 h 80°C

For curing resins, accelerators used were A K Z O NL49ST and Scott Beder E and catalysts used were Interox SD2 and Scott Beder M

To determine the effect of resin type on failure times, pipes were produced with resins possessing a range of mechanical properties and differing corrosion resistance. These resins included Beetle 870, a HET-acid based polyester supplied by BIP and Atlac 382-05A, a bisphenol resin supplied by Atlas Chemicals. Both of these resins are chemically resistant but very brittle. Somewhat tougher chemically resistant resins used were Crystic 600 PA, an epoxy modified bisphenol supplied by Scott Bader, and Derakane 411-45, a vinyl ester resin supplied by Freeman Chemicals. Impolex TS00, a general purpose terephthalic polyester supplied by ICI Ltd, and Crystic 272 a standard isophthalic supplied by Scott Bader, both reasonably tough resins, were also tested, as was a fiexibilized resin system consisting of Crystic 272 with a 30% by weight addition of a flexible resin, Crystic 586. The pipes used for these tests were wound with Fibreglass Equerove 23/47 2400 Tex E-glass rovings, except the T500 pipe which was made with Fibreglass Suprewind 20/70 2400 Tex E-glass rovings. The pipes were wound with a nominal wall thickness of 2 ram. Two pipes were produced to evaluate the effect of postcuring the resin on stress corrosion failure. These pipes were wound oversized and subsequently machined to a constant thickness of 2 nun. This was done in an attempt to any errors due to variations in thickness between the pipes. The pipe without post-cure was allowed to cure for 24 h at room temperature before testing. Pipes were also produced from Owens Coming ECR-glass rovings supplied by Norwegian Glasfiber. These pipes were wound with Crystic 272 resin, again to a wall thickness of 2 ram. The specimens used for gel-coat tests were produced from non-standard pipe. A problem often encountered in environmental testing is that of preferential attack at the exposed cut edges. In ring compression testing, the epoxy resin used to bond the polyvinylidene chloride sheets to the pipe effectively seals the cut edges of the specimen. When pipes are supplied with a gel-coat, the increased time scale of the

COMPOSITES . JU LY 1983

experiment may result in diffusion of the test liquid through the epoxy or through the epoxy-pipe interface, to the cut edge of the pipe, before diffusion can occur through the gel-coat to the underlying fibres. This would result in premature stress corrosion cracking from the edge and invalidate the test. The pipe construction used was designed to eliminate this possibility. Gel-coated pipes were prepared in three stages. Initially, a thin gel-coat layer was brushed onto the mandrel and a constant thickness was maintained using a doctor blade. The gel-coat resin used was Crystic 69 PA supplied by Scott Bader. The gel-coat was allowed to gel before starting the second stage of winding the glass rovings. Filament winding was performed as normal except that discrete, 70 mm lengths of pipe were wound at intervals along the mandrel. The third stage consisted of brushing additional resin over the gaps left between the rovings, so as to consolidate the pipe. The matrix resin used for these pipes was Crystic 272 and the glass was Equerove 23/47 2400 Tex rovings. Test specimens were prepared by slicing the pipe through the pure resin bands between filament wound sections. The edge rovings of the specimens are in this way always protected by a resin barrier greater in extent than the gel-coat itself, and premature edge failure is avoided. The structure of such a gel-coated test specimen is shown schematically in Fig. 1. Gel-coated pipes had a nominal thickness of 2.25 ram, which included a 0.25 mm gel-coat. A number of gel-coated pipes were pre-cracked before stress corrosion testing. These pipes were subjected to a diametrical compression in an Instron testing machine, that was just sufficient to produce gel-coat cracking at position B as shown in Fig. 2. The pipes were then unloaded and used for stress corrosion testing in the normal way. Stress corrosion testing was performed using a ring compression test based on ASTM 3681, illustrated schematically in Fig. 2 and described in more detail elsewhere.9 The load is

255

Hoop windings

Pureresin

,o • o o • e o6"o o•

• • • o"~ll

Table 2. Times to failure of hoop wound pipe in 0.65 M HCI at 20°C

--...,

boooeooooooooeo~

I[

~u9 --

emam

OII

11.20 10.10 8.27 6~7 5.46 3.94

1.15 1.05 0.88 0.74 0.59 0.43

331 302 252 213 170 123.6

50.5 89 180 318 1385 3547

Cryztic 600 PA (epoxy modified bin-phenol)

9.9 8.6 6.5 5.7 4.2

0.95 0.83 0.65 0.56 0.43

337 294 232 199 154

82 270 1290 2380 4900

Derakane 411-45 (vinyl-ezted

17.5 13~ 9.4 9.2 7.5 6.3 5.4 5.0 4.1

1.55 1.27 0~8 0.86 0.72 0.62 0.53 0.49 0.40

572 467 325 317 267 230 190 181 149

20 27 825 292 537 1130 2500 7430 14250

Beetle 870 (HET-ecid)

17.3 13.0 11.6 9.9 7.7 5.2 3.8 3.2

1.63 1.30 1.07 0.97 0,77 0.52 0.38 0.33

553 431 364 335 262 177 131 112

I mpolex T500 (terephthalic)

31.0 26.0 21.0 17.3 12.3 10.6 8.4 4.8

2.10 1.90 1.60 1.35 1.00 0.85 0.70 0.40

780 695 580 490 362 316 252 149

3 2 4 31 98 225 690 12750

Cryntic 272 (bophthalic)

20.1 19.4 14.2 10.5 8.6 8.8 5.3

1.71 1.59 1.22 0.90 0.76 0.76 0.50

595 549 424 314 265 271 169

8 23 76 190 1420 1550 6250

Cryntic 272 + 30% Cryztic 586 (flexibilized itophthalic)

18.3 14.3 13.0 11,3 9.5 5.6 7.2 5.9

1.57 1.20 1.16 1.00 0.80 0.50 0.64 0.55

570 436 420 362 292 177 231 199

32 190 237 625 1220 6840 11500 14000

Resin

D (%)

(%)

(MN/m2)

tl

(mint)

Gel-coot ( 0 . 2 5 m m ) / Atlac 382-05A (bit-phenol) ~eooooooooooooo/.

%, : o o o o o o o o o o o o o e / ~ 6 : ' ~Epoxy

II

/ Polyvinylidenechloride sheel"

Fig. 1 Schematicrepresentation of the construction of gel-coated pipe specimens applied using a cantilever beam and the displacement of the ring is monitored continuously using an LVDT transducer and a PET micro-computer. In previous work 9'z3'z4 it has been shown that failure involves several distinct stages and that the time to failure can be characterized in a number of different ways. In this paper the time to the onset of rapid stress corrosion crack growth, tl, was determined and taken as the time to failure. Atl stress corrosion tests were performed in 0.65 M HCI. A charge of 50 cm s was added to each test cell and this covered the bottom of the specimen to a depth of about 15 mm. Ali tests were carried out at 20°C. After stress corrosion failure, certain pipes were sectioned to remove parts of the fractured material for examination. The fracture surfaces were ultrasonicaUy cleaned to remove debris and then coated with gold before examination in a Philips 501 scanning electron microscope.

6.5 18 27 56 154 624 1400 6400

RESULTS The basic test data is obtained as time to failure, tz for differing initial values of ring displacement AD/D, where D is the average pipe diameter. This data is listed in Tables 2-5.

Previous work has identified the stress acting parallel to the fibre, o,, as the controlling parameter for stress corrosion crack growth. For the fibres at the inner surface of the pipe

~

Lood

Fig. 2 Testrig used for ring compression tests

256

COMPOSITES. JULY 1983

Table 3. Time to failure of machined hoop pipe in 0.65 M HCI at 20°C

Resin

~D -D

= ~fvf

where Em is the resin modulus, Ef is the modulus of the fibres, and Vf is the fibre volume fraction. emax

011

tl

(%)

(MN/m 2)

(min)

(%)

Crystic 272 (post-cured)

12.6 10.7 7.2 4.9 4.0

1.16 1.00 0.69 0.48 0.39

327.2 281.6 193.4 135.0 109.2

34 152 330 7305 48500

Crystic 272

17.9 14.7 11.0 9.5 8.8 7.0 5.7 5.4 4.3

1.60 1.34 1.03 0.89 0.83 0.67 0.55 0.52 0.42

583.8 489.5 375.4 326.7 303.9 244.4 199.5 190.3 154.4

29 148 267 316 2320 3688 9600 12300 24420

(no post-cure)

Table 4. Times to failure of ECR-glass pipe in 0.65 M HCI at 20°C

Resin

Crystic 272

~D -~-

era=

(%)

Oll

t1

(%)

16.6 14.0 12.4 10.4

2.2 1.84 1.72 1.47

660 545 511 435

744 2400 8640 98000

(MN/m 2)

(mins)

Table 5. Times to failure of gel-coated hoop wound pipe in 0.65 M HCI at 20°C

AD Resin

-5(%)

emax

Oil

tl

Crystic 272 with

17.1 16.7 13.9 12.2

1.57 1.54 1.32 1.16

512 501 425 377

10750 7900 132000 240000

17.6

1.61

525

4

15.7 10.7 7.4

1.46 1.03 0.73

475 334 237

30 240

intact gel-coat of Crystic 69 PA Crystic 272 with pre-crecked gel-coat of Crystic 69 PA

+ ~ = (1 - v f )

(%)

(MN/m2)

(mins)

11

in contact with the test acid, Oil is a maximum at point B, shown in Fig. 2, and can be calculated from the maximum surface tensile strain at this point, emax using oil = E emax where E is the modulus of the laminate in the fibre direction and is calculated using the rule of mixtures expression:

COMPOSITES. VOL 14. NO 3. JULY 1983

emax is a function of both the initial pipe deflection and the pipe wall thickness, and was determined using previously obtained calibration curves. 9 In the case of the gel-coated pipes, the relevant value of emax is not the maximum surface tensile strain, but the maximum strain in the fibres nearest the inner surface. For the 2.25 nun thick pipes containing a 0.25 mm gel-coat, emax was determined by ignoring the gel-coat and using the calibrated values of emax for a 2 mm thick pipe subjected to the same deflection. The data from the stress corrosion tests are presented in Figs 3-6 plotted as log all vs log tl. The time to failure decreases with an increase in stress and in most cases there is a linear relationship between log oil and log tt. Fig. 3 shows the dependence of time to failure, tz, on applied stress for pipe with different resin matrices. The chemically resistant resins and, in particular, the very brittle Beetle 870 and Atlac 382-05A, were found to be inferior to the general purpose isophthalic and terephthalic resins, Crystic 272 and Impolex TS00. The flexibilized resin system Crystic 272 + 30% Crystic 586 showed the greatest resistance to stress corrosion cracking. The stress corrosion performance of machined pipes, with and without post-cure, is compared in Fig. 4. The pipes without post-cure took longer to fail than post-cured pipe at similar stresses. A linear relationship exists between log on and log h for the post-cured pipe but it is debatable whether such a relationship holds for the pipe without post-cure. The two sets of results may converge at very low stresses. The effect of using ECR-glass in place of conventional E-glass fibres is shown in Fig. 5. ECR-glass pipe exhibits an increase in failure time for a given stress of the order of three decades of time compared with E-glass. The nature of the failure process is however the same and the fracture surfaces of E-glass and ECR-glass failures, tested at equivalent stresses, are very similar. The results of testing gel-coated pipes are shown in Fig. 6. The provision of an intact gel-coat barrier, 0.25 mm thick, resulted in an increase in failure time of the order of three decades of time. However, if the gel-coat is pre.cracked, time to failure is actuaUy reduced by one decade of time compared to un-coated pipe.

MICROSCOP Y OF GEL-COA TED PIPE FA I L URES

The fracture surfaces of gel-coated pipes were examined using scanning electron microscopy. Stress corrosion cracks are generally characterized by their planar nature. ~'s,t°'11 In the case of gel-coated specimens, the crack through the gel-coat lay in the same plane as that through the underlying reinforced layers. The fracture surface of a pipe subjected to a nominal maximum fibre stress of 500 MN/m2 is shown in Fig. 7. The gel-coat layer, remote from the crack itself, is undamaged, shows no signs of manufacturing defects such as pin-holes, and does not exhibit signs of chemical attack.

257

3

A N

E

Z

:E

Z

:E v

_J

C, C r y s t i c 6 O O P A

0

o F, Crystic 272 eG, Crystic 2 7 2 + 3 0 % 5 8 6 I I I I t 0 I 2 3 4 Log time, ?1 (min)

/

-~

~

I 5

/

6

I 0

o E-glass • ECR-glass I I

I 2

I 3

I 4

I 5

6

Log time, 11 (rain) Fig. 3 Dependence o f time t o failure, t I , on stress parallel t o the fibres f o r pipe with a range o f resin types, tested in 0.65 M HCI at

Fig. 5 Dependance o f time t o failure on stress parallel t o the fibres f o r pipe with E-glass and ECR-glass rovings, tested in 0.65 M HCI at 20°C

20°C

Examination of the fracture markings on the resin gel-coat and fibre-reinforced layers enabled the localized direction of crack growth to be determined, as marked~in Fig. 7. Typical resin markings that are used to assess the direction of crack growth are shown in Fig. 8, which is a higher magnification view of area A in Fig. 7. Mapping the direction of crack growth reveals that gel-coat fracture did not originate at the free inner surface of the pipe, but rather at, or near, the interface between the gel-coat and reinforced layers. For the specimen shown in Fig. 7, it appears that fracture originated at the interface, at position I, shown at higher magnification in Fig. 9. The fracture surface of another specimen is shown in Fig. 10. Once again, the localized directions of crack growth are marked. In this case, the fracture appears to have originated within the reinforced layer, at position 0.

DIscussION Conventional wisdom would dictate the use of a chemically resistant resin such as Atlac 382 or Beetle 870 for use in acidic environments. The results presented here dearly show that in the presence of stress, and at 20°C, these resins are not suitable and that flexible resins provide better stress corrosion resistance.

Do

Previous work has shown that stress corrosion crack growth can occur at very low stresses. 13 Under such conditions debonding does not occur at the growing crack tip, the conventional laminate toughening mechanisms cannot operate and the resin can offer a proportionately greater contribution to the toughness of the material than during fracture in air. H o ~ and Hull~s have studied the micromechanisms of stress corrosion crack growth in pipe with different resin matrices in detail in an earlier paper. It was concluded that the behaviour of the resin at the crack tip is of more importance in restricting stress corrosion cracking than resin chemical resistance. A schematic model describing the possible events during stress corrosion crack growth was proposed and this is illustrated in Fig. 11. This model suggests that resin fracture toughness, Kic, is a critical parameter in determining the rate of crack propagation. Table 6 lists the fracture toughness values of the resins used in this work. Reference to Fig. 3 will show that, in general, the greater the resin toughness, the longer the time to failure of the pipe. It should be noted that the beneficial effect of using tough resins results from an improved laminate resistance to crack

o a

2

J -J

•

V Gel-coot • No gel-coot n Crocked gel-coot

• Post-cured • No post-cured I0

I I

I 2

I

5

I

4

I

5

6

Log time, ?1 (rain) Fig. 4

Dependence o f time t o failure on stress parallel t o the fibres

for machined pipe with and w i t h o u t post-cure, tested in 0.66 M HCI

at 20°C

258

I q

I I

I 2

I 3

4

I 5

I 6

Log time, tl (rain) Fig. 6 Dependence o f time t o failure on streN parallel t o the fibres f o r pipes with intact gel-coat=, cracked gel-coat= and no gel-coat=, tested in 0.65 M HCI at 20°C

COMPOSITES. JULY 1983

reasonable to assume that resin toughness would also have been substantially reduced. Accordingly, laminates without a post-cure might be expected to perform better under stress corrosion conditions than post-cured laminates, as the tougher matrix will provide a greater resistance to crack growth in an analogous fashion to the fiexibilized resins discussed earlier. The physical and chemical changes within the resin that result from post-curing are subject to debate. Marshall and co-workers2° have suggested that residual styrene will plasticize the resin and that the change in resin properties upon post-curing results from the reduction in residual styrene levels rather than any significant increase in cross-linking. They report that the chemical resistance of vinyl-ester resins, Derakane 41 I--45 and 470-36, in dilute HCI is not affected by post-curing, while the rates of acid permeation through the resins are increased. They conclude that residual styrene can inhibit the uptake of HCI at low concentrations. The lower acid diffusion rates may be another factor contributing to the superior stress corrosion performance found in laminates without post-cure. It is reasonable to conclude that post-curing is not essential for GRP subjected to stress corrosion conditions. However, it

Fig. 7 Scanningelectron micrograph showing the fracture surface of a gel~:oated pipe after stress corrosion. The arrows mark the local directions of crack growth. Position I marks the likely crack nucleation site, at the interface between the gel-coat and the fibre reinforced layer

propagation in acid, and not from any improved laminate resistance to microcracking produced by the initial loading. The specific test conditions and specimens used do not produce microcracking during loading. Improved laminate resistance to microcracking will however be an added advantage from the use of tough resins, particularly in more realistic laminates based on CSM. The rate of acid diffusion through the resin is likely to be another important parameter influencing the time to failure in stress corrosion. Hogg and Hull Is found that toughening a resin by excessive additions of a flexibilizing agent could result in a loss of stress corrosion resistance, presumably due to an increase in the rate of acid diffusion. Accordingly, a resin should not be selected for use in stress corrosion applications on the sole basis of its fracture toughness. Post-curing is usually regarded as beneficial for laminates that are to be subjected to aggressive environments in service. Once again, however, it was found that this did not hold for the specific case of acidic stress corrosion at 20°C. The effect of post-curing a polyester resin is to increase stiffness, strength and creep resistance, and decrease strain to failure and toughness. Loader 19 has recently studied the effects of post-cure on Atlac 382. A cure cycle of 3 hours at 100°C reduced residual styrene levels to 0.15% compared with 9.8% before post-cure. This post-cure resulted in increases in tensile strength of 30%, tensile modulus of 50% and a reduction in strain to failure of 50%. It is

COMPOSITES. JULY 1983

Fig. 8 Scanningelectron microgreph showing area A, Fig. 7 at higher magnification. The parabolic markings, P, on the resin fracture surface indicate the localized direction of crack growth (arrowed). The uppermost fibre from the reinforced layer is shown at position F

Fig. 9 Scanningelectron micrograph showing the likely crack initiation site, marked I in Fig. 7, at higher magnification

259

the gel-coated pipe results as drawn in Fig. 6, indicates a relationship: Time to failure a

1 n

Oil

where n ~ 12. The corresponding value of n for polyester resin in HCI has been reported to be ~. 80. 21 Consequently, it seems unlikely that the gel coat fracture is simply a stress rupture phenomenon. It is probable that acid diffusion through the gel-coat results in the attack of the fibres, producing flaws that extend into the matrix and initiate gel-coat rupture.

Fig. 10 Scanning electron micrograph showing the fracture surface of a gel-coated pipe in which crack nucleation is thought to have occurred within the fibre-reinforced layer at position 0. The subsequent directions of localized crack growth are marked

A simple check can be made to ensure that acid is diffusing at a suitable rate for the model to be feasible. The expression ~/Dt, where D is the diffusion coefficient, provides a 'rule of thumb' value for the average distance travelled by a diffusing ion after time t. Typical values of D for HCI diffusion into polyester resins lie in the range 2 - 4 x 109 cm 2/s.22

should be noted that many of the effects of post-curing,

eg increased stiffness, are very desirable. Furthermore, a

~ m D

post-cured laminate possesses a well defined set of properties and is not so susceptible to fabricating conditions, such as workshop temperature, as a laminate without post-cure. To ensure a consistent product, it would seem advisable for designers to specify a resin system on the basis of its postcured properties rather than on its ill-defined properties without post-cure. ECR-glass has shown itself to be significantly better than conventional E-glass in stress corrosion. With the current safety factors used in GRP design, the use of ECR-glass would effectively remove the problem of acidic stress corrosion at 20°C. However, stress corrosion of ECR-glass does occur and this may become a problem if safety factors are reduced and GRP is subjected to higher service loadings in the future. The behaviour of ECR-glass at higher temperature and with different concentrations of acid is as yet unknown. Work is in progress to determine whether the matrix can play such a significant role in restricting stress corrosion crack growth in ECR laminates as was found in E-glass laminates.

c

d

Fig. 11 Schematic representation of stress corrosion crack growth illustrating the role of the matrix. (a) If the matrix is brittle, fibre fracture may result in simultaneous resin fracture, up to the next fibre. (b) If the matrix is tough only limited crack growth may occur in the resin. A further increment,of acid diffusion is required to fracture the next fibre. (c) Subsequent fracture of the second fibre may result in back-cracking of the matrix, resulting in a coalescence of the fracture surface. (d) If the matrix is sufficiently tough end ductile, fracture of the second fibre may not result in fracture of the resin ligaments which may exhibit plastic flow and cavitation.

The results have shown that an intact gel-coat can provide an effective barrier to stress corrosion. The ultimate failure in gel-coated specimens could arise in two ways. Either creep rupture of the gel-coat could occur, thus providing the acid with access to the underlying fibres, or the acid could diffuse through the intact gel-coat to initiate stress corrosion cracking in the fibre-reinforced layers.

Table 6. work

Microscopy has revealed that fracture initiates at, or near, the interface between the gel-coat and the fibre-reinforced layers. This could indicate that the acid diffused through the intact gel-coat, as gel-coat creep rupture would be expected to start at the inner pipe surface where tensile stresses are at a maximum. However, it is possible that gel-coat creep rupture could have originated at a discontinuity, eg a void, between the gel-coat and reinforced layers. Once the gel-coat has cracked, the subsequent stress corrosion fracture will be very rapid, such that the time to failure of the pipe is effectively the time to failure of the gel-coat. The slope of the least squares fit line through

Crystic 272

0.62

Crystic 272 + 30% Crystic 586

0.77

Derakane 411-45

0.75

Crystic 600 PA

0.49

Beetle 870

0.46

Atlac 382-05A

0.45

260

The fracture toughness of the resins used in this

Resin

Fracture toughness, Kic (MN/m 3/2)

Impolex T500

Not available

The values for KIC quoted here were obtained from double torsion tests at 20°C in air 17

COMPOSITES . JULY 1983

I f x / D t is made equal to 0.025 cm, the gel-coat thickness, a value for t is obtained that ranges from 5200 - 2600 mins.

This is in keeping with the minimum time to failure obtained for gel-coated pipes of 7900 mins. It should also be noted that there is evidence to suggest that the acid diffusion rate will increase in resins subjected to high tensile strains. 22 The results obtained from pipes with cracked gel-coats have serious implications for the use of unreinforced gel-coats in practice. If a gel-coat is cracked in service the subsequent time to failure o f the structure would be very short indeed. An example would be a vessel designed to withstand 19 years (107 rains) at the operating stress level. Gel-coat fracture, possibly induced by impact, eg the 'dropped spanner', would result in failure within two hours. If a resin gel-coat is to be used, it should be tough enough to withstand considerable service maltreatment. In practice, gel-coats are frequently used in combination with reinforcement such as C-glass tissue and various organic fibre veils. The performance of these reinforced gel-coats has not yet been studied, but it is felt that any forms of glass reinforcement must be suspect in a stress corrosion environment.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11

CONCLUSIONS The correct choice of resin can significantly affect the time to failure of a laminate. Tough resins with a low permeability to acid are the most suitable for protection against stress corrosion. The resin chemical resistance does not appear to be a critical factor.

12

3 7th Ann Conf SPI Reinforced Plastics/Composites Institute 1982 paper 9-D

13

Post-curing the laminate does not improve stress corrosion resistance, although this practice may be desirable for other design considerations.

14

The use of acid resistant (ECR) glass fibres reduces the rate of stress corrosion cracking dramatically.

15

Resin gel-coats will delay stress corrosion cracking by providing a barrier through which the acid must diffuse before fibre attack can commence. Great care must be employed in the application of resin gel-coats and their subsequent service treatment, as cracked or damaged gelcoats will drastically reduce the life o f a component.

16

The above conclusions are based on testing performed on unidirectional specimens at room temperature. It is uncertain at present whether these conclusions can be applied to CSM or fabric composites, or to stress corrosion at other temperatures.

17 18 19

22 The author is grateful to the Science and Engineering Research Council for financial support, to the various companies who have supplied both material and advice, and to colleagues in the Department of Metallurgy and Matefiais Science, University of Liverpool, without whose help this work could not have been done.

COMPOSITES. JULY 1983

Hogg, PJ. and Hull, D. 'Micromechanisms of crack growth in composite materials under corrosive environments' Metal Science 14 (1980) 441 Hull, D. and Hogg, PJ. 'Nucleation and propagation of cracks during strain corrosion of GRP', in Advances in Composite Materials, volume 1' edited by A.R. Bunsell et al, (Pergamon Press, Paris, 1980) 543 Roberts, R.C. 'Environmental stress cracking of GRP. Implications for reinforced plastics process equipment' Composites 13 No 4 (October 1982) pp 389-392 Hull, D., Lesg, MJ. and Spencer, B. 'Failure of glass/polyester filament wound pipe' Composites 9 No 1 (January 1978) pp 17-24 Gatward, C.H. unpublished work (Liverpool University, UK) Hogg, PJ. and Hull, D. 'Role of matrix properties on the stress corrosion of GRP' BPF Reinforced Plastics Congress, 1983 paper 29 Loader, T.R. 'The effect of post-cure on the mechanical properties of Atlac 382-05' Atlas Chemical Industries Internal Report

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A CKNOWLEDGEMENTS

'Chemical resistance of thermosetting resins used in glass fiber reinforced structures' ASTM C 581 (American Society for Testing and Materials, Philadelphia, PA, USA) 'Specification for vessels and tanks in reinforced plastics' BS 4994 (1973) Norwood, L.S. and Miilman, A.F. 'Strain limited design criteria for reinforced plastic process equipment' Composites 11 No 1 (January 1980) pp 39--45 Butt, L.T. and Wright, D.C. 'Use of Polymers in Chemical Plant Construct'on' (Applied Science, London, 1980) Norwood, L.S., Edgell, D.W. and Hankin, A.G. 'Blister performance of GRP systems in aqueous environments' BPF Reinforced Plastics Congress, 1980 paper 39 Hogg, PJ. and Hull, D. 'Corrosion and environmental deterioration of GRP', in 'Developments in GRP Technology, volume 1' edited by B. Harris (Applied Science) in press Collins,I-LH.'Strain corrosion cracking of GRPlaminates' Plastics and Rubber, Materials and Applications 6 (1978) p 6 Roberts, R.C. 'Design strain and failure mechanism of GRP in a chemical environment' BPF Reinforced Plastics Congress, 1978 paper 19 I-Iogg,PJ., Hull, D. and Spencer, B. 'Stress and strain corrosion of glass reinforced plastics' Composites 12 No 3 (July 1981) pp 166-172 Jones, F.R., Rock, LW., Wheatley, A.R. and Bailey, LE. 'Environmental stress corrosion cracking of GRP' BPF Rein. forced Plastics Congress, 1982 paper 32 Hogg, PJ., Hull, D. and Legg, MJ. 'Failure of GRPin corrosive environments', in 'Composite Structures' edited by I.H. Marshall (Applied Science, London, 1981) p 106 Marshall,G.P., Kisbenyi, M., Harrison, D. and Pinzelli, R. 'Environmental stress corrosion of chemically resistant polyester resins and.glass reinforced laminates in acids' Proc

Bimh-Kisbenyi, M., Harrison, D., Eym, D.M. and Marshall, G.P. 'The effect of post-cure on environmental stress corrosion of vinyl-ester laminates and gel-coat layers' BPF Reinforced Plastics Congress, 1980 paper 44 Hutchinson, LM. private communication (Nottingham University, UK) Marshall,J.M., Marshall, G.P. and Pinzelii, R.F. 'The diffusion of liquid into resins and composites' Proc 37th Ann Conf, SPI Reinforced Plastics/Composites Institute, 1982 paper 9-C

AUTHOR Dr Hogg is with the Department of Metallurgy and Materials Science, The University o f Liverpool, PO Box 147, Liverpool L69 3BX, UK.

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