Plastics Reinforcerl l v Se lel Wires Glassfibers have been widely used f o r reinforcing plastics,
but steel wires may proue to give a stronger laminate. Evidence presented here supports the claim of steel ouer glass fibers
t is a famous claim for glass fiber reinforced plastics that, weight for weight, they are stronger than steel. This statement is normally reinforced with figures which run approximately as follows: A glass fiber laminate may have a strength of 8000 kg./sq. cm. with a density of 1.95 grams/cu. cm. The specific strength is therefore about 4000 cm. Steel, on the other hand, has a tensile strength of something like 6000 kg./sq. cm. with a density of 8 grams/cu. cm. and, therefore, the specific strength is 750 cm. This statement does not, however, take into consideration strength values which are commercially available, nor does it take into consideration fatigue and long term values. No reputable glass reinforced plastics fabricator will use as much as 5000 lb./sq. in. (about 350 kg./sq. cm.), and well known manufacturers are known to go down as low as 2000 lb./sq. in. (about 140 kg./sq. cm.). If we compare this maximum value of 350 kg./sq. cm., which gives a specific strength of only 175 cm., with the strength of a hard drawn steel wire which may be something like 400,000 lb./sq. in. (24,710 kg./sq. cm.) and a density of 8 grams/cu. cm., we obtain a specific strength of something like 3100 cm. This looks different indeed. This argument in favor of glass is, in many cases, only a harmless sales argument, as the majority of reinforced plastics are stressed slightly, or not at all. They mainly have the duty of keeping themselves in position as, for instance, in the case of corrugated roofing material, cabs for trucks, machine guards, and ducting, However. for some jobs like stressed sandwich skins or filament wound equipment-pipes, tanks, and poles-where strength, both absolute and specific, is essential, it is important to produce a reinforcement material better than glass. At the same time it has to be commercially viable and this, for the time being, precludes the use of
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beryllium wires, &tal whiskers, carbon fibers, and the rest. Marshall of M I T reported (2) that steel wires gave excellent reinforcement values, provided the problem of adhesion of the resin could be solved. He found, however, that it was extremely difficult to work even with a small number of loose wires. Wire Sheet
A solution of this problem which, strength for strength, is considerably cheaper than glass has been found in what for the time being we call the “Wire Sheet.” The wire sheet is a n arrangement of hard drawn steel wires, generally of approximately 0.25-mm. (10 mil.) diameter on a suitable supporting subbase which can be paper, glass cloth, or asbestos, but is generally a tissue of bonded synthetic resin fibers. The wire sheet which can be, and is being, produced in “endless” rolls with varying numbers of parallel wires per inch-normally 10, 20, 30, 40, and 60, and in widths of up to 1 meter-can be used like glass mat or glass cloth for low pressure laminates, and in the form of tapes for filament winding like glass rovings. Strength of Laminates
T h e strength of a glass fiber laminate depends on the type of glass, the shape in which it is offered (roving, cloth, mat), its surface finish, the type of resin, the details of the manufacturing process and may show short time strength values of anything from 2000 to 24,000 kg./sq. cm. The position with wire sheet laminates is somewhat different. The thickness of a laminate, incorporating a given number of layers of wire sheet, depends on the laminating pressure or on the winding tension (Figure 1). T h e strength of such a laminate is, for any given number of wires per square inch, comVOL. 5 9
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TOTAL STRENGTH 14,336 LB. (APPROX. 1000 KG.)
1.1 MM. 4.6
IN.
SAMPLE THICKNESS
- 1 2 LAYERS OF 0.010
in. WIRE
+
BASE
4.4
342
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330
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800
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0.7
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Fzgure 7. pressure
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40 50 LAMINATING PRFSSURE
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Dependence of %ire sheet laminate thickness on laminating
pletely independent of the fabricating pressure or tension and the resulting cross section (Figure 2). I t is simply a linear function of the content of steel, and a much more reasonable way of assessing the strength is to say that a given cross section contains a certain number of steel wires, each of which has a strength of approximately 14.5 kg. (31 lb.). The total strength of this particular cross section will therefore be the number of wires X 14.5 kg. To put it another way: T o calculate the number of wires needed to guarantee a certain laminate strength figure, we divide this figure by 14.5, and add our safety margin. I t is today quite easily possible to incorporate into a laminate something like 25 to 28 layers per centimeter thickness and, when using a material with 60 wires per inch (24 wires per centimeter), to obtain a breaking strength of 28 X 24 X 14.5 or 9750 kg./sq. cm. T h e adhesion of resin to glass presents problems, as evidenced in the literature (7). It appears that polyester resins, for instance, will bond to glass only mechanically ( 3 ) , whereas the bonding of epoxy resin may be gravely affected by fatigue and ingress of water. T h e strength of adhesion of polyester resins to the steel wires of the wire sheet is of the order of 60 kg./sq. cm., which means that the adhesion of resin to about 35-mm. length of wire will equal the breaking strength of the wire. As a consequence a well matched laminate will, on failure, break cleanly (Figure 3), but wires ill be pulled out of the laminate if laminating resin and bonding resin (the resin used for joining the wire to the subbase) are matched improperly (Figure 4). All short time breaking tests of dry laminates in the beginnine; gave some trouble as it was rather difficult to lead the stresses from the testing machine into the laminate. However, these tests have shown that the 84
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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_ _ 0.08 0.09 0.10 0.11
10
KG./CMz
THICKNESS
0.12 0.13 0.14 0.15IN.
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 MM.
Figure 2. Dependence of &ire sheet laminate strength on thickness
laminates would break at their theoretical strength or sometimes up to 57, above it. A hard drawn steel wire will show a fatigue limiti x . , a complete flattening of the curve (IVoehler curve), in the region of 40% to 38% of the short time ultimate load. ,4wire sheet laminate will show a curve which is completely identical with that of the original wire (Figure 5) and it can, therefore, be stated with all confidence that a wire sheet laminate, even if used under cyclic loading, can be stressed permanently and safely to 30Y0 of its short time ultimate load. For a laminate just described which has a strength of 9750 kg./sq. cm., we come to an admissible permanent load of 2925 kg./ sq. cm. (about 41,600 lb./sq. in.). This is about nine times more than the best glass fiber laminates will tolerate permanently. Also it is more than twice the value to which a steel structure can be designed. Figure 6 shows comparative results of cyclic testing. Wet Strength
Wet strength cyclic tests were also made with wire and have shown here again the existence of a definite fatigue curve with a limit of approximately 22Yc,. The reason is simple-IVe have a typical case of surface flaws (Griffiths' flaws), the surface energy of which is reduced when in contact with water and which, as a consequence, will lead to fracture at levels above 227,. As steel 011 the whole \vi11 have no internal flaws, unlike glass ("C"
F. J a r a y is a chartered consulting engineer w i t h headquarters in Worcester, England. This article is based on a paper given at Achema, Frankfurt, Germany, in AUTHOR Francis
June 1967.
Figure 4. Pulled wires after break of poorly matched laminate
Figure 3. Clean break of well matched laminate
flaws) ( 7 ) and as water cannot migrate into steel [again unlike the case of glass ( S ) ] , the wet fatigue of steel will not be a progressive matter. We have made wire sheet laminates and have fatigued them wet. While these experiments have not yet been concluded, the wire and wire sheet laminate curves are very close to each other, and it is expected that the curve of the wet wire sheet laminate will closely approximate the wet fatigue curve of the wire itself, in the same way as the dry wire and wire sheet laminate curve coincided (Figure 5). Experiments are in progress to assess the effect of silane added to the laminating resin with a view to increasing wet strength. I t is possible that this will prevent the wire itself from becoming wet and, while it is not expected that the ultimate fatigue limit will be improved upon materially, it is expected that the fatigue curve will be somewhat flatter than that of the wire itself. Young's modulus of elasticity in tension, E , for hard drawn steel wire is greater than 2 X l o 6 kg./sq. cm. On a wire sheet laminate the value of E is in strict proportion to the steel content, with the resin contributing virtually nothing. A good wire sheet laminate will, therefore, have a n E modulus of approximately 0.6 to 0.7 X l o 6 kg./sq. cm. (about l o 7 lb./sq. in.). This, of course, is many times higher than that of glass and, in fact, is reached only by beryllium and excelled by carbon fiber laminates. Pipes of Wire Sheet
An important outlet for wire sheet is the production of pipes by filament winding. I t is, of course, possible to make pipes by winding wire sheet at any angle between 0 ' and 90'. Experiments have shown that the Pipes made from wire sheet (although there apparently may be
Figure 5. Dry cycling of wire sheet laminate and wires 9 100 5 90 80 70
= 60 8 50 U
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104 REVERSALS OF STRESSES
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Figure 6. Cyclic testing of wire, wire sheet laminate, and glassjber laminate VOL. 5 9
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large wire-free and resin filled gaps in the pipe) burst very close to or even above the theoretical strength-Le., the sum total of the strengths of the individual wires. When the pipe bursts, unlike a glass pipe, it will not weep. A glass fiber pipe will, when used for comparatively high pressures and for a long time, need a liner, but the necessity of a liner for a wire sheet pipe is greatly reduced. Inasmuch as an important outlet for the wire sheet is the filament winding of poles, we have made axial compression tests on such pipes. Pipes wound with a circumferential and an axial system of fibers failed by dis-
Figure 7. Disintegration failure of pipe wound with circumferential and axial fiber sytem
rarely consist of fewer than 4000 individual fibers; the resin layer will, therefore, have a thickness which, over extremely small distances, may vary from zero to quite substantial values. The voids [the best laminate is alleged to have something like 57, voids (5)], filled with air, will assist the setting up of stresses and act as starting points for the corrosion of glass fibers. Superimposed external stresses will produce strains which will open up cracks and lead to progressive failure-this explains why modern American practice does not tolerate strains in the laminate in excess of O.lYc, though allegedly the resin itself should show infinitely larger strains before breaking I n contrast, a wire sheet laminate will have individual wires M-hich are comparatively thick. say 250 microns, compared with 9 or 10, which are not bundled or twisted, and which offer the resin a simple and clean surface of attachment. The resin on setting has, therefore, much more freedom to mol-e and contract, and the resultant laminate will be completely free from stresses as can be shown by the absence of birefringence when viewed under polarized light. I t is most interesting that such birefringence does not even arise when the laminates are stressed. Extreme Conditions
We have tried to find whether boiling in water has any influence on the strength of the laminate. Boiling for two and six hours, while in one or two cases leading to the twisting of laminates, had no effect on short time ultimate strength. Boiling up to 200 hours unfortunately led to the hydrolysis of the resin and these experiments will have to be repeated. Similarly, while these experiments ha\-e not yet been completed it appears that a wire sheet laminate will be capable of usage up to the maximum temperature that resins can stand. This at present is about 200’ to 250’ C., but experiments are planned with other laminatiny materials which promise much higher temperature resistance. Outlook
Figure 8. Resin shear failure of p;Pe wound helicaib ut 45’
integration rather than by buckling on reaching and exceeding the full theoretical load (Figure 7 ) . Pipes which had been helically wound at an angle of 45’ failed by shear in the resin itself (Figure 8 ) . The reason for this behavior is simple. A glass fiber laminate, el-en if not stressed by external forces, will always show that the resin has residual shrinkage stresses ( 4 ) which may be from the maximum positive to the maximum negative and which, most likely, will cause very large numbers of microcracks. These assist ingress of water and water vapor into the laminate and onto the glass with consecutive and catastrophic effects. This effect on glass is accentuated by the fact that the resin has to penetrate the tight glass fiber bundles which 86
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
While it is now possible to make extremely strong and very cheap storage tanks, pipes and silos of virtually any diameter, it seems likely that within a year or two it will also becomes feasible to make distillation columns and other high temperature, high pressure chemical engineering equipment. I n short, it appears that when using the wire sheet it is possible to make most of those laminates which have been promised and expected, a hope unfullfilled by glass fiber laminates. REF E REN C ES (1) Kier, J., Conference on Research Projects in Reinforced Plastics, London, March 1965, paper H . (2) Marshall D. W. ”Researrh on [Vire tVound Composite Materials,” School of Engineerink (Prof.’Frcd McGarry), MIT, Cambridge, Mass., November 1 7 6 2 . (3) McGarry, Fred, SPI Conference 1 9 5 9 , paper 1 2 E . ( 4 ) Ourwater, John. SPI Conferences 1960 and 1961, paper 19B. ( 5 ) Paul. J.T., Jr., Thompson, J . B., SPI Conference 1965, paper 120. (6) Schmitz, G. K., Metcalfe, A. G., “Stress Corrosion of Glass Fibers,” I x n . ENC.CIIEY.PROD.RES.DEVEI.OP., 5 (l), 1 (1966). ( 7 ) Vogel, G . E., et al., SPI Conferencc 1967, paper 13B
