WO1988002671A1

WO1988002671A1 - Stiffening of plastics panels

Info

Publication number: WO1988002671A1
Application number: PCT/GB1987/000726
Authority: WO
Inventors: David Bashford; Ian James Alexander; Richard Austin Panther
Original assignee: The Secretary Of State For Defence In Her Britanni
Priority date: 1986-10-16
Filing date: 1987-10-16
Publication date: 1988-04-21
Also published as: AU8079687A; GB8624832D0

Abstract

Improved method for attaching a ''Top-Hat'' stiffener (2) to a plastics panel (1) particularly in boat construction which can be subject to considerable shock sufficient to cause stiffener-panel separation in conventional construction. A longitudinal foamed urethane former (5), rectangular in x-section is attached to the panel. Toughened flexi-resin fillets (7) are located at the intersections of the sides of the former and the surface of the panel. GRP layers (3) are then formed over the former so as to overlay the panel surface on opposite sides of the former to produce the ''Top-Hat''. The flexi-resin at the heels of the side limbs of the ''Top-Hat'' frame reduce the tendency for separation of the frame from the panel. The GRP rovings adjacent to the fillets advantageously are also impregnated with the same toughened resin as the fillets (7).

Description

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STIFFENING OF PLASTICS PANELS

The invention relates to the attachment of stiffening members to GRP and other plastic panels.

In large structures incorporating plastics panels it is necessary to provide stiffening members attached to the panels to increase the structural rigidity. Acommonly used stiffener is a "top-hat" stiffener, being "top-hat" shaped in cross section with flanges for attachment to the panels. The "top- hat" stiffener may be made by first bonding a core of polyurethane foam to the panel and then building up layers of GRP over this core to form the "top-hat" stiffener. When used in ship and boat building to stiffen GRP panels, separation of panels and stiffening members is a problem. To counter the tendency for separation the only satisfactory method is to bolt the stiffener flanges to the panels using, for example, titanium bolts. The material and installation costs of this are very high. The bolts do not stop or inhibit crack formation but arrest propagation of cracks to prevent total separation.

The object of the present invention is to provide an improved method of attaching stiffener frames to panels.

The invention provides a method of stiffening a plastics panel comprising the 'successive steps of: a) attaching a longitudinal foam core stiffening former, substantially rectangular in cross section, to the panel; b) applying fillets lengthwise of the former at the junction of the sides of the former and the panel, the fillets being made of a tough flexible resin adhesive with high strain capability; c) laying over the former and fillets into overlapping relationship with the surface of the panel on opposite sides of the former one or more layers of a reinforcing material impregnated with a thermosetting resin. The inventors have shown that the separation of "top-hat" stiffeners from GRP panels is determined firstly by the fillet material and secondly by the resin in the secondary bond-line between stiffener and panel. Conventional isophthalic polyester resin does not perform well as it is brittle with low strain to failure. The design of "top-hat" stiffeners ensures a considerable stress concentration exists at the "heels" where the stiffener first meets the panel.

By using a tough flexible resin fillet at the "heels" of the "top-hat" a considerable improvement in structural integrity results. In addition, adhesion can be improved still further by applying at least one layer of material impregnated with the tough flexible resin around the corner of the heel. In a more complex arrangement the layer may extend over the stiffener from one side of the panel to the other.

Preferably the tough resin is a urethane acrylate comprising unsaturated oligomers dissolved in a monomer such as styrene cured by an additionmechanism ega peroxide catalyst and heat or a peroxide catalyst and an accelerator.

In order to apply the fillets the tough resin is preferably made into a paste by adding a resin thickening agent such as a fumed silica, eg Aerosil (TM) , Cabosil CTH) to increase the viscosity of the resin. This produces a paste which although reducing the high strain capabilityof the tough resinwhen cured is still above 50% elongation at break. Advantageously the fillet comprises a mixture of 20 parts by weight of resin to one part by weight of the fumed silica.

Shock trials have shown that panels stiffened by adding toughened resin fillets perform as well, if not better, than titanium bolt reinforced constructions.

The inventionwill now be described, by way of example only, with reference to the attached drawings of which: FIGURE 1 is a sectional view through a panel stiffened according to one arrangement of the invention;

FIGURE 2 shows schematically a "top-hat" stiffener test arrangement;

FIGURE 3 shows a graph of displacement against applied pulling load for a standard stiffened panel with and without titanium bolts;

FIGURE 4 illustrates stiffener arrangements with additional woven rovings applied;

FIGURES 5-9 illustrate various modifications of the stiffener- panel joint;

FIGURE 10 shows representative load vexsus displacement curves with various toughened fillet geometries and stiffener flange heel compliances; and

FIGURE 11 shows a load versus displacement curve for a further embodiment.

The basic "top-hat" configuration for a GRP structure is shown in Figure 1. A glass fibre reinforced (GRP) panel 1 is reinforced by aGRP stiffener 2. The stiffener 2 is an elongated body "U" shaped in cross-section and provided with lateral flanges 3 which are adhered to the upper surface 4 of the panel 1. The stiffener 2 is formed by adhering a foamed polyurethane former 5 along panel lines where stiffening is required. Polyester resin impregnated glass woven rovings are applied over the former 5 with additional uni-directional rovings to thicken the top. In order to achieve correct lay-up of rovings at the angles between the panel 1 and the former 5 it has been proposed that radiused fillets 7 are first applied after attaching the former 5. The fillets 7 are made of a trowellable, non-slumping paste of polyester resin thickened with milled glass fibres.

In this arrangement it was necessary to provide bolts through the flanges 3 and the panel 1 to arrest cracks which tended to form and propagate at the secondary bond-line region 8 between the flanges and the panel 1.

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The inventors have discovered that by using a suitable adhesive for the fillets 7 crack initiation at the bond-lines between the "top-hat" stiffener and the panel can be prevented. A material is used which has high strain to failure capability. A resin adhesive comprised of unsaturated urethane acrylate in a monomer such as styrene is used because it can have an extremely high elongation at break. This is mixed with fumed silica (Aerosil 200(TM)) to thicken the resin to a paste consistency. 15 to 20 parts by weight of resin to one part of colloidal silica was found to be optimum. This paste can be shaped to give the appropriate radius for the fillet 7 on the "top-hat" section. The colloidal silica has been found to thicken the resin without reducing its resistance to- failure after curing, to below 50% elongation at break. A suitable resin is Crestomer 1152PA supplied by Scott Bader Company Limited.

Figure 2 illustrates a slow pull-off experimental technique adapted to assess stiffened panels according to the invention. Each "top-hat" variant was tested on a number of specimens. The specimen 21 was clamped by means of "a square cross-section mild steel bar 22 centrally applied to the panel 23. An evenly distributed load 24 was applied over the area of the frame top of the stiffener 25 via a shackle 26. A displacement rate of 1mm per inch was used..

Representative load versus displacement curves for specimens without the toughened fillets according to the invention are shown in Figure 3. The solid curve 31 represents an un-reinforced specimen and the broken curve 32 represents a similar specimen with titanium bolts reinforcing the flange to panel joints. Up to an applied load of about 14KN only minor cracking of the fillets 27 occurred. These fillets were a polyester resin thickened with milled glass fibres. At about 14KN (14.3KN average for the unbolted) a crack propagated down the secondary bond line 28 between the stiffener flange 29 and the base panel 23. This designated complete failure for the un- reinforced specimens 31, while for reinforced specimens 32 the

SUBSTITUTE SHEET bolts restrained crack propagation.

Six variants of the Figures 2 and 3 specimens were produced with additional woven rovings 41 and 42 applied to the base panel and stiffener heels respectively as shown in Figure 4. These rovings were impregnated with the same toughened resin used for the fillets of the present invention. The fillets 43 as for the specimens above were made from a paste with milled glass fibres, but the polyester resin was replaced by a tough resin which was Crestomer 1027 supplied by Scott Bader Company Limited. The attached Table provides a description of each version together with the average peak load capability.

Specimen No of Base No Of web/ Average

Plate Cloths Flange Cloths Peak Load( N)

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A 1 W.R. 2 W.R. 19.8

B 1 W.R. 4 W.R. 19.8

C 3 W.R. 2 W.R. 19.1

D 3 W.R. 4 W.R. 19.2

E 6 W.R. 2 W.R. 21.3

F 6 W.R. 4 W.R. 25.0

Specimens A-F show an average improvement of about 50% as a result of the use of additional woven rovings. The inventors have showed that the action of the toughened resin rovings is to slow down crack propagation rather than to delay its onset.

Use of the fillets according to the invention was first demonstrated with a number of specimens of modified type D and F. The pull-off performance was greatly improved with maximum failure loads in the range 39 to 49 KN. Significantly, no cracks

SUBSTITUTE SHEET were initiated before these peak loads and failure was very rapid and catastrophic. The arrangement for the modified F specimens is shown in Figure 5.

Figures 6 to 9 show representations of specimens L-P. The L specimen shown in Figure 6 is similar to the F specimen shown in Figure 5 except that the toughened resin plies 61 extend almost to the end 62 of the stiffener flange and the fillet 63 has an altered design.

VariantM shown in Figure 7 has a similar fillet design 71 to the L specimen, while the N and P specimens both have a larger radius fillet 81 and 91 with the P fillet being larger than the other specimens as shown in Figures 8 and 9. Also -in the M-P arrangements the tough resin (Crestomer 1027) was incorporated in the first four plies 72,82,92, of the 12-ply "top-hat" stiffeners.

Representative load versus displacement curves for the specimens L-P are shown in Figure 10. The results showed the following: a) maximum loads to failure approached 40 KN; b) there was a non-linear displacement with respect to load beyond about 20 KN; and c) there was a non-catastrophic mode of failure as a crack initiated before the maximum load and propagated very slowly before complete flange separation, accounting for the rounding of the peaks in Figure 10. b) and c) are thought to be due to observed incomplete curing of the specimens tested. Subsequent specimens have not exhibited problems b) and c) .

Results show that most of the improved pull-off resistance is due to the use of a toughened fillet, however, further small benefits can be obtained by using additional toughened resin impregnated plies. The optimum performance was obtained by using a thixotropic tough resin Crestomer lllOPA derived from the resin used in the other arrangements, and thickened with Aerosil 200(TM) . The specimen, designated V, had additional

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plies, as illustrated in Figure 4, extending 50mm to either side of the fillet 43. The specimen V had 3 additional base plies and 4 additional flange plies with the fillet 43 dimensioned: (20 + t)mm radius and 20mm triangle, where t is the thickness of the stiffener wall. The average maximum load for a number of examples of this specimenVwas found to be 68.8 +/- 0.7 KN and the work done to maximum load was 890.1 +/- 26.4 J. In all cases the specimens distorted elastically to the maximum load when, upon crack initiation, flange separation was immediate and catastrophic as shown in Figure 11.

Stiffened panels according to the invention have the capacity to store large amounts of elastic energy, exceeding the loads at which the base panels of bolt-reinforced specimens fractured. Where good shear support is required for example to ensure the total integrity of a ship excessive use of toughened resins for bonding stiffeners to the panel should be avoided since the shear support may be lostwith resins of low tensile and shear modulus.

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Claims

1. A method of stiffening a plastics panel (1) comprising the successive steps of: a) attaching' a longitudinal foam core stiffening former (5), substantially rectangular in cross section, to the panel (1) ; b) applying fillets (7) lengthwise of the former (5) at the junction of the sides of the former and the panel, the fillets (7) being made of a tough flexible resin adhesive with high strain capability; c) laying over the former and fillets into overlapping relationship with the surface of the panel on opposite sides of the former one or more layers of a reinforcing material (6) impregnated with a thermosetting resin.

2. Amethod as claimed in claim1 wherein there is added at least one layer (42) of material impregnated with the tough flexible resin around the corner of the heel.

3. Amethod as claimed in claim 2 wherein the layer (42) extends over the stiffener from one side of the panel (1) to the other.

4. A method as claimed in any one preceding claim wherein the tough resin is a urethane acrylate comprising unsaturated oligomers dissolved in styrene as the reactivemonomer, cured by a peroxide catalyst..

5. A method as claimed in any one preceding claim wherein the tough resin is made into a paste by adding a resin thickening agent in finely divided form to produce a colloidto increase the viscocity of the resin.

6. A method as claimed in claim 5 wherein colloidal silica is used.

7. Amethod as claimed in claim 6 wherein the fillet comprises a mixture of 20 parts byweight of resin to one part byweight of the colloidal silica.

8. A reinforced plastics panel comprising a panel member (1) and a reinforcing member (2), characterised in that the reinforcing member comprises: a) a longitudinal foam core stiffening former (5), substantially rectangular in cross section, to the panel (1); b) fillets (7) lengthwise of the former (5) at the junction of the sides of the former and the panel, the fillets (7) being made of a tough flexible resin adhesive with high strain capability; and c) one or more layers of a reinforcing material (6) impregnated with a thermosetting resin laying over the former and fillets into overlapping relationship with the surface of the panel on opposite sides of the former.

9. A reinforced plastics panel as claimed in claim 8 wherein there is added at least one layer (42) of material impregnated with the tough flexible resin around the corner of the heel.

10. A reinforced plastics panel as claimed in claim9 wherein the layer (42) extends over the stiffener from one side of the panel (1) to the other.

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