GB2281299A

GB2281299A - Composite materials containing fibres

Info

Publication number: GB2281299A
Application number: GB9317152A
Authority: GB
Inventors: Kevin Steven Fancey
Original assignee: University of Hull
Current assignee: University of Hull
Priority date: 1993-08-18
Filing date: 1993-08-18
Publication date: 1995-03-01
Anticipated expiration: 2013-08-18
Also published as: GB9317152D0; GB2281299B

Abstract

A composite material comprises a matrix material which may be of polymeric, ceramic or metallic material reinforced with fibres. The fibres, e.g. nylon fibres, are subject to an applied tensile stress prior to incorporation into the matrix. After incorporation, the retained stress, resulting from the viscoelastic properties of the fibres, imparts compressive forces on the surrounding matrix. This inhibits crack propagation.

Description

COMPOSITE MATERIALS The invention relates to composite materials and in particular composite materials in which a matrix is reinforced with fibres.

The matrix material may, for example, be a polymeric material or a ceramic material. The fibres can be polymeric fibres or polymeric fibres mixed with stronger fibres such as glass or carbon fibres. The term "fibres" in this specification is intended to encompass any suitable reinforcing rod or fibre-type material.

One function of such fibres in the matrix is to halt crack propagation through the composite since, as the crack reaches a fibre, the crack is blunted by the fibre. Thus the presence of the fibres makes the composite more crack-resistant than the matrix on its own. Accordingly, the fracture toughness is improved.

According to a first aspect of the invention, there is provided a method of forming a composite material comprising subjecting reinforcing fibres to tensile stress and then incorporating the stressed fibres into a matrix material.

The incorporation into the matrix of fibres that are under tensile stress induces residual compressive stresses in the matrix surrounding the fibres when the fibres are incorporated into the matrix. These residual compressive stresses impede crack propagation before the crack reaches the fibre.

The matrix may be a polymeric material or a ceramic material or a metallic material.

The stressed fibres may be of a polymeric material. The matrix can also include other fibres such as glass fibres or carbon fibres.

The method may comprise taking a continuous length of said fibre material, subjecting said continuous length to tensile stress and then cutting said continuous length into fibres prior to incorporation of said stressed fibres into said matrix.

Preferably the matrix material is a cold-setting material.

According to a second aspect of the invention, there is provided a matrix material reinforced with pre-stressed fibres, the fibres subjecting the matrix to compressive stresses on incorporation into the matrix.

The following is a more detailed description of an embodiment of the invention, by way of example, reference 'being made to the accompanying drawing which is a schematic cross-sectional view through a portion of a matrix showing a fibre incorporated into a matrix and the propagation of a crack through the matrix.

The composite material is formed by a matrix 10 within which is incorporated a fibre 11. The matrix may be any suitable polymeric material or any suitable ceramic material or any suitable metallic material.

The fibre is a polymeric fibre such as nylon 6.6. Prior to incorporation into the matrix, the fibre 11 is subject to an applied tensile stress. On release of the applied tensile stress, the viscoelastic characteristics of the fibre material will cause recovery of the resulting strain in the fibre to be time-dependent. Depending on the conditions of the previously applied tensile stress (such as the magnitude of the applied stress and duration of the applied stress), recovery of a large proportion of the resulting strain may require minutes or hours. This allows time for the fibres to be incorporated into the matrix in such a stressed condition. Methods for incorporating such fibres into matrices are well known in the art and will not be described in detail.

Immediately after incorporation into the matrix, the retained stress in the fibre results in the fibre continuing to attempt strain recovery, which causes the matrix material around the fibres to be subjected to compressive forces due to the linkage between the fibres and the matrix. The linkage between the fibres and the matrix may be improved by, for example, increasing the surface roughness of the fibres.

As seen in the Figure, this has the effect of reducing crack propagation since, as the crack propagates into the compressive zone of matrix material around the fibre, further propagation of the crack is impeded.

The matrix material is preferably a material which does not require elevated temperatures during moulding or forming since elevated temperatures during moulding or forming might adversely affect the stress retention of the fibres. An example of a suitable matrix material would be a cold setting resin such as epoxy. Other matrix materials which would normally require elevated temperatures during moulding or forming could be alternatively produced at lower temperatures by physical vapour deposition or chemical vapour deposition. The attainment of lower temperatures during matrix formation by these vapour deposition methods would be facilitated by the use of ionisation assistance or plasma assistance.

The fibres need not be stressed individually. A continuous length of the fibre material could be stressed and then chopped into individual fibres prior to moulding into the matrix.

Claims

1. A method of forming a composite material comprising subjecting reinforcing fibres to tensile stress and then incorporating the stressed fibres into a matrix material.

2. A method according to claim 1 wherein the matrix is a polymeric material or a ceramic material or a metallic material.

3. A method according to claim 1 or claim 2 wherein the stressed fibres are of a polymeric material.

4. A method according to any one of claims 1 to 3 wherein the matrix includes other fibres such as glass fibres or carbon fibres.

5. A method according to any one of claims 1 to 4 and comprising taking a continuous length of said fibre material, subjecting said continuous length to tensile stress and then cutting said stressed continuous-length into stressed fibres prior to incorporation of the stressed fibres into said matrix.

5. A method according to any one of claims 1 to 4 and comprising taking a continuous length of said fibre material, subjecting said continuous length to tensile stress and then cutting said stressed continuous length into stressed fibres prior to incorporation of the stressed fibres into said matrix.

6. A method according to any one of claims 1 to 5 and comprising roughening the surface of said fibres prior to the incorporation of said fibres in said matrix.

7. A method according to any one of claims 1 to 6 wherein the matrix material is a cold-setting material.

8. A method of forming a composite material substantially as hereinbefore described with reference to the accompanying drawing.

9. A matrix material made by the method of any one of claims 1 to 8.

10. A matrix material reinforced with pre-stressed fibres6 the fibres subjecting the matrix to compresszve stresses on incorportion into the matrix.

Amendments to the claims have been filed as follows 1. A method of forming a composite material comprising subjecting viscoelastic reinforcing fibres to tensile stress in the viscoelastic region thereof, removing the tensile stress and then incorporating the stressed fibres into a matrix material.