GB1331431A - Composite materials - Google Patents

Abstract

1331431 Composite material; concrete reinforcement J G MORLEY 2 Sept 1970 [6 Sept 1969 29 Jan 1970] 44181/69 and 4257/70 Headings E1K and E1W A composite article which is to be subjected to tensile stress comprises at least two members one within the other, wherein substantially the whole area of interfacial contact between the members is such that the local shear strength of the interfacial bond is reduced as the local tensile stress in the article increases. In the embodiment of Fig. 1, this is accomplished by providing an inner member 5 in the form of a helically twisted fibre or wire, e.g. of square section, the diameter of the helix being slightly greater than the diameter of a bore 6 in a matrix 7 into which bore the wire is inserted by being forcibly pulled through, the pulling force stretching the helix sufficiently to reduce its diameter to that of the bore. The ends of the fibre or wire are secured to the matrix so that if the matrix is subsequently put in tension the frictional grip between the fibre or wire and the bore 6 is reduced, and if the matrix fractures the fibre or wire is able to perform work as it stretches before the frictional force is reduced to zero. Preferably, the tensile strength of the fibre or wire is such that the fibre or wire will fail a little after the frictional force has reduced to zero. The matrix may be a metal or plastic tube or a block of material. In a modification (Fig. 2, not shown) the inner member is a rod of double wedge section bent to sinusoidal shape in one plane, the bore being of corresponding double wedge section. A plurality of inner members may be provided where the matrix is a block of material. In the embodiment of Fig. 3, the inner member is a straight fibre or wire 5 of any suitable material surrounded by a helically wound wire 10 closely fitting the bore 6 in the matrix. When tension is applied to the matrix, the helically wound wire 10 does not substantially change in diameter, thus resisting contraction of the bore 6 and allowing the fibre or wire 5 to stretch, the reduction in diameter of the fibre or wire 5 consequent upon this causing a reduction in the frictional force between the inner and outer members. In a further embodiment (Fig. 4, not shown), the inner member is again a straight fibre or wire (11) but in this case it is surrounded by a coating (12) of a material having a different Poisson's ratio, whereby upon stretching of the assembly the coating (12) reduces in thickness to a greater extent than the difference between the diameters of the fibre or wire (11) and the bore (6), so reducing the friction force. The inner and outer members may be bonded together, the type of bonding used providing a reduction in bond strength as the assembly is tensioned. The inner member may be of stainless steel or of glass fibres embedded in a polymeric resin. The outer member may be of a polymer or concrete. Spaces at the interfacial region may be filled with a viscous medium. Surface irregularities at the interface may be reduced by chemical etching.