Classifications

• • D—TEXTILES; PAPER
  • D04—BRAIDING; LACE-MAKING; KNITTING; TRIMMINGS; NON-WOVEN FABRICS
  • D04B—KNITTING
  • D04B21/00—Warp knitting processes for the production of fabrics or articles not dependent on the use of particular machines; Fabrics or articles defined by such processes
  • D04B21/14—Fabrics characterised by the incorporation by knitting, in one or more thread, fleece, or fabric layers, of reinforcing, binding, or decorative threads; Fabrics incorporating small auxiliary elements, e.g. for decorative purposes
  • D04B21/18—Fabrics characterised by the incorporation by knitting, in one or more thread, fleece, or fabric layers, of reinforcing, binding, or decorative threads; Fabrics incorporating small auxiliary elements, e.g. for decorative purposes incorporating elastic threads
• • A—HUMAN NECESSITIES
  • A43—FOOTWEAR
  • A43B—CHARACTERISTIC FEATURES OF FOOTWEAR; PARTS OF FOOTWEAR
  • A43B1/00—Footwear characterised by the material
  • A43B1/02—Footwear characterised by the material made of fibres or fabrics made therefrom
  • A43B1/04—Footwear characterised by the material made of fibres or fabrics made therefrom braided, knotted, knitted or crocheted
• • A—HUMAN NECESSITIES
  • A43—FOOTWEAR
  • A43B—CHARACTERISTIC FEATURES OF FOOTWEAR; PARTS OF FOOTWEAR
  • A43B23/00—Uppers; Boot legs; Stiffeners; Other single parts of footwear
  • A43B23/02—Uppers; Boot legs
  • A43B23/0205—Uppers; Boot legs characterised by the material
• • A—HUMAN NECESSITIES
  • A43—FOOTWEAR
  • A43B—CHARACTERISTIC FEATURES OF FOOTWEAR; PARTS OF FOOTWEAR
  • A43B23/00—Uppers; Boot legs; Stiffeners; Other single parts of footwear
  • A43B23/02—Uppers; Boot legs
  • A43B23/04—Uppers made of one piece; Uppers with inserted gussets
  • A43B23/045—Uppers with inserted gussets
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T442/00—Fabric [woven, knitted, or nonwoven textile or cloth, etc.]
  • Y10T442/40—Knit fabric [i.e., knit strand or strip material]
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T442/00—Fabric [woven, knitted, or nonwoven textile or cloth, etc.]
  • Y10T442/40—Knit fabric [i.e., knit strand or strip material]
  • Y10T442/413—Including an elastic strand
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T442/00—Fabric [woven, knitted, or nonwoven textile or cloth, etc.]
  • Y10T442/40—Knit fabric [i.e., knit strand or strip material]
  • Y10T442/45—Knit fabric is characterized by a particular or differential knit pattern other than open knit fabric or a fabric in which the strand denier is specified

Definitions

• the current invention relates to the manufacture of knitted auxetic fabrics.
• the Poisson's ratio of a material is a measure of its expansion or contraction in a direction perpendicular to an applied strain. Materials with a positive Poisson's ratio contract in a direction perpendicular to an applied tensile strain whereas materials having a negative Poisson's ratio expand in a direction perpendicular to an applied tensile strain. Materials having a negative Poisson's ratio are known as auxetic materials.
• auxetic properties for example foams (US 4,668,557), fibres (WOOO/53830) and honeycombs.
• auxetic materials often naturally form synclastic curves and may therefore provide an improved fabric for clothing manufacture. Auxetic fabrics may also find many other applications in which a thin material having auxetic properties is desirable.
• WO2009/002479 describes a variety of net fabrics, some having negative Poisson's ratios. Net fabrics are only suitable for use in particular applications, and are not useful as general-purpose fabrics due to the large open spaces, as seen in Figures 3 and 4, that characterise net fabrics. Furthermore, many of the fabrics disclosed in that document do not have any elastic component, and therefore would not return to their original shape after deformation, making them impractical for most uses.
• an auxetic solid knitted fabric comprising an auxetic component knitted from at least a first type of fibre, and a stabilising component knitted from at least a second type of fibre, wherein the first and second fibre types have different mechanical properties.
• Figure 1 shows schematic diagrams of structures that have been shown to be auxetic
• Figure 2 shows a schematic diagram of an auxetic fabric
• Figure 3 shows a schematic diagram of the fabric shown in Figure 3 after deformation
• Figure 4 shows a schematic diagram of the fabric shown in Figure 3 after non-auxetic deformation by stretching of the ribs in the x-direction;
• Figure 5 shows a schematic diagram of a further fabric displaying auxetic properties
• Figure 6 shows a schematic diagram of an auxetic shape including indications of dimensions used in the equations
• Figure 7 shows a graph of calculated values for the Poisson's ratio as a function of the angle ⁇
• Figure 8 shows a schematic diagram of an auxetic shape including indications of dimensions used in the equations
• Figure 9 shows a graph of Poisson's ratio as ⁇ and I vary, with ⁇ and m remaining constant
• Figure 10 shows a graph of Poisson's ratio against a ratio of l/m calculated using equation (2);
• Figure 11 shows a first stitch pattern;
• Figure 12 shows a photograph of a fabric knitted using the first stitch pattern
• Figure 13 shows a schematic diagram of a testing system
• Figure 14 shows a series of photographs of the fabric shown in Figure 12 undeformed and under 10% strain
• Figure 15a shows plots of width and length for the fabric of Figure 13 subject to tensile load application along the x direction;
• Figure 15b shows a plot of the transverse strains of the middle 4 width sections of Figure 15a as a function of axial strain
• Figures 16a to d show plots of length and width against time for the fabric shown in Figure 13;
• Figures 17a to d show plots of widthwise strain against lengthwise strain for the fabric shown in Figure 13;
• Figure 18 shows a second stitch pattern
• Figure 19 shows a photograph of a fabric knitted using the second stitch pattern
• Figure 20 shows a series of photographs of the fabric shown in Figure 18 undeformed and under 10% strain
• Figures 21 show plots of length and width against time for the fabric shown in Figure 18;
• Figures 22a to d show plots of widthwise strain against lengthwise strain for the fabric shown in Figure 18;
• Figure 23 shows a third stitch pattern
• Figure 24 shows a photograph of a fabric knitted using the third stitch pattern
• Figure 25 shows a series of photographs of the fabric shown in Figure 24 undeformed and under 10% strain
• Figures 26a to d show plots of length and width against time for the fabric shown in Figure 24;
• Figures 27a to d show plots of widthwise strain against lengthwise strain for the fabric shown in Figure 24;
• Figure 28 shows a fourth stitch pattern
• Figure 28a shows a loop diagram from the stitch pattern of Figure 28
• Figure 29 shows a photograph of a fabric knitted using the fourth stitch pattern
• Figure 30 shows a series of photographs of the fabric shown in Figure 28 undeformed and under 10% strain
• Figures 31a to d show plots of length and width against time for the fabric shown in Figure 28;
• Figures 32a to d show plots of widthwise strain against lengthwise strain for the fabric shown in Figure 28;
• Figure 33 shows photographs of a 12 gauge fabric made using the stitch pattern of Figure 28.
• FIG 1 shows schematically two geometric patterns that have been shown to be auxetic.
• the auxetic effect may be achieved at a macroscopic level by a number of mechanisms including rib or shape rotation or flexure.
• rib or shape rotation or flexure For example, the cellular frameworks of reentrant struts (ribs) of Figure 1 deform by rib flexing or rotation to expand their re-entrant side(s).
• Modern knitting machine technology allows the formation of complex, and relatively arbitrary, shapes formed by the fibre paths within a knitted fabric.
• the current invention provides fabrics, and methods of manufacture of fabrics, that display auxetic behaviour by reproducing the geometry of auxetic structures in the knit fabric structures.
• Figure 2 shows a schematic diagram of a knitted fabric having auxetic properties in the y direction - that is, a tensile load applied in the y direction causes an increase in width in the x direction.
• the fabric is also auxetic in the x direction.
• the fabric is formed of two components - an auxetic component 30 and a stabilising component 31.
• dashed lines are utilised to differentiate types of fibres, or fibres forming different components of the fabric. The non-continuous nature of those lines does not indicate that the fibres are not continuous, but are simply used to differentiate between fibres for clarity in the absence of the ability to use colour.
• the fabric shown schematically in Figure 2 is knitted from fibres to form the fabric from selected fibres.
• many techniques may be utilised to knit a fabric having the design shown in Figure 2 and examples of possible patterns and systems are provided below.
• the auxetic component is formed using relatively high modulus fibres and the stabilising component is formed using relatively low modulus, elastic, fibres.
• the fibres of the two components may be knitted together, one component may be laid into the other one, or a combination of knitting and laying in may be utilised between the two components.
• the auxetic component deforms such that the Poisson's ratio of the fabric is negative.
• the stabilising component acts to return the fabric to its relaxed, unloaded state.
• the fibres of the stabilising component are stretched by the application of the load in the x-direction, thereby allowing movement of the vertices of the auxetic component to provide the deformation described above.
• the stabilising fibres also move in the y direction as the vertices at which they are attached are moved by the auxetic component.
• the fibres of the stabilising component (which, as noted previously, are elastic) act to pull the auxetic component back to the rest state shown in Figure 2.
• the fabric therefore returns to its original structure and can provide repeated performance.
• Deformation during this second phase is predominantly by stretching of those fibres of the auxetic component aligned along the loading (x) direction and rotation of those fibres of the auxetic component oriented at an angle to the loading (x) direction.
• the modulus of those fibres is larger than the fibres of the stabilising component and therefore the modulus of the material in this second phase is larger than in the first, negative Poisson's ratio, phase.
• deformation in this second phase is dominated by the fibres of the auxetic component and therefore the stabilising component has little, or no, effect on this second phase behaviour.
• each re-entrant triangle For tensile loading along the y direction, the base of each re-entrant triangle first moves to a straight line configuration and then adopts a convex rather than the original concave (re-entrant) shape upon further loading.
• Rotation of the fibres in a convex (as opposed to re-entrant) triangular network formed by the auxetic component leads to extension along the length in the y direction being accompanied by a reduction in width along the x direction. Deformation during the second phase for loading along y is then predominantly by rotation of the fibres of the auxetic component.
• the knitted fabric shown schematically in Figure 2 has a negative Poissons, that is it is auxetic, in a first phase of stretching of the fabric.
• stretching of the fabric is by stretching of the fibres of the stabilising component and by rotation of the fibres of the auxetic component about their vertices. This rotation causes the re-entrant triangles of the auxetic component to deform towards regular triangles, and thus the fabric to have a negative Poisson's ratio.
• the modulus of the material during the first phase is defined by the modulus of the fibres of the stabilising component, and the resistance of the fibres of the auxetic component to the rotation movement.
• the behaviour of the fabric will depend on the relative behaviours of the fibres and components they form.
• the fibre of the auxetic component must have a sufficiently high modulus compared to the resistance to rotation of that fibre about its vertices, such that the shapes of the auxetic component deform in preference to the fibres of the auxetic component stretching.
• the elasticity of the fibres of the stabilising component must be sufficiently high compared to the resistance of the fibres of the auxetic component to rotation that the stabilising component can return the auxetic component to its original configuration after release of the stretching force. It is therefore likely that the modulus of the fabric will be dominated by the modulus of the stabilising component. Similarly the modulus of the fibres of the auxetic component should be sufficiently high compared to the resistance to rotation such that the auxetic component deforms by rotation rather than buckling of the fibres forming the component.
• Figure 5 shows a schematic of a further fabric expected to demonstrate auxetic properties.
• the fabric of Figure 5 is similar to the fabric of Figures 3 to 5 in that it is formed of an auxetic component and a stabilising component.
• the layout is related to that of Figures 3 to 5 in that it is obtained by the removal of one of the sides from each of the re-entrant triangles to combine two adjacent triangles into a single shape.
• the resulting shapes are similar to the bow-tie shapes utilised in auxetic honeycomb structures.
• deformation of the fabric can be assumed to be solely due to rotation of the fibres of the auxetic component about its vertices. That is, solely due to a transformation from Figure 2 to Figure 3, with no stretching of the fibres of the auxetic component and no resistance to rotation at the vertices such that no bending of the fibres of the auxetic component occur.
• Figure 7 shows a graph of calculated values for the Poisson's ratio as a function of the angle ⁇ .
• the model predicts negative Poisson's ratios for ⁇ ⁇ 90°. Equation (1 ) suggests that in order for the structure to be auxetic ⁇ must be ⁇ 90°. However, if other deformation mechanisms are considered negative Poisson's ratios may also be obtained for ⁇ > 90°.
• Figure 8 shows the symbols used in the following discussion.
• a possible deformation mechanism, in structures having ⁇ > 90°, is for ribs AB and BC to stretch as angle ⁇ decreases ( ⁇ and m remain constant), thereby compensating for an increase in AC due to the decrease in ⁇ .
• the Poisson's ratio can be shown to be calculated by equation (2).
• Figure 9 shows a graph of Poisson's ratio as ⁇ and I vary, with ⁇ and m remaining constant.
• Figure 10 shows a graph of Poisson's ratio against a ratio of l/m calculated using equation (2). As seen in that figure negative Poisson's ratios may be obtained for certain values of l/m.
• Equation (2) and Figures 9 and 10 demonstrate that negative Poisson's ratios may be obtainable for values of ⁇ > 90° for certain deformation mechanisms.
• the deformation mechanism is defined by the relative properties of the fibres and knit structure.
• a number of stitch patterns were designed for implementation on a knitting machine to knit structures corresponding to the structure illustrated schematically in Figure 2 and to explore various parameters which may affect the Poisson's ratio of the resulting fabric.
• Figure 11 shows a first stitch pattern designed to manufacture a fabric functionally equivalent to that shown schematically in Figure 2.
• the pattern utilises three types of fibres, two to form the auxetic component and one to form the stabilising component.
• the auxetic component is laid into the stabilising component which is knitted using open loop stitches.
• the stitch pattern can be implemented using three guide bars set as shown in Table 1.
• the nomenclature 0-2 refers to a jump of one needle.
• a fabric was knitted using the stitch pattern shown in Figure 11.
• An 18 gauge machine was utilised with the fibres shown in Table 2.
• Figure 12 shows a photograph of the knitted fabric produced using the stitch pattern of Figure 11 and the settings of Tables 1 and 2. As can be seen the auxetic component of the fabric does not have the re-entrant triangle configuration of Figure 2 as intended, but instead the base of the triangle is actually directed out of the shape.
• the fabric was heat set prior to mechanical testing. Test samples (15cm long by 5 cm wide) were cut along and perpendicular to the warp (X) direction, and also at ⁇ 45° to the warp direction. Fiducial markers were placed on the samples using marker pen to enable accurate measurement of strains by optical means during mechanical testing. Testing of the fabric was performed using the system shown in Figure 13. The fabric concerned was loaded into the jaws of the universal testing machine for measurement of its stress/strain characteristics, and a camera system enabled the measurement of dimensional changes from the fiducial markers on the sample during the application of a load to the fabric. The fabric was measured for uniaxial loads applied in the X and Y directions, and also at -45° to the X direction (direction 1 ) and +45° to the X direction (direction 2).
• Figure 15 shows a series of photographs taken during testing of the fabric.
• the photos in the left hand column show fabric unloaded, while the photos in the right hand column show the fabric with a 10% length-wise strain.
• Each of the rows relates to one of the testing directions, as noted in the row heading.
• Videoextensometry data provided measurements of the length and width of the fabric while a longitudinal strain is applied.
• Figure 15a shows the width and length videoextensometry data for the fabric of Figure 13 subject to tensile load application along the x direction (along the warp direction).
• I and Io are the length and original length in the direction of interest.
• Figure 15b shows the transverse strains of the middle 4 width sections as a function of axial strain.
• Figures 16a to d show graphs, for the four orientations (x, y, -45°, +45° respectively), of length and average width against time as the fabric samples were stretched in the apparatus shown in Figure 13. The middle 4 of 10 width measurements were averaged to give the average width the data shown in Figure 16.
• Figures 17a to d show graphs, for the four orientations (x, y, -45°, +45° respectively), of average widthwise strain against lengthwise strain derived from the data of Figure 16.
• Figure 18 shows a second stitch pattern designed to manufacture a fabric equivalent functionally to that shown schematically in Figure 2.
• an additional fibre is utilised in the stabilising component of the fabric.
• a second elastomeric fibre is knitted to join the bases of the triangle shapes in each column.
• this fibre is knitted using closed loop stitches which was intended to provide a fabric which is more stable in both length and width directions.
• the stitch pattern can be implemented using four guide bars set as shown in Table 4.
• a fabric was knitted using the stitch pattern shown in Figure 18.
• An 18 gauge machine was utilised with the fibres shown in Table 5.
• Figure 19 shows a photograph of the knitted fabric produced using the stitch pattern of Figure 18 and the settings of Table 4 and Table 5. As can be seen the auxetic component of the fabric does not have the re-entrant triangle configuration of Figure 2 as intended, but the base of the triangles is significantly flatter than was achieved with the previous stitch pattern.
• Figure 20 shows a series of photographs taken during testing of the fabric.
• the photos in the left hand column show fabric unloaded, while the photos in the right hand column show the fabric with a 10% length-wise strain.
• Each of the rows relates to one of the testing directions, as noted in the row heading.
• Figures 21 a to d show graphs, for the four orientations (x, y, -45°, +45° respectively), of length and average width against time as the fabric samples were stretched in the apparatus shown in Figure 13.
• the four central widths were averaged to produce the averaged width data shown in Figure 16 to minimise possible artefacts due to edge effects associated with the width sections nearest the grips of the testing machine.
• Figures 22a to d show graphs, for the four orientations (x, y, -45°, +45° respectively), of average widthwise strain against lengthwise strain calculated as described previously from the data of Figure 21.
• Figure 23 shows a third stitch pattern designed to manufacture a fabric equivalent functionally to that shown schematically in Figure 2.
• This stitch pattern is a variation of that shown in Figure 18 in that the fibres forming the bases of the triangles are knitted into the fabric using closed loop stitches in contrast to being laid-in in the previous pattern. These changes were intended to achieve more stability at the cross-over points and more stably anchor the hinges.
• the stitch pattern can be implemented using four guide bars set as shown in Table 7.
• Table 7 A fabric was knitted using the stitch pattern shown in Figure 23. An 18 gauge machine was utilised with the fibres shown in Table 8.
• Figure 24 shows a photograph of the knitted fabric produced using the stitch pattern of Figure 23 and the settings of Table 7 and Table 8. This fabric is denser and tighter than the previous samples.
• Figure 25 shows a series of photographs taken during testing of the fabric.
• the photos in the left hand column show fabric unloaded, while the photos in the right hand column show the fabric with a 10% length-wise strain.
• Each of the rows relates to one of the testing directions, as noted in the row heading.
• Figures 26a to d show graphs, for the four orientations (x, y, -45°, +45° respectively), of length and average width against time as the fabric samples were stretched in the apparatus shown in Figure 13.
• the four central widths were averaged to produce the averaged width data shown in Figure 26 to minimise possible artefacts due to edge effects associated with the width sections nearest the grips of the testing machine.
• Figures 27a to d show graphs, for the four orientations (x, y, -45°, +45° respectively), of average widthwise strain against lengthwise strain calculated as described previously from the data of Figure 26.
• the measurements of the fabric were repeated a number of times and the results averaged to provide the Poisson's ratio values given in Table 9 below.
• This fabric sample therefore demonstrates auxetic behaviour in at least one direction, and a zero or negative Poisson's ratio in a second direction.
• the auxetic properties measured in the vi 2 an d v 2 i directions is consistent with auxetic behaviour arising from the structure shown schematically in Figure 5.
• One of the long struts is aligned close to the loading direction, while the other long strut is redundant in terms of Poisson's ratio, but may contribute to the stiffness of the material.
• Figure 28 shows a fourth stitch pattern designed to manufacture a fabric equivalent functionally to that shown schematically in Figure 2.
• This stitch pattern is a variation of that shown in Figure 23 in that the first fibres of the stabilising component are now knitted in a Tricot stitch with both closed and open stitches, in contrast to the pillar stitch used in the previous patterns. This modification was intended to produce a more isotropic fabric.
• Figure 28a shows a loop diagram expected from the stitch pattern shown in Figure 28.
• the stitch pattern can be implemented using four guide bars set as shown in Table 10.
• a fabric was knitted using the stitch pattern shown in Figure 28.
• An 18 gauge machine was utilised with the fibres shown in Table 1 1.
• Figure 29 shows a photograph of the knitted fabric produced using the stitch pattern of Figure 28 and the settings of Table 10 and Table 1 1.
• Figure 30 shows a series of photographs taken during testing of the fabric.
• the photos in the left hand column show fabric unloaded, while the photos in the right hand column show the fabric with a 10% length-wise strain.
• Each of the rows relates to one of the testing directions, as noted in the row heading.
• Figures 31 a to d show graphs, for the four orientations (x, y, -45°, +45° respectively), of length and average width against time as the fabric samples were stretched in the apparatus shown in Figure 13.
• the images of the fabric provided width data at ten points along the length of the sample.
• the four central widths were averaged to produce the averaged width data shown in Figure 31 to minimise possible artefacts due to edge effects associated with the width sections nearest the grips of the testing machine.
• Figures 32a to d show graphs, for the four orientations (x, y, -45°, +45° respectively), of average widthwise strain against lengthwise strain calculated as described previously from the data of Figure 31.
• This fabric sample therefore demonstrates auxetic behaviour in two directions.
• One of the long struts is aligned close to the loading directions which demonstrated auxetic properties, while the other long strut is redundant in terms of Poisson's ratio, but may contribute to the stiffness of the material.
• a further fabric was knitted using the stitch pattern of Figure 28 at 12 gauge and using the settings shown in Table 13.
• Figure 33 shows photographs of the front and back of the knitted fabric produced using the stitch pattern of Figure 28 and the settings of Tables 13 and 14.
• Figure 34 shows a further variation of a stitch pattern for knitting an auxetic fabric.
• This pattern is a variation on that shown in Figure 23 with a Tricot stitch being used for Guide Bar 2.
• the fibres described in relation to Figure 23 may be utilised with this stitch pattern.
• the fabric was tested by applying a tensile strain at approximately 45 degrees to the warp direction and monitoring the width of the fabric. It was observed that as the fabric extended in length, its width increased, clearly demonstrating auxetic behaviour.
• the knitted fabrics disclosed herein comprise an auxetic component and a stabilising component.
• the auxetic component comprises shapes that provide an auxetic behaviour and are formed of fibres that are of a relatively higher modulus that the fibres of the stabilising component.
• the stabilising component acts to restore the auxetic component to its resting shape after the fabric has been deformed. The fabric can therefore return to its original shape and provide auxetic behaviour on subsequent stretchings.
• the material disclosed herein is not therefore a 'single shot' material but may find applications where the continued performance is required.
• a fabric could be knitted using only the auxetic component described herein to provide a one-shot fabric. Such fabrics may find application where no repeat performance is required. Modification of the knitting patterns described herein may be required to produce such one-shot fabrics.
• the behaviour of the fabric is defined by the interaction and relative characteristics of the stabilising and auxetic components of the fabric.
• the changes in the knit patterns between the example fabrics described herein affect those relative characteristics and as demonstrated by the measurements this affects the performance of the fabrics.
• Pattern four also introduces a lateral interaction of the stabilising component with the auxetic component pattern for the red stabilising fibre such that it acts to pull the centre of the re-entrant side of the triangles out of the triangle, thereby acting to expand the fabric in the lateral direction. It is thought this may contribute to the improved negative Poisson's ratio seen for this fabric.
• the fabrics described herein are solid fabrics, as contrasted to net fabrics, which are significantly different types of material.
• Net fabrics have large open spaces between stitches to create a net structure, whereas the fabrics described herein only have minor open spaces within the stitches forming the fabric.
• open spaces are bounded by stitches of the fabric, whereas in the current fabrics any open spaces are bounded by straight yarns.
• Net fabrics are generally formed of tricot course and chain courses in which the yarn stitches meander across the fabric to create the open spaces that characterise a net fabric in contrast to the solid fabrics described by the current invention.
• Figure 2 shows a schematic diagram of an auxetic structure comprising a reinforcement component as described in relation to Figure 1 , and a recovery component.
• the reinforcement and recovery components are joined at the vertices of the reinforcement shape at which they coincide.
• the recovery component therefore acts to pull the base of the arrowhead shape together.
• the arrowheads expand as shown in Figure 3, and the recovery component is stretched.
• the recovery components are exerting a force across the base of the arrowheads which acts to return the structure to the form shown in Figure 2.
• a structure which returns to its original shape, and displays repeated auxetic behaviour is therefore provided by the structure shown in Figure 2.
• the structure of Figure 2 operates due to the combination of a high modulus reinforcement component having auxetic properties in conjunction with an elastic, low modulus, recovery component.
• the reinforcement component provides the negative Poisson's ratio when a strain is applied and the recovery component returns the reinforcement component back to its original shape upon removal of the deforming strain.
• This principle can be applied to a range of auxetic structures as shown schematically in Figure 5.
• the reinforcement and recovery components may be provided using any suitable materials and may be formed using any suitable manufacturing techniques. A number of examples of sheet materials will now be described which embody the principles set out above.
• the materials forming the auxetic component and their recovery component are not necessarily fibres, but may be any suitable materials providing the required mechanical properties.
• the recovery component may be a matrix component into which the auxetic component is embedded.
• an auxetic knitted fabric may be embedded in a neoprene matrix to form a composite material.
• the neoprene matrix may be formed to complete enclose the knitted structure, or the knitted structure may be on the surface of the neoprene.
• Neoprene is used as an example only and any comparable material may be utilised for the matrix component as dictated by the requirements of the resulting material.
• the auxetic component is formed from carbon fibre and the recovery component is formed from a low modulus fibre such as aramid or polyethylene.
• the auxetic component is provided by a fibre or monofilmaent stitched to an elastic sheet which provides the recovery component.
• the vertices at either end of the re-entrant side of the auxetic component are fixed to the recovery element.
• a sewing machine may be used to form the auxetic component on the elastic sheet, or for a thicker or harder recovery element holes may be pre drilled and the auxetic element threaded through the holes.
• the auxetic component may be stapled or bonded to the recovery component.
• the auxetic component may be formed by etching a metallic element pre-laminated on the recovery component.
• a suitable material for the recovery component may be a neoprene sheet.
• the auxetic component is provided by a carbon fibre, stitched to a substrate made from a commercially available composite material such as that made from aramid fibres in an elastomeric resin matrix, so that the vertices at either end of the re-entrant side of the auxetic component are fixed to the recovery element.
• the auxetic component may be created on a thin sheet of laminate and then embedded in further layers of laminate to create a thicker sheet. Several layers of the auxetic component may be produced and embedded between further layers of the recovery element to form a multi layer laminated structure.
• the auxetic component may be formed using a fibre placement and bonding technique, and the recovery component is bonded to the vertices as described above.
• the auxetic component may be cut or fabricated from sheet material, for example a thin sheet of steel.
• the recovery component may comprise a crimped wire mesh which is bonded or stitched to the vertices of the auxetic component.
• the auxetic component is a honeycomb material and the recovery component is an elastic component joining the vertices of the cells.
• the honeycomb may be cut from a sheet material or extruded through a die.
• the recovery component may be elastic fibres joining vertices or may be a sheet bonded to the honeycomb.
• the sheet material and/or honeycomb may have a significant thickness.
• the vertices of the auxetic component may be formed as flexible sections of the component, or may be formed as specific hinge elements. The components may therefore be formed of a plurality of discrete components joined into the overall structure.
• the recovery component may also be provided by the auxetic component.
• the auxetic component may deform by hinging at the vertices, as shown in Figures 1 and 2. Those vertices may be made such that the deformation towards Figure 2 is elastic and thus when the strain is removed the auxetic component has internal forces which attempt to return it to the undeformed state. Similarly, deformation may occur by rib flexing. Elastic flexing may be utilised to provide the recovery force to return the auxetic component to the undeformed state.
• auxetic component and auxetic component are formed as one component, the action of the recovery component being provided by elastic deformation of the component.
• elastic deformation at the rib hinges, or of the ribs.
• the scale of these example materials may be tailored to the specific example for which they are to be utilised and the materials and techniques used to manufacture them.
• the unit cells may be comparable to the size provided by the knitted fabrics or may be significantly smaller or larger.
• the thickness of the sheet material is defined by the materials from which the sheet is manufactured and it is not envisaged that all examples will be thin as per the fabrics, but thicker sheets are explicitly contemplated where this is appropriate for the materials and manufacturing techniques.
• the properties of the materials described herein are dependent on the relative properties of the materials forming the materials.
• Various deformation mechanisms have been described, for example rib flexing, rotation and stretching.
• the selected materials will define the mode of deformation and hence the properties of the resulting sheet material.
• Either element may be formed using a smart material such as material having a high expansion coefficient or be an electrically activated material known as a piezoelectric.
• auxetic sheet material comprising an auxetic component formed from a first material, and a recovery component formed from a second material, wherein the first and second materials have different mechanical properties
• auxetic sheet material is not exclusively an auxetic knitted fabric, comprising an auxetic component knitted from at least a first type of fibre, and a stabilising component knitted from at least a second type of fibre, wherein the first and second fibre types have different mechanical properties.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Materials Engineering (AREA)
• Textile Engineering (AREA)
• Knitting Of Fabric (AREA)

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• 2010
  • 2010-04-30 EP EP20100718676 patent/EP2454407A1/de not_active Withdrawn
  • 2010-04-30 US US13/318,249 patent/US20120129416A1/en not_active Abandoned
  • 2010-04-30 WO PCT/GB2010/050719 patent/WO2010125397A1/en active Application Filing

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