Abstract:
A one-piece fiber reinforcement for a reinforced polymer is described. In an embodiment, a one-piece reinforcement is fabricated by first assembling an interior randomly oriented fiber layer between two exterior aligned fiber layers. With all layers in face to face contact, a preselected number of fibers from the aligned layer is conveyed out of its aligned layer and threaded into at least the random fiber layer so that the conveyed fibers engage and mechanically and frictionally interfere with the random fibers. The fibers may be conveyed from one aligned layer to the other for yet greater interference. The interfering fibers serve to secure and interlock the layers together, producing a one-piece reinforcement which, when impregnated with a polymer precursor, shaped and cured may be incorporated in a polymer reinforced composite article.

Description:
TECHNICAL FIELD 
     This invention pertains to methods of fabricating a one-piece fiber reinforcement from a number of co-extensive, separate and distinct reinforcing fibrous structures arranged in layered fashion. The method involves intermingling fibers from the different layers so that at least some of the fibers from each layer are inserted into an adjacent layer and bridge the original layer interface to engage with, and interfere with, at least some of the fibers of at least one other layer to interlock the layers. The invention, applied over substantially the entire lateral extent of the individual reinforcements, may be used to fabricate reinforcements with improved resistance to delamination and simplify manufacture of fiber reinforced polymer composites. 
     BACKGROUND OF THE INVENTION 
     There is increasing interest in substituting high performance lightweight reinforced composite components and structures comprising a polymer matrix with a suitable reinforcement for stamped sheet metal components in vehicles. Suitable polymers are often thermosets, such as epoxies, vinyl-esters or polyesters, or thermoplastics, such as polypropylene or poly amide, and suitable reinforcements include structural fibers such as carbon, glass or aramid fibers. Such fibers may be randomly oriented and arranged or aligned along one or more preferred directions. 
     Individual carbon fibers may range from about 5 to 10 micrometers in diameter with 7 micrometer diameter fibers being especially common. Individual glass fibers may range from about 7 to 30 micrometers, depending in part on the grade of glass. In many applications, particularly those employing aligned fibers, assemblages of commonly-oriented fibers, variously called tows or roving, are used. Such carbon fiber assemblages may contain as few as 1000 or as many as 50,000 or more fibers, while glass fiber assemblages may include up to 200,000 or more fibers. 
     For fabric applications, aligned fibers may be assembled into one of two fabric structures for ease of application: a woven cloth or a non-woven fabric often called a non-crimp or stitch bonded fabric. A woven cloth employs tows of a first orientation which alternately overlie and underlie fiber tows of a second orientation, usually at about 90° to the first orientation. The weave may be tight, with adjacent tows positioned about a millimeter or less apart, or loose, with adjacent tows spaced up to about 10 millimeters apart. In an alternative structure, a number of spaced apart fiber rovings, individually fed from their respective spools, may be simply laid alongside one another in a ply, and temporarily secured and locked into place, by stitching, using, for example, a polyester yarn. Such stitching generally extends over the length and breadth of the reinforcement ply and is usually accomplished with a stitch beam which incorporates a plurality of needles and has a suitable motion to enable both simple chain stitches and other more complex stitches, for example tricot stitches. In many cases multiple coextensive plies are laid atop one another and the rovings of all of the plies are secured in a single stitching operation. Often the plies are placed with the fiber orientations of adjacent plies rotated one from another to render the in-plane properties less directional, or more isotropic, in the multi-ply reinforcement than in each ply individually. The weight of each ply is determined by the bulk of the roving and the spacing between adjacent roving bundles. These, non-woven reinforcements are called stitch bonded fabrics or non crimp fabrics, often abbreviated as NCF. 
     One common example of a multi-ply NCF is a 4-layer grouping of fibers arranged at 0°, +45°, −45° and 90° respectively with substantially equal numbers of fibers in each orientation. A 2-layer NCF with fibers arranged at +45° and −45° also finds wide application. Of course this description of such a multi-ply NCF is intended to be exemplary and not limiting. It will be appreciated that variations in the number of plies, in the number of orientations, in the angular alignment of the fibers within any ply and in the fiber density in each orientation are comprehended by the terminology non-crimp fabric, stitch bonded fabric, NCF, NCF fabric or aligned fiber layer as used in this specification. 
     Such fabric reinforcements, woven or non-woven, may be impregnated with a suitable polymer resin, placed in a mold, shaped and then cured, typically at modestly elevated temperature, say about 150° C., to form the desired polymer composite. It will be appreciated that the above-listed sequence of operations may be modified for different molding processes. For example, preforms may be placed in a mold with resin already impregnated, or the resin can be added after the preform is in the mold via resin infusion, resin transfer molding, or structural resin injection molding. Thermoplastic or thermoset sheets or materials with comingled strands of thermoplastic and reinforcing fiber may also be employed. 
     Commonly, more than one fabric reinforcement may be required to develop the desired properties in the composite. These reinforcements may be stacked atop one another, while possibly rotating or offsetting one layer with respect to another, with the goal of developing greater isotropy, or lack of directionality in properties, at least in the plane of the reinforcement. 
     Reinforcing layers in which the reinforcing fibers are randomly oriented such as by directed fiber preforming or Programmable Powered Preform Process (P4™ preforming), or one or more layers of continuous strand mat such as Owens Corning 8610 or chopped strand mat also find application. Such reinforcements may, by virtue of the fibers being oriented over all possible orientations, offer more isotropic properties than even a multilayer NCF fabric reinforcement. 
     One suitable configuration for a multilayer fiber-based polymer composite reinforcement is a layer of randomly-oriented fibers sandwiched between two layers of aligned fibers, which may be assembled as NCF (non crimp fabric) layers or woven layers. But, such multilayer reinforcements are also multi-piece, and require that each reinforcement layer be placed and positioned individually, complicating manufacturing. 
     There is therefore need for a one-piece reinforcement which facilitates manufacturing of fiber reinforced polymer composite articles and at least meets the performance objective of multilayer, multi-piece reinforcements. 
     SUMMARY OF THE INVENTION 
     A layered, one-piece fiber reinforcement suitable for use in a reinforced polymer composite is formed from a plurality of layered, coextensive individual reinforcements in face to face contact. The individual reinforcements may include at least an oriented layer of woven or non-woven reinforcing fabric with oriented fibers and a layer of randomly oriented fibers. A needle punch or similar technique is used to pull or push a preselected portion of the fibers of a layer and insert them into at least an adjacent layer where they may engage with the fibers of the adjacent layer. It is preferred that the fibers engage the layers substantially uniformly over substantially the entire extent of the layer. Frictional interaction and mechanical interference between the fibers from the differing layers will hold, bind and interlock at least adjacent reinforcing layers to one another and render a one-piece reinforcement with enhanced interlayer strength. In reinforcements with more than two layers it may be preferred to thread fibers through all the layers of the reinforcement so that all layers are interlocked. A one-piece reinforcement is thereby effected from a plurality of reinforcing layers. The one-piece reinforcement simplifies manufacturing of fiber reinforced polymer composite articles and provides improved properties over the same arrangement of non-interlocked reinforcing layers. 
     For example, in an embodiment, a 3-layer reinforcement includes two aligned fiber layers, which may, for example, be NCF layers, with a random fiber layer positioned between them. The random fiber layer may comprise continuous or chopped fibers. A preselected number of fibers from a first aligned fiber layer is pulled or pushed through the random fiber layer and inserted into or through the second aligned fiber layer to frictionally and mechanically securely bind all of the layers together. Optionally, fibers may also be pulled or pushed from the second aligned fiber layer, through the random fiber layer, to the first aligned fiber layer to further secure the layers and effect a one-piece reinforcement. Such extensive fiber rearrangement is not a requirement and fibers may be pulled or pushed from only the random layer to be inserted in one of the aligned layers, or vice versa. 
     Such a reinforcement, by virtue of those fibers extending out of the plane of reinforcing layer and directed through the reinforcement thickness, will impart enhanced interfacial strength at the layer interfaces to a reinforced polymer article. Such increased interfacial strength may suppress delamination and enhance the energy adsorption afforded by the article under severe loading. This benefit may also obtain with layered chopped strand mat or continuous strand mat reinforcements. Because the location of such load application may be indeterminate, the layers should be bound together over substantially their entire extent with the engaging fibers generally uniformly distributed over the entire area of the layer(s). But it may be preferred to concentrate the engaging fibers at load application sites if these may be predicted, for example by simulation or modeling, or are known from experience or experiment. 
     Needle punching employs an elongated tool, with a shaft incorporating at least one feature adapted to engage and capture fibers when the tool is moved in a first direction, and, when the tool is moved in the reverse direction, release the captured fibers. The tool, which may be needle-shaped with a diameter of from about 0.5 to 1 millimeter, is operated with a reciprocating motion so that it is repeatedly inserted into, and withdrawn from, a fiber-containing layer. In a tool with a plurality of fiber-capturing features, these will typically be distributed along the length and/or around the cross-section of the tool shaft. Generally the fiber capturing features, for example barbs, hooks or flukes, are arranged for unidirectional fiber capture. That is, a fiber in sliding contact with the tool shaft will be captured and retained by the fiber capturing feature under only one of the tool&#39;s reciprocating motions. Often the fiber-capturing feature is oriented to capture fibers as the needle or tool is inserted into a fiber layer so that with each insertion of the tool, fibers captured by the barb(s) or similar, during an early part of the stroke will be pushed more deeply into the fiber layer as the tool continues to advance. At the end of the tool stroke, as the tool reverses direction and is withdrawn, the fiber will disengage from the unidirectional fiber capturing feature but will be held in place through frictional engagement with other fibers or through mechanical interference with other fibers. Because the fiber capturing feature is unidirectional, the tool is ill-oriented and unsuited to capture any further fibers during retraction, and so may be readily withdrawn. Repeated insertions and withdrawals, usually accompanied by lateral movement of the tool to previously unprocessed areas, will promote increasing engagement, entanglement and interference between the fibers from the upper and lower sections of the layer. This procedure may be continued until the layers are secured to one another by a suitable number of inserted fibers across substantially their entire extent. Generally the number of inserted fibers per unit area will be substantially uniform across the extent of the reinforcement but a greater areal density of inserted fibers may be employed in more highly stressed regions if required. 
     Higher productivity may be achieved through the use of multiple tools, operated independently or ganged together in a common fixture. When multiple tools are employed the tools may be supported by plates incorporating a plurality of close-fitting holes suitably positioned to receive the tools. Also the fabric layer may be supported on a similar, hole-containing, tool receiving plate or on a fiber array oriented parallel to the tool or on a support body which may be penetrated by the tool without damage to the tool, such as a solid or foam soft rubber body. 
     Although a common embodiment employs fiber capturing features oriented to enable fiber capture during only one of the up-down strokes of a reciprocating tool, tool variants suitable for fiber capture on both of the up and down strokes may be employed. 
     The strength of a joint formed between layers in a layered one-piece reinforcement will depend, primarily on the number of fibers of each layer which interferingly engage with the fibers of the abutting layer and so will generally depend on the number of tool strokes. If fiber-engaging features are distributed along the length of the tool, the extent of fiber engagement and interference may also depend on the length of the tool stroke. Commonly such needle punch or similar procedures may be applied from only a single side so that the tool will always enter the layer stack on a particular surface of a particular layer. But, to achieve more robust fiber intermingling, the procedure may also be applied from both sides of the stack. Where such two-side needle punching is preferred it may be carried out either by using two sets of opposing tools or by using a single tool set from one direction and then interchanging the top and bottom surfaces of the stack and performing a second needle punching operation. 
     Such a one-piece reinforcement is effective in imparting increased strength and fracture resistance to a reinforced polymer article. Most reinforced polymer components are substantially two-dimensional in character with a thickness appreciably less than their lateral extent. Planar reinforcements are usually oriented to enhance lateral properties and are assembled one atop the other without interconnection. After impregnating the layers with a polymer resin and curing of the composite, the layers are secured to one another by only whatever thickness of polymer is trapped between them. Under high impact loads, if the polymer fractures or releases from one or other of the layers, delamination or separation of the reinforcement layers may occur. Once delaminated the layers are rendered incapable of providing mutual support and act independently, diminishing their effectiveness. 
     With the one-piece reinforcement of the present invention, fibers from one layer may be inserted into at least an adjacent layer so that these fibers serve as reinforcements which extend between and span layers. These inserted fibers will be oriented out of the plane of the reinforcing layers, commonly, but without limitation, within ±10° of perpendicular to the layer interface, and, after curing, secured in position by adhesion between the fibers and the polymer. Further, these fibers, in contrast to the fibers in the reinforcement layers, will follow a tortuous path which will be effective in resisting fiber pull-out from the polymer matrix. With the inter-layer reinforcement contributed by these layer-spanning fibers, the reinforcement will be less likely to delaminate under severe loads and so may provide enhanced performance over assemblies of reinforcing layers without such layer-spanning interlocking fibers. 
     The fiber content of such a one-piece reinforcement may include all commonly-used reinforcing fibers including, but not limited to, carbon fibers or glass fibers, as well as aramid fibers. 
     A fiber reinforced polymer article containing such a one-piece reinforcement may be fabricated by the following steps (though, depending on the particular molding process used, not necessarily in this order): assembling a layered reinforcement by stacking a plurality of generally planar, generally coextensive fiber-containing reinforcements atop one another in face to face relation; conveying a preselected portion of the fibers from at least one layer of the layered reinforcements out of the plane of the reinforcement and pulling or pushing them into to at least a second layer of reinforcement to secure the reinforcements together and repeating until all layers are bound to one another; impregnating the reinforcement with a suitable polymer precursor in sufficient quantity to wet all of the fibers and to fill a mold cavity; shaping the polymer precursor-impregnated reinforcement to a preselected geometry suitable for production of the article and thereby forming a pre-preg; placing the pre-preg in a mold with an interior cavity defining the desired article shape; closing the mold to induce the prepreg to conform to the shape of the die cavity, to compact the prepreg and to displace and distribute polymer precursor throughout the mold cavity; and curing the polymer precursor in the shaped pre-preg to form the fiber reinforced polymer article. One-sided vacuum-bag or autoclave molding may also be employed. 
     These and other aspects of the invention are described below, while still others will be readily apparent to those skilled in the art based on the descriptions provided in this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows, a schematic representation of a number of aligned and random reinforcements suitable for reinforcing a reinforced polymer article.  FIG. 1A  shows, in plan view, a woven reinforcement;  FIG. 1B  shows in perspective view, a non-woven reinforcement;  FIG. 1C  shows, in plan view, a continuous mat reinforcement; and  FIG. 1D  shows in plan view a chopped mat reinforcement. 
         FIGS. 2A-G  show, in cross-section, a series of schematic representations of a needle-punch tool entering and withdrawing from a 3-layer reinforcement with two aligned fiber outer layers and a random fiber inner layer. The tool is adapted to capture a fiber on removal from the reinforcement. 
         FIGS. 3A-F  show, in cross-section, a series of schematic representations of a needle-punch tool entering and withdrawing from a three layer reinforcement with two aligned fiber outer layers and a random fiber inner layer. The tool is adapted to capture a fiber on entering the reinforcement. 
         FIGS. 4A-B  shows, schematic illustrations of two bi-directional needle punch tools adapted to capture fibers on both entering and withdrawing from a fibrous body. 
         FIGS. 5A-C  shows, in cross-section, the operation of a reconfigurable needle punch tool in which the fiber capture feature may be: retracted— FIG. 5A ; oriented to capture fibers— FIG. 5B ; and oriented to disengage fibers— FIG. 5C .  FIG. 5D  shows, in fragmentary cross-section, an alternative fiber capture feature design. 
         FIG. 6  shows, in quasi-perspective view a three-layer reinforcement stack with fiber loops extending though the layers to interlock and bind the layers together. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Fiber reinforced polymer composite articles find increasing application where low mass and high strength are required. Often the composite reinforcement is itself a composite of several different, generally coextensive reinforcing fiber layers stacked or layered atop one another. The reinforcements may be aligned woven or non-woven fibers, or randomly arranged and positioned fibers which may be continuous, or chopped. Illustrative examples of such reinforcements are shown in  FIGS. 1A-D  and may comprise without limitation, carbon fibers, glass fibers, and aramid fibers. 
       FIG. 1A  shows a portion of a woven reinforcement  10  comprising warp fibers  12  arranged into tows  14  which alternately overlie and underlie weft fibers  16  arranged into tow(s)  18 .  FIG. 1B  shows a four layer non-crimp fabric (NCF)  20  in which roving layers  25 ,  27 ,  29 ,  31  containing oriented spaced-apart roving  24 ,  26 ,  28 ,  30  each containing fibers  22  (shown only once for clarity) are laid down in layered fashion and secured by tricot stitching  32 . 
       FIG. 1C  shows a random continuous fiber mat  34  in which a plurality of continuous fibers  36 , have been laid down in a generally random manner in a generally planar, but layered configuration.  FIG. 1D  shows a random chopped fiber mat  38  in which lengths of chopped fiber, for example fiber  40  (shown in heavier weight line for clarity) have been randomly arranged to form a generally planar, but layered, array. It will be appreciated that although the fiber density shown in both of  FIGS. 1C and 1D  is relatively low for ease of viewing, typical fiber mats may have many more overlying fibers and may have appreciable thickness. 
       FIGS. 2A-G  show how a group of three discrete and initially unattached fiber reinforcing layers,  50 ,  52 ,  54  may be secured into a one-piece reinforcement by the action of tool  56  with fiber engaging feature  58 . Reinforcing layers  50 ,  54  are aligned fiber reinforcement layers, here depicted, without limitation or restriction, as four-layer stackups of aligned reinforcing fiber rovings similar to the NCF shown in  FIG. 1B . Reinforcing layers  50  and  52  could equally be NCF fabrics with fewer or greater layers of woven fabrics without limitation. Without limitation, reinforcing layer  52  is shown as a random fiber layer similar to either of the continuous fiber or chopped fiber mats shown in  FIGS. 1C and 1D . 
     In  FIG. 2A , point  59  of tool  56 , moving as indicated by arrow  57  and guided by opening  64  in top plate  60  just penetrates the upper fiber layer  51  of aligned fiber reinforcement  50 . Continued motion of tool  56 , shown at  FIGS. 2B-D  progressively drives tool  56  through, successively, reinforcing layers  50 ,  52  and  54 , until, as shown at  FIG. 2D , tool point  59  emerges from lower fiber layer  55  of aligned fiber reinforcement  54  and just engages opening  63  in lower support  62 . Throughout tool advance, no fiber capture occurs because capture surface  68  and guidance surface  66  which together define fiber capture feature  58  are not arranged to engage and capture any of the fibers encountered by tool  56  as it advanced in the direction of arrow  57 . At  FIG. 2E , after point  59  has penetrated lower aligned fiber reinforcement  70 , the direction of motion of tool  56  has reversed and is now indicated by arrow  57 ′, enabling capture surface  68  to engage with fibers from roving layer  53  of lower aligned reinforcement  70 , or, as shown, all of roving layer  53  to form and carry roving loop  53 ′ upward through reinforcing layers  52  and  50  as shown in  FIGS. 2F and 2G . Continued motion of tool  56  in direction of arrow  57 ′ will fully disengage tool  56  from opening  64  in upper plate  60  so that by relaxing tension on loop  53 ′ to disengage tool capture surface  68  from loop  53 ′, tool  56  may be moved laterally to fully disengage loop  53 ′ from capture feature  58  so that the process may be repeated. Loop  53 ′ remains in the position shown in  FIG. 2G , inserted into, and engaging, reinforcement layers  52  and  50 . 
     The direction of motion of tool  56  has been shown as generally perpendicular to the plane of the fiber reinforcements. This is not intended to limit the invention which also comprehends the use of inclined or slanted tools. It will be appreciated that any inclination of the tool will also be manifested in the orientation of the fiber loop(s). 
       FIGS. 3A-F  show, in an alternate embodiment, how a similar group of three discrete and initially unattached fiber reinforcing layers,  150 ,  152 ,  154  may be secured into a one-piece reinforcement by the action of tool  156  with fiber engaging feature  158 . Similarly to  FIGS. 2A-G , reinforcing layers  150 ,  154  are aligned fiber reinforcement layers, with four-layer stackups of aligned reinforcing fiber rovings similar to the NCF shown in  FIG. 1B . Reinforcing layer  152  is shown as a random fiber layer similar to either of the continuous fiber or chopped fiber mats shown in  FIGS. 1C and 1D . 
     The particular number, arrangement and character (aligned or random fiber; woven or non-woven; number of fibers or plies per layer) of the layers shown in  FIGS. 3A-F  are exemplary only and no limitation of the scope of the invention is intended or should be inferred from the particular reinforcing layer arrangement shown. 
     In  FIG. 3A , point  159  of tool  156  moving in a direction  157  is shown penetrating several of plies of reinforcing layer  150  while fiber capture feature  158  has not yet engaged the upper ply  151  of layer  150 . Tool  156  may, like tool  56  shown in  FIGS. 2A-G , be supported by openings in a top plate and by openings in a lower support like those shown as  64  in (top) plate  60  and openings  63  in (lower) support  62  in  FIGS. 2A-G . These features have been omitted from  FIGS. 3A-F  for clarity. At  FIG. 3B  tool  156  has further advanced in the direction of arrow  157  so that capture surface  168  of fiber capture feature  158  has engaged a fiber loop  153  from reinforcing layer  151  of reinforcement  150 . Fiber loop  153  is guided into contact with capture surface  168  by guidance surface  166 . With continued advance in direction of arrow  157 , shown at  FIG. 3C , tool  156  penetrates deeper into the reinforcement stack defined by reinforcement layers  150 ,  152  and  154  engaging a second thread loop  153 ′ originating in ply  151 ′ of layer  150 . At  FIG. 3D , fiber loops  153  and  153 ′ have been pulled out of the plane of layer  150  and extended through the reinforcement stack and below ply  155  of layer  154 . In so doing, fiber loops  153  and  153 ′ bridge the interfaces between layers  150  and  152  as well as between layers  152  and  154 . Fiber loops  153  and  153 ′ are directed generally perpendicular to the plane of layer  151 . As tool  156  is withdrawn in the direction of arrow  157 ′ ( FIGS. 3D-F ), displaced fiber loops  153  and  153 ′, now inserted into reinforcing layers  152 ,  154  are frictionally and interferingly engaged by the fibers of reinforcing layers  154  and  152 . The frictional restrain applied to loops  153  and  153 ′ causes them to disengage from fiber capture feature  158  and remain in their displaced configuration. Tool  156  may be laterally displaced and re-inserted into the reinforcement stack to repeat this process until a suitable and predetermined number of fibers has been inserted into adjacent reinforcing layer(s). 
     Repeated application of the processes shown in  FIGS. 2A-G  and  3 A-F will result in a plurality of fibers or fiber tows or roving which will extend through the thickness of the reinforcement stackup. These fibers, through frictional engagement with other fibers in the stackup and/or through mechanical interference with other fibers in the stackup will induce sufficient cohesion between the reinforcement layers to render a one-piece reinforcement. The processes shown in  FIGS. 2A-G  and  3 A-F are intended to illustrate the interaction between an individual tool and the individual layers. To obtain a generally uniform areal density of fibers or tows extending through the layers, an individual tool may be repeatedly inserted and removed while following a path which traverses substantially the entire area of the reinforcement. A second approach, suitable for reinforcements of more limited area, is to employ a plurality of tools, suitably positioned on a common support or fixture, so that all of the tools may be inserted and extracted in concert. A combined approach may be adopted for more extensive reinforcements. A multi-tool fixture may be repeatedly employed and stepped over substantially the entire area of the support until a suitable, and suitably uniform, density of fibers or tows extending through the layers is obtained. 
     Both top plate  60  and lower support  62  have been shown as sheet-like or plate-like bodies with openings positioned to accept reciprocating tool  56 . Top plate  60  serves to guide and support tool  56  which may, if cylindrical in cross-section have a diameter of less than 0.5 millimeters or so and may break or bend if not supported. But lower plate  62  serves to support the workpiece and ensure that tool  56  is driven into the workpiece rather than bodily displacing it. Alternate designs of lower support  62  may be employed. For example the lower support may be a solid or porous body, capable of penetration by tool  56 , which does not appreciably dull tool point  59 , such as rubber or rubber foam. Alternatively an array of (relatively) widely spaced upwardly pointing fibers or thin columns may be used. With this design the fibers or columns may be present in sufficient number and density to support the workpiece but suitably positioned and spaced apart to at least minimize the possibility of contact between a descending tool and the support columns. In a related design the support columns may be made compliant so that any tool-support contact on tool advance merely deflects or moves the support aside temporarily, enabling to return to its undeflected configuration as the tool is withdrawn. 
     Because the fiber-capturing action of the tools shown in  FIGS. 2A-G  and  3 A-F occurs at different stages of the stroke a bi-directional tool, incorporating both fiber capturing features  58  and  158  may be employed. A representative tool  256  is shown in  FIG. 4A  and includes both of fiber capturing features  58  and  158 . In operation, fiber capturing tool  158 , closer to tool point  259 , will engage the upper surface of a reinforcement stack first and begin to convey fibers from the upper surface view of the stack to the lower surface. As tool  256  advances deeper into the stack, fiber capture feature  58  will be immersed in the stack but, due to its orientation will be unable to engage with or capture any fibers. When the tool reaches the limit of its advance stroke and begins to retract, fiber capture feature  158  will release those fibers which it was conveying and fiber capture feature  58  will capture fibers and begin to convey them toward the upper surface. The stroke of the tool and the placement of the fiber capturing features on the tool, in conjunction with the thickness of the reinforcement stack will determine the origin and extent of the fiber loops. Generally however it is preferred that the loops extend through the entire thickness of the stack for greatest cohesion across all layers. 
       FIG. 4B  illustrates a second configuration for such a tool. It will be appreciated that, to be effective a tool should induce as little damage to the fibers and fiber breakage as possible and for this reason a tool with a small cross-section of say between about 0.5 and 1 millimeter is preferred. But tool geometry will also influence the likelihood of fiber damage from the tool. The angular nature of the tool point  259  and fiber capture features  58 ,  158  shown in  FIG. 4A  may create stress concentrations or otherwise cut or damage fibers. In  FIG. 4B , tool  356  is shown with rounded end  359  which may be effective in laterally displacing fibers to enable insertion and passage of tool  356  rather than potentially cutting or otherwise weakening fibers with pointed end  259 . Similarly fiber capture features  58 ′ and  158 ′, though still suited to capture and retain fibers, are shown as having a more rounded, or curved appearance, in both directions to minimize stress concentrations and promote fiber conveyance with minimal damage. 
     The tool designs shown have exhibited a fixed geometry and relied on the directionality of the fiber capture process to disengage the tool from the fiber when fiber conveyance is terminated, generally when the fibers from one layer have been pulled or pushed through and inserted into at least a second layer.  FIGS. 5A-D  shows an illustration of a variable geometry tool which may also be effective in conveying fibers through and across layers but may be more effective in minimizing the potential for fiber damage. 
     The tool  100 , shown, at  FIG. 5A  in a configuration suitable for penetrating a workpiece consists of a generally cylindrical pin  78  slidably restrained within the inner surface  79  of a hollow cylindrical casing  80 . A plurality of fiber capturing features  88  are pivotally pinned, near extremity  92 , to cylindrical pin  78  at pivots  94  and engage one of a like plurality of openings  82  in casing  80 . The relative positions of pivot  94 , attached to pin  78 , and opening  82 , located in casing  80  determine the orientation of fiber capturing feature  88 . Changing the relative positioning of pivots and openings by moving pin  78  relative to casing  80  enables generally simultaneous adjustment of the orientations of all of the fiber capturing features  86  as shown in  FIGS. 5A and 5B . In  FIG. 5A , pin  78  and casing  80  are so arranged that fiber capturing features  88 , are supported on pin  94  on one end, and on lower edge  84  of opening  82 , near its other extremity  90 . Thus fiber capturing features  88  are near fully retracted into casing  80  so that only a portion extends beyond casing  80 . In such configuration tool  100  may be directed into a workpiece in direction of arrow  96  with little likelihood of imparting damage to a workpiece fiber  98  in contact with casing exterior surface  81  from fiber capturing features  88 . 
     In  FIG. 5B  the direction of tool  100  motion is reversed as indicated by arrow  104 . Also pin  78  has been displaced, relative to casing  80 , in the direction of arrow  91  and so likewise displacing fiber capturing feature  88  in the direction of arrow  91 . Because of its engagement with opening  82 , feature  88  will be guided by upper opening edge  86  or by lower opening edge  84  so it rotates outboard and into a more suitable fiber capturing orientation as well as extending so that extremity  90  of feature  88  protrudes beyond outer surface  81  of casing  80 . In this configuration, features  88  are well-suited to engage any fibers  98  adjacent to outer casing surface  81  as shown at  FIG. 5B . 
     Yet further relative motion of pin  78  with respect to casing  80  as shown at  FIG. 5C  may result in further extension of fiber capturing feature  88  and also in its further rotation to an orientation in which it is not properly inclined to capture and retain fibers. In this configuration fiber  98 , upon continued motion of tool  100  in the direction of arrow  104 , fiber  98  may ‘roll off’ and disengage from feature  88 . Resetting tool  100  to the configuration of  FIG. 5A  by moving pin  78 , with respect to casing  80 , in a direction opposite that of arrow  90  enables the cycle to be repeated. Depending on the angular range of motion of fiber capturing feature  88 , it may be feasible to have it operate bidirectionally. With only modest further rotation, fiber capturing feature  88  may be oriented to capture fibers if the direction of motion of tool  100  is reversed. Thus tool  100  may be operated unidirectionally or bidirectionally. 
     Fiber damage may be further minimized through the use of a more smoothly curved fiber capture feature such as the ‘comma-shaped’ design  88 ′ shown, in fragmentary view, in both retracted (solid line) and extended (broken line) configuration in  FIG. 5D . 
       FIG. 6  shows a depiction of a one-piece layered reinforcement in quasi-perspective view. A three-layer stack  300  comprises reinforcing layers  250 ,  252  and  254  represented as woven fiber layers,  250 ,  254  and chopped fiber mat layer  252 . These layers are interlocked and bound together by a plurality (not shown) of fiber loops which may include one or more of the individual representative loop configuration  253 ,  353 ,  253 ′,  353 ′,  253 ″,  353 ″, all shown in heavy line for clarity. Loop  353 , extending from woven layer  254 , passes through and is inserted between the fibers of layer  252  and is further inserted into the woven fibers of layer  250 . Loop  253 , extending from woven layer  250 , passes through and is inserted between the fibers of layer  252  and is further inserted into the woven fibers of layer  254 . Less extensive loops  253 ′ and  353 ′ originate in random fiber layer  252  and are inserted into woven fiber layers  250  and  254  respectively, while loops  253 ″ and  353 ″, originating in woven fiber layers  250  and  254  respectively extend only partway through the stack and are inserted into random layer  252 . The representation shown in  FIG. 6  is illustrative and not limiting. For example, other fiber layer configurations may be employed and such alternate fiber layer configurations may enable other loop configurations than those shown. Also and not all possible loop configurations may be found in a specific reinforcement. 
     A fiber reinforced polymer article containing such a one-piece reinforcement may be fabricated by the following steps, which need not necessarily be performed in the order listed—in particular, it may be preferred to charge the reinforcement with polymer precursor after the reinforcement has been placed in a mold: assembling a layered reinforcement by stacking a plurality of generally planar, generally coextensive fiber-containing reinforcements atop one another in face to face relation; conveying a preselected portion of the fibers from at least one layer of the layered reinforcements out of the plane of the reinforcement and pulling or pushing them out of the plane of the layer across at least one layer boundary to insert the fibers into at least a second layer of reinforcement to secure the reinforcement layers together; and repeating until all layers are bound to one another by fibers extending from one layer and engaging with at least an adjacent layer; impregnating the reinforcement with a suitable polymer precursor in sufficient quantity to wet all of the fibers and to fill a mold cavity; shaping the polymer precursor-impregnated reinforcement to a preselected geometry suitable for production of the article and thereby forming a pre-preg; placing the pre-preg in a mold with an interior cavity defining the desired article shape; closing the mold to induce the prepreg to conform to the shape of the die cavity, to compact the prepreg and to displace and distribute polymer precursor throughout the mold cavity; and curing the polymer precursor in the shaped pre-preg to form the fiber reinforced polymer article. 
     One-sided vacuum-bag or autoclave molding may also be employed. In this case the pre-preg may be positioned in one-half of a mold cavity and pressure applied to induce the pre-preg to conform to the mold shape. 
     The practice of the invention has been illustrated through reference to certain preferred embodiments that are intended to be exemplary and not limiting. The full scope of the invention is to be defined and limited only by the following claims.