Patent Publication Number: US-2020282694-A1

Title: Composite structure and method of forming thereof

Description:
FIELD 
     The field relates generally to composite structures and, more specifically, to a punch formed blade stringer composite structure having enhanced impact and crack propagation resistance. 
     BACKGROUND 
     The fabrication of multi-layer laminate structures generally includes bonding layers of metallic (e.g., aluminum, titanium, or corrosion resistant steel (CRES)) and/or non-metallic (e.g., carbon fiber, boron, or fiberglass) reinforcement material together with a matrix material to form a rigid structure. The reinforcement material strengthens and stiffens the laminate structure, and the matrix material supports the reinforcement material after a curing process. Multi-layer laminate structures generally have a high strength-to-weight ratio and may be formed in a variety of shapes and sizes. At least some known aircraft components are fabricated from multi-layer laminate structures of non-metallic composite materials such as carbon-fiber-reinforced polymer (CFRP). The composite materials are used in combination with metallic materials, such as aluminum, titanium, and/or steel, to reduce the overall weight of the aircraft. Reducing the overall weight generally contributes to increasing the fuel efficiency of the aircraft. However, common multi-layer laminate structures fabricated from CFRP may be susceptible to damage, such as the formation of micro-cracks and delamination of the structure during service and/or manufacturing thereof. Known damage to such structures may be small and difficult to detect during scheduled maintenance. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     BRIEF DESCRIPTION 
     In one aspect, a composite structure includes a multi-layer laminate including a plurality of layers of material, and at least one crack barrier layer extending within the plurality of layers of material. The multi-layer laminate is folded to define a first web region and a second web region in a face-to-face relationship, and a folded tip region defined between the first web region and the second web region. The at least one crack barrier layer is configured to restrict crack propagation from spreading through the plurality of layers of material in the folded tip region. 
     In another aspect, a stringer includes a web including a first web region, a second web region in a face-to-face relationship with the first web region, and a folded tip region defined between the first and second web regions. A base extends from the web. The base includes a first base region extending from the first web region, and a second base region extending from the second web region. The web and the base are formed from a multi-layer laminate including a plurality of layers of material and at least one crack barrier layer extending within the plurality of layers of material. The at least one crack barrier layer is configured to restrict crack propagation from spreading through the plurality of layers of material in the folded tip region. 
     In yet another aspect, a method of forming a composite structure includes forming a multi-layer laminate including a plurality of layers of material and at least one crack barrier layer extending within the plurality of layers of material, and folding the multi-layer laminate to define a first web region and a second web region in a face-to-face relationship, and a folded tip region defined between the first web region and the second web region. The method also includes curing the multi-layer laminate, wherein the at least one crack barrier layer is configured to restrict crack propagation from spreading through the plurality of layers of material in the folded tip region. 
     Various refinements exist of the features noted in relation to the above-mentioned aspects of the present disclosure. Further features may also be incorporated in the above-mentioned aspects of the present disclosure as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments of the present disclosure may be incorporated into any of the above-described aspects of the present disclosure, alone or in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of an example composite structure. 
         FIG. 2  is an enlarged view of a folded tip region of the composite structure shown in  FIG. 1 . 
         FIG. 3  illustrates a series of process steps for forming the composite structure shown in  FIG. 1 . 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION 
     The embodiments described herein relate to a punch formed blade stringer composite structure having enhanced impact and crack propagation resistance. The composite structure may be used as a stringer in an aircraft structure. The composite structure is formed from a multi-layer laminate structure that has been folded to define a base, a web, and a folded tip region in the web. At least some known composite manufacturers cut the folded tip region from the web to reduce the risk of cracks or delamination, that initiate in the folded tip region, from propagating to the remainder of the composite structure. In the example embodiment, the multi-layer laminate structure is formed from a plurality of layers of material and a crack barrier layer extending within the plurality of layers of material. The crack barrier layer is positioned to facilitate restricting crack propagation from spreading through the plurality of layers in the folded tip region. In addition, in some embodiments, a detection layer is extended across the folded tip region to facilitate providing impact damage detection. The resulting composite structure has increased strength, a more robust tip resistant to lateral impacts, and barely visible impact damage detection capability. 
       FIG. 1  is an illustration of an example composite structure  100 . In the example embodiment, composite structure  100  includes a web  102  and a base  104  extending perpendicularly relative to web  102 . Alternatively, web  102  is oriented obliquely relative to base  104 . Web  102  includes a first web region  106 , a second web region  108  in a face-to-face relationship with first web region  106 , and a folded tip region  110  defined between first web region  106  and second web region  108 . Base  104  includes a first base region  112  extending from first web region  106 , and a second base region  114  extending from second web region  108 . First base region  112  includes a first tapered end  116 , and second base region  114  includes a second tapered end  118 . In some embodiments, composite structure  100  is coupled to a skin panel  120  of an aircraft (not shown), or a separate base charge (not shown), for example. Composite structure  100  facilitates supporting, and increasing the rigidity of, skin panel  120 . 
       FIG. 2  is an enlarged illustration of folded tip region  110 . In the example embodiment, composite structure  100  includes a multi-layer laminate  122  that includes a plurality of layers  124  of material, and at least one crack barrier layer  126  extending within the plurality of layers  124  of material. For example, multi-layer laminate  122  has a first surface  128  and an opposing second surface  130 . As will be described in more detail below, multi-layer laminate  122  is folded on itself to form composite structure  100 . When folded, first surface  128  is exposed to an ambient environment, and second surface  130  in first web region  106  and in second web region  108  is in face-to-face contact for defining a central core  132 . 
     The at least one crack barrier layer  126  is positioned between outer surface  128  and central core  132 . For example, the plurality of layers  124  and crack barrier layer  126  are aligned coterminously in at least one dimension of composite structure  100 . More specifically, in one embodiment, layers  124  and crack barrier layer  126  extend from first tapered end  116  to second tapered end  118  such that the at least one crack barrier layer  126  extends throughout folded tip region  110 . Alternatively, crack barrier layer  126  has a shorter length and is limited to extending within certain regions of composite structure  100 , such as only extending across folded tip region  110 . Cracks and/or delamination may initiate within central core  132  and propagate through multi-layer laminate  122 . As such, the at least one crack barrier layer  126  is positioned to facilitate restricting crack propagation from spreading through the plurality of layers  124  in folded tip region  110 . 
     In the example embodiment, the at least one crack barrier layer  126  includes a first crack barrier layer  134  and a second crack barrier layer  136  spaced from each other within multi-layer laminate  122 . First crack barrier layer  134  is positioned closer to central core  132  than second crack barrier layer  136 , and second crack barrier layer  136  is positioned closer to outer surface  128  than first crack barrier layer  134 . In addition, first crack barrier layer  134  is positioned closer to central core  132  than to outer surface  128 . As such, first crack barrier layer  134  is positioned a predetermined distance D from central core  132 . First crack barrier layer  134  restricts crack propagation that may have initiated at central core  132  from spreading beyond predetermined distance D. In one embodiment, the layup location of first crack barrier layer  134  within multi-layer laminate  122  is determined as a function of a threshold thickness of folded tip region  110 . As such, a crack (not shown) in multi-layer laminate  122  is restricted from propagating to a length greater than the threshold thickness. The threshold thickness may be about 50 percent, about 40 percent, or about 25 percent of a total thickness T of folded tip region  110 . In addition, second crack barrier layer  136  is positioned a distance from first crack barrier layer  134  to provide redundant crack propagation protection in the event crack propagation extends beyond first crack barrier layer  134 . 
     Layers  124  of material and crack barrier layer  126  may be fabricated from any material that enables composite structure  100  to function as described herein. In the example embodiment, layers  124  have a first structural configuration, and crack barrier layer  126  has a second structural configuration different from the first structural configuration. As such, positioning crack barrier layer  126  fabricated from different material than the remainder of layers  124  in multi-layer laminate  122  facilitates forming a discontinuity within multi-layer laminate  122 , which facilitates reducing the spread of crack propagation therein. 
     In addition, layers  124  and crack barrier layer  126  are selected for inclusion in multi-layer laminate  122  based on resin compatibility and thermal expansion considerations. For example, layers  124  and crack barrier layer  126  may be pre-impregnated with resin, which is selected to enable sufficient compatibility for forming multi-layer laminate  122 . Layers  124  and crack barrier layer  126  may be impregnated with the same resin or a different resin. In addition, layers  124  and crack barrier layer  126  are selected such that the materials of layers  124  and crack barrier layer  126  have a difference in coefficient of thermal expansion less than a predetermined threshold. The predetermined threshold may be 25 percent, 20 percent, or 10 percent, based on an overall difference in coefficient of thermal expansion values of the material of layers  124  and crack barrier layer  126 . As such, layers  124  and crack barrier layer  126  are selected to facilitate forming multi-layer laminate  122  that is structurally sound with a reduced likelihood of delamination from occurring therein. In one embodiment, layers  124  are fabricated from a unidirectional, pre-impregnated, carbon fiber material, and crack barrier layer  126  is fabricated from a bidirectional, pre-impregnated, carbon fiber material. The bidirectional carbon fiber material may be a woven sheet of carbon fiber having fibers oriented in any direction that enables crack barrier layer  126  to function as described herein. For example, the woven sheet may have a 0/90 or 45/45 degree fiber orientation. 
     In the example embodiment, multi-layer laminate  122  also includes a detection layer  138  extending across folded tip region  110 , and across at least a portion of first web region  106  and second web region  108 . Detection layer  138  may be fabricated from any material that enables composite structure  100  to function as described herein. In one embodiment, detection layer  138  is fabricated from a glass fiber-reinforced plastic material, also commonly referred to as fiberglass. The composition of detection layer  138  is selected to facilitate visualizing impact damage induced to composite structure  100 . For example, fiberglass is generally more brittle than CFRP material such that the application of the same force to fiberglass or CFRP material would be more readily visible and more easily detectable on the fiberglass. 
     Detection layer  138  also facilitates protecting folded tip region  110  from impact damage. For example, referring to  FIG. 1 , folded tip region  110  may be susceptible to encountering different types of impacts, such as a first impact  140 , a second impact  142 , and a third impact  144 . First impact  140  is generally axially aligned with web  102 , second impact  142  is oriented at about 45 degrees relative to web  102 , and third impact  144  is oriented generally laterally to web  102  at about 10 degrees relative to web  102 . Without detection layer  138 , composite structure  100  having folded tip region  110  is generally capable of sustaining non-critical impact damage from third impact  144  having a force less than a predetermined threshold. The addition of detection layer  138  enables composite structure  100  to be capable of sustaining non-critical impact damage from first impact  140  and second impact  142  having forces less than a predetermined threshold. As such, folded tip region  110  provides robust shear resistance to facilitate withstanding lateral impact damage induced by third impact  144 , and detection layer  138  augments the robustness of folded tip region  110  to facilitate withstanding damage induced by first impact  140  or second impact  142 . Folded tip region  110  is also resistant to interlaminar buckling. 
       FIG. 3  illustrates a series of process steps for forming composite structure  100  (shown in  FIG. 1 ). In the example embodiment, a punch-forming apparatus  146  is used to form composite structure  100 . Punch-forming apparatus  146  includes a punch tool  148  and a die  150 . Die  150  includes a pair of die blocks  152  spaced a distance from each other to define a cavity  154  therebetween. Punch tool  148  is translatable relative to die  150 , and die blocks  152  are translatable relative to each other to adjust the size of cavity  154 . 
     In the example embodiment, punch tool  148  is initially positioned a distance from die  150 . A first process step  156  includes forming multi-layer laminate  122  and positioning multi-layer laminate  122  on die  150 . Multi-layer laminate  122  is positioned to extend across cavity  154 . A second process step  158  includes translating punch tool  148  towards die  150  to facilitate forcing multi-layer laminate  122  into cavity  154  and initiate folding multi-layer laminate  122  to define folded tip region  110 . Once punch tool  148  is fully inserted into cavity  154 , punch tool  148  is removed therefrom in a third process step  160  such that only multi-layer laminate  122  remains within cavity  154 . A fourth process step  162  includes translating die blocks  152  towards each other to facilitate further folding multi-layer laminate  122  such that first web region  106  and second web region  108  are in a face-to-face relationship. Multi-layer laminate  122  may then be removed from die  150  and cured to form composite structure  100 , or cured in-situ within die  150 . 
     Example embodiments of a composite structure folded to define a folded tip region having a crack barrier layer extending therein are described above in detail. Aspects of the composite structure are not limited to the specific embodiments described herein, but rather, components of the composite structure may be used independently and separately from other components described herein. For example, aspects of the composite structure may be included in any composite structure where inhibiting crack propagation from spreading therein is desired. 
     When introducing elements of the present disclosure or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” “containing” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of terms indicating a particular orientation (e.g., “top”, “bottom”, “side”, etc.) is for convenience of description and does not require any particular orientation of the item described. 
     As various changes could be made in the above constructions and methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawing[s] shall be interpreted as illustrative and not in a limiting sense.