Designed Defects in Composites

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Designed Defects in Laminate Composites Abstract Novel methods and kits for creating defects in composite materials, as well as an engineered part or structure with one or more designed defects, are disclosed. The disclosed inventions advantageously permit the intentional introduction of one or multiple defects of any desired size and shape, either between the same two chosen laminae or on different layers within a multi-layer composite, having mechanical and thermal properties that are akin to those found for actual defects. One disclosed method involves creating a mask having a cut-out in the size and shape of a desired defect, placing the mask at a location on a first substrate where the defect is desired, applying a release agent, removing the mask, and adding a second substrate over the first substrate with release agent. The first substrate may be a cured lamina or an uncured lamina pre-impregnated with resin having a barricade in the size and shape of the outline of the desired defect. Background The present disclosure relates generally to composites and particularly to designed defects in a laminate composite. In fiber-based laminated composites of any kind, the basic building material is composed of thin sheets (or laminae) of the main structural component such as carbon fiber, glass fiber, Kevlar® or the like. An engineered part or structure is created by “laying up” successive layers of lamina to achieve the final design. A resin, epoxy, or the like is used to hold the laminae in place and provide rigidity. The lamina is available as either pre-impregnated with the resin (so called “prepreg” material) or in its raw form without any resin, with the resin added during the manufacturing process through a variety of means. The engineered part or structure may have a complex shape, which is usually achieved by the use of a mold into which the laminae are laid. The final step to creating the composite part or structure is to cure the resin, the method of which is dependent on the resin used. With any method of composite creation, great care is taken during manufacturing to ensure that the resin fully envelops or saturates the laminae to ensure bonding between the laminae and within each lamina. Defects such as porosity (small voids) and delaminations (physical separation or gap between laminae) must be avoided to ensure the part or structure meets the design specifications and maintains structural integrity throughout its designed life. In addition to manufacturing defects, a delamination may also result from an unintentional impact event, for example. As defects are a common problem occurring in laminated composite materials, it is important to determine whether a defect exists in laminated parts or structures. Myriad techniques exist for identifying a defect including nondestructive testing or inspection methods. Such techniques include vibrothermography, lock-in thermography, pulsed thermography, and ultrasound-based methods, for example. Regardless of the technique, there is a need for a “standard” test specimen so various techniques can be compared to assess their effectiveness at locating defects. An appropriate test specimen may also be used as a demonstration or a calibration standard for a manufacturer of nondestructive inspection equipment. Similarly, manufacturers of composite materials may also wish to create samples with intentional, artificial defects for testing a material's strength when defects are present or otherwise quantify the decrease in performance due to a defect, or for helping their customers identify a defect. A conventional method of creating a delamination defect involves the insertion of a thin plastic (usually polytetrafluoroethylene (“PTFE”)) membrane in the composite sample to simulate the actual defect. For certain inspection techniques, this conventional method may be acceptable. However, for others, namely vibrothermography, lock-in thermography, pulsed thermography, and perhaps some ultrasound-based techniques, such a conventional method may not suffice because of the difference in mechanical and thermal properties between a real delamination and a plastic membrane or insert. Accordingly, novel methods and kits that intentionally introduce one or multiple defects of any desired size and shape, either between the same two chosen laminae or on different layers within a multi-layer composite, having mechanical and thermal properties that are akin to those found for actual defects, as well as an engineered part or structure with one or more such designed defects, are therefore desired. Summary One exemplary embodiment of the disclosed subject matter is a method of creating a defect in a composite including creating a mask having a cut-out in the size and shape of a desired defect, placing the mask at a location on a first substrate where the defect is desired, applying a release agent, removing the mask, and adding a second substrate over the first substrate with release agent. The first and second substrates may be laminae. The first substrate may be a cured lamina or an uncured lamina pre-impregnated with resin having a barricade in the size and shape of the outline of the desired defect. Another exemplary embodiment of the disclosed subject matter is a method comprising curing a first lamina pre-impregnated with resin, creating a mask having a cut-out in the size and shape of a desired defect, placing the mask at a location on the first lamina where the defect is desired, applying a release agent, removing the mask, adding a second lamina pre-impregnated with resin, and curing the second lamina pre-impregnated with resin. Yet another exemplary embodiment of the disclosed subject matter is a kit comprising a first lamina, a mask having a cut-out in the size and shape of a desired defect, a release agent, and a second lamina comprising an uncured ply pre-impregnated with resin. The first lamina may be a cured fiber-reinforced material. The mask may be a plastic sheet. The first lamina may include cross-linked polymers in the outline of the size and shape of the desired defect.