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Method and apparatus to detect and characterize a defect in a fiber preform residing in a preform-receiving cavity of a mold or simulated mold of the type used for liquid composite molding (e.g., resin transfer molding) to make a composite component or structure. The defect may result from fiber pre

Method and apparatus to detect and characterize a defect in a fiber preform residing in a preform-receiving cavity of a mold or simulated mold of the type used for liquid composite molding (e.g., resin transfer molding) to make a composite component or structure. The defect may result from fiber preform misplacement in the cavity, accidental inclusions, preform density variations, and/or distortion of the preform. Transient gas or other fluid pressures at multiple locations of the cavity are measured during changing of fluid pressure in the cavity. The measured transient pressures, or data derived therefrom, are analyzed to determine membership of the fiber preform in one of two or more groups that include a standard defect-free group and a defective group, whereby the method determines if the fiber preform or its position in the cavity deviates from the standard defect-free group.

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We claim: 1. A method of detecting a defect associated with a fiber preform in a cavity, comprising providing a changing fluid pressure in the cavity containing the fiber preform, measuring time-varying fluid pressures over time at one or more locations of the cavity during changing of the fluid pr

We claim: 1. A method of detecting a defect associated with a fiber preform in a cavity, comprising providing a changing fluid pressure in the cavity containing the fiber preform, measuring time-varying fluid pressures over time at one or more locations of the cavity during changing of the fluid pressure in the cavity, analyzing the measured fluid pressures, or data derived therefrom, to determine membership of the fiber preform in one of two or more groups that include a standard defect-free group and a defective group, determining a defect-free or defective condition of the fiber preform or its position in the cavity using group membership, and conveying the condition to a user for accepting or rejecting the fiber preform or its position in the cavity. 2. The method of claim 1 wherein the changing fluid pressure involves decreasing gas pressure in the cavity over time. 3. The method of claim 2 wherein the gas pressure is decreased in the cavity to ambient pressure after the cavity has been gas pressurized to superambient pressure. 4. The method of claim 1 wherein the changing fluid pressure involves increasing gas pressure in the cavity over time. 5. The method of claim 1 wherein time-varying fluid pressure is measured at one or more ports communicated to the cavity. 6. The method of claim 1 wherein analyzing the fluid pressures, or results derived therefrom, involves statistical analysis to detect an anomaly associated with the fiber preform or its position in the cavity. 7. The method of claim 1 wherein said data derived from the measured time-varying fluid pressures comprises local permeability data of the preform. 8. The method of claim 1 wherein the defective group is associated with a type of defect in the preform or its placement in the cavity. 9. The method of claim 8 wherein the defective group is associated with misplacement of the preform in the cavity. 10. The method of claim 8 wherein the defective group is associated with a race tracking defect associated with a periphery of the preform. 11. The method of claim 8 wherein the defective group is associated with an anomaly in the preform itself. 12. The method of claim 11 wherein the anomaly is associated with an accidental inclusion present in the preform. 13. The method of claim 11 wherein the anomaly is associated with a density variation in the preform. 14. The method of claim 11 wherein the anomaly is associated with a shearing distortion of the preform. 15. The method of claim 1 wherein the analyzing of the measured time-varying fluid pressures, or data derived therefrom, is conducted using a data base generated from previous tests of fiber preforms wherein the fiber preforms have been grouped by existence and severity of defect. 16. The method of claim 1 wherein the preform is evaluated using a one dimensional variation of transient fluid pressures. 17. The method of claim 1 including, after determining the defect-free condition, accepting the preform or its position in the cavity for resin injection. 18. The method of claim 1 including, after determining the defective condition, rejecting the preform or its position in the cavity. 19. A method of detecting a defect associated with a fiber preform in a preform-receiving cavity of a mold or simulated mold, comprising providing a decreasing or increasing gas pressure in the cavity containing the fiber preform, measuring time-varying gas pressures over time at multiple locations of the cavity spaced apart along a dimension of the preform during decreasing or increasing of the gas pressure in the cavity, analyzing the measured time-varying pressures, or data derived therefrom, to determine membership of the fiber preform in one of two or more groups that include a standard defect-free group and a defective group, determining a defect-free or defective condition of the fiber preform or its position in the cavity using group membership, and conveying the condition to a user for accepting or rejecting the preform or its position in the cavity. 20. A method of detecting a defect associated with a fiber preform in a cavity, comprising the steps of changing fluid pressure in the cavity containing the fiber preform, measuring time-varying fluid pressures over time at one or more locations of the cavity during changing of the fluid pressure in the cavity, determining a defect-free or defective condition of the fiber preform or its position in the cavity using the time-varying fluid pressures or data derived therefrom, and conveying the condition to a user for accepting or rejecting the preform or its position in the cavity. 21. Apparatus for detecting a defect associated with a preform in a cavity, comprising a mold or simulated mold having a preform-receiving cavity for receiving a preform, means for changing fluid pressure in the cavity over time, and means for measuring time-varying fluid pressures over time at one or more locations of the cavity to provide time-varying fluid pressures that are analyzable to determine a defect-free or defective condition of the preform or its position in the cavity that is conveyed to a user for accepting or rejecting the preform or its position in the cavity. 22. The apparatus of claim 21 wherein the means for changing fluid pressure comprises a source of pressurized fluid communicated to the cavity via a valve. 23. The apparatus of claim 22 wherein the valve vents the cavity to ambient pressure after the cavity is pressurized to superambient pressure. 24. The apparatus of claim 21 wherein the means for measuring the time-varying pressures comprises one or more ports communicated to the cavity at the one or more locations and a fluid pressure measuring device at the one or more ports. 25. The apparatus of claim 24 wherein the fluid pressure measuring device is a pressure transducer.