Composite structures with damage detection capability

1. A polymer-matrix fiber-reinforced composite structure, the structure comprising:

a plurality of layers each comprising a polymer-matrix composite, the plurality of layers comprising a first layer;

wherein the first layer comprises a first polymer-matrix and a first fluorescent agent interspersed through the first polymer-matrix, the structure being configured such that damage to one of the layers results in a change in fluorescence observable from the at least first layer at the point of damage.

2. The structure of claim 1 wherein the first layer is an outermost layer of the structure.

3. The structure of claim 1 wherein the first layer is between different layers.

4. The structure of claim 1 wherein the plurality of layers further comprises a second layer, where in the second layer comprises a second polymer-matrix and a second fluorescent agent interspersed throughout the second polymer-matrix.

5. The structure of claim 4 wherein the first fluorescent agent and the second fluorescent agent comprise the same compound.

6. The structure of claim 4 wherein the first fluorescent agent and the second fluorescent agent are not the same compound.

7. The structure of claim 1 wherein the first fluorescent agent is selected from the group consisting of anthra-thioxantene, thioxanthene benzanthrone, anthraquinone series compounds, xanthene, benzothioxanthene-dicarboximide, aminoketone series compounds, hydroxycoumarin, coumarin, coumarin 480, glycidyl-oxcoumarin, benzopyran derivatives, azomethine derivatives, naphthalimide derivative, perylene derivatives, and combinations thereof.

8. The structure of claim 1 wherein the first polymer-matrix is a thermoset material or a thermoplastic material.

9. The structure of claim 1 wherein the polymer-matrix fiber-reinforced composite structure is reinforced by carbon fibers, glass fibers, organic fibers, or a combination thereof.

10. The structure of claim 1 wherein the polymer-matrix fiber-reinforced composite structure is an aircraft part.

11. The structure of claim 10 wherein the polymer-matrix fiber-reinforced composite structure is a driveshaft.

12. A method for detecting damage in a polymer-matrix fiber-reinforced composite structure, the structure comprising a plurality of layers each comprising a polymer-matrix composite, the plurality of layers comprising a first layer, wherein the first layer comprises a first polymer-matrix and a first fluorescent agent interspersed through the first polymer-matrix, the structure being configured such that damage to one of the layers results in a change in fluorescence observable from the at least first layer at the point of damage, the method comprising:

applying light having a wavelength selected to induce fluorescence in the first fluorescent agent to the polymer-matrix fiber-reinforced composite structure;

detecting a detected fluorescence pattern; and

comparing the detected fluorescence pattern to a predetermined normal fluorescence pattern without damage.

13. The method of claim 12 further comprising removing the polymer-matrix fiber-reinforced composite structure from ambient light prior to applying the light having the wavelength to induce fluorescence in the first fluorescent agent.

14. The method of claim 12 wherein the wavelength corresponds to ultraviolet (UV) light.

15. The method of claim 12 wherein the fluorescence occurs in a visible light spectrum.

16. The method of claim 12 wherein the predetermined normal fluorescence pattern is non-fluorescence in the polymer-matrix fiber-reinforced composite structure, and fluorescence in the detected fluorescence pattern indicates damage.

17. The method of claim 12 wherein the predetermined normal fluorescence pattern is fluorescence in the polymer-matrix fiber-reinforced composite structure, and non-fluorescence or reduced fluorescence in the detected fluorescence pattern indicates damage.

18. The method of claim 12 wherein the plurality of layers further comprises a second layer, wherein the second layer comprises a second polymer-matrix and a second fluorescent agent interspersed through the second polymer-matrix; and

wherein the presence or absence of fluorescence corresponding to the first fluorescent agent and the second fluorescent agent in the detected fluorescence pattern indicates a depth of damage.

19. A method for making a polymer-matrix fiber-reinforced composite structure, the structure comprising a plurality of layers each comprising a polymer-matrix composite, the plurality of layers comprising a first layer, wherein the first layer comprises a first polymer-matrix and a first fluorescent agent interspersed through the first polymer-matrix, the structure being configured such that damage to one of the layers results in a change in fluorescence observable from the at least first layer at the point of damage, the method comprising:

providing a polymer;

mixing the first fluorescent agent into the polymer to form a fluorescent-based polymer;

arranging a layup using the fluorescent-based polymer; and

making the polymer-matrix fiber-reinforced composite structure by curing or solidifying the layup;

wherein the first fluorescent agent is interspersed throughout the first polymer-matrix.

20. The method of claim 19 wherein the polymer-matrix comprises thermoset material or a thermoplastic material and wherein making the polymer-matrix fiber-reinforced composite structure comprises curing the thermoset material or solidifying the thermoplastic material.