X-ray digital image correlation

1. A method of X-ray digital image correlation, the method comprising: aligning cameras of an X-ray imaging system, by

performing an initial alignment with a light source co-located with an X-ray source and a translucent medium positioned between the light source and a camera of the plurality of cameras, wherein the translucent medium is used in place of a scintillator that will ultimately be used to collect X-ray images;

removing the translucent medium;

performing two-dimensional calibration on the camera to identify distortion parameters and image center;

orienting the camera such that the X-ray source is aligned at the image center;

placing the translucent medium back between the light source and the camera;

determining an exact target position; and

replacing the translucent medium with a scintillator panel, wherein the scintillator panel is flat and positioned perpendicular to a camera optical axis, wherein the X-ray imaging system comprises two X-ray sources pointed at a target area at a stereo angle and two corresponding X-ray detectors behind the target area, wherein the cameras form part of the X-ray detectors;

tuning the X-ray imaging system to determine power levels of the X-ray sources that maximize image contrast and signal-to-noise ratio;

calibrating the X-ray imaging system to determine intrinsic and extrinsic stereoscopic imaging parameters of the X-ray imaging system;

placing a specimen in the target area after calibrating the X-ray imaging system, wherein a random contrast pattern comprising an X-ray attenuating material is deposited on the specimen;

taking, with the X-ray imaging system, a number of stereoscopic X-ray images of the specimen over a number of time steps;

preprocessing, by a computer system according to the power levels of the X-ray sources, the stereoscopic X-ray images to maximize image contrast and signal-to-noise ratio and remove background objects; and

determining, by the computer system, from the stereoscopic X-ray images, kinematic quantities of the specimen according to changes in the random contrast pattern deposited on the specimen over a number of successive stereoscopic X-ray images.

2. The method of claim 1 , further comprising applying at least one of motion, loading, or deformation to the specimen concurrently with taking the X-ray images.

3. The method of claim 1 , wherein the kinematic quantities comprise at least one of:

shape;

displacement;

strain; or

strain-rate.

4. The method of claim 1 , wherein the target area comprises one of:

a shock tube;

a wind tunnel;

a radiant heater; or

a fire environment.

5. The method of claim 1 , wherein the random contrast pattern deposited on the specimen comprises titanium or tantalum as the X-ray attenuating material.

6. The method of claim 1 , wherein tuning the X-ray sources comprises:

collecting a number of X-ray images with a matrix of different current and voltages settings;

computing an image contrast ratio for each X-ray image;

computing an image signal of each X-ray image;

computing an image noise of each X-ray image;

computing a signal-to-noise ratio of each X-ray image;

computing a final image quality metric for each X-ray image; and

selecting current and voltage settings for the X-ray sources according to the final image quality metrics of the X-ray images.

7. The method of claim 1 , wherein calibrating the X-ray imaging system comprises

collecting light-field images that account for artifacts present on the cameras and scintillator surfaces in the X-ray detectors as well as non-uniform X-ray illumination;

taking an N-frame average of the light-field images;

removing outlier pixels according to an M-pixel outlier filter;

placing a dot-grid calibration target on a stage assembly, wherein a dot grid comprises a repeating pattern of X-ray attenuating material, and wherein the stage assembly rotates, tilts, and translates the dot-grid calibration target throughout a defined calibration volume;

concurrently taking stereoscopic X-ray images of the dot-grid calibration target at each position of the stage assembly;

preprocessing the X-ray images of the dot-grid calibration target to enhance image contrast and signal-to-noise ratio; and

performing a preliminary stereo-system calibration to determine preliminary extrinsic and intrinsic parameters of the X-ray imaging system.

8. The method of claim 7 , wherein the dot-grid calibration target comprises a printed circuit board, and wherein X-ray attenuating pads are made of gold or copper.

9. The method of claim 7 , wherein preprocessing the X-ray images of the dot-grid calibration target comprises:

taking an N-frame average;

removing outlier pixels; and

re-ranging a histogram of the images, wherein pixel intensities are scaled such that dark dots are at 10% of maximum image intensity and light background is at 90% of maximum image intensity.

10. The method of claim 7 , further comprising:

replacing the dot-grid calibration target with a speckled calibration target on the stage assembly, wherein the stage assembly rotates, tilts, and translates the speckled calibration target throughout the defined calibration volume;

concurrently taking stereoscopic X-ray images of the speckled calibration target at each position of the stage assembly;

preprocessing the X-ray images of the speckled calibration target to enhance image contrast and signal-to-noise ratio;

correlating the X-ray images of the speckled calibration target according to the preliminary stereo-system calibration; and

performing a final stereo-system calibration according to results of the X-ray image correlation of the speckled calibration target and the dot-grid calibration target combined to determine the intrinsic and extrinsic stereoscopic imaging parameters.

11. The method of claim 10 , wherein speckled calibration target comprises a flat plate with a random digital image correlation speckle pattern formed of X-ray attenuating material.

12. The method of claim 10 , wherein preprocessing the X-ray images of the speckled calibration target comprises:

taking an N-frame average;

removing outlier pixels; and

correcting the X-ray images of the speckled calibration target with the light-field images.

13. The method of claim 1 , further comprising automatically selecting a subset of X-ray images from all X-ray images taken by the X-ray imaging system by:

determining an average image brightness of the X-ray images;

performing course image search of the X-ray images;

upon detection of average image brightness above a specified threshold, switching from

course image search to fine image search, wherein the X-ray images are stepped through one by

one until a brightness of the X-ray images is no longer above the specified threshold;

averaging all X-ray images found during fine image search into a single averaged X-ray

image; and

saving the averaged X-ray image.

14. A method of X-ray digital image correlation, the method comprising:

tuning an X-ray imaging system, wherein the X-ray imaging system comprises two X-ray sources pointed at a target area at a stereo angle and two corresponding X-ray detectors behind the target area, and wherein tuning determines power levels of the X-ray sources that maximize image contrast and signal-to-noise ratio;

calibrating the X-ray imaging system, wherein calibration comprises:

taking stereoscopic X-ray images of a dot-grid calibration target mounted on a motorized stage assembly as the stage assembly rotates, tilts, and translates the dot-grid calibration target throughout a defined calibration volume;

preprocessing the stereoscopic X-ray images of the dot-grid calibration target to enhance image contrast and signal-to-noise ratio;

replacing the dot-grid calibration target with a speckled calibration target on the stage assembly;

taking stereoscopic X-ray images of a speckled calibration target as the stage assembly rotates, tilts, and translates the speckled calibration target throughout the defined calibration volume;

preprocessing the stereoscopic X-ray images of the speckled calibration target to enhance image contrast and signal-to-noise ratio; and

correlating the stereoscopic X-ray images of the speckled calibration target to determine intrinsic and extrinsic stereoscopic imaging parameters of the X-ray imaging system;

placing a specimen in the target area after calibrating the X-ray imaging system, wherein a random contrast pattern comprising an X-ray attenuating material is deposited on the specimen; taking, with the X-ray imaging system, a number of stereoscopic X-ray images of the specimen over a number of time steps;

applying at least one of motion, loading, or deformation to the specimen concurrently with taking the stereoscopic X-ray images;

preprocessing, by a computer system according to power levels of the X-ray sources, the stereoscopic X-ray images to maximize image contrast and signal-to-noise ratio and remove background objects; and

determining, by the computer system, from the stereoscopic X-ray images, kinematic quantities of the specimen according to changes in the random contrast pattern deposited on the specimen over a number of successive stereoscopic X-ray images.

15. An X-ray digital image correlation system, comprising:

an X-ray imaging system comprising:

two X-ray sources pointed at a target area at a stereo angle, wherein power levels of the X-ray sources are set to maximize image contrast and signal-to-noise ratio; and two corresponding X-ray detectors behind the target area, wherein each X-ray detector comprises a camera and a scintillator;

a computer system programmed to:

tune the X-ray imaging system to determine power levels of the X-ray sources that maximize image contrast and signal-to-noise ratio;

calibrate the X-ray imaging system to determine intrinsic and extrinsic stereoscopic imaging parameters of the X-ray imaging system;

take a number of stereoscopic X-ray images of a specimen in the target area over a number of time steps, wherein a random contrast pattern comprises an X-ray attenuating material is deposited on the specimen;

preprocess the stereoscopic X-ray images according to the power levels of the X-ray sources to maximize image contrast and signal-to-noise ratio and remove background objects;

determine, from the stereoscopic X-ray images, kinematic quantities of the specimen according to changes in the random contrast pattern deposited on the specimen over a number of successive stereoscopic X-ray images and system by:

select a subset of X-ray images from all X-ray images taken by the X-ray imaging determining an average image brightness of the X-ray images;

performing course image search of the X-ray images;

upon detection of average image brightness above a specified threshold, switching from course image search to fine image search, wherein the X-ray images are stepped through one by one until a brightness of the X-ray images is no longer above the specified threshold;

averaging all X-ray images found during fine image search into a single averaged X-ray image; and

saving the averaged X-ray image.

16. The X-ray digital image correlation system of claim 15 , further comprising a loading device configured to apply motion, loading, or deformation to the specimen during X-ray imaging.

17. The X-ray digital image correlation system of claim 15 , wherein the kinematic quantities comprise at least one of:

shape;

displacement;

strain; or

strain-rate.

18. The X-ray digital image correlation system of claim 15 , wherein the target area comprises one of:

a shock tube;

a wind tunnel;

a radiant heater; or

a fire environment.

19. The X-ray digital image correlation system of claim 15 , wherein the random contrast pattern deposited on the specimen comprises titanium or tantalum as the X-ray attenuating material.