Optics: measuring and testing – Material strain analysis
Reexamination Certificate
1999-08-06
2001-12-04
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Material strain analysis
C073S762000
Reexamination Certificate
active
06327030
ABSTRACT:
BACKGROUND OF THE INVENTION
The subject invention relates to the field of strain measurements. Examples of applications which can benefit from the subject technology include engineering analysis of strain on complex geometries, design analysis, and non destructive testing of structures. Accordingly, the subject invention can be utilized in the automotive, aerospace, civil structures, and sporting goods fields, as well as many others. Specific embodiments of the subject invention pertain to a strain-sensitive coating, a strain field mapping system, and a method for conducting strain analysis. The method and apparatus of the subject invention are particularly useful in the field of full-field strain analysis.
In the field of structural analysis, the ability to determine of the stresses which a structural body is experiencing can provide important feedback in the design and construction of such structural bodies. Typically, surface strain on the structural member provides information on the stresses that the body is experiencing. This information leads to the identification of stress concentrations, over stressed areas, and general stress mapping for design and comparison to predictive methods. Currently, a number of methods exist for measuring such surface strain, including point and full-field methods.
The point methods include electrical resistance strain gauge methods and methods employing electro-optic sensors and optical methods. These methods typically require the affixing of a plurality of sensors at various locations on a structural body, or stepping the sensor across the structural body, such that when the structure is stressed each sensor or step indicates the surface strain at one point. In order to determine the strain over an entire body, numerous sensors located at critically stressed points on the surface are required or numerous steps are required for movable sensors. Accordingly, these point methods can be cumbersome, making it difficult to determine the stresses over an entire surface of a structure.
A number of surface measurement techniques have been developed to overcome the limitations of the point methods, including brittle coatings, photoelastic coatings, moire methods, interferometric methods, and digital image correlation methods. Each of these methods can be useful for certain applications but have characteristics which limit their usefulness. Brittle coatings typically provide good qualitative information about the principal stress directions on objects. Some limitations of brittle coatings are that the part can only be tested in one loading configuration, it only provides limited quantitative information, and methods for automated data collection do not exist. Photoelastic coatings provide only the shear stress and principal stress direction information on objects and are typically cumbersome to apply to large bodies since the coating process is time consuming. Moiré methods are typically limited to flat objects and are not used on complex geometries. Interferometric methods such as holographic interferometry, electric speckle pattern interferometry and shearography require sophisticated vibration isolation greatly reducing their applicability. Digital image correlation methods lack the sensitivity required to test parts in the material linear range.
Accordingly, there still remains a need for a method and system which can easily and accurately measure full field strain on complex shaped structures. Even more advantageous would be a method and system which can provide real-time dynamic strain measurements, even at low strain levels.
BRIEF SUMMARY OF THE INVENTION
The subject invention relates to a strain-sensitive coating, a strain measurement system, and a method for determining mechanical strains on substrate materials. In a preferred embodiment of the subject invention, the system includes a luminescent strain-sensitive coating and a strain field mapping system which can be used to create a strain map of the mechanical strain on a substrate material.
The luminescent strain-sensitive coating of the subject invention preferably utilizes a polymer-based binder coating. The subject coating can incorporate one or more luminescent compounds. These luminescent compounds can absorb light from an illumination source and then emit light such that the light emitted by these luminescent compounds can emanate from the coating and be detected. In a specific embodiment, one or more luminescent dye(s) can be incorporated with, or added to, the binder. In a preferred embodiment the luminescent dye in the coating emits light over a band of wavelengths whose center is different from the wavelength band of illumination used to excite the dye. In a specific embodiment, the emitted light is red shifted from the illumination light.
The subject coating can be applied to a substrate material via one or more of a variety of methods. Such methods include, but are not limited to, electrostatic deposition, chemical vapor deposition, painting, spincoating, and dip coating. Preferably, the coating can be formulated so that it can be applied using aerosol technology to produce a thin film on a substrate material such as metal, polymer, ceramic, or composite. After the subject coating has been applied to the structural body, it can undergo a curing process. This curing process can include, for example, exposure to various environmental conditions for a variety of time durations.
In a preferred embodiment, a dry lake-bed pattern of cracks can be generated in the coating during the curing process. When the substrate material is subsequently subjected to strain, the crack morphology (e.g., crack opening, crack depth, crack density, crack length, and/or crack orientation) changes as a result of the strain in the substrate. Accordingly, the change in crack morphology can be used to investigate the strain in the underlying material by, for example, studying the changes in one or more of the characteristics of the light emanating from the coating. As an example, this crack morphology change can influence the overall intensity of the emitted light from the coating. In a specific embodiment, the intensity of the emitted light from the coating is a function of the strain state of the underlying material. In a full-field sense the emitted intensity can be used to map strain information on the surface of structural bodies and complex parts.
In another embodiment, the coating can be formulated and cured such that it is essentially uncracked after the curing process. In this embodiment, cracks can be produced during the application of mechanical strain. The density of cracks, as well as other aspects of the crack morphology, can change as a result of the strain in the substrate. Again, the change in crack morphology can be used to investigate the strain in the substrate by, for example, measuring changes in one or more characteristics of the light emanating from the coating. For instance, as the number of cracks increases, the intensity of light emitted from the coating can also increase. In addition, the intensity of the emitted light can also be a function of the crack morphology as previously described. The morphology of the crack pattern can then change as a result of changing strain in the underlying material. Accordingly, the emitted light intensity of the coating can be correlated to the strain in the underlying substrate material. The state of strain can thus be mapped on the surface of the substrate material.
The subject invention also pertains to a strain field mapping system for producing a map of the mechanical strain field on an underlying substrate. Changes in luminescence characteristics such as intensity, due to the strain field, can be imaged by a variety of techniques. In a specific embodiment the spatial resolution of the imaging equipment can be adjusted such that individual cracks in the coating are not observed in the image, but, rather, an average intensity that is related to strain is observed over a region. The method of the subject invention can then capture an image
Brennan Anthony B.
Carroll Bruce
El-Ratal Wissam
He Liu
Hubner James
Rosenberger Richard A.
Saliwanchik Lloyd & Saliwanchik
University of Florida
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