Optics: measuring and testing – Material strain analysis – With polarized light
Reexamination Certificate
2001-05-21
2003-11-18
Frech, Karl D. (Department: 2876)
Optics: measuring and testing
Material strain analysis
With polarized light
C356S032000, C356S073000, C073S800000
Reexamination Certificate
active
06650405
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of stress and strain measurement, specifically to an improved method for detecting and viewing of stress and strain in objects or parts.
BACKGROUND OF THE INVENTION
Stress and Strain Detection and Measurement
Stress and strain detection and measurement is an important field of engineering and is used in almost every area of manufacture and construction where a knowledge of the stresses and strains being experienced by an object are important. By knowing the stresses in a part, failure modes and service life can be predicted and failure analysis can be performed. With this knowledge, parts can be redesigned to be lighter, stronger, or less expensive. “Stress” and “strain” are sometimes used interchangeably in the following descriptions, since one can be determined if the other is known and a stress-strain diagram is available.
Strain, e, is a dimensionless response to stress expressed as a fraction e=&Dgr;L/L
o
where L
o
is the original length of the object and &Dgr;L is the change in length of the object when stress is applied. Stress, s, is a measure of force per unit area given by F/A where F is the force being applied and A is the area it is being applied to. Because stress cannot be measured directly in practice, strain is measured instead. The stress in an object is related to the strain by the Young's Modulus, E, which is given by the following relationship:
E
=
s
e
el
=
(
F
/
A
)
(
Δ
L
/
L
o
)
=
stress
/
strain
.
Knowing the Elastic Modulus of a given material, the stress in the material can be determined by measuring the strain. Traditionally, stress and strain measurements have been accomplished by a number of different methods. Some of these methods are described below:
Strain Gages
Strain gages are small electronic devices that measure strain through a change in resistance. The resistance, R, of a wire is a function of the size of the wire as well as of the material as follows:
R
=
ρ
(
L
A
)
,
where L is the length of the wire, A is the cross-sectional area of the wire, and &rgr; is electrical resistivity, a property of the material. As the length of the wire L increases and the cross-sectional area A decreases, the resistance R increases. This property can be exploited to measure strain with a strain gage. By measuring the increase in resistance of a length of a thin wire attached to a part, the strain in the part can be determined and the stress calculated.
Unfortunately, strain gages have a number of disadvantages. First, applying a strain gage to a part can be difficult. Second, the electrical signal produced by a strain gage is very small and must be amplified. Amplification can lead to noise problems and loss of accuracy. Another significant disadvantage of strain gages is they can only measure strain in one direction. A different strain gage must be used for every different direction in which strain is to be measured. Finally, strain gages can only measure localized strain. That is, the strain gage can only measure strain exactly at the point where the gauge is applied. As such, strain gages require prior knowledge of the stress and strain distribution in the part and the direction of strains in order to be most effective.
Brittle Lacquer
Brittle lacquer is a brittle coating that cracks easily under tensile strain. The lacquer is applied to the unstressed part. When the part is stressed, the brittle lacquer cracks, starting at the areas of highest strain. Brittle lacquer is difficult to work with and does not provide a quantitative measure of the stress and strain. As such, the brittle lacquer method can only indicate which areas of a part are experiencing stress and strain. Also, only one test is possible with a given application of brittle lacquer. Once the brittle lacquer has cracked, the coating must be stripped off and reapplied for subsequent tests.
Fiber Optics
Fiber-optics can be used to measure stress and strain by detecting the change in length of all optical fiber. In theory, the operation of a fiber-optic strain gage is similar to the operation of a strain gage that measures change in resistance. In the case of a fiber-optic strain gage, a change in the transmissibility of light is being measured. Fiber-optic strain gages possess the same disadvantages as standard strain gages: they are difficult to apply and can only measure localized strain in one direction. As such, prior knowledge of the stress and strain field in the part is required.
Because of the disadvantages and the complexity of strain gages, brittle lacquer, and fiber-optics, these techniques for measuring stress and strain are typically used only at the product development stage. Manufactured products generally do not come with built-in strain gages for monitoring stresses and strains, although this might be desirable in some cases. For instance, monitoring the stresses and strains in a production aircraft part would be useful to help predict failures of that part and to schedule maintenance on that part. Moreover, the cost of these particular methods of detecting stress and strain make them somewhat prohibitive even at the product development stage.
Photoelastic Techniques
A different class of stress and strain measurement techniques which have been used for a number of years are known as photoelastic techniques. Photoelastic techniques exploit the photoelastic properties of certain materials to detect stress and strain. The speed of propagation of light in transparent materials is generally slower than in a vacuum or in air. The ratio of the speed of light in a given material to the speed of light in a vacuum is called the index of refraction of that material. In homogeneous materials, the index of refraction is constant regardless of the direction of propagation or plane of vibration of the light. In other materials, strain in the material causes the index of refraction to change depending on the direction of propagation of light. These materials, which can be optically isotropic when unstrained, become optically anisotropic when strain is present.
Materials which become optically anisotropic when stressed are known as photoelastic materials. The change in index of refraction relative to index axis in the material can typically be related to the stress and strain in the material by observing and quantifying the photoelastic effect. The photoelastic effect is caused by alternately constructive and destructive interference between light rays which have undergone relative retardation, or phase shift, in the stressed photoelastic material. When illuminated with polarized light and viewed through a polarizing filter, fringe patterns become visible in the photoelastic material that reveal the overall stress and strain distribution in the part and show the locations and magnitudes of the stresses and strains in the part. A person skilled in the art of photoelastic analysis can interpret and measure these patterns.
Photoelastic techniques have the advantage of being a full-field measurement technique. The strain over the entire surface of the part can be measured. Furthermore, the measurement technique is not directional. Unlike strain gages, a photoelastic coating can detect strain regardless of the direction of that strain. As such, prior knowledge of the directions and magnitude of the strain in the part before applying the photoelastic coating is not required.
Photoelastic techniques are an excellent technique for stress and strain analysis with many advantages over other methods of analysis. However, the application of photoelastic coatings to parts is problematic. Presently, photoelastic materials are available in sheets and plate form for application to flat parts. To perform the analysis, a sheet of photoelastic material must be carefully cut to shape and bonded to the part. The part is then subjected to test forces. While being subjected to the test forces, the part can be viewed through a reflection polariscope to determine the direction and magnitude of the stresses and st
Ellens Mark William
Lam Duhane
Frech Karl D.
Kim Ahshik
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