Measuring and testing – Specimen stress or strain – or testing by stress or strain... – Specified electrical sensor or system
Reexamination Certificate
2001-06-06
2003-03-04
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
Specified electrical sensor or system
Reexamination Certificate
active
06526835
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for characterizing physical properties of a test piece, and related methods.
In many circumstances, it is desirable to determine the physical properties of an object or structure. For example, in a manufacturing environment, it may be advantageous to determine selected physical properties of a product so as to verify that the product meets the minimum standards for its intended purpose. Similarly, it may be advantageous to determine the physical properties of a product in order to adjust a manufacturing process, so that a consistent and high-quality product may be produced. Furthermore, by evaluating the physical properties of a particular type of product while adjusting various process parameters, it is possible to optimize the process parameters for a particular product or process. For example, for the manufacture of a drawn polymer material, such parameters as the temperature of the molten material, the amount of draw-down, the through-put, and the viscosity of the base material may be determined and optimized through proper evaluation of physical properties of the finished product.
A wide variety of physical properties may be measured. One useful property is the elastic modulus. The elastic modulus is a ratio of the stress applied to an object or structure over strain exhibited by that object or structure.
Stress is defined as the applied force divided by the resisting area of the object. It will be appreciated that an object may be subjected to many sorts of stress, such as shear, compression, torsion, and tensile. It will also be appreciated that the resisting area of an object or structure depends on both the shape of the object and the type or direction of stress applied to it.
Strain is a relative deflection of the object in question in response to the applied stress. An object may similarly exhibit various sorts of strains depending on the stresses applied.
Regardless of the type of stress and strain, however, the elastic modulus may be understood to be an indication of an object's response to applied forces.
One particularly useful elastic modulus is the Young's modulus. The Young's modulus is a ratio of the applied tensile stress &dgr; over the exhibited tensile strain &egr;. Although a wide variety of other elastic moduli may of course be measured, it is enlightening to consider as a specific example the Young's modulus and devices known for measuring it.
Several devices are known for measuring Young's modulus. Conventional devices include destructive (“test until failure”) systems and dynamic mechanical analyzers, commonly known as DMAs. Neither type of device is entirely satisfactory, however.
Conventionally, the Young's modulus of an object is determined destructively by measuring the deflection of the object while applying a gradually increasing force to the object until it “fails”, that is, deforms plastically or fractures. The stress-strain relationship is then determined for the sample. The stress-strain relationship is plotted as a curve, and a straight-line fit of the curve is approximated. The slope of the line may be used as an approximation of Young's modulus.
This approach has numerous disadvantages. For example, a useful approximation of Young's modulus can only be determined if the stress-strain relationship is linear or nearly linear over a broad range. That is, the material must have a large and generally uniform range of elastic deformation. Although some materials have such properties, many others do not. In particular, many composite materials do not exhibit linear stress-strain relationships.
Also, the destruction of the object being tested is inherent in the test method. In order to obtain sufficient useful data for the straight-line fit, the sample must be tested across essentially its entire range of elastic deformation. Therefore, it is necessary to increase the applied force until the sample either deforms plastically or fractures. In either event, the sample is destroyed, and is unavailable for sale, further testing, or other purposes.
Because conventional destructive methods require a substantial range of stress-strain data to determine a useful value of Young's modulus, destructive testing is generally not suitable for determining Young's modulus at a particular stress, or for a narrow band of stresses. Conventional destructive testing typically produces only an average value for the entire range of applied stress.
In addition, destructive tests are generally limited to samples of standard size and shape. This is the case for several reasons. First, in order to calculate the stress on a sample, the resisting area of the sample must be known. Thus, in order to determine the Young's modulus of an arbitrarily-shaped product using conventional destructive methods, the geometry of the product must be carefully measured, and the area calculated. For complex shapes, this is a considerable difficulty.
Second, the Young's modulus of an object depends not only on its material and its resisting area but also on its shape. For example, a U-shaped beam generally exhibits a higher Young's modulus than a flat strip, even if the strip and the beam have the same resisting area and are made from the same material. In many cases, especially for complex shapes, it is impractical or impossible to calculate in advance the effect of a particular shape on the Young's modulus. Thus, in order to compare test subjects without performing complex corrections due to varying geometry, it is generally necessary to test a sample of standard size and shape instead of an actual product.
Although this simplification is convenient, the use of samples as opposed to actual products produces difficulties. For example, tests on samples tend to be inaccurate for orthotropic materials, that is, materials that have a directionally non-uniform structure. One common example is wood, which has a grain that is stronger in some directions than in others. Other orthotropic or partially orthotropic materials include composites, laminates, etc. Orthotropic materials pose difficulties for conventional tests for several reasons.
First, the stress and strain of orthotropic materials do not always vary linearly with increasing dimension. That is, doubling the cross-sectional area of a sample of an orthotropic material may not double the stress required to achieve a given strain, even if the material composition and shape are kept exactly the same.
Second, orientation is important for orthotropic materials. For example, an object composed of many laminated layers will have a very different Young's modulus if the layers are oriented parallel to the direction of stress than if they are perpendicular to the direction of stress. Because it may be inconvenient or impossible to produce a test sample that is representative of the orientation in which the actual product will be used, the accuracy of the test becomes questionable at best.
Dynamic Mechanical Analyzers operate according to a different principle. As previously described, the Young's modulus is a measure of material stiffness. The Young's Modulus of an object, along with the object's geometry, determine the stiffness coefficient of that object. In turn, the stiffness coefficient of an object determines the frequency at which it will vibrate. By proper analysis, it is therefore possible to determine the Young's modulus of an object if the geometry and a frequency of vibration of the object are known.
In order to obtain data, the object must of course be made to vibrate. However, the vibration must be at the free vibrational frequency for the object, also known as its natural frequency. The free vibrational frequency of an object is the frequency at which it will vibrate if it is initially disturbed but not subsequently subjected to additional forces. Such vibrations are known as free vibrations.
If an object is driven at its free vibrational frequency, also known as its harmonic
Andersen Corporation
Merchant & Gould P.C.
Noori Max
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