Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
2001-02-23
2002-10-29
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
C073S804000
Reexamination Certificate
active
06470756
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for measuring stress within an object. More specifically, the invention relates to a method for determining residual stress within materials.
2. Relevant Technology
The composition of natural and man made materials is well understood. We know that materials are made up of atoms that are bonded by atomic forces to form solids. Atoms may also combine with other atoms to form molecules. These molecules may be bonded closely together, as with solids, or far apart, as with gases. The state of the material, solid, liquid, or gas, depends on the amount of energy the molecules have. Because molecules and atoms have no known structural connection, they are free to move and act in response to the attraction forces of the molecules and atoms around them. These attraction forces may be conceptualized as very small springs which connect each atom, and/or molecule. The strength or weakness of the attraction forces depends largely on the amount of energy the atom is exposed to. Because the molecules and atoms of an object are exposed to different amounts of energy atoms at one location within an object may be pulled or pushed by other atoms.
These push and pull forces are known as stress. Stress is a force divided by an area. The force is the atomic attraction force and the area is a measured area of an object. In common elements such as iron or copper, the atoms are atomically bond together in a very tight lattice. Because different atoms of an object are exposed to varying amounts of energy, the stress of one or more atoms on neighboring atoms varies throughout the material. The different magnitudes of stress throughout an object are known as residual stress.
The name comes primarily from the fact that residual stress is the stress remaining within an object as a result of forming, shaping or otherwise processing the object. Generally, objects are formed by exposure to a change in energy, or heat, and an application of pressure. For example, diamonds are carbon atoms which have undergone extreme heat and pressure. The heat and pressure apply stress to the diamond. When the heat and pressure are removed, the atoms making up the diamond react by pulling and pushing each other. The push and pull within the diamond is residual stress, the stress left after application of the formation stress.
Similarly, man made objects experience some form of processing before the final product or object is ready for use. Particularly with metals, there are a number of processing methods which may be used to form, shape and refine the elements into metallic objects made of one element or composite objects made of two or more elements. For example, steel is made from combining iron and carbon. Other steel alloys are made using iron, carbon, and other natural or man made elements. Processing methods for creating steel or other metallic alloys include, forging, case hardening, quenching, welding, bonding, casting, extruding, and the like. These methods involve heating the elements and applying pressure to shape the object. From steel other objects may be created. These objects may require that the steel be rolled, hammered, stamped, drilled, machined, or otherwise shaped.
Whenever an object is exposed to a change in temperature, or application of pressure, the atoms within the object react by increasing or decreasing their attraction on neighboring atoms. This changes the residual stress within the object. Residual stress may not be a major concern in large applications of an object, such as the blade of a shovel. But, residual stresses contributes to failures from fatigue, fracture, distortion, wear, creep, stress corrosion cracking and the like. Material may also be processed to make parts of machines where a variation in size or performance of a part may cause serious problems.
For example, military fighter planes are generally made of metal and metal alloy parts. Due to the stress and change in temperature operation of the fighter places on the parts, the parts are precision machined and engineered to exact specifications. The problem is the residual stresses within a part may cause the part to distort, or deform during use, or prior to installation such that the part is unusable. This may be very costly. One estimate suggests that the cost of distorted scrapped fighter plane parts is around $263 million.
Techniques do exist for calculating the residual stress within an object. In the hole drilling method, a strain gauge rosette is placed on a free surface of the object. Then, a hole is drilled in the middle of the rosette. The strain gauge rosette is then used to measure the strain of the surface once the hole is cut. In the layer removal method, changes in one existing surface are measured after a layer of material is removed from an opposite surface. In the compliance method, the strain is measured near or opposite a successively extended slot. Other techniques such as Moire interferometry also exist.
These techniques as well as the method of the present invention, relate to elastic materials. Elastic materials are those which deform in response to internal or external stresses placed on the object. All materials are elastic to some degree. These techniques measure the residual stress by calculating how much the material deformed after some of the material is removed through removing a layer, drilling a hole, and the like. The material deforms once a hole or other material is removed because the residual stress within the material is freed through creation of the hole, cut, or layer.
These techniques are related because each performs the measurements on a pre-existing surface of the object. The problem is that measuring a change in a pre-existing surface is not as precise as measuring the new free surface created by cut, hole, or layer removal. Because stress is not being released at the pre-existing surface, any changes in the pre-existing surface are approximations of the deformation at the newly created surface which caused the measured deformation at the pre-existing surface. In other words, the accuracy of measurements taken at a pre-existing surface is limited due to the remoteness between where the residual stress is relieved and where the measurements are made.
Additionally, because the measurements are indirect representations of the displacement at the newly formed surface, the calculations to arrive at the residual stress at the newly created surface are complex. The calculations involve theoretically complex and tedious inversion calculations simply to determine the residual stress in a single dimension. Completing the calculations and reducing the measurements to the residual stresses may take several weeks, particularly if an analyst needs to know the residual stress in two or three dimensions.
Measuring the new free surface would allow one to calculate the residual stress which existed prior to removal of the material. However, there is not a reference point which may be used to determine the amount of change in the new free surface after the material is removed. In addition, other techniques may avoid measuring deformation at the new free surface because the drill bit, or cutting tool introduced residual stress into the material during the removal process.
Therefore, it would be an advancement in the art to provide a system and method for determining the residual stress within an elastic object at a new free surface where the residual stress is being relieved. It would be a further advancement to provide a system and method for determining the residual stress within an elastic object such that the calculations are simple and direct rather than inversion calculations involving complex theory. It would be another advancement in the art to provide a system and method for determining the residual stress within an elastic object such that an analyst does not need special training or expertise to perform the measurements.
SUMMARY OF THE INVENTION
The invention is a system and method for determining th
Madson & Metcalf
Noori Max
The Regents of the University of California
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