Patent Application
20100316437
This invention relates to a method and system for compensating for the differences in the coefficient of thermal expansion of materials fastened together, and in particular to a method and system in which one material can be fastened to another material with no resultant thermal induced stress.
When the temperature of a solid material is increased the material expands resulting in an increase in volume. When the temperature of a solid material is decreased the material contracts resulting in a decrease in volume. It is a well know fact that solid materials typically expand in response to heating and contract upon cooling. Materials expand because an increase in temperature results in greater thermal vibration of the atoms in the material and subsequently an increase in the average separation distance between adjacent atoms. The dimensional response of a solid material due to a change in temperature is characterized by the coefficient of thermal expansion (CTE). The linear CTE describes the magnitude of expansion as a function of increased temperature and is expressed as follows:
where
where
where
It is a common practice in the development or design of a part or system to pick a material with the best properties for the given application. These properties can be physical such as strength and density or economical such as cost and availability. Differing materials are often used in one mechanical system to optimize different parts of the system. In these systems different material parts will be fastened together. These differing materials will have differing thermal expansions. When this system experiences a change in temperature the differing materials will have differing amounts of thermal strain or thermally induced change in length. Since the materials are rigidly fastened together this strain becomes a stress in the parts based on Hooke's law,
σ=E(ε1−ε2)
where
Most mechanical systems only require that the fastened joint does not fail under the thermal stresses. This is easily accomplished by altering the shape and size of the fastened components and the fasteners. However, for stress sensitive instruments, very small stresses in the components can cause instrument errors and failure.
Current methods to compensate for the stress and deformation of joined structures generally address linear expansion and contraction. For example, steel railroad rails, if laid in contact end to end would buckle on hot days as the result of thermal expansion. Most tracks are built from pieces of steel supported by wooden ties, and laid with a space between the ends. This space provides a buffer for thermal expansion, allowing the rail to elongate without contacting the next rail. Another example of a method to compensation for thermal expansion is expansion joints. Bridges are built with metal expansion joints, which contain gaps between bridge sections that allow for expansion and contraction without causing faults in the overall structure of the bridge.
For cryogenic optical testing chambers in which high performance Silicon Carbide mirrors are used, it is desired to reduce mirror errors caused by thermal gradients. However, the chamber is built primarily of aluminum and stainless steel which have CTEs that are much different than that of the Silicon Carbide mirrors. Mounting the Silicon Carbide mirrors to the chamber and expecting good performance over a 250 K range is difficult to achieve.
When materials with different CTE are joined, stress and deformation occurs as the result of changes in temperature. Systems have been designed to incorporate a material with an intermediate CTE to reduce the magnitude of the thermal stresses. For example, struts can be designed with a controlled thermal expansion structure that maintains a constant length over a temperature range. If the surfaces of two materials with different CTEs need to be joined, one or more different materials with intermediate CTEs are usually sandwiched between the two surfaces to distribute the thermal stresses at the interfaces, thus resulting is a lower overall stress. Other available technologies to fasten materials with different CTEs involve using flexures to absorb the thermal elastic strain associated with differential thermal expansion rates.
Deficiencies with current methods and technologies for addressing thermal induced stresses in joined structures are associated with the linear nature of the solutions which are not applicable to non-linear structures. For non-linear structures, the use of an intermediate layer distributes the thermal induced stress to the interfaces but only reduces the magnitude of the thermal stress in the structure. Therefore a thermal expansion compensation method and system is needed to compensate for, and eliminate, thermal induced stresses in systems composed of materials with different coefficients of thermal expansion.
A thermal compensation method and system that eliminates thermal induced stresses in structures composed of different materials fastened together is disclosed. The system utilizes three compensation mounts made from a material with a coefficient of thermal expansion (CTE) that is between that of the two materials being fastened together. The size of the mounts and fastening locations are calculated based on the coefficient of thermal expansion for the two materials being fastened together and the mount material. The geometry of the mounts can be “C” shaped to allow for fastening two planar surfaces without introducing a large thickness increase to the structure. This system allows for materials to be fastened together, and when placed in an environment with temperature fluctuations, the system experiences no or insignificant amounts of thermal induced stresses. Fastening of materials includes all joining methods used to join separate parts together. This includes but is not limited to the use of bolts, screws, rivets, welding, and adhesive bonding.
Understanding that drawings depict only certain preferred embodiments of the invention and are therefore not to be considered limiting of its scope, the preferred embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
In the following description, numerous specific details are provided for a thorough understanding of specific preferred embodiments. However, those skilled in the art will recognize that embodiments can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In some cases, well-known structures, materials, or operations are not shown or described in detail in order to avoid obscuring aspects of the preferred embodiments. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in a variety of alternative embodiments. Thus, the following more detailed description of the embodiments of the present invention, as represented in the drawings, is not intended to limit the scope of the invention, but is merely representative of the various embodiments of the invention. The invention will be described referring to mounting a silicon carbide component to an aluminum base utilizing an aluminum alloy for the compensation mount members, although the method and system can be applied to any alloys, ceramics, polymers and other materials.
The use of low thermal expansion, brittle materials such as silicon carbide (SiC) in optical instruments has increased the need for improved mounting techniques. The disclosed thermal expansion compensation system utilizes an intermediate thermal expansion material to compensate for the CTE mismatch between the materials being joined. The availability of aluminum alloys with CTEs as low as 12 ppm/K allows the compensation mount for optical instruments to be low mass, low cost and low profile.
A thermal expansion compensation system is necessary for space based optical components, such as the case in which a low CTE telescope material (i.e. SiC, Si, glass, composite) is fastened to an aluminum 6061 bus. Space bound platforms for optical sensors are most often built out of aluminum. Aluminum is lightweight and economical. Aluminum honeycomb composite panels are very light weight and very stiff. The major deficiency of aluminum is that is has a very high coefficient of thermal expansion, around 23 ppm/K. More and more precision instruments such as precision space telescopes are being built out of high performing exotic materials such as SiC and beryllium. SiC is very stiff and brittle. SiC has a low CTE of about 3 ppm/K. In order to mount a SiC telescope to an aluminum honeycomb bench, by existing methods, significant flexures are needed to reduce the thermal elastic stress caused by the great CTE differences of the materials. These flexures require significant volume and are only able to reduce some of the stress. The presently disclosed thermal expansion compensation method and system eliminates the joint's thermal elastic stress based on the compensation mounts fastened between the materials.
The mount material for space based optical platforms can be a low CTE aluminum such as RSA 419 or 443 (CTE's 13-16 ppm/K). Hybrid material telescopes are often used for various reasons, including a thermalizing optical performance, reducing cost, adaptive optics and for other performance needs. Utilizing a hybrid telescope over a large temperature range can be a very difficult engineering problem as the thermal elastic stresses will deform optical surfaces and reduce performance. Utilizing the disclosed thermal expansion compensation method and system, a 6061 aluminum mirror with a CTE of 23 ppm/K can be fastened to a composite mount with a CTE of 2.5 ppm/K and have essentially no surface deformation due to the thermal elastic stress from room temperature to near liquid nitrogen cryogenic temperatures.
Aluminum mirrors which are economical to produce can be fastened to a SiC structure and perform well over a large temperature range using the disclosed thermal expansion compensation technique. In some cases glass mirrors are preferred but this often requires an expensive graphite fiber composite structure. Glass mirrors could be used in an aluminum structure with the use of the disclosed thermal expansion compensation method and system. With both of these hybrid systems, special attention is necessary to assure optical athermal performance over the operating temperature range. Lenses could be utilized to provide this optical athermalization.
In one embodiment, shown in
The distance from the respective holes to the center of the thermal expansion compensation system, identified as “A” and “B” in
αM×(A+B)=αH×B+αL×A
where
Fabrication tolerances and coefficient of thermal expansion uncertainty and nonlinearity over the anticipated temperature range may produce some negligible thermal strain which may not be compensated for. It is important that the designer understand the thermal expansion behavior of all three materials in the joint over the temperature range of interest to best compensate for the differing thermal strains. The “C” mount members can have various cross sectional geometries. They can be machined from standard flat plate materials or, for weight savings, can be machined to have two flanges separated by a web similar to an “I” beam to increase their moment of inertia. This way the “C” mounts can be lightweight and very stiff. The “C” shaped geometry allows the compensation system to be constructed without overlapping the mounts, thus maintaining a low profile joint.
This joint is not typical in that the three separate “C” mounts are only fastened to other parts with one fastener at each end of the “C” mount. This minimal amount of fasteners allows rotation in the joint, reducing the stiffness and locating precision of the mount. Dowel pins separated some distance from the fastener cannot be used as they will create a fixed length (distance from the fastener to the dowel pin) that will cause thermal elastic strain in the joint. For this reason for pinning to be successful, the fastener to pin distance must be zero. To satisfy this requirement a hollow pin can be used with the fastener extending through the center of the pin. Both pin and fastener are then coaxial with zero fixed distance between their centerlines. Utilizing hollow pins will enable locating precision in the joint.
The thermal expansion compensation method and system described herein utilizes a “C” shaped geometry for the compensation mount member. The system can be implemented with other appropriate geometries such as, but not limited to, those illustrated in
The fastening method described herein to connect the compensation mount members to the two materials consists of a screw through a dowel pin. Any type of fastener known to those skilled in the art can be used, such as but not limited to, screws, bolts, pins, dowels, etc. In addition adhesives and other non hardware joining methods can be used effectively.
The thermal expansion compensation system 40 can be constructed using three linear members 43, 44, 45 as shown in
Without further elaboration, it is believed that one skilled in the art can use the preceding description to utilize the invention to its fullest extent. Therefore the examples and embodiments disclosed herein are to be construed as merely illustrative and not a limitation of the scope. It will be obvious to those skilled in the art that many changes, such as types of materials, and member and component shapes, may be made to the details of the above described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.
This application claims priority to U.S. Patent Application No. 61/187,309 filed on Jun. 16, 2009, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61187309 | Jun 2009 | US |
