IMPROVED MANUFACTURED STRUCTURAL MEMBER HAVING A HOLE Field of the Invention The present invention relates to improved manufactured structural materials having a hole which include strengthened regions proximate to the hole.

Background Structural members having holes are widely used in many applications. In many situations, the holes are necessary to provide access, inspection, or connection.

However, holes present in load-bearing structural members can cause stress concentrations that often lead to failure of the structural material proximate to the holes.

For example, airplanes have holes for wiring, fuel and hydraulic lines. Similarly, holes are common in boats, buildings, automobiles, homes and other structures that have functions beyond simply sheltering or containing. Engineers typically compensate for the weaknesses caused by these holes by increasing the thickness of the material around them. In one example, ship builders add extra material around portholes in hulls to guard against structural weakness or failure.

However, thickening structural material around holes adds weight. This is generally, a significant problem for airplanes and spacecraft that need to be as light as possible. A rule of thumb in the aerospace industry is that reducing the weight of a plane by one pound saves 10 pounds of fuel, so techniques to maintain aircraft strength without adding weight are clearly needed. This is particularly true for spacecraft that have extremely high launch costs.

Similar to thickening around holes, others have used reinforcement rings, such as grommets, comprising a different material, such as an elastic material, disposed around

the circumference of holes to alleviate stress concentrations around the holes.

However, reinforcement rings add weight and cost. Additionally, reinforcement rings have limited application to holes having a relatively shallow depth compared to the diameter of the hole.

Others have attempted to use piezoelectric actuators bonded to the surface of a material near a hole with a voltage applied to the piezoelectric actuators to reduce stress concentrations. However, this approach is difficult to implement due to the need to provide an applied voltage and the production cost due to the required step of bonding the piezoelectric actuators to the material surface. In addition, such an approach can lead to reliability problems, such as debonding of the actuator from the metal surface.

SUMMARY OF THE INVENTION A manufactured integral structural member includes a bulk material having at least one hole therein. The member provides at least one strengthening region integrated with the bulk material having an elastic modulus which is different from a bulk elastic modulus provided by the bulk material. The strengthening region provides an increased strength proximate to the hole compared to the member having a bulk elastic modulus throughout. The strengthening region can be remote from the hole and provide an elastic modulus greater than the bulk elastic modulus.

In one embodiment where the bulk material provides a modulus-strength exponent ratio (P/b) of < 0.75, the strengthening region can be remote from the hole.

Additionally, the at least one strengthening region can include a plurality of strengthening regions isolated from one another. In another embodiment, the bulk material can provide a modulus-strength exponent ratio (ß/b) of < 1.0 and the strengthening region can be disposed along an edge of the hole. Additionally, the bulk material can provide a modulus-strength exponent ratio 0.75 < ( (3/b) < 1.0 and-the at least one strengthening region can include at least one strengthening region disposed along an edge of the hole and at least one strengthening region disposed remote from the hole. The hole can include a deep depression. Additionally, a manufactured integral structural member can also include at least one MEMS Device disposed at least one of in and on the bulk material where the MEMS Device includes the strengthening region.

A method for forming a manufactured integral structural member includes the step of providing a bulk material having at least one hole therein where the bulk material has a bulk elastic modulus. The method also includes the step of forming at least one strengthening region integrated with the bulk material having an elastic modulus which is different from the bulk elastic modulus where the strengthening region provides an increased strength proximate to the hole compared to the member having a bulk elastic modulus throughout.

The method can also include the step of selecting a location for the strengthening region based on a modulus-strength exponent ratio (ß/b) of the bulk material. The location can be remote from the hole if a modulus-strength exponent ratio (ß/b) of the bulk material is < 0.75. Additionally, the locations can be disposed along an edge of the hole if a modulus-strength exponent ratio (ß/b) of the bulk material is > 1.0. Also, the location of at least one strengthening region can include at least one strengthening region disposed along an edge of the hole and at least one strengthening region

disposed remote from the hole if the bulk material provides a modulus-strength exponent ratio (p/b) of 0.75 < (ß/b) < 1.0.

The forming step can include selectively modifying density of at least one region of the bulk material to form the strengthening region. The forming step can also include selectively modifying porosity of at least one region of the bulk material to form the strengthening region. Further, the forming step can include selectively modifying microstructure of at least one region of the bulk material to form the strengthening region.

The forming step can include the steps of selectively removing at least a portion of the bulk material in the strengthening region and depositing additional material in the strengthening region. The forming step can also include at least one selected from the group consisting of selective chemical vapor deposition and selective physical vapor deposition. Additionally, the step of providing a bulk material having at least one hole therein can include providing a bulk material having at least one deep depression therein. The method can also include the step of determining elastic modulus distribution to be provided to the strengthening region.

BRIEF DESCRIPTION OF THE DRAWINGS A fuller understanding of the present invention and the features and benefits thereof will be accomplished upon review of the following detailed description together with the accompanying drawings, in which: Figure 1 illustrates a manufactured structural member having at least one strengthening region integrated with the bulk material having an elastic modulus which is different from a bulk elastic modulus provided by the bulk material for increasing the strength of the structural member proximate to the hole, according to an embodiment of the invention.

Figure 2 (a) illustrates a schematic of a functionally graded exemplary structural member for a bulk material having a modulus-strength exponent ratio (p/b) of < 0.75 and Figure 2 (b) illustrates a schematic of a functionally graded exemplary structural member for a bulk material having a modulus-strength exponent ratio (P/b) of < 0.75 having two (2) isolated strengthening regions proximate to an elliptical hole, while Figure 2 (c) illustrates a plot of optimized normalized elastic modulus as a function of distance from the hole for materials having (ß/b) =0.5.

Figure 3 (a) illustrates a schematic of a functionally graded exemplary structural member for a bulk material having a modulus-strength exponent ratio (ß/b) of > 1.0, while Figure 3 (b) illustrates a plot of optimized normalized elastic modulus as a function of distance from the hole for materials having (R/b) =1.5.

Figure 4 (a) illustrates a schematic of a functionally graded exemplary structural member for a bulk material having a modulus-strength exponent ratio 0.75 < (P/b) < 1.0.

DETAILED DESCRIPTION OF THE INVENTION Referring to Figure 1, a manufactured integral structural member 100 according to an embodiment of the invention is shown. The manufactured structural member 100 includes a bulk material 105 having at least one hole 115 therein. At least one strengthening region 110 is integrated with the bulk material 105 and disposed proximate to the hole 115. The hole 115 can include a complete passage through the bulk material 105 and can also include an absence of material from one surface of bulk material 105 that does not form a complete passage to an opposite surface of the bulk material 105. The depth of a hole can be varied and can range from a fraction of the size of the hole 115, to a depth equaling the full depth of the manufactured structural member 100. Additionally, the hole 115 can include a deep depression whose depth is at least twice the average diameter.

Since structural member 100 includes regions having differing mechanical properties, structural member 100 is referred to herein as comprising a"functionally graded"material. The strengthening region 110 has an elastic modulus which is different from the bulk elastic modulus provided by the bulk material 105. Thus, the strength of regions proximate to holes in structural members which are normally the weakest regions thereof can be improved by the invention to approach the strength of the bulk material without a hole.

As used herein, the phrase"manufactured structural member"refers to structural members not formed by biological processes. Bone is an example of a structural member formed by a biological process. The term"integral"refers to one piece structural member, such as members including doped regions, alloyed regions, etched and filled regions, and irradiated (e. g. laser irradiated) regions. Integral members may be contrasted with structural members having two or more separate components. For example, a structural member having two components includes a bulk material and a separate ring layer disposed over and adhered to a hole provided by the bulk material.

As used herein, the phrase"proximate to the hole"refers to a distance from the hole no more than the larger of five (5) times the length of the long axis and ten (10) times the geometric average of the lengths of the short axis and the long axis.

Additionally, an important materials property is termed the tensile elastic modulus, or Young's Modulus. This parameter is generally given the symbol E, as it is herein. The elastic modulus (herein referred to as"E") of a material describes the relationship between the stress and strain of a given material.

Although Figure 1 illustrates a single strengthening region 110 surrounding a single deep depression 115, the invention is not limited in this regard. For instance, Figure 2 (a) illustrates one embodiment of a manufactured integral structural member 200 including a bulk material 205, a strengthening region 210, and a hole 215.

Additionally, the invention is not limited to the hole (or deep depression) having any particular geometry, such as a cylinder with circular ends as shown in Figure 2 (a). For example, Figure 2 (b), illustrates another embodiment of a manufactured integral structural member 250 including a bulk material 255, a strengthening region 260, and a hole 265 with an elliptical shape.

As also shown in Figure 2 (b), the bulk material 255 can include multiple strengthening regions 260. Figure 2 (b) illustrates that strengthening regions 260 are not limited to any particular shape and that it is also not necessary for strengthening regions 260 to completely surround the hole 265. For example, while Figure 2 (a) illustrates one uniform annular strengthening region 210 that surrounds the hole 215, Figure 2 (b) illustrates two crescent shaped strengthening regions 260 that do not completely surround the hole 265. Furthermore, although Figure 2 (a) and 2 (b) appear to illustrate the strengthening region 210 and 260 at a surface of the bulk material 205 and 255, respectively, the respective strengthening regions 210 and 260 can also be deposed through the entire depth of the bulk material 205 and 255, can be embedded between the two surfaces of the bulk material 205 and 255, and can be deposed at one surface and extending through depth of the bulk material 205 and 255 just short of the opposite surface.

The location and properties of the strengthening region 210 can be based on the modulus-strength exponent ratio. As an illustrative example described below, the modulus-strength exponent ratio can be calculated for various densities produced through suitable processes. Nevertheless, any material parameter that can be varied that affects both the E and the strength (a), such as strain hardening, in accordance with the equations below can be used (instead of density (p) ) to determine the modulus- strength exponent ratio.

For a given material, E and strength (a) are related to apparent density (p) by power law functions shown below : <BR> <BR> E= a pb<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> #allow=α#ß

where a and a are proportionality constants Through algebraic manipulation of the above equations, the strength (a) can be expressed as a function of the E as follows: #allwo=C0=Eß/b where Co is a proportionality constant.

The ratio (P/b) from the equation above is the modulus-strength exponent ratio, herein referred to as the"RATIO". For a bulk material in which the density (p) can be varied, P and b can be experimentally determined via industry standard stress-strain tests. Using the industry standard stress-strain tests, the E and strength (a) can be determined for different values of the density (p) of the material. Nevertheless, the range of available density (p) variation can be affected by the process used to vary the density (p). With the values of E and strength (a) obtained for the various density (p) values, strength (a) versus density (p) can be plotted using equation aanow= a p a and E versus density (p) can be plotted using equation E= a pb. From these two plots and as used herein, p and b, and their respective RATIO, can be determined for any bulk material within the range of available density (p) variation available for the processes used to vary the density (p).

Once (3 and b are obtained for a given material, optimal location (s) of the strengthening region (s) and the E distribution in the strengthening region can be determined, preferably using structural analysis and optimization software. One example of suitable software for such computations includes GENESIS@ version 7.2.

GENESIS@ is a fully integrated structural analysis and design optimization software.

Analysis is based on the finite element analysis (FEA) for static, normal modes, direct and modal analysis, and buckling analysis. Design optimization is based on the advanced approximation concepts approach to find an optimum design efficiently and reliably. Actual optimization is performed by the well established DOT and BIGDOT optimizers. GENESIS@ software is produced by Vanderplaats Research and Development, Inc., located in Colorado Springs, Colorado.

Accordingly, the location of the strengthening region and its E can be based the RATIO of the material. Figure 2 (a) illustrates a schematic of a functionally graded exemplary structural member for a bulk material 205 having a RATIO of < 0.75. For a bulk material 205 exhibiting such a RATIO, optimization software determined that by

providing a strengthening region 210 of greater elastic modulus compared to E of the bulk material 205 located remote from hole 215, the load path is directed away from regions of the bulk material 205 around hole 215. A remote location from the hole can include any portion of the material not adjacent to the hole 215, but proximate to the hole 215. Thus, the manufactured structural member 200 is strengthened without modifying bulk material 205 adjacent to the hole 215.

In another embodiment, Figure 2 (b) illustrates a schematic of a functionally graded exemplary structural member 250 for a bulk material 255 having a RATIO of < 0.75 including two (2) isolated strengthening regions 260 proximate to an elliptical hole 265. In this embodiment, the strengthening regions 260 form crescent shaped regions, yet the strengthening regions 260 are not limited to any particular shape.

As an example, bulk material 255 can be determined to have a RATIO equal to 0.5. The location of the strengthening region 260 for such a bulk material 255 can be determined from Figure 2 (c). Figure 2 (c) illustrates a plot of optimized normalized E as a function of distance from the hole 265 for materials having a RATIO equal to 0.5 and shows that the relative modulus of the strengthening region 260 increases as the distance from the center of the hole 265 increases and is greatest at a distance of (2) times the radius of the hole 265. Additionally, the E of the strengthened region 260 decreases, starting at a distance of (2) times the radius of the hole 265, until reaching the E of the bulk material 255 at a distance of (3) times the radius of the hole 265.

One example of a material generally having a RATIO of > 1.0 is cold rolled iron alloy. As is known in the industry, the density of different regions of an integrated material comprised of an iron alloy can be varied by cold rolling, various heat treatments, and various other processes which can provide selective density adjustments.

Additionally, any process In another embodiment, a bulk material 305 can be determined to have a RATIO of > 1.0. Figure 3 (a) illustrates a schematic of a functionally graded exemplary structural member 300 for a bulk material 305 having a RATIO > 1.0. For a bulk material 305 exhibiting such a RATIO, structural analysis and optimization software determined that by providing an integrated strengthening region 310 which provides greater E compared to the E of the bulk material 305 adjacent to a hole 315, the structural member 300 around the hole 315 can provide greater strength. Such an increase in strength is achieved without adding a separate material, such as a grommet, or without significant weight increase which results from the thickening the bulk material 305 around the hole 315.

As an example, bulk material 305 can be determined to have a RATIO equal to 1.5. The optimum location of the strengthening region 310 for such a bulk material 305 can be determined from Figure 3 (b). Figure 3 (b) illustrates a plot of optimized normalized E as a function of distance from the hole 315 for bulk materials 305 having a RATIO equal to 1.5. This figure demonstrates that a surrounding region 310 can be adjacent to the hole 315. Although Figure 3 (a) appears to have a strengthening region 310 of uniform E distribution, Figure 3 (b) clearly illustrates that an optimal strengthening region 310 can have a non-uniform E distribution in which the E of the strengthening region 310 decreases monotonically until reaching the E of the bulk material 305 as distance increases from the hole 315.

One example of a bulk material 305 having a RATIO equal to 1.5 can be an aluminum foam. As illustrative of one of the multiple materials and manufacturing process to selectively alter the density of the bulk material 305, the porosity of a foam can be selectively altered to provide regions of varied density, i. e. strengthening regions 310. Consequently, the strengthening regions 310 can be at least partially based upon the spatial porosity distribution within the bulk material 305. Generally, a decrease in the porosity of a material will increase the E of the bulk material 305, resulting in a material of greater strength. Therefore, the porosity of regions of the material 305 can be increased or decrease to achieve, respectively, a greater or lesser E. Therefore, some regions of a foam can be processed to produce relatively lower density and E when compared to the density and E of the unprocessed regions. As is known in the industry, there are a variety of manufacturing processes to locally vary porosity.

In yet another embodiment, a bulk material 405 can be determined to have a RATIO in the range of 0.75 to 1.0. Figure 4 (a) illustrates a schematic of a functionally graded exemplary structural member 400 for a bulk material 405 having a RATIO in the range of 0.75 to 1.0. For a bulk material 405 exhibiting a RATIO in the range of 0.75 to 1.0, the bulk material 405 can have multiple strengthening regions 410, such as 41 Oa and 41 Ob. Nevertheless, the invention is not limited to two strengthening regions 41 Oa and 41 Ob as the bulk material can also include other strengthening regions (not shown).

As shown in Figure 4 (a) the strengthening regions 41 Oa and 41 Ob can be located adjacent to the hole 415 and at some distance away from the hole 415.

As briefly discussed earlier, an integrated bulk material with selected regions of E different from a bulk material can be produced by a variety of available manufacturing processes. Some processes locally vary the E of a region by altering the microstructure of the bulk material to change the density of that region. A few non-exhaustive

illustrative examples of these processes include cold rolling, heat treatment, and laser annealing. Additionally, other manufacturing processes can produce an integrated bulk material with selected regions of varied E by altering the porosity of the material to effect changes in density, such as for foam materials. Also, other manufacturing processes that can selectively alter the density of particular region within a bulk material are possible, such as removing a region and filing the region with material deposited by processes including chemical vapor deposition and physical vapor deposition.

Furthermore, although the invention is generally applicable for application on the macro-scale, such as for plane and spacecraft wings, the invention is also applicable to application on the micro-scale. For example, the invention can be used to reinforce holes and deep depressions present in certain MEMS based devices, such as MEMS sensors. The MEMS device can be disposed in or on the bulk material, where the MEMS device includes the strengthening region. A non-exhaustive list of manufacturing processes to selectively effect changes in the E of particular micro-scale and nano-scale regions include, conventional electronic circuit manufacturing techniques, such as etching, photolithography, ion-implementation, deposition, ion-beam processing, and laser processing.

In accordance with the inventive arrangements, a method for forming a manufactured integral structural member is provided. The method includes providing a bulk material having at least one hole therein, the bulk material having a bulk elastic modulus. The method also includes the step of forming at least one strengthening region integrated with the bulk material having an elastic modulus which is different from the bulk elastic modulus where the strengthened region provides an increased strength proximate to the hole compared to the member having a bulk elastic modulus throughout. Additionally the step of providing a bulk material having at least one hole therein can include providing a bulk material having at least one deep depression therein.

The method can also include the step of selecting a location for the strengthening region based on a RATIO of the bulk material. If the RATIO of the bulk material is < 0.75 the location can be remote from the hole. If the RATIO of the bulk material is > 1.0 the location can be disposed along an edge of the hole. If the RATIO of the bulk material is in the range of 0.75 to 1.0, the location of at least one strengthening region can include at least one strengthening region disposed along an edge of the hole and at least one strengthening region disposed remote from the hole.

In another embodiment, the forming step can include selectively modifying density of at least one region of the bulk material to form the strengthening region. The forming step can include selectively modifying porosity of at least one region of the bulk material to form the strengthening region. The forming step can also include selectively modifying microstructure of at least one region of the bulk material to form the strengthening region. Additionally, the forming step includes the steps of selectively removing at least a portion of said the bulk material in the strengthening region and depositing additional material in the strengthening region. The forming step can include at least one selected from the group consisting of selective chemical vapor deposition and selective physical vapor deposition.

Additionally, the method can include the step of determining elastic modulus distribution to be provided to the strengthening region.

It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description as well as the examples which follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.