Abstract:
Impact resistant components and methods of protecting structures from impacts. The components are interposed between a potential point of impact and a structure to be protected. They comprise a shape memory alloy (SMA) exhibiting pseudoelastic behavior, and having a high strain to failure.

Description:
[0001]    This application claims benefit of priority of U.S. Provisional Application No. 60/353,383, filed Feb. 1, 2002, which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention is directed to novel materials and methods for increasing energy absorption in materials to improve impact resistance. The materials of the invention are expected to be particularly useful in the manufacture of impact-resistant aircraft components.  
         BACKGROUND OF THE INVENTION  
         [0003]    Both military and civil passengers and crew are demanding higher levels of safety during air travel. For the military, the focus is on achieving mission objectives while protecting crew in the event of a crash. The seat must isolate the occupant from crash loads up to levels determined to be survivable. The seats must be designed with a minimum of weight and cost while meeting these performance targets.  
           [0004]    Similarly, in civil aircraft, the standards for crash safety are also improving. The FAA has increased the required survivable crash load levels in recent years to support the agency&#39;s efforts to reduce the overall risk to aircraft passengers. One way to support this goal is to increase the survivability for the occupants in the event of an accident.  
           [0005]    To improve the survivability of aircraft and other vehicular accidents, a wide variety of impact resistant designs have been developed. For example, U.S. Pat. Nos. 4,509,621 to Desjardins et al., 4,523,730 to Martin, 4,718,719 to Brennan, 4,861,103 to Vallee, 4,911,381 to Cannon et al., 5,152,578 to Kiguchi, 5,273,240 to Sharon, 5,730,492 to Warrick et al., and 5,813,649 to Peterson et al. all discuss mechanical designs for resisting impact by absorbing crash energy. These designs can be further enhanced by the selection of appropriate materials to maximize energy absorption during impact.  
           [0006]    Strain energy absorption is measured as the area under a stress-strain curve to fracture. Generally speaking, highly ductile materials (i.e., materials with high failure strains) are preferred for impact absorption applications, because of their high strain energy absorption. Reversible elastic behavior makes negligible contribution to strain energy absorption for most metals. The elastic regime accounts for most of the energy absorption for a few other materials in use for impact absorption, such as carbon-reinforced polymer composites, but breaking strength can be more difficult to predict for these materials, which may be more sensitive to minor defects.  
           [0007]    A need still exists for a strain energy absorbing material having a significant reversible deformation regime, as well as a higher total energy absorption than conventional materials.  
         SUMMARY OF THE INVENTION  
         [0008]    In one aspect, the invention comprises an energy-absorbing component, comprising a structural member comprising a shape memory alloy (SMA), wherein the SMA exhibits pseudoelastic behavior in response to impact loading. The component is part of a structure subject to impact loading, such as an aircraft, an automobile, a mine-resistant vehicle, a down-hole drill, a blast shield, or a building. The component may, for example attach a seat or an instrument to an aircraft. The SMA may have a strain to failure of 50%, 60%, 70%, 80%, or 90%. The reversible pseudoelastic strain may be at least 3%, at least 5%, at least 10%, or at least 15%. The material properties of the SMA may be determined by a secondary anneal of about 550° C.-800° C., which may follow cold-working of the SMA.  
           [0009]    In another aspect, the invention comprises a method of protecting a structure from impact loading, by interposing a structural member between the, structure and a potential point of impact, where the structural member comprises a shape memory alloy (SMA), that exhibits pseudoelastic behavior in response to impact loading. The structure may be, for example, an aircraft, an automobile, a mine-resistant vehicle, a down-hole drill, a blast shield, or a building. The component may, for example attach a seat or an instrument to an aircraft. The SMA may have a strain to failure of 50%, 60%, 70%, 80%, or 90%. The reversible pseudoelastic strain may be at least 3%, at least 5%, at least 10%, or at least 15%. The material properties of the SMA may be determined by a secondary anneal of about 550° C.-800° C., which may follow cold-working of the SMA. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0010]    The invention is described with reference to the several figures of the drawing, in which,  
         [0011]    [0011]FIG. 1 is a typical stress-strain curve for a shape memory alloy;  
         [0012]    [0012]FIG. 2 is a chart of specific strain energy absorption for several materials;  
         [0013]    [0013]FIG. 3 compares the stress-strain curves of 4130 steel and Nitinol; and  
         [0014]    [0014]FIG. 4 is a micrograph of pre-test and post-test deformation of martensitic sheet test articles with and without additional heat treatment. 
     
    
     DETAILED DESCRIPTION  
       [0015]    According to the invention, pseudoelastic memory alloy materials are used to improve the energy absorption of mechanical components used in structures subjected to impact loads. The materials of the invention may be used in any structure potentially subject to impact loading (e.g., mine resistant vehicles, blast protection shields, personnel armor, drilling operations subject to water-hammer disturbance, buildings subject to seismic activity, automobiles, and trains), but are expected to find particular utility in aerospace applications, because of their very high specific strain absorption (strain absorption divided by density).  
         [0016]    Shape memory alloys (SMAs) are metal alloy materials that have the ability to return to their original shape after being deformed. All SMAs have two distinct crystal structures, or phases, with the phase present being dependent on the temperature and the amount of stress applied to the SMA. The two phases generally are martensite, which exists at lower temperatures, and austenite at higher temperatures. The exact structure of these two phases depends on the type of SMA. The most commonly used type is called Nitinol. Nitinol is an alloy of two component metals, nickel (Ni) and titanium (Ti), which are mixed in an approximate ratio of 55% by weight Ni and 45% by weight Ti, and annealed to form a part in the desired shape.  
         [0017]    Shape memory alloys possess two material properties that work together to provide shape memory. The first material property is an austenite to martensite transition in the SMA. This is a solid-to-solid phase transition from an austenite phase with high symmetry (such as a cubic molecular structure) to a martensite phase with lower symmetry (such as tetragonal or monoclinic structures). The second property of a shape memory alloy is the ability of the low-symmetry martensite structure to be deformed by twin boundary motion. A twin boundary is a plane of mirror symmetry in the material. If the twin boundary is mobile, as in certain martensite structures, the motion of the boundary can cause the crystal to rearrange and thus accommodate strain.  
         [0018]    Pseudoelasticity (also known as superelasticity) uses the same deformation mechanisms as shape memory, but occurs without a change in temperature. Instead, the transformation is induced by stress alone. Applied stress can overcome the natural driving force that keeps the material at equilibrium in the austenite phase. By applying stress to the material, it can be converted into the martensite phase, and the crystal structure will strain to accommodate the applied stress. When this stress-energy is greater than the chemical driving force of stabilization in the austenite phase, the material will transform to the martensite phase and be subject to a large amount of strain. When the stress is removed, the material returns to its original shape in the austenite phase, since martensite is not thermodynamically stable above the transition temperature in the absence of stress. This superelastic behavior is fully reversible and does not require any change in temperature.  
         [0019]    Superelastic Shape Memory Alloys exhibit a stress-induced phase transformation resulting in a constant stress plateau with corresponding large strain amplitude. In the data shown in FIG. 1, the plateau occurs at 20 ksi and extends from 1% to 7% strain. Once the phase transformation is complete, the material again behaves elastically and then yields at approximately 90 ksi and 12%. Perhaps the most significant attribute is the elongation at failure, which can exceed 60%.  
         [0020]    For impact resistance (crashworthy) designs, strain energy absorption can be a useful metric for evaluating structural materials. The strain energy metric incorporates both the ductility and the strength in one metric. For aerospace applications, density is also critical. Dividing the strain energy by the density yields the specific strain energy.  
         [0021]    The chart in FIG. 2 compares the specific strain energy of Nitinol with some common high performance structural materials: 4130 steel (180 ksi strength), 2024-T4 aluminum, and a carbon fiber reinforced polymer (CFRP). The bars illustrate the elastic energy (lower section) and the plastic energy (upper section) as well as the total energy (sum). The elastic energy absorbed by the conventional metal alloys is too small to be visible on the chart. Nitinol&#39;s elastic behavior stands out, exceeding even the much lighter CFRP, as a result of its superelasticity. However, it is in the total (elastic+plastic) energy that the Nitinol really excels. Its combination of strength and ductility can exceed that of the other materials by more than a factor of 5.  
         [0022]    We have developed a highly deformable Nickel Titanium (NiTi) alloy with potential for a variety of applications. The development has focused on optimizing the material&#39;s inherent strain energy absorbing capability for enhancing the crashworthiness of aircraft seats. The NiTi is being integrated in the seat support structure to reduce the loads on an occupant in a crash, thus enhancing survivability. NiTi is known for its unique superelastic and shape memory capabilities. The high deformation capability adds a new dimension to the functionality of this material.  
         [0023]    [0023]FIG. 3 illustrates the unique high-elongation behavior achieved with specialized material processing. As much as 60% elongation or more can be attained. The NiTi behavior also differs significantly from a typical high strength steel alloy in the way it elongates. The steel will neck down with large deformation occurring in the local region of the necking, where failure eventually occurs. In contrast, the deformation in the NiTi is uniform along the entire length of the beam. This means that energy can be absorbed uniformly throughout a volume of material and that high deformations can be controlled more uniformly.  
         [0024]    The stress-strain plot also shows the remarkable differences between the NiTi and a typical high-strength steel alloy. The total strain energy (area under curve) is much greater for the NiTi, as a result of the high deformation. Further, high strain rates do not appear to significantly degrade the energy absorbing capability of these materials.  
         [0025]    The ductility of NiTi SMAs can be further enhanced by cold-working and/or annealing processes.  
         [0026]    Cold-Working  
         [0027]    The cold-working process is used to enhance the properties of the alloy after it is hot-formed. The microstructure of the alloy changes during cold-working, making the grain size smaller. Reducing grain size or refining the alloy improves both strength and ductility. This is why wrought forms of the material perform better than cast forms. In order to achieve the desired strength and ductility for this application, the material is preferably cold worked and heat treated.  
         [0028]    Cold work introduces dislocations in the material and causes work-hardening. Cold working is also directional—properties are enhanced primarily along the axis that the material is being worked (e.g., rolled or drawn). As the material is cold-worked, the material will eventually reach a point where further deformation will cause fracture. In order to cold work further, the material must first be annealed. With cold rolling operations, a 30% area reduction is a typical limit before annealing. Cold working in combination with proper heat treatment will set the substructure of the material that will define the mechanical behavior. Texture is also used to describe the material microstructure, referring to the orientation of planar crystals and the direction of the crystal axes. For example, cold drawing of wire aligns the crystals and maximizes strength along the wire axis.  
         [0029]    Secondary Annealing  
         [0030]    DiCarlo, et al., have described in U.S. Pat. No. 6,106,642, incorporated by reference herein, a process for heat treating Nitinol to improve the flexibility of arterial stents. We have used this process to produce a wire showing over 70% strain at break, significantly higher than a standard wire. The secondary annealing will be included in the primary test matrix as a parameter for evaluation.  
         [0031]    Typically in the processing of NiTi, the material is cold worked and then fully annealed at 700-750° C. to cause recrystallization. New crystals form and the grain size gets smaller and more oriented. Then a final anneal is done for straightening or shape setting at a lower temperature, around 500° C. for 1-5 minutes. This is a recovery process in which dislocations do not recrystalize but instead line up and form subgrains. A stable network of dislocations is formed, referred to as a “recovery structure”. This yields good transformation properties. DiCarlo et al. disclose a secondary anneal, heat treating at up to 750° C. AFTER doing the shape set.  
         [0032]    According to our invention, by using cold-working and/or a secondary anneal, Nitinol having the highest possible failure strain is produced. This material can be incorporated into many existing designs for energy-absorbing components, such as those disclosed in the Background of the Invention above. Because the energy absorption for reversible (pseudoelastic) deformation is fairly large for this material, moderate impacts (e.g., those experienced during hard landings or severe turbulence) that would require replacement of many energy-absorbing components made of conventional materials will not require replacement of the materials of the invention. In addition, these materials can accommodate much greater strains in the elastic/plastic regime, and thus can absorb substantially more crash energy than would be possible using conventional metal or polymer composite materials. Further, structural integrity can be maintained under more severe conditions, resulting in greater crashworthiness, because of the high absolute strength at failure of these materials.  
       EXAMPLES  
       [0033]    We have compared superelastic and martensitic alloys with the baseline material of 4130 steel (a chromium-molybdenum steel typically used for pressure vessels and for aircraft structural components) by tensile testing of sheet specimens. The alloys were obtained from Shape Memory Applications, Inc. (San Jose, Calif.), and have the properties shown in Table 1.  
                           TABLE 1                               Austenite Peak   Active Austenite       Alloy   Composition   Temperature   Temperature                   superelastic (S)   55.8% Ni,   −15-5° C.   10-20° C.           balance Ti       martensitic (M)   55.1-55.5% Ni,    45-95° C.   45-95° C.           balance Ti                  
 
         [0034]    [0034]FIG. 4 is a micrograph showing pre- and post-test deformation of the martensitic (shape memory) specimens under standard treatment and under high elongation treatment. The measured data are shown in Table 2.  
                                                     TABLE 2                                   Specific   Improvement           Elongation   Strength   Energy   over Baseline       Material   (%)   (ksi)   (ft-lbs/in)   (%)                                Baseline 4130   8.3   169   3,570   0       steel sheet       S alloy sheet   11.0   102   2,830   −21       M alloy sheet   23.9   124   7,850   +120       Improved S alloy   28.3   107   8,660   +142       sheet       Improved S alloy   58.0   114   19,300   +440       wire       Improved M alloy   60.2   120   19,300   +440       sheet       Improved M alloy   74.3   147   26,200   +630       wire                  
 
         [0035]    The “improved” superelastic and martensitic sheets were subjected to two iterations of 30% cold work (with an anneal in between), and then were secondarily annealed at 650° C. for 10 minutes. The “improved” superelastic and martensitic wire were annealed in the as-drawn condition at 700° C. for 10 minutes. It will be seen that while the martensitic (shape memory) alloy was substantially better than 4130 steel even in the original state, the treatment of the invention dramatically further improved performance.  
         [0036]    Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.