Materials

Monazite Deformation Twinning Research

AFRL scientists have significantly advanced the understanding of a phenomenon called deformation twinning, a major materials deformation mechanism that is particularly important at low temperatures and high strain rates. Working with industry, laboratory researchers successfully identified five deformation twin modes in monazite, a complex mineral with low symmetry. They were able to explain the existence of these modes using fundamental principles that should ultimately prove useful for the prediction of deformation twinning in more complex systems. These studies help scientists obtain the knowledge required to create better tools for analyzing the composition and application potential of minerals and other natural materials essential both to the development of national defense systems and to the research and development of dynamic new commercial products.

Posted in: Briefs, Materials
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Dielectric Coolants

The Joint Strike Fighter (JSF) is the Department of Defense's affordable next-generation strike aircraft designed to meet the needs of the Air Force (AF), Navy, Marines, and US allies. Currently in development by Lockheed Martin, the multimission, supersonic, JSF aircraft will provide all services with enhanced lethality and survivability and reduced vulnerability (see figure). The JSF's unique, multiple-variant design pushes the threshold of fighter technology far beyond current limitations. The AF variant of the technology takes multirole fighter performance to new levels, offering improved stealth, increased range on internal fuel, and advanced avionics. The JSF's advanced avionics, as well as its flight control, target acquisition, and other sophisticated electronic systems rely on high-performance coolants to ensure proper operation. Designers employ dielectric coolants to dissipate heat from high-energy electronic components and therefore consider these fluids critical to aircraft operation and safety.

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New Capability to Characterize the Mechanical Properties of Explosive Materials

Improved targeting accuracy and the long-standing desire to minimize collateral damage are causing current and future munitions to become much smaller. As munitions size decreases, the explosive materials packed within bomb cases begin to carry a significant portion of the structural loads experienced by the warhead. In an ongoing program effort to determine the mechanical properties of explosives and other energetic materials, scientists at AFRL's High Explosives Research and Development (HERD) facility (Eglin Air Force Base, Florida) acquired a miniaturized split Hopkinson pressure bar (MSHPB) (see Figure 1). Designed and built by Mr. Clive Siviour under the guidance of Drs. John Field, Bill Proud, and Stephen Walley (of the United Kingdom's University of Cambridge, Physics and Chemistry of Solids Group), the MSHPB is capable of strain rates up to 105 s-1 in material samples. AFRL's European Office of Aerospace Research and Development sponsored the project.

Posted in: Briefs, Materials, Materials properties, Hazardous materials
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Ceramic Matrix Composites Research

AFRL scientists characterized and evaluated the high-temperature mechanical behavior of fiber-reinforced ceramic matrix composite (CMC) materials used in aerospace structural applications. Researchers examined four principal characteristics of a porous matrix composite that General Electric developed for the aerospace industry. Their evaluations resulted in an increased understanding of the materials and their potential for applications in military and commercial aerospace products.

Posted in: Briefs, Materials, Research and development, Ceramics, Composite materials
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Characterizing Mechanical Properties at the Microscale

Scientists from AFRL, Pratt & Whitney Aircraft, and General Electric Aircraft Engines, working under the Defense Advanced Research Projects Agency's Accelerated Insertion of Materials (AIM) program, have invented a new method for characterizing the single-crystal properties of aerospace alloys using micron-size test samples. The research team based the new characterization method on focused ion beam (FIB) microscopy and a commercially available nanoindentation-based test instrument. Further development of these methodologies, in conjunction with their continued integration with simulation methods devised under the AIM program, will enable engineers to consider local changes in material microstructure and their effect on properties in the design process. The integration of advanced mechanical property measurements, materials representation, and simulation methods will dramatically decrease the time required for new materials insertion and will transform microstructure into a design variable for engineered systems. These advancements will directly benefit combat systems and readiness.

A deformed single crystal of pure nickel after measurement of critical resolved shear stress under single-slip conditionsA primary challenge to the rapid insertion of new materials into the design cycle is the need to understand both the intrinsic properties of an engineering material at the microscopic level and the influence of defects on these properties at the macroscopic level. Historically, scientists have been unable to develop model parameters or validate continuum materials behavior models that are based upon discrete microstructural information. Continuum crystal plasticity models are at the frontier of techniques that incorporate direct microstructural information. However, a major deficiency of these models is the need to obtain required input information: the single-crystal mechanical properties of individual grains, or microconstituents. Acquiring this information is particularly difficult when such parameters must reflect the subtleties of material process history or the local influence of material defects.

Under the AIM program, AFRL researchers have sought to measure the single-crystal mechanical properties, such as the critical resolved shear stresses and strain hardening rates, of micro- and nanoscale samples extracted from relevantly processed structural alloys (see figure). Scientists are currently developing direct methods to automatically and rapidly characterize both the mechanical response of relevant microstructural elements and the stochastic nature of material property variation to establish the mechanical properties of a material's representative volume elements (RVE).

It is essential for scientists building continuum models to quickly determine the mechanical properties of RVEs in order to quantify the inherent variability in material properties, the observed variability in experimental measurements, and the uncertainty in predicted properties. They can then establish "confidence metrics" for the data they incorporate into the designer's knowledge base. Without such confidence, scientists can add new materials (or old materials in new applications) to the knowledge base only after extremely difficult and costly testing.

The new characterization method uses FIB milling to isolate and prepare single-crystal mechanical test specimens from individual grains, or precipitates, of a conventionally processed alloy. Scientists then move the prepared specimens to a conventional nanoindenter device outfitted with a flatpunch indentation tip. The nanoindenter imposes uniaxial compression on the microsamples and records highfidelity load-displacement measurements as the samples deform. With the development of this novel mechanical behavior test capability, researchers now envisage sampling the local mechanical effects of material microstructure and statistically incorporating these results in improved constitutive response surfaces, which could be used in simulations of critical component features.

Dr. Dennis M. Dimiduk, Dr. Michael D. Uchic, and Dr. Peter S. Meltzer (Anteon Corporation), of the Air Force Research Laboratory's Materials and Manufacturing Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm. Reference document ML-H-04-10.

Posted in: Briefs, Materials
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Fire-Resistant Hydraulic Fluid

An AFRL-developed fire-resistant hydraulic fluid recently completed a B-52 flight test, and based on successful test results, systems engineers from Oklahoma City Air Logistics Center (OC-ALC) will adopt the fluid (MILPRF- 87527) for use in over 90% of the aircraft's hydraulic systems. OC-ALC engineers will conduct further tests to determine whether they can also convert the hydraulic systems controlling the B-52's landing gear and wingtip protection struts to the fire-resistant fluid. AFRL expects the improved fluid's higher flash point and reduced flammability to increase the B-52 aircraft's survivability and overall operational safety. Further, the fluid's associated thermal stability measurements and fluid film thickness data indicate it performs well over extended periods of time in hightemperature environments and in temperatures as low as -65°F.

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Predicting the Composition of Metallic Glasses

AFRL scientists made significant progress in developing bulk metallic glasses to improve the durability and performance of aerospace components. They also successfully created working scientific models that can predict the composition of new metallic glasses, a capability that helps researchers determine in advance whether a particular glass can be manufactured in bulk form. As a direct result of their effort, researchers recently discovered several new bulk metallic glasses. Their work also led to the successful development of a new technique to illustrate the topology of amorphous (noncrystalline) metal alloys.

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Innovative Processes Strengthen F-15E Vertical Stabilizers

AFRL materials engineers resolved a potentially serious problem that affects the operational life of F-15E vertical stabilizers. Working with engineers from Warner Robins Air Logistics Center (WR-ALC), Georgia, they successfully developed, demonstrated, and transitioned an adhesive bonding process and nondestructive inspection (NDI) technique that ensure the stiffening doublers attached to both sides of the aircraft's two vertical stabilizers remain adequately secured (see figure on next page). Successful transition of these innovative methods restores the operational life of the vertical stabilizers.

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