Dynamic compressive strength and mechanism of failure of Al-W fiber composite tubes with ordered mesostructure

Significance Statement

Accurate component selection and mesostructure in metal composites affect their properties to satisfy particular requirements for several applications subject to dynamic deformation, fragmentation and possible reaction. For instance, a combination of high density and high strength is necessary for penetrators. A good example for reinforcement is tungsten fibers, which have high density and strength, and can be implemented to reinforce matrix metals to increase their density.

Tungsten fibers can be used to reinforce aluminum matrix for various applications. A significant difference in the acoustic impedances between tungsten and aluminum makes their composites desirable for acoustic filters. When exposed to quasi-static compression force, aluminum matrix composites reinforced with ductile tungsten fibers configured in the axial direction exhibit buckling failure mode which is different from kinking for aluminum matrix composites reinforced with fragile alumina fibers.

University of California San Diego researchers Professor Vitali Nesterenko, Professor Kenneth Vecchio and Dr. Po-Hsun- Chiu investigated the dynamic characteristics of aluminum alloy-tungsten fibers using split Hopkinson pressure bar. In their study, the composite tubes had periodic configuration of tungsten fibers in axial and hoop arrangements processed using Hot and Cold Isostatic Pressing methods. Their work is now published in International Journal of Impact Engineering.

The researchers developed a novel processing method allowed to synthesize fully dense aluminum tungsten tubes with a high content of tungsten fibers oriented in axial and hoop directions. However, aluminum alloy lost its hardness after Hot Isostatic Pressing. Therefore, additional heat treatment was necessary to restore its strength.

The developed method with specific pressure, soaking time, temperature and cooling under pressure helped the authors to overcome tungsten fragmentation during pressure treatment, realize the required strength and density in the composites, curtail aluminum and tungsten reactions and maintain tungsten fibers periodic alignment in tubular test pieces.

They found that samples with high hardness of aluminum matrix and samples without heat treatment attained their highest strength at the same strains. Critical strains for the fracture in quasi-static conditions were closer to the corresponding values in dynamic tests. For both conditions, stresses dropped with strains after reaching their maximum. This could be attributed to identical fracture mechanism in the two conditions irrespective of strain rate differences between them.

Specimens with higher aluminum matrix hardness posted a higher compressive (750MPa) strength although they deformed at a lower strain rate. Samples without heat treatment posted a compressive strength of 600MPa. Measured micro-hardness for heat treated specimens almost doubled compared to samples without heat treatment. Increasing the micro-hardness of the aluminum matrix after heat treatment didn’t yield a similar strength increase indicating that the principle failure mechanism was due to tungsten fibers, whose properties were not affected by the heat treatment.

This study observed the dynamic strength of tested specimens was quite high as compared to measured values in quasi-static tests indicating that compressive strength of composites specimens was sensitive to strain rate. Strain rate sensitivity was attributed to the tungsten fibers.

In contrast, the observed dynamic failure was attributed to buckling of tungsten fibers which were configured in the axial direction. This was initiated by the initial fracture of the circumferential fibers. The researchers also found that the instability of the tungsten fibers was similar for both dynamic and quasi-static tests: kinking at one face of the specimen in the direction of the incident bar and buckling in the central section.

failure-Al-W-fiber-composite-tubes-ordered-mesostructure-Advances-in-Engineering

About The Author

Po-Hsun Chiu is a post-doctoral researcher at University of California, San Diego in the Department of Mechanical and Aerospace Engineering. He received his B.S. from National Cheng Kung University in Taiwan and Ph.D. from University of California, San Diego both in Materials Science and Engineering. He has 18 publications including 8 refereed journal articles. He gave presentations in 4 conferences and received American Physical Society- Shock Compression of Condensed Matter travel award in 2013. He has expertise in high pressure, high temperature processing techniques – Hot and Cold Isostatic Pressing and in dynamic testing using Split Hopkinson Pressure Bars, Drop Weights Test, explosively driven Thick-Walled Cylinder test, Explosively Driven Fragmentation Test, and in material characterization techniques using Optical Microscope, Scanning Electron Microscope, and X-ray Diffraction analysis. His research interests focus on processing and deformation mechanisms and fragmentation of composite materials at high strain rate.

About The Author

Professor Vecchio completed his undergraduate education in 1983 at Carnegie-Mellon University in Metallurgical Engineering and Materials Science and his Master of Science degree in Metallurgy and Materials Science at Lehigh University in 1985. Professor Vecchio received his Ph.D. in Materials Science and Engineering from Lehigh University in 1988. His Ph.D. research made use of a combination of electron microscopy and fatigue/fracture mechanics. In 1988, Professor Vecchio joined the Department of Applied Mechanics and Engineering Sciences at the University of California at San Diego as an assistant professor, and is now full professor in the Department of Mechanical and Aerospace Engineering at UC San Diego. For ten years he served as the Director of the Electron Optics and Microanalysis Facility for the Jacobs School of Engineering at UCSD. In 2000, Professor Vecchio was the recipient of the Marcus Grossman Young Author Award from ASM-International and elected Fellow of ASM International in 2009. His early research focused on structure-property relations in advanced materials with emphasis on applications in dynamic loading events for both civilian and defense-related fields.

Prof. Vecchio has a strong interest in the use of computational materials science, material informatics, and materials analytics as an accelerated path to new materials development. Professor Vecchio led the creation of the Department of NanoEngineering at UCSD, and in 2007 was named its first Chair. He has lead an aggressive recruiting effort to attract some of the top scientist and engineers to join the Department of NanoEngineering, and has spearheaded the formation of its graduate and undergraduate education programs.

About The Author

Vitali F. Nesterenko is a Distinguished Professor and Chair of the Department of Mechanical and Aerospace Engineering, University of California, San Diego. He received his Doctor in Physics and Mathematics in 1989 from Russian Academy of Sciences. He joined UCSD in 1994 as visiting Research Scientist and in 1996 as faculty. Professor Nesterenko’s research interests include strongly nonlinear wave dynamics of low dimensional metamaterials (particulate and discrete systems with strongly nonlinear interaction).

He pioneered the concept of “sonic vacuum” and theoretically and experimentally discovered a new strongly nonlinear solitary wave. His research interests also include micromechanics of powder deformation under dynamic and quasi-static loading; shear instability and fragmentation of heterogeneous materials in dynamic conditions; shear induced chemical reactions in condensed materials; shock/blast mitigation using soft heterogeneous materials; mechanics of densification and sintering of pressure assisted advanced ceramics and alloys.

He and his co-authors successfully processed and tested high-gradient heterogeneous material based on Ti alloy for ballistic applications, developed high pressure processing method for bulk magnesium diboride and solenoids with critical current suitable for size scaling and complex shapes. Recently reactive materials based on Al-W system were successfully processed and tested. He is the author of 152 papers in archival Journals and of the book “High-Rate Deformation of Heterogeneous Materials” published by Springer Verlag, NY.

He is a Fellow of the American Physical Society elected for “pioneering contribution to strongly nonlinear wave propagation in granular materials, through the discovery of a new solitary wave, and to shock (localized shear) mesomechanics in porous and heterogeneous media”.

Reference

Po-Hsun Chiu1, Kenneth S. Vecchio2, and Vitali F. Nesterenko1,3. Dynamic compressive strength and mechanism of failure of Al-W fiber composite tubes with ordered mesostructured. International Journal of Impact Engineering, volume 100 (2017), pages 1-6.

Show Affiliations
  1. Materials Science and Engineering Program
  2. Department of NanoEngineering
  3. Department of Mechanical and Aerospace Engineering, University of California, San Diego, CA 92093-0411, USA

 

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