RT&L FOCUS AREA(S): Hypersonics TECHNOLOGY AREA(S): Materials, Weapons OBJECTIVE: Develop an innovative Kolsky tension bar system to characterize the dynamic tensile behavior of brittle materials at high strain rates. DESCRIPTION: Tension-induced dynamic fragmentation is often observed on the back side of the penetration/perforation targets as a result of stress wave reflection from the free surface. Tensile/spall damage generated during the fragmentation process significantly compromises the penetration resistance of the target material, which leads to reduced protection capability. This unique challenge is further exacerbated by the lack of a reliable dynamic tensile characterization system for brittle materials. Over the past several decades, some efforts have been made to develop a Kolsky bar-based dynamic tensile testing technique using the ideas of spall tension [1,2], split tension [3,4], and direct tension [5,6]. A common draw back of all these techniques is that the specimens rarely reach the state of stress equilibrium or constant strain-rate deformation at the time of fracture due to extremely small tensile failure strain for most of the brittle materials. The goal of this topic is to develop a Kolsky tension bar system capable of extending the tensile strain of brittle specimens during dynamic tests through the implementation of a pre-compression mechanism. This idea leverages the relatively large compressive failure strain for brittle materials to effectively provide additional time for the tensile specimens to achieve dynamic stress equilibrium and constant strain-rate deformation. The proposed new Kolsky tension bar technique needs to provide a wide range of adjustable pre-compressive stresses to accommodate different types of brittle materials (high-strength concrete, armor glass/ceramics, composites, etc.) for the Army's applications. PHASE I: Demonstrate the design concept for the new Kolsky tension bar system, and the unique mechanism of applying/controlling the static pre-compressive stress to tensile specimens. Develop a reliable specimen attachment technique to ensure 1. rigid, steady specimen/bar connection, and 2. smooth and continuous transition from compressive to tensile loading conditions. Demonstrate the feasibility of tailoring the dynamic loading wave to achieve stress equilibrium and constant strain rate deformation for the test specimen. Deliver a report documenting the research and development efforts along with a detailed description of the proposed prototype Kolsky tension bar. Phase I focuses on the conceptual design of the prototype. Only the most effective and promising design that addresses all the aforementioned requirements will be given consideration for a Phase II award. PHASE II: Manufacture a prototype of the Kolsky tension bar system proposed in Phase I. Demonstrate the pre-compression mechanism at several designated stress levels, and the loading wave tailoring capability aimed at achieving desired dynamic stress equilibrium and constant strain rate deformation. Demonstrate the specimen attachment technique that is capable of handling the rapid transition from compression to tension. Phase II will also require live demonstration tests using the prototype system. The materials of choice are fiber-reinforced and non-reinforced high-strength concrete, or other brittle material with limited tensile strain capacity and clear applications for the Army. These materials are chosen due to their extremely low tensile failure strength (and strain) compared to other brittle materials. Reporting and documentation: (1) the CAD drawings, operation manual, and safety guideline for the prototype system; (2) the verification procedure that demonstrates the experimental results meet the desired testing conditions. In addition to the reporting and documentation, Phase II also requires delivery of a well-tuned, fully functioning pre-compression Kolsky tension bar system. PHASE III DUAL USE APPLICATIONS: The development of a pre-compression Kolsky tension bar system benefits a broad range of military applications such as vehicular/body armor design, testing of high-strength/ultra-high-strength concrete for impact and blast protection, development of composite gun barrel for light-weight gun system, etc. This new technique also has potential to satisfy the needs of academic research on dynamic tensile behavior of other emerging brittle materials, and clear applications for the Army through the various materials R&D efforts in support of several modernization priorities through the CFTs. REFERENCES: Klepaczko JR, Brara A (2001) An experimental method for dynamic tensile testing of concrete by spalling. Int J Impact Eng 25:387-409 Erzar B, Forquin P (2010) An experimental method to determine the tensile strength of concrete at high rates of strain. Exp Mech 50:941-955 John R, Antoun T, Rajendran AM (1992) Effect of strain rate and size on tensile strength of concrete. In: Shock Compression of Condensed Matter 1991 (S.C. Schmidt, R.D. Dick, J.W. Forbes, D.G. Tasker, eds.), Proceedings of the American Physical Society Topical Conference, pp. 501-504 Ross CA, Tedesco JW, Kuennen ST (1995) Effects of strain rate on concrete strength. ACI Mat J 92:37-47 Birkimer DL, Lindemann R (1971) Dynamic tensile strength of concrete materials. ACI J, January 47-49 Goldsmith W, Sackman JL, Ewert C (1976) Static and dynamic fracture strength of Barre granite. Int J Rock Mech Min Sci & Geomech Abstr 13:303-309 KEYWORDS: split Hopkinson bar, brittle material, Kolsky tension, dynamic tensile behavior
