IMR: Development/Acquisition of a Mixed-Mode Fracture Testing Instrument for Research and Education in Adhesion Science

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

Adhesive bonding has become an essential means for joining components in a wide range of applications, including automotive, aerospace, civil infrastructure, biomedical, and microelectronic fields. Satisfactory performance of these bonds requires retaining structural integrity over the service life where they are often subjected to dead loads, impact, and/or fatigue, while exposed to environmental challenges such as temperature and humidity. A significant need exists for improved understanding of the fracture resistance of bonded joints, and how these properties can be incorporated into meaningful and robust design procedures for bonded structures. Joints often fail by fracture propagating under some combination of mode I (opening), mode II (forward shear), and mode III (tearing) loading. Because fracture energies depend on mode mixity, comprehensive failure envelopes for a range of mode mixities are generated by conducting pure and mixed mode tests. This proposal seeks funding to develop a unique instrument capable of easily varying the mode mix for fracture testing of adhesively bonded beam specimens. Currently, different mode mixities are achieved by using different test configurations, increasing complexity and obscuring meaningful comparisons. Several fixtures have been developed to vary the mode mixity over a limited range for a given specimen geometry, but these techniques are cumbersome to use and limited in their applicability. These complications are a major hindrance to developing an improved understanding of the effects of mode mixity on fracture properties and locus of failure. These limitations will be largely overcome by the unit proposed herein, which offers significant potential for new scientific insights gained through use of a convenient and efficient test method relevant to many fields. The proposed instrument will be built around a customized load frame complete with dual actuators, load cells, displacement transducers, controllers, and data acquisition system. By independently adjusting the magnitude and phase of the actuators, any desired fracture energy and mode mixity may be applied to commonly used, ASTM standard, double cantilever beam specimens. Because the mode mixity can be easily changed during a test, one can investigate the effects of mode mix, even as a debond propagates within a single specimen. The instrument will have unique scientific and engineering capabilities for characterizing mixed mode fracture, developing fracture envelopes, and investigating the complex interactions between stress state and spatially varying material properties and how they affect locus of failure. The unit is expected to be useful in many areas of adhesive utilization and can also be readily extended to other disciplines, such as the study of interlaminar properties of composites or other laminated materials, important for many aerospace, automotive, and infrastructure applications. In addition to the scientific merits, companies producing or using adhesives and composite materials for many industrial fields are expected to gain from the insights that can conveniently be obtained with this simple unit. Because the specimens are already an ASTM standard and are easily fabricated, barriers will be reduced for the use and broader adoption of this technology. In essence, we will be able to gain a great deal of additional information about the material performance using specimens that are already in common use. A graduate student and an undergraduate student will participate in the development effort, obtaining significant experience in instrument design, construction, and calibration, along with computer interfacing and programming skills. The unit will be used by a diverse group of students and faculty associated with our interdisciplinary Center for Adhesive and Sealant Science. This unique research capability will nicely complement the wide array of equipment we have available for characterizing adhesion and composite properties, and is expected to attract significant interest from current sponsors as well as potential sources of future funding, including industry and government laboratories. The unit will offer a very flexible instrument to enhance the research of mechanical properties of adhesives, and also provide useful new insights related to polymer and surface science in this interdisciplinary field of adhesion. %%% Adhesive bonding has become an essential means for joining components in a wide range of applications, including automotive, aerospace, civil infrastructure, biomedical, and microelectronic fields. Satisfactory performance of these bonds requires retaining structural integrity over the service life where they are often subjected to dead loads, impact, and/or fatigue, while exposed to environmental challenges such as temperature and humidity. A significant need exists for improved understanding of the fracture resistance of bonded joints, and how these properties can be incorporated into meaningful and robust design procedures for bonded structures. Joints often fail by fracture propagating under some combination of tensile and shear mode loadings. Because fracture energies depend on mode combinations, comprehensive failure envelopes for a range of mode combinations are generated by conducting pure and mixed mode tests. These provide important understanding of the failure process, and avoid non-conservative design space. This proposal seeks funding to develop a unique instrument capable of easily varying the mode mix for fracture testing of adhesively bonded beam specimens. Currently, different mode combinations are achieved by using different test configurations, increasing complexity and obscuring meaningful comparisons. Several fixtures have been developed to vary the mode combination over a limited range for a given specimen geometry, but these techniques are cumbersome to use and limited in their applicability. These limitations will be largely overcome by the unit proposed herein, which offers significant potential for new scientific insights gained through use of a convenient and efficient test method relevant to many fields. The proposed instrument will be built around a customized load frame complete with dual actuators, load cells, displacement transducers, controllers, and a data acquisition system. By independently adjusting the magnitude and phase of the actuators, any desired fracture energy and mode mix may be applied to commonly used, double cantilever beam specimens. The instrument will have unique scientific and engineering capabilities for characterizing mixed mode fracture for design and scientific applications. The unit is expected to be useful in many areas of adhesive utilization and can also be readily extended to other disciplines, such as the study of interlaminar properties of composites or other laminated materials, important for many aerospace, automotive, and infrastructure applications. In addition to the scientific merits, companies producing or using adhesives and composite materials for many industrial fields are expected to gain from the insights that can conveniently be obtained with this simple unit. Because the specimens are already an American Society for Testing and Materials standard and are easily fabricated, barriers will be reduced for the use and broader adoption of this technology. In essence, we will be able to gain a great deal of additional information about the material performance using specimens that are already in common use. The development of the device will promote the training and research effort in adhesion science, scientific instrument design and programming for a diverse group of undergraduate and graduate students associated with faculty in our interdisciplinary Center for Adhesive and Sealant Science.