David A. Dillard - US grants

A new technique to accurately measure interfacial energies between arbitrary materials systems will be studied. The technique relies on a soft structure (a bent elastica or a pressurized blister) for the measurements, instead of the commonly used soft material. Solutions for a bent elastica brought into contact with a rigid substrate will be modified to account for the perturbations which result from weak interfacial attractions. Experimental large-scale prototypes for the configurations well be constructed and tested to evaluate the techniques and validate the mathematical models. Small prototypes will be compared with values obtained from the Johnson-Kendall-Roberts (JKR) technique for some of the more limited materials systems for which this technique is applicable.

NSF PROPOSAL # 0122124, ADVANCED REPLACEMENTS FOR MECHANICAL FASTENERS IN HOUSING CONSTRUCTION FOR HIGH WIND ZONES

ABSTRACT

Adhesives offer several design benefits over conventional mechanical fasteners including nails and rivets. The acrylic foam tape is a unique adhesive product that requires no curing and yet offers substantial property advantages for certain semi-structural applications. Virginia Tech proposes to implement an innovative assembly process in the construction industry through the development of materials, design, and application databases. The assembly process will be based on generic acrylic foam tapes and the test bed will be shear walls and diaphragms in light-frame construction for wind-critical areas, a very large market segment. The assembly process meets PATH (Partnership for Advancing Technology in Housing) goals of promoting housing affordability, durability, and wind-damage resistance. PATH's technical areas addressed are "advanced panel systems" and "whole-house and building process redesign."
The fundamental research component of the project will generate material data with emphasis on tape durability. Numerical structural modeling and cyclic tests of the assemblies will provide a basis for showing the adequacy of the tape to resist the dynamic loads associated with high wind events. A design methodology will be developed to enable field engineers to recommend and direct successful tape applications on construction sites. Shear wall and diaphragm models will be constructed for testing and demonstration.
Walls and diaphragms will be designed to take full advantage of load re-distribution capabilities, increased flexibility and damping, and improved fatigue resistance provided by the tapes. The advantages of the resilient foam tape in wind critical applications will be highlighted as meeting the PATH goal of improved disaster resistance. The application of the tape increases overall system stiffness, increases the resistance of the roof sheathing to wind uplift from hurricane loading, and tape sealing reduces water damage, the major property damage under hurricane conditions. In addition to simplifying the assembly/construction process, the design may offer enhanced performance including longer life, better appearance, reduced transportation costs, and environmentally friendly alternatives over the use of conventional fastening systems. Collaboration with the National Association of Home Builders (NAHB) through the PATH program will facilitate timely interactions with end users of technologies developed in the project and will speed up widespread adoption. A national homebuilder company will build a demonstration home if laboratory tests show feasibility.
The output of the project will be materials, joint performance, and application data on a simple but innovative assembly process. The databases and demonstration models will be focused on specific applications for wind-critical areas but will be applicable to a broad range of advanced adhesive tapes for use in housing construction.

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.
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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.

EEC-0552738
David A. Dillard


This REU award for a Site on materials and processes for proton exchange membrane (PEM) fuel cells supports 12 engineering and science students each year for three years in a 12-week research experience at Virginia Polytechnic Institute and State University. The primary goal of this REU site is to nurture a diverse group of talented undergraduate students to develop an appreciation for research and scholarship, a broader base of knowledge, a better understanding of the research process, and a desire to contribute to advances in science and engineering through graduate research and professional careers.

Students will be involved in research projects related to new fuel cell materials development; new methods for materials processing that yield high performance fuel cell assemblies; new techniques and theories for materials characterization; and new approaches for performance modeling that reveal the impact of materials and design on system performance. This program will build on Virginia Techs existing strengths in each of these research focus areas and their long history of successful REU programs which have engaged a large and diverse student population. The study of materials and processes for PEM fuel cells will offer an exceptional vehicle for introducing students to independent research, while exposing them to an important and rapidly developing technical field.

This site is supported by the Department of Defense in partnership with the NSF REU program.

CMMI 0826143

Fracture of Adhesive Bonds under Mixed Mode Loading:
Experiments in a Dual Actuator Load Frame and Numerical Simulations

D. A. Dillard, D. C. Ohanehi, R. C. Batra, J. G. Dillard
Virginia Tech, Blacksburg, Virginia

Under prior NSF support, a novel dual-actuator load frame was developed with capabilities to easily vary the fracture mode (relative amount of shear and opening displacements imposed) for fracture testing of adhesively bonded beams. The unique capabilities allow for a wide range of studies investigating fracture of adhesive systems and practical engineering joints. The current project will investigate the effect of changing different fracture modes on fracture energies and crack propagation under slow, cyclic fatigue, and impact loading situations; how applied stress state interacts with chemical surface treatments in determining failure; and the development of fracture envelopes useful in engineering design. New numerical techniques will be developed and used to model the material response under the applied loading conditions and also guide specific experiments to gain new insights into the fracture of bonded beams under different loading conditions.

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.
The improved understanding of bond failure obtained through this research offers opportunities for safer and more durable bonded structures. Two doctoral and several undergraduate students will participate in the proposed experimental and numerical modeling effort, obtaining significant experience in testing, mathematical modeling, numerical analysis, and effects of loading rates and adherend surface pretreatment on bonded joint performance. The research will train scientists who will design safer, lighter, stronger and more economical bonded joints. Results will be disseminated through premier technical journals, at conferences, and a web site.

Around 80 percent of women experience vaginal tears during labor, when the diameter of the vagina has to increase from 2.5 cm to 9.5 cm in order to allow the passage of a full-term baby. Complications associated with vaginal tears include postpartum hemorrhaging, fecal incontinence, urinary incontinence, and dyspareunia. Current evaluation methods of vaginal tears after childbirth are qualitative and rely on the expertise and training of obstetricians and midwives. This reliance upon subjective assessments often leads to incorrect diagnosis and inadequate treatment that severely compromise the quality of life of women. In order to establish new quantitative metrics for evaluating vaginal tears, a seamless experimental, theoretical, and computational characterization of the mechanical properties of the vagina -- including the tear resistance properties -- will be studied. This project will examine fundamental mechanical properties and response of this very under-studied tissue. The knowledge gained will support improved understanding of the response of vaginal tissue to the natural stretch that occurs during childbirth, and this will then support improved diagnostic, assessment, and treatment methods. Undergraduate and graduate students, mostly from underrepresented groups, will be recruited to participate in this highly interdisciplinary research and education program, which involves synergistic exchanges between the medical and STEM fields. In addition, in order to inspire, prepare, and empower undergraduate women to pursue graduate studies in engineering and help close the gender gap, a new one-day workshop offering preparatory information about graduate school will be organized at Virginia Tech.

In order to meet these project goals, entire vaginal canals from virgin and late pregnant rats will be subjected to inflation tests to induce tearing, and the resulting large inhomogeneous deformations will be measured using the digital image correlation technique. By using multi-photon microscopy and second harmonic generation imaging, the collagen fiber organization in the near- and far-fields of the tears will be measured. The experimental data will guide the development of new microstructurally-based constitutive models for the vaginal wall that are validated using a highly efficient data-driven reduced order modeling approach. Finally, new mechanics-based metrics for vaginal tear evaluation will be established following the theory of critical distances approach, using both the experimental data and data-driven models. This will include markers for levator ani trauma, predictors for maternal trauma, and protocols for pelvic floor trainer devices.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.