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1. A RADIATION SENSITIVE ADHESIVE SYSTEM FOR FUNCTIONALLY GRADED COMPOSITE JOINTS

   https://doi.org/10.12783/asc36/35946  

   HURVITZ, SAMUEL B.; STAPLETON, SCOTT; HUSSEINI, JAMAL (September 2021, Proceedings of the American Society for Composites Technical Conference)

   Ozden, O. (Ed.)

   Adhesively bonded composite joints can help reduce weight in structures and avoid material damage from fastener holes, but stress concentrations formed at the edges of the adhesive bond line are a main cause of failure. Stress concentrations within the adhesive can be reduced by lowering the stiffness at these edges and increasing the stiffness in the center of the joint. This may be achieved using a dual-cure adhesive system, where conventional curing is first used to bond a lap joint, after which high energy radiation is applied to the joint to induce additional crosslinking in specific regions. Anhydride-cured epoxy resins have been formulated to include a radiation sensitizer enabling the desired cure behavior. Tensile testing was performed on cured systems containing varying levels of radiation sensitizer in order to evaluate its effects on young’s modulus as a function of radiation dose. 

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2. Functionally Graded Adhesives Joints with Enhanced Strength

   https://doi.org/10.2514/6.2019-0234  

   Cassano, Alessandro G.; Stapleton, Scott E.; Schmidt, Daniel F. (January 2019, AIAA SciTech Forum)

   Functionally graded adhesive bondlines are currently being researched to relax stress concentrations at the re-entrant corner of bonded joints and improve the strength of joints. Bi-adhesive joints have been under development for some time, but lately adhesives with continuous gradation have been shown to theoretically enable more stress reductions and greater strength benefits. Several researchers have shown the potential to create a working adhesive gradation system with very promising results, but adhesive stability over long periods of time has proven difficult to realize. Nearly as important as adhesive development are analysis methods for functionally graded adhesive joints, since the gradation must be designed to yield beneficial results. Therefore, this work addresses the potential gains provided by design of functionally graded adhesive joints driven by finite element analysis. A parametric study on a strap joint with homogenous adhesive is conducted to highlight parameters which influence the global strength of an adhesively bonded joint. A statistical approach is used to identify significant correlations between strength and adhesive material parameters. Results from the statistical study are applied to drive strategies to create joints with optimized gradation and validated by failure analysis within the finite element model. A strap joint is analyzed as example of the potential gain of functionally graded joints. 

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3. Evolution of the gekkotan adhesive system: Does digit anatomy point to one or more origins?

   https://doi.org/doi:10.1093/icb/icz006  

   Russell, A. P.; T. Gamble, T. (January 2019, Integrative and comparative biology)

   Synopsis Recently-developed, molecularly-based phylogenies of geckos have provided the basis for reassessing the number of times adhesive toe-pads have arisen within the Gekkota. At present both a single origin and multiple origin hypotheses prevail, each of which has consequences that relate to explanations about digit form and evolutionary transitions underlying the enormous variation in adhesive toe pad structure among extant, limbed geckos (pygopods lack pertinent features). These competing hypotheses result from mapping the distribution of toe pads onto a phylo- genetic framework employing the simple binary expedient of whether such toe pads are present or absent. It is evident, however, that adhesive toe pads are functional complexes that consist of a suite of integrated structural components that interact to bring about adhesive contact with the substratum and release from it. We evaluated the competing hypotheses about toe pad origins using 34 features associated with digit structure (drawn from the overall form of the digits; the presence and form of adhesive scansors; the proportions and structure of the phalanges; aspects of digital muscular and tendon morphology; presence and form of paraphalangeal elements; and the presence and form of substrate compliance-enhancing structures). We mapped these onto a well-supported phylogeny to reconstruct their evolution. Nineteen of these characters proved to be informative for all extant, limbed geckos, allowing us to assess which of them exhibit co- occurrence and/or clade-specificity. We found the absence of adhesive toe pads to be the ancestral state for the extant Gekkota as a whole, and our data to be consistent with independent origins of adhesive toe pads in the Diplodactylidae, Sphaerodactylidae, Phyllodactylidae, and Gekkonidae, with a strong likelihood of multiple origins in the latter three families. These findings are consistent with recently-published evidence of the presence of adhesively-competent digits in geckos generally regarded as lacking toe pads. Based upon morphology we identify other taxa at various locations within the gekkotan tree that are promising candidates for the expression of the early phases of adhesively-assisted locomotion. Investigation of functionally transitional forms will be valuable for enhancing our understanding of what is necessary and sufficient for the transition to adhesively-assisted locomotion, and for those whose objectives are to develop simulacra of the gekkotan adhesive system for biotechnological applications. 

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4. Characterization of Functionally Graded Adhesives Using Radiation Curing

   https://doi.org/https://doi.org/10.2514/6.2021-1403  

   Najafian, S.; Cassano, A. G.; Stapleton, S. E.; Schmidt, D. F (January 2021, AIAA Scitech 2021 Forum)

   null (Ed.)

   Grading adhesive properties across a bondline can lead to more unniform stresses and increased strength without altering the geometry of the adherends. In this research, radiation sensitizers have been added to adhesives to create a secondary cross-linking possibility that is activated with radiation. In this way the adhesive stiffness and strength can be controlled by controlling the exposure to radition. In this paper, a system of grading adhesive properties is introduced and the double cantilver beam test results show that the gradation not only changes stiffness and strength, but also mode I fracture properties. Additionally, specimens were created with graded properties along the bondline and test results will be presented in the final paper. 

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5. Finite element modeling with subject‐specific mechanical properties to assess knee osteoarthritis initiation and progression

   https://doi.org/10.1002/jor.25338  

   Lampen, Nathan; Su, Haoyun; Chan, Deva D.; Yan, Pingkun (April 2022, Journal of Orthopaedic Research)

   Finite element models of the knee can be used to identify regions at risk of mechanical failure in studies of osteoarthritis. Models of the knee often implement joint geometry obtained from magnetic resonance imaging (MRI) or gait kinematics from motion capture to increase model specificity for a given subject. However, differences exist in cartilage material properties regionally as well as between subjects. This paper presents a method to create subject-specific finite element models of the knee that assigns cartilage material properties from T2 relaxometry. We compared our T2-refined model to identical models with homogeneous material properties. When tested on three subjects from the Osteoarthritis Initiative data set, we found the T2-refined models estimated higher principal stresses and shear strains in most cartilage regions and corresponded better to increases in KL grade in follow-ups compared to their corresponding homogeneous material models. Measures of cumulative stress within regions of a T2-refined model also correlated better with the region's cartilage morphology MRI Osteoarthritis Knee Score as compared with the homogeneous model. We conclude that spatially heterogeneous T2-refined material properties improve the subject-specificity of finite element models compared to homogeneous material properties in osteoarthritis progression studies. Statement of Clinical Significance: T2-refined material properties can improve subject-specific finite element model assessments of cartilage degeneration. 

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