Processing-structure-property relationships in material extrusion additive manufacturing are complex, non-linear, and poorly understood. In this work, we designed an informatics workflow for the collection of high pedigree data from each stage of the fused filament fabrication (FFF) printing process. In conjunction with a design of experiments, we applied the workflow to investigate the influences of processing parameters on weld strength across three commercially available FFF printers. Environmental, material, and print conditions that may impact performance were monitored to ensure that relevant data were collected in a consistent manner. Acrylonitrile butadiene styrene (ABS) filament was used to print ASTM D638-14 Type V tensile bars. Data were analyzed using multivariate statistical techniques, including principal component analysis. The magnitude of the effects of extrusion temperature, layer thickness, print bed temperature, and print speed on the tensile properties of the final print were determined. The results demonstrated that printer selection is important and changes the impact of print parameters.

This research was supported in part by the US Army Education Outreach – Science and Engineering Apprentice Program - College Qualified Leaders Program at the US Army Research Laboratory and was administered by Academy of Applied Science (under contract number W911SR-15-2-0001).

The Diels-Alder reaction was used to yield thermal reversibility of the bonding between a partially furan-functionalized epoxy thermosetting matrix and a maleimide-treated glass fiber. Under ambient temperature conditions, the covalent bond forming product reaction dominates, but this reaction reverses at elevated temperatures to allow for interfacial healing. Single-fiber microdroplet pull-out testing was used to characterize the coupled effects of healing temperature and the glass transition temperature (T-g) of the epoxy on interfacial strength recovery. In particular, the roles of mobility and reaction kinetics were independently varied to understand the individual effects of both.

: The selection and substitution of materials is the keystone of successful engineering. Ground vehicle armor represents a complex and broad spectrum of possible designs that are continually evolving to meet the protection needs imposed by everemerging threats. Adhesive selection plays a critical role in lightweight armor design. Hence, it is vital to capture, consolidate, and organize adhesive data in a meaningful way for both engineering design and material advancement. Many adhesives have been available from the commercial market over the years. Those intended for aerospace applications tend to have the highest pedigree engineering criteria defined within existing databases. The Army s adhesive needs push the quest for desirable properties well outside of the aerospace regime, which makes a trial and error selection approach both costly and time consuming. The vastness and variance in candidate adhesives and their potential applications for the Army create an overly complex material selection problem. The goal of this research is to capitalize on modern database and materials informatics capabilities to facilitate the advancement of adhesion science at a faster rate.

Ionic liquid gels (ILGs) for potential use in ion transport and separation applications were generated via a free radical copolymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and N,N'-methylene(bis)acrylamide (MBA) using 1-ethyl-3-methylimidazolium ethylsulfate (IL) as a room temperature ionic liquid solvent medium. The AMPS and MBA monomer solubility window in the IL in the temperature range of 25 to 65 °C was determined. In situ ATR-FTIR showed near complete conversion of monomers to a cross-linked polymer network. ILGs with glass transition temperatures (T(g)s) near -50 °C were generated with T(g) decreasing with increasing IL content. The elastic moduli in compression (200 to 6600 kPa) decreased with increasing IL content and increasing AMPS content while the conductivities (0.35 to 2.14 mS cm⁻¹) increased with increasing IL content and decreasing MBA content. The polymer-IL interaction parameter (χ) (0.48 to 0.55) was determined via a modified version of the Bray and Merrill equation.

Diels-Alder (DA) chemistry is increasing popular due to its simplicity and efficiency, however, one concept that has yet to be thoroughly explored is incorporation of DA linkages within materials for the development of polymeric phase change materials It is well established that the retro-DA reaction results in a large endotherm. which could be a potential energy sink for phase change materials Hydroxyl-terminated polybutadiene (HTPB) was selected as a prepolymer and modified with different DA linkages Cured materials were prepared upon addition of diisocyanates and their physical properties of the cured elastomers were investigated using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and theological testing The resulting thermally responsive HTPB (TR-HTPB) exhibited the ability to absorb energy and flow at elevated temperatures Furthermore, they displayed unusual responses to repeated thermal cycling, including changes in thermal absorption and rheological characteristics

Adhesion of the reinforcement to the polymer matrix is essential for load transfer from the polymer matrix to the reinforcement material in fiber-reinforced composites. The reversible Diels–Alder reaction between a furan-functionalized epoxy-amine thermosetting matrix with a maleimide-functionalized glass fiber was used to impart remendability at the polymer–glass interface for potential application in glass fiber-reinforced composites. At room temperature the Diels–Alder adduct is formed spontaneously and above 90°C the adduct breaks apart to reform the original furan and maleimide moieties. Healing of the interface was investigated with single fiber microdroplet pull-out testing. Following complete failure of this interface, significant healing was observed, with some specimens recovering over 100% of the initial properties. Healing efficiency was not affected by the distance of displacement, with an overall average of 41% healing efficiency. Up to five healing cycles were successfully achieved. It is expected that a glass fiber-reinforced composite of maleimide-sized glass within a furan-functionalized network will demonstrate extension of fatigue life.

The Diels-Alder reaction was used to develop a reversibly cross-linking gel as a healing agent for traditional epoxy-amine thermosets. Direct application of the reversibly cross-linking network to a crack surface in an epoxy-amine thermoset resulted in the recovery of 37% of the initial epoxy-amine network's strength. Composites in which the reversibly cross-linking gel was incorporated as a secondary particulate phase recovered 21% of the initial composite strength after the first healing cycle, with healing possible up to five times.

Self-healing materials are particularly desirable for load-bearing applications because they offer the potential for increased safety and material lifetimes. A furan-functionalized polymer network was designed that can heal via covalent bonding across the crack surface with the use of a healing agent consisting of a bismaleimide in solution. Average healing efficiencies of approximately 70% were observed. The healing ability of fiber-reinforced composite specimens was investigated with flexural, short beam shear, and double cantilever beam specimens. It was found that solvent amount and maleimide concentration play key roles in determining healing efficiency.

Materials that can recover mechanical properties following failure offer increased safety and service life. Moreover, successful development of such systems would reduce factors of safety required in design thus reducing weight. Inspiration for remendable materials comes from nature where the ability to heal is a characteristic of living organisms and a prime example of autonomy. The development of self-healing polymeric material systems could therefore enable new multifunctional composite systems used in military applications. Two approaches for healing polymer networks have captured much attention. In one method, polymer networks are made to self-heal by adding particles filled with uncured resin. The resin held within the particles is released upon crack formation and cures to mend the damage. The other mechanism for healing relies on inherent reversibility of bonds found designed into polymer networks. The work to be presented is part of the Drexel-ARL Army Materials Center of Excellence (MCOE) program for polymers. Our approach to design self-healing composites combines advantages of healing via encapsulation and healing via reversible bonds. Incorporation of a healing agent allows for crack healing while maintaining the desirable physical and mechanical properties of the base thermoset. Reversible bonding of the healing agent provides the ability to crack heal cracks multiple times. We report on the development of two healing systems for epoxy-amine thermosets based on the thermoreversible Diels-Alder reaction of furan and maleimide. In one, crack healing of a traditional epoxy- amine thermoset is induced by thermally reversible crosslinking of a secondary phase. The secondary phase can be in the form of microspheres or fibrous mats. In the other, furan functionalization of an epoxy-amine thermoset allows for in-situ crack healing of this thermoset with a bismaleimide solution. Both phenomena occur at room temperature and minimal pressure and significant load recovery is possible multiple times in a given location. In both methods significant strength recovery is observed. In the second close to complete recovery is observed. In this case strength recovery is postulated to be the result of both physical and chemical bonding across the crack surface. Physical bonding is caused by solvent-mediated swelling and subsequent interlocking of crack surfaces, while chemical bonding results from the Diels-Alder reaction of furan and maleimide. This form of the Diels-Alder reaction is reversible, forming a ring structure at room temperature and reforming the respective diene and dienophile between 60 and 90°C. The degree of strength recovery is linked to a detailed kinetic analysis of the reversible reaction obtained using near IR spectroscopy. Moreover, the polymer network design is based on this analysis and takes into account intrinsic kinetics as well as diffusion limitations that limit the approach to equilibrium compositions at processing and healing temperatures.

: Quantitative determination of adhesive performance when bonding to ceramic substrates has traditionally been a challenge. The brittleness of ceramic materials limits the ability to easily machine these substrates into the specific geometries required for rigorous adhesive fracture energy measurements. In this research, a mixed-mode loading scheme was implemented using an asymmetric wedge test configuration to study the effects of adhesion, promoting surface treatments on fracture energy and bond line durability of titanium bonded to alumina using a structural epoxy film adhesive. This testing scheme limited bending to the more compliant titanium and minimized deformations in the thicker alumina. Additionally, machining the alumina was avoided due to the relative simplicity of the testing configuration. X-ray photoelectron spectroscopy showed that sandblasting the ceramic surface yielded improved reactivity toward the sol-gel adhesion promoter used in this study, which resulted in increased hot/wet bond durability. Weibull modulus calculations and field emission-scanning electron microscopy imaging of the ceramic fracture surfaces showed that surface flaws induced while sandblasting the ceramic did not decrease the flexural strength.

In this work, the surface modifications of various polymer films due to helium–oxygen dielectric barrier discharge (DBD) exposure operating under atmospheric pressure are reported. The polymer films studied include ultra high molecular weight polyethylene, polyamide, polytetrafluoroethylene and polyimide. Experimental results reveal increased hydrophilicity and surface energy of the plasma exposed polymers. This is attributed to the presence of oxygen containing groups grafted onto the surface during plasma treatment, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis. Scanning electron microscopy (SEM) data show the appearance of micro depressions, the size of which depends on the chemical structure and the treatment time, suggesting that mild etching occurs in a predicted fashion. Most importantly, this uniform modification occurs within a few seconds of exposure, time comparable to continuous on-line industrial processing.

: Evaluation of bond strength between dissimilar materials such as those in ceramic-metallic systems has to date been qualitative, offering no quantitative comparative results. This was due primarily to the difficulties inherent in handling and machining brittle materials such as ceramics. In this study a testing scheme was developed to circumvent the mechanical property inconsistencies of a metallic-ceramic system and allow quantitative evaluation of bond strength in a titanium/alumina composite system. Using a carefully designed half-wedge configuration, results are compared to a standard full-wedge test and show comparable bond strength and strain energy release rate values. Additionally, the effects of surface treatments on the ceramic-metallic bond strength are also evaluated with the half-wedge configuration as well as the effects of surface treatment on the mechanical integrity of the alumina ceramic.