The Effect of Thin Film Adhesives on Mode I Interlaminar Fracture Toughness in Carbon Fiber Composites with Shape Memory Alloy Inserts.

Shape Memory Alloy (SMA) was placed within Polymer Matrix Composite (PMC) panels alongside film adhesives to examine bonding. Double cantilever beam (DCB) testing was performed using ASTM D5528. C-scanning was performed before testing, modal acoustic emissions (MAE) were monitored during testing, and microscopy performed post-test. Data was analyzed using modified beam theory (MBT), compliance calibration (CC) and modified compliance calibration (MCC) methods. Fracture toughness for control specimens was higher than previously reported due to fiber-bridging. Specimens with SMAs and adhesives stabilized crack propagation. Results revealed SMA-bridging; a phenomenon mimicking fiber-bridging which increased the load and fracture toughness of SMA specimens.

The Effect of Thin Film Adhesives on Mode II Interlaminar Fracture Toughness in Carbon Fiber Composites with Shape Memory Alloy Inserts.

A single sheet of nickel-titanium (NiTi) shape memory alloy (SMA) was introduced within an IM7/8552 polymer matrix composite (PMC) panel in conjunction with multiple thin film adhesives to promote the interfacial bond strength between the SMA and PMC. End notched flexure (ENF) testing was performed in accordance to ASTM D7905 method for evaluation of mode II interlaminar fracture toughness (GIIC) of unidirectional fiber-reinforced polymer matrix composites. Acoustic emissions (AE) were monitored during testing with two acoustic sensors attached to the specimens. The composite panels examined using scanning electron microscopy techniques after part failure. GIIC values for the control composite samples were found to be higher than those of samples with embedded SMA sheets. The presence of adhesives bonded to SMA sheets further diminished the GIIC values. AE values revealed poor bonding of the panels, with little to no signals during testing.

The Effects of Fiber Orientation and Adhesives on Tensile Properties of Carbon Fiber Reinforced Polymer Matrix Composite with Embedded Nickel-Titanium Shape Memory Alloys.

Tensile tests of Nickel-titanium (NiTi) shape memory alloys (SMA) embedded within carbon fiber reinforced polymer matrix composite (CFRP/PMC) laminates were evaluated with simultaneous monitoring of modal acoustic emissions (MAE). Three different layup configurations utilizing two different thin film adhesives were applied to bond the materials. Ultimate tensile strengths, strains, and moduli were obtained along with cumulative AE energy of events and specimen failure location. Scanning electron microscopy was used to examine the break areas of the specimens post-test. Microscopy was used to validate failure locations revealed from MAE analysis. 90° plies in the outer ply gave the strongest acoustic signals as well as the cleanest fracture of the specimens tested. Overlapping 0° ply layers surrounding the SMA was found to be the best scenario to prevent failure of the specimen itself.

Tailoring properties of cross-linked polyimide aerogels for better moisture resistance, flexibility, and strength.

Combinations of rigid and flexible aromatic diamines were used to tailor the properties of octa(aminophenyl)-silsesquioxane (OAPS) cross-linked polyimide aerogels. 2,2'-Dimethylbenzidine (DMBZ) or p-phenylenediamine (PPDA) was used in combination with the more-flexible diamine, 4,4'-oxydianiline (ODA). The amount of rigid diamine was varied from 0% to 100% of the total diamines in the backbone. The resulting aerogels vary in density, shrinkage, porosity, surface area, mechanical and thermal properties (depending on the type of diamine and the proportions of rigid diamine to flexible diamine used). Replacing ODA with PPDA increases shrinkage that occurs during gelation and processing, while increasing the DMBZ fraction decreases shrinkage. Replacing ODA with 50 mol% of DMBZ maintains the flexibility of thin films, while the moisture resistance of the aerogels is greatly improved.

Mechanically strong, flexible polyimide aerogels cross-linked with aromatic triamine.

Polyimide gels are produced by cross-linking anhydride capped polyamic acid oligomers with aromatic triamine in solution and chemically imidizing. The gels are then supercritically dried to form nanoporous polyimide aerogels with densities as low as 0.14 g/cm(3) and surface areas as high as 512 m(2)/g. To understand the effect of the polyimide backbone on properties, aerogels from several combinations of diamine and dianhydride, and formulated oligomer chain length are examined. Formulations made from 2,2'-dimethylbenzidine as the diamine shrink the least but have among the highest compressive modulus. Formulations made using 4,4'-oxydianiline or 2,2'dimethylbenzidine can be fabricated into continuous thin films using a roll to roll casting process. The films are flexible enough to be rolled or folded back on themselves and recover completely without cracking or flaking, and have tensile strengths of 4-9 MPa. Finally, the highest onset of decomposition (above 600 °C) of the polyimide aerogels was obtained using p-phenylene diamine as the backbone diamine with either dianhydride studied. All of the aerogels are suitable candidates for high-temperature insulation with glass transition temperatures ranging from 270-340 °C and onsets of decomposition from 460-610 °C.

Polyimide aerogels cross-linked through amine functionalized polyoligomeric silsesquioxane.

We report the first synthesis of polyimide aerogels cross-linked through a polyhedral oligomeric silsesquioxane, octa(aminophenyl)silsesquioxane (OAPS). Gels formed from polyamic acid solutions of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), bisaniline-p-xylidene (BAX) and OAPS were chemically imidized and dried using supercritical CO(2) extraction to give aerogels having density around 0.1 g/cm(3). The aerogels are greater than 90 % porous, have high surface areas (230 to 280 m(2)/g) and low thermal conductivity (14 mW/m-K at room temperature). Notably, the polyimide aerogels cross-linked with OAPS have higher modulus than polymer reinforced silica aerogels of similar density and can be fabricated as both monoliths and thin films. Thin films of the aerogel are flexible and foldable making them an ideal insulation for space suits, and inflatable structures for habitats or decelerators for planetary re-entry, as well as more down to earth applications.

Structure-property relationships in porous 3D nanostructures: epoxy-cross-linked silica aerogels produced using ethanol as the solvent.

Cross-linking silica aerogels with organic groups has been shown to improve the strength over un-cross-linked aerogels by as much as 2 orders of magnitude. Previous cross-linking chemistry has been developed using solvents specifically chosen to dissolve the monomers and accommodate the reaction temperature. Because the process of making the aerogels requires so much solvent, it is of interest to consider less toxic solvents such as ethanol to increase safety and enhance scale up. To this end, two different epoxy precursors with suitable solubility in ethanol were evaluated as cross-linkers for silica gels prepared from (3-aminopropyl)triethoxysilane and tetraethylorthosilicate. In addition, 1,6-bis(trimethoxysilyl)hexane (BTMSH) was used as an additive in the underlying silica structure to add flexibility to the aerogels. It was found that the ethanol-derived aerogels exhibited more shrinkage than those prepared from other solvents but that including BTMSH in the aerogels significantly reduced this shrinkage. Inclusion of BTMSH also imparted the ability of the aerogel monoliths to recover elastically when compressed up to 50% strain. In addition, optimized cross-linked aerogels prepared in this study have mechanical properties comparable to those using other more undesirable solvents and cross-linkers.