Nonaqueous Fluoride/Chloride Anion-Promoted Delamination of Layered Zeolite Precursors: Synthesis and Characterization of UCB-2

The delamination of layered zeolite precursor PREFER is demonstrated under mild nonaqueous conditions using a mixture of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride in N,N-dimethylformamide (DMF) as solvent. The delamination proceeds through a swollen material intermediate which is characterized using powder X-ray diffraction (PXRD). Subsequent addition of concentrated HCl at room temperature leads to synthesis of UCB-2 via delamination of the swollen PREFER material and is characterized using PXRD, transmission electron microscopy (TEM), and argon gas physisorption, which shows lack of microporosity in UCB-2. ^(29)Si magic angle spinning (MAS) NMR spectroscopy indicates lack of amorphization during delamination, as indicated by the entire absence of Q^2 resonances, and ^(27)Al MAS NMR spectroscopy shows exclusively tetrahedral aluminum in the framework following delamination. The delamination process requires both chloride and fluoride anions and is sensitive to solvent, working well in DMF. Experiments aimed at synthesizing UCB-2 using aqueous conditions previously used for UCB-1 synthesis leads to partial swelling and lack of delamination upon acidification. A similar lack of delamination is observed upon attempting synthesis of UCB-1 under conditions used for UCB-2 synthesis. The delamination of PREFER is reversible between delaminated and swollen states in the following manner. Treatment of as-made UCB-2 with the same reagents as used here for the swelling of PREFER causes the delaminated UCB-2 material to revert back to swollen PREFER. This causes the delaminated UCB-2 material to revert back to swollen PREFER. Altogether, these results highlight delamination as the reverse of zeolite synthesis and demonstrate the crucial role of noncovalent self-assembly involving the zeolitic framework and cations/anions/structure-directing agent and solvent during the delamination process

Delaminated areas beneath organic coating: A local electrochemical impedance approach

Local electrochemical impedance mapping was used to investigate delamination phenomena at the steel/epoxy-vinyl primer interface. The delamination occurred from an artificial defect(cutter scribing) and from ageing in a salt spray chamber. The samples were taken from the salt spray chamber after 20, 30 and 50 days of exposure. To observe delamination after ageing, the corrosion product layers were removed by a cathodic polarization at −1.5 V/SCE for 4 h. A non-aged reference sample was tested for comparison. Mapping was performed at 5 kHz. Initiation and propagation of the delamination were clearly observed. The delaminated surface areas measured by visual observations after the removal of the coating were lower than those determined by local electrochemical impedance mapping. The delamination mechanisms were discussed with reference to literature data

Delamination growth analysis in quasi-isotropic laminates under loads simulating low-velocity impact

A geometrically nonlinear finite-element analysis was developed to calculate the strain energy released by delamination plates during impact loading. Only the first mode of deformation, which is equivalent to static deflection, was treated. Both the impact loading and delamination in the plate were assumed to be axisymmetric. The strain energy release rate in peeling, G sub I, and shear sliding, G sub II, modes were calculated using the fracture mechanics crack closure technique. Energy release rates for various delamination sizes and locations and for various plate configurations and materials were compared. The analysis indicated that shear sliding (G sub II) was the primary mode of delamination growth. The analysis also indicated that the midplane (maximum transverse shear stress plane) delamination was more critical and would grow before any other delamination of the same size near the midplane region. The delamination growth rate was higher (neutrally stable) for a low toughness (brittle) matrix and slower (stable) for high toughness matrix. The energy release rate in the peeling mode, G sub I, for a near-surface delamination can be as high as 0.5G sub II and can contribute significantly to the delamination growth

Delamination failure in a unidirectional curved composite laminate

Delamination failure in a unidirectional curved composite laminate was investigated. The curved laminate failed unstably by delaminations developing around the curved region of the laminate at different depths through the thickness until virtually all bending stiffness was lost. Delamination was assumed to initiate at the location of the highest radial stress in the curved region. A closed form curved beam elasticity solution and a 2-D finite element analysis (FEA) were conducted to determine this location. The variation in the strain energy release rate, G, with delamination growth was then determined using the FEA. A strength-based failure criteria adequately predicted the interlaminar tension failure which caused initial delamination onset. Using the G analysis the delamination was predicted to extend into the arm and leg of the laminate, predominantly in mode I. As the initial delamination grew arould the curved region, the maximum radial stress in the newly formed inner sublaminate increased to a level sufficient to cause a new delamination to initiate in the sublaminate with no increase in applied load. This failure progression was observed experimentally

A new test methodology based on structural resonance for mode I fatigue delamination growth in an unidirectional composite

A specific device has been set up to test by vibration resonance the mode I fatigue delamination growth onset of composite laminates. This test system, based on the DCB test specimen, is a mass-spring-specimen dynamic system designed to resonate. The defined operating conditions allow performing delamination propagation tests under imposed load and stopping the test under reproducible conditions, identical to the ones recommended in the ASTM-D6115 standard. This system allows fatigue tests to be driven up to 100Hz, reducing the time taken by a factor of ten without detrimental heat being generated in the material. The effect of frequency on the fatigue delamination growth on mode I has been investigated through a comparison with standard tests performed at 10Hz. A decrease in resistance to the propagation of delamination is observed with the increase in frequency for the composite studied. This frequency effect seems to be a strain rate effect and was taken in consideration by using dynamical critical energy restitution rate for the G-N curve plotting

Analysis of delamination in unidirectional and crossplied fiber composites containing surface cracks

A two-dimensional hybrid stress finite element analysis is described which was used to study the local stress field around delamination cracks in composite materials. The analysis employs a crack tip singularity element which is embedded in a matrix interlayer between plies of the laminate. Results are given for a unidirectional graphite/epoxy laminate containing a delamination emanating from a surface crack through the outside ply. The results illustrate several aspects of delamination cracks: (1) the localization of the singular stress domain within the interlayer; (2) the local concentration of stress in the ply adjacent to the crack; (3) the nature of the transverse normal and interlaminar shear stress distributions; and (4) the relative magnitudes of K sub 1 and K sub 2 associated with the delamination. A simple example of the use of the analysis in predicting delamination crack growth is demonstrated for a glass/epoxy laminate. The comparisons with experimental data show good agreement

Delamination resistant composites by interleaving bio-based long-chain polyamide nanofibers through optimal control of fiber diameter and fiber morphology

In this work an innovative electrospinning system is proposed that simultaneously has an adequate temperature resistance, a high increase in mode I (þ51%) and mode II (þ96%) delamination performance and can be commercially produced. Interleaving nanofibrous veils can potentially solve the issue of the limited delamination resistance encountered in composite laminates, but industrial upscaling has always been impeded by one or more critical factors. These constraining factors include a limited temperature stability of the nanofibers, a lack in simultaneous mode I and II delamination performance increase and the complexity of the electrospinning system because non-commercial polymers or specialty nanofibers (e.g. coaxial) are required. In this paper, a robust electrospinning system is proposed that is the first to overcome all major hurdles to make nanofiber toughening industrially viable. A new class of nanofibers based on biosourced polyamide 11 and its poly(ether-block-amide) co-polymers is used to deal with those shortcomings. The nanofibers have tuneable diameters down to 50 nm and cross-section morphologies ranging from circular to ribbon-shaped. The key to this work is the fundamental underpinning of the toughening effect using a broad range of interleaves with different mechanical and thermal properties, fiber diameters and fiber morphologies, all produced from the same bio-based base polymer. Generally, round and thin nanofibers performed better than larger and ribbon-like fibers. The relationship between the fiber morphology and the delamination performance is further underpinned using detailed analysis of the fracture surface. Ultimately, this results in a range of optimized nanofibrous veils capable of improving the delamination resistance considerably without suffering from the aforementioned drawbacks