Lynn S. Penn

Damage Detection by Piezoelectric Patches in a Free Vibration Method

An experimental program was undertaken to evaluate the feasibility of using piezoelectric patches for damage detection in composite materials. The fact that damage development can cause shifts in the natural frequencies of a structural component suggested an impulse-frequency response approach, in which free vibration was initiated by a single external mechanical pulse and was sensed by piezoelectric patches. Patches in both surface-bonded and embedded configurations were tried. The investigation was conducted on unidirectional glass fiber/epoxy laminated plates containing controlled levels of damage, present as interply delaminations. The data obtained from the piezoelectric patches showed the expected decrease in frequency of the natural vibrational modes with increase in damage. The attractiveness of this method lies in its convenience and its potential for use in the field.

Development of a single fiber fragmentation test for high strain rates

Abstract A new, high-strain-rate test of the fiber–matrix interface is described. The test is an adaptation of the single fiber fragmentation test, which has been conducted only at low strain rates up to now. Like its low-rate counterpart, the new, high-rate test is intended for providing comparisons of fiber surface treatments used in interface tailoring. The new test will be valuable whenever interface tailoring is the strategy used for enhancement of composite performance at high rates. Test validation was carried out on three different fiber–matrix systems, with low-rate results on those systems used as controls. The high-rate test was found to provide a reliable evaluation of whether interface improvements demonstrated at low rate are also evident at high rate.

Use of the Free Vibration Spectrum to Detect Delamination in Thick Composites

A free vibration method was evaluated for damage detection in thick composite plates, whereby vibration was initiated by a single external mechanical pulse and was sensed by piezoelectric patches bonded to the surface of the composite plate. The investigation was conducted on unidirectional fiber glass/epoxy plates approximately 0.387 in (==10 mm) thick, containing controlled delaminations. The free vibration method described in this paper was found relatively insensitive for identifying small to medium delaminations in thick plates. Consequently, several frequency computations using a finite element model were performed to determine the influence of delamination size and location for a single delamination.

Evidence for relative radius of gyration as the criterion for selective diffusion behavior of polymer brushes.

When exposed to a dilute solution of free species, the polymer brush functions as a selective barrier to diffusion. Experiments with linear polymer chains and dendrimers of various sizes demonstrated that the selection criterion is relative size, i.e., radius of the free species (radius of gyration for linear chains and simple radius for dendrimers) relative to the radius of gyration of the chains composing the brush. This suggests that linear chains do not necessarily assume extended conformations as they diffuse into a brush but have conformations similar to those of nanoscale spherical inclusions.

Toughening of impenetrable interfaces by monodisperse tethered polymer chains: effect of areal attachment density

Experiments were conducted to evaluate the effect of areal attachment density of monodisperse tethered chains on the toughness of an impenetrable interface. The interfacial toughness was found to be sensitive to the areal attachment density, with a reduction in attachment density enhancing the toughness. These findings are consistent with the notion that when the tethered chains are in the stretched conformation, the areal attachment density must be reduced to permit mixing with the matrix free chains. We conclude that the observed improvements in interfacial toughness were a result of increased mixing, i.e. molecular level interaction, between tethered and matrix chains.

Toughening of impenetrable interfaces by polydisperse tethered polymer chains: effect of tethered chain length

Experiments were conducted to evaluate the effect of polydisperse tethered chains on the toughness of an impenetrable interface. The results indicate that tethered chains improved the toughness of such interfaces - an improvement that we attribute to an interaction between the tethered chains and the matrix free chains. An increase in the tethered chain length relative to the free chain length, plus the natural polydispersity of the tethered chains, appears to underlie this beneficial interaction.

Chain end analysis of bisphenol A polysulfone and its relation to molecular weight

The preparation of amine-terminated polysulfone by step polymerization of the monomers bisphenol A and dichlorodiphenyl sulfone in the presence of end-capping reagent 4-aminophenol was investigated. A persistent problem with end-capping strategy as applied to step polymerization is the presence of end groups other than those introduced by the end-capping reagent. These unintended end groups, which can persist in the polymer product even when 100% of the end-capping reagent has reacted, are associated with a proportionate decrease in polymer chain length. This situation renders quantitative analysis of a single type of end group invalid as a method for molecular weight determination. The presence of unintended end groups does not appear to correlate with a particular set of reaction conditions; unintended end groups were found to occur in polymerizations conducted under strong base conditions (NaOH), under weak base conditions (K2CO3), and with a wide range of monomer feed ratios. A scheme for unambiguous quantification of chain ends and molecular weight for end-capped polysulfone is described.

Evaluation of the potential usefulness of in situ fiber optic sensors for column monitoring in chromatographic separations and purifications

Fiber optic sensors show promise for real-time monitoring of local conditions inside chromatography columns. Experiments on a Sephadex G-50 column, with blue dextran as the test analyte, showed that insertion of a single fiber or a lattice of fibers did not interfere with column fluidics, provided the fiber diameter was within the same order of magnitude as the diameters of the particles composing the column packing. Experiments on a Sephadex G-50 column, with fluorescein as the test analyte, showed that column packing particles do not interfere with the sensors ability to accurately reflect differences in fluorescence produced by fluorescein under different conditions (e.g., concentration, pH). Experiments with fluorescein on a G-50 column also showed that fiber optic sensors can be used to monitor local analyte concentrations within a column and to diagnose column problems which lead to reduced column efficiency.

Improvement of Interfaces by Tethered Polymer Chains. Part I: Evaluation of the Interface in a Single-Fiber Composite at Low and High Strain Rates

A study of the effect of tethered chains on the fiber-matrix interface in a single-fiber composite at low and high strain rates was conducted. The materials system studied was composed of glass fiber, polysulfone matrix, and polysulfone tethered chains. Single fiber fragmentation tests demonstrated that the tethered chains were beneficial to the interface not only at a typical low strain rate, but also under impact at an estimated strain rate >2000 times higher. It is believed that the tethered chains, by entangling with the matrix chains, provide energy dissipation mechanisms that operate over a wide range of time scales to improve the performance of the interface.

Improvement of Interfaces by Tethered Polymer Chains. Part II: Evaluation of a Particulate Composite at Very High Strain Rate

A study of the effect of tethered chains at the fiber-matrix interface on the performance of a particulate composite at very high strain rate was conducted. The materials system studied was composed of glass beads, polysulfone matrix, and polysulfone tethered chains. The entanglement of the tethered chains with the matrix in the composite specimen was confirmed by physico-chemical analysis. Although the tethered chains had been shown to have a beneficial effect on the interface in a single-fiber, model composite, they were found to have no effect whatsoever on the macroscopic behavior of the particulate composite at very high strain rate (150/s).