Advanced technology for minimum weight pressure vessel system

Bosses were made of fiber/resin composite materials to evaluate their potential in lightweight pressure vessels. An approximate 25% weight savings over the standard aluminum boss was achieved without boss failures during burst tests. Polymer liners and metal liners are used in fiber composite pressure vessels for containment of gases. The internal support of these liners required during the filament winding process has previously been provided by dissolvable salt mandrels. An internal pressurization technique has been developed which allows overwinding the liner without other means of support and without collapse. Study was made of several additional concepts including styrene/Saran, styrene/flexible epoxy

High-performance fiber/epoxy composite pressure vessels

Activities described include: (1) determining the applicability of an ultrahigh-strength graphite fiber to composite pressure vessels; (2) defining the fatigue performance of thin-titanium-lined, high-strength graphite/epoxy pressure vessel; (3) selecting epoxy resin systems suitable for filament winding; (4) studying the fatigue life potential of Kevlar 49/epoxy pressure vessels; and (5) developing polymer liners for composite pressure vessels. Kevlar 49/epoxy and graphite fiber/epoxy pressure vessels, 10.2 cm in diameter, some with aluminum liners and some with alternation layers of rubber and polymer were fabricated. To determine liner performance, vessels were subjected to gas permeation tests, fatigue cycling, and burst tests, measuring composite performance, fatigue life, and leak rates. Both the metal and the rubber/polymer liner performed well. Proportionately larger pressure vessels (20.3 and 38 cm in diameter) were made and subjected to the same tests. In these larger vessels, line leakage problems with both liners developed the causes of the leaks were identified and some solutions to such liner problems are recommended

Improved Signal-to-noise Wideband Acoustic/ultrasonic Contact Displacement Sensors for Wood and Polymers

Research leading to a significant improvement in the signal-to-noise sensitivity of wideband acoustic/ultrasonic contact displacement sensors for wood and polymers is described. Design principles for such high-sensitivity sensors are reviewed. Comparisons of response between ceramic and polymer piezoelectric elements are made on low modulus specimens, A new, practical high-sensitivity sensor is characterized and its signal-to-noise sensitivity is compared to that of an existing commercial wide band displacement sensor. The comparisons were made for polymer, maple, and redwood samples. Optimization of the piezoelectric element in the new sensor is considered. The typical increased sensitivity of the new sensor is about 30 dB over the existing commercial sensor

Finite Element and Plate Theory Modeling of Acoustic Emission Waveforms

A comparison was made between two approaches to predict acoustic emission waveforms in thin plates. A normal mode solution method for Mindlin plate theory was used to predict the response of the flexural plate mode to a point source, step-function load, applied on the plate surface. The second approach used a dynamic finite element method to model the problem using equations of motion based on exact linear elasticity. Calculations were made using properties for both isotropic (aluminum) and anisotropic (unidirectional graphite/epoxy composite) materials. For simulations of anisotropic plates, propagation along multiple directions was evaluated. In general, agreement between the two theoretical approaches was good. Discrepancies in the waveforms at longer times were caused by differences in reflections from the lateral plate boundaries. These differences resulted from the fact that the two methods used different boundary conditions. At shorter times in the signals, before reflections, the slight discrepancies in the waveforms were attributed to limitations of Mindlin plate theory, which is an approximate plate theory. The advantages of the finite element method are that it used the exact linear elasticity solutions, and that it can be used to model real source conditions and complicated, finite specimen geometries as well as thick plates. These advantages come at a cost of increased computational difficulty, requiring lengthy calculations on workstations or supercomputers. The Mindlin plat theory solutions, meanwhile, can be quickly generated on personal computers. Specimens with finite geometry can also be modeled. However, only limited simple geometries such as circular or rectangular plates can easily be accommodated with the normal mode solution technique. Likewise, ..

Acoustic Emission Under Biaxial Stresses in Unflawed 21-6-9 and 304 Stainless Steel

Acoustic emission (AE) testing has been carried out with uniaxial and biaxial (2:1 stress ratio) stressing of smooth samples of 21-6-9 and 304 stainless steel (SS). Uniaxial testing was done with simple tensile and compression samples as well as with the special biaxial specimens. Biaxial tensile stressing was accomplished with a specially designed specimen, which had been used previously to characterize AE in 7075 aluminum under biaxial stressing. Results were obtained for air-melt and for vacuum-melt samples of 21-6-9 SS. The air-melt samples contain considerably more inclusion particles than the vacuum-melt samples. For the 304 SS, as received material was examined. To allow AE correlations with microstructure, extensive characterization of the 21-6-9 microstructure was carried out. Significant differences in AE occur in biaxially stressed specimens as compared to uniaxially stressed samples. 15 figures, 3 tables

Assessment of impact damage in Kevlar{reg_sign}-epoxy, filament-wound spherical test specimens by acoustic emission techniques

The results of a study of the acoustic emission (AE) behavior of impact-damaged, spherical, composite test specimens subjected to thermal cycling and biaxial mechanical loading are presented. Seven Kevlar{reg_sign}-epoxy, filament-wound, spherical composite test specimens were subjected to different levels of impact damage. The seven specimens were a subset of a group of 77 specimens made with simulated fabrication-induced flaws. The specimens were subjected to two or three cycles of elevated temperature and then hydraulically pressurized to failure. The pressurization regime consisted of two cycles to different intermediate levels with a hold at each peak pressure level; a final pressurization to failure followed. The thermal and pressurization cycles were carefully designed to stimulate AE production under defined conditions. Both impacted and nonimpacted specimens produced thermo-AE (the term given to emission stimulated by thermal loading), but impacted specimens produced significantly more. Thermo-AE was produced primarily by damaged composite material. Damaged material produced emission as a function of both rising and falling temperature, but the effect was not repeatable. More seriously damaged specimens produced very large quantities of emission. Emission recorded during the static portion of the hydraulic loading cycles varied with load, time, and degree of damage. Static load AE behavior was quantified using a newly developed concept, the event-rate moment, and various correlations with residual strength were attempted. Correlations between residual strength, long-duration events, and even-rate moments were developed with varying degrees of success