A nondestructive evaluation method is desired for ensuring the 'as manufactured' and 'post service' quality of graphite/epoxy instrumentation rack shells. The damage tolerance and geometry of the racks dictate that the evaluation method be capable of identifying defects, as small as 0.25 inch2 in area, over large acreage regions, tight compound radii and thickness transition zones. The primary defects of interest include voids, inclusions, delaminations and porosity. The potential for an IR thermographic inspection to replace ultrasonic testing for qualifying the racks as 'defect free' is under investigation. The inspection process is validated by evaluating defect standard panels built to the same specifications as the racks, except for the insertion of artificially fabricated defects. The artificial defects are designed to closely match those which are most prevalent in the actual instrumentation racks. A target defect area of 0.0625 inch2 was chosen for the defect standard panels to ensure the ability to find al defects of the critical size.
The measurement and control of cleanliness for critical surfaces during manufacturing and in service operations provides unique challenges in aerospace. For re-usable propulsion systems, such as the solid rocket motors, the current thrust for environmentally benign processes has had a major impact on programs designed for maintaining quality in the production of bondline surfaces. The major goal is to improve upon our ability to detect and identify possible contaminants which are detrimental to the integrity of the bondline. This effort requires an in-depth study of the possible sources of contamination, methodologies to detect and identify contaminants, discriminate between contaminants and chemical species caused by environmental conditions, and the effect of particular contaminants on the bondline integrity of the critical surfaces. This presentation will provide an overview of several optical methods used to detect and identify contamination on critical surfaces, currently being performed by the Surface Contamination and Analysis Team at Marshall Space Flight Center. The methods under development for contamination monitoring include FTIR and Near-IR SPectrometry, UV Fluorescence, and Variable Angle Spectroscopic Ellipsometry. Comparisons between these methods and the current primary tool, optical stimulation of electron emission for on-line inspection will be presented. Experiments include quantitative measurement of silicone and Conoco HD2 greases, metal hydroxides, tape residues, etc. on solid rocket motor surfaces.
The concurrent inspection of calibrated test panels manufactured with artificially created, although realistically behaving flaws is essential to providing confidence in the thermographic inspection process of advanced composite structure. For honeycomb type composite structures of principle interest is identifying delamination and disbond type defects along the bondline between the core and faceplate, as well as within the faceplate itself. To ensure that these types of flaws will be caught during the inspection cycle of a structural component the test panels must have similar behaving artificial defects. A common practice for the manufacture of artificial flaws in test panels is the use of embedded Teflon tape, or other release agents, for force an unbond condition within the laminate. These procedures though, yield results that are questionable, since one is not sure whether or not the inspection process is identifying the unbond or the inserted materials. Several fabrication methods are compared and contrasted in this paper, for controlling the degree of disbond to simulate defects resulting from mishandling or manufacturing errors without the need for inserting foreign materials in the laminate. These results are also compared to those obtained by inspecting a composite inter-tank test structure which used Teflon tape as the means to simulate critically sized defects.
Samples of ZBLAN optical fiber were heated to the pulling and crystallization temperature in microgravity aboard a sounding rocket and on the ground at 1g. This was done in order to better understand the effects of gravity on the crystallization behavior of ZBLAN fibers. Samples heated in 1g at both temperatures crystallized. Samples heated to the crystallization temperature in microgravity were contaminated with water upon re-entry. Samples heated to the pulling temperature showed no evidence of crystallization in microgravity.
The requirement to increase our understanding and control of processes has accelerated development of chemical sensor and analyzer technology. Analytical chemists anticipated the requirement to reduce the time between sampling and reporting the results. Multivariate statistical analyses when implemented on dedicated computers controlling modern instruments provide a mechanism for real time monitors. Implementation of these advanced techniques of analytical chemistry can also provide protection from environmental contaminants. The University of Alabama in Huntsville Laboratory For Inline Process Analyses has developed UV-visible-near infrared spectrophotometric methods that provide immediate, in-situ analyses. An appropriate light source illuminates the sample through a fiber optic. A second fiber then returns the reactance signal to the spectrophotometer. The spectrophotometer and computer are portable and can be used in a plant or by a field scientist. Implementation of computer programs based on multivariate statistical algorithms make possible obtaining immediate and reliable information from long data sets that may contain large amounts of extraneous information, for example, noise and/or analytes that we do not wish to control.
Space based materials processing experiments can be enhanced through the use of IVA robotic systems. A program to determine requirements for the implementation of robotic systems in a microgravity environment and to develop some preliminary concepts for acceleration control of small, lightweight arms has been initiated with the development of physical and digital simulation capabilities. The physical simulation facilities incorporate a robotic workcell containing a Zymark Zymate II robot instrumented for acceleration measurements, which is able to perform materials transfer functions while flying on NASA's KC-135 aircraft during parabolic maneuvers to simulate reduced gravity. Measurements of accelerations occurring during the reduced gravity periods will be used to characterize impacts of robotic accelerations in a microgravity environment in space. Digital simulations are being performed with TREETOPS, a NASA developed software package which is used for the dynamic analysis of systems with a tree topology. Extensive use of both simulation tools will enable the design of robotic systems with enhanced acceleration control for use in the space manufacturing environment.