Tack Measurements of Prepreg Tape at Variable Temperature and Humidity
NASA’s Advanced Composites Project has established a goal of achieving a 30% reduction in the timeline for certification of primary composite structures for application on commercial aircraft. Prepreg tack is one of several critical parameters affecting composite manufacturing by automated fiber placement (AFP). Tack plays a central role in the prevention of wrinkles and puckers that can occur during AFP, thus knowledge of tack variation arising from a myriad of manufacturing and environmental conditions is imperative for the prediction of defects during AFP. A full design of experiments was performed to experimentally characterize tack on 0.635 cm (0.25 in) wide slit-tape tow IM7/8552-1 prepreg using probe tack testing. Several process parameters (contact force, contact time, retraction speed, and probe diameter) as well as environmental parameters (temperature and humidity) were varied such that the entire parameter space could be efficiently evaluated. Mid-point experimental conditions (i.e., parameters not at either extrema) were included to enable prediction of curvature in relationships and repeat measurements were performed to characterize experimental error.
Collectively, these experiments enable determination of primary dependencies as well as multi-parameter relationships. Slit-tape tow samples were controllably adhered to the bottom plate of a rheometer using a fixture that prevented modification of the region to be interrogated with a polished stainless steel probe. The probe surface was slowly brought into contact with the prepreg surface until a pre-determined normal force was achieved (2-30 N). After a specified dwell time (0.02-10 s), during which the probe substrate interaction was maintained under displacement control, the probe was retracted from the surface (0.1-50 mm/min). Initial results indicated a clear dependence of tack strength on several parameters, with a particularly strong dependence on temperature and humidity. Although an increase in either of these parameters reduces tack strength, a maximum in tack was predicted to occur under conditions of low temperature and moderate humidity. The reasons for this are currently being evaluated and one possible explanation may be an increase in the “wettability” of the uncured resin without a concomitant increase in the “dewetting” of the probe surface upon retraction.
2017-12-07, 11:30 - 12:30
Trainer: Christopher John Wohl Jr., Ph.D.
Christopher J. Wohl, Ph.D. is a senior research scientist in the Advanced Materials and Processing Branch at the NASA Langley Research Center in Hampton, VA. He has experience in a variety of fields including physical, polymeric, and organic chemistry, laser spectroscopy, kinetics, and surface and interfacial sciences. He has researched the generation, characterization, and modification of novel materials for adhesion mitigation involving surface engineering and self-organizing systems. He has also been involved with process identification and modification for surface preparation of adhesively bonded interfaces including both composite and metallic substrates. He received a B.S. (Chemistry) from Virginia Polytechnic Institute and State University in 2002 and a Ph.D. (Physical Chemistry) from Virginia Commonwealth University in 2006. He is the author or coauthor of 28 peer reviewed journal articles, 59 other referenceable publications (conference proceedings), 22 invention disclosures, 13 patent applications, and 3 issued patents. He has received several awards including an Aviation Week Laureate Award in 2016, the NASA ARMD Associate Administrator Award in 2016, a NASA Group Achievement Award in 2014 for flight testing insect adhesion mitigation coatings, the NASA Langley H.J.E. Reid Award (2nd place) for an outstanding publication in 2013, the NASA Langley Engineering Directorate Innovation Award in 2011 (part of a six member team), the American Chemical Society’s Young Chemist Committee Leadership Award in 2010, among others.
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