The correct way to use the Mylar strip is to measure from the center-width locations of the boundary line around the circumference of the now-deformed ellipse. Measuring from inside-to-inside or outside-to-outside is wrong! With a fuzzy ellipse boundary line (old stencil, poor gridding technique, etc.), it is not hard to make measurement errors more severe than just measuring the outside-to-outside dimensions of a crisp circle/ellipse. The width of the line forming the boundary of an etched circle is about 0.008″ which is also the thickness of the lines of the Mylar strip commonly used to measure the deformed ellipse manually (the diverging railroad tracks). If you are measuring inside-to-inside of a ellipse that was formed after starting with a 0.100″ diameter circle that was stretched 20% in one direction, you’ll measure the major axis as 0.120″-0.008″, or 0.112 inch, which is 12% on the major strain axis. You’ll also run the risk of measuring the minor strain wrong at 0.100”-0.008”, or 0.092 inch, which corresponds to a minor strain of -8%. Similarly, if you are measuring the outside-to-outside dimensions, you’ll wind up with 28% on the major strain axis and + 8% on the minor axis. A poor technique can turn a correct (20%, 0%) reading into anything between (28%,8%) and (12%,-8%)! Unless you are measuring from exactly the center- width position on the line making up the circumference of the circle/ellipse, you can get vastly different results, confusing the strain analysis interpretation.
The Importance of a Crisp Grid Pattern and Proper Grid Measuring Technique
About the Author: Danny Schaeffler
Danny received his Bachelor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and a Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA. While at Drexel, Danny held engineering and research positions with the David Taylor Research Center (Annapolis, MD) and Hoeganaes Corporation (Riverton, NJ).
After receiving his Doctorate, Danny started at the LTV Steel Technology Center (Independence, OH), where he was a member of the Customer Technical Center, focused on customer-based problem solving in the areas of corrosion, formability, and surface contaminants. He then transferred into the Automotive Development Group, focused on formability analysis and training; materials selection and optimization; tooling development and production launches for new vehicle programs; customer materials & manufacturing cost reduction efforts; forming/denting/structural computer simulation; and technical awareness and communications with the automotive manufacturers and their Tier One / Tier Two companies. Danny's first position in the ADG was as the engineer responsible for all transplant accounts, and then moved to having responsibility for the Ford Motor account, before being promoted to the Group Manager. During his time with LTV Steel, Danny was active in AISI and Auto/Steel Partnership activities, serving as Chairman of the Standardized Dent Resistance Test Project as well as participating in other committees. Danny is the founding member of Engineering Quality Solutions, Inc.
Danny has over a dozen publications and is a member of ASM International, SAE International, and is a former President of the North American Deep Drawing Research Group (www.NADDRG.org).