The Importance of a Crisp Grid Pattern
and Proper Grid
Measuring Technique
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.