Device provides accurate measurement of strain on flexible membranes and fabrics. It is compact and lightweight, has strain-amplification capability up to five, and has an accuracy better than one percent

Alley, V. L., Jr.

Mchatton, A. D.

English

NASA Technical Reports Server

December 1975	 B75-10300 
NASA TECH BRIEF 
Langley Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Amplifying Ribbon Extensometer 
The accurate measurement of strain on flexible 
membranes and/or fabrics has long been a challeng-
ing objective. High-strain measurement capability is 
required in mapping strains in parachute canopies, in 
stressed fabric structures, balloons, sails, and 
inflatables in general. Numerous measuring devices 
are available, but are, in general, oversophisticated 
and too bulky and heavy for the base upon which they 
are attached. In many devices, the gage extraction 
forces are so large that the strain field to be measured 
is grossly altered by the gage installation. 
A new flexible-ribbon extensometer can function 
successfully with extraction forces of 1 percent of the 
ultimate membrane force of the base material on
which it is installed. It is compact and lightweight, 
has a strain-amplification capability up to five and 
has an accuracy better than 1 percent. 
A number of configurations are possible, but the 
simplest is an amplified version and modification of 
the Ranes nonelectrical strain gage reported in NASA 
Compilation SP-5926 (02), Measurement Technology, 
page 28. That extensometer is fabricated from a 
highly-elastic base ribbon. Some of the elastic warp 
yarns are removed on equal spacing as required and 
are replaced by a relatively inelastic yarn called the 
indicator filament. An amplification factor of three is 
made possible by reeving the indicator filament as 
shown in the illustration. 
	
Elastic	 Color 
Half	 Band
	 Interface 
	
Loop \ 
.._-_--- -4— 	 Strain 
Scale
Removed 
Surface	 Knot	
Indicator
 
(or Tie) 
Reset 
Tall 
Half 
Loop 
Amplifying Ribbon Extensometer Installed
(continued overleaf) 
This document was prepared under the sponsorship of the National
	
Government assumes any liability resulting from the use of the 
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https://ntrs.nasa.gov/search.jsp?R=19750000300 2020-03-19T20:16:22+00:00Z
The indicator filament enters the ribbon at the knot 
and exits only for the half loops. The turns are 180 
degrees and are facilitated by the semicircular 
fabrications ofsurgical tibing called "half loops." For 
gains higher than five, units of concentric half loops 
of varying- radii are fabricated, and successful gages 
with gains as large as 10 have been made. The 
indicator filament is dyed a different color, extending 
from the zero point on the strain scale to the end of 
the reset tail. A suitable scale is printed- on the base 
ribbon to permit direct reading of strain by observing 
the position of the color interface on the indicator 
filament relative to the printed scale. The illustration 
shows a strain reading of 0.13. 
For large values of amplification, the observable, 
detectable, or indicated measurement provides in 
creased accuracy, particularly if both the strain and 
the base ribbon are small. The amplification level is 
limited, however, by the friction in the half loops and 
between the base ribbon and the filament. The gage 
extraction force increases essentially exponentially 
with the gain and becomes too large for most fabric 
and film applications as the gain exceeds five. 
Successful gages for gains with amplification 
factors of 10 are called "open-faced gages" or 
"partially-open-faced gages." For such gages the 
friction is reduced by not threading or by threading 
only the tail end of the indicator filament through the 
base ribbon. When there is no threading, the 
recording end of the indicator filament is tensioned 
and constrained to the maximum experienced strain 
by adding a unique tensioner to the half loops. 
For the type of gage illustrated, the encapsulation 
of the indicator filament within the base ribbon 
minimizes the possibility of the recorded strain being 
altered by external factors such as wind, surface 
abrasion, or dynamic motions. The low profile is 
interrupted only when the indicator filament exits the 
ribbon weave to effect the turns at the' half loops.
For open-faced gages, the encapsulation feature of the 
ribbon must be replaced by encasing the half loops 
and reeved filaments in an envelope sleeve of 
high-elongation, low-resistance material such as 
rubber dental dam or Hytrel. 
While being primarily of value in aerospace 
applications, in design and research test functions, it 
is conceivable that such devices could be useful where 
maximum displacements of strain excursions are 
required for critically packaged- items, for recording 
and verification of maximum shipping dynamics on 
delicate packaged devices, for accounting for life 
consumed -from strain of known cyclic frequency, or 
for biomedical measurements of mobility and strain of 
muscles. The amplification feature is acquired in a 
passive way, at a low cost, with light weight, an 
essentially planar geometry, and a reset capability. In 
addition, the extensometer is easy to install. - 
Note:. 
Requests-for further information may be directed 
to:
Technology Utilization Officer 
Langley Research Center	 . 
Mail Stop 139-A
	 . 
Hampton, Virginia 23665 . 
Reference: B75-10300  
Patent status: - 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
Patent Counsel	 -, 
-	 Langley Research Center 
Mail Stop 313 
Hampton, Virginia 23665 
Source: Vernon L. Alley, Jr., and

Austin D. McHatton
(LAR-11825)
. 
-.	 .	
.	 Categories: 06 (Mechanics) 
B75-10300	 04 (Materials)


Amplifying ribbon extensometer

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