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The Challenge
A major supplier of geotextiles needed a way of measuring
strain when testing in accordance with ASTM D4595. The geotextile
is a woven material that is used for erosion control in reinforced
embankments. The test method covers the measurement of tensile properties
using a 200 mm wide strip of woven material.
A 100 mm gauge-length extensometer was required because
the customer used roller type grips to hold the material. The strain
could not be reliably measured from grip separation because the
gauge length is unknown with roller type grips.
Moreover, a contacting type extensometer would be inadequate for several reasons:
- Material damage caused by pin insertion
- Difficulty with the set up
- Errors resulting from looseness or play
- Extensometer damage from high energy release failures.
The Solution
Instron's video extensometer uses a standard video
camera with a 350 mm field of view to track contrasting lines applied
to the material. The two contrasting lines, which are separated
by approximately 100 mm, are applied to the material using a felt
pen or by applying a thin adhesive tape.
Application of the marks is not critical because
the system measures the separation of the marks before the beginning
of each test. Measured strain is obtained from the average separation
of the two marks, in contrast to a contacting extensometer or laser
extensometer which measures the strain at a single point. The marks
can be any length up to 87 mm. For more information see ASTM publication
STP 1379 “Wide Width Geotextile Testing with Video Extensometry”
by David Jones.
System Characteristics
- Model 4484 150 kN electromechanical test instrument
- 200 mm wide roller grips
- Non-contacting video extensometer
- Series IX™ software for test control and data analysis
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