CALCULATING STRAIN AT FAILURE FOR YARNS AND TEXTILES USING THE EFFECTIVE GAUGE LENGTH METHOD
L
S dL
dS
Materials Testing Hints and Tips
The stress/strain curve is reasonably linear from 90% of the failure load up to failure for most highly-twisted yarns and tightly-woven textiles. This characteristic can be used to extrapolate a known strain at 90% to obtain the real strain at failure. The strain calculated using the initial grip separation at gauge length is in error due to grip penetration. The Effective Gauge Length (EGL) is a value by which the increase in grip separation must be divided to obtain the real strain of the specimen. The EGL can be determined by conducting a calibration test where two marks have been placed on the specimen at a known separation, L. The initial grip separation, S is noted. The test is now performed but stopped at 90% of the failure load. At this point the changes in separation dL and dS are noted. Then:
dL = true strain L
6.4
also:
dL dS = L EGL
therefore: EGL = dS.L dL So for subsequent tests, with an initial separation, S:
Strain at failure =
dS (failure) EGL
This technique can only be used for calculating strain at failure, because of the varying degrees of grip penetration as the specimen properties change during a test.
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