|
Determination of Effective Gage Length
If the pressure of the grips on the clamped specimen is not uniform throughout
the whole of the clamped length, a certain amount of specimen extension may take
place within the grips. When this happens, if the elongation is measured and
a calculation of percentage elongation is based on the separation between the
grip at the start of the test, the resulting figure will be in error.
Extension of the specimen within the grips is referred to as grip penetration.
It may not be apparent, from examining the load-elongation curve, whether grip
penetration has occurred or not, since it will not produce a stick-slip effect,
but it is proportional to the applied load. Because grip penetration is proportional
to load, the load-elongation curve will remain smooth and apparently normal.
A method of determining the presence and magnitude of possible grip penetration
is to plot elongation against gage length for a given applied force. If the
resulting line, when extrapolated to zero gage length, does not pass through
the origin, but gives a positive displacement on the elongation axis, then
this is the result of grip penetration. It is essential when performing these tests
to always test the specimen at the same strain rate since certain materials are
strain rate sensitive, i.e. the longer the gage length, the faster the required
crosshead speed and the ratio between gage length and crosshead speed will be
a constant.
The intercept gives a value Ej representing elongation within the grip at a
specified load. The quantity AE represents the true elongation for corresponding
gage length or grip separation. When calculating elongation from a load vs.
strain graph, the value Ej should be subtracted from the total elongation as
read directly from the chart before dividing this value by the gage length or
grip separation figure.
|