GRIP PENETRATION EFFECTS

Determining Effective Gauge Length

If grip pressure on the clamped specimen is not uniform throughout the clamped length, a certain amount of specimen extension may take place within the grips. When this happens, if the elongation is measured from the test curve 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 since it will not produce a stick-slip effect, but it is proportional to the applied load. As grip penetration is proportional to load, the load elongation curve will remain smooth and apparently normal.

A method for determining the presence and magnitude of possible grip penetration is to plot elongation against gauge length for a given applied force (Fx). If the resulting line, when extrapolated to zero gauge 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. For example, the longer the gauge length, the faster the required crosshead speed, and the ratio between gauge length and crosshead speed will be constant.

Calculating Effective Gauge Length

The intercept gives a value Ej representing elongation within the grip at a specified load. The quantity AE represents the true elongation for a corresponding gauge length or grip separation. When calculating elongation from a load versus strain graph, the value Ej should be subtracted from the total elongation before dividing this value by the gauge length or grip separation figure.