How To Determine Effective Gauge Length
If gripping pressure on the clamped specimen is not uniform throughout the clamped area, 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 precentage 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 determininng the presence and magnitude of possible grip penetration is to plot elongation against gauge length for a given 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 restult 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 the gauge length and the 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.
If gripping pressure on the clamped specimen is not uniform throughout the clamped area, 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 precentage 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 determininng the presence and magnitude of possible grip penetration is to plot elongation against gauge length for a given 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 restult 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 the gauge length and the 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.
2 comments:
Hi, Interesting theory here but what about deformation of the load cell, displacement of the wedge in case of wedge grip clamps and so on?
It look strange to me that you assign this “error” only to specimen extension in the clamps and not to all other factors that influence a tensile test negatively.
Thanks for any reaction
Best regards
Hello Anonymous
You are correct there is some load cell deflection, there is compliance of the frame, and there are loses in the drivetrain. Unavoidable in any mechanical tester of this sort. These are in most cases minimal compared to the topic at hand of “Grip Penetration Effects”. Regardless all these loses are inherently included, and no additional steps are required to the “Calculating Effective Gauge Length” procedure.
Best regards;
Frank
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