12Determination of percentage total elongation at maximum force (Agt)

The method consists of determining on the force-extension diagram obtained with an extensometer, the extension at maximum force ( $\Delta L_{\mathrm{m}}$ ).

Some materials exhibit a flat plateau at maximum force. When this occurs, the percentage total elongation at maximum force is taken at the mid-point of the flat plateau (see figure 1).

The extensometer gauge length shall be recorded in the test report.

The percentage total elongation at maximum force is calculated by the following formula:

$A_{\mathrm{gt}}=\frac{\Delta L_{\mathrm{m}}}{L_{\mathrm{e}}} \times 100$

If the tensile test is carried out on a computer controlled testing machine having a data acquisition system, the elongation is directly determined at the maximum force.

For information, a manual method is described in annex H.