Strain at Break A

During tensile tests on metals, the initial gauge length L₀ increases until the point of break, up to length L_u. The initial gauge length-related permanent extension ΔL = L_u - L₀ is designated as strain at break A and specified as a percentage.

The strain at break A_t can only be determined with an extensometer, which remains on the specimen up to and including the point of break, and measures the extension of the specimen.

In the past, strain at break A was normally measured manually, while today it is also measured with extensometers. For automatic measurements, correct determination of the point at which the specimen breaks (point of break) is therefore of significant importance.

Formula for calculation of strain at break A:

Strain at break A = ΔL / L₀ x 100 [%]

If during the elongation up to the point of break, necking occurs, we differentiate between uniform elongation and necking.

• With uniform elongation, the strain is in large part distributed evenly over the gauge length until the maximum force is reached.
• In the case of necking, the specimen only continues to elongate in the area where the necking occurs after the maximum force has been reached.

When the break occurs at the center of the gauge length, the measured strain values are the highest and most accurate, because when the break occurs in the center, the deformation of the two halves of the specimen is even. If the break shifts towards the specimen grip, the spread of necking is impeded by the clamped shoulder ends of the specimen that are not part of the deformation. This results in lower strain at break values.

For better reproducibility of the measured values, so-called proportional specimens are often used for tensile tests. In these proportional specimens, the initial gauge length L₀ and the initial cross section S₀ are at a set ratio to each other.

Formula for flat specimens:
L₀ = k * √S₀

For flat specimens, the factor 5.65 is used internationally for k. Alternatively, a value of k=11.3 (for long specimens) can also be used.

Formula for round specimens:
L₀ = k * d₀

For round specimens, a value of k=5 (for short specimens) or alternatively k=10 (for long specimens) is common. Depending on the k value, the strain at break is indicated as A₅ (k=5) or A₁₀ (k=10).

The strain at break ε_b for polymer materials is the last recorded strain value before the stress drops to less than or equal to 10% of the strength.

In case of breaks above the yield point, the strain is specified as the nominal strain at break ε_tb. The nominal strain at break ε_tb is the last recorded nominal strain value before the stress drops to less than or equal to 10% of the strength. The nominal strain is determined from the measured values of the original gauge length L_e (distance between the clamps).