Elongation, also called elongation at break or percentage elongation (symbol A), is the increase in gauge length of a tensile specimen measured after fracture, expressed as a percentage of its original length.
How Elongation Is Measured
DIN EN ISO 6892-1 defines the tensile test on metallic materials at room temperature and, with it, the formula A = (Lu – L0) × 100 / L0, where L0 is the original gauge length and Lu the gauge length after the broken specimen halves are fitted back together. The gauge length reference matters: A5 uses a proportional round specimen with L0 equal to five times its diameter, while A80 uses a fixed 80 mm gauge length regardless of specimen diameter. The two values are not directly comparable for the same material.
Elongation in Foundry Practice
Elongation is the standard measure of ductility, and cast iron grades sit at opposite ends of the scale. Grey cast iron with lamellar graphite (EN-GJL) shows practically no elongation at break, because the graphite lamellae act as internal notches that trigger brittle fracture with almost no plastic deformation. Ductile cast iron with spheroidal graphite (EN-GJS) behaves differently: its rounded graphite nodules do not concentrate stress the same way, so the matrix can deform measurably before it fails. The elongation value is even written into the EN-GJS grade name itself, directly after the tensile strength figure.
Elongation Across EN-GJS Grades
| Grade (DIN EN 1563) | Min. Tensile Strength | Min. Elongation A5 |
|---|---|---|
| EN-GJS-400-15 | 400 N/mm² | 15% |
| EN-GJS-500-7 | 500 N/mm² | 7% |
| EN-GJS-600-3 | 600 N/mm² | 3% |
| EN-GJS-700-2 | 700 N/mm² | 2% |
These are the minimum values for separately cast test samples, which DIN EN 1563 defines independently of the wall thickness of the actual casting. For samples cast onto the component itself, the standard sets lower minimum values as the casting’s wall thickness increases, because thicker sections cool more slowly and coarsen the graphite structure. Check the standard’s cast-on sample tables before applying a grade’s headline figures to a specific wall thickness.
Frequently Asked Questions About Elongation
What counts as a good elongation value for cast iron?
It depends entirely on the grade and the matrix structure. Ferritic ductile iron grades reach the highest elongation, up to around 15% or more, while pearlitic and higher-strength grades trade elongation for tensile strength. Grey cast iron stays close to 0% regardless of grade.
What is the difference between A5 and A80?
Both are elongation values, but with different gauge length definitions. A5 scales the gauge length to the specimen diameter, A80 fixes it at 80 mm no matter the diameter. The resulting percentages are not interchangeable for the same batch of material.
Does a high elongation value also mean good impact toughness?
Not necessarily. Elongation describes slow, quasi-static ductility from a tensile test. Impact toughness under sudden loading is measured separately, typically with a Charpy test, and the two properties do not always move together.
Standard Reference
DIN EN ISO 6892-1 defines the test method and the elongation formula. DIN EN 1563 sets the minimum elongation values for each ductile cast iron grade, built directly into the grade designation.
Further Terms
- Tensile strength – the paired figure in the same tensile test and in the EN-GJS grade name
- Vermicular graphite – a graphite shape between lamellar and spheroidal, with intermediate elongation
- Cementite – a hard, brittle phase that lowers elongation wherever it appears in the matrix
- Quality symbols – a separate marking system, not to be confused with elongation values